In addition, Rorγt+ ILC numbers were also reduced upon specific deletion of AhR in Rorγt-expressing cells (including ILCs) [[56]]. Together these data indicate that the effects of AhR-deficiency on

Rorγt+ ILCs are cell intrinsic. Interestingly, the reduction of Rorγt+ ILC numbers, induced by ablation of AhR, was observed only after birth. see more During fetal development, and early after birth, the ILC22 numbers in AhR-deficient mice are comparable to those in wild type mice, indicating that AhR is not required for development of these cells [[54]]. However, after weaning, the numbers of Rorγt+ ILCs in AhR-deficient mice steadily decrease [[54]]. Maintenance of ILC numbers is not a consequence of AhR activation by products of colonizing microbiota, because the difference in ILC22 numbers between wt and AhR-deficient animals is not affected by treatment with a mix of antibiotics [[54]]. Also, the observation that germ-free animals do not show reductions in gut residing Rorγt+ ILC numbers [[55, 57]] is consistent with the notion

that products from commensals are not required for the maintenance of these cells. It is controversial whether dietary products are the AhR ligands responsible for the maintenance of gut-residing Rorγt+ ILCs, as observed for IELs [[53]]. In one study, it was found that mice fed with a diet free of AhR-binding phytochemicals showed decreased numbers of Rorγt+ ILCs, causing a lack of CPs and Suplatast tosilate ILFs [[55]]. Addition of indole-3-carbinol, a dietary product, restored the Rorγt+ ILC numbers [[55]]. Another study, however, suggested that endogenous Small molecule library solubility dmso AhR ligands, including the tryptophane catabolite kynurenine, were potent regulators of Rorγt+ ILC maintenance as removal of dietary AhR ligands in that study did not disturb Rorγt+ ILC homeostasis and function [[56]]. The differences may be due to different types of controlled diets used by the different groups.

Further experiments should aim to resolve these discrepancies. The mechanisms by which AhR controls Rorγt+ cell numbers are not fully understood. Microarray analysis of Rorγt+ cells from wt and AhR-deficient mice suggested that Notch 1 is a downstream target of AhR [[56]]. Consistent with this, administration by gavage of the toxin TCCD (2,3,7,8-tetrachlorodibenzo-p-dioxin) resulted in the upregulation of Notch1 and Notch2 in gut Rorγt+ ILCs. Evidence for a role of Notch in AhR-mediated maintenance of Rorγt+ ILCs was provided by the observation that mice deficient for RBP-Jk, an essential partner of Notch, showed substantially reduced numbers of NKp46-expressing Rorγt+ ILCs and, although less prominently, of CD4+ Rorγt+ ILCs (LTi cells) also [[56]]. However, there were differences between the AhR- and RBP-Jk-deficient mice, in that in the latter, cryptopatches and ILFs were largely intact, whereas they were greatly reduced in AhR-deficient mice [[56]].