Deconstructing the allo-specific memory B cell response Allo-specific B cells and their antibody products are strongly predict with acute and chronic allograft rejection, especially in "sensitized" transplant recipients with pre-existing donr-specific antibodies (DSA). The source of antibody in the sensitized recipients derives from two distinct cellular pools: constitutive production of allo-specific antibody from the long lived plasa cell and de novo production from reactivated memory B cells in the recall response. While the long lived plasma cells can be viewed as a static population that produces a finite amount of antibody (life time times secretion rate), memory B cells represent a highly dynamic population that re- cycle indefinitely to produce bursts of antibody and to reseed the long lived plasma cell and memory B cell pools. Thus, we posit that understanding the roles of memory B cells under circumstances of transplantation, especially the mechanisms that regulate the dynamic behavior of allo-specific memory B cells, will ultimately prove to be important for controlling transplant rejection. Our work has uncovered a central regulator of B cell differentiation that controls the identity of differentiated B cells as a function of antigen affinity/avidity of the B cell antigen receptor (BCR). The Irf4 transcription factor controls the generation of plasma cells (PC) and Germinal Center B (GC B) cells by activating the expression of the rate limiting transcription factors important for those cell fates, Blimp-1 and Bcl6, respectively. We hypothesize that Irf4 plays a similarly critical role in controlling the generation of memory B cells as well as in controlling the dynamics of memory B cell reactivation. Furthermore, we have optimized a strategy to follow the fate of individual allogeneic MHC-specific B cells responding to transplants in mice. This technology has enabled us to quantify the proportions of PC, GC B, and memory B cells after primary and secondary immunizations. Therefore we propose to define the life cycle of allo-antigen specific memory B cells and how that may be altered by costimulation blockade, to identify the conditions with which memory B cells reactivate, and to determine the impact of memory B cells on mechanisms of humoral rejection.
Memory B cells are a dynamic cell type that recycles indefinitely into future immune responses to produce bursts of high affinity antibody and to reseed the long lived plasma cell and memory B cell pool. In this proposal, we combine single cell tracking technology with novel genetic tools to better understand the life cycle of allogeneic MHC-specific memory B cells and how that may be perturbed by costimulation blockade as well as the rules by which memory B cells reactivate during new immune responses. These studies will provide new insight into the associations of alloantibody and increased incidence of transplant rejection as well as into new immunosuppressive strategies to control B cells and their antibody products.