Folicullar lymphoma

Lymphomas are the seventh most common cancer in
western countries. The incidence is higher in industri-
alized countries than in developing countries, higher
in adults than in children, and higher in men than in
women1
. As indicated in the fourth edition of the
WHO Classification of Tumours of Haematopoietic
and Lymphoid Tissues2
, lymphomas are distinguished
into precursor cell and mature cell neoplasms. Mature
cell neoplasms are classified as B cell lymphomas, T cell
lymphomas and Hodgkin lymphoma. B cell lymphomas
are named according to the location of the related cell
type within the lymphoid follicle3
and include follicu-
lar lymphoma (FL) and diffuse large B cell lymphoma
(DLBCL) (Fig. 1). FL (also known as follicle-related B cell
lymphoma) is a B cell malignancy with a morphological
appearance resembling a follicular lymphoid structure.
FL cells maintain the same state of differentiation as that
of germinal centre (GC) B cells within the secondary lym-
phoid follicle. Similarly, mantle cell lymphoma is related
to the mantle zone, and marginal zone lymphoma (MZL)
is related to the marginal zone of a lymphoid follicle3
.
FL is characterized by a follicular (nodular) pattern
of growth as well as by varying degrees of diffuse neo-
plastic growth. Earlier names for the disease, although
no longer in use, reflected its appearance. It was termed
nodular lymphoma in the 1956 Rappaport classification4
and centroblastic/centrocytic lymphoma, referring to
its cytology, in the 1974 Kiel classification5
. In the 1994
Revised European–American Lymphoma (REAL)
Classification6
, it was called follicular centre lymphoma
because it is a systemic nodal neoplasm composed of
centroblasts and centrocytes of the GC in the follicle. FL
usually presents in supradiaphragmatic and abdominal
lymph nodes, and eventually disseminates to extranodal
deposits in the bone marrow and, less frequently, in
other organs7
. Approximately 15 years ago, the treatment
of patients with FL was revolutionized by the introduc-
tion of therapeutic monoclonal antibodies, in particular
rituximab, which targets the B cell marker CD20 (ref.7
).
Today, these biological drugs are administered either
alone or in combination with chemotherapy.
FL displays marked heterogeneity as it comprises
several morphological variants and specific subtypes3,8
.
Morphological variants should be recognized to avoid
misdiagnosis, whereas specific FL subtypes should be
identified owing to their different clinical courses2,3
.
Morphological variants and specific clinical subtypes
are not mutually exclusive designations. Although FL
is usually an indolent disease with a median overall
survival of >15 years, FL remains an incurable malig-
nancy. A particularly vexing clinical problem is that
~20% of patients progress or relapse in the first 2 years
following treatment initiation with a dismal prognosis
(5-year progression-
free survival (PFS) of ~60%).
Follicular lymphoma
Antonino Carbone1
*, Sandrine Roulland2
, Annunziata Gloghini3
, Anas Younes4
,
Gottfried von Keudell4
, Armando López-Guillermo5
and Jude Fitzgibbon6
Abstract|Follicularlymphoma(FL)isasystemicneoplasmofthelymphoidtissuedisplaying
germinalcentre(GC)Bcelldifferentiation.FLrepresents~5%ofallhaematologicalneoplasmsand
~20–25%ofallnewnon-Hodgkinlymphomadiagnosesinwesterncountries.Tumorigenesisstarts
inprecursorBcellsandbecomesfull-blowntumourwhenthecellsreachtheGCmaturationstep.
FLisprecededbyanasymptomaticpreclinicalphaseinwhichpremalignantBcellscarryinga
t(14;18)chromosomaltranslocationaccumulateadditionalgeneticalterations,althoughnotall
ofthesecellsprogresstothetumourphase.FLisanindolentlymphomawithlargelyfavourable
outcomes,althoughafractionofpatientsisatriskofdiseaseprogressionandadverseoutcomes.
OutcomesforFLintherituximaberaareencouraging,with~80%ofpatientshavinganoverall
survivalof>10years.PatientswithrelapsedFLhaveawiderangeoftreatmentoptions,including
severalchemoimmunotherapyregimens,phosphoinositide3-kinaseinhibitors,andlenalidomide
plusrituximab.Promisingnewtreatmentapproachesincludeepigenetictherapeuticsandimmune
approachessuchaschimericantigenreceptorT celltherapy.Theidentificationofpatientsathigh
riskwhorequirealternativetherapiestothecurrentstandardofcareisagrowingneedthatwill
helpdirectclinicaltrialresearch.ThisPrimerdiscussestheepidemiologyofFL ,itsmolecularand
cellularpathogenesisanditsdiagnosis,classificationandtreatment.
1
Centro di Riferimento
Oncologico di Aviano IRCCS,
Aviano, Italy.
2
Aix Marseille University,
CNRS, INSERM, Centre
d’Immunologie de Marseille-
Luminy, Marseille, France.
3
Department of Diagnostic
Pathology and Laboratory
Medicine, Fondazione IRCCS
Istituto Nazionale Tumori,
Milan, Italy.
4
Lymphoma Service,
Memorial Sloan Kettering
Cancer Center, New York,
NY, USA.
5
Department of Hematology,
Hospital Clínic, IDIBAPS,
Barcelona, Spain.
6
Barts Cancer Institute,
Queen Mary University of
London, London, UK.
*e-mail: acarbone@cro.it
https://doi.org/10.1038/
s41572-019-0132-x
1
PRIMER
NAture RevIewS | DISEASE PRIMERS | Article citation ID: (2019) 5:83
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Lymph node
Secondary follicle
Primary
follicle
Medulla
Paracortex
Cortex
Naive
B cell
Antigen
Dark zone Light zone
Centroblast
Centrocyte
FDC
MHC II
TCR
TFH
cell
Apoptosis
GC B cell-like
DLBCL
Activated
B cell-like DLBCL
DLBCL NOS
Long-lived
plasma cell
Memory B cell
Plasmablast
Mantle zone Germinal centre
Marginal zone
Dark zone
re-entry
Clonal expansion
BCR diversification
Somatic hypermutation
No or low-affinity BCR
Antigen
BCR
Affinity
selection
Multiple
myeloma
M-MCL
Differentiation
and class switch
recombination
Competition
for T cell help
MZL FL
BL
UM-MCL
Fig. 1 | Origin of mature B cell lymphomas. B cell lymphomas are cancers that develop from the malignant
transformation of B lymphocytes at various stages of ontogeny. Most are of mature B cell origin, and revolve around the
germinal centre (GC) reaction, a critical step in which B cells are subject to intense proliferation and genomic remodelling
processes — namely , somatic hypermutation and class-switch recombination — to generate memory B cells and plasma
B cells that produce high-affinity antibodies. From naive B cells to memory B cells, most differentiation steps are associated
with a malignant B cell subtype (defined as the cell of origin (COO)) on the basis of classic histological definitions and gene
expression profiling. The COO assumes that B cell malignancies are ‘frozen’ at a given B cell differentiation stage arising in
a particular location of the B cell follicle. For example, follicular lymphoma (FL) is a follicle-related B cell lymphoma that
is considered the malignant counterpart of normal ‘frozen’ GC B cells. Unmutated mantle cell lymphoma (UM-MCL)
originates from mantle zone B cells, marginal zone lymphoma (MZL) resembles marginal zone B cells whereas Burkitt
lymphoma (BL) resembles dark zone B cells. Based on the COO, distinct diffuse large B cell lymphoma (DLBCL) molecular
subtypes are defined as not otherwise specified DLBCL (DLBCL NOS), whereas, the GC B cell-like DLBCL corresponds to
B cells that are arrested at various stages of the GC transit (from dark zone to light zone B cells) and the activated B cell-like
DLBCL seems to derive from GC B cells en route to plasma cell differentiation, resembling plasmablasts. BCR , B cell
receptor ; FDC, follicular dendritic cell; M-MCL, mutated mantle cell lymphoma; MHC, major histocompatibility complex;
TCR , T cell receptor ; TFH, follicular T helper. Adapted from ref.44
, Springer Nature Limited.
Moreover, FL frequently transforms into an aggres-
sive subtype (transformed FL (t-
FL)) resembling GC-
derived DLBCL, which is associated with poor clinical
outcomes9
. Risk stratification methods for identifying
these patients have recently been proposed10–12
.
In this Primer, we describe the epidemiology of FL,
the molecular and cellular pathogenesis of the disease,
its diagnosis, classification and treatment, and how the
disease affects quality of life. We briefly describe in situ
follicular neoplasia (ISFN), morphological variants of
2 | Article citation ID: (2019) 5:83 www.nature.com/nrdp
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FL and specific subtypes of FL. Additionally, we dis-
cuss optimized treatment approaches to limited and
advanced-
stage disease and for patients with relapsing
or transformed disease.
Epidemiology
FL represents ~5% of all haematological neoplasms.
It is the second most common subtype of non-
Hodgkin
lymphoma, accounting for ~20–25% of all new non-
Hodgkin lymphoma diagnoses in western countries
with an age-
standardized incidence of 2–4 per 100,000
person-years13–15
. In 2016, 13,960 new cases of FL were
estimated in the United States13
; ~2,220 new cases are
diagnosed each year in the United Kingdom16
and
~2,500 in France17
. FL is slightly more common in men
than in women (sex ratio of 1.2:1). It is more common
in older people, with a median age range of 60–65 years
at the time of diagnosis and a progressive increase in
incidence starting from 35 years of age and peaking at
70 years. In the United States between 2007–2016, the
incidence of typical FL ranged from 2.8 per 100,000
person-
years for people between 20 and 64 years of age to
14.7 per 100,000 person-
years for people >65 years of age.
In the paediatric and young adult population, FL is
an extremely rare disease. Since the 2016 classification
of lymphoid neoplasms18
, paediatric-
type FL is consid-
ered a separate entity to FL and presents as localized
lymphadenopathy. Unlike typical FL, paediatric-
type
FL exhibits an excellent prognosis with a slow poten-
tial for recurrence or progression; complete remission
is typically achieved in most cases after therapy19
. On
the basis of limited series, paediatric-type FL comprises
<6.5% of childhood lymphomas, with a median age
range of 7.5–11.7 years at diagnosis and a male to female
ratio of 4:1 (ref.19
). Risk factors for the paediatric-
type
FL remain undocumented to date.
Globally, FL is one of the non-
Hodgkin lymphomas
for which trends in incidence had been rising considera-
bly across all regions and ethnicities20
. Data from the US
National Surveillance, Epidemiology, and End Results
(SEER) Program show that the age-
standardized inci-
dence of FL increased through the 1970s and 1980s at
3–4% per year and reached a maximum of 3.4 per 100,000
person-years in 2003. Since then, the incidence stabilized
and then began to slowly decline by ~0.5–2% per year
in western countries21
. The reasons for this continu-
ous decline in incidence are not understood but might
be partly due to changes in lifestyle or environmental
exposure.
