The liver as a lymphoid organ annu rev

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The Liver as a
Lymphoid Organ
Ian Nicholas Crispe
David H. Smith Center for Vaccine Biology and Immunology, Aab Institute for Biomedical
Research, University of Rochester Medical Center, Rochester, New York 14642;
email: nick crispe@urmc.rochester.edu

Annu. Rev. Immunol. 2009. 27:147–63

Key Words

The Annual Review of Immunology is online at
immunol.annualreviews.org

antigen presentation, hepatitis, Kupffer cells, innate immunity,
sinusoid, stellate cells

This article’s doi:
10.1146/annurev.immunol.021908.132629
Copyright c 2009 by Annual Reviews.
All rights reserved
0732-0582/09/0423-0147$20.00

Abstract
The liver receives blood from both the systemic circulation and the
intestine, and in distinctive, thin-walled sinusoids this mixture passes
over a large macrophage population, termed Kupffer cells. The exposure of liver cells to antigens, and to microbial products derived from the
intestinal bacteria, has resulted in a distinctive local immune environment. Innate lymphocytes, including both natural killer cells and natural
killer T cells, are unusually abundant in the liver. Multiple populations
of nonhematopoietic liver cells, including sinusoidal endothelial cells,
stellate cells located in the subendothelial space, and liver parenchymal
cells, take on the roles of antigen-presenting cells. These cells present
antigen in the context of immunosuppressive cytokines and inhibitory
cell surface ligands, and immune responses to liver antigens often result in tolerance. Important human pathogens, including hepatitis C
virus and the malaria parasite, exploit the liver’s environment, subvert
immunity, and establish persistent infection.

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INTRODUCTION
sinusoid: thin-walled
blood space through
which blood passes in
the liver
Kupffer cell:
intravascular
macrophage lining the
liver sinusoids


DC: dendritic cell
TLR: Toll-like
receptor
APC: antigenpresenting cell

The liver stands at a hemodynamic conﬂuence.
In distinctive, thin-walled vessels termed sinusoids, oxygenated blood from the arterial system mixes with portal venous blood returning from the intestine. The sinusoids contain
a diversity of immunologically active cell types,
including both lymphocytes and myeloid cells
(Figure 1). The Kupffer cells form a large intravascular macrophage bed and, with liver dendritic cells (DCs), come in both immunogenic
and tolerogenic forms. The liver also contains
diverse lymphocytes, including T cells, natural
killer T (NKT) cells, and natural killer (NK)
cells.
Portal venous blood contains the products
of digestion, along with antigens and microbial products that originate from the bacteria in the small and large intestine. Among

Hepatocytes
HSC

mDC/
pDC

KC
LSEC

these bacterial products is lipopolysaccharide
endotoxin (LPS), derived from the cell walls of
Gram-negative bacteria. Under normal conditions, LPS is undetectable in the systemic circulation, but it is present at up to 1.0 ng/ml
in portal venous blood (1). The cells of the
hepatic sinusoids express the LPS receptor and
effectively remove this molecule so that the
systemic circulation is protected from endotoxemia (2). Many cells of the innate immune
system express the LPS receptor, which consists of Toll-like receptor-4 (TLR4) together
with the molecules CD14 and MD2; engagement of this receptor on most cell types delivers a strong activating signal. However, in the
liver these receptors are continuously exposed
to low levels of LPS, resulting in altered responsiveness to an LPS challenge. At the same
time, the adaptive immune cells of the liver are
exposed to food-derived antigens, the majority
of which are harmless. The continuous presence, under normal conditions, of both TLR
ligands and antigens has resulted in a distinctive
set of mechanisms to maintain self-tolerance
yet deliver immunity to infection. Liver immunity features a local concentration of overlapping innate immune mechanisms, together
with the capacity of unusual cell types to act
as antigen-presenting cells (APCs). The liver’s
resident immune cells are not passive in the face
of continuous exposure to antigens and LPS;
instead they exist in a state of active tolerance,
which results in liver allograft tolerance (3) but
also creates a window of vulnerability for welladapted pathogens. This state of tolerance is
metastable; the right combination of signals can
reverse tolerance and activate immunity locally.

Figure 1

INNATE IMMUNITY
IN THE LIVER

Antigen-presenting cells (APCs) in liver sinusoids. The liver contains multiple
subsets of dendritic cells, including myeloid and plasmacytoid dendritic cells
(mDCs and pDCs). There are abundant mononuclear phagocytes in the form
of Kupffer cells (KCs), and these can express costimulatory molecules. In
addition, there are two additional populations of APCs in the form of liver
sinusoidal endothelial cells (LSECs) and hepatic stellate cells (HSCs). The
evidence for APC function in each of these cell types is summarized in the text.

The LPS from intestinal bacteria is not the only
immune stimulus to which the liver is exposed.
Pattern-recognition receptors in the liver
sense the presence of enteric pathogens. These
include cell surface and endosomal TLRs, cytoplasmic nucleotide-binding oligomerization

148

Crispe


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domain (NOD)-like receptors, and RNA
helicases, including retinoic acid inducible
protein-I (RIG-I). The TLRs recognize a
diverse array of bacterial and viral molecules,
including bacterial lipopeptides (heterodimers
of TLR1 and TLR2, and of TLR2 with
TLR6); LPS and ﬂagellin (TLR4 and TLR5,
respectively); and exogenous dsRNA, viral
ssRNA, and bacterial unmethylated DNA
(TLR3, TLR7, TLR8, and TLR9). The NOD
receptors recognize bacterial peptidoglycans,
whereas the RIG-I molecules recognize
structural features of viral ssRNA. These
receptors, their speciﬁcity, and their roles in
host defense were recently reviewed (4, 5).
From the perspective of a global discussion of
liver immunity, the key point is that signals
from these diverse receptors converge on two
signaling pathways. Thus, all of the TLRs
except TLR3 transmit signals via the adaptor
protein MyD88 (myeloid differentiation
factor-88), which results in the activation of the
kinases p38, JNK, and IκB kinase, leading to
NF-κB activation. NOD receptors also activate
NF-κB. The TLR4 receptor complex, in addition to activation via MyD88, recruits the
adaptor protein TRIF (TIR-domain containing
adaptor recruiting interferon-β), which acts via
TBK1 (TRAF family member–associated NFκB activator–binding kinase 1) to cause phosphorylation and nuclear localization of IRF-3
(IFN regulatory factor 3), the transcription
factor that drives synthesis of type 1 interferon
(IFN). Ligation of TLR3 selectively activates
this signaling pathway. Similarly, RIG-I and
its homolog MDA-5 promote the activation
of mitochondrial IPS-1 (IFN-β promoter
stimulator 1), resulting in IRF-3 activation
and type 1 IFN secretion. These pathways
are optimized such that pattern-recognition
receptors engaged by bacterial products generally promote NF-κB activation, whereas those
pathways activated by viral infection strongly
induce IFN-β. The continuous, low-level
stimulation of the former pathway is one of the
distinctive features of the liver environment
(Figure 2).

NK cells are present at higher frequency in
the liver than in most tissues. Thus, in human
liver leukocytes obtained by elution from donor
livers (6), in cell suspensions obtained from human liver tissue (7), and in the cells isolated
from the mouse liver by enzymatic digestion
(8), NK cells make up as many as 50% of liver
lymphocytes. Similarly to NK cells elsewhere,
these cells respond both to cytokine activation
and to engagement of an excess of activating
receptors over inhibitory receptors (9). Once
activated, they manifest their function through
cytokine synthesis and cytotoxicity. NK cells express two key adaptor molecules: DAP10 and
DAP12. DAP10 associates with the activating
NKG2D receptor, and the ITAM (immunoreceptor tyrosine-based activation motif)-bearing
DAP12 adaptor protein associates with several
receptors, including the CD94-NKG2C heterodimer and the Ly49H receptor. Ligands for
NKG2D are expressed in the liver under diverse circumstances. Low amounts of NKG2D
ligands are expressed constitutively in the liver
(20), and these ligands can be upregulated after viral infection or transformation of hepatocytes. The NKG2D ligands include MHC class
I–related proteins A and B (MICA and MICB),
which are expressed on human hepatocellular
cancer cells (10), and mouse retinoic acid early
inducible-1 (RAE-1), which is transcriptionally upregulated by cytomegalovirus infection
(11). Liver NK cells are induced to synthesize
IFN-γ in response to IL-12 (12) and to manifest
perforin-dependent cytotoxicity in response to
the Kupffer cell–produced cytokine IL-18 (13).
They are also cytotoxic owing to the expression
of TRAIL (TNF-receptor apoptosis-inducing
ligand), which can engage death receptors that
are induced on hepatocytes by hepatitis B virus
(HBV) (14).
The liver lymphocytes also contain an unusually high frequency of NKT cells. These
include both canonical NKT cells expressing
an invariant T cell receptor (TCR) that binds
to CD1d complexed with α-galactosylceramide
and the noncanonical NKT cells that recognize
other ligands. Liver NKT cells are abundant

www.annualreviews.org • The Liver as a Lymphoid Organ

HBV: hepatitis B virus

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Constitutively engaged
by gastrointestinal ligands
Bacterial
peptidoglycans

