Repeated ultraviolet irradiation induces the expression of Toll-like receptor 4, IL-6, and IL-10 in neonatal human melanocytes

Background: Human melanocytes express Toll-like receptor 4 (TLR4), which regulates ultraviolet (UV)-induced cutaneous immunosuppression in Langerhans cells. Lipopolysaccharide (LPS) stimulation increases melanocyte pigmentation and TLR4 expression, while inducing local innate inflammatory responses.

Aims: We investigated whether UV radiation induces TLR4 expression in neonatal human melanocytes (NHMs) and how this affects the immune system.

Methods: We cultured NHMs with LPS treatment or with one-time or repeated UVA or UVB exposure, and investigated and compared the effects on TLR4 expression, melanin contents, and cytokine production.

Results: NHMs in the resting state did not express TLR4. LPS stimulation induced TLR4 expression and increased pigmentation. TLR4 expression was not detected after single-dose UVA or UVB treatment, but pigmentation increased. Repeated UV treatment induced TLR4 expression and increased pigmentation. LPS stimulation and repeated UV treatment increased IL-6 secretion, and repeated UVB treatment increased IL-10 secretion.

Conclusion: These results suggest that human melanocytes may actively participate in UV-induced immune modulation.