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

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.

Structure of the defensin NsD7 in complex with PIP2 reveals that defensin : lipid oligomer topologies are dependent on lipid type

“…Current evidence points to key aspects of the mechanism of action of human defensins being conserved across species . Human α‐defensin 6 and β‐defensin 1 have both been demonstrated to fibrilize in vivo to form nanonet traps that hinder the mobility of bacteria, however, the oligomerization triggers and the required components have not been clearly identified. Furthermore, we recently demonstrated that human β‐defensin 3 contains a motif that binds PIP 2 to mediate tumor cell permeabilization similar to that of the plant defensins .…”

Bacterial Periplasmic Oxidoreductases Control the Activity of Oxidized Human Antimicrobial β-Defensin 1

“…bacter baumannii. Conversely, Gram-positive bacteria were resistant against hBD1ox (10,11), which indicates that hBD1ox activity may target a mechanism that is specific to Gram-negative bacteria. Interestingly, under anaerobic conditions, the hBD1ox completely lost its antimicrobial activity ( Fig.…”

“…Previous data from our group demonstrated that hBD1red is active against Grampositive and Gram-negative bacteria by targeting mainly the bacterial membrane and trapping bacteria in a net structure (11). Due to the different activity spectra of hBD1red and hBD1ox under various environmental conditions, we here aimed to investigate whether the redox-active periplasm of Gram-negative bacteria contributes to the observed difference in hBD1 activity.…”

“…Reduced hBD1 (hBD1red) has a distinct antimicrobial profile and function compared to that of the native form. Additionally, as an independent mechanism, the reduced peptide provides broad protection by entrapping bacteria in extracellular net structures, preventing bacterial invasion (11). HBD1 is secreted as an oxidized peptide and can be reduced by present reductases such as thioredoxin (12) or a reducing environment.…”

“…However, it is likely that in vivo a mixture of reduced and oxidized hBD1 (hBD1ox) exists. HBD1ox, which contains three closed disulfide bonds, shows no antimicrobial activity against tested Gram-positive bacteria (11) but specific activity against the Gram-negative bacterium Escherichia coli (10). Yet, the distinct exact antimicrobial mechanisms of oxidized hBD1 are still unknown.…”