Incidence patterns for FL contrast dramatically
among geographical regions and ethnic groups. FL
incidence is highest in developed, high-
income coun-
tries such as the United States, Australia, South Africa
and the countries of western Europe22
. FL is 2–3 times
less common in Asian populations, with an estimated
age-
standardized incidence varying between 0.4 and
1.1 per 100,000 person-
years in Japan and South Korea.
In the United States, FL incidence was 59% lower in
black individuals, 27% lower in Hispanic individuals
and 66% lower among individuals of Asian and Pacific
Island descent than in non-
Hispanic white individu-
als based on American Cancer Society registry data
(2011–2012)13
. Strikingly, a higher incidence has been
observed amongst individuals of Asian descent born in
the United States than in those born in Asia, which sup-
ports an environmental component in the risk of FL in
addition to genetic and ethnic factors23
.
Important developments have been observed in the
treatment of FL over the past two decades. With the intro
duction of rituximab, the 5-year survival of patients
with FL has improved from 70% in the 1990s to 88.4%
today. Despite therapeutic advances, heterogeneity in
the clinical behaviour of FL is increasingly recognized.
Patients with FL who were treated upfront with immuno-
chemotherapy and who did not progress within
24 months of diagnosis (event-
free survival at 24 months
(EFS24)) had similar survival to the sex-
matched and
age-
matched general population24
. Median survival for
newly diagnosed patients with FL might be as high as
15–20 years24
. Patients experiencing disease progression
within 2 years after induction therapy containing rituxi-
mab have an increased risk of premature death25
. Recent
cohort studies in the United States and France showed
that FL-
related mortality remained the leading cause of
death in the first decade after diagnosis. This finding
was particularly true for patients who did not achieve
EFS24, for those who presented with a high Follicular
Lymphoma International Prognostic Index (FLIPI) and
for those with t-
FL25
.
Genetic susceptibility
Several studies have implicated family history in FL
aetiology26
. A large pooled case–control study from the
InterLymph Consortium comprising 3,530 FL cases
and 22,639 controls concluded that participants with
a family history of non-
Hodgkin lymphoma (all types)
had a two-
fold greater risk of developing FL than par-
ticipants without such history27
. In addition, an analysis
of population-
based cancer registry data from Sweden
showed that first-
degree relatives of patients with FL
have a fourfold higher risk of developing FL28
. Although
both studies support a genetic aetiology for FL, neither
study could distinguish this risk as attributable to a
shared familial susceptibility or a shared environment.
Direct evidence for inherited susceptibility comes
from genome-
wide association studies that found that
multiple, independent single nucleotide variants (also
known as single nucleotide polymorphisms (SNPs)) in
human leukocyte antigen (HLA) class I and II genes on
chromosome 6p21.3 significantly influenced FL risk29
.
These findings highlight a central role for antigen pres-
entation in FL pathogenesis29
. In addition, FL risk was
strongly associated with homozygosity at HLA class II
loci, suggesting that decreased HLA diversity reduces
the diversity of antigens presented and, accordingly,
might provide an advantage for the tumour to escape
immune detection30
. A large meta-
analysis of genome-
wide association studies reported new FL susceptibil-
ity loci outside the HLA region. Interestingly, all these
SNPs are localized near potential and proven oncogenes:
BCL2 at chromosome 18q21.33 (an antiapoptotic gene
frequently disrupted by the t(14;18) translocation in FL),
CXCR5 at chromosome 11q23.3 (encoding a member
of the chemokine receptor family with a major role in
3
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GC B cell organization and follicular T helper (TFH)cell
migration)31
, ETS1 at chromosome 11q24.3 (encoding a
transcription factor that regulates the activity of genes
involved in B cell growth and GC B cell differentiation
such as PAX5 and PRDM1)32
, and PVT1 at chromo-
some 8q24 (which is downstream of the MYC locus
and encodes five noncoding microRNAs (miR-1204,
miR-1205, miR-1206, miR-1207 and miR-1208) with
suspected oncogenic properties)33
. Additionally, SNPs
located at the region of LPP and BCL6 at chromosome
3q28 (between the lipoma preferred partner gene, which
is implicated in lipoma development, and the BCL6
oncogene, which encodes the master regulator of the
GC reaction) have also been implicated34
. These results
suggest that germline variation in pathways involved
in B cell apoptosis, GC function or modulations in FL
cell–tumour microenvironment (TME) interactions
influence disease progression35
.
Environmental and occupational exposure
Only a few, modest and sex-
specific associations of risk
with environmental, lifestyle and occupational exposure
have been reported, suggesting that FL has a complex,
multifactorial aetiology36
. A large pooled analysis from
the InterLymph Consortium of 19 case–control studies
revealed several associations that increase the risk of
FL: having a high body mass index as a young adult,
working as a spray painter or being a medical doctor for
>10 years27
. However, these associations remain to be fur-
ther investigated in occupational studies to evaluate the
specific role of the individual exposures. Additionally,
increased FL risks have been recurrently reported among
women with a medical history of Sjögren syndrome
and women who are current cigarette smokers. Reasons
for this sex-
specific and smoking-
linked increased risk
are not clear, although results are consistent across mul-
tiple studies. Thus, such putative associations might
reflect genetic variation, hormonal exposure or occupa-
tional exposures between men and women. By contrast,
reduced risk of FL was observed in individuals with a
history of an atopic disorder, blood transfusions, high
exposure to sunlight or ultraviolet radiation37
and occu-
pation as a university or higher education teacher27
for
reasons that are unclear.
Many studies have assessed associations between FL
and either farming or occupational pesticide exposure,
but results are inconsistent27,38–41
. Nevertheless, a causal
association between farming and FL seems biologically
plausible. An epidemiological study with stratification
of cases with non-
Hodgkin lymphoma based on t(14;18)
translocation status revealed an increased risk associ-
ated with exposure to certain pesticides for t(14;18)+
non-
Hodgkin lymphoma but not for t(14;18)–
cases42
.
Specifically, the risk for t(14;18)+
non-
Hodgkin lym-
phoma was significantly elevated amongst farmers who
used animal insecticides, crop insecticides, herbicides
and fumigants compared with farmers who never used
pesticides. Although this study did not focus on FL risk,
t(14;18) is the main genetic event in >85% of FL cases.
Along this line, a case–control study of exposed farmers
and controls from the general population revealed an
increased prevalence of t(14;18) translocations in the
blood of exposed farmers. These high clonal t(14;18)+
FL precursor cells in the blood of seemingly healthy
exposed farmers provide a potential molecular con-
nection between agricultural pesticide exposure, clonal
expansion of cells harbouring the t(14;18) translocation
and FL risk. These findings deserve further evaluation43
.
Mechanisms/pathophysiology
Pathogenesis
FL follows a complex development over many decades
in initially asymptomatic individuals (Fig. 2), which leads
to large interpatient and intratumoural heterogeneity.
FL is considered a prototypical B cell malignancy of GC
origin based on its molecular and cellular features. FL
cells express typical GC B cell markers such as BCL6,
activation-induced deaminase (AID) or CD10, harbour
the presence of AID-mediated immunoglobulin somatic
hypermutation (a process critical for immunoglobulin
gene diversification and generation of high-
affinity anti-
bodies) and have a close resemblance to developmentally
blocked GC B cells as demonstrated by bulk transcrip-
tional profiling44,45
. Interestingly, the FL cell of origin is
being re-
evaluated using single-
cell transcriptomics46,47
.
A recent study identified that the gene co-
expression
patterns characterizing normal GC functional dynamics
were lost in single FL cells, indicating a desynchroniza-
tion of the GC molecular programme in tumour cells.
Although the mechanisms underlying this desynchro-
nization remain to be identified, FL cells ‘transit’ with
high clonal dynamics between several functional states,
which remain to be fully characterized, contrary to the
assumed dogma that FL cells were frozen or blocked at
a specific GC B cell differentiation state. Integrating FL
functional heterogeneity and patients' clinical features
may reveal subsets of potential diagnostic, prognostic
or theranostic interest46
.
Although the origin of FL malignant transformation
is being investigated, the natural history of the disease
clearly does not initiate in the GC but begins earlier
during B cell ontogeny in the bone marrow. Thus, FL
represents an attractive model to study the molecular
events leading to lymphoid malignancies. A typical
example is the t(14;18) (q32;q21) translocation — the
genetic hallmark and most recurrent feature in >85%
of FL cases — which occurs early in pre-
B cells in the
bone marrow as a result of a repair failure during V(D)
J recombination (the process that allows the assembly
of the immunoglobulin gene components to form the
Bcellreceptor)48,49
.Asaconsequence,theBCL2oncogene
is placed under the transcriptional control of immuno
globulin heavy chain (IGH) regulatory regions, leading
to an ectopic overexpression of the BCL2 protein from
the initial stages of B cell differentiation. After antigen
encounter, B cells in the GC undergo multiple rounds
of genetic diversification of immunoglobulin genes by
AID, mutation of its B cell receptor (BCR) and strin-
gent selection for the expression of high-
affinity anti-
bodies. Thus, only rare cells in which somatic mutations
improve BCR affinity will be preferentially selected and
receive survival signals as a result of their increased pro-
cessing of antigen at the follicular dendritic cell (FDC)
surface and presenting processed peptides to TFH cells,
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which drive antibody affinity maturation. B cells over-
expressing BCL2 due to the t(14;18) translocation fail to
undergo apoptosis during affinity maturation, retaining
potentially low to moderate affinity cells early during
lymphomagenesis.
A large proportion (>70%) of healthy individuals
carry low levels of t(14;18)+
cells (one cell per million) in
the blood and/or lymphoid tissues (including the bone
marrow)50–52
. The vast majority of these individuals will
never develop FL, indicating that BCL2 deregulation
alone is insufficient to directly transform B lympho-
cytes. Thus, the GC B cells must accumulate further
oncogenic hits during the later phases of B cell matura-
tion53
. Longitudinal follow-up studies in epidemiological
cohorts of healthy individuals have shown persistence
of t(14;18)+
B cells in the blood for many years54
, which
has a tendency to increase with ageing51,52
, immuno-
modulation due to certain infectious conditions (such
as hepatitis C virus)55
and environmental exposures43,56
.
