Paneth cell α-defensin 6 (HD-6) is an antimicrobial peptide

Abstract

Defensins protect human barriers from commensal and pathogenic microorganisms. Human α-defensin 6 (HD-6) is produced exclusively by small intestinal Paneth cells but, in contrast to other antimicrobial peptides (AMPs) for HD-6, no direct antibacterial killing activity has been detected so far. Herein, we systematically tested how environmental factors, like pH and reducing conditions, affect antimicrobial activity of different defensins against anaerobic bacteria of the human intestinal microbiota. Remarkably, by mimicking the intestinal milieu we detected for the first time antibacterial activity of HD-6. Activity was observed against anaerobic gut commensals but not against some pathogenic strains. Antibiotic activity was attributable to the reduced peptide and independent of free cysteines or a conserved histidine residue. Furthermore, the oxidoreductase thioredoxin, which is also expressed in Paneth cells, is able to reduce a truncated physiological variant of HD-6. Ultrastructural analyses revealed that reduced HD-6 causes disintegration of cytoplasmic structures and alterations in the bacterial cell envelope, while maintaining extracellular net-like structures. We conclude that HD-6 is an antimicrobial peptide. Our data suggest two distinct antimicrobial mechanisms by one peptide: HD-6 kills specific microbes depending on the local environmental conditions, whereas known microbial trapping by extracellular net structures is independent of the reducing milieu.

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

“…We previously described the formation of nets by HD6, as well as its antimicrobial activity, to be independent of cysteines [17]. We therefore investigated if this was also true for hBD1red.…”

“…In contrast to HD6, for which Chu et al described the formation of nets to be dependent of the presence of bacterial components, this is different for hBD1red. Also, while both redox forms of HD6 are able to form nets [17], only the reduced form of hBD1 shows this ability. HBD1 has 6 cysteines that form 3 disulphide bridges, characteristic for defensins, under oxidizing conditions.…”

“…Similar to hBD1, Paneth cell HD6 also shows strong antimicrobial activity after the reduction of its disulphide bonds [17]. The same basic principle of broadened activity is also true for Psoriasin, a S100 protein with strong presence especially in the skin [18].…”

Antimicrobial activity of human α-defensin 5 and its linear analogs: N-terminal fatty acylation results in enhanced antimicrobial activity of the linear analogs

“…A subsequent study indicated that HD6 exhibited antibacterial activity against commensal anaerobic bacteria under reducing conditions [26]. HD5 is a key contributor to host defense.…”

Human α-Defensin 6: A Small Peptide That Self-Assembles and Protects the Host by Entangling Microbes

“…For α-defensins, a comparison of HD5 and HD6 exemplifies this notion because HD5 exhibits broad-spectrum microbiocidal activity whereas HD6, at least in the oxidized form, exhibits negligible cell killing ability. 19,22,35,38,41 …”