Graft-versus-leukaemia Immunity is Retained Following Treatment with Post-transplant Cyclophosphamide Alone or Combined with Tocilizumab in Humanised Mice

Objectives: Donor haematopoietic stem cell transplantation treats leukaemia by inducing graft-versus-leukaemia (GVL) immunity. However, this benefit is often mitigated by graft-versus-host disease (GVHD), which is reduced by post-transplant cyclophosphamide (PTCy) alone or combined with tocilizumab (TOC) in humanised mice. This study established a preclinical humanised mouse model of GVL and investigated whether PTCy alone or combined with TOC impacts GVL immunity.

Methods: NOD--IL2Rγ mice were injected with 2 × 10 human peripheral blood mononuclear cells (hPBMCs) on day 0 and with 1 × 10 THP-1 acute myeloid leukaemia cells on day 14. In subsequent experiments, mice were also injected with PTCy (33 mg kg) or Dulbecco's phosphate buffered saline (PBS) on days 3 and 4, alone or combined with TOC or control antibody (25 mg kg) twice weekly for 28 days. Clinical signs of disease were monitored until day 42.

Results: Mice with hPBMCs from three different donors and THP-1 cells showed similar survival, clinical score and weight loss. hCD33 leukaemia cells were minimal in mice reconstituted with hPBMCs from two donors but present in mice with hPBMCs from a third donor, suggesting donor-specific GVL responses. hPBMC-injected mice treated with PTCy alone or combined with TOC (PTCy + TOC) demonstrated prolonged survival compared to control mice. PTCy alone and PTCy + TOC-treated mice with hPBMCs showed minimal hepatic hCD33 leukaemia cells, indicating sustained GVL immunity. Further, the combination of PTCy + TOC reduced histological damage in the lung and liver.

Conclusion: Collectively, this research demonstrates that PTCy alone or combined with TOC impairs GVHD without compromising GVL immunity.