Mechanisms of Graft-versus-Host Disease Prevention by Post-transplantation Cyclophosphamide: An Evolving Understanding

Abstract

Post-transplantation cyclophosphamide (PTCy) has been highly successful at preventing severe acute and chronic graft-versus-host disease (GVHD) after allogeneic hematopoietic cell transplantation (HCT). The clinical application of this approach was based on extensive studies in major histocompatibility complex (MHC)-matched murine skin allografting models, in which cyclophosphamide was believed to act via three main mechanisms: (1) selective elimination of alloreactive T cells; (2) intrathymic clonal deletion of alloreactive T-cell precursors; and (3) induction of suppressor T cells. In these models, cyclophosphamide was only effective in very specific contexts, requiring particular cell dose, cell source, PTCy dose, and recipient age. Achievement of transient mixed chimerism also was required. Furthermore, these studies showed differences in the impact of cyclophosphamide on transplanted cells (tumor) versus tissue (skin grafts), including the ability of cyclophosphamide to prevent rejection of the former but not the latter after MHC-mismatched transplants. Yet, clinically PTCy has demonstrated efficacy in MHC-matched or partially-MHC-mismatched HCT across a wide array of patients and HCT platforms. Importantly, clinically significant acute GVHD occurs frequently after PTCy, inconsistent with alloreactive T-cell elimination, whereas PTCy is most active against severe acute GVHD and chronic GVHD. These differences between murine skin allografting and clinical HCT suggest that the above-mentioned mechanisms may not be responsible for GVHD prevention by PTCy. Indeed, recent work by our group in murine HCT has shown that PTCy does not eliminate alloreactive T cells nor is the thymus necessary for PTCy's efficacy. Instead, other mechanisms appear to be playing important roles, including: (1) reduction of alloreactive CD4+ effector T-cell proliferation; (2) induced functional impairment of surviving alloreactive CD4+ and CD8+ effector T cells; and (3) preferential recovery of CD4+ regulatory T cells. Herein, we review the history of cyclophosphamide's use in preventing murine skin allograft rejection and our evolving new understanding of the mechanisms underlying its efficacy in preventing GVHD after HCT. Efforts are ongoing to more fully refine and elaborate this proposed new working model. The completion of this effort will provide critical insight relevant for the rational design of novel approaches to improve outcomes for PTCy-treated patients and for the induction of tolerance in other clinical contexts.

Reconstitution of T Cell Subsets Following Allogeneic Hematopoietic Cell Transplantation

“…Proliferation of innate immune cells is already higher within CB grafts and innate IR has been shown to be more rapid after CBT compared to BMT/PBT [2,17,53]. In contrast to ATG, posttransplant Cy is suggested to induce T helper cell dysfunction rather than elimination and thymic clonal deletion, thereby reducing both acute and chronic GvHD occurrence [55,56]. Together, adapting the dosing schemes of drugs used as standard-of-care or simply adjusting the order of different treatment modalities can be considered "low hanging fruit" for the improvement of survival chances post-HCT.…”

Acute Graft‐versus‐Host Disease: Emerging Insights and Updates into Detection, Prevention, and Treatment

“…Early studies determined a calcineurin inhibitor (tacrolimus or cyclosporine) combined with methotrexate to be the standard for acute GVHD prevention. 36,37 Other options, such as combination sirolimus plus tacrolimus, 38 rabbit antithymocyte globulin, 39 or posttransplantation cyclophosphamide as single agent or in combination with a calcineurin inhibitor 40 have also proven to be effective regardless of donor type. However, given the wide variability in incidence and severity of acute GVHD, other preventative strategies are needed.…”

How to Make an Immune System and a Foreign Host Quickly Cohabit in Peace? The Challenge of Acute Graft-Versus-Host Disease Prevention After Allogeneic Hematopoietic Cell Transplantation

“…This was made possible thanks to the development of innovative platforms for GVHD prevention in this peculiar high alloreactivity setting. Among them, the advent of posttransplant cyclophosphamide (PTCy) has revolutionized this procedure and can be considered as one of the major advances in the field of alloHCT over the past two decades (135)(136)(137)(138)(139). This approach, designed by the John Hopkins University group in Baltimore, consists in the administration of (one or) two boluses of high dose cyclophosphamide (Cy, a nitrogen mustard alkylating agent) shortly after alloHCT (day +3 and/or +4) followed by MMF/tacro prophylaxis (starting from day +5).…”

“…This approach, designed by the John Hopkins University group in Baltimore, consists in the administration of (one or) two boluses of high dose cyclophosphamide (Cy, a nitrogen mustard alkylating agent) shortly after alloHCT (day +3 and/or +4) followed by MMF/tacro prophylaxis (starting from day +5). The initial rationale of this strategy mostly assumed to be a cytotoxic and selective depletion of highly proliferative T eff (supposed to be the newly primed alloreactive T cell clones during the first days after the graft infusion) (Figure 1), while preserving resting hematopoietic stem cells and non-alloreactive T cells (such as anti-infectious memory T cells) (135). Additional researches further demonstrated that PTCy also induces central tolerance by additional intrathymic clonal deletion of alloreactive T cell precursors (140,141).…”

“…Additional researches further demonstrated that PTCy also induces central tolerance by additional intrathymic clonal deletion of alloreactive T cell precursors (140,141). Moreover, it was recently suggested that beyond these effects on T eff , PTCymediated protection against GVHD also (and mainly) relies on the promotion of T reg and the induction of tolerance (135,141). T reg are indeed less sensitive than T eff to the Cy cytotoxic effects due to their higher expression of aldehyde dehydrogenase (the major detoxifying enzyme for cyclophosphamide) (142).…”