Activation of the aryl hydrocarbon receptor inhibits neuropilin-1 upregulation on IL-2-responding CD4(+) T cells

Neuropilin-1 (Nrp1), a transmembrane protein expressed on CD4(+) T cells, is mostly studied in the context of regulatory T cell (Treg) function. More recently, there is increasing evidence that Nrp1 is also highly expressed on activated effector T cells and that increases in these Nrp1-expressing CD4(+) T cells correspond with immunopathology across several T cell-dependent disease models. Thus, Nrp1 may be implicated in the identification and function of immunopathologic T cells. Nrp1 downregulation in CD4(+) T cells is one of the strongest transcriptional changes in response to immunoregulatory compounds that act though the aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor. To better understand the link between AhR and Nrp1 expression on CD4(+) T cells, Nrp1 expression was assessed in vivo and in vitro following AhR ligand treatment. In the current study, we identified that the percentage of Nrp1 expressing CD4(+) T cells increases over the course of activation and proliferation in vivo. The actively dividing Nrp1(+)Foxp3(-) cells express the classic effector phenotype of CD44(hi)CD45RB(lo), and the increase in Nrp1(+)Foxp3(-) cells is prevented by AhR activation. In contrast, Nrp1 expression is not modulated by AhR activation in non-proliferating CD4(+) T cells. The downregulation of Nrp1 on CD4(+) T cells was recapitulated in vitro in cells isolated from C57BL/6 and NOD (non-obese diabetic) mice. CD4(+)Foxp3(-) cells expressing CD25, stimulated with IL-2, or differentiated into Th1 cells, were particularly sensitive to AhR-mediated inhibition of Nrp1 upregulation. IL-2 was necessary for AhR-dependent downregulation of Nrp1 expression both in vitro and in vivo. Collectively, the data demonstrate that Nrp1 is a CD4(+) T cell activation marker and that regulation of Nrp1 could be a previously undescribed mechanism by which AhR ligands modulate effector CD4(+) T cell responses.