AID
Overview
Activation-induced cytidine deaminase (AID, encoded by AICDA) is the enzyme responsible for initiating both somatic hypermutation (SHM) and class switch recombination (CSR) of immunoglobulin genes. AID deaminates cytidine residues in DNA, creating uracil mismatches that are processed by error-prone repair pathways to introduce point mutations (SHM) or double-strand breaks leading to isotype switching (CSR). Traditionally considered a GC-associated enzyme, AID expression has been documented at extrafollicular sites, indicating that SHM and CSR can occur outside germinal centers.
In this wiki, AID is significant as the marker whose preservation — despite Bcl-6⁺ GC B cell loss — demonstrates that B cell activation and antibody diversification continue through the extrafollicular pathway when GCs are absent.
Key Points from Literature
- AID⁺ B cells preserved in COVID-19 despite GC loss: Post-mortem COVID-19 lymph nodes and spleens showed marked reduction in Bcl-6⁺ GC B cells but quantitative preservation of AID⁺ B cells (both absolute numbers and as % of CD19⁺ cells). AID⁺ cells were diffusely distributed in COVID-19 tissue rather than concentrated in GC structures as in controls. This dissociation demonstrates that AID expression and the capacity for SHM/CSR are maintained in the absence of GC formation, consistent with extrafollicular B cell activation (see Kaneko2020 - GC Loss and TFH Block in COVID-19, n=11 COVID + controls, multi-color immunofluorescence).
- EF SHM at GC-comparable rates in murine model: William2002 demonstrated somatic hypermutation at ~0.3 mutations per gene per generation at the splenic T zone–red pulp border in MRL/lpr mice, in clusters completely lacking FDC networks. AID-dependent mutation occurred at this extrafollicular site while GCs in the same spleens harboured no mutated clones of the tracked specificity. This establishes that the enzymatic machinery for SHM (AID) can operate effectively at EF sites (see William2002 - Extrafollicular Somatic Hypermutation in Autoimmune Mice, in vivo murine model, 8 mice, 305 sequences).
Contradictions & Debates
- The preservation of AID⁺ B cells in COVID-19 demonstrates that T cell-mediated B cell activation continues extrafollicularly. However, AID expression alone does not reveal whether SHM is occurring at high rates — the Kaneko2020 study did not measure SHM directly in the tissue. The low SHM observed in COVID-19 convalescent antibodies (cited in the paper) is consistent with AID acting in an EF context with limited rounds of mutation.
Related Pages
Bcl-6, Somatic Hypermutation, Class Switch Recombination, Extrafollicular Response, Germinal Center