Synergistic venetoclax and anti-CD3 therapy for control of alloreactive response in hematopoietic stem cell transplantation.

T cell-based immunotherapies are effective for treating hematological malignancies and promoting lifelong graft tolerance after solid organ transplantation. Recipient preconditioning resulting in profound lymphodepletion is a key to preventing graft rejection and maximizing T cell engraftment. The 2 standard lymphodepleting agents, antithymocyte globulin and cyclophosphamide chemotherapy, are associated with major adverse effects in patients. This research investigates a new therapeutic approach that combines anti-CD3 (aCD3) therapy with venetoclax, as a BCL-2 inhibitor and T cell immunoglobulin and mucin 1 blockade to achieve optimal T cell lymphodepletion and immune tolerance in preclinical hematopoietic stem cell transplantation models. This conditioning regimen strongly induces T cell apoptosis, particularly in naïve and central memory CD8+ T cells, and simultaneously enhances regulatory T cells and tolerogenic dendritic cells. This combination therapy improved donor engraftment, promoted allograft tolerance, and prevented graft-versus-host disease in murine hematopoietic stem cell transplantation and pancreatic islet transplantation models. This aCD3 targeted approach is a more precise and less toxic lymphodepletion strategy, as demonstrated by reduced organ damage and inflammatory response, compared to posttransplant cyclophosphamide. Our study introduces a novel aCD3-based conditioning approach which enables successful engraftment, mixed chimerism, and tolerance induction.