Apparent self-tolerance of commensal-specific B cells and their responses to infection

About This Grant

Project summary/abstract: How the immune system maintains a balance between containing the intestinal microbiota and limiting inflammation is an important field of study with relevance to numerous human diseases. The immune system must also balance the need for having a diverse B cell receptor (BCR) repertoire with maintaining self-tolerance. Tolerance of self-reactive B cells is enforced during B cell maturation in the bone marrow (BM) and peripherally through B cell anergy, which limits their lifespan and makes them hyporeactive. Commensal microbes add complexity to the idea of tolerance, as they are an important source of foreign antigen, but are not necessarily harmful. It is rarely considered whether the continuous presence of commensal-derived antigen could have tolerogenic effects on B cells. This proposal describes a new model where we find that a microbial antigen specifically tolerizes B cells, and will test the ability of the tolerized microbial antigen-specific B cells to respond to commensal antigen or infection. We have generated three BCR-knock-in (KI) mouse strains expressing lipopolysaccharide (LPS)-specific BCRs of either low- or high- affinity heavy chains, or a high-affinity light chain, collectively referred to as 33a KIs. These BCRs were derived from an expanded B cell clone that was responding to infection with Salmonella, but also binds a portion of small-intestinal commensal microbes. 33a KI B cells show hallmarks of self-tolerance mechanisms including clonal deletion, receptor editing, and anergy. Critically, transferring BM from SPF mice into lethally irradiated germ-free mice improved B cell development, demonstrating microbially derived antigen can contribute to B cell tolerance. Strict B cell tolerance could generate immunological blind spots for certain microbial antigens, while conversely a lack of tolerance could cause overwhelming inflammation at barrier sites. Intriguingly, we observe that 33a KI B cells form germinal centers in the Peyer’s patches (PPs) at steady-state, and a robust splenic extrafollicular response to systemic Salmonella infection, demonstrating that despite tolerization, the B cells can respond in certain contexts. The central goal of this proposal is to define the fundamental properties of this novel type of B cell response during homeostasis and following infection. In Aim 1 we will determine whether 33a B cells undergo classical germinal center responses in the PP during homeostasis, including the formation of long-lived memory B cells and plasma cells. In Aim 2, we will define the 33a B cell response to infection, and test whether the nature of antigenic stimulation impacts their response using oral infection with mutant Salmonella strains and a closely related pathogen, Yersinia pseudotuberculosis. This project tests a new conceptual framework that microbial antigen can contribute to both B cell tolerance and inflammation as needed, which will in turn have important implications for understanding many human disease conditions. The knowledge and skills I will attain from the proposed work will be broadly applicable and help me achieve my overall goal of becoming an independent investigator studying the immune balance of tolerance and protection.