Bacterial
lipopeptides

TLR2/6

TLR1/2

Constitutively quiescent,
activated by virus infection

Flagellin

LPS

TLR5

dsRNA

Viral ssRNA

TLR4


RIG-1
NOD-1
NOD-2

MyD88

TLR3
TRIF

NF-κB

IL-10

MDA-1

IPS -1
IRF-3

TNF-α, IL-1,
IL-6, IL-12, IL-18

IFN-α
IFN-β

Figure 2
Effects of microbial products. The model explains the effects of the liver environment on patternrecognition receptors. The presence of detectable LPS in portal blood suggests that other gut-derived
microbial molecules may be present also. We therefore propose the model that NOD proteins and the
subsets of TLRs that recognize microbial products are constitutively engaged in the liver. In contrast,
receptors for viral elements (TLR3, RIG-I, MDA-1) are not engaged under normal conditions. We propose
that this changes the balance between NF-κB and IRF-3-dependent signaling pathways. In this diagram,
heterodimeric TLRs are indicated as TLR1/2 and TLR2/6.

both in mouse (15) and in human (6). These
cells were recently ﬁlmed patrolling the hepatic sinusoids, based on their expression of
a GFP reporter molecule driven by the endogenous CXCR6 promoter (16). Despite
their thymic origin (17) and expression of a
TCRαβ generated by V(D)J recombination,
these CD1d-reactive T cells show evolutionary
convergence with leukocytes expressing innate
pattern-recognition receptors; their TCRs recognize glycolipid antigens that are conserved
features of bacterial cell walls. These include
glycosphingolipids from the soil bacterium,
Sphingomonas sp. (18), and a diacylglycerol derived from the pathogenic spirochaete Borrelia burgdorferi (19). The NKT cells may also
have the potential to respond to the bacterial
cell wall components derived from the intestinal bacteria, and this could account not only

HCV: hepatitis C
virus

150

Crispe

for their abundance in the liver, but also for
their expression of markers of activation (6).
Like many other T cells, the NKT cells express
DAP10- and DAP12-associated receptors, and
the NKG2D receptors on these cells are implicated in immunopathology in a mouse model
of hepatitis B in which RAE-1 is induced to engage these receptors (20).
The signiﬁcance of innate immunity in the
defense of the liver is evident from the multiple adaptations through which pathogens subvert it. Thus, HCV RNA interacts with RIG-I
to activate NF-κB and IFN-β secretion; however, HCV also subverts this pathway because
its NS3/4 protease cleaves the IPS-1 adaptor
protein of RIG-I signaling (21). IRF-3 may also
be activated through the TLR3 pathway, but
HCV NS3/4 also targets this pathway through
the cleavage of TRIF (22). Strikingly, this


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mechanism of innate immune escape is also
seen in hepatitis A virus infection, where the
protease 3ABC cleaves IPS-1 (also known as
MAVS), again resulting in subversion of NF-κB
activation (23). This convergence supports the
argument that the IRF-3 pathway plays a critical role in antiviral immunity in liver. Apart
from inhibiting innate immune signaling pathways within the infected cells, HCV targets NK
cells though a completely independent mechanism. The HCV envelope protein, E2, binds to
CD81 on human NK cells, and this results in
suppression of their cytotoxic function and of
IFN-γ synthesis (24). An important feature of
HCV is therefore both a concerted attack on
IFN-β synthesis and a major cellular source of
IFN-γ. Because the pathogen so actively subverts IFN delivery and function, it is reasonable
to suppose that IFN-responsive genes play a key
role in anti-HCV immunity. It is not, therefore, surprising that the mainstay of treatment
is high-dose exogenous IFN-α.
Complex protozoan pathogens also manipulate the phagocytic function of Kupffer cells.
Thus, the malaria parasites Plasmodium sp. enter Kupffer cells as part of the process by which
they cross liver endothelium and gain access to
hepatocytes. Evidence in favor of the mechanism comes from direct visualization of malaria
sporozoite behavior in vivo (25) and from
the observation that parasitization of the liver
is reduced in osteopetrotic mice, which lack
mature macrophages (26). As part of their interaction, the malaria sporozoites disable the
Kupffer cells’ respiratory burst by increasing
intracellular cyclin AMP (27). This effect is
mediated by an abundant malaria protein, the
circumsporite protein (CSP), which binds to
a Kupffer cell’s surface receptor, LRP-1 (the
low-density lipoprotein receptor-related protein). Thus, the most likely model is that
the malaria sporozoite’s CSP engages LRP-1,
inducing cyclic AMP and suppressing the Kupffer cell’s normal response to phagocytosed
pathogens. This converts the Kupffer cells from
effective elements in innate host defense into
portals through which the parasite traverses the
endothelium (28).

WORLD PREVALENCE OF LIVER DISEASE
Malaria causes severe disease in 500 million people each year;
many more are infected, and 40% of the world population is at
risk. There is no effective vaccine.
More than 350 million people have chronic infection with
HBV, which results in one million deaths per year from cirrhosis
and liver cancer. A recombinant subunit vaccine is effective, so
this total will probably decline. Around 180 million people are
infected with HCV, and, of these, 130 million are chronically
infected and at risk for cirrhosis and liver cancer. There is no
effective vaccine.

THE DIVERSITY OF POTENTIAL
ANTIGEN-PRESENTING CELLS
The liver contains plasmacytoid DCs (pDCs)
and myeloid DCs (mDCs), and pDCs are more
abundant than they are in lymphoid tissue.
These pDCs are a major source of IFN-α, consistent with the importance of innate immune
mechanisms in the liver. But in addition, the
mouse liver contains two other identifiable subsets of DCs, the CD8α+ DCs (29) and the
less well-defined natural killer DCs (NKDCs)
(30), neither of which has yet been identified in humans. In addition to synthesizing
IFN-α, pDCs synthesize both IL-10 and IL-12.
In LPS-treated mice, liver pDCs synthesized
less IL-12 than did splenic pDCs (31) and were
poor APCs compared with splenic DCs (32).
Conversely, liver mDCs synthesized IL-10, and
this was increased in HCV patients (33). As discussed below, diverse other potential APCs in
the liver respond to TLR ligation by secreting
IL-10.
The liver contains a large macrophage population; these are the Kupffer cells. In addition
to their role as phagocytes, these cells express
MHC and costimulatory molecules, rendering them potential APCs (Figure 3). However,
relatively little work has addressed the APC
function of Kupffer cells. Early experiments
suggested that Kupffer cells were primarily immunosuppressive. Thus, addition of Kupffer
cells to a mixed leukocyte reaction performed
in the presence of low arginine resulted in

sporozoite: the
developmental stage of
the malaria parasite
that infects liver cells

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TNF-α

HSC
PD-L1

KC


LPS
Proteins
Particulates

TLR4

IL-10
IL-12/18

IFN-γ
T cell
NK cell

LSEC

Figure 3
Immunology of Kupffer cells. These cells can take up LPS, proteins, and
particulates from the blood and secrete a number of cytokines, including
TNF-α, IL-12, and IL-18, but also IL-10. The balance between IL-12/-18 and
IL-10 production regulates NK cell activity. Kupffer cells express PD-L1 and
have the capacity to inactivate T cells, a function shared with other liver APCs.
Both T cells and NK cells secrete IFN-γ, which powerfully activates Kupffer
cells. (Abbreviations: KC, Kupffer cell; HSC, hepatic stellate cell.)

