B-Cell Immunology Study Notes

Quick Revision: Centroblasts in Ten Points

1. Where: centroblasts sit in the dark zone of germinal centres in secondary lymphoid tissue (lymph nodes, spleen, Peyer’s patches).
2. What they do: proliferate rapidly and diversify their antibody genes.
3. Somatic hypermutation: the enzyme AID (activation-induced cytidine deaminase) introduces point mutations into immunoglobulin variable-region genes.
4. Next stage: centroblasts stop dividing and become centrocytes, which move to the light zone.
5. Selection: centrocytes capture antigen held on follicular dendritic cells and present it to T follicular helper (Tfh) cells.
6. Survival: cells whose receptors bind antigen better receive more Tfh help (e.g. CD40L, IL-21) and survive; others die by apoptosis.
7. Cyclic re-entry: selected cells can return to the dark zone for further rounds of division and mutation.
8. Outcome: the result is affinity maturation — antibodies that bind more tightly over the course of a response.
9. Exits: germinal centre B cells leave as long-lived plasma cells or memory B cells; class-switch recombination (also AID-dependent) changes antibody isotype.
10. Clinical link: several B-cell lymphomas (e.g. follicular lymphoma, many diffuse large B-cell lymphomas and Burkitt lymphoma) originate from germinal centre B cells; the transcription factor BCL6 is central to the germinal centre programme.

Summary for revision only — educational information, not medical advice.

Advanced Centroblast Biology: Somatic Hypermutation and Affinity Maturation

Deep dive into the molecular biology of centroblasts and their role in adaptive immunity

Centroblasts represent one of the most biologically remarkable cell types in the human body. These large, rapidly dividing B lymphocytes occupy the dark zone of germinal centers in secondary lymphoid organs — lymph nodes, spleen, and mucosa-associated lymphoid tissue (MALT) — where they undergo two extraordinary processes simultaneously: clonal expansion and targeted mutagenesis of their antibody genes.

Activation-Induced Cytidine Deaminase (AID): The Engine of Mutation

The key enzyme enabling somatic hypermutation (SHM) in centroblasts is activation-induced cytidine deaminase (AID), encoded by the AICDA gene. AID is a DNA deaminase that converts cytosine (C) to uracil (U) in single-stranded DNA. In the context of immunoglobulin genes, this preferentially targets specific DNA sequence motifs called WRCY hotspots (where W=A/T, R=purine, C=target cytosine, Y=pyrimidine).

When the resulting U:G mismatch is processed by the cell's DNA repair machinery, several outcomes are possible:

• Replication over U: If replicated before repair, U pairs with A, resulting in a C→T transition mutation — the most common type of SHM mutation.
• Uracil-DNA glycosylase (UNG) processing: UNG removes the uracil, leaving an abasic site. Error-prone DNA polymerase eta (pol η, encoded by POLH) fills in the gap, generating A/T mutations — explaining the characteristic accumulation of mutations at A-T basepairs in SHM.
• Mismatch repair (MMR) processing: The MSH2-MSH6 complex recognizes the G:U mismatch and recruits exonuclease 1 (Exo1) plus error-prone polymerases, spreading mutations beyond the initiating deamination event.

The overall SHM mutation rate in centroblasts reaches approximately 10^-3 per basepair per cell division — roughly one million times higher than the spontaneous mutation rate in other somatic cells. This extraordinarily elevated rate is precisely targeted to the variable regions of immunoglobulin heavy (IgH) and light chain (IgL) genes by cis-regulatory elements in the immunoglobulin loci.

The Dark Zone and Light Zone Architecture

The germinal center is organized into two histologically and functionally distinct zones:

• Dark zone (DZ): Contains centroblasts — large, rapidly cycling B cells with dispersed chromatin and scant cytoplasm. The dark zone lacks follicular dendritic cells (FDCs) and T cells. Centroblasts in the dark zone proliferate and undergo somatic hypermutation of their immunoglobulin genes. The master transcription factors CXCR4 and FOXO1 maintain the centroblast in the dark zone identity.
• Light zone (LZ): Contains centrocytes — smaller, non-dividing B cells with condensed chromatin. Centrocytes express high levels of BCR surface immunoglobulin and are tested against antigen displayed on the surface of follicular dendritic cells (FDCs). FDCs capture antigen-antibody complexes through Fc receptors and present native antigen to centrocyte BCRs. Centrocytes also interact with T follicular helper (Tfh) cells through CD40-CD40L and cytokine signaling. The transcription factor CXCR5 and IRF4 are upregulated in light zone cells.

Selection and Cyclic Re-entry

Centrocytes in the light zone undergo stringent selection based on their ability to bind antigen through their mutated BCRs. Those centrocytes whose mutations improved antigen binding affinity receive strong survival signals through BCR cross-linking and Tfh cell help (IL-21, IL-4, CD40L). Those with reduced or abrogated antigen binding capacity fail to receive these signals and undergo apoptosis — a process called negative selection that eliminates low-affinity variants.

Positively selected centrocytes face a fate decision mediated by the strength of signals received:

• Recycling to the dark zone: Centrocytes receiving moderate BCR signal and low amounts of Tfh help re-enter the dark zone as centroblasts for additional rounds of mutation, potentially acquiring further affinity-enhancing mutations.
• Plasma cell differentiation: Centrocytes receiving very strong BCR signal upregulate BLIMP-1 (encoded by PRDM1), which represses PAX5 and drives differentiation into antibody-secreting plasma cells — either short-lived plasmablasts that quickly produce antibody locally, or long-lived plasma cells that migrate to the bone marrow and produce antibody for years.
• Memory B cell formation: Centrocytes receiving moderate BCR signal combined with strong Tfh help upregulate BCL6 and differentiate into long-lived memory B cells that circulate and provide rapid secondary responses upon re-exposure to antigen.

Clinical Significance: Lymphoma and Therapeutic Targets

The extraordinary biology of centroblasts — rapid proliferation, AID-mediated mutagenesis, and DNA repair errors — makes germinal center B cells particularly prone to oncogenic transformation. Key malignancies that arise from centroblasts include:

• Diffuse large B-cell lymphoma (DLBCL), GCB subtype: The germinal center B-cell-like subtype of DLBCL arises from centroblasts with BCL6 overexpression (maintaining the centroblast proliferative state) and often BCL2 translocation or amplification (preventing apoptosis). BCL6 inhibitors are in clinical development.
• Follicular lymphoma (FL): Arises from centrocytes with t(14;18) translocation placing BCL2 under IgH enhancer control, preventing apoptosis during normal germinal center selection. FL cells continue to circulate through germinal center reactions, accumulating additional mutations.
• Burkitt lymphoma: Arises from centroblasts with t(8;14) or variant translocations placing MYC under IgH enhancer control. MYC is a major driver of centroblast proliferation, and its dysregulation drives the extremely rapid growth rate (doubling time ~24h) of Burkitt lymphoma.