Ubiquitously expressed Human Beta Defensin 1 (hBD1) forms bacteria-entrapping nets in a redox dependent mode of action.

Judith Raschig,Adam A Strömstedt,Jan Wehkamp,Daniela Mailänder-Sánchez,Anne Berscheid,Nisar P Malek,Jürgen Berger,Heike Brötz-Oesterhelt,Lioba F Courth,Denise M Monack

doi:10.1371/journal.ppat.1006261

Abstract

Ever since the discovery of endogenous host defense antimicrobial peptides it has been discussed how these evolutionary conserved molecules avoid to induce resistance and to remain effective. Human ß-defensin 1 (hBD1) is an ubiquitously expressed endogenous antimicrobial peptide that exhibits qualitatively distinct activities between its oxidized and reduced forms. Here, we explore these antimicrobial mechanisms. Surprisingly, using electron microscopy we detected a so far unknown net-like structure surrounding bacteria, which were treated with the reduced but not the oxidized form of hBD1. A transmigration assay demonstrated that hBD1-derived nets capture bacteria and inhibit bacterial transmigration independent of bacterial killing. The presence of nets could completely prevent migration of hBD1 resistant pathogens and are stable in the presence of human duodenal secretion with a high amount of proteases. In contrast to HD6, cysteins are necessary for net formation. This redox-dependent function serves as an additional mechanism of action for hBD1 and differs from net formation by other defensins such as Paneth cell-derived human α-defensin 6 (HD6). While hBD1red and hBD1ox have distinct antimicrobial profiles and functions, only the reduced form provides additional host protection by entrapping bacteria in extracellular net structures preventing bacterial invasion. Better understanding of the modes of action of endogenous host peptides will help to find new antimicrobial strategies.

Highlights

The innate immune system is the primary barrier against commensal invasion and microbial infections
The human body is constantly challenged by commensal and by pathogenic bacteria
Antimicrobial molecules of the innate immune system protect the human body from bacterial overgrowth and keep human microbiota in balance

Summary

Introduction

The innate immune system is the primary barrier against commensal invasion and microbial infections. Antimicrobial peptides (AMPs) are key effector molecules of the innate immune system protecting the human body from bacterial overgrowth, thereby retaining a balanced microbiota and fending off commensals and pathogens [1,2]. Oxidized hBD1 (hBD1ox), which contains three closed disulfide bonds, showed no antimicrobial activity against most microbes, with the exception of a few Gram-negative bacteria including E. coli [12]. Chileveru et al described a different mechanism for human Paneth cell alpha-defensin 5 (HD5) [15]. They reported that HD5 enters the cytoplasm of E. coli and other Gram-negative bacteria, leading to morphological changes like blebbing, clumping and cell elongation [15], followed by the loss of bacterial viability. Paneth cell HD6 was reported to form net-like structures, entrapping S. typhimurium, thereby preventing the translocation of these bacteria into the intestinal epithelium [16]

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