LSEC: liver
sinusoidal endothelial
cell
stellate cell: a
distinctive liver cell
type, located between
the liver endothelial
cells and the
hepatocytes

152

immunosuppression, mediated in part by PGE2
(34). Kupffer cells may also mediate suppression
through their synthesis of nitric oxide (35) and
respond to TLR4 ligation by secreting IL-10
(36). In a liver transplant model, Kupffer cells
expressed FasL, leading to alloreactive CD4+ T
cell apoptosis (37). On the basis of such experiments, investigators have invoked Kupffer cells
to explain such diverse phenomena as oral tolerance, portal vein tolerance, and liver allograft
tolerance. However, Kupffer cells may also act
as effective APCs. In HCV infection, human
Kupffer cells became MHC I and II high, expressed CD40 and CD80, and formed clusters
with CD4+ T cells, consistent with their acting
as APCs (38). Perhaps the capacity of Kupffer
cells to stimulate or inhibit T cell activation depends on the signals to which these cells have
been exposed. In a study of liver NK cell activation by human Kupffer cells, the selective
Crispe

activation of either the TRIF pathway or the
MyD88 pathway of TLR signaling resulted in
the predominant expression of either IL-18 or
IL-10, leading to higher or lower levels of NK
cell activation (39).
The liver sinusoidal endothelial cells
(LSECs) have been implicated in antigen presentation (Figure 4). These endothelial cells
are unusual in several respects: They do not secrete an organized basement membrane, and
they are perforated by numerous fenestrations,
clustered into sieve plates. These cells express
the scavenger receptor, which renders them
competent to take up circulating proteins. They
also express MHC class I and class II and costimulatory molecules including CD40, CD80,
and CD86, giving them the surface characteristics of highly active stimulatory APCs, such
as DCs (40). However, they respond to TLR4
ligation with the secretion of IL-10, to which
they also respond by downregulating their APC
functions (41), and their main effect seems to be
the induction of T cell tolerance. Thus, when
LSECs were isolated from mice that received
ovalbumin parenterally or orally, they engaged
T cells, resulting in immune deviation to a
CD4+ T regulatory phenotype, or in CD8+
T cell tolerance (42, 43).
Hepatic stellate cells reside in the subendothelial space of Disse and constitute the primary site for the storage of vitamin A. They regulate hepatic sinusoidal blood flow and can also
transdifferentiate into myofibroblasts during
the process of liver fibrosis (44). Recent studies suggest that these cells also belong among
the liver APCs (Figure 5). Thus, they express
MHC class I, MHC class II, and CD1d, and
they have the potential to respond to innate immune signals though their expression of TLR4,
CD14, and MD2, which renders them LPS
responsive (45, 46). Ex vivo, stellate cells can
activate NKT cells and classical T cells (47),
although their coexpression of the inhibitory
molecule PD-L1 also renders them capable of
T cell inactivation, leading to tolerance (48).
With so many potential liver cell subsets manifesting APC activity ex vivo, the issue of cell purification becomes particularly


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acute. Classical experiments by Steinman and
colleagues (49) demonstrated that, among
spleen cells, the most powerful APC activity
was concentrated in a very rare subset of cells,
the DCs; indeed, the alleged APC activity of
other subsets of spleen cells, including splenic
macrophages, could have been attributed to
rare, contaminating DCs. This same concern
applies to studies that document the APC action of ex vivo liver cell subsets, but now we have
more complexity. Which of the APC functions
of a culture of stellate cells may be attributed
to rare, contaminating DCs or LSECs? And in
the case of the LSECs, how many contaminating DCs or Kupffer cells is enough to account
for their ability to activate or to silence a T cell
response? The optimum reagents for the analysis of the significance of APC function would be
a series of transgenic mice in which molecules
of interest, such as MHC class I or MHC class
II molecules, are expressed or selectively inactivated under the control of promoters of absolute cell-type specificity. These ideal tools
are not yet at hand; nevertheless, we can draw
some solid conclusions from the extant in vivo
experiments.

LOCAL PRIMING OF T CELLS
IN THE LIVER
The presence in the liver of so many distinct
subsets of cells with APC function raises the
question of whether T cells are in fact activated
locally in vivo. The distinctive architecture of
the hepatic sinusoids permits circulating T cells
to make direct contact with underlying hepatocytes and also with stellate cells, as well as with
LSECs and intravascular Kupffer cells. Such interactions have in fact been revealed by electron
microscopy (50), providing a structural basis for
primary T cell activation by hepatocellular antigens. This idea was first supported by experiments in which CD8+ T cells made a rapid,
antigen-driven, local, intrahepatic immune response in transgenic mice expressing the HBV
genome in both liver and other tissues (51)
and subsequently in transgenic mice expressing
nonself MHC class I molecules (52). In addi-

TNF-α

HSC

CD95L
TLR4
LPS
Proteins

MHC I/II
CD40/80/86

IL-10
Scavenger-R
Mannose-R

T

Trapping,
Tregs,
tolerance

ICAM-1
VCAM-1 VAP-1

LSEC

Transcytosis?

Figure 4
Immunology of liver sinusoidal endothelial cells (LSECs). These cells respond
to LPS via TLR4 and can acquire circulating proteins via the scavenger
receptor and the mannose receptor. LSECs may transport proteins across
themselves, and into hepatocytes, a process termed transcytosis. They process
and present antigens in association with costimulatory ligands (CD40, CD80,
and CD86) but respond to ambient LPS by secreting IL-10, biasing T cells
toward tolerance. These cells also express multiple adhesion molecules,
including ICAM-1 (intercellular adhesion molecule-1), VCAM-1 (vascular cell
adhesion molecule-1), and VAP-1 (vascular adhesion protein-1), all of which
are implicated in T cell retention in the liver sinusoids. Thus, these cells can
promote immune tolerance, both through the local trapping of activated
T cells and the induction of regulatory T cells. This concept is summarized in
the diagram as “trapping, Tregs, tolerance.”

tion to transgenic antigens, CD8+ T cells made
a local response to antigen delivered specifically
to hepatocytes using an AAV-2 vector, and this
resulted in the subsequent seeding of activated
cells to the lymph nodes and the spleen (53).
Furthermore, the transplanted mouse liver, depleted of bone marrow–derived APCs, was fully
competent to activate naive CD8+ T cells in
response to a peptide antigen (54). Abundant
evidence therefore supports the concept that
the liver is a secondary lymphoid organ, acting
as a site of primary T cell activation.
In such experiments, it is not always clear
which cell population is the main APC. The
most compelling evidence that hepatocytes

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TLR4

HSC
IL-10

CD1d
NKT

TGF-β1
LPS
T


KC
LSEC

Figure 5
Immunology of hepatic stellate cells. Stellate cells perceive LPS via TLR4 and
also respond to multiple cytokines, including IL-10 and TGF-β1. They express
a high surface density of the nonclassical MHC class I–like molecule CD1d and
thereby activate NKT cells.

parenchymal cell:
the most abundant cell
type in the liver, also
known as hepatocytes
cross-presentation:
transfer of an antigen
from the antigenexpressing cell to a
distinct APC, resulting
in T cell engagement

154

(also termed liver parenchymal cells) themselves act as primary APCs comes from
parallel studies in vivo and in vitro, using allogeneic MHC antigens. In liver allograft experiments, cytotoxic T cells underwent spontaneous apoptosis in the transplanted liver (55),
and both hepatocytes and nonparenchymal cells
activated, then caused apoptosis of, the activated T cells in vitro (56). Similarly, both activation and apoptosis of T cells were observed
in vivo following the adoptive transfer of alloreactive T cells to transgenic mice expressing
a nonself MHC class I molecule on hepatocytes
(52, 57). Furthermore, the T cell apoptosis was
also manifest in vitro when T cells were cultured with hepatocytes, leading to the hypothesis that a feature of liver tolerance was “death
by neglect,” the engagement of T cells by APCs
deficient in costimulatory activity (58).
In these experiments, the lack of susceptibility to cross-presentation of intact MHC
molecules was key in the interpretation of the
data and strongly suggested that hepatocytes
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themselves were engaging the CD8+ T cells.
However, it is also noteworthy that in all these
studies the final outcome was T cell inactivation
or apoptosis. In a distinct transgenic model, the
deliberate priming of antigen-specific T cells
at an extrahepatic site was not enough to break
tolerance to a transgenic liver antigen expressed
in hepatocytes, supporting the argument for an
active state of antigen-driven local tolerance
(59).
The presentation of cell-intrinsic MHC
antigens by hepatocytes constitutes direct presentation. Similarly, the in vitro analysis of
the APC functions of stellate cells dealt primarily with direct presentation of cellular
antigens, or the presentation of exogenous
peptides (47). In vivo experiments revealed the
capacity of stellate cells to act as immunosuppressive APCs, protecting pancreatic islet allografts against rejection (60). In this capacity,
also, stellate cells were presenting their intrinsic antigens. In contrast, we know that LSECs
are capable of presenting exogenous antigens
encountered in vivo. This is the case for soluble antigens given parenterally or orally (42,
43). The LSECs also appear to be capable of
true cross-presentation, in which tumor cell–
derived antigen was acquired by LSECs and
resulted in CD8+ T cell tolerance (61). The
mechanism of this kind of tolerance is likely
to involve PD-L1, based on antibody blocking
experiments (48); in addition, LSECs deficient
in this molecule failed to induce CD8+ T cell
tolerance (62).
In addition to all these nonclassical APCs,
DCs also traffic through the liver. Thus, classical mDC precursors expressing CD11c, but
not B220, are recruited to the liver during
granulomatous inflammation initiated by Propionibacterium acnes and subsequently detected
first in the Disse space, then in granulomas,
and subsequently in lymphoid aggregates in the
portal tracts (63). These movements were orchestrated by chemokines, with CCL3 driving
the initial localization to the granulomas, and
CCL21 causing the subsequent relocalization
to portal-associated lymphoid tissue (PALT)
(64). This certainly suggests, though it does not


