Effect of a diet containing a mixture of soybean isoflavones and soyasaponins on contact hypersensitivity and gut microbiota in mice

Dietary soybean isoflavones (SIs) and soyasaponins (SSs) individually exert inhibitory effects on contact hypersensitivity (CHS), which is an animal model of allergic contact dermatitis (ACD); however, the beneficial effects of diets containing a mixture of SIs and SSs on CHS remain unclear. Here, we investigated the CHS-suppressive effects of diets containing a mixture of SIs and SSs at physiologically relevant doses, and the role of the gut microbiota. The diets attenuated ear swelling and reduced the number of Gr-1-positive cells infiltrating the auricle tissues. Ear swelling was inhibited in the SI-and SS-treated mice compared to the SI-treated mice. Compared to the CHS controls, the levels of chemokine (C-X-C motif) ligand 2 and triggering receptor expressed on myeloid cells-1 (TREM-1) were lower in the auricle tissues of SI- and SS-treated mice, whereas TREM-1 production was not suppressed in SI- or SS-treated mice. In addi-tion, vancomycin treatment had little effect on the CHS-inhibitory activity of SI and SS diets, whereas ciprofloxacin and metronidazole treatments blocked the effects of SI and SS diets. These antibiotics exhibited differing effects on the microbiota composition deduced from the bacterial 16S ribosomal RNA gene sequences. Therefore, con-suming a mixture of SIs and SSs has ACD-preventive effects and is affected by the gut microbiota.