The commensal microbiota establishes a mutualistic relationship with the gut immune system by activating B cells via T cell-dependent (TD) and T cell-independent (TI) pathways that generate secretory immunoglobulin A (SIgA) with high and low affinity for antigen, respectively. High-affinity (HA)-SIgA emerges from a relatively well- understood TD pathway involving CD40L and is generally viewed as essential for gut homeostasis due to its specific recognition of bacterial cells. However, recent studies indicate that also (LA)-SIgA from a poorly understood TI pathway specifically recognizes commensal bacteria. Additional evidence shows that HA-SIgA coats colitogenic bacteria in patients with inflammatory bowel disease (IBD), raising the possibility that gut homeostasis requires balanced HA-SIgA and LA-SIgA responses. This proposal will combine an in-depth analysis of primary immunodeficiency (PID) specimens with studies of knockout, germfree and gnotobiotic mouse models to dissect the regulation, microbiota reactivity and function of LA-SIgA and HA-SIgA responses. Preliminary data show that TI production of LA-SIgA involves CD40-independent activation of gut B cells by TACI, a BAFF/APRIL receptor that triggers IgM-to-IgA class switching through the kinase mTOR. Additional preliminary evidence shows that TACI deficiency impairs microbiota diversity and gut homeostasis by decreasing the coating of bacterial cells by LA-SIgA. Here, we hypothesize that TACI and CD40 orchestrate LA-SIgA and HA-SIgA responses through mTOR-regulated TI and TD pathways targeting non-overlapping consortia of gut bacteria. Three specific aims are proposed. Aim 1 is to elucidate the regulation of LA-SIgA and HA-SIgA responses by TACI and CD40 and dissect their reactivity for the gut microbiota. Aim 2 is to characterize the composition and function of bacterial communities targeted by LA-SIgA and HA-SIgA antibodies; Aim 3 is to dissect the functional interplay of LA-SIgA and HA-SIgA responses induced by TACI and CD40 with B cell signals from mTOR. This kinase activates B cells by engaging TACI through MyD88. The proposed studies (Project 3) will take advantage of cells, stool and tissues made available by this consortium and of the complementary and integrative expertise of the Cunningham-Rundles group, which evaluates the role of TACI in B cell proliferation and differentiation (Project 1), the Meffre group (Project 2), which explores the role of TACI and other key antibody- regulating molecules in B cell tolerance, and the Casanova group (Project 4), which seeks to identify new causative genes in PIDs.