Purpose: Graft versus host disease (GVHD) is the major complication of allogeneic bone marrow transplantation (BMT) and limits the therapeutic efficacy of this modality. Although the role of natural T-regulatory cells (nTreg) in attenuating GVHD has been extensively examined, the ability of induced T-regulatory cells (iTreg) to mitigate GVHD is unknown. The purpose of this study was to examine the ability of in vitro and in vivo iTregs to abrogate GVHD.

Experimental Design: We examined the ability of in vitro differentiated and in vivo iTregs to reduce the severity of GVHD in a clinically relevant mouse model of BMT. The effect of blockade of interleukin (IL) 6 signaling on the efficacy of these Treg populations was also studied.

Results: In vitro differentiated iTregs fail to protect mice from lethal GVHD even when administered at high Treg:effector T-cell ratios. Lack of GVHD protection was associated with loss of Foxp3 expression and in vivo reversion of these cells to a proinflammatory phenotype characterized by secretion of IFN-γ. Phenotypic reversion could not be abrogated by blockade of IL-6 signaling or by in vitro exposure of iTregs to all-trans retinoic acid. In contrast, the in vivo induction of iTregs was significantly augmented by IL-6 blockade and this resulted in reduced GVHD.

Conclusion: Instability of Foxp3 expression limits the utility of adoptively transferred iTregs as a source of cellular therapy for the abrogation of GVHD. Blockade of IL-6 signaling augments the ability of in vivo iTregs to prevent GVHD but has no effect on in vitro differentiated iTregs. Clin Cancer Res; 17(12); 3969–83. ©2011 AACR.