A single dose of a CD137 antibody-drug conjugate protects nonhuman primate allogeneic HCT recipients against acute GVHD Available to Purchase

• In NHPs, a single CD137-ADC dose post-HCT eliminates PD-1+ effector T-cells, enabling reconstitution of non-pathogenic T-cell subsets.

• The CD137-ADC offers long-term aGVHD-free survival but was associated with a higher risk of rhLCV (EBV homolog) reactivation and disease.

Rapid CD137 upregulation on alloreactive T-cells upon allogeneic stimulation suggests that their selective elimination could prevent acute graft-versus-host disease (aGVHD) after allogeneic hematopoietic stem cell transplantation (HCT). Here, we developed a novel aGVHD prophylactic regimen consisting of a single dose of an anti-CD137 antibody-drug conjugate (CD137-ADC) administered on the day of transplant without additional immunosuppression. The CD137-ADC depleted both human and non-human primate (NHP) activated T-cells and proved highly effective in preventing xenogeneic aGVHD in mice receiving human peripheral blood mononuclear cells (PBMC), as well as in NHP undergoing MHC-haploidentical HCT. Flow cytometry analysis of NHP T-cells indicated specific depletion of activated PD-1+ CD4 and CD8 T-cells, while sparing naïve and PD-1-OX40+ memory T-cell subsets during the first week after HCT. CD137-ADC-treated NHP recipients demonstrated robust hematopoietic and immune reconstitution. Hallmarks of T-cell recovery after CD137-ADC, which were associated with long-term aGVHD-free survival, included reconstitution of CD4 memory T-cells expressing TRAIL, terminally-differentiated CD8 T-cells expressing CX3CR1, and CD4 FoxP3+ Tregs - cell types not expected to be involved in aGVHD pathogenesis. CD137-ADC-treated recipients demonstrated a higher risk of reactivation of rhLCV (the rhesus macaque EBV analogue), which was associated with reconstitution of follicular helper T-cells, interferon signaling-associated memory, and gamma-delta T-cell subsets. This reactivation was controllable with rituximab administration. These results document effective depletion of alloreactive T-cells and prevention of aGVHD following a single dose of CD137-ADC, suggesting that clinical translation should be carefully explored.