472 Differential inflammatory responses induced by Cutibacterium acnes are not dependent on strain-types but are associated with the presence of a linear plasmid

Abstract

Cutibacterium acnes (C. acnes, formerly Propionibacterium acnes), is normally well tolerated by human skin. Different strain-types of C. acnes are enriched in acne, and it has been hypothesized that strain-types may promote disease. To study the host immune response against strain-types and the genetic basis for such differences, we generated a library of >1000 C. acnes isolates from swabs of lesional and non-lesional sites of acne patients and healthy volunteers. C. acnes strain-level identity was determined by high resolution sequence typing and the inflammatory potential of each isolate was assessed by high-throughput screening of IL-8 secretion from SEB-1 sebocytes and normal human keratinocytes (NHEK) exposed to sterile bacterial supernatants. Consistent with prior findings, a higher frequency of C. acnes phylotype IA1 strains were recovered from acne lesional sites (specifically C1 and C2 sequence types (ST)). However, non-lesional sites from acne patients were found to have K1 and K2 STs, similar to healthy skin. Furthermore, and contrary to predictions, many of the K1/K2 STs from healthy skin and non-lesional acne promoted greater IL-8 production compared to lesional C1/C2 STs (p=0.012). Moreover, clonal isolates from the same ST promoted different cytokine responses, with high and low cytokine-inducing strains identified within K1, C1, C2 and H1 STs. The capacity of identical STs to promote different inflammatory responses was validated by intradermal injection of sterile supernatants into mice and subsequent qRT-PCR for IL-8, CXCL1 and IL-1β. The functional differences within these strains could not be attributed to protease or lipase activity, short chain fatty acid or porphyrin production. Instead, whole genome sequencing revealed the presence of a linear plasmid only in the pro-inflammatory strains. Several putative virulence genes are present in this plasmid. We conclude that a virulence plasmid is most highly associated with the proinflammatory activity of C. acnes.