Exploiting the VISTA Pathway to Prevent Acute GVHD and Control Steroid Refractory Disease

Abstract Despite multi-drug prophylaxis, aGVHD affects 20-70% of allogeneic hematopoietic cell transplant (allo-HCT) patients. V-region Immunoglobulin-containing Suppressor of T cell Activation (VISTA), a negative checkpoint regulator expressed on resting naïve mouse and human T cells. A single dose of agonist VISTA monoclonal antibody (mAb) on allo-HCT day 0 causes antigen-specific operational tolerance by deletion and anergy in resting naïve T cells that have concurrent T cell receptor (TCR) signals and 90-100% long-term survival. VISTA has been linked to induced Treg generation, expansion, stability and maintenance. In acute graft-vs-host disease (aGVHD), agonist mAb increases peripheral Tregs; the extent to which pTregs contribute to operational tolerance will be explored (aim 1A). Limited data exist for aGVHD prevention. Polyclonal CD4 T cells are the dominant aGVHD effectors in MHC disparate aGVHD models and TCR signaling is critical for deletion/anergy; studies are proposed using high affinity donor TCR transgenic and polyclonal CD4 and/or CD8 T cells will assess operation tolerance and long-term survival in MHC and minor antigen disparate models. Tetramers will track polyclonal and monoclonal donor allospecific T cells in agonist mAb treated mice. Frequently used calcineurin inhibitors (CNIs) may alter TCR signals below a threshold needed for agonist mAb effects; testing is required before translation (aim 1C). A unique feature of VISTA is downregulation with T cell activation. Non-alloreactive T cells and alloreactive T cells escaping agonist mAb induced deletion/anergy may permit generation of leukemia-specific T cells (aim 1B). Steroids are first line therapy for aGVHD patients but only half the patients have day 28 complete responses; 1-year survival rate for steroid refractory (SR) aGVHD patients is dismal. In mice and patients, we show myeloid cell infiltration is 2.5-fold higher than T cells in gut, a primal aGVHD organ in SR aGVHD. VISTA is expressed on myeloid cells at >10-fold higher levels than T cells. Agonist mAb inhibits myeloid cell chemotaxis and reprograms inflammatory monocytes/macrophages into anti- inflammatory cells. We hypothesize that agonist mAb reprograming of gut monocytes/macrophages can treat SR aGVHD (aim 2). Our central hypothesis is agonist VISTA mAb has dual uses for aGVHD prevention, inducing T cell operational tolerance, and SR aGVHD therapy, reprogramming myeloid cells to be anti- inflammatory. Our aims will test the hypotheses that: Operational tolerance induced by agonist mAb allospecific deletion/anergy of donor T cells depends on in vivo Treg induction, permits unaffected T cells to generate leukemia-specific responses, and is subverted by day 0 inflammation or CNIs initiated pre-transplant (aim 1). In aim 2, we will test the hypothesis that agonist mAb reprograms inflammatory monocytes and macrophages in the gut to be anti-inflammatory and locally release of immune suppressive cytokines, ameliorating SR aGVHD without systemic side-effects of exogenous cytokines. Our studies are foundational for translation of Noelle’s agonist human VISTA mAb for aGVHD prevention and SR aGVHD therapy.