Why some infections have a role whereas others do not
is unknown. Indeed, most detectable circulating or resi-
dent t(14;18)+
cells constitute a clonal population of atyp-
ical memory B cells57
that have already transited through
the GC and acquired the characteristics of FL, such as
markers of developmentally blocked GC centrocyte cells
(CD10+/–
CXCR4lo
CD27+
Ki67lo
BCL6+
) and imprints of
AID-
mediated genomic instability, while paradoxically
gaining extensive dissemination and homing abilities in
multiple distant lymphoid organs (including the bone
marrow)43,58,59
. Interestingly, these FL-
like cells (FLLCs:
a cell phenotypically resembling FL cells that will never
commit to become a FL cell) preferentially retain the
expression of surface IgM constant regions to form their
BCRs despite ongoing class switch recombination to IgG
on the nonproductive translocated allele, indicating a
selective pressure to maintain surface IgM expression
in FL. Importantly, such an allelic paradox is a charac-
teristic of overt FL and suggests that malignant B cells
benefit from the proliferation and survival pathways that
are triggered by this type of BCR57
.
Multiple studies have shown that chronic and repeti-
tive immune responses trigger reactivation and re-entry
of normal IgM+
memory B cells (and to some extent IgG+
memory B cells) into the GC reaction60–63
. One study
linked the subversion of memory B cell dynamics to fol-
licular lymphomagenesis by demonstrating that, unlike
normal memory B cells, BCL2+
memory B cells support
multiple iterative GC entries upon repeated antigen
challenges, which promotes AID-
induced mutations
in a mutagenic environment and, in turn, propagates
clonal evolution towards FL progression58
. One direct
consequence of this multi-
hit model of FL genesis is the
Additional
mutational hits
Iterative GC transits,
clonal expansion
and dissemination
t(14;18)+
committed
CPC
Initiating
mutations?
BCL2+
Uncoupling selection
from diﬀerentiation
Time
FLLC
Naive
cell
t(14;18)+
Additional
mutational hits
FL tumour cell
CPC
clonal
evolution
Relapse 3
t-FL
divergent
evolution
Relapse 1
Linear
evolution
Relapse 2
Divergent
evolution
Transformation
Treatment
ISFN
Aquisition of
mutations Germinal
centre
Iterative
GC re-entry,
clonal expansion,
genomic
instability and
dissemination
Antigen
recall
FLLC
variants
Apoptosis
IgM>IgG
memory
t(14;18)+
GC
re-entry
Antigen
stimulation
Pre-B cell
HSPC
Bone marrow
Germinal centre
Selection
t(14;18)
Healthy or subclinical disease Primary tumour Relapse
Diagnosis
Fig. 2 | A model for the stepwise evolution of FL. Primary overt follicular lymphoma (FL) emerges from early mutated
cancer precursor cells (CPCs) engaged in a dynamic process of re-entry into the germinal centre (GC), evolving and
disseminating over decades in asymptomatic individuals. Such early clones are also likely at the origin of post-treatment
relapses. FL-like cells (FLLCs) are present in most healthy individuals and will never progress to FL (despite dissemination
to different organs and sharing some genotypic and phenotypic features with FL). By contrast, CPCs are more evolved and
committed to ultimately give rise to FL. In situ follicular neoplasia (ISFN) represents an early precursor lesion that might
progress into FL at a low rate of progression (5%), although its relationship with CPCs remains unknown. In FL , spatial and
temporal genomic analysis revealed that relapse events (relapse 2) or transformation to faster-growing high-grade FL
(t-FL; relapse 3) arise predominantly by divergent evolution of a common mutated CPC that clonally diverges through
the acquisition of distinct genetic events. Relapse events (that is, relapse 1) arising from a direct clonal evolution
of the dominant FL clones present at the diagnosis also exist and characterize mostly early progression specimens.
HSPC, haematopoietic stem and precursor cell. Adapted with permission from ref.255
, Wiley-VCH.
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potential existence of committed FL cancer precursor
cells (CPCs) in ‘asymptomatic’ individuals that will
develop and clonally expand for extended periods of
time, ultimately evolving into FL64
. The point at which
an FLLC becomes a committed FL CPC (might likely be
the origin of FL relapses and transformation) is currently
under study. In the largest case–control study conducted
so far, clonal relationships based on the t(14;18) junc-
tion signature were established between paired samples
issued from prediagnostic blood and subsequent FL
biopsy collected up to 10 years apart. In ~20% of ‘healthy’
individuals who went on to develop FL, commitment to
FL progression was preceded by an elevated frequency
of t(14;18)+
cells in the blood. Investigators estimated
a 23-fold higher risk of subsequent FL development in
seemingly healthy individuals with t(14;18) frequencies
of >1 per 10,000 cells, a risk estimate strongly significant
regardless of time to disease progression65
.
Additional compelling evidence for the existence
of FL CPCs arises from two clinical observations of
donor–recipient pairs who developed FL 3–10 years
after bone marrow transplantation or donor lymphocyte
infusion66,67
. Both donor and recipient lymphomas were
clonally related and shared identical BCL2–IGH junc-
tions, BCR rearrangements and the same set of second-
ary FL genetic alterations, demonstrating that a direct
transfer of FL CPC from the donor’s bone marrow to the
recipient gave rise to overt FL years later66,67
. In addition,
spatial and temporal phylogenetic analyses of paired FL
samples (for example, samples from lymph nodes versus
bone marrow and samples at diagnosis versus relapse
or transformation) have now clearly established the
prevalent mode of clonal dynamics and progression in
FL. At diagnosis, relapse or transformation, the bulk of
tumour cells share only a restricted set of early found-
ing genetic mutations potentially present in CPCs but
differ by a large set of mutations that are unique to each
disease event, reflecting divergent evolution pattern68–71
.
Altogether, the available data converge towards an FL
lymphomagenesis scenario — now widely adopted by in
the field — whereby primary overt FL and subsequent
transformation or relapse would emerge from a ‘hidden’
reservoir of committed, common precursor clones that
evolve and widely disseminate over decades in asymp-
tomatic individuals46
. This subclonal and independent
evolution leads to considerable interpatient and intra-
tumour clonal heterogeneity. To date, CPCs remain
an elusive population that also evade immunochemo
therapeutic strategies; these cells likely represent a
minor subset of the tumour subpopulation that might
be absent from the bulk or be undetectable with exis
ting methods69
. Our ultimate goal is to target this CPC
popu
lation to eradicate FL. Thus, future efforts should
focus on isolating and delineating the oncogenic cir-
cuitries as well as characterizing the immune and stro-
mal determinants that sustain the survival and clonal
dynamics of this founder FL population.
Considering the large proportion of healthy individ-
uals who harbour low levels of t(14;18)+
FLLCs in the
blood but will never develop FL, the sole detection of
the t(14;18) translocation cannot be used for screening
purposes to enable successful early detection. A t(14;18)
frequency >1 in 10,000 cells in the blood from healthy
individuals years before diagnosis has been shown to
be a first predictive biomarker for FL and to identify a
population at high risk of developing FL65
. However, this
biomarker remains imperfect as it identifies only 25%
of future patients and requires the testing of millions of
individuals to identify a few. In the future, further char-
acterization of the genomic landscape of FLLCs versus
CPCs in apparently healthy people may help to refine
the CPC genomic identity and thereby contribute to dis-
criminating individuals at risk of FL development years
before malignant transformation from those who will
never develop the disease. Additionally, characterization
of the CPCs might provide a rational basis for mutation-
based therapies to directly target the CPC population
for the treatment of indolent FL to avoid recurrence or
relapses after therapy.
Tumour microenvironment
Due to a lack of functional models, many considerations
are based on indirect evidence, are extrapolated from
physiological GC to FL, and await functional proof.
In the early stages of lymphomagenesis, neoplastic cells
invade existing lymphoid follicles, interact with the
GC microenvironment and emit signals for their own
survival and proliferation72
. FL differs from almost all
other lymphomas by growing in a permanent three-
dimensional structure (follicles) and not exclusively
as diffuse sheets devoid of any arrangement, probably
caused by the lymphoma–TME interaction. The TME
in FL, regardless of the involved site, includes immuno
competent lymphoid cells, stromal cells and compo-
nents of the extracellular matrix (Fig. 3). Sclerosis may
be found within neoplastic follicles, although rarely.
FLtumourcellsproliferateinaGC-likeTME,incloseasso-
ciation with non-
neoplastic lymphoid and nonlymphoid
cells. The cellular interactions in the TME are similar to
normal GC B cells in the follicular microenvironment
during normal immune reactions73
.
Cells of the TME and neoplastic B cells engage in
reciprocal crosstalk via various signalling pathways. For
example, FL cells express the transmembrane receptor
CD40; its ligand, CD40L, is expressed on TFH cells or
found circulating in soluble form74
. TFH cells also secret
cytokinessuchasIL-4andIL-21,whichfavourthegrowth
and survival of FL cells; receptors for these growth factors
are expressed on FL cells. Furthermore, chemokines such
as CXC-
chemokine ligand 12 (CXCL12) and CXCL13
secreted by stromal cell subsets (including FDCs) bind
to CXCR5 on TFH cells and FL cells.
Additional pathways important in tumour cell growth
and survival include the tumour necrosis factor (TNF)
ligandmemberOX40LanditsreceptorOX40,andmacro-
phage migration inhibitory factor and its receptor
CD74. In the pathogenesis of FL, unusual stimulation
of the BCR by the innate immune system occurs through
the introduction of N-
linked glycosylations by soma
tic hypermutation in the variable region of the BCR.
Tumour cells evade the antitumour immune response
via signals from a variety of cells such as TFH cells, cyto-
toxic T lymphocytes and macrophages. The tumour
cells express CD70, which can lead to the conversion
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of T helper cells to CD4+
CD25+
regulatory T (Treg) cells
upon binding its ligand (CD27). Thus, TME cells not
only support the growth and survival of FL cells, but also
promote immune evasion mechanisms including reduc-
tion of tumour immunogenicity, inhibition of immune
effectors and infiltration by immunosuppressive cells75
.
The cellular composition of the TME varies depend-
ing on where the FL grows. In lymph node localization
of FL, FDCs, fibroblastic reticular cells and TFH cells are
the main components of the TME72
. In bone marrow
localization of FL, mesenchymal stromal cells are the
main stromal component of the TME, whereas the main
component of the bone marrow niche is myeloid cells.
Mesenchymal stromal cells communicate with neoplas-
tic B cells and recruit monocytes which, in turn, stimu
late M2 polarization, a process by which macrophages
activate different functional programmes in response
to signals from their microenvironment. M2 is a subset
of CD163+
macrophages that have anti-
inflammatory
properties76
. High levels of IL-4 are likely to polarize the
tumour-
associated macrophages to the M2 type with
upregulated dendritic cell-
specific ICAM-3-grabbing
nonintegrin (DC-
SIGN) expression and subsequent
engagement of mannosylated surface immunoglobulin
on FL cells.