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prove, that DCs localize to areas of lymphoneogenesis in the inflamed liver and engage T
cells there. In humans, the same chemokine,
CCL21, is expressed by vascular endothelium in
portal areas of the liver, but not in lymph node
vessels (65). On this basis, investigators argue
that PALT is a distinct immunological compartment; sadly, the acronym lacks distinctiveness
because it has been applied to lymphoid tissue
in the human prostate (66) and in the chicken
pineal gland (67). But the existence of PALT,
in the hepatic context, adds to the complexity
of liver immunology. In addition to examining
diverse APCs resident in the sinusoids, we also
need to consider more conventional lymphoid
tissue threaded through the liver in the network
of portal tracts.

THE RISE AND FALL OF
LIVER-PRIMED T CELLS
Whereas the immune system is competent to
eliminate infection with hepatitis A virus, a state
of persistent infection is a common outcome
in HBV infection and the usual outcome in
HCV infection. We may therefore ask why liver
immune responses frequently fail. Does this
simply indicate the sophistication of several
well-adapted pathogens, or, in some sense, is
the liver to blame?
In the case of chronic HCV infection,
antigen-specific CD8+ T cells frequently assume a stunned or exhausted phenotype, in
which they express a low level of the IL-7
receptor-α (CD127) and a high level of the inhibitory receptor PD-1 (68). In several infections, including HIV and HCV in humans and
lymphocytic choriomeningitis virus in mice,
CD8+ T cells with this phenotype are able
neither to secrete IFN-γ nor to make IL-2
(69–71). In the case of HCV, another striking
feature of chronic infection is selective IL-10
production by CD4+ T cells (72). Furthermore,
chronic HCV in humans may be associated with
very weak or absent CD4+ T cell responses
(73), whereas in chimpanzees, viral escape mutants that prevent CD4+ T cell recognition fa-

vor chronicity (74), suggesting that the CD8+
T cells may be incapacitated owing to the lack
of CD4+ T cell help, a state termed “helpless”
(75). To what extent, then, are stunning, exhaustion, PD-1 expression, IL-10 intoxication, and
helplessness all effects of the liver environment?
As far as PD-1/PD-L1 interactions are concerned, the liver seems to be a preferential site
of action. PD-L1 is expressed on several liver
cell types (76). Mice deficient in PD-L1 developed an immunoinflammatory hepatitis caused
by CD8+ T cells, suggesting a key role for
PD-L1 in regulating both CD8+ T cell abundance and immunopathology in the liver (77).
The immunosuppressive effects of IL-10 secreted by LSECs, Kupffer cells, and liver pDCs
have already been emphasized, along with the
interpretation that this is an effect of low-level
TLR4 ligation. Why should this bias exist? One
possibility is that the LPS-responsive cells of
the liver are manifesting a mechanism that has
evolved to suppress chronic immune inflammation. Thus, in an acute immune response,
IL-10 synthesis occurs after the peak of proinflammatory cytokines and may help to restore
the system to a resting state. Under conditions
of chronic activation, this mechanism predominates, limiting tissue injury. In the liver, the
continuous presence of low levels of LPS may
emulate chronic inflammation, calling forth
IL-10 as a regulatory response.
Does the liver promote CD8+ T cell
helplessness? The liver’s unique vasculature
permits circulating T cells to engage with
hepatocytes (50), which act as APCs (56, 78).
Therefore, we can consider the liver as a tissue
that favors CD8+ T cell priming on cells that
can neither prime nor be engaged by CD4+ T
cells. The helpless phenotype would naturally
follow from direct priming of HCV-specific
and possibly also HBV-specific CD8+ T cells
on hepatocytes. In fact, there is one report
of apparently directly primed CD8+ T cells
in the context of liver transplantation. A liver
expressing HLA-A2 was transplanted into an
HLA-A2-negative recipient, who subsequently
developed HLA-A2-restricted anti-HCV


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CD8+ T cells (79). This would be consistent
with direct priming on newly infected donor
hepatocytes. A further test of the inadequacy
of liver-primed CD8+ T cells comes from a
comparison of the response of TCR-transgenic
T cells to a neo-self-MHC antigen expressed
either exclusively in the liver or also in lymph
nodes. Extrahepatic priming resulted in
fully differentiated CD8+ T cells that could
localize to the liver and cause autoimmune
immunopathology. In contrast, the exclusively
liver-primed CD8+ T cells were relatively
innocuous (80).
In summary, multiple mechanisms account
for the rise and fall of T cells specific for liver
pathogens, but many of these effects are linked
to the liver’s unique immunobiology. This is a
clear case of contributory negligence; HBV and
HCV are subtle and devious pathogens, to be
sure, but the liver offers them opportunities for
immune subversion.

TGF-β1

IL-10
Trapping,
FasL,
TRAIL,
phagocytosis

T

T

HSC

PD-L1

KC
LSEC

Figure 6
Mechanisms of T cell tolerance in liver. The expression of adhesion molecules
facilitates the trapping of activated T cells in liver sinusoids, where they may
undergo apoptosis owing to FasL and TRAIL expressed on Kupffer cells and
may also be phagocytosed. In addition, T cells that recognize antigen in the
liver are exposed to immunosuppressive cytokines, including IL-10 and
TGF-β1, and to inhibitory ligands, including PD-L1 (also known as B7-H1).
156

Crispe

HOW DOES THE LIVER INDUCE
SYSTEMIC TOLERANCE?
In view of the bias toward tolerance when T
cells encounter antigens in the liver (summarized in Figure 6), it is not surprising that such
a large organ can impose systemic immune tolerance. This phenomenon was first recognized
in the context of allogeneic liver transplantation. In the classic experiments conducted at
the University of Oxford, renal transplants between unrelated pigs were promptly rejected,
whereas liver transplants between equally unrelated pigs were generally accepted. Strikingly,
the transplantation of a kidney and a liver from
the same donor enhanced the survival of the
kidney. In the half-century since the description of these “Strange English Pigs”1 (3, 81),
this phenomenon has not been fully explained.
Many explanations have been considered. For
example, serial transplantation experiments implicated passenger leukocytes as playing a role
in the induction of kidney allograft rejection
and suggested that their loss plays a role in
the induction of allograft tolerance (82). However, the loss of passenger leukocytes cannot
be implicated in the tolerance associated with
liver transplantation because of the abundance
of long-lived donor hematopoietic cells within
the liver graft (83). It was therefore reasonable
to propose that, unlike other passenger leukocytes, those originating in the liver were tolerogenic. Investigators (84, 85) suggested that the
detection of a low frequency of graft-derived
leukocytes in multiple tissues of a liver transplant recipient (microchimerism) explains liver
transplantation tolerance, but the survival of
these cells would be equally well explained if
the liver were imposing allospecific tolerance
by some other mechanism.
If recirculating passenger leukocytes were an
effect rather than a cause of liver allograft tolerance, what other mechanisms would be candidates? The sessile Kupffer cells, LSECs, and

1

“Strange English Pigs” was the title of an anonymous editorial in The Lancet, November 1, 1969, on the topic of liver
allograft tolerance.


