Abstract
The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that plays multiple roles in regulation of immune and inflammatory responses. The ability of certain AhR ligands to induce regulatory T cells (Tregs) has generated interest in developing AhR ligands for therapeutic treatment of immune-mediated diseases. To this end, we designed a screen for novel Treg-inducing compounds based on our understanding of the mechanisms of Treg induction by the well-characterized immunosuppressive AhR ligand, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). We screened a ChemBridge small molecule library and identified 10-chloro-7H-benzimidazo[2,1-a]benzo[de]Iso-quinolin-7-one (10-Cl-BBQ) as a potent AhR ligand that was rapidly metabolized and not cytotoxic to proliferating T cells. Like TCDD,10-Cl-BBQ altered donor CD4+ T cell differentiation during the early stages of a graft versus host (GVH) response resulting in expression of high levels of CD25, CTLA-4 and ICOS, as well as several genes associated with Treg function. The Treg phenotype required AhR expression in the donor CD4+ T cells. Foxp3 was not expressed in the AhR-induced Tregs implicating AhR as an independent transcription factor for Treg induction. Structure-activity studies showed that unsubstituted BBQ as well as 4, 11-dichloro-BBQ were capable of inducing AhR-Tregs. Other substitutions reduced activation of AhR. Daily treatment with 10-Cl-BBQ during the GVH response prevented development of GVH disease in an AhR-dependent manner with no overt toxicity. Together, our data provide strong support for development of select BBQs that activate the AhR to induce Tregs for treatment of immune-mediated diseases.
Highlights
The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that is gaining increasing attention as an important regulator of many aspects of immunological function
10-Cl-BBQ induced a battery of genes that are known to be associated with AhR activation by TCDD (Fig. S1) [24]. 10-Cl-BBQ did not inhibit proliferation of activated T cells in vitro or in vivo (Fig. S2), nor did it induce acute hepatic toxicity in mice (Table S1)
Based on studies with TCDD, these AhRTregs are induced by direct activation of AhR in donor CD4+ T cells [4,12] and differ from the Foxp3+ Tregs that are induced indirectly via AhR signaling in dendritic cells [3,9]
Summary
The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that is gaining increasing attention as an important regulator of many aspects of immunological function. An AhRdependent Treg phenotype emerged during the initial activation and expansion of the naıve allospecific CD4+ donor T cells and was characterized by high expression of CD25 and CTLA-4 but no Foxp, implicating AhR as a unique transcription factor driving Treg development [4] These AhR-induced Tregs (AhRTregs) showed potent suppression of naıve and allogeneic T cell proliferation in vitro and increased expression of several genes that have been associated with other types of Tregs [4,13]. These Foxp3+ Tregs emerge over several days or weeks in different autoimmune disease models and may develop indirectly via AhR-dependent induction of tolerogenic dendritic cells [3,9] Both types of Tregs likely contribute to control of autoimmune diseases following treatment with AhR ligands [4,5,15,16]
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