Lecture 25 - B cell development

Question 1

Where do B cells develop

Answer 1

Bone marrow (then migrate to secondary lymphoid tissues)

Question 2

Where does repertoire generation occur

Answer 2

Bone marrow

Question 3

Where does selection of b cells that bind to specific antigens for further development take place

Answer 3

(Proliferation, isotype switching, somatic hypermutation/affinity maturation) occurs in secondary lymphoid tissues

Question 4

How are cells at different stages of development identified

Answer 4

different combinations of CD proteins on their surface

eg. CD127 is the α chain of the receptor for interleukin-7. (CD= cluster of differentiation)

Question 5

What is the pathway for Pro-B cells development

Answer 5

Pluripotent Haematopoietic stem cell (CD34)

Common Lymphoid Progenitor (CD34 + CD10)

B-cell precursor (CD34, CD10, CD127)

Pro-B cell (CD34, CD10, CD127, CD19)

Question 6

Describe the development of B cells guided by stroll cells in bone marrow

Answer 6

Lymphoid progenitor cell use Very late antigen 4 (VLA-4) (an integral) to bind to the adhesion molecule VCAM-1 (vascular cell adhesion molecule) on stroll cells

This and other CAMs promote binding of kit on the b-cell to stem cell factor (SCF) on stromal cell.

Activation of Kit causes B cell to proliferate

B cells at later stage of maturation require interleukin-7 (IL-7) to stimulate overall growth and proliferation

Question 7

The development of B cells in the bone marrow proceeds through stages defined by the rearrangement and expression of the immunoglobulin genes (SLIDE 5)

Answer 7

In stem cells, the immunoglobulin (Ig) genes are in the germline configuration. The first rearrangements are of the heavy-chain (H-chain) genes. Joining DH to JH defines the early pro-B cell, which becomes a late pro-B cell on joining VH to DJH. Expression of a functional μ chain defines the large pre-B cell. Large pre-B cells proliferate, producing small pre-B cells in which rearrangement of the light-chain (L-chain) gene occurs. Successful light-chain gene rearrangement and expression of IgM on the cell surface define the immature B cell.

Question 8

What does rearrangement of immunoglobulin heavy-chain genes in pro-B cells give rise to productive and nonproductive rearrangements

Answer 8

A productive rearrangement enables the B cell to proceed to the next stage of development. Rearrangements can occur successively at the H-chain genes on both chromosomes, and if neither is successful the cell dies.

Question 9

How is a pre-B-cell receptor distinguished from the B cell receptor

Answer 9

absence of a κ or λ immunoglobulin light chain and the presence instead of the surrogate light chain composed of the VpreB and λ5 polypeptides. The pre-B-cell receptor is in low abundance at the cell surface and is largely retained inside the cell in membrane-enclosed vesicles, from which it generates signals that lead to the cessation of heavy-chain gene rearrangements. In addition to forming the two Ig-like domains of the surrogate light chain, VpreB and λ5 have extensions that cause oligomerization of pre-B-cell receptors and the transduction of signals necessary for pre-B-cell survival.

Question 10

slide 7?

Question 11

Timing of the production of proteins involved in immunoglobulin gene rearrangement and expression during B-cell development.

Answer 11

The rearrangement of immunoglobulin genes and the expression of the pre-B-cell receptor and cell-surface IgM require several categories of specialized proteins at different times during B-cell development. Examples of such proteins are listed here, with their expression during B-cell development indicated by red shading. The protein kinase FLT3 (Fms-like tyrosine kinase 3) is a cell-surface receptor on stem cells that, on binding to the cytokine FLT3 ligand produced by bone marrow stromal cells, receives signals that cause the stem cell to differentiate into common lymphoid progenitors. CD19 is a subunit of the B-cell co-receptor, which cooperates with the antigen receptor to produce activating signals when antigen is bound. CD45 is a cell-surface protein phosphatase that modulates signals from a B-cell receptor that has bound specific antigen. CD43, CD24, and BP-1 are cell-surface markers that help to distinguish different stages in B-cell development. The contributions of the other proteins listed here are explained in the text.

Question 12

Why don’t self-reactive B cells normally produce autoantibodies, and what happens if this control fails?

Answer 12

Some B cells produced in the bone marrow recognize self-antigens. Normally, these cells are prevented from maturing and becoming plasma cells through tolerance mechanisms, which reduce the risk of producing autoantibodies.
If these tolerance mechanisms fail, self-reactive B cells can become activated, produce autoantibodies, and lead to autoimmune disease.

Question 13

What is receptor editing and what are its possible outcomes in self-reactive B cells?

