Antimicrobial peptides: Defending the mucosal epithelial barrier

Abstract

The recent epidemic caused by aerosolized SARS-CoV-2 virus illustrates the importance and vulnerability of the mucosal epithelial barrier against infection. Antimicrobial proteins and peptides (AMPs) are key to the epithelial barrier, providing immunity against microbes. In primitive life forms, AMPs protect the integument and the gut against pathogenic microbes. AMPs have also evolved in humans and other mammals to enhance newer, complex innate and adaptive immunity to favor the persistence of commensals over pathogenic microbes. The canonical AMPs are helictical peptides that form lethal pores in microbial membranes. In higher life forms, this type of AMP is exemplified by the defensin family of AMPs. In epithelial tissues, defensins, and calprotectin (complex of S100A8 and S100A9) have evolved to work cooperatively. The mechanisms of action differ. Unlike defensins, calprotectin sequesters essential trace metals from microbes, which inhibits growth. This review focuses on defensins and calprotectin as AMPs that appear to work cooperatively to fortify the epithelial barrier against infection. The antimicrobial spectrum is broad with overlap between the two AMPs. In mice, experimental models highlight the contribution of both AMPs to candidiasis as a fungal infection and periodontitis resulting from bacterial dysbiosis. These AMPs appear to contribute to innate immunity in humans, protecting the commensal microflora and restricting the emergence of pathobionts and pathogens. A striking example in human innate immunity is that elevated serum calprotectin protects against neonatal sepsis. Calprotectin is also remarkable because of functional differences when localized in epithelial and neutrophil cytoplasm or released into the extracellular environment. In the cytoplasm, calprotectin appears to protect against invasive pathogens. Extracellularly, calprotectin can engage pathogen-recognition receptors to activate innate immune and proinflammatory mechanisms. In inflamed epithelial and other tissue spaces, calprotectin, DNA, and histones are released from degranulated neutrophils to form insoluble antimicrobial barriers termed neutrophil extracellular traps. Hence, calprotectin and other AMPs use several strategies to provide microbial control and stimulate innate immunity.

Keywords: LL-37; antimicrobial peptides/proteins; calprotectin; defensins; disease; epithelium; health.

Figure 1

A schematic view of oropharyngeal candidiasis showing the complex participants in the infection. The oral microbiome associated with candidiasis is different from the oral microbiome in health [86]. The outgrowth of C. albicans hyphae may be facilitated by or drive the changes associated with the dysbiotic microbiome. C. albicans in a dysbiotic community leads to inflammation and upregulation of many AMPs including calprotectin, LL-37, and defensins. At the site of infection, the expression of AMPs, the dysbiotic microbiome, and the outgrowth of C. albicans can individually or collectively signal for infiltration by immune cells, including neutrophils, macrophage, and T cells. Neutrophils and macrophage phagocytose Candida. Neutrophil degranulation provides the components for neutrophil extracellular traps, which contain the spread of Candida amid high concentrations of AMP calprotectin [67].

Figure 2

Localization of calprotectin at the epithelial barrier specifies function. (A) Calprotectin released from degranulated neutrophils and epithelial cells complex with DNA and histones to form antimicrobial neutrophil extracellular traps. (B) When calprotectin is released from cells in a high calcium concentration inflammatory environment, the soluble AMP forms heterotetramers, providing increased affinity for trace metal divalent cations. Successful sequestration of the trace metals from microbes results in reduced growth and “nutritional immunity.” (C) Calprotectin localized within the cytoplasm of epithelial cells or neutrophils appear to protect against invasive microbes that seek to reside intracellularly as part of their life cycle. Whether intracytoplasmic calprotectin with AMP activity exists as monomers, heterodimers. or heterotetramers, assuming increased cytoplasmic calcium upon release from intracellular stores, remains to be studied. (D) Released calprotectin can engage a range of receptors on cells in the inflammatory environment. Engagement with specific receptors link this AMP with a range of innate cellular immune responses.