ABSTRACT Multiple diseases, including graft-versus-host disease, transplant rejection, rheumatoid arthritis, and lung fibrosis are known to be driven by pathological activation of T cells. While T cell activation is a key part of many immune responses, this process can become pathological when T cells inaccurately recognize a patient’s own tissues or in the context of tissue transplantation. While immunomodulatory drugs including corticosteroids and cyclosporine are FDA-approved, these agents act on many immune cell types, leading to broad immunosuppression and severe side effects. Past high-throughput screening efforts identified and validated small molecule ‘Selective Inhibitors of T Cell Activation (SITCAs)’ that function in vitro and in vivo without influencing inflammatory responses in other cell types. While these molecules suggested the potential for novel T cell-selective immunomodulatory agents, lack of understanding of their cellular targets prevented further drug discovery efforts. Exportin-1 (XPO1) catalyzes nuclear-to-cytoplasmic transport of hundreds of proteins and also has established roles in regulating the centromere and transcription. The highly toxic natural product Leptomycin was used to establish that blocking XPO1-mediated nuclear export led to cancer cell death, and later efforts led to FDA approval of selinexor, a Selective Inhibitor of Nuclear Export (SINE), for multiple myeloma patients who have failed at least four prior therapies. Our data establish that multiple Selective Inhibitors of T Cell Activation also target XPO1, but with novel pharmacology: these ‘partial antagonists’ inhibit XPO1’s novel role in the T cell activation process but have minimal effects on nuclear export and are substantially less cytotoxic. These data suggest that XPO1 represents a promising new target for blocking pathological T cell activation, and that the novel partial antagonist profile is desirable to avoid on-target cytotoxicity associated with existing XPO1 modulators. This proposal seeks to understand and optimize XPO1 partial antagonists for application in immune- mediated diseases. First, we seek to use structural and functional assays to understand how different small molecules that bind the same site of XPO1 show such divergent effects on cellular phenotypes including nuclear export and cell viability. In Aim 2, we will establish the cellular mechanisms by which XPO1 modulators block T cell activation, with the hypothesis that dissociation from chromatin of XPO1, NFAT transcription factors, and other chromatin factors plays a central role. Finally, we will use medicinal chemistry to optimize the partial antagonist profile and evaluate leading partial antagonists in preclinical models of T cell function, including using human primary T cells and in a mouse model of lung fibrosis in which T cells are known to play a role. Together these studies will extend XPO1 as a therapeutic beyond late-stage cancer patients by optimizing novel p...