The document summarizes B cell development, activation, and differentiation. It is divided into three main stages: 1) generation of mature B cells, 2) activation of mature B cells, and 3) differentiation into plasma and memory B cells. The process involves an antigen-independent maturation phase and an antigen-dependent activation and differentiation phase where B cells proliferate, undergo affinity maturation and class switching in germinal centers. Cytokines also play a role in B cell differentiation.

1. B cell development, activation
and differentiation

2. The process is divided into three stages
– Generation of mature immunocompetent B
cells (maturation)
– Activation of mature B cells
– Differentiation into plasma and memory B
cells

3. • Antigen-independent phase
– This process is an orderly
sequence of Ig-gene in the absence
of antigen
– These naive B cells are carried to
the secondary lymphoid organs
(spleen and lymph nodes)
• Antigen-dependent phase
– Upon activation cells proliferate
(clonal expansion) and differentiate
to generate plasma and memory B
cells.
– This involves affinity maturation
(increase in the average affinity of
antibodies) and class switching
(change in isotype of the antibody
produced by the B cell)
Overview of B-cell development and differentiation
B cell development and
differentiation

4. • The generation of mature B cells starts in
embryo and continues throughout life
• Before birth, yolk sac, fetal liver and fetal
bone marrow are major sites of B-cell
maturation
• After birth generation of mature B cells
occurs in bone marrow
B cell maturation

5. Progenitor B cells proliferate
in bone marrow
• Progenitor B cells (pro-B cells)
proliferate within bone marrow,
filling extravascular spaces
between large sinusoids
• Proliferation and differentiation of
pro-B cells into precursor B cells
(pre-B cells) requires
microenvironment provided by
bone marrow stromal cells
• If pro-B cells are removed from
bone marrow and cultured in
vitro, they will not progress to
mature B cell stages unless
stromal cells are present
• Stromal cells interact directly
with pro-B and pre-B cells and
secrete various cytokines (IL-7)
which support developmental
process
Bone-marrow stromal cells are required for maturation
of progenitor B cells into precursor B cells

6. Ig-gene rearrangment produces
immature B Cells
• First to occur in the pro-B cell stage is a heavy-chain DH-to-JH gene rearrangement
followed by a VH-to-DHJH rearrangement
• If first heavy-chain rearrangement is not productive, then VH-DH-JH rearrangement
continues on the other chromosome
• Upon completion of heavy-chain rearrangement, the cell is classified as a pre-B cell.
• Development of a pre-B cell into an immature B cell requires a productive light-
chain gene rearrangement
• Because of allelic exclusion, only one light-chain isotype is expressed on the
membrane of a B cell
• Completion of a productive light-chain rearrangement commits the now immature B
cell to a particular antigenic specificity determined by cell’s heavy-chain VDJ
sequence and light-chain VJ sequence
• Immature B cells express mIgM (membrane IgM) on cell surface

7. • Recombinase enzymes RAG-1 and
RAG-2 are required for both heavy
and light-chain gene rearrangements
• Expressed during the pro-B and pre-B
cell stages
• Enzyme terminal deoxyribonucleotidyl
transferase (TdT) which catalyzes
insertion of N-nucleotides at the DH-JH
and VH-DHJH coding joints is active
during the pro-B cell stage
• Because TdT expression is turned off
during part of pre -B cell stage when
light-chain rearrangement occurs, N-
nucleotides are not usually found in
VL-JL coding joints
Overview of B-cell development and maturation

8. Thymus dependent and independent antigens
have different requirements for response
• Thymus-dependent (TD) antigens
requires direct contact with TH
cells
• Thymus-independent (TI) antigens
are divided into types 1 and 2
– TI-1 antigens (e.g.,
lipopolysaccharide) are
polyclonal B-cell activators
– TI-2 antigens (e.g., bacterial
flagellin) activate B cells by
extensively crosslinking mIg
receptor
Properties of thymus-dependent and thymus-independent antigens

