Toll‐like receptors 2 and 3 enhance melanogenesis and melanosome transport in human melanocytes

Summary Because little is known about how the innate immune response influences skin pigmentation, we examined whether Toll‐like receptor ( TLR ) agonists participate in melanogenesis and melanosome transportation. We observed that TLR 2/2 agonist HKLM and TLR 3 agonist Poly(I:C) increased the amount of extracellular melanin from primary human epidermal melanocytes. HKLM , but not Poly(I:C), increased the melanogenic genes such as tyrosinase and dopachrome tautomerase. Poly(I:C) increased the expression of Rab27A, a molecule that facilitates melanosome transport to perimembranous actin filament. UVB irradiation induced Rab27A and melanosome transportation in a similar manner of Poly(I:C). Si RNA for TLR 3 or Rab27A suppressed the perimembranous accumulation of Gp100‐positive vesicles in melanocytes and decreased melanin transfer to neighboring keratinocytes induced by both Poly(I:C) and UVB . These results suggest that the microenvironment in the epidermis and innate immune stimuli, such as microbiome and ultraviolet represented here by TLR 2 and TLR 3 agonists, could affect the melanogenesis in human melanocytes.