Complement C3 deficient mice show more severe imiquimod-induced psoriasiform dermatitis than wild-type mice regardless of the commensal microbiota.

The complement active product, C3a, and the receptor C3aR comprise an axis that exerts various biological functions, such as protection against infection. C3a is highly expressed in the inflamed skin and blood from patients with psoriasiform dermatitis. However, the role of the C3a/C3aR axis in psoriasiform dermatitis remains unclear because conflicting results using C3^-/- mice have been published. In this study, to elucidate the contribution of commensal microbiota in C3^-/- and wild-type (WT) mice were subjected to imiquimod-induced psoriasiform dermatitis under different housing conditions. C3^-/- mice showed increased epidermal thickness and keratinocyte proliferation markers in the inflamed ear compared to WT mice upon treatment with IMQ. These inflamed phenotypes were observed in both cohoused and separately housed conditions, and antibiotic treatment did not abolish the aggravation of IMQ-induced psoriasiform dermatitis in C3^-/- mice. These results suggested that the difference of commensal microbiota is not important for the C3-involved psoriasiform dermatitis. Keratinocyte hyperproliferation is a major feature of the inflamed skin in patients with psoriasiform dermatitis. In vitro experiments showed that C3a and C3aR agonists inhibited keratinocyte proliferation, which was abolished by introduction of a C3aR antagonist. Collectively, these results suggest that the C3a/C3aR axis plays a critical role in psoriasiform dermatitis development by inhibiting keratinocyte proliferation, regardless of the regulation of the commensal microbiota.