B Cell Development and Immunoglobulin Gene Rearrangement

B Cell Ontogeny and Activation

• B cell development is tied to immunoglobulin (Ig) gene rearrangements
• B cell ontogeny involves steps with checkpoint mechanisms, enzymes, and molecules
• Clonal proliferation and differentiation are key processes
• Lymphocyte development results in antigen receptor expression

B Cell Receptor (BCR)

• BCR has light chain, heavy chain, and Igα, Igβ components
• BCR signaling involves ITAMs
• Recognition of BCR is crucial

B Cell Development

• B-cell development occurs on bone marrow stromal cells
• SDF-1 (CXCL12) retains stem cells
• Cell signals (growth, proliferation, differentiation) happen
• Apoptosis is blocked for continued cell development
• IL-7, produced by bone marrow stromal cells, promotes proliferation and survival of progenitor cells; it's crucial for differentiation

BCR Generation and B Cell Development

• Light and heavy chains are rearranged
• Germline DNA is rearranged
• The rearrangement results in the formation of an active gene for the heavy and light chains
• The variable regions of heavy and light chains create diversity
• Heavy and light chains form an antibody with variable and constant regions

H Chain Rearrangement in Early Pro-B Cells

• H-chain gene rearrangement is a crucial step in B-cell development
• The process of H-chain rearrangement is important for survival signaling
• H-chain rearrangement leads to pre-BCR formation
• Failure to successfully rearrange H chains can lead to apoptosis

Pre-B Cells and Pre-BCR

• Pre-B cells express the pre-BCR
• Pre-BCR signaling drives proliferation of large pre-B cells
• Pre-BCR signaling is independent of antigen binding
• Pre-BCR signaling leads to activation of Btk(Bruton's tyrosine kinase) which promotes cell survival, proliferation, and maturation

X-linked Agammaglobulinemia (XLA)

• XLA is a disease caused by a mutation in BTK
• Mutation in BTK results in a lack of mature B-cell production and antibodies
• Leads to an increased risk of infection
• Patients would need immunoglobulin (Ig) replacement therapy

L Chain Rearrangement (Small Pre-B Cell)

• L-chain rearrangement produces a small pre-B cell
• The μ heavy chain, surrogate light chains, and pre-B-cell receptor are on the cell surface
• The μ chain is in the endoplasmic reticulum after VJ rearrangement
• The light chain (A or K) and IgM are on the surface

Successful V-J Rearrangement (BCR Expression)

• Successful V-J rearrangement leads to BCR expression
• Cells with complete H+L rearranged IgM, structure, Igα and Igβ proteins on the surface
• Signaling through the BCR complex triggers PI3K
• This triggers cell survival but not proliferation

Summary of BCR Gene Rearrangement

• The process of BCR generation involves selecting V, D, J, and C segments from the germline DNA
• Heavy chains are rearranged into polypeptides
• Light chains are rearranged into polypeptides
• The four chains assemble to produce an immunoglobulin

IgM+ Immature B Cells Circulate

• IgM+ immature B cells circulate to the spleen
• They interact with cells in the follicle to mature
• The mature B-cells circulate to secondary lymphoid structures

B Cell Maturation in the Spleen

• Immature B cells leave the bone marrow and reach the spleen
• They migrate to the T-cell zone of the white pulp
• Transitional B cells are formed

Site of Negative Selection in the Spleen

• T1 transitional B cells undergo negative selection to eliminate self-reactive B cells
• Negative selection involves exposure to self-antigens
• Survivors become T2 transitional B cells

T1 Transitional B Cells Recognizing Self-Ag

• T1 transitional B cells recognizing self-Ag are eliminated
• T2 transitional B cells in the follicle interact with follicular dendritic cells (FDCs)
• Naïve mature B cells are the result

Interaction of Surviving B Cells with FDCs and Naïve Mature B Cells

• Interaction with FDCs and cytokines drive B-cell maturation
• Mature B cells recirculate between lymph, blood, and secondary lymphoid tissues
• Interaction is crucial for final maturation

Marginal Zone B cells Generated

• T2 transitional B cells migrate to the marginal zone
• Develop into marginal zone B cells
• Rapid IgM response to blood antigens

B Cell Engagement of Antigen

• Antigen binding induces movement of BCR to the antigen contact site to increase the encounter between BCR and antigen

Follicular Dendritic Cells (FDCs)

• FDCs have an extensive dendrite surface area
• FDCs display large quantities of antigens
• FDCs have no phagocytic activity
• FDCs preserve antigens on their surface for months or years

BCR Engagement and Activation

• Initial BCR engagement triggers movement of BCR to contact areas
• Clustering of BCRs on the same antigen is necessary for crosslinking
• Signaling molecules such as Igα and Igβ containing cytoplasmic tails (ITAMs) are activated
• Src kinase phosphorylation of ITAMs in Igα and Igβ occurs
• Syk kinase binds to phosphorylated ITAMs, leading to downstream signaling

B Cell Activation and Additional Signals

• B cell activation requires co-receptor signaling, crucial in B-cell activation
• Components include CR2 (CD21), CD19, and CD81
• CR2 recognizes iC3b and C3d of complement fragments
• CD19 is a signaling chain
• CD81 binds CD19 and aggregates the co-receptor and BCR

B cell activation and additional signals -cont'd

• Signal transduction via co-receptor pathway enhances BCR signaling 10,000-fold or more

Outcomes of FOB BCR Recognition of Antigen

• Activation of B cells can trigger proliferation and expression of anti-apoptotic factors
• Antigen presentation via MHC class I and II molecules is enhanced
• Cytokine receptors are stimulated for signaling
• Expression of CCR7 and downregulation of CXCR5 changes cell migration

B-1 and B-2 Cells

• B-1 cells arise from fetal liver and are found in the peritoneal and pleural cavities
• B-1 cells do not require T-cell help to produce IgM
• B-2 cells arise from bone marrow and are located in secondary lymphoid organs
• B-2 cells require T-cell help to undergo class switching and somatic hypermutation