Thrombin receptor activating peptide-6 decreases acute graft- versus -host disease through activating GPR15

G-protein coupled receptor 15 (GPR15) is expressed on T-cells. We previously reported knockout of GPR15 increased acute graft- versus -host disease (G v HD) 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 G v HD 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 (G v T) efficacy against A20 lymphoma cells. SALLRN, a mutant of TRAP-6, preserved the anti-acute G v HD 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 G v HD in mice without impairing G v T efficacy.