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stellate cells all have the capacity to present antigens to T cells, along with cosignals including
IL-10 and PD-L1 that induce tolerance. Therefore, we may conjecture that liver allograft tolerance is induced by this mechanism, with persistent microchimerism as a by-product. In this
model, allospecific precursor T cells would enter the transplanted liver, undergo activation
by liver-specific APCs, and then either undergo
paralysis or deletion owing to liver-specific local signals. We might consider how this mechanism could account for the effect of the liver in
oral tolerance. The delivery of a protein antigen, usually ovalbumin, into the stomach results in systemic tolerance, particularly of Th1
CD4+ T cells and CD8+ T cells. If the venous drainage of the gut is surgically diverted,
this oral tolerance is lost (86). In the context
of an oral tolerance model, isolated and ex vivo
cultured LSECs interacted with ova-specific T
cells, activating them but then causing them to
deviate their immune function toward an antiinflammatory pattern of cytokine synthesis (43).
While the caveats already raised in relation to
the absolute purity of cultured liver APCs certainly apply to these experiments, they make
a prima facie case for the induction of systemic
tolerance by liver APCs. Similarly, systemic immune tolerance can be induced by the injection
of APCs into the portal vein (87), although it is
unclear whether these APCs take up residence
in the liver and become tolerogenic or whether
cross-presentation by resident liver APCs
accounts for this effect.
The induction of systemic tolerance by liver
APCs has been attributed both to peripheral
deletion and to the induction of antigen-specific
Tregs. Supporting the deletion model is the
abundant evidence that activated, circulating
CD8+ T cells are sequestered in the liver, even
in the absence of antigen. This is most strikingly
illustrated in the context of influenza infection,
where both virus RNA and protein are limited
to the respiratory system, but influenza-specific
CD8+ T cells are found in the liver, associated with Kupffer cell–rich inflammatory foci
and with subclinical hepatocyte damage (88).
This sequestration depends on integrin lig-

ands, including ICAM-1 (intercellular adhesion
molecule-1) and VCAM-1 (vascular cell adhesion molecule-1) (89), and, like other aspects of
liver tolerance, it may be driven by LPS acting
via TLR4 (90). The evidence that this form of
antigen-independent CD8+ T cell sequestration results in systemic tolerance is thin; however, in the context of a CD8+ T cell response
to antigen-pulsed DCs, the interruption of the
TLR4-dependent element of T cell trapping in
the liver resulted in exaggerated systemic immune responses, followed eight weeks later by
an enhanced secondary response. On the basis
of this experiment, John et al. (91) proposed the
model that the liver regulates the magnitude of
CD8+ T cells responses. However, it is abundantly clear that deletion is not the fate of every
T cell that enters the liver because liver-derived
T cells can repopulate systemic memory (92).
It has been argued that this observation invalidates the model that the liver acts as a sink
for activated CD8+ T cells (93), but this argument fails to take into account the distinction
between activated blast T cells and CD8+ memory T cells. The data are best reconciled by a
model in which recently activated lymphoblasts
preferentially localize to hepatic sinusoids owing to adhesion molecules expressed on LSECs,
and then recruit and are killed by Kupffer
cells, creating a “sink” for excess T cell blasts.
Resting memory cells, in contrast, may be overrepresented in the liver owing to their adhesion receptors, but they do not activate Kupffer cells nor are they phagocytosed. These cells
find the liver quite hospitable, as we have argued
previously (94).
The alternative to a deletion model is the
idea of active regulation by suppressor T cells,
also known as Tregs. These cells exist in a
variety of subsets, some of which arise as
Tregs in the thymus, whereas others differentiate from apparently uncommitted peripheral CD4+ and CD8+ T cell precursors. The
impact of liver transplantation on these cells
is controversial. A perfusate of human liver
was enriched in CD4+ 25+ FoxP3+ CD127low
cells, and such donor-derived Tregs were
found in the circulation of liver transplant

Treg: regulatory
T cell

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recipients, an aspect of the phenomenon of
microchimerism (95). Investigators agree that,
after liver transplantation in humans, the
overall frequency of Tregs in the circulation
falls (96, 97). Among subsets of CD4+ 25+
T cells, the number of cells that expressed
high amounts of CD127 increased in human
liver transplant recipients with stable allografts,
whereas the number of CD4+ 25+ 127low cells
with regulatory function decreased (98); this
is hard to reconcile with the idea that the
CD127low regulatory subset was maintaining
tolerance. However, in mouse liver transplantation, CD4+ 25+ FoxP3+ CTLA4+ Tregs increased in abundance after liver grafting, and
depletion of these cells using an anti-CD25 antibody caused acute rejection of the graft, consistent with an important role for these kinds
of Tregs in maintaining liver allograft tolerance and also, by extrapolation, systemic tolerance (99). It does not follow that mouse and
human are fundamentally different. The most
straightforward resolution of the apparent conflict between mouse and human data is that
spontaneous liver allograft acceptance involves
CD4+ 25+ FoxP3+ Tregs but that, in human,
liver allografts are tolerated because of immunosuppression and despite the depletion of
Tregs by immunosuppressive drugs.


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THE LIVER AS A LYMPHOID
ORGAN: CONCLUSIONS
The liver is an important site for primary T cell
activation, but this takes place in an environment biased toward tolerance. Several mech-

anisms contribute to this suppressive milieu.
First, the constitutive exposure of liver cells to
traces of endotoxin and other microbial products results in the down-modulation of costimulatory molecules and the synthesis of IL-10
by Kupffer cells and LSECs. Second, the open
architecture of the liver endothelium results in
ready access of naive T cells to diverse subsets
of APCs, including hepatocytes. This may result in selective CD8+ T cell priming without
concomitant CD4+ T cell activation, resulting
in a “helpless” phenotype that leads to longterm CD8+ T cell dysfunction and a lack of
immune memory. Third, the liver endothelium
expresses adhesion molecules that facilitate the
sequestration of circulating activated T cells,
particularly CD8+ T cells. This gives the liver
a role in systemic immunoregulation. An important epiphenomenon resulting from these
mechanisms is liver allograft tolerance.
Because of the high threshold for the initiation of an adaptive T cell response in the liver,
innate immune mechanisms assume greater significance. Abundant NK cells and NKT cells
may be activated by pathogen-associated structures via TLRs, invariant TCRs, or alternative sensing systems such as RIG-I, and by
cytokines. Kupffer cells respond to inflammatory cytokines such as IFN-γ by the synthesis
of their own inflammatory mediators: TNF-α,
IL-12, and IL-18. These in turn deliver positive signals to both adaptive and innate immune
cells. Awash with potential activating signals,
the liver’s immune system is held in a baseline
state of active tolerance, which can be reversed
by sufficiently strong pathogen-specific signals.

SUMMARY POINTS
1. The liver receives blood from the intestine, which is rich in microbial products. These
engage TLRs, which modify innate immunity in the hepatic environment.
2. The liver lymphocytes are enriched in CD8+ T cells, activated T cells, memory T cells,
NKT cells, and NK cells.
3. Multiple cell populations can act as APCs in the liver. These include hepatocytes, endothelial cells, and subendothelial stellate cells, as well as several subsets of dendritic
cells.

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4. The liver is rich in immunosuppressive cytokines including IL-10, and several liver cell
subsets express the inhibitory ligand PD-L1. The consequence of this is that many encounters between T cells and liver APCs end in immune tolerance.

FUTURE ISSUES


1. The mechanisms of antigen presentation and T cell activation by the many subsets of
liver APCs must be clarified. In particular, do these cell types form distinctive synapses
with T cells? Which of them engage in cross-presentation of hepatocellular antigens?
What mechanisms promote such cross-presentation?
2. In hepatitis B, hepatitis C, hepatocellular cancer, and the liver stage of malaria, how are
hepatocellular antigens presented to the immune system?
3. Which immune mechanisms favor the elimination of hepatocellular antigens? Which
mechanisms cause liver immunopathology? How can we promote the former, while
limiting the effects of the latter?

