Kaempferol exerts anti-inflammatory effects by accelerating Treg development via AhR-mediated and PU.1/IRF4-dependent transactivation of the Aldh1a2/RALDH2 gene in dendritic cells
Retinaldehyde dehydrogenase 2 encoded in the Aldh1a2 gene (RALDH2) is expressed in intestinal dendritic cells (DCs) in mesenteric lymph nodes (MLNs) and plays a crucial role in Treg development by producing retinoic acid. In the present study, we screened food ingredients and identified kaempferol, a flavonoid, as the compound that most effectively up-regulated RALDH2 expression in DCs. The development of Foxp3+ cells from OT-II-derived naive CD4+ T cells was enhanced by a co-culture with OVA peptide-pulsed bone marrow-derived DCs (BMDCs) in the presence of kaempferol or by a pre-treatment of BMDCs with kaempferol. The involvement of the aryl hydrocarbon receptor (Ahr) in the effects of kaempferol on Aldh1a2 expression was demonstrated by in vitro experiments using siRNA and various agonistic or antagonistic compounds of AhR. AhR suppressed Aldh1a2 gene expression and kaempferol inhibited the AhR activity by functioning as an antagonist of AhR. The kaempferol treatment also increased the protein levels of PU.1 and IRF4, which are essential for transcription of the Aldh1a2 gene in DCs, and accelerated the subsequent recruitment of PU.1 to the Aldh1a2 gene. The kaempferol-induced increase in PU.1 protein levels occurred in an AhR-dependent and transcription-independent manner, whereas the kaempferol treatment transactivated the Irf4 gene in DCs. The frequency of DCs exhibiting RALDH2 activity in the MLNs of mice was increased by the intraperitoneally (i.p.) administration of an AhR antagonist or by the oral administration of astragalin, a 3-O-glucoside of kaempferol. We also observed an increase in Tregs in the Peyers patches of C57BL/6 mice i.p. administered kaempferol-treated BMDCs. We utilized an OVA-induced food allergy model of Balb/c mice to examine the effects of kaempferol in vivo and confirmed that the rapid decrease in body temperature and allergic diarrhea observed just after the OVA challenge were significantly suppressed in mice administered kaempferol.