Disease progression. Upon antigen stimulation, TFH cells
upregulate the expression of CD40L, which binds CD40
on B cells. This engagement activates nuclear factor-
κB
(NF-
κB) canonical pathways and induces expression
of the transcription factor interferon regulatory fac-
tor 4 (IRF4) in B cells. IRF4 in turn downregulates the
expression of BCL6, enabling terminal differentiation of
GC B cells to post-
GC lymphocytes77,78
. The BCL6 gene
encodes B cell lymphoma 6 protein, which is expressed
in GC B cells79
and is considered as the master regulator
of normal GC formation77
. Downregulation of BCL6 is
necessary for B cell terminal differentiation. CD40, a
member of the tumour necrosis factor receptor (TNFR)
family, induces B cell growth and differentiation. In nor-
mal conditions, the CD40–CD40L pathway is involved
in GC formation80
; in FL, this pathway plays a part in
regulating the growth of CD40+
neoplastic GC B cells
and the surrounding CD40L+
TFH cells, revealing a close
functional similarity with their normal GC B and T cell
counterparts74
. The regulation of tumour growth entails
enhancing other survival pathways such as those involv-
ing NF-
κB, Janus kinase (JAK) and signal transducer and
activator of transcription (STAT) signal transduction81
.
FL tumours are enriched with TFH cells, which stim-
ulate CD40 signalling in FL cells leading to increased
secretion of IL-4 (refs82,83
). IL-4 in turn activates STAT6-
mediatedsignallinginFLcells,enhancingcellsurvival82,83
.
In addition, IL-4 produced by TFH cells stimulates stro-
mal cells (particularly FDCs) to increase the secretion of
CXCL12, which regulates B cell trafficking between the
dark and light zones of the normal GC31,84
. In vitro experi-
mentsreportedthatCXCL12increasedrecruitment,hom-
ing and migration of FL cells85
; this mechanism might be
responsible for tumour cell dissemination in vivo.
In normal physiology, CD4+
and CD8+
T cells express
programmed cell death 1 (PD-1), a member of the CD28
receptor family that regulates immune tolerance by pro-
motingapoptosisofantigen-
specificT cellswhiledecreas-
ing apoptosis of Treg cells. In the FL microenvironment,
PD-1 is expressed on both dysfunctional CD4+
and CD8+
T cells and on fully functional TFH cells. PD-1+
TFH cells
co-
expressing CD10 secrete high amounts of IL-4, IL-21,
and TNF, thereby stimulating the growth of FL cells83
.
CD10+
TFH cellsalsoprovidesignals,suchasCC-chemokine
ligand 17 (CCL17) and CCL22, which decrease the
immuneresponsetothetumourbyrecruitingTreg cells86,87
.
Finally, neutrophils (cells of the innate immune system)
Follicular
T helper
cell
FL
tumour
cell
CXCR5
CD40L
CD40
IL-4
Macrophage
(M2)
CD163+
Follicular
dendritic
cell
CXCR5
Stromal
cell
CXCL13
CXCL12
IL-21
PD-1
PD-L1 (?)
BCR
N-glycan
IL-21R
IL-4R
Neutrophil
CD74
MIF
PD-L1
IL-4R
CD40
DC-SIGN
IL-4R
TCR
MHC II
CXCR4
BAFF
BAFFR
Treg
cell
FOXP3+
CD70
CD27
CXCR5
CXCL13
CD8+
CTL
PD-1
MHC I
CD74
Fig. 3 | The FL tumour microenvironment. The follicular lymphoma (FL) tumour cell
resides within a microenvironment with infiltrating inflammatory cells. These cells
express ligands that bind receptors expressed on the FL cell membranes, activating
pathways in the FL cells that support growth and survival. Beneficial signals for growth
and survival include cytokines, such as IL-4 and IL-21 (secreted by follicular T helper cells),
or CXC-chemokine ligand 12 (CXCL12) and CXCL13 (secreted by stromal cell subsets
including follicular dendritic cells), CD40 and its cognate ligand CD40L; macrophage
migration inhibitory factor (MIF) and its cognate ligand CD74. B cell activating factor
(BAFF), which binds to its receptor BAFFR , is also produced by follicular dendritic cells
and has been proposed to contribute to the antiapoptotic effect of stromal cells on
normal and malignant germinal centre B cell growth256
. B cell receptor (BCR) stimulation
by the innate immune system occurs through N-glycans via the acquisition of
N-glycosylation sites in the variable region of the BCR . Tumour cells overcome the
antitumour immune response from CD4+
T helper cells, CD8+
cytotoxic T lymphocytes
(CTLs) and macrophages. Programmed cell death 1 ligand 1 (PD-L1) is expressed by
macrophages, and PD-L1 expression by malignant B cells is currently unclear. Cells of
the innate immune system, including neutrophils, have also been implicated as active
elements in the tumour microenvironment in FL through the activation of supportive
stromal cells. FL cells also produce chemokines as a result of crosstalk with follicular
T helper cells, recruiting CD4+
CD25+
regulatory T (Treg) cells that further reduce the
immune response to the tumour. DC-SIGN, dendritic cell-specific ICAM-3-grabbing
nonintegrin; FOXP, Forkhead box P; MHC, major histocompatibility complex;
PD-1, programmed cell death 1; TCR , T cell receptor.
7
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in the TME activate stromal cells88
, which may sustain
FL growth by specifically triggering NF-
κB-dependent
polarization of tumour-
supportive stromal cells88
.
Taken together, these observations suggest that FL
cellssubverttheTMEtotheirownadvantagewhileescap-
ing immune surveillance. Of particular interest, some of
these functions are mediated by genetic alterations that
favour interactions with surrounding protumoural TME
components. This role has been well demonstrated by
loss-
of-function mutations of TNFRSF14 (encoding TNF
ligand superfamily member 14) in 30–40% of patients
with FL89–91
and more frequently in the paediatric-
type FL, which favours intrinsic BCR signalling but
also the activation of a TFH cell–stromal cell tumour-
permissive loop92
. Additionally, the introduction and
selection in 95% of patients with FL of N-
glycosylation
sites within the variable region of immunoglobulin
trigger antigen-
independent BCR activation and sur-
vival signals by interacting with DC-
SIGN-expressing
tumour-associated macrophages93–97
.
Additional genetic characteristics
In line with the multi-
hit model of FL pathogenesis,
next-
generation sequencing has led to the identifica-
tion of candidate genetic lesions that promote FL clonal
evolution in addition to the founder BCL2 transloca-
tions69–71,98–100
(Table 1). The frequency of genetic alter-
ations affecting FL is variable, but for any single gene
the frequency is higher in FL than in DLBCL not oth-
erwise specified of the GC B cell-
like subtype, which
expresses CD10 and BCL6 (refs99,101–110
). Amongst
the most prevalent lesions are genetic alterations in
chromatin-
modifying genes that are present alone or
in combination in almost every patient. Mutations in the
H3K4 histone methyltransferase KMT2D (also known
as MLL2) are the most frequent abnormality, found in
80–90% of patients with FL98,111
. Mutations in enhancer
of zeste homologue 2 (EZH2), the catalytic subunit of the
polycomb PRC2 complex involved in histone methyla-
tion, are found in >25% of cases70,111
, whereas mutations
in genes encoding histone acetyltransferases, namely
Table 1 | Genetic alterations affecting at least 10% of cases of FL
Gene Alterations (effect) Frequency in
FL (%)
Effect or function Refs
Proliferation
KMT2D Mutation (↓) 80–90 Histone modification; tumour suppressor 10,104,111,245
IgHV, IgLV Mutation (↑) 75–90 N-glycosylation of IgV region of BCR; BCR signalling 93,97
RB1 Deletion (↓) 12 Impairment of cell cycle control 124
CDK4 Copy number gain (↑) 29 Impairment of cell cycle control 124
BCL6 Translocation (↑) 6–15 Transcription factor ; tumour progression 103,105,119,120,259,260
Mutation (↑) 47
H1–2, H1–4 Mutation (↓) 44 Chromatin remodelling 70,104
MEF2B Mutation (↓) 13–15 Transcription factor ; transcriptional activator 10,104,111
EP300 Mutation (↓) 10–20 Histone modification 10,70,100,111
SESN1 Epigenetic silencing (↓) ~20 Promotion of mTOR activity 261
RRAGC ATP6V1B2, ATP6AP1
Mutation (↑) 17 mTORC1 survival signal 262
EZH2 Mutation (↑) 7–30 Histone modification 10,99,103,104,106
ARID1A Mutation (↓) 15 Chromatin remodelling 10,104,111
GNA13 Mutation (↓) ~10 B cell growth and lymphoma cell dissemination 10,104,110,111,245
SGK1 Mutation (↓) ~10 Deregulation of FOXO transcription factors and NF-κB 10,103,104,111
FOXO1 Mutation (↑) ~10 Transcription factor ; survival and proliferation 103,104
CARD11 Mutation (↑) 10 Increased BCR signalling 10,70,104,111
STAT6 Mutation (↑) 10 Activation of JAK–STAT signalling 10,103,109
Survival
BCL2 Translocation (↑) 80–90 Suppression of apoptosis 101–103,125
Mutation (↑) 50
TNFAIP3 Mutation (↓) 2–26 Loss of tumour suppressor 104,107,108
Immune evasion
EPHA7 Deletion (↓) 70 Tumour suppressor 103,124
Epigenetic silencing (↓)
TNFRSF14 Mutation (↓) 18–50 Tumour suppressor ; increased BCR signalling 89,91,103,104,245
CREBBP Mutation (↓) 33–70 Histone modification; tumour suppressor 10,100,103,104,111,245
↑, gain of function; ↓, loss of function; BCR , B cell receptor ; FL , follicular lymphoma; FOXO, Forkhead box O; JAK , Janus kinase; mTOR , mechanistic target of
rapamycin; mTORC1, mechanistic target of rapamycin complex 1; NF-κB, nuclear factor-κB; STAT, signal transducer and activator of transcription.
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CREBBP and EP300, are found in >65% and >15% of
FL cases, respectively100
. By changing the equilibrium
between active and repressive histone marks of tran-
scription, inactivating mutations of KMT2D, CREBBP
and EP300 and gains of function of EZH2 results in a
shift from transcriptional activation towards aberrant
repression of gene transcription (loss of H3K4me1
and H3K27ac enhancer activation marks and gain of
H3K27me3 promoter repressive marks, respectively)
interfering with key gene expression programmes that
govern normal GC selection, exit and differentiation.
Consequently, aberrant GC B cells accumulate and
fail to properly respond to exit signals from the GC
microenvironment (reviewed elsewhere73,78,81
).
