Low Dose Post-transplant Cyclophosphamide and Sirolimus Induce Mixed Chimerism with CTLA4-Ig or Lymphocyte Depletion in an MHC-Mismatched Murine Allotransplantation Model

Abstract

Abstract Allogeneic hematopoietic stem cell transplantation (allo-HCT) offers a curative option for patients with non-malignant hematological diseases. High-dose post-transplant cyclophosphamide (PT-Cy, 200 mg/kg) and sirolimus (Sir, 3 mg/kg) synergistically induce stable mixed chimerism. Further, Sir and cytotoxic T lymphocyte-associated antigen-4 immunoglobulin (CTLA4-Ig) promote immune tolerance and allograft survival. Here, in a major histocompatibility complex (MHC)-mismatched allo-HCT murine model, we combined CTLA4-Ig and T-cell depleting anti-Thy1.2 with a lower dose of PT-Cy. We demonstrate that low-dose PT-Cy and Sir combined with anti-Thy1.2, with or without CTLA4-Ig, induced similar donor chimerism levels compared to high-dose PT-Cy and Sir. Engrafted groups displayed significantly reduced frequencies of recipient-specific interferon-γ-producing T cells and natural killer (NK) cells. Interestingly, an increased frequency in regulatory T cells (Tregs) was found in engrafted mice except when CTLA4-Ig is combined with low-dose PT-Cy. Splenocytes collected from engrafted mice showed no proliferation upon restimulation with Balb/c stimulators. Collectively, low-dose PT-Cy and Sir combined with Thy1.2 with or without CTLA4-Ig induced mixed chimerism and Treg mediated donor unresponsiveness and reduced inflammatory T cells and B cells. In combination with CTLA4-Ig or lymphocyte depletion, low-dose PT-Cy and Sir may be considered in future regimens to reduce graft rejection in patients who undergo allo-HCT.