The largest mucosal surface in the body is in the gastrointestinal (GI) tract, a location that is heavily colonized by normally harmless microbes. A key mechanism required for maintaining a homeostatic balance between this microbial burden and the lymphocytes that densely populate the GI tract is the production and trans-epithelial transport of poly-reactive IgA1. Within the mucosal tissues, B cells respond to cytokines, sometimes in the absence of T cell help, undergo class switch recombination (CSR) of their Immunoglobulin (Ig) receptor to IgA, and differentiate to become plasma cells (PC)2. However, IgA-secreting PC likely have additional attributes that are needed for coping with the tremendous bacterial load in the GI tract. Here we report that IgA+ PC also produce the anti-microbial mediators TNFα and iNOS, and express many molecules that are commonly associated with monocyte/granulocytic cell types. The development of iNOS-producing IgA+ PC can be recapitulated in vitro in the presence of gut stroma, and the acquisition of this multi-functional phenotype in vivo and in vitro relies on microbial co-stimulation. Deletion of TNFα and iNOS in B-lineage cells resulted in a reduction in IgA production, altered diversification of the gut microbiota and poor clearance of a gut-tropic pathogen. These findings reveal a novel adaptation to maintaining homeostasis in the gut, and extend the repertoire of protective responses exhibited by some B lineage cells.
Activation-induced cytidine deaminase (AID) mutates cytidine to uridine at immunoglobulin loci to initiate secondary antibody diversification but also causes genome-wide damage. We previously demonstrated that AID has a relatively low catalytic rate. The structure of AID has not been solved. Thus, to probe the basis for its catalytic lethargy we generated a panel of free or DNA-bound AID models based on eight recently resolved APOBEC structures. Docking revealed that the majority of AID:DNA complexes would be inactive due to substrate binding such that a cytidine is not positioned for deamination. Furthermore, we found that most AID conformations exhibit fully or partially occluded catalytic pockets. We constructed mutant and chimeric AID variants predicted to have altered catalytic pocket accessibility dynamics and observed significant correlation with catalytic rate. Data from modeling simulations and functional tests of AID variants support the notion that catalytic pocket accessibility is an inherent bottleneck for AID activity.
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