Several studies have looked at the functional con-
sequences of B cell-
intrinsic FL founder alterations in
murine models, validating their tumour suppressive
role in vivo and the acceleration of FL formation in the
context of BCL2 overexpression92,112–115
. However, several
lines of evidence have also demonstrated a pivotal role
for some of those (epi)genetic mutations as cell-extrinsic
modulators of the TME92,98,116
. Deletion of herpesvirus
entry mediator (Hvem) in a chimeric mouse model with
a VavPBcl2 genetic background showed that HVEM loss
triggers the amplification of TFH cells, leading to high lev-
els of TNF and lymphotoxin. These two nonredundant
factors are involved in lymphoid stromal cell differenti-
ation and maintenance and favour lymphoid stromal cell
activation, including FDCs and fibroblastic reticular
cells92
. Similarly, loss of CREBBP in patients with FL and
mouse models in combination with overexpression of
BCL2 have been shown to facilitate immune evasion by
downregulating antigen presentation and major histo-
compatibility complex (MHC) II expression, associated
with reduced T cell infiltration98,112,115
. A similar observa-
tion was made in human lymphomas and murine models
carrying mutant EZH2Y641
with reduced MHC expression
in favour of a tumour progression model associated with
mutation-
driven acquired immune escape117
. Of clini-
cal relevance, histone deacetylase 3 (HDAC3)-selective
inhibitors fully reverse mutant CREBBP aberrant epi-
genetic programming, resulting in the restoration of
immune surveillance due to induction of the interferon
pathway and antigen presentation genes118
. By contrast,
EZH2 inhibitors restored MHC expression in EZH2-
mutant lymphoma cell lines117
. These data strongly argue
for the development of synergistic therapies that com-
bine immunotherapies with epigenetic reprogramming
to enhance tumour recognition and elimination; such a
strategy could also be conceptually attractive to target
the CPC population70,71,98
.
Other common mutations in FL affect transcriptional
regulation, including alterations in BCL6, STAT6 and
BCL2. In FL, BCL6 is thought to contribute to lympho-
magenesis as it is recurrently affected by chromosomal
translocations or altered by somatic mutations119,120
,
resulting in its constitutive expression. BCL6 rearrange-
ment without BCL2 translocation was reported in the
subgroup of high-
grade FL cases121
. Translocation involv-
ing BCL6 — most often t(3;14)(q27;q32)/BCL6–IGH
— have been identified in 10–15% of cases of FL122,123
.
Somatic mutations of BCL6 also occur commonly116
.
Mutations in STAT6 have been reported in >10% of
FL cases109
. These mutations hyperactivate IL-4, JAK
and STAT6 signalling and have been proposed to be
involved in driving FL pathogenesis81,109
. Genomic alter-
ations impairing the retinoblastoma pathway have been
found in ~50% of FL cases124
. This pathway involves
retinoblastoma-
associated protein (encoded by RB1),
a negative regulator of cellular growth that participates
in cell cycle control. These alterations include gains
on chromosome 12, leading to upregulation of cyclin-
dependent kinase 4 (CDK4). Chromosome 12 gains
have been shown to be involved in FL pathogenesis by
impairing the cell cycle124
.
Finally, mutation of BCL2 has been described along
with BCL2 translocation125
. In the small proportion of
cases with no BCL2 translocation, no clear driving altera-
tion has been identified. Differences between BCL2+
and
BCL2–
FL have been identified by whole-
exome sequenc-
ing, SNP profiling, gene expression profiling and assess-
ment of N-glycosylation sites126
. These analyses revealed
that typical FL mutations also affected t(14;18)–
FL cells,
but N-
glycosylation sites were detected considerably
less frequently. Furthermore, t(14;18)–
FL cells demon-
strate a strong and exclusive enrichment of the immune
response signature, suggesting increased crosstalk with
the TME. Additionally, deletion of chromosome 1p36
with concomitant TNFRSF14 mutations has been found
in some FL-specific subtypes, such as the paediatric type
and primary cutaneous follicle centre lymphoma, and in
some t(14;18)–
FL cases127–129
.
BCR signalling in FL survival
Another critical player in FL pathogenesis, although
rarely targeted by somatic mutations, is the BCR sig-
nalling pathway. Survival of FL cells requires signals
from a functional and structurally intact BCR on the
cell surface despite an ongoing somatic hypermutation
process acting throughout the disease course. Ongoing
somatic hypermutation of the FL BCR, therefore, favours
a supportive mechanism independent of the presence of
cognate antigen81
. Acquisition of N-
glycosylation sites
in >80% of BCRs in FL might explain this sustained
antigen-
independent BCR signalling through an inter-
action with carbohydrate moieties at the surface of the
BCR and lectins found within the TME95,96
. An alterna-
tive mechanism to explain sustained BCR signalling in
the absence of antigen was uncovered using genome-
wide CRISPR functional genomic screens in t-
FL cell
line models130
. This study demonstrated that the BCR
and its proximal kinases SYK and LYN were essential
for survival of most GC-
related lymphomas (including
t-FL), indicating that the BCR signal in t-
FL cells is akin
to a particular form of ‘tonic’ BCR signalling whereby
BCR, together with its co-
receptors CD19, SYK and
LYN, trigger downstream activation of the phospho-
inositide 3-kinase (PI3K)–AKT survival pathway, with-
out engaging the kinase BTK and NF-
κB as do normal
antigen-
activated B cells. This BCR signalling mode in
t-
FL is highly reminiscent of tonic signalling required
to maintain the viability of naive mouse B cells in an
antigen-
independent and NF-
κB-independent man-
ner130
. Consequently, these t-
FL genetic vulnerabilities
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have important clinical implications as they provide
rational therapeutic strategies for the use of proxi-
mal BCR inhibitors (SYK or LYN inhibitors) or PI3K
inhibitors to disrupt BCR signalling in t-
FL cells rather
than BTK inhibitors. Indeed, clinical trials of FL have
mirrored these results, demonstrating clinical efficacy
and US FDA approval in the refractory and relapse FL
settings for PI3K inhibitors such as idelalisib or copana-
lisib131,132
, whereas the clinical response to the BTK
inhibitor ibrutinib is modest133
.
Diagnosis, screening and prevention
Despite progress in the diagnosis and treatment of FL,
global variation exists in the availability and/or accessi-
bility of diagnostic tests and drugs for cancer between
developing and developed countries. Health disparities
such as higher death rates and higher rates of advanced
diagnoses in people from low socioeconomic groups and
those who live in geographically isolated areas remain.
These disparities are mainly caused by diagnostic delay
and are not related to the global variation in diagnostic
procedures134,135
.
Principal signs of FL consist of enlargement of lymph
nodes in the neck or abdomen. Enlarged lymph nodes of
the abdomen are typically found incidentally on imaging.
Symptoms include fatigue, fever or night sweats, weight
loss or recurrent infections. However, most patients
with FL have no obvious symptoms of the disease at
the time of diagnosis. Rarely, FL is found outside the
lymph nodes. Extranodal FL can cause a variety of symp-
toms depending on its location; for example, patients
with bone marrow disease might develop anaemia,
thrombocytopenia and/or neutropenia.
Specimens for diagnosis
The diagnosis of lymphoma is usually made via analysis
of histological sections obtained from surgically excised
lymph nodes (Fig. 4). Excisional lymph node biopsy is
the reference method for formulating a diagnosis of
lymphoma as well as measuring levels of prognostic
biomarkers7
. In case of recurrence, if the patient can
withstand the procedure, repeating biopsy on the new
location can ascertain any changes in the molecular
characteristics of the lesion and exclude transformation
to DLBCL or another tumour type. The histological defi-
nition of t-
FL implies the evolution towards a high-grade
lymphoma, usually DLBCL. No histological parameters
are known that can predict the risk of histological trans-
formation at diagnosis. The histological transformation
can be associated with clinical transformation that is
defined by rapid progression, treatment resistance and
poor prognosis.
In patients who are elderly, have comorbidities or
whose abnormal lymph nodes are deep, the diagnosis
is based on needle biopsy performed under CT guid-
ance136
. However, the usefulness of needle biopsy (both
core needle biopsy and fine needle aspiration biopsy)
for diagnosing FL is debated. The smaller tissue samples
obtained with needle biopsies might limit the ability to
differentiate FL from MZL and lymphoid hyperplasia,
and might hinder the detection of areas of histological
transformation136
.
Needle biopsy of bone marrow is performed as part
of staging a patient with an FL diagnosis. Bone mar-
row involvement in FL is found in most patients137
and
immunohistochemical analysis of specimens from the
bone marrow helps to identify lymphoma infiltration,
but this finding is not specific to FL. In fact, a large frac-
tion of DLBCLs involving the lymph nodes display a pat-
tern of bone marrow infiltration similar to low-
grade
FL-like infiltration7,136
. Blood tests to evaluate blood
cell count, to measure lactate dehydrogenase levels and
screenfor virusessuch asHIV,hepatitisBvirusand hepa-
titis C virus, and imaging (CT and PET) are routinely
performed as a part of staging to plan the treatment7
.
Diagnosis and classification
Neoplastic follicles in FL contain small or medium-sized
B cells with a cleaved shape (centrocytes) and larger,
noncleaved B cells (centroblasts, which have a moderate
amount of cytoplasm). These tumour cells are admixed
with different numbers of reactive T cells, FDCs and
histiocytes, plus occasional macrophages, granulocytes
and plasma cells138
. The presence of centroblasts is used
to determine the grade of FL following the WHO cri-
teria (Table 2), which was introduced in the early 2000s
after petition from researchers and clinicians. Grade is
determined from the number of centroblasts per high-
power microscopy field (×40 objective, 0.159 mm2
).
However, the grading system and its relationship to
prognostication has been debated. Currently, the WHO
does not recommend distinguishing between grade
1 and 2 (the most frequent grades, containing up to
15 centroblasts per field). Additionally, a Grade 3A FL
withdiffusegrowthcontaining>15centroblastsperhigh-
power field should be classified as DLBCL according to
the WHO system, but this has neither been confirmed
nor has any biological difference been provided for
this approach.
FL is usually easy to establish in the biopsy spec-
imen because the disease effaces the normal lymph
node architecture. Neoplastic follicles typically not only
a b
Fig. 4 | Cellular features of the germinal centre in health and disease. a | Microscopic
features of the normal germinal centre environment. Three stains simultaneously detect
different proteins in formalin-fixed, paraffin-embedded sections of a reactive lymph
node. Triple staining for CD20 (teal), CD3 (purple) and Ki67 (yellow , indicating
proliferation) cells shows the topographic relationship between CD3+
T cells, CD20+
B cells, and Ki67+
follicular cells. Ki67+
cells co-express CD20. Magnification ×20.
b | Multiplexed immunohistochemistry shows that the neoplastic nodules are composed
of CD20+
B cells (teal). CD3+
T cells (purple) are mainly interfollicular, whereas Ki67+
cells
(yellow) are within the follicles and co-express CD20. Magnification ×10.
