Review of the Innate Immune Response in Acne vulgaris: Activation of Toll-Like Receptor 2 in Acne Triggers Inflammatory Cytokine Responses

Acne vulgaris is a common disorder that affects 40–50 million people in the USA alone. The pathogenesis of acne is multifactorial, including hormonal, microbiological and immunological mechanisms. One of the factors that contributes to the pathogenesis of acne is <i>Propionibacterium acnes</i>; yet, the molecular mechanism by which <i>P. acnes</i> induces inflammation is not known. Recent studies have demonstrated that microbial agents trigger cytokine responses via Toll-like receptors (TLRs). TLRs are pattern recognition receptors that recognize pathogen-associated molecular patterns conserved among microorganisms and elicit immune responses. We investigated whether TLR2 mediates <i>P. acnes-</i>induced cytokine production in acne. Using transfectant cells we found that TLR2 was sufficient for NF-ĸB activation in response to <i>P. acnes</i>. In addition, peritoneal macrophages from wild-type, TLR6 knockout and TLR1 knockout mice, but not TLR2 knockout mice, produced IL-6 in response to <i>P. acnes.</i><i>P. acnes</i> induced activation of IL-12 and IL-8 production by primary human monocytes, and this cytokine production was inhibited by anti-TLR2-blocking antibody. Finally, in acne lesions, TLR2 was expressed on the cell surface of macrophages surrounding pilosebaceous follicles. These data suggest that <i>P. acnes</i> triggers inflammatory cytokine responses in acne by activation of TLR2. As such, TLR2 may provide a novel target for the treatment of this common skin disease.