9. Two types of signals drive B cells
into and through cell cycle
• Naive or resting B cells are in G0 stage
of cell cycle
• Activation drives the resting cell into cell
cycle progressing through G1, S, G2 and
M phase
• The events could be grouped into
– Competence signals which drive B
cells from G0 into early G1 phase to
receive next level of signals
– Progression signals which drive
cells from G1 into S phase and
ultimately to cell division and
differentiation (M phase)
• Competence is achieved by signal 1 and
signal 2 and are generated by different
pathways with TI and TD antigens
• These include signals generated when
multivalent antigens bind and crosslink
mIg
An effective signal for B-cell activation involves two
distinct signals induced by membrane events

10. Transduction of activating signals
involves Ig-α/Ig-β heterodimers
• Compartmentalization of
function within receptor
subunits
• Activation by membrane-
associated Src protein tyrosine
kinases
• Assembly of a large signaling
complex with proteintyrosine-
kinase activity
• Recruitment of other signal-
transduction pathways
• Changes in gene expression
Some of the many signal-transduction pathways activated by the BCR

11. B cell co-receptor complex can
enhance B cell responses
• B cell co-receptor is a complex of three
proteins
– CD19 (member of the immunoglobulin
superfamily, has a long cytoplasmic tail and
three extracellular domains)
– CR2 (CD21) (a receptor of C3d, a breakdown
product of the complement system)
– TAPA-1 (CD81)
• 104 molecules of mIgM had to be engaged
by antigen for B cell activation to occur when
co-receptor was not involved (only 102
molecules required when co-receptor was
engaged)
• Mice were immunized with either unmodified
lysozyme or a hybrid of hen’s egg lysozyme
to C3d (fusion protein responses were 1000
to 10,000 times greater than to lysozyme
alone)
B-cell coreceptor is a complex of three cell membrane
molecules: TAPA-1 (CD81), CR2 (CD21), and CD19

12. In vivo sites for induction of humoral
responses
• In vivo activation and differentiation of B cells occurs in
defined anatomic sites
• Blood borne antigens are filtered by spleen, whereas
antigens from tissue spaces are filtered by regional
lymph nodes
• As antigen percolates through cellular architecture of a
node, it encounter antigen presenting cells
• Once antigen mediated B cell activation takes place,
small foci of proliferating B cells form at edges of T cell
rich zone
• Few activated B cells, together with a few TH cells
migrate from foci to primary follicles which develop into
secondary follicles
• The secondary follicles provide a specialized
microenvironment for interactions between B cells,
activated TH cells and follicular dendritic cells
• The follicular dendritic cells have Fc receptors and
complement receptors which retain antigen-antibody
complexes for long periods of time
• Activated B cells may migrate towards the center of the
secondary follicle and form a germinal center
Schematic diagram of a peripheral lymph node showing
anatomic sites at which various steps in B-cell ctivation,
proliferation, and differentiation occur

13. Germinal centers and antigen induced B cell
differentiation
• Germinal centers arise within 7-10 days after
initial exposure to a thymus dependent antigen
• Proliferating B cells (centroblasts) appear in
germinal centers as a well defined dark zone
• Centroblasts give rise to centrocytes which are
small, nondividing B cells expressing membrane
Ig
• Centrocytes move from dark zone into a region
containing follicular dendritic cells called light
zone
• Germinal centers are important for affinity
maturation, class switching and formation of
plasma cells and memory B cells
• Affinity maturation was first noticed by H. N.
Eisen and G.W. Siskind when they immunized
rabbits with the hapten carrier complex DNP-
BGG
• Average affinity of anti-DNP antibodies increased
about 140-fold from 2 weeks to 8 weeks
Overview of cellular events within germinal centers

14. B cells and germinal center
Rapidly dividing and mutating B cells
deep in the center
Antigen-retaining FDCs
Macrophage
capturing dead B
cells
B cells being
tested for
antibody affinity
Dark zone
Light zone
centrocytes
centroblasts

15. Role of cytokines in B cell differentiation
• Differentiation of B cells is
dependent on cytokines
• IL-2 is an inducer of
proliferation for B cells
• Involved in class
switching, etc. The interactions of numerous cytokines with B cells generate signals
required for proliferation and class switching during the differentiation
of B cells into plasma cells