Role of the aryl hydrocarbon receptor in thymocyte emigration in vivo.

The aryl hydrocarbon receptor (AHR) is a ligand-dependent member of the PAS-bHLH-family of nuclear receptors. Anthropogenic ligands include environmental contaminants such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Over-activation of the AHR causes thymus atrophy and immunosuppression. Signaling via the AHR changes the thymocyte differentiation program at several checkpoints, in particular within the CD4-CD8- double-negative (DN) thymocyte subset. Here, we show that AHR over-activation led to the preferential emigration of DN thymocytes to the periphery and accumulation in the spleen. Some of these recent thymic emigrants (RTE) had a novel "activated immature" phenotype (CD3-TCRbeta-CD25+/intCD44-CD45RB+/intCD62L+CD69- cells). Gene expression profiling of DN RTE revealed 15 genes that were up-regulated more than threefold by TCDD, including the S100A9 gene. Exposure of S100A9 null mice to TCDD showed a role for this protein in AHR-mediated thymic egress.