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replace the lymph node structure, but also infiltrate
the lymph node capsule and extend into the perinodal
adipose tissue. Neoplastic follicles are round and of
homogeneous appearance, unlike the normal reactive
follicles involved in immune reactions. Neoplastic fol-
licles sometimes merge to form confluent nodules or
diffuse areas of disease. Although these histological pat-
terns are useful for a diagnosis of malignancy, it is the
cell morphology that is essential for diagnosing the FL
type. Four patterns of FL are recognized by the WHO
classification3
in histological examinations: the follicular
pattern (tumour tissue is >75% follicular); the follicular
and diffuse pattern (of which 25–75% of the specimen
is of the follicular pattern); a pattern containing foci of
FL (of which the follicular proportion is <25%); and a
diffuse pattern (absence of follicular areas). A neoplasm
with a purely follicular pattern, even if composed of
centroblasts alone, is considered FL.
By immunohistochemistry, the typical immuno
phenotypes observed in FL are BCL2+
, BCL6+
, CD3–
,
CD5–
, CD10+
, CD20+
, CD23+/–
, CD43–
, cyclin D1–
and
Ki67+
. In rare cases, CD10–
or BCL2–
are detected. In
young patients with localized, BCL2–
disease, paediatric-
type FL should be considered. Immunohistochemistry
helps to distinguish FL from other CD20+
small, mature
B cell lymphomas, namely small lymphocytic lymphoma
and chronic lymphocytic leukaemia (CD5+
, CD23+
),
mantle-
cell lymphoma (CD5+
, cyclin D1+
) and MZL and
lymphoplasmacytic lymphoma (BCL6–
, CD5–
, CD10–
).
The Ki67 proliferation index in FL generally correlates
with histological grade136
. A Ki67 index >30% is associa
ted with more aggressive clinical behaviour136
, but there
is no evidence to support the basing of therapeutic deci-
sions on this finding. With flow cytometry, the typical
immunophenotypes are CD5–
, CD10+
, CD19+
, CD20+
,
CD23–
and a positive expression for immunoglobu-
lin κ-
chain or immunoglobulin λ-
chain. In difficult
cases, molecular and cytogenetic tests are also required,
which include immunoglobulin gene rearrangements
(clonality testing) and BCL2 translocation (t(14;18)
or variants).
In some cases, information on the immunoglobulin
gene clonality of the B cell infiltrate can increase the
reliability of a diagnosis of FL established on a histo-
logical and immunophenotypical basis. Investigations
for molecular characterization include the study of the
BCR variable region by PCR, and the study of BCL2 gene
translocation by fluorescence in situ hybridization.
Immunophenotypic and cytogenetic findings. For an
immunohistochemical diagnosis of FL, testing should
be performed for BCL2, BCL6, CD3, CD5, CD10, CD20,
CD23, CD43, cyclin D1 and Ki67 expression. Positive
findings for BCL2, BCL6, CD10 and CD20 are indic-
ative of FL (Fig. 5). Care must be taken in the immuno
diagnostic work-
up of FL when evaluating BCL2 and
CD10, which show decreased intensity and frequency of
staining as the grade of FL increases (Table 2). The inten-
sity of CD10 expression tends to decrease from ISFN
to grade 3B FL, which could be linked to tumour pro-
gression mechanisms139
. While a subset of cases is truly
BCL2–
, false-negative results can occur if a specimen has
mutations of the BCL2 epitope recognized by the anti-
BCL2 antibody used; these samples would test positive
whenotheranti-BCL2antibodiesareused136
.Cytogenetic
analysis of FL specimens reveals the t(14;18)(q32;q21)
translocation in most cases; fluorescence in situ hybrid-
ization is a valuable method for detecting the resulting
IgH–BCL2 fusion gene in fixed paraffin-
embedded
tissue sections.
In situ follicular neoplasia. ISFN is characterized by
t(14;18)+
GC B cells with high expression of BCL2 in
abnormal lymph node follicles that occasionally pro-
gress to clinically overt disease140
. This early lymphoid
neoplasia also shares other genetic alterations with FL,
including mutations in EZH2, TNFRSF14 or KMT2D,
and deletion of chromosome 1p36 (refs141,142
). This
condition has an intrafollicular growth pattern because
mutant B cells localize within the GC but do not invade
the surrounding structures143
(Fig. 6). ISFN is occasion-
ally observed in lymph nodes excised for suspected lym-
phoma or other reasons, and it is sometimes observed in
patients who eventually develop FL144
.
Histological and immunophenotypic features helpful
in recognizing ISFN comprise intact lymph node archi-
tecture and a normal follicle size. Follicular involvement
is limited to the GC, and the cellular composition is
entirely made of centrocytes that strongly express BCL2
and CD10 (ref.145
). Even though the involved follicles are
usually scattered throughout the lymph node, they have
intact follicular cuffs and sharp follicular edges.
ISFN has uncertain clinical behaviour. This lesion
may be associated, at diagnosis or during follow-
up,
with T and B cell lymphomas other than FL, includ-
ing splenic marginal zone lymphoma, classic Hodgkin
lymphoma, DLBCL, chronic lymphocytic leukaemia or
small lymphocytic lymphoma, and peripheral T cell lym-
phoma146,147
. Furthermore, association with nonlymphoid
malignancies (for example, melanoma, prostatic carci-
noma, breast carcinoma, gastrointestinal stromal tumour
and paraganglioma) has been observed148–150
.
Morphological variants of FL. A morphological variant
of FL can be of any histological pattern. An accurate
diagnosis of FL requires knowledge of its morpholog-
ical variants and how to distinguish them from other
Table 2 | WHO grading system for FL
Grade CB:HPF
ratio
Ki67
index
Phenotypic and cytogenetic observations
1 0–5 <20% In up to 90% of cases, tumour cells express CD10
and BCL2 and have BCL2 translocation
2 6–15 <20% In up to 90% of cases, tumour cells express CD10
and BCL2 and have BCL2 translocation
3A >15 >20% Centrocytes are present. In up to 75% of cases,
tumour cells express CD10 and BCL2 and have BCL2
translocation
3B >15 >20% Diffuse areas of centroblasts are present. In a few
cases, tumour cells express CD10 and BCL2 and
have BCL2 translocation
Follicular lymphoma (FL) is graded according to cytological features. CB, centroblast;
HPF, high-
power microscopy field (×40 objective, 0.159 mm2
). Data from refs3,136
.
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diseases (Box 1). In the variant termed partial involve-
ment by FL, neoplastic follicles (partially involving the
lymph node) alternate with non-
neoplastic follicles.
This variant should be distinguished from both ISFN
and reactive lymph node hyperplasia. Partial involve-
ment by FL is associated with limited stage disease and
good prognosis151
. In the floral variant, neoplastic folli-
cles are abnormally shaped as expanded mantle zones
enter into the centre of the neoplastic follicles152,153
.
This variant resembles the non-
neoplastic entity ‘pro-
gressive transformation of GC’152–154
, MZL or nodular
lymphocyte-
predominant Hodgkin lymphoma152,153
.
FL with monocytoid (marginal zone) B cell differen-
tiation is yet another variant with a large number of
monocytoid B cells in the marginal zone or perifollicular
area. This variant is associated with a high disease
stage and poor prognosis and should not be confused
with MZL155
. Finally, signet ring cell FL is a variant in
which lymphoma cells are present within follicles and
interfollicular areas. Tumour cells have clear, vacuo-
lated cytoplasm and an eccentric nucleus, and should be
distinguished from other carcinoma cells, for example,
gastric carcinoma.
Subtypes of FL. New clinicopathologic and genetic
features identifying specific FL subtypes have been
recognized in the revised 2017 WHO classification3,18
.
Although most FLs are characterized by BCL2 trans
location, a subset of cases lacks such rearrangements and
instead has a variety of other genetic features including
IRF4 expression, BCL6 alterations and absence of CD10
expression139,156
. The specific subtypes in general are
uncommon3
and the clinicopathologic and genetic fea-
tures of FL subtypes3,8,18,157,158
can be classified as primary
nodal and primary extranodal.
The primary nodal subtypes include paediatric-
type
FL that is characterized by a lack of BCL2 rearrange-
ment, localized disease and neoplastic B cells with a
high proliferation index. Despite its name, it can also
occur in adults (and the other types can occur in chil-
dren as well). This disease is recognized by the WHO
as a distinct entity with unique features3,18,19
. The most
frequently mutated genes reported in paediatric-
type FL
are TNFRSF14, MAP2K1 and IRF8 (ref.159
). Large B cell
lymphoma with IRF4 gene rearrangements is a provi-
sional entity that is distinct from paediatric-type FL and
DLBCL18
; it is a localized disease with frequent involve-
ment of the cervical lymph nodes or Waldayer’s tonsillar
ring. CD5+
FL constitutes ~5% of FL cases and carries
a higher risk of transformation. All grades of FL are
represented in this FL type, which exhibits a higher rate
of transformation to DLBCL than typical CD5–
types of
FL18
. Finally, Epstein–Barr virus-
positive FL occurs in
elderly patients without evidence of immunodeficiency.
This type is associated with high-
stage disease and is a
grade 3 FL18
.
The primary extranodal types include testicular
FL, which is observed in the paediatric population
and, rarely, in adulthood. It is characterized by a lack
of BCL2 translocation, and has a favourable progno-
sis3,136
. Duodenal-
type FL typically presents as solitary
or multiple polypoid tumours and displays features that
overlap with ISFN. The neoplastic cells express BLC2,
CD10 and CD20, and have a low proliferation index.
The clinical course of this subtype is indolent, but it
might spontaneously regress3,136
. Finally, primary cuta-
neous follicle centre lymphoma represents ~50% of all
B cell lymphomas of the skin and mainly affects middle-
aged adults. BCL2 translocation is reported in 10–40%
of cases and is often associated with a follicular growth
pattern. The prognosis, regardless of the growth pattern
and cytology, is usually favourable with survival of >95%
at 5 years3
.
Histological transformation. Histological transfor-
mation of FL into an aggressive lymphoma can occur
during the course of the disease and is observed at the
time of rebiopsy. The lymphoma histological type most
often observed at the time of rebiopsy is DLBCL, but FL
can also transform into other types of high-
grade B cell
lymphoma160,161
.
