Hematopoietic stem cell transplantation (HSCT) is the standard of care for various malignant and non-malignant hematologic diseases and has been used for other diseases such as autoimmune diseases and transplant tolerance induction. In order to achieve implantation of donor HSCs, it is necessary to first deplete the host's own HSCs. However, currently used clearing pretreatment regimens such as radiotherapy are associated with severe myelosuppression and other toxicities such as infections and infertility due to their non-selective nature. Therefore, a safer treatment option is needed.
Tatsuo Kawai's team from Harvard Medical School published an article in Science Translational Medicine titled "Selective Bcl-2 inhibition promotes hematopoietic chimerism and allograft tolerance without myelosuppression in nonhuman primates". Using a clinically relevant non-human primate (NHP) model, they found that selective inhibition of B-cell lymphoma 2 (Bcl-2) effectively depletes peripheral lymphocytes and partially removes HSCs, while preserving myeloid cells and Treg cells. Although Bcl-2 inhibition alone is not sufficient to induce hematopoietic chimerism, the addition of a Bcl-2 inhibitor to the pretreatment regimen promotes hematopoietic chimerism and renal allograft tolerance at half the previously administered dose of systemic irradiation. Thus, selective inhibition of Bcl-2 can induce hematopoietic chimerism without myelosuppression, thus making HSCT more suitable for a variety of clinical indications.
Previous work noted that inhibition of Bcl-2 circumvents the problem of myelosuppression while promoting mixed chimerism of hematopoietic cells in mice. Based on this, the authors conducted a study using Venetoclax, an FDA-approved BCL-2 inhibitor (Bcl-2i). To assess the effect of Bcl-2i in the induction of apoptosis, the authors cultured peripheral blood cells and myeloid cells in vitro with different concentrations of Venetoclax. In vitro assays showed dose-dependent activation of caspase-3/7 and apoptosis of T, B, and NK cells. In contrast, no significant changes were observed in granulocytes and monocytes. In vivo testing of NHP observed dose-dependent apoptosis of HSCs and hematopoietic progenitor cells, in addition to yielding results consistent with those in vitro. These findings suggest that Venetoclax is effective in inducing apoptosis in peripheral lymphocytes and HSCs, while preserving myeloid cells.
These results led the authors to postulate that Venetoclax, as part of a comprehensive pretreatment regimen for kidney bone marrow transplantation (CKBMT) that also includes myelosuppression, could generate hematopoietic chimerism and mitigate the effects of systemic irradiation (TBI). In group A, NHPs received the standard pretreatment regimen of 3-Gy TBI, 7-Gy local thymic irradiation, and pretransplant anti-thymocyte globulin (ATG), while in group B, the models received the standard pretreatment regimen with anti-CD154 monoclonal blocking and cyclosporine A (CyA). Group B had no TBI, Group C received TBI at a decreased dose of 1.5 Gy, and Group D received no TBI at all.
Analysis of these four groups revealed that individuals in group C were present exhibiting significant chimerism, whereas few donor chimeric cells were present in group B and no chimerism was induced at group D, indicating that TBI is an unnecessary link. No granulocytopenia, thrombocytopenia, or anemia was observed in group C recipients. In contrast, allogeneic cytopenia was observed in group A recipients who did not receive Bcl-2i treatment.
In addition, six of the eight recipients in group A achieved long-term survival without immunosuppression (> 300 days), and although the allografts were stably tolerated and half of these recipients were rejection-free, the remaining three grafts eventually failed due to the development of donor-specific antibodies (DSA) and chronic rejection. In group B, only a low degree of chimerism was achieved, with four of the five recipients experiencing acute rejection within 6 months, and although the remaining recipients in this group remained alive, DSA had developed. In group C, a higher degree of mixed chimerism resulted in more stable allograft tolerance, with five of the six recipients achieving long-term survival (> 300 days) without rejection. Kaplan-Meier analysis revealed significant differences between groups B and C in renal allograft survival (P < 0.05) and rejection-free allograft survival. However, in group D without TBI, long-term survival was not achieved in either recipient due to acute rejection.
The study concludes that targeted inhibition of Bcl-2 with Venetoclax increases hematopoietic chimerism without myelosuppression, leading to steady, long-term immunosuppression-free renal allograft survival in a clinically relevant NHP transplantation model. These findings not only pave the way for the development of less toxic pretreatment regimens that can be utilized in conjunction with HSCT to treat a wide variety of diseases, but they also eliminate a key barrier to the induction of allograft renal tolerance in existing approaches.