Debbie Watson

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‐ scid ‐IL2Rγ null mice were injected with 2 × 10 7 human peripheral blood mononuclear cells (hPBMCs) on day 0 and with 1 × 10 6 THP‐1 acute myeloid leukaemia cells on day 14. In subsequent experiments, mice were also injected with PTCy (33 mg kg −1 ) or Dulbecco's phosphate buffered saline (PBS) on days 3 and 4, alone or combined with TOC or control antibody (25 mg kg −1 ) 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.