G-protein coupled receptor 15 (GPR15) is expressed on T-cells. We previously reported knockout of GPR15 increased acute graft-versus-host disease (GvHD) in mice. In this study, we identified thrombin receptor activating peptide-6 (TRAP-6, peptide sequence: SFLLRN) as an activator of GPR15. GRP15 and β-arrestin2 were needed for TRAP-6-mediated inhibition of mixed lymphocyte reactions. TRAP-6 decreased acute GvHD in allotransplant models in mice, an effect dependent on GPR15-expression in donor T-cells. RNA-seq and protein analyses indicated TRAP-6 increased binding of β-arrestin2/TAB1 and inhibited phosphorylation of TAK1 and NF-κB-P65. GPR15 is expressed differently on CD4+ T-cells and CD8+ T-cells. TRAP-6 inhibited phosphorylation of NF-κB-P65 in CD4+ T-cells but increased granzyme B expression in CD8+ T-cells. TRAP-6 decreased acute GvHD without inhibiting graft-versus-tumor (GvT) efficacy against A20 lymphoma cells. SALLRN, a mutant of TRAP-6, preserved the anti-acute GvHD effect but avoided the adverse effects of TRAP-6. TRAP-6 and SALLRN also decreased allogeneic and xenogeneic reactions induced by human blood mononuclear cells. In conclusion, TRAP-6 activated GPR15 on T-cells and decreased acute GvHD in mice without impairing GvT efficacy.
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