Posttransplantation cyclophosphamide prevents graft-versus-host disease by inducing alloreactive T cell dysfunction and suppression

TLDR

Posttransplantation cyclophosphamide has dramatically improved outcomes in allogeneic hematopoietic cell transplantation, yet its mechanisms were largely inferred from MHC‑matched murine models. The study aimed to elucidate how posttransplantation cyclophosphamide prevents graft‑versus‑host disease. To do so, the authors created a T‑cell–replete, MHC‑haploidentical murine HCT model (B6C3F1→B6D2F1) to assess alloreactive T‑cell elimination, intrathymic clonal deletion, and suppressor T‑cell induction. PTCy does not eliminate alloreactive T cells or require the thymus; instead it induces functional impairment of these cells and rapidly activates suppressive mechanisms, notably the preferential expansion of alloantigen‑specific regulatory T cells, thereby preventing GVHD and prompting a paradigm shift with direct clinical implications.

Abstract

Posttransplantation cyclophosphamide (PTCy) recently has had a marked impact on human allogeneic hematopoietic cell transplantation (HCT). Yet our understanding of how PTCy prevents graft-versus-host disease (GVHD) largely has been extrapolated from MHC-matched murine skin-allografting models that were highly contextual in their efficacy. Herein, we developed a T cell–replete, MHC-haploidentical, murine HCT model (B6C3F1→B6D2F1) to test the putative underlying mechanisms: alloreactive T cell elimination, alloreactive T cell intrathymic clonal deletion, and suppressor T cell induction. In this model and as confirmed in four others, PTCy did not eliminate alloreactive T cells identified using either specific Vβs or the 2C or 4C T cell receptors. Furthermore, the thymus was not necessary for PTCy's efficacy. Rather, PTCy induced alloreactive T cell functional impairment, which was supported by highly active suppressive mechanisms established within one day after PTCy that were sufficient to prevent new donor T cells from causing GVHD. These suppressive mechanisms included the rapid, preferential recovery of CD4+CD25+Foxp3+ regulatory T cells, including those that were alloantigen specific, which served an increasingly critical function over time. Our results prompt a paradigm shift in our mechanistic understanding of PTCy. These results have direct clinical implications for understanding tolerance induction and for rationally developing novel strategies to improve patient outcomes.