Staphylococcus epidermidis Antimicrobial δ-Toxin (Phenol-Soluble Modulin-γ) Cooperates with Host Antimicrobial Peptides to Kill Group A Streptococcus

Kenshi Yamasaki,Yuping Lai,Anna L. Cogen,Frank R. DeLeo,Judy E. Kim,Richard L. Gallo,Victor Nizet,Jun Muto,Katheryn M. Sanchez,Laura Crotty Alexander,Jackelyn Tanios

doi:10.1371/journal.pone.0008557

Abstract

Antimicrobial peptides play an important role in host defense against pathogens. Recently, phenol-soluble modulins (PSMs) from Staphylococcus epidermidis (S. epidermidis) were shown to interact with lipid membranes, form complexes, and exert antimicrobial activity. Based on the abundance and innocuity of the cutaneous resident S. epidermidis, we hypothesized that their PSMs contribute to host defense. Here we show that S. epidermidis δ-toxin (PSMγ) is normally present in the epidermis and sparsely in the dermis of human skin using immunohistochemistry. Synthetic δ-toxin interacted with neutrophil extracellular traps (NETs) and colocalized with cathelicidin while also inducing NET formation in human neutrophils. In antimicrobial assays against Group A Streptococcus (GAS), δ-toxin cooperated with CRAMP, hBD2, and hBD3. In whole blood, addition of δ-toxin exerted a bacteriostatic effect on GAS, and in NETs, δ-toxin increased their killing capacity against this pathogen. Coimmunoprecipitation and tryptophan spectroscopy demonstrated direct binding of δ-toxin to host antimicrobial peptides LL-37, CRAMP, hBD2, and hBD3. Finally, in a mouse wound model, GAS survival was reduced (along with Mip-2 cytokine levels) when the wounds were pretreated with δ-toxin. Thus, these data suggest that S. epidermidis–derived δ-toxin cooperates with the host-derived antimicrobial peptides in the innate immune system to reduce survival of an important human bacterial pathogen.

Highlights

Antimicrobial peptides (AMPs) are critically important in the host’s defense against infections
Since injured skin rapidly accumulates neutrophils at sites of infection and injury, and these cells act in part to protect the skin through the formation of neutrophil extracellular traps (NETs) containing antimicrobial peptides [6,20], we evaluated if dtoxin from surface S. epidermidis could interact with NETs and contribute to their activity. d-toxin was added to PMA-induced neutrophil extracellular traps (NETs) in culture
Addition of dtoxin to these cells showed that d-toxin bound to the NETs and colocalized with cathelicidin endogenously released from the neutrophil (Figure 1b)

Summary

Introduction

Antimicrobial peptides (AMPs) are critically important in the host’s defense against infections. Cathelicidins are an important class of mammalian AMPs as cathelicidin-deficient mice have enhanced susceptibility to skin and other infections, providing evidence that AMPs are critical to the innate immune defense[1,2,3,4]. AMPs have been shown to be important in multiple cell types, including, but not limited to, macrophages, neutrophils, mast cells, and epithelial cells such as skin keratinocytes. Pathogens come into contact with AMPs via host secretory mechanisms and phagocytosis. It has been suggested that NETs entwine and kill circulating pathogens in a web of DNA, histones, and AMPs

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Conclusion