In severe strep skin infections, the body floods the wound with the antimicrobial peptide LL‑37, but the bacteria produce an enzyme (SpeB) that chops up LL‑37, making it ineffective at the infection site.

Severe soft tissue infections, such as necrotizing fasciitis and severe cellulitis, caused by group A streptococci (GAS) are rapidly progressing life-threatening infections characterized by massive bacterial loads in the tissue even late after the onset of infection. Antimicrobial peptides are important components of the innate host defense, and cathelicidins have been shown to protect against murine necrotic skin infections caused by GAS. However, it has been demonstrated that the streptococcal cysteine protease SpeB proteolytically inactivates the human cathelicidin LL-37 in vitro. Here we have investigated the expression of LL-37 and its interaction with GAS and SpeB during acute severe soft tissue infections by analyses of patient tissue biopsy specimens. The results showed large amounts of LL-37, both the proform (hCAP18) and the mature peptide, in the tissue. Confocal microscopy identified neutrophils as the main source of the peptide. A distinct colocalization between the bacteria and LL-37 could be noted, and bacterial loads showed positive correlation to the LL-37 levels. Areas with high LL-37 levels coincided with areas with large amounts of SpeB. Confocal microscopy confirmed strong colocalization of GAS, SpeB, and LL-37 at the bacterial surface. Taken together, the findings of this study provide in vivo support of the hypothesis that SpeB-mediated inactivation of LL-37 at the streptococcal surface represents a bacterial resistance mechanism at the infected tissue site in patients with severe GAS tissue infections.