Sirolimus, Post–Transplant Cyclophosphamide, and CTLA4-Ig Maintain Stable Mixed Chimerism In a Mismatched Murine Model

Abstract

Abstract Hematopoietic stem cell transplantation (HSCT) is the only curative option for patients with sickle cell disease (SCD). We sought to extend our haploidentical HSCT regimen with nonmyeloablative conditioning. The objective of our study was to assess the ability of the immunosuppressive agent CTLA4-Ig to induce mixed chimerism in the haploidentical HSCT setting when combined with sirolimus, post-transplant cyclophosphamide (PT-Cy), or both. We used a mismatched mouse model with BALB/c donors and C57BL/6 recipients. Recipient mice received 200 cGy TBI on day of the transplant and were conditioned with or without sirolimus, PT-Cy, and/or CTLA4-Ig. Our data show that when CTLA4-Ig alone or CTLA4-Ig/PT-Cy are given, donor cells are not detected. However, mixed chimerism is maintained in mice who received either sirolimus/PT-Cy or sirolimus/PT-Cy given together with CTLA4-Ig over 60 days. Additionally our data indicate a significant increase in recovery of donor cells within CD8 T-cell, CD19 B-cell, and CD11b myeloid cell populations in groups conditioned with sirolimus/PT-Cy, or sirolimus/PT-Cy/CTLA4-Ig as compared to groups conditioned with CTLA4-Ig (p=0.003; p=0.002; p=0.049) alone or CTLA4-Ig/PT-Cy (p=0.001; p<0.0001; p=0.001) at 30 days, respectively. These data demonstrate that sirolimus in combination with PT-Cy and CTLA4-Ig is synergistic in an MHC-mismatched mouse model. The novel tri-drug group establishes mixed chimerism while conditioning with CTLA4-Ig alone or CTLA4-Ig/PT-Cy does not. The data also identify induction of stable mixed chimerism in specific cellular subpopulations within the sirolimus/PT-Cy and sirolimus/PT-Cy/CTLA4-Ig groups, indicating that these cells may play an important role in HCST.