Answer 13

Receptor editing is a tolerance mechanism in which immature self-reactive B cells in the bone marrow undergo additional light-chain gene rearrangements to change their antigen specificity.
If a new non–self-reactive receptor is produced, the B cell survives and continues developing. If editing fails and the cell remains self-reactive, it undergoes apoptosis

Question 14

Describe the general route of B-cell circulation through secondary lymphoid tissue

Answer 14

B cells circulating in the blood enter the lymph-node T-cell areas via a high endothelial venule (HEV). From there they pass into a primary lymphoid follicle. If the B cells do not encounter their specific antigens, they leave the follicle and exit from the lymph node in the efferent lymph. The circulation route is the same for immature and mature B cells, which all compete with each other to enter primary follicles.

Question 15

How do immature B cells become mature B cells in secondary lymphoid tissues?

Answer 15

Immature B cells enter secondary lymphoid tissues through high endothelial venules (HEVs), attracted by CCL21 and CCL19.

They compete to enter a primary follicle, where CXCL13 from follicular dendritic cells (FDCs) attracts them.
Inside the follicle, interaction with FDC surface molecules (and survival factors like BAFF) promotes their final maturation.

If they mature and do not encounter their antigen → they exit and recirculate.

If they fail to enter a follicle → they continue recirculating briefly and then die.

Question 16

Describe the cycle of how B cells mature

Answer 16

Chemokine CCL21 attracts immature B cells to HEV

CCL21 and CCL19 attract B cells into lymph node

CXCL13 attracts B cells into primary follicle

Interactions with follicular dendritic cells drive B-cell maturation

Mature B cells recirculate between lymph, blood and secondary lymphoid tissues

Question 17

Where and how are naïve B cells activated in a lymph node?

Answer 17

Naïve B cells enter lymph nodes via HEVs and bind antigen delivered in lymph.
They internalize and process the antigen, then present it on MHC II to a cognate CD4⁺ helper T cell at the T–B border.
With T-cell help, B cells become activated and form a primary focus of proliferating cells.

Question 18

What are the possible fates of activated B cells?

Answer 18

Activated B cells can:
Differentiate rapidly into plasma cells (early response; often low-affinity antibodies).
Enter a germinal center, where they proliferate, undergo somatic hypermutation and affinity maturation, and may isotype switch.
High-affinity B cells then become long-lived plasma cells (medullary cords or bone marrow) or memory B cells.

Question 19

What is isotype (class) switching in B cells?

Answer 19

Isotype switching is a recombination process that changes the heavy-chain constant (C) region of an antibody without altering the VDJ antigen-binding region.
This allows a B cell to produce a different antibody class (e.g., IgG, IgA, IgE) while keeping the same antigen specificity.

Question 20

How does isotype switching occur at the DNA level?

Answer 20

Repetitive switch (S) regions lie upstream of heavy-chain C genes (except δ).
AID (activation-induced deaminase) targets these regions, creating DNA breaks.
Recombination between two S regions deletes the intervening DNA as a circular fragment and places the VDJ region next to a new C gene (e.g., switching from μ to γ1).
The first switch is usually from IgM (μ) to another isotype.

Question 21

What determines the specialized functions of different immunoglobulin isotypes?

Answer 21

Each antibody isotype has distinctive constant (C) region properties that determine its effector functions, such as:
Opsonization
Complement activation
Placental transfer
Mucosal immunity
These functional differences allow antibodies with the same antigen specificity to perform different immune roles.

Question 22

What is opsonization and how do antibodies contribute to it?

Answer 22

Opsonization is the process by which antibodies enhance phagocytosis.
Antibodies can:
Directly act as opsonins by binding Fc receptors on phagocytes.
Indirectly promote opsonization by activating complement.
(Note: IgG2 functions as an opsonin only in individuals with a specific Fc receptor variant.)

Question 23

How do antibodies neutralize toxins?

Answer 23

Antibodies bind to bacterial toxins and block their ability to attach to and enter host cells (neutralization).
The antibody–toxin complex is then recognized by Fc receptors on macrophages, leading to phagocytosis and degradation.

Question 24

How does antibody opsonization promote phagocytosis of bacteria?

Answer 24

IgG binds to antigens on a bacterium, coating its surface (opsonization).
The exposed Fc regions bind Fc receptors on macrophages, triggering phagocytosis and destruction of the bacterium.

Question 25

How do antibodies enhance bacterial clearance via complement?

Answer 25

Antibody bound to a bacterium activates the complement system, leading to deposition of C3b on the bacterial surface. Macrophages bind both C3b and antibody Fc regions, resulting in enhanced phagocytosis and degradation.