B Cell Rab7 Mediates Induction of Activation-Induced Cytidine Deaminase Expression and Class-Switching in T-Dependent and T-Independent Antibody Responses

Class switch DNA recombination (CSR) is central to the maturation of the Ab response because it diversifies Ab effector functions. Like somatic hypermutation, CSR requires activation-induced cytidine deaminase (AID), whose expression is restricted to B cells, as induced by CD40 engagement or dual TLR-BCR engagement (primary CSR-inducing stimuli). By constructing conditional knockout Igh(+/C)gamma(1-cre)Rab7(fl/fl) mice, we identified a B cell-intrinsic role for Rab7, a small GTPase involved in intracellular membrane functions, in mediating AID induction and CSR. Igh(+/C)gamma(1-cre)Rab7(fl/fl) mice displayed normal B and T cell development and were deficient in Rab7 only in B cells undergoing Igh(C)gamma I1-cre gamma 1-S gamma 1-C gamma 1-cre transcription, as induced-like Igh germline I gamma 1-S gamma 1-C gamma 1 and I epsilon-S epsilon-C epsilon transcription-by IL-4 in conjunction with a primary CSR-inducing stimulus. These mice could not mount T-independent or T-dependent class-switched IgG1 or IgE responses while maintaining normal IgM levels. Igh(+/C)gamma(1-cre)Rab7(fl/fl) B cells showed, in vivo and in vitro, normal proliferation and survival, normal Blimp-1 expression and plasma cell differentiation, as well as intact activation of the noncanonical NF-kappa B, p38 kinase, and ERK1/2 kinase pathways. They, however, were defective in AID expression and CSR in vivo and in vitro, as induced by CD40 engagement or dual TLR1/2-, TLR4-, TLR7-, or TLR9-BCR engagement. In Igh(+/C)gamma(1-cre)Rab7(fl/fl) B cells, CSR was rescued by enforced AID expression. These findings, together with our demonstration that Rab7-mediated canonical NF-kappa B activation, as critical to AID induction, outline a novel role of Rab7 in signaling pathways that lead to AID expression and CSR, likely by promoting assembly of signaling complexes along intracellular membranes.