Gut IgA Class Switch Recombination in the Absence of CD40 Does Not Occur in the Lamina Propria and Is Independent of Germinal Centers

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Abstract
Conflicting findings have recently been presented as to the sites and sources of B cells that undergo class switch recombination (CSR) to IgA in the gut. In this study we provide compelling evidence in CD40−/− mice demonstrating that IgA CSR can be independent of CD40 signaling and germinal center formation and does not occur in the gut lamina propria (LP) itself. We found that CD40−/− mice had near normal levels of gut total IgA despite lacking germinal centers and completely failing to raise specific responses against the T cell-dependent Ags cholera toxin and keyhole limpet hemocyanin. The Peyer’s patches in CD40−/− mice expressed unexpectedly high levels of activation-induced cytidine deaminase mRNA and germline α transcripts, but few postswitch circular DNA transcripts, arguing against significant IgA CSR. Moreover and more surprisingly, wild-type mice exhibited no to low IgA CSR in mesenteric lymph nodes or isolated lymphoid follicles. Importantly, both strains failed to demonstrate any of the molecular markers for IgA CSR in the gut LP itself. Whereas all of the classical sites for IgA CSR in the GALT in CD40−/− mice appeared severely compromised for IgA CSR, B cells in the peritoneal cavity demonstrated the expression of activation-induced cytidine deaminase mRNA comparable to that of wild-type mice. However, peritoneal cavity B cells in both strains expressed intermediate levels of the germinal center marker GL7 and exhibited no germline α transcripts, and only three of 51 mice analyzed showed the presence of postswitch circular DNA transcripts. Taken together, these findings strongly argue for alternative inductive sites for gut IgA CSR against T cell-independent Ags outside of the GALT and the nonorganized LP.