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 XP