Introduction: Induction of hematopoietic chimerism by donor bone marrow transplantation (BMT) is essential for achievement of allograft tolerance in clinical HLA mismatched kidney transplantation, while myelosuppressive toxicity of the radiation is yet to be solved. We have recently found that chimerism can be achieved with minimal total body irradiation in nonhuman primates (NHPs) by inhibiting Bcl-2 (anti-apoptotic protein) with Venetoclax (ABT-199). However, a minimal dose of TBI was still required for chimerism induction. Since Mcl-1, another member of the Bcl-2 family proteins, is highly expressed in hematopoietic stem cells (HSC) in bone marrow (BM), we hypothesized that Mcl-1 inhibition might delete the host HSC niche, thus facilitating donor bone marrow engraftment without TBI requirement.

Methods: HSC (CD34+CD90+CD45RA-) counts and Colony Forming Units (CFU) of BM aspirates were measured after treatment with ABT-199 (10mg/kg X11) alone (n=3), Mcl-1 inhibitor (S63845, 5mg/kg X 5) alone (n=3), or combination of both (n=3). Three recipients treated with the combination underwent BMT from the MHC mismatched donor, and were also treated with ATG (3 doses pre-transplant) and anti-CD154 and a 28 day-course of cyclosporine (Fig. 1a). Another NHP underwent skin transplantation seven weeks after BMT with the same regimen including S63845 and ABT-199.

Results: Monotherapy of ABT-199 or S63845 depleted CD34+ BM cells to 50 ± 13% and to 42±17% of pre-treatment levels, respectively. CFUs were also suppressed to 58 ± 8.6% and to 46 ± 17%, respectively. Combining ABT-199 with S63845 depleted more HSCs (22 ± 11%) with near complete depletion in two animals. CFUs were also effectively suppressed to 23 ± 8%(Fig. 1b). Since dual inhibition of both Mcl-1 and Bcl-2 most effectively depleted HSCs, BMT was perfomred using the combination of S63845 and ABT-199 (Fig. 1a). After conditioning, all recipients successfully developed multilineage chimerism, which lasted for 3 months, despite immunosuppression was completely discontinued at one month after BMT (Fig. 1c). Skin transplantation showed donor-specific tolerance with accepted donor skin graft and rejected two different third-party skin graft (Fig 1d).

Conclusion: Dual inhibition of Mcl-1 and Bcl-2 effectively depleted BM HSC, leading to successful hematopoietic chimerism induction without myeloablative treatments and donor-specific tolerance. This approach may set the path for the development of a novel and clinically applicable protocol for induction of hematopoietic chimerism without myeloablative treatments.