Role of <em>Cutibacterium acnes </em>in the Aetiopathogenesis of Sarcoidosis: Current Insights and Future Study Directions

Cutibacterium acnes (C. acnes) is a commensal bacterium of the skin microbiota which can transform itself into a pathogen depending on the peculiar susceptibility and permissivity of the host: C. acnes is the sole microorganism so far to be found in the specific organ lesions of sarcoidosis, and C. acnes-induced activation of T helper type 1 (Th1) immune responses is generally higher in patients with sarcoidosis than in healthy individuals. This bacterium acts as an opportunistic agent in several inflammatory conditions other than sarcoidosis, such as prostate cancer (PCa) and prosthetic joint infections. Both innate and adaptive immunity appear involved in the pathogenesis of C. acnes-mediated sarcoid lesions and a key-role is played by host toll-like receptors (TLR)-2, -4, and -6, NOD-like receptors (NLR), and monocytes/macrophages cytoplasmic receptors. This review aims at summarizing the updated knowledge about the potential cause-effect relationship existing between C. acnes and sarcoidosis, addressing issues of future research directions and novel therapeutic strategies in the management of a complex disease as sarcoidosis.