Early window of susceptibility of allospecific CD4+T cells to AhR-induced suppression reflects excess production of IL-2, over-expression of regulatory genes and repression of cell cycle genes.

Abstract Activation of AhR in CD4+ T cells leads to suppression of Th1-, Th2- and Th17-mediated immune responses. In allograft models, the first 3 days of the immune response represent a critical window for AhR activation during which significant changes in donor CD4+ T cells occur. However, little is known about the mechanisms that underlie this early window of CD4+ T cell sensitivity to AhR activation and whether limiting AhR signaling to this early window is sufficient to suppress the allograft response. We used a rapidly metabolized AhR ligand, 10-Cl-BBQ, to show that early AhR activation alone is indeed sufficient to suppress GVHD. Early activation of AhR was also sufficient to increase the frequency of CD4+ Foxp3+ T cells on day 15 of the GVH response. AhR activation induced an early increase in IL-2 production by donor CD4+ T cells that was responsible for the Tr1-like phenotype on day 2 and the Foxp3+ Treg phenotype on day 15. However, blockade of IL-2 signaling, which prevented the induction of Foxp3+ Tregs, failed to alter AhR-induced suppression of GVHD. Global gene expression analysis of purified alloresponding donor CD4+ T cells on day 2 and 3 of the GVH response revealed up-regulation of many genes associated with regulatory T cells and down-regulation of genes involved in cell cycle progression. Reduced cell cycling was validated by reduced BrdU incorporation on day 4 of the GVH response. These results are consistent with a model of AhR signaling during naïve CD4+ T cell activation leading to induction of unique AhR-Tregs that induce premature cell-cycle arrest in effector CD4+ T cells; the lack of mature CD4+ T helper cells prevents the maturation and differentiation of allospecific CD8+ CTL leading to suppression of GVHD.