UVB, cis-urocanic acid and skin microbiome
The skin microbiome plays an important role in health and disease. We have recently shown that microbes living on the skin regulate the immunomodulatory properties of ultraviolet (UV) radiation, but the underlying mechanisms remain to be uncovered. Using a pre-clinical model, we report that acute UVB radiation induces a transient and significant restructuring of bacterial communities on the skin via one of its major photoproducts, cis-urocanic acid (cis-UCA). Certain bacteria, such as Staphylococcus epidermidis, use urocanase (HutU) to degrade and metabolize cis-UCA in order to proliferate. This in turn affects the concentration of cis-UCA, limiting its ability to suppress adaptive immune responses and induce tolerance to environmental antigens. Interestingly, addition of a topical urocanase inhibitor restricts the metabolism of cis-UCA by HutU+ bacteria and restores immunosuppression. Overall, these results illustrate mechanistic interactions between skin bacteria and solar radiation and open new avenues to enhance the beneficial effects of phototherapy protocols.