Proof of a clonal relationship between low-
grade
FL and a subsequent aggressive lymphoma is required
to define histological transformation160
. Tumours that
arise by histological transformation of a low-
grade FL
share many features with de novo lymphomas, such as
BCL2, CD10 and BCL6 expression, but histologically
transformed tumours often contain areas of persistent
a c
b
Fig. 5 | Immunophenotype of follicular lymphoma. Photomicrographs from a single patient of formalin-fixed,
paraffin-embedded tissue sections. a | BCL2 immunostaining is intense, showing BCL2+
B cells in follicular areas.
Magnification ×10. b | CD23+
follicular dendritic cells are loosely and irregularly aggregated within the neoplastic
follicles. Arrows indicate irregularly aggregated follicular dendritic cells. Original magnification ×10. c | Double stain
(purple) simultaneously detects BCL2+
B cells and CD23+
follicular dendritic cells that are irregularly aggregated within
the neoplastic follicle. Magnification ×15.
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low-grade lymphoma161
. Genetic alterations, including
MYC translocations, TP53 mutations and copy number
loss, might be involved in histological transformation11,162
(see below, Outlook). Histological transformation is not
associated with a specific histological feature, genetic
feature or subtype of FL.
Management
Prognosis
Follicular lymphoma is characterized by a waxing and
waning disease course. Given the heterogeneity of the
disease course, several models have been proposed to
predict treatment outcome, such as PFS or EFS, and over-
all survival of patients with FL. The FLIPI incorporates
five factors, namely age, stage, haemoglobin levels, lac-
tate dehydrogenase levels and number of involved nodal
areas.UsingFLIPI,patientscanbegroupedintothreerisk
groups: low risk (score 0–1), intermediate risk (score 2)
and high risk (score 3–5), predicting 10-year overall sur-
vival of 70%, 50%, and 35%, respectively163
. Although
this score was developed in the pre-rituximab era, it has
maintained its validity164
. Subsequently, a FLIPI2 prog-
nostic model was developed using prospective data from
942 patients treated in the rituximab era. The model also
used five risk factors (age, bone marrow involvement,
haemoglobin levels, diameter of the largest node and
β2-microglobulin levels) and identified three risk groups
with differential outcomes165
. More recently, a simplified
prognostic model was generated from the findings of
the PRIMA study (PRIMA-
prognostic index or PRIMA-
PI), using serum β2-microglobulin levels and an assess-
ment of bone marrow involvement to generate three
risk groups with different overall survival and EFS24
(ref.166
). However, this model has not been validated
in prospective studies.
Taking advantage of the availability of genome
sequencing results, a new prognostic model (m7-FLIPI)
incorporated clinical data and mutational status of
seven genes (EZH2, ARID1A, MEF2B, EP300, FOXO1,
CREBBP and CARD11) to predict treatment outcome
of front-line immunochemotherapy regimens (R-CHOP
(comprising rituximab, cyclophosphamide, doxorubicin,
vincristine and prednisolone) and R-
CVP (comprising
rituximab, cyclophosphamide, vincristine and predni-
solone)10
. Approximately 50% of patients with high-risk
FLIPI were reclassified as having a low risk based on the
m7-FLIPI. This model needs further prospective valida-
tion in other widely used regimens, such as bendamus-
tine. Furthermore, the limited use of DNA sequencing
has precluded the wide application of this model in
clinical practice.
Early-stage disease
Since FL is largely asymptomatic, the majority of patients
present with advanced-
stage disease, with only 20–30%
presenting with stage I or II disease167
. Typically, early-
stage FL is treated with external beam radiotherapy with
or without systemic therapy, which imparts excellent dis-
ease control leading to long-
term complete remission
in ~50% of patients168–170
(Fig. 7). Most studies reported
before the introduction of modern imaging might have
underestimated the treatment outcome in patients who
were graded as stage I or II FL, as some cases might
have been classified as advanced-
stage malignancy
with modern imaging171
. Although combined modal-
ity therapy (combination of radiotherapy and systemic
immunotherapy or chemotherapy) has been shown to
improve PFS compared with radiotherapy alone, it had
no impact on improving overall survival, possibly due
c
d
a b
Fig. 6 | In situ follicular neoplasia. Tumour progression from in situ follicular neoplasia to early follicular lymphoma (FL)
or overt FL. a | The strongly immunostained BCL2+
B cells are confined to the germinal centre. The BCL2 staining in this
population is more intense than that exhibited by the surrounding mantle cells. Magnification ×20. b | A lymph node
displaying overt FL and early FL. Magnification ×0.2. c | Foci of early FL in which BCL2+
cells expand outside the follicle
without a defined mantle zone. Magnification ×2. d | Foci of overt FL showing intense immunostaining of BCL2 within the
neoplastic follicle. Magnification ×2. All panels are of formalin-fixed, paraffin-embedded tissue sections.
Box 1 | Morphological variants of FL
• Partial involvement by follicular lymphoma (FL)
(differential diagnoses: in situ follicular neoplasia and
reactive lymph node hyperplasia)
• Floral variant (differential diagnoses: progressively
transformed germinal centres, marginal zone
lymphoma and nodular lymphocyte-predominant
Hodgkin lymphoma)
• FLwithmonocytoid(marginalzone)Bcelldifferentiation
variant(differentialdiagnosis:marginalzonelymphoma)
• Signetringcellvariant(differentialdiagnosis:carcinoma)
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to the efficacy of subsequent therapy172–174
. In selected
cases, initial observation (watch and wait) may be a rea-
sonable choice, especially in patients with substantial
comorbidities175
.
Advanced-stage disease
The majority of patients (70–80%) with FL present with
advanced-
stage disease (stage III and IV), which is con-
sidered incurable but is associated with long median
survival. Given the slow growth rate, incurability and
adverse effects of therapy, asymptomatic patients with
low tumour burden may be considered for deferral of
therapy initiation (watch and wait) until they become
symptomatic176,177
. An alternative strategy is to treat these
patients with single-
agent rituximab, which might fur-
ther delay the need for initiating subsequent therapy by
an average of 8 years178
.
Patients with high tumour burden or with disease-
related symptoms are typically treated with chemo
immunotherapy, with or without maintenance therapy
with an anti-
CD20 monoclonal antibody179–181
. The
most widely used front-
line regimen is rituximab plus
bendamustine or R-
CHOP, but other regimens are also
used182–184
(Fig. 7). Several studies evaluated the role of
giving single-
agent anti-
CD20 antibody after induction
chemoimmunotherapy, typically administered every
2monthsfor2years(maintenancetherapy).Consistently,
maintenance therapy with anti-CD20 monoclonal anti-
bodies has been shown to prolong PFS185,186
, but with-
out affecting overall survival. One study showed that
substituting rituximab in the front-
line regimen with
obinutuzumab, a glycoengineered type II anti-
CD20
monoclonal antibody, prolonged PFS but still had no
overall survival benefit187
. Given the lack of survival
benefit, the use of anti-CD20 maintenance after chemo-
immunotherapy regimens should be considered based
on assessment of the risks and benefits and patient
preferences.
Initial phase II studies of lenalidomide (an immune
modulatory agent)and rituximab, a chemotherapy-
free regimen, demonstrated high overall and complete
response rates in patients with relapsed or treatment-
naive FL188–190
. These results generated enthusiasm
for the possibility of substituting standard chemo
immunotherapy regimens with less-
toxic regimens188–190
.
Although a subsequent randomized phase III trial
demonstrated that lenalidomide plus rituximab is as
effective as standard chemotherapy-
based regimens190
,
investigation of the long-
term efficacy and safety is
needed before this regimen is widely adopted.
Response assessment
The Lugano classification is used for initial evaluation,
staging and response assessment of all types of lym-
phomas, including FL191
. Recently, a more simplified
response evaluation in lymphoma (RECIL) model was
established based on analysing 47,828 imaging meas-
urements from 2,983 adult and paediatric patients with
lymphoma enrolled in 10 multicentre clinical trials192
.
RECIL uses the sum of longest diameters of a maximum
of three target lesions as opposed to the Lugano criteria
that uses bidimensional measurements of six target
lesions. Both models use multiple imaging modalities to
determine remission status. In recent years, new molec-
ular tests have been investigated to determine remis-
sion status, including circulating tumour DNA193,194
.
However, these tools remain investigational, and require
standardization and prospective validation before
incorporating them into clinical practice.
Relapsed or refractory disease
The majority of patients will have disease relapse or pro-
gression after initial therapy. It is important to rule out
disease transformation to a more aggressive histology,
which occurs at a rate of 1–3% per patient per year195–197
.
Two reports suggested that early disease progression
within 12–24 months of initial diagnosis or start of
therapy is associated with poor survival9,198
. However,
this poor prognosis might be primarily driven by trans-
formed disease. The optimal therapy of patients with
progression within 24 months without disease transfor-
mation remains undetermined, but such patients might
not need more-
intensive therapy. However, patients
with early progression with histological transforma-
tion may benefit from more-
intensive therapy, such as
stem cell transplantation. In one retrospective study,
consolidation with either autologous or allogeneic stem
cell transplantation resulted in similar outcomes, with
durable remissions achieved in almost half of patients199
.
However, autologous stem cell transplantation was
less toxic and, therefore, it is more widely used in this
setting199
.
Localized
radiotherapy
• Anti-CD20 monoclonal antibody ±
chemotherapy
• Anti-CD20 monoclonal antibody ±
chemotherapy and/or radiotherapy
Observation,
if clinically
indicated
• Anti-CD20 monoclonal antibody (in those
with low tumour burden or in elderly patients)
• Anti-CD20 monoclonal antibody +
chemotherapy (± maintenance anti-CD20)
Stage III or stage IV
Stage I or stage II (bulky), or non-contiguous stage II
Stage I or
contiguous
stage II
(non-bulky)
Follicular lymphoma
Fig. 7 | Standard first-
line therapy of follicular lymphoma. The Lugano Classification191
includes two stages: limited
(previously Ann Arbor stages I and II non-bulky257
) and advanced (previously Ann Arbor stages III or IV257
). This classification
is now incorporated into the 8th edition of the TNM Classification of Malignant Tumors by the Union for International
Cancer Control258
.
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Patients with relapsed FL have a wide range of
treatment options, including different chemoimmu-
notherapy regimens, PI3K inhibitors, lenalidomide
plus rituximab, and novel therapies being tested within
clinical trials200–205
. To date, three different PI3K inhib-
itors have been approved in the United States for the
treatment of FL: idelalisib (for the δ-
isotype), copan-
lisib (for the α- and δ-
isotypes) and duvelisib (for the
γ- and δ-
isotypes)132,206,207
. Lenalidomide plus rituximab
shows significant activity in patients with relapsed FL,
and was also recently approved for the treatment of
patients with relapsed and refractory FL205,208,209
. In a
randomized, double-
blind, phase III study, lenalidomide
plus rituximab gave a PFS of 39.4 months compared with
14.1 months for rituximab and placebo in patients
with previously treated FL (n = 295) and MZL (n = 63)
(HR 0.46; 95% CI 0.34–0.62; P < 0.0001)205
.