DISCLOSURE STATEMENT
The author is not aware of any affiliations, memberships, funding, or financial holdings that might
be perceived as affecting the objectivity of this review.
LITERATURE CITED
1. Lumsden AB, Henderson JM, Kutner MH. 1988. Endotoxin levels measured by a chromogenic assay in
portal, hepatic and peripheral venous blood in patients with cirrhosis. Hepatology 8:232–36
2. Catala M, Anton A, Portoles MT. 1999. Characterization of the simultaneous binding of Escherichia coli
endotoxin to Kupffer and endothelial liver cells by flow cytometry. Cytometry 36:123–30
3. Calne RY, Sells RA, Pena JR, Davis DR, Millard PR, et al. 1969. Induction of immunological
tolerance by porcine liver allografts. Nature 223:472–76
4. Thompson AJ, Locarnini SA. 2007. Toll-like receptors, RIG-I-like RNA helicases and the antiviral innate
immune response. Immunol. Cell Biol. 85:435–45
5. Abreu MT, Fukata M, Arditi M. 2005. TLR signaling in the gut in health and disease. J. Immunol.
174:4453–60
6. Tu Z, Bozorgzadeh A, Crispe IN, Orloff MS. 2007. The activation state of human intrahepatic lymphocytes. Clin. Exp. Immunol. 149:186–93
7. Norris S, Collins C, Doherty DG, Smith F, McEntee G, et al. 1998. Resident human hepatic lymphocytes
are phenotypically different from circulating lymphocytes. J. Hepatol. 28:84–90
8. Crispe IN. 1996. Isolation of mouse intrahepatic lymphocytes. Curr. Protoc. Immunol. 3:22–28
9. Lanier LL. 2008. Up on the tightrope: natural killer cell activation and inhibition. Nat. Immunol. 9:495–502
10. Jinushi M, Takehara T, Tatsumi T, Kanto T, Groh V, et al. 2003. Expression and role of MICA and MICB
in human hepatocellular carcinomas and their regulation by retinoic acid. Int. J. Cancer 104:354–61
11. Lodoen M, Ogasawara K, Hamerman JA, Arase H, Houchins JP, et al. 2003. NKG2D-mediated natural
killer cell protection against cytomegalovirus is impaired by viral gp40 modulation of retinoic acid early
inducible 1 gene molecules. J. Exp. Med. 197:1245–53
12. Nguyen KB, Salazar-Mather TP, Dalod MY, Van Deusen JB, Wei XQ, et al. 2002. Coordinated and
distinct roles for IFN-αβ, IL-12, and IL-15 regulation of NK cell responses to viral infection. J. Immunol.
169:4279–87

3. The discovery of liver
tolerance, leading to the
recognition of the liver
as a distinct
immunological site.

159


ANRV371-IY27-06

ARI

16 February 2009

22. A nice example of
how HCV-encoded
proteins attack immune
signaling pathways.

160

9:31

13. Dao T, Mehal WZ, Crispe IN. 1998. IL-18 augments perforin-dependent cytotoxicity of liver NK-T
cells. J. Immunol. 161:2217–22
14. Dunn C, Brunetto M, Reynolds G, Christophides T, Kennedy PT, et al. 2007. Cytokines induced during
chronic hepatitis B virus infection promote a pathway for NK cell-mediated liver damage. J. Exp. Med.
204:667–80
15. Halder RC, Seki S, Weerasinghe A, Kawamura T, Watanabe H, Abo T. 1998. Characterization of NK
cells and extrathymic T cells generated in the liver of irradiated mice with a liver shield. Clin. Exp. Immunol.
114:434–47
16. Geissmann F, Cameron TO, Sidobre S, Manlongat N, Kronenberg M, et al. 2005. Intravascular immune
surveillance by CXCR6+ NKT cells patrolling liver sinusoids. PLoS Biol. 3:e113
17. Bendelac A. 1995. Positive selection of mouse NK1+ T cells by CD1-expressing cortical thymocytes.
J. Exp. Med. 182:2091–96
18. Long X, Deng S, Mattner J, Zang Z, Zhou D, et al. 2007. Synthesis and evaluation of stimulatory
properties of Sphingomonadaceae glycolipids. Nat. Chem. Biol. 3:559–64
19. Kinjo Y, Tupin E, Wu D, Fujio M, Garcia-Navarro R, et al. 2006. Natural killer T cells recognize
diacylglycerol antigens from pathogenic bacteria. Nat. Immunol. 7:978–86
20. Vilarinho S, Ogasawara K, Nishimura S, Lanier LL, Baron JL. 2007. Blockade of NKG2D on NKT
cells prevents hepatitis and the acute immune response to hepatitis B virus. Proc. Natl. Acad. Sci. USA
104:18187–92
21. Johnson CL, Owen DM, Gale M Jr. 2007. Functional and therapeutic analysis of hepatitis C virus NS3.4A
protease control of antiviral immune defense. J. Biol. Chem. 282:10792–803
22. Ferreon JC, Ferreon AC, Li K, Lemon SM. 2005. Molecular determinants of TRIF proteolysis
mediated by the hepatitis C virus NS3/4A protease. J. Biol. Chem. 280:20483–92
23. Yang Y, Liang Y, Qu L, Chen Z, Yi M, et al. 2007. Disruption of innate immunity due to mitochondrial
targeting of a picornaviral protease precursor. Proc. Natl. Acad. Sci. USA 104:7253–58
24. Tseng CT, Klimpel GR. 2002. Binding of the hepatitis C virus envelope protein E2 to CD81 inhibits
natural killer cell functions. J. Exp. Med. 195:43–49
25. Frevert U, Engelmann S, Zougbede S, Stange J, Ng B, et al. 2005. Intravital observation of Plasmodium
berghei sporozoite infection of the liver. PLoS Biol. 3:e192
26. Baer K, Roosevelt M, Clarkson AB Jr, van Rooijen N, Schnieder T, Frevert U. 2007. Kupffer cells are
obligatory for Plasmodium yoelii sporozoite infection of the liver. Cell. Microbiol. 9:397–412
27. Usynin I, Klotz C, Frevert U. 2007. Malaria circumsporozoite protein inhibits the respiratory burst in
Kupffer cells. Cell. Microbiol. 9:2610–28
28. Pradel G, Frevert U. 2001. Malaria sporozoites actively enter and pass through rat Kupffer cells prior to
hepatocyte invasion. Hepatology 33:1154–65
29. O’Connell PJ, Morelli AE, Logar AJ, Thomson AW. 2000. Phenotypic and functional characterization
of mouse hepatic CD8α+ lymphoid-related dendritic cells. J. Immunol. 165:795–803
30. Pillarisetty VG, Katz SC, Bleier JI, Shah AB, Dematteo RP. 2005. Natural killer dendritic cells have
both antigen presenting and lytic function and in response to CpG produce IFN-γ via autocrine IL-12.
J. Immunol. 174:2612–18
31. Abe M, Tokita D, Raimondi G, Thomson AW. 2006. Endotoxin modulates the capacity of CpG-activated
liver myeloid DC to direct Th1-type responses. Eur. J. Immunol. 36:2483–93
32. De Creus A, Abe M, Lau AH, Hackstein H, Raimondi G, Thomson AW. 2005. Low TLR4 expression by
liver dendritic cells correlates with reduced capacity to activate allogeneic T cells in response to endotoxin.
J. Immunol. 174:2037–45
33. Averill L, Lee WM, Karandikar NJ. 2007. Differential dysfunction in dendritic cell subsets during chronic
HCV infection. Clin. Immunol. 123:40–49
34. Callery MP, Mangino MJ, Flye MW. 1991. Arginine-speciﬁc suppression of mixed lymphocyte culture
reactivity by Kupffer cells—a basis of portal venous tolerance. Transplantation 51:1076–80
35. Roland CR, Walp L, Stack RM, Flye MW. 1994. Outcome of Kupffer cell antigen presentation to a
cloned murine Th1 lymphocyte depends on the inducibility of nitric oxide synthase by IFN-γ. J. Immunol.
153:5453–64
Crispe


