CD4 and neoplasm: AhR signaling activation has pleiotropic effects on the regulation of the immune response, including upregulating IDO1 expression in tumor cells, upregulating programmed cell death 1 (PD-1) expression in CD8+ T cells, upregulating forkhead box P3 (FOXP3) expression in CD4+ T cells, impairing CD8+ T cells proliferation, and increasing the proportion of Tregs10–13.