Selective Inhibitors of T Cell Activation Target Exportin-1 at Cys528 to Suppress Pathological T Cell Activation

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 partial antagonists of XPO1.

Grant Number: 5R01AI171104-03
NIH Institute/Center: NIH
Principal Investigator: Drew Adams