Samson, Luke

Abstract Allogeneic stem cell transplantation (alloSCT) is a highly effective, curative therapy for leukemia yet does not provide a survival benefit above autologous SCT in patients with multiple myeloma (MM). To explore this, we developed preclinical models of SCT using C57Bl/6 recipient mice and either C57Bl/6 (ASCT) or C3H.SW (alloSCT) donor grafts. Importantly, these models recapitulated the clinical setting whereby alloSCT provided superior outcomes, compared to ASCT, in recipients bearing MLL-AF9-driven acute myeloid leukemia (AML) but not in those bearing Vk*MYC-MM. Interestingly, we found that MM-specific, T cell-mediated immunity was generated after ASCT, which failed due to MM-induced T cell exhaustion. MM relapse after ASCT could be prevented by TIGIT or PD-1 targeted immune checkpoint inhibition or via depletion of suppressive CSF1-R+ myeloid cells. Conversely, after alloSCT, T cell exhaustion was driven principally by alloantigen and CD8 T cells expressed high levels of PD-1, TIGIT and TIM-3. Furthermore, Vk*MYC myeloma exploited this alloantigen-driven T cell exhaustion via expression of high levels of PD-L1 and CD155 (the cognate ligands for PD-1 and TIGIT), which were expressed minimally on MLL-AF9 AML. To exploit alloSCT in MM, we used post-transplant cyclophosphamide (PT-Cy) to delete high affinity alloreactive T cells that generate the exhausted donor T cell pool. Subsequent administration of CD137 agonists enhanced T cell activation and cytolytic activity within bone marrow, without exacerbating GVHD. Thus, PT-Cy provides a platform for optimizing immunotherapy after alloSCT.