ANRV371-IY27-06

ARI

16 February 2009

9:31

36. Knolle P, Schlaak J, Uhrig A, Kempf P, Meyer zum Buschenfelde KH, Gerken G. 1995. Human
Kupffer cells secrete IL-10 in response to lipopolysaccharide (LPS) challenge. J. Hepatol. 22:226–
29
37. Miyagawa-Hayashino A, Tsuruyama T, Egawa H, Haga H, Sakashita H, et al. 2007. FasL expression in
hepatic antigen-presenting cells and phagocytosis of apoptotic T cells by FasL+ Kupffer cells are indicators
of rejection activity in human liver allografts. Am. J. Pathol. 171:1499–508
38. Burgio VL, Ballardini G, Artini M, Caratozzolo M, Bianchi FB, Levrero M. 1998. Expression of costimulatory molecules by Kupffer cells in chronic hepatitis of hepatitis C virus etiology. Hepatology 27:1600–6
39. Tu Z, Bozorgzadeh A, Pierce RH, Kurtis J, Crispe IN, Orloff MS. 2008. TLR-dependent cross talk
between human Kupffer cells and NK cells. J. Exp. Med. 205:233–44
40. Knolle PA, Limmer A. 2001. Neighborhood politics: the immunoregulatory function of organ-resident
liver endothelial cells. Trends Immunol. 22:432–37
41. Knolle PA, Uhrig A, Hegenbarth S, Loser E, Schmitt E, et al. 1998. IL-10 down-regulates T cell activation
by antigen-presenting liver sinusoidal endothelial cells through decreased antigen uptake via the mannose
receptor and lowered surface expression of accessory molecules. Clin. Exp. Immunol. 114:427–33
42. Limmer A, Ohl J, Kurts C, Ljunggren HG, Reiss Y, et al. 2000. Efficient presentation of exogenous
antigen by liver endothelial cells to CD8+ T cells results in antigen-specific T-cell tolerance.
Nat. Med. 6:1348–54
43. Limmer A, Ohl J, Wingender G, Berg M, Jungerkes F, et al. 2005. Cross-presentation of oral antigens
by liver sinusoidal endothelial cells leads to CD8 T cell tolerance. Eur. J. Immunol. 35:2970–81
44. Friedman SL. 2008. Hepatic stellate cells: protean, multifunctional, and enigmatic cells of the liver. Physiol.
Rev. 88:125–72
45. Paik YH, Schwabe RF, Bataller R, Russo MP, Jobin C, Brenner DA. 2003. Toll-like receptor 4 mediates
inflammatory signaling by bacterial lipopolysaccharide in human hepatic stellate cells. Hepatology 37:1043–
55
46. Seki E, De Minicis S, Osterreicher CH, Kluwe J, Osawa Y, et al. 2007. TLR4 enhances TGF-β signaling
and hepatic fibrosis. Nat. Med. 13:1324–32
47. Winau F, Hegasy G, Weiskirchen R, Weber S, Cassan C, et al. 2007. Ito cells are liver-resident
antigen-presenting cells for activating T cell responses. Immunity 26:117–29
48. Yu MC, Chen CH, Liang X, Wang L, Gandhi CR, et al. 2004. Inhibition of T-cell responses by hepatic
stellate cells via B7-H1-mediated T-cell apoptosis in mice. Hepatology 40:1312–21
49. Nussenzweig M, Steinman R. 1980. Contribution of dendritic cells to stimulation of the murine syngeneic
MLR. J. Exp. Med. 151:1196–212
50. Warren A, Le Couteur DG, Fraser R, Bowen DG, McCaughan GW, Bertolino P. 2006. T lymphocytes
interact with hepatocytes through fenestrations in murine liver sinusoidal endothelial cells. Hepatology
44:1182–90
51. Ando K, Guidotti LG, Cerny A, Ishikawa T, Chisari FV. 1994. CTL access to tissue antigen is restricted
in vivo. J. Immunol. 153:482–88
52. Bertolino P, Bowen DG, McCaughan GW, Fazekas de St Groth B. 2001. Antigen-specific primary
activation of CD8+ T cells within the liver. J. Immunol. 166:5430–38
53. Wuensch SA, Pierce RH, Crispe IN. 2006. Local intrahepatic CD8+ T cell activation by a nonself-antigen
results in full functional differentiation. J. Immunol. 177:1689–97
54. Klein I, Crispe IN. 2006. Complete differentiation of CD8+ T cells activated locally within the transplanted liver. J. Exp. Med. 203:437–48
55. Qian S, Lu L, Fu F, Li Y, Li W, et al. 1997. Apoptosis within spontaneously accepted mouse
liver allografts: evidence for deletion of cytotoxic T cells and implications for tolerance induction.
J. Immunol. 158:4654–61
56. Lee YC, Lu L, Fu F, Li W, Thomson AW, et al. 1999. Hepatocytes and liver nonparenchymal cells induce
apoptosis in activated T cells. Transplant. Proc. 31:784
57. Bertolino P, Trescol-Biemont MC, Rabourdin-Combe C. 1998. Hepatocytes induce functional activation
of naive CD8+ T lymphocytes but fail to promote survival. Eur. J. Immunol. 28:221–36
58. Bertolino P, Trescol-Biemont MC, Thomas J, de St Groth BF, Pihlgren M, et al. 1999. Death by neglect
as a deletional mechanism of peripheral tolerance. Int. Immunol. 11:1225–38

36. This paper links
LPS, Kupffer cells, and
IL-10, setting up a
major paradigm for liver
tolerance.

42. A classic paper
documenting the APC
activity of LSECs.

47. This paper adds
stellate cells to the
diverse inventory of
liver APC.

55. This paper reveals
the apoptosis of T cells
infiltrating a liver
allograft, arguing for
peripheral deletion as a
key aspect of liver
tolerance.

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68. The lack of effective
T cell immunity to
HCV is linked to the
“exhausted” phenotype.

80. This paper suggests
that intrahepatic T cell
priming is incomplete,
resulting in poor
function, in contrast to
lymph node priming.

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59. Limmer A, Sacher T, Alferink J, Kretschmar M, Schonrich G, et al. 1998. Failure to induce organspecific autoimmunity by breaking of tolerance: importance of the microenvironment. Eur. J. Immunol.
28:2395–406
60. Chen CH, Kuo LM, Chang Y, Wu W, Goldbach C, et al. 2006. In vivo immune modulatory activity of
hepatic stellate cells in mice. Hepatology 44:1171–81
61. Berg M, Wingender G, Djandji D, Hegenbarth S, Momburg F, et al. 2006. Cross-presentation of antigens
from apoptotic tumor cells by liver sinusoidal endothelial cells leads to tumor-specific CD8+ T cell
tolerance. Eur. J. Immunol. 36:2960–70
62. Diehl L, Schurich A, Grochtmann R, Hegenbarth S, Chen L, Knolle PA. 2008. Tolerogenic maturation of
liver sinusoidal endothelial cells promotes B7-homolog 1-dependent CD8+ T cell tolerance. Hepatology
47:296–305
63. Yoneyama H, Matsuno K, Zhang Y, Murai M, Itakura M, et al. 2001. Regulation by chemokines of
circulating dendritic cell precursors, and the formation of portal tract-associated lymphoid tissue, in a
granulomatous liver disease. J. Exp. Med. 193:35–49
64. Yoneyama H, Ichida T. 2005. Recruitment of dendritic cells to pathological niches in inflamed liver.
Med. Mol. Morphol. 38:136–41
65. Grant AJ, Goddard S, Ahmed-Choudhury J, Reynolds G, Jackson DG, et al. 2002. Hepatic expression of
secondary lymphoid chemokine (CCL21) promotes the development of portal-associated lymphoid tissue
in chronic inflammatory liver disease. Am. J. Pathol. 160:1445–55
66. Di Carlo E, Magnasco S, D’Antuono T, Tenaglia R, Sorrentino C. 2007. The prostate-associated lymphoid
tissue (PALT) is linked to the expression of homing chemokines CXCL13 and CCL21. Prostate 67:1070–80
67. Mosenson JA, McNulty JA. 2006. Characterization of lymphocyte subsets over a 24-hour period in PinealAssociated Lymphoid Tissue (PALT) in the chicken. BMC Immunol. 7:1
68. Radziewicz H, Ibegbu CC, Fernandez ML, Workowski KA, Obideen K, et al. 2007. Liverinfiltrating lymphocytes in chronic human hepatitis C virus infection display an exhausted phenotype with high levels of PD-1 and low levels of CD127 expression. J. Virol. 81:2545–53
69. Gruener NH, Lechner F, Jung MC, Diepolder H, Gerlach T, et al. 2001. Sustained dysfunction of antiviral
CD8+ T lymphocytes after infection with hepatitis C virus. J. Virol. 75:5550–58
70. Day CL, Kaufmann DE, Kiepiela P, Brown JA, Moodley ES, et al. 2006. PD-1 expression on HIV-specific
T cells is associated with T-cell exhaustion and disease progression. Nature 443:350–54
71. Barber DL, Wherry EJ, Masopust D, Zhu B, Allison JP, et al. 2006. Restoring function in exhausted CD8
T cells during chronic viral infection. Nature 439:682–87
72. Godkin A, Jeanguet N, Thursz M, Openshaw P, Thomas H. 2001. Characterization of novel HLA-DR11restricted HCV epitopes reveals both qualitative and quantitative differences in HCV-specific CD4+
T cell responses in chronically infected and nonviremic patients. Eur. J. Immunol. 31:1438–46
73. Wedemeyer H, He XS, Nascimbeni M, Davis AR, Greenberg HB, et al. 2002. Impaired effector function
of hepatitis C virus-specific CD8+ T cells in chronic hepatitis C virus infection. J. Immunol. 169:3447–58
74. Puig M, Mihalik K, Tilton JC, Williams O, Merchlinsky M, et al. 2006. CD4+ immune escape and
subsequent T-cell failure following chimpanzee immunization against hepatitis C virus. Hepatology 44:736–
45
75. Janssen EM, Droin NM, Lemmens EE, Pinkoski MJ, Bensinger SJ, et al. 2005. CD4+ T-cell help controls
CD8+ T-cell memory via TRAIL-mediated activation-induced cell death. Nature 434:88–93
76. Iwai Y, Terawaki S, Ikegawa M, Okazaki T, Honjo T. 2003. PD-1 inhibits antiviral immunity at the
effector phase in the liver. J. Exp. Med. 198:39–50
77. Dong H, Zhu G, Tamada K, Flies DB, van Deursen JM, Chen L. 2004. B7-H1 determines accumulation
and deletion of intrahepatic CD8+ T lymphocytes. Immunity 20:327–36
78. Bertolino P, Trescol-Biemont MC, Rabourdin-Combe C. 1998. Hepatocytes induce functional activation
of naive CD8+ T lymphocytes but fail to promote survival. Eur. J. Immunol. 28:221–36
79. Lauer GM. 2005. Hepatitis C virus-specific CD8+ T cells restricted by donor HLA alleles following liver
transplantation. Liver Transpl. 11:848–50
80. Bowen DG, Zen M, Holz L, Davis T, McCaughan GW, Bertolino P. 2004. The site of primary
T cell activation is a determinant of the balance between intrahepatic tolerance and immunity.
J. Clin. Invest. 114:701–12
Crispe


