Project Abstract Human immunology has changed radically in the past decade, accelerated, in part, by the COVID-19 pandemic. Despite this progress, we still lack knowledge of how to specifically induce optimal vaccine-induced T and B cell responses. One major gap in human immunology is the inability to precisely interrogate individual immune pathways in antigen-specific T and B cell responses through gain- or loss-of-function. Such studies have remained largely the domain of mouse immunology. However, humans are routinely treated with specific immune modifying therapeutics. Many of these same individuals also receive vaccines that allow examination of a specific adaptive immune response with known timing and well-defined antigens. Thus, it is now possible to capture the human experiment where the human immune system has been specifically perturbed by a therapeutic while another immune stimulus, such as a vaccine, is delivered. With well-designed patient cohorts selected for receipt of drugs that target a pathway of interest, key mechanistic gain- and loss-of-function type studies can be performed in humans. Here, we focus on the PD-1 inhibitory receptor pathway and vaccine-induced immunity. PD-1 is highly expressed in germinal centers (GC) by GC T follicular helper (TFH) cells, and the PD-1 ligands PD-L1 and PD-L2 are expressed by GC B cells. PD-1 is also upregulated by activated CD8 T cells during priming in humans. The role of the PD-1 pathway in human vaccine immunity, however, is largely unknown. Our preliminary data generated during the previous U19 cycle demonstrate the feasibility of this approach with promising data on effects of PD-1 on vaccine-induced TFH responses. Thus, the studies proposed will address a major gap in knowledge in human immunology: how does PD-1, expressed highly by vaccine induced immune cells, regulate vaccine-induced humoral and cellular immunity. We will test the central hypothesis that disruption of the PD-1 pathway through precision immunotherapy leads to changes in magnitude, differentiation state, repertoire, quality, and memory for GC-dependent TFH and B cell responses as well as GC-independent CD8 T cell responses. First, we will test whether loss of PD-1 signals alters the quality and/or magnitude of GC- associated CD4 T cell and B cell responses to vaccination. Second, we will ask whether loss of PD-1 signals alters CD8 T cell priming, repertoire and memory CD8 T cell differentiation following vaccination. This work will provide new insights into how a key immune regulatory pathway controls vaccine induced immunity using “mouse-like” loss-of-function studies in humans. This work will have implications for improving vaccines and for the fundamental regulation of immunological memory in humans. Moreover, this approach could be a platform for future studies capturing specific human immune perturbation to interrogate new immunobiology. By its nature Project 3 will be highly interactive with Project 1 and 2 and ...