Post-transplant cyclophosphamide prevents while depleting proliferating regulatory T cells

Graft-versus-host disease (GVHD) remains a serious limitation of allogeneic he-matopoietic cell transplantation (allo-HCT). While post-transplant administra-tion of cyclophosphamide (PTCy) is increasingly used as GVHD prophylaxis, its precise mechanisms of action and its impact on graft-versus-leukemia effects have remained debated. Here, we studied the mechanisms of xenogeneic GVHD (xGVHD) prevention by PTCy in different humanized mouse models. We observed that PTCy attenuated xGVHD. Using flow cytometry and single -cell RNA-sequencing, we demonstrated that PTCy depleted proliferative CD8* and conventional CD4* T cells but also proliferative regulatory T cells (Treg). Further, T-cell receptor b variable region sequencing (TCRVB) analyses demonstrated that highly xenoreactive T-cell clones were depleted by PTCy. Although Treg frequencies were significantly higher in PTCy-treated than in control mice on day 21, xGVHD attenuation by PTCy was not abrogated by Treg depletion. Finally, we observed that PTCy did not abrogate graft -versus-leukemia effects.

Automated summary

Post-transplant administration of cyclophosphamide (PTCy) can attenuate xenogeneic graft-versus-host disease (xGVHD) by depleting proliferative T cells and highly xenoreactive T-cell clones, while not affecting graft-versus-leukemia effects.