ANRV371-IY27-06

ARI

16 February 2009

9:31

81. Calne RY, White HJ, Binns RM, Herbertson BM, Millard PR, et al. 1969. Immunosuppressive effects of
the orthotopically transplanted porcine liver. Transplant. Proc. 1:321–24
82. Welsh KI, Batchelor JR, Maynard A, Burgos H. 1979. Failure of long surviving, passively enhanced kidney
allografts to provoke T-dependent alloimmunity. II. Retransplantation of (AS X AUG)F1 kidneys from
AS primary recipients into (AS X WF)F1 secondary hosts. J. Exp. Med. 150:465–70
83. Ng IO, Chan KL, Shek WH, Lee JM, Fong DY, et al. 2003. High frequency of chimerism in transplanted
livers. Hepatology 38:989–98
84. Qian S, Demetris AJ, Murase N, Rao AS, Fung JJ, Starzl TE. 1994. Murine liver allograft transplantation:
tolerance and donor cell chimerism. Hepatology 19:916–24
85. Starzl TE, Demetris AJ, Trucco M, Ramos H, Zeevi A, et al. 1992. Systemic chimerism in human female
recipients of male livers. Lancet 340:876–77
86. Yang R, Liu Q, Grosfeld JL, Pescovitz MD. 1994. Intestinal venous drainage through the liver is a
prerequisite for oral tolerance induction. J. Pediatr. Surg. 29:1145–48
87. Gorczynski RM. 1992. Immunosuppression induced by hepatic portal venous immunization. Immunol.
Lett. 33:67–77
88. Polakos NK, Cornejo JC, Murray DA, Wright KO, Treanor JJ, et al. 2006. Kupffer cell-dependent
hepatitis occurs during influenza infection. Am. J. Pathol. 168:1169–78
89. John B, Crispe IN. 2004. Passive and active mechanisms trap activated CD8+ T cells in the liver.
J. Immunol. 172:5222–29
90. John B, Crispe IN. 2005. TLR-4 regulates CD8+ T cell trapping in the liver. J. Immunol. 175:1643–50
91. John B, Klein I, Crispe IN. 2007. Immune role of hepatic TLR-4 revealed by orthotopic mouse liver
transplantation. Hepatology 45:178–86
92. Polakos NK, Klein I, Richter MV, Zaiss DM, Giannandrea M, et al. 2007. Early intrahepatic accumulation
of CD8+ T cells provides a source of effectors for nonhepatic immune responses. J. Immunol. 179:201–10
93. Bertolino P, Bowen DG, Benseler V. 2007. T cells in the liver: there is life beyond the graveyard. Hepatology
45:1580–82
94. Crispe IN. 2003. Hepatic T cells and liver tolerance. Nat. Rev. Immunol. 3:51–62
95. Demirkiran A, Bosma BM, Kok A, Baan CC, Metselaar HJ, et al. 2007. Allosuppressive donor
CD4+ CD25+ regulatory T cells detach from the graft and circulate in recipients after liver transplantation. J. Immunol. 178:6066–72
96. San Segundo D, Fabrega E, Lopez-Hoyos M, Pons F. 2007. Reduced numbers of blood natural regulatory
T cells in stable liver transplant recipients with high levels of calcineurin inhibitors. Transplant. Proc.
39:2290–92
97. Demirkiran A, Kok A, Kwekkeboom J, Kusters JG, Metselaar HJ, et al. 2006. Low circulating regulatory
T-cell levels after acute rejection in liver transplantation. Liver Transpl. 12:277–84
98. Codarri L, Vallotton L, Ciuffreda D, Venetz JP, Garcia M, et al. 2007. Expansion and tissue infiltration of
an allospecific CD4+ CD25+ CD45RO+ IL-7Rαhigh cell population in solid organ transplant recipients.
J. Exp. Med. 204:1533–41
99. Li W, Carper K, Liang Y, Zheng XX, Kuhr CS, et al. 2006. Anti-CD25 mAb administration prevents
spontaneous liver transplant tolerance. Transplant. Proc. 38:3207–8


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Contents

Volume 27, 2009

Frontispiece
Marc Feldmann p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p x
Translating Molecular Insights in Autoimmunity into Effective
Therapy
Marc Feldmann p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p1
Structural Biology of Shared Cytokine Receptors
Xinquan Wang, Patrick Lupardus, Sherry L. LaPorte,
and K. Christopher Garcia p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 29
Immunity to Respiratory Viruses
Jacob E. Kohlmeier and David L. Woodland p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 61
Immune Therapy for Cancer
Michael Dougan and Glenn Dranoff p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 83
Microglial Physiology: Unique Stimuli, Specialized Responses
Richard M. Ransohoff and V. Hugh Perry p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p119
The Liver as a Lymphoid Organ
Ian Nicholas Crispe p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p147
Immune and Inflammatory Mechanisms of Atherosclerosis
Elena Galkina and Klaus Ley p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p165
Primary B Cell Immunodeficiencies: Comparisons and Contrasts
Mary Ellen Conley, A. Kerry Dobbs, Dana M. Farmer, Sebnem Kilic,
Kenneth Paris, Sofia Grigoriadou, Elaine Coustan-Smith, Vanessa Howard,
and Dario Campana p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p199
The Inflammasomes: Guardians of the Body
Fabio Martinon, Annick Mayor, and Jürg Tschopp p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p229
Human Marginal Zone B Cells
Jean-Claude Weill, Sandra Weller, and Claude-Agn` s Reynaud p p p p p p p p p p p p p p p p p p p p p p267
e

v

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Aire
Diane Mathis and Christophe Benoist p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p287
Regulatory Lymphocytes and Intestinal Inflammation
Ana Izcue, Janine L. Coombes, and Fiona Powrie p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p313
The Ins and Outs of Leukocyte Integrin Signaling
Clare L. Abram and Clifford A. Lowell p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p339


Recent Advances in the Genetics of Autoimmune Disease
Peter K. Gregersen and Lina M. Olsson p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p363
Cell-Mediated Immune Responses in Tuberculosis
Andrea M. Cooper p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p393
Enhancing Immunity Through Autophagy
Christian Munz p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p423
¨
Alternative Activation of Macrophages: An Immunologic Functional
Perspective
Fernando O. Martinez, Laura Helming, and Siamon Gordon p p p p p p p p p p p p p p p p p p p p p p p p451
IL-17 and Th17 Cells
Thomas Korn, Estelle Bettelli, Mohamed Oukka, and Vijay K. Kuchroo p p p p p p p p p p p p p p485
Immunological and Inflammatory Functions of the Interleukin-1
Family
Charles A. Dinarello p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p519
Regulatory T Cells in the Control of Host-Microorganism Interactions
Yasmine Belkaid and Kristin Tarbell p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p551
T Cell Activation
Jennifer E. Smith-Garvin, Gary A. Koretzky, and Martha S. Jordan p p p p p p p p p p p p p p p591
Horror Autoinflammaticus: The Molecular Pathophysiology of
Autoinflammatory Disease
Seth L. Masters, Anna Simon, Ivona Aksentijevich, and Daniel L. Kastner p p p p p p p p p621
Blood Monocytes: Development, Heterogeneity, and Relationship
with Dendritic Cells
Cedric Auffray, Michael H. Sieweke, and Frederic Geissmann p p p p p p p p p p p p p p p p p p p p p p p p669
Regulation and Function of NF-κB Transcription Factors in the
Immune System
Sivakumar Vallabhapurapu and Michael Karin p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p693

vi

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