Novel therapies
Promising new treatment approaches include epigenetic
modulating drugs such as tazemetostat, an inhibitor of
EZH2 taken orally (ref.210
). A gain-
of-function EZH2
mutation at tyrosine 641 (Y641) increases methylation
and silences several genes involved in B cell differen-
tiation, cell cycle and tumour suppression. EZH2 is
required for GC formation and somatic EZH2 muta-
tions promote lymphoid transformation211
. In a trial of
tazemetostat, patients with relapsed or refractory FL har-
bouring an EZH2 mutation had a 71% overall response
rate (ORR) whereas patients without this mutation had
a 33% ORR212
. Additionally, several HDAC inhibitors,
including abexinostat, vorinostat and mocetinostat, have
demonstrated clinical activity in patients with relapsed
or refractory FL, with response rates in the range of
12–50%213–217
.
With the success of anti-
CD20 therapy, new immuno
therapy approaches are being investigated, includ-
ing bispecific antibodies, antibody–drug conjugates,
immune-
checkpoint inhibitors and chimeric antigen
receptor (CAR) T cell therapy. The bispecific anti-
body mosunetuzumab, which targets CD20 and CD3,
redirects and recruits endogenous T cells to the prox-
imity of CD20-expressing B cells; it has a promising
clinical activity in patients with relapsed or refractory
FL, with an ORR of 61%218
. Polatuzumab vedotin, an
antibody–drug conjugate targeting the CD79b com-
ponent of the BCR, carries the microtubule inhibitor
monomethyl auristatin E as its payload. When com-
bined with rituximab, polatuzumab vedotin induces an
ORR of 70% (45% complete response)219
. Another strat-
egy is that of Hu5F9-G4, an antibody targeting CD47
(which is overexpressed on cancer cells), which enables
killing of tumour cells by macrophages by disrupting
the inhibitory effect of CD47 on macrophage phago-
cytosis. When combined with rituximab, Hu5F9-G4
induced an ORR and complete response of 71% and
43%, respectively, in patients with FL220
. Finally, initial
experience with CAR T cell therapy targeting the pan-B
cell CD19 antigen reported a 70% complete response
at 6 months in relapsed or refractory FL213,214,219,221,222
.
However, this treatment modality should be reserved
for patients with a high medical need to justify the
cost and treatment-
related toxicity. Also of interest are
patients with histological trans
formation, or patients
with refractory disease, after exhausting the currently
available approved therapies.
Quality of life
FL is a disease in which patients experience multiple
relapses between disease-
free periods. Patients with FL
who do not need therapy at diagnosis and those with
prolonged responses after conventional treatments have
excellent outcomes, with a life expectancy similar to that
of healthy individuals223
. By contrast, patients with early
relapse or progression and those whose disease under-
goes histological transformation have a dismal progno-
ses and need intensive therapy9,198,224,225
. Most of these
patients die from disease progression, whereas many
long-
term responders eventually die from unrelated
causes. Several time-
related surrogates for outcome
(such as progression of disease at 24 months or complete
response at 30 months) can be used to identify high-risk
patients9,198,223,225,226
. Histological transformation, in the
rituximab era, occurs in ~1–3% of patients per year227
and,evenwithnewtherapies,suchaneventisanominous
prognostic sign228
.
Patients with FL have an increased risk of develop-
ing other medical conditions, including infections and
secondary neoplasms, although less frequently than in
other lymphoproliferative disorders229
. During chemo-
therapy, infections are typically bacterial. With the use
of immunotherapy such as rituximab, reactivation of
hepatitis B virus and other viral infections is possible230
.
In fact, maintenance therapy had to be stopped in ~4%
of patients due to infectious complications185
. The use of
purine analogues, including fludarabine and benda-
mustine, has been associated with immunodeficiency-
related infections, particularly when patients receive
maintenance therapy with an anti-
CD20 antibody after
induction therapy187,231,232
. Furthermore, in patients
with relapsed or refractory disease, the use of PI3K
inhibitors including idelalisib has been associated
with immunodeficiency-
related infections, includ-
ing cytomegalovirus and Pneumocystis jiroveci233–237
.
Additionally, both the disease itself and the therapies
given may increase the risk of secondary malignan-
cies, including myelodysplasia and acute leukaemia.
According to three recent series, the cumulative inci-
dence of secondary malignancy is ~8–10%, whereas the
rate of haematological malignancy is 3%238–240
. The risk
of secondary neoplasms due to immunosuppressive or
immunoregulatory drugs has emerged as a matter of
concern231
. Finally, treatments such as stem cell trans-
plantation and CAR T cell therapies have specific toxic-
ities, including cytokine release syndrome, neurotoxicity
or infectious complications, of which clinicians should
be aware.
Several quality of life studies have been performed
in patients with FL, mainly in the setting of clinical tri-
als185,187
. Not surprisingly, quality of life in disease-
free
patients was found to be similar to that of the general
population, whereas patients with relapsed, active dis-
ease had the lowest scores in health-
related quality of
life in terms of physical, emotional, functional and social
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well-being scores241
. Interestingly, maintenance with
rituximab after induction did not substantially impair
quality of life185
. In a chronic disease such as FL with
long survival, the late effects of therapy on quality of life
should also be considered but, thus far, studies of this
type have not been reported. Avoiding chemotherapy
could improve quality of life by diminishing the inci-
dence of adverse effects of the treatment. Nevertheless,
chemotherapy-
free regimens are not without risk.
Indeed, new and previously unknown adverse effects are
emerging from newer regimens. Thus, the risk to bene-
fit ratio should always be considered when a change in
therapy is being planned.
Outlook
Transformation of FL to a more aggressive disease now
affects <10% of all patients227,242–244
although it remains
the major cause of death25
. However, therapy does have
a cost, considering that a substantial number of patients
will die from treatment-
related toxicity or secondary
malignancies25
. In parallel, genetic landscapes, includ-
ing longitudinal69–71,245
and spatial profiling68
, provide a
portrait of the coding genome and recognized mutations
in components of the epigenome. We can also postulate
the existence of a rare population of B cells called the
CPCs57,70,245,246
, which are responsible for propagating
each new episode of FL. This progenitor cell model of
FL is appealing as it offers an explanation for the pro-
tracted and incurable nature of the disease. Anti-
CD20
antibody maintenance therapy and consolidative stem
cell transplants may deplete or control this cell popu-
lation sufficiently well to increase the length of time to
disease progression, without removing them entirely.
Our new understanding of FL enables a ‘reset’ of our
research priorities, focusing our efforts on how best
to manage high-
risk patients, use existing and novel
therapies appropriately, and de-
escalate therapy for
low-risk patients.
Biological definition of FL
Rapid progress has been made in assessing the B cell-
specific and lymphoma-specific roles of genes mutated
in FL92,112,113,115
. Conceivably, mutations might affect a
defined set of downstream genes and pathways, but
information is scant on how individual gene muta-
tions work together, and whether the dependence
on a specific mutation is maintained throughout the
course of the disease. Better understanding of these
networks could offer an explanation for the mod-
est efficacy of certain strategies including the BCL2
inhibitor venetoclax247
and the BTK inhibitor ibruti-
nib128
. We are critically lacking an understanding of
why mutations in genes encoding components of the
epi-machinery (KMT2D, CREBBP, EZH2, ARID1A,
EP300 and histone linkers) seem to be fundamental
in FL70,71,99,100,106,111,245
. Indeed, whether these muta-
tions have entirely individual roles is debatable, with
an alternative possibility that epigenetic deregulation
may create ‘B cell chaos’ that leads to cell-to-cell varia-
tions in the deposition of individual chromatin marks,
facilitating increases in intratumour heterogeneity
and escape mechanisms248
. Although the first reports
of single-cell analysis in FL have been made47,249
, future
studies will shed light on the dichotomy between nor-
mal and malignant B cells and how their individual
interactions with the TME influence survival in FL.
As many epigenetic mutations occur early in FL70,71,245
and can be linked by evolution of the CPC population,
epigenetic therapies could conceivably be an impor-
tant buffer to reduce gene expression heterogeneity and
curtail the repopulating potential of this B cell pool.
Characterization and targeting of CPCs must, there-
fore, be at the forefront of research efforts, and are a
critical first step in devising novel approaches that can
more effectively target this B cell fraction with the goal
of achieving disease ‘cure’.
Identifying and managing high-risk FL
Clinical trial programmes in FL should be directed
towards high-
risk patient groups. Identifying these
patients at the time of diagnosis and predicting the
clinical trajectory of FL are challenging; existing novel
approaches are largely reliant on diagnostic biop-
sies. New prognostic tools, including the m7-FLIPI10
,
23-gene11
and PRIMA-
PI166
, have merit but are unlikely
to be sufficiently robust alone to inform clinical prac-
tice. The prospect of dynamic monitoring, with post-
treatment PET–CT imaging250,251
in conjunction with the
serial measurement of biomarkers, most notably circu-
lating tumour DNA193
, may help better define a high-risk
group of patients.
Considering the clinical and molecular heterogeneity
of FL, not all high-
risk patients are the same and their
outcomes will vary depending on the underlying tumour
genetics, patterns of clonal evolution, composition of the
TME and the nature and timing of subsequent FL or t-
FL
relapse. Indeed, focusing attention on discrete high-risk
groups separately could be beneficial — for example,
those who experience early transformation compared
with patients who progress with recurrent low-
grade
disease.
The identification of high-
risk patients will help
direct clinical trial activity in FL, whereby alternative
strategies to the current standard of care are required.
A plethora of active agents in FL are available, emerg-
ing from early, nonrandomized trials of therapies
ranging from small molecule inhibitors213,247,252–254
to
CAR T cells222
, all of which could conceivably benefit
subgroups of high-
risk patients. It is indeed important
to acknowledge that opportunities have been largely
missed in trials of FL to assess whether specific biomark-
ers identify cohorts responsive to a particular treatment,
with the notable exception of the finding of a preferential
response to the EZH2 inhibitor tazemetostat in patients
with EZH2-mutated FL253,254
.
Evaluation of new technical innovations, from single-
cell studies46,47,62
to omics profiling approaches, is needed
to gather additional insights into the biology and vulner-
abilities of FL. Moreover, comprehensive sampling, over
the entirety of the FL disease trajectory, is essential to
fully capitalize on the expanding array of technological
strategies at our disposal.
Published online xx xx xxxx
Primer
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