PROJECT SUMMARY/ABSTRACT This K99 Career Pathway to Independence in Blood Science Award details a five-year training program to advance Dr. Bhavya Doshi’s career goal of becoming an independent translational physician-scientist in coagulation. The proposed application combines approaches using tissue culture and animal models with investigations using patient samples to address the current limitations in the understanding of the immune response to factor VIII (FVIII). Dr. Doshi’s career development and training objectives are geared to foster this research proposal and her overall career goal of understanding basic mechanisms that drive disease to develop targeted treatment and prevention strategies. During the award period, Dr. Doshi will engage with a robust team of scientists in hemostasis and immunology to build her immunologic and translational research skills, learn genomic skills necessary for her future career aspirations, and continue to develop her expertise in coagulation and inhibitors. Under the guidance of her mentors, Dr. Rodney Camire and Dr. Michael Milone, these training objectives will be met by a combination of didactic course work and workshops, participation in seminar series, hands-on research experience, and mentoring by her advisory committee. Her advisory committee is composed of world-renowned scientists with extensive mentoring experience and diverse and complementary scientific expertise. They are all versed in bringing basic research findings to the bedside. Hemophilia A (HA) is caused by a mutation in the F8 gene leading to dysfunction or deficiency of coagulation FVIII. The development of neutralizing antibodies (“inhibitors”) to FVIII is the leading cause of morbidity and mortality in patients with HA. Dr. Doshi’s preliminary studies in HA patients and mice are the first to support that the cytokine B cell activating factor (BAFF) is potentially a biomarker and/or regulator of the FVIII immune response. This proposal seeks to probe basic mechanisms in FVIII-specific B cell generation and how BAFF contributes to this humoral response in order to leverage these findings for therapeutic application. Aim 1 seeks to determine the location, kinetics, and types of B cell responses to FVIII in mice by using a novel method to identify FVIII-specific B cells and subsequently determine what happens to this compartment with immunomodulatory strategies, including anti-BAFF. As BAFF is both systemically and locally produced by fibroblast cells and hematopoietically-derived immune cells, respectively, Aim 2 investigates the source of BAFF and its effect on the T cell response to FVIII. Finally, Aim 3 assesses whether genetic drivers of cytokine levels for BAFF or T helper cytokines drive phenotypic changes in T and B cell subsets that lead to inhibitor generation and/or persistence in HA patients. Together, these studies will inform the immune compartments that are critical to the FVIII immune response and establish the preclini...