CUTANEOUS BIOLOGY: Induction of toll-like receptors by Propionibacterium acnes

Background: The bacterium Propionibacterium acnes is involved in the induction and maintenance of the inflammatory phase of acne. Recent studies have found that keratinocytes express toll-like receptors (TLRs) implicated in immediate immunity. No studies have, to date, been carried out on the action of P. acnes upon TLR activation in keratinocytes. Objectives: Focusing on the inflammatory phase of acne, to clarify the role of P. acnes in immediate immunity by inducing expression of TLR-2 and TLR-4 by keratinocytes. We also studied how the secretion and expression of matrix metalloproteinase (MMP)-9 is induced by P. acnes. Methods: The work was carried out on two levels: in vivo with the study of the expression of TLR-2 and TLR-4 proteins in biopsies of acne lesions and in vitro on cultured keratinocyte monolayers to study the modulating effects of P. acnes on the expression of TLR-2 and TLR-4 and also on the expression and secretion of MMP-9. Results: Our findings reveal that in vivo TLR-2 and TLR-4 expression is increased in the epidermis of acne lesions. In vitro, an increase in TLR-2 and TLR-4 expression by human keratinocytes occurred in the first hours of incubation with bacterial fractions as well as an increase of the expression and secretion by the keratinocytes of MMP-9, which plays a role in inflammation. Conclusions: This work demonstrates that P. acnes induces TLR expression and that this mechanism could play an essential role in acne-linked inflammation. These receptors could be involved notably in acute acne.