Investigations of the AhR-induced Treg : transcriptional regulation and suppressive mechanisms Public Deposited

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  • The aryl hydrocarbon receptor (AhR) has recently been described as a novel therapeutic target, given the potent suppression of multiple immune-mediated diseases following activation by the prototypic ligand, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). In the parent-into-F1 graft-versus-host (GVH) model, suppression of the cytotoxic T-lymphocyte (CTL) response is associated with the presence of CD25⁺CTLA-4⁺IL-10⁺Foxp3[superscript neg] regulatory donor CD4⁺ T-cells (AhR-Tregs). These AhR-Treg have a suppressive capacity greater than thymus-derived natural Treg. Therefore it is hypothesized that suppression of the CTL response is mediated by the AhR-Treg. The purpose of these studies was two-fold: 1) to identify mechanisms by which TCDD-activated AhR induces AhR-Tregs and 2) to determine the mechanism by which AhR-Tregs suppress the CTL response. To address the first aim, we hypothesized that the AhR, a transcription factor, may directly influence gene expression during CD4⁺ T-cell activation and differentiation, thereby driving their differentiation into AhR-Treg. T-cell differentiation is dependent upon expression of certain transcription factors and lineage-specific genes for proteins and cytokines. Therefore, gene expression associated with CD4⁺ T-cell activation and/or differentiation was evaluated in CD4⁺ T-cells cultured under different polarizing conditions. Unexpectedly, treatment with TCDD did not alter lineage-specific genes that drive CD4⁺ T-cell differentiation. Apart from the known AhR-regulated genes Cyp1a1, Cyp1b1 and Ahrr, only expression of Il22 was inappropriately up- regulated across all conditions tested. The increase in Il22 expression translated to increased IL-22 protein n. Of the 49 genes evaluated there were no other changes in gene expression consistent across polarizing conditions. This suggests that the role of AhR is superimposed upon the cytokine milieu, and therefore may have been obscured by the use of strongly polarizing cytokines. Ultimately, the results cast doubt on the use of in vitro conditions to recapitulate in vivo events, although the role of IL-22 in AhR biology remains of interest. To address the second part of my hypotheses, the suppressive mechanism(s) of the AhR-Treg were examined, focusing primarily on the increased expression of CTLA-4, CD25 and IL-10 seen on day 2 of the GVH response in previous studies. Expression of CTLA-4 induces the IFNγ - IDO tolerogenic pathway. Expression of CD25, the high-affinity subunit for the IL-2R is associated with regulatory function, through induction of tolerogenic cytokines and sequestration of IL-2. The immunosuppressive cytokine IL-10 is known to inhibit T-cell activation and expansion. Independent blockade of CTLA-4 and IDO function revealed that the AhR-Treg do not appear to use the IDO tolerogenic pathway to mediate suppression. Furthermore, antibody-mediated blockade of neither CD25 nor IL-10 were able to alleviate TCDD-mediated immunosuppression. During these studies we discovered that treatment with TCDD induced Foxp3, the transcription factor for Treg, in donor CD4⁺ and CD8⁺ T-cells 15 days after adoptive transfer. Interestingly, the up-regulation of Foxp3 was also seen in un-activated host cells, indicating non-specific regulation of Foxp3 by TCDD-activated AhR. Furthermore, concurrent blockade of CD25 and Foxp3 had no effect on TCDD's ability to suppress the GVH response. In sum, these studies suggest that the AhR-Treg is a novel regulatory cell, with characteristics of a Treg and increased IL-22 production. Regulatory T-cells have a wide repertoire of suppressive mechanisms, including production of TGFβ1 and TGFβ3, sequestration of IL-2 and granzyme B. Understanding the mechanism of suppression utilized by the AhR-Treg is crucial to develop the AhR as a novel therapeutic target. Future studies will continue to evaluate the potential for the AhR-Treg to use alternative mechanisms to suppress T-dependent immune diseases.
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