B cell Rab7 mediates induction of AID expression and class-switching in T-dependent and T-independent antibody responses (IRM10P.605)

Abstract IgH class switch DNA recombination (CSR) is central to the maturation of the antibody response, as it diversifies antibody biologic effector functions. By constructing conditional KO Igh+/Cγ1-creRab7fl/fl mice, we have previously identified a B cell-intrinsic role for Rab7, a small GTPase that regulates endosome maturation, in CSR and antibody responses. In Igh+/Cγ1-creRab7fl/fl B cells, Rab7 gene is ablated within 48 h of stimulation by IL-4 plus CD154 or LPS, resulting in reduced CSR to IgG1 and IgE. Here, we have addressed the mechanisms underlying the role of Rab7 in CSR. Once Rab7 was ablated, B cells were also defective in CSR to IgA, indicating that Rab7 regulates the central CSR machinery in an Ig isotype-independent manner. Indeed, expression of AID, as essential to CSR, was reduced in Rab7 KO B cells and enforced AID expression rescued CSR. Finally, super-resolution imaging of CD154-stimulated B cells revealed that Rab7 formed foci, within which CD40 was enriched. These findings, together with our demonstration that Rab7 mediated canonical NF-κB activation, as critical to AID induction, outline a novel role of Rab7 in specific signaling pathways that lead to AID expression and CSR, likely by promoting assembly of signaling complexes along intracellular membranes.