Abstract Microbiota are required for the development of secondary lymphoid tissues, including gut- associated lymphoid tissues (GALT). Interactions between GALT and the microbiota are especially important in rabbits, as B cells proliferate and mutate their V(D)J genes in response to select commensal bacteria. Further, VHa B cells, identified because they utilize VH1, are selectively expanded in GALT as a result of specific commensal bacteria. In contrast, other B cells, designated VHn, do not expand in GALT. VHa and VHn B cells differ primarily in the external surface of framework regions (FR) 1 and 3, and we propose that VHa-specific amino acids on the external surfaces of FR1 and FR3 form a binding motif that together with TLR signaling through MyD88, mediates selective expansion of VHa B cells by binding bacterial cell surface molecules. We hypothesize that this innate-like, non-antigen-specific stimulation through the B cell receptor (BCR) facilitates selective expansion of VHa B cells and diversification of the primary Ab repertoire. In Aim 1, we will test this hypothesis by identifying bacteria in GALT that selectively bind to and activate VHa but not VHn B cells. Aim 2 will test if the identified bacteria specifically drive expansion of VHa B cells in vivo; and in Aim 3, we will explore the mechanism by which the bacteria stimulate VHa B cells. The results will elucidate a new mechanism by which bacteria interact with B cells to promote development of the immune system.