Antimicrobial peptides as defense molecules at the interface of the host and bacteria

Abstract

Microbes surround us. However, most of the time we are unaware of them. Our epithelial surfaces, which provide the physical barrier and separate us from the environment, are usually free of signs of a battle taking place. Over the past several years, it has been realized that the epithelium is capable of mounting its own battery of defensive chemicals. Besides the constituents of fluids, such as tears, saliva, and sweat that are secreted onto the surface of epithelia, antimicrobial peptides defend this barrier. The aim of this project was to investigate antimicrobial components at different body surfaces like lung, skin and intestine.

In the first study the human beta defensin-2 was discovered as the major antimicrobial peptide from lung epithelial cells against Pseudomonas aeruginosa, a bacterium often affecting the lungs of patients suffering from cystic fibrosis. It was demonstrated that mucoid forms of P. aeruginosa as well as the proinflammatory cytokines IL-1beta and TNF-alpha are able to induce the expression human beta defensin-2 in respiratory epithelial cells.

In the second study the four human beta defensins hBD1-4 were examined in keratinocytes. The expression of three defensins, namely hBD2, -3 and -4 were inhibited by co-cultivation with retinoic acid. These defensins exhibited upregulation during keratinocyte differentiation and stimulation by proinflammatory cytokines and bacterial contact. Co-incubation with retinoic acid exhibited the induction of hBD2, -3 and -4. Since retinoic acid is used topically in certain skin diseases and leads sometimes to bacterial superinfection, the here described inhibitory effect of retinoic acid on the regulation of defensins could explain this phenomenon.

In the third study it was demonstrated that most of the antimicrobial components in the small intestine of mice localize to the mucus layer building an effective shield between the bacteria in the intestinal lumen and the epithelial cells. Mucus preparation exhibited broad antimicrobial activity. Besides alpha defensins, other antimicrobial active factors like lysozyme, phospholipase A2 and ribosomal proteins could be identified. The results of this study imply that the mucus is not only a physical barrier but serves as a structure to keep antimicrobial peptides and proteins in high concentration. Recent studies have suggested an important role of antimicrobial peptides and the mucus in inflammatory bowl diseases.

In the fourth study the heparin/heparan sulfate interacting protein (HIP), which is similar to the ribosomal protein L29, was identified as one of the major antimicrobial active substance in lung extracts by HPLC-fractionation, N-terminal sequencing and mass-spectrometry analysis. HIP/RPL29 was also detected in extracts of the small intestine. HIP/RPL29 exhibited broad antimicrobial activity and notably against P. aeruginosa strains. The HIP/RPL29 protein was found to be localized specifically to the epithelial surface of the lungs and intestines by immunohistochemistry. We suggest that HIP/RPL29 fulfils a function as an abundant antimicrobial factor of the epithelial innate defense shield against invading bacteria both in lungs and in the small intestine.