Regulatory T cells (Tregs) suppress graft-versus-host disease (GVHD) while preserving a beneficial graft-versus-leukemia (GVL) effect. Thus, their use in allogeneic stem cell transplantation (SCT) provides a promising strategy to treat GVHD. However, 3 obstacles prevent their routine use in human clinical trials: (1) low circulating number of Tregs in peripheral blood, (2) loss of suppressor function after in vitro expansion, and (3) lack of Treg-specific surface markers necessary for efficient purification. FOXP3 is exclusively expressed in Tregs and forced expression in CD4+CD25− T cells can convert these non-Tregs into Tregs with functional suppressor function. Here, we show that the FDA-approved hypomethylating agents, decitabine (Dec) and azacitidine (AzaC), induce FOXP3 expression in CD4+CD25− T cells both in vitro and in vivo. Their suppressor function is dependent on direct contact, partially dependent on perforin 1 (Prf1), but independent of granzyme B (GzmB), and surprisingly, Foxp3. Independence of Foxp3 suggests that genes responsible for the suppressor function are also regulated by DNA methylation. We have identified 48 candidate genes for future studies. Finally, AzaC treatment of mice that received a transplant of major histocompatibility complex mismatched allogeneic bone marrow and T cells mitigates GVHD while preserving GVL by peripheral conversion of alloreactive effector T cells into FOXP3+ Tregs and epigenetic modulation of genes downstream of Foxp3 required for the suppressor function of Tregs.

Decitabine is a hypomethylating agent that irreversibly inhibits DNA methyltransferase I, inducing leukemic differentiation and re-expression of epigenetically silenced putative tumor antigens. We assessed safety and efficacy of decitabine maintenance after allogeneic transplantation for acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS). Decitabine maintenance may help eradicate minimal residual disease, decrease the incidence of graft-versus-host disease (GVHD), and facilitate a graft-versus-leukemia effect by enhancing the effect of T regulatory lymphocytes. Patients with AML/MDS in complete remission (CR) after allotransplantation started decitabine between day +50 and +100. We investigated 4 decitabine doses in cohorts of 4 patients: 5, 7.5, 10, and 15 mg/m(2)/day × 5 days every 6 weeks, for a maximum 8 cycles. The maximum tolerated dose (MTD) was defined as the maximum dose at which ≤ 25% of people experience dose-limiting toxicities during the first cycle of treatment. Twenty-four patients were enrolled and 22 were evaluable. All 4 dose levels were completed and no MTD was reached. Overall, decitabine maintenance was well tolerated. Grade 3 and 4 hematological toxicities were experienced by 75% of patients, including all patients treated at the highest dose level. Nine patients completed all 8 cycles and 8 of them remain in CR. Nine patients died from relapse (n = 4), infectious complications (n = 3), and GVHD (n = 2). Most occurrences of acute GVHD were mild and resolved without interruption of treatment; 1 patient died of acute gut GVHD. Decitabine maintenance did not clearly impact the rate of chronic GVHD. Although there was a trend of increased FOXP3 expression, results were not statistically significant. In conclusion, decitabine maintenance is associated with acceptable toxicities when given in the post-allotransplantation setting. Although the MTD was not reached, the dose of 10 mg/m(2) for 5 days every 6 weeks appeared to be the optimal dose rather than 15 mg/m(2), where most hematological toxicities occurred.

The clinical goal of allogeneic hematopoietic stem cell transplantation (allo-HSCT) is to minimize GVHD while maintaining GvL. Here, we show that interferon γ receptor-deficient (IFNγR−/−) allogeneic Tconv, which possess normal alloreactivity and cytotoxicity, induce significantly less GVHD than wild-type (WT) Tconv. This effect is mediated by altered trafficking of IFNγR−/− Tconv to GVHD target organs, especially the gastrointestinal (GI) tract. We show that the chemokine receptor CXCR3 is induced via IFNγR-mediated signaling and partially contributes to the trafficking of WT Tconv to GVHD target organs. Indeed, CXCR3−/− Tconv recapitulate the reduced GVHD potential of IFNγR−/− Tconv in a minor-mismatched GVHD model. Most importantly, IFNγR−/− (and CXCR3−/−) Tconv mediate a robust and beneficial GvL effect. In addition, we show that IFNγR−/− regulatory T cells (Tregs) are fully suppressive in vitro although defective in suppressor function in vivo and that WT Tregs suppress GVHD in vivo only when allogeneic Tconv produce interferon γ (IFNγ), suggesting that the IFNγR signaling pathway is the major mechanism for both Tregs and Tconv to migrate to GVHD target organs. Finally, pharmacologic inhibition of IFNγR signaling with inhibitors of JAK1/JAK2, which are mediators of IFNγR signaling, results in the decreased expression of CXCR3 and reduced GVHD and improved survival after allo-HSCT and this effect is mediated by altered trafficking of Tconv to GVHD target organs.

We have recently reported that interferon gamma receptor deficient (IFNγR−/−) allogeneic donor T cells result in significantly less graft-versus-host disease (GvHD) than wild-type (WT) T cells, while maintaining an anti-leukemia or graft-versus-leukemia (GvL) effect after allogeneic hematopoietic stem cell transplantation (allo-HSCT). We demonstrated that IFNγR signaling regulates alloreactive T cell trafficking to GvHD target organs through expression of the chemokine receptor CXCR3 in alloreactive T cells. Since IFNγR signaling is mediated via JAK1/JAK2, we tested the effect of JAK1/JAK2 inhibition on GvHD. While we demonstrated that pharmacologic blockade of JAK1/JAK2 in WT T cells using the JAK1/JAK2 inhibitor, INCB018424 (Ruxolitinib), resulted in a similar effect to IFNγR−/− T cells both in vitro (reduction of CXCR3 expression in T cells) and in vivo (mitigation of GvHD after allo-HSCT), it remains to be determined if in vivo administration of INCB018424 will result in preservation of GvL while reducing GvHD. Here, we report that INCB018424 reduces GvHD and preserves the beneficial GvL effect in two different murine MHC-mismatched allo-HSCT models and using two different murine leukemia models (lymphoid leukemia and myeloid leukemia). In addition, prolonged administration of INCB018424 further improves survival after allo-HSCT and is superior to other JAK1/JAK2 inhibitors, such as TG101348 or AZD1480. These data suggest that pharmacologic inhibition of JAK1/JAK2 might be a promising therapeutic approach to achieve the beneficial anti-leukemia effect and overcome HLA-barriers in allo-HSCT. It might also be exploited in other diseases besides GvHD, such as organ transplant rejection, chronic inflammatory diseases and autoimmune diseases.