2-4 Development of B Cells and Generation of Diversity

Question 1

What are some ways in which B cells diversify immunoglobulin before they encounter antigens (naive B cells)?

Answer 1

• Combinational: DNA sequence for V region = combinations of V+J (light chains) or V+J+D (heavy chains) segments
• Junctional: Random somatic recombination of gene segments in antigen-binding sites (+P,N nucleotides)
• Alternative mRNA splicing (IgM and IgD)

Question 2

How do naive B cells achieve junctional diversity?

Answer 2

Recombination activation genes (RAGs) join V, D, and J regions via gene signal sequences, creating palindromic (P) sequences. (Both RAG1 and RAG2 are necessary, or you can’t make B cells [no immunoglobulin] or T cells [no receptor].)

Joining is imprecise and error-prone, often resulting in multiple reading frames. Terminal deoxytransferase (TdT) randomly adds N nucleotides to open ends of DNA (without a template) in heavy chains.

Question 3

With regard to the order of B cell development, how do chain rearrangements take place?

Answer 3

• Heavy chain rearrangements first
• 1 chromosome at a time
• Checkpoint in pre-B cells before making light chain: surrogate light chain sends signal that H chain is good
• Lots of editing and re-editing
• Many, many cells apoptose during this process

Question 4

How does selective mRNA splicing diversify B cells?

Answer 4

Naive B cells express both IgM and IgD on their surfaces. Differential splicing, based on different activation/stimulation, decides whether a B cell is:

• IgM or IgD (before antigen binding)
• Transmembrane or secreted (after antigen binding; excision of hydrophobic transmembrane sequence)

Question 5

What are some ways in which B cells diversify immunoglobulin after they encounter antigens?

Answer 5

• Alternative mRNA processing: transmembrane → secreted
• Class switching: IgM/IgD → other isotypes, but same antigen specificity (AID)
• Somatic hypermutation: accelerated selection process for high antigen affinity (AID)

Question 6

How and why does class switching occur in B cells?

Answer 6

During an immune response, when IgG/IgE/IgA effector functions become suitable, it is advantageous to make use of the existing IgM/IgD antigen specificity. Accordingly, T cells stimulate Activation-Induced cytidine Deaminase (AID), which selectively targets switch regions, loops them out, and irreversibly recombines such that a variable region is joined to a novel constant region (unless the switch is to IgD).

T-independent antigens do not stimulate class switching because they do not stimulate T cells.

Question 7

What is somatic hypermutation?

Answer 7

A process that selects for high antigen affinity. Occurs in B cells, NOT T cells.

During class switching, Activation-Induced cytidine Deaminase (AID) catalyzes an unusually high rate of mutation, which selects for AA mutations that improve antigen binding.

Question 8

How does the mechanism of T cell formation compare to the mechanism of B cell formation?

Answer 8

T cell and B cell formation are similar, except that T cells lack AID and, therefore, cannot undergo somatic hypermutation.

Just as class switching in B cells involves irreversible changes to the DNA sequence, T cells CANNOT become γ:δ T cells after the α-chain locus is rearranged, because the δ-chain locus is located within the α-chain locus.