Regulation of mammalian defensin expression by Toll-like receptor-dependent and independent signalling pathways

Members of the Toll-like receptor family of innate immunity pattern-recognition receptors are abundant in the male rat reproductive tract

Protecting developing and maturing spermatozoa and reproductive tissues from microbial damage is an emerging aspect of research in reproductive physiology. Bacterial, viral, and yeast infections of the testis and epididymis can hinder maturation and movement of spermatozoa, resulting in impaired fertility. Toll-like receptors (TLRs) are a broad family of innate immunity receptors that play critical roles in detecting and responding to invading pathogens. Objectives of this study were to determine if organs of the rat male reproductive tract express mRNAs for members of the TLR family, to characterize expression patterns for TLRs in different regions of the epididymis, and to determine if TLR adaptor and target proteins are present in the male reproductive tract. Messenger RNA for Tlr1-Tlr9 was abundantly expressed in testis, epididymis, and vas deferens, as determined by RT-PCR, while Tlr10 and Tlr11 were less abundantly expressed. Tlr mRNA expression showed no region-specific patterns in the epididymis. Immunoblot analysis revealed relatively equal levels of protein for TLRs 1, 2, 4, and 6 in testis, all regions of the epididymis and vas deferens, and lower levels of TLRs 3, 5, and 9-11. TLR7 was primarily detected in the testis. The TLR adapter proteins, myeloid differentiation primary response gene 88 and TLR adaptor molecule 1, as well as v-rel reticuloendotheliosis viral oncogene homolog and NFKBIA, were prominent in testis, epididymis, and vas deferens. The abundant expression of a majority of TLR family members together with expression of TLR adaptors and activation targets provides strong evidence that TLRs play important roles in innate immunity of the male reproductive tract.

In vivo beta-defensin gene expression in rat gingival epithelium in response to Actinobacillus actinomycetemcomitans infection

BACKGROUND AND OBJECTIVE

Human beta-defensins have been identified in the oral cavity and are predicted to play a role in the defense against pathogenic bacteria. Homologous rat beta-defensins (RBDs) have been identified, but their expression in the oral cavity has not been examined. Therefore, the aim of this study was to investigate the expression of innate immune mediators in the rat gingival epithelium.

MATERIAL AND METHODS

Rats were pretreated with antibiotics to depress the normal oral flora, followed by the introduction of Actinobacillus actinomycetemcomitans in their food to allow colonization and the development of periodontal disease. At various time points, animals were killed and the gingival epithelium was extracted. Semiquantitative reverse transcription-polymerase chain reaction was performed to measure RBD and Toll-like receptor (TLR) mRNA levels.

RESULTS

Three beta-defensins (RBD-1, -2 and -5) and two TLRs (TLR-3 and -4) are expressed in normal rat gingival epithelium. After the introduction of A. actinomycetemcomitans, RBD-1 and RBD-2 mRNA levels increased for the first week followed by a return to basal levels. No change in TLR mRNA levels was observed.

CONCLUSION

The rat model provides a good system for experimental analysis of the innate immune response to periopathogenic bacteria in the oral cavity, as well as the potential role of beta-defensins in the host response to colonization.

Multispecific myeloid defensins

PURPOSE OF REVIEW

This review describes recent progress in our understanding of defensins and their contributions to innate immunity. Defensins are small, cysteine-rich endogenous antibiotic peptides. Human neutrophils contain large amounts of three alpha-defensins (HNP-1-HNP-3), and smaller amounts of a fourth, HNP-4. Monocytes and macrophages generally lack defensins, but they release messengers that induce the synthesis of beta-defensins in epithelial cells.

RECENT FINDINGS

In addition to their antimicrobial and immunomodulatory effects, HNP-1-HNP-3 possess antiviral and toxin-neutralizing properties. Induction of beta-defensins in epithelial cells is mediated by cell-surface Toll-like receptors or cytoplasmic peptidoglycan receptors that can recognize pathogen-associated molecules. Mutations in Nod2, a cytoplasmic peptidoglycan receptor, are associated with reduced levels of intestinal alpha-defensins and ileal Crohn's disease. Human defensin genes show marked copy-number polymorphism. High level constitutive expression of defensins may afford protection against HIV-1 and other defensin-sensitive pathogens. Theta-defensins (cyclic octadecapeptides found in nonhuman primates) have impressive antiviral and antitoxic properties.

SUMMARY

The multiple properties of defensins contribute to human innate immunity against bacteria, bacterial toxins, and viruses.

Structure-dependent functional properties of human defensin 5

The mucosal epithelium secretes a variety of antimicrobial peptides that act as part of the innate immune system to protect against invading microbes. Here, we describe the functional properties of human defensin (HD) 5, the major antimicrobial peptide produced by Paneth cells in the ileum, in relation to its structure. The antimicrobial activity of HD-5 against Escherichia coli proved to be independent of its structure, whereas the unstructured peptide showed greatly reduced antimicrobial activity against Staphylococcus aureus. We find that HD-5 binds to the cell membrane of intestinal epithelial cells and induced secretion of the chemokine interleukin (IL)-8 in a concentration- and structure-dependent fashion. Incubation of HD-5 in the presence of tumor necrosis factor alpha further increased IL-8 secretion synergistically, suggesting that HD-5 may act as a regulator of the intestinal inflammatory response.

Defensins and cathelicidins in gastrointestinal infections

PURPOSE OF REVIEW

To review recently published studies presenting novel and relevant information on antimicrobial peptides in gastrointestinal infections.

RECENT FINDINGS

Defensins and cathelicidins are important antimicrobial peptides expressed by the gastrointestinal epithelium. Their localization and regulation have been the focus of current research establishing the relevance of these peptides both in counteracting an attack by pathogens as well as in controlling the endogenous bacterial flora. In the small intestine, Paneth cell alpha-defensins maintain a low level of microorganisms and regulate the composition of the bacterial flora. In contrast, a constitutive beta-defensin can be found in nearly all gastrointestinal tissues. Other relevant beta-defensins as well as human cathelicidin are inducible by inflammation or infections. Thus Helicobacter pylori enhances defensin expression in the gastric mucosa and Campylobacter jejuni and Salmonella provoke a similar response in the colon. Other pathogenic bacteria may suppress the antimicrobial peptide response as an escape strategy. Notably, the therapeutic induction of cathelicidins alleviates experimental shigellosis, suggesting a future role of endogenous antibiotics in medical therapy.

SUMMARY

These recent findings together with a better understanding of underlying mechanisms involved in the regulation and biology of antimicrobial peptides will open up new therapeutic avenues to battle infections.

Curli biogenesis and function

Curli are the major proteinaceous component of a complex extracellular matrix produced by many Enterobacteriaceae. Curli were first discovered in the late 1980s on Escherichia coli strains that caused bovine mastitis, and have since been implicated in many physiological and pathogenic processes of E. coli and Salmonella spp. Curli fibers are involved in adhesion to surfaces, cell aggregation, and biofilm formation. Curli also mediate host cell adhesion and invasion, and they are potent inducers of the host inflammatory response. The structure and biogenesis of curli are unique among bacterial fibers that have been described to date. Structurally and biochemically, curli belong to a growing class of fibers known as amyloids. Amyloid fiber formation is responsible for several human diseases including Alzheimer's, Huntington's, and prion diseases, although the process of in vivo amyloid formation is not well understood. Curli provide a unique system to study macromolecular assembly in bacteria and in vivo amyloid fiber formation. Here, we review curli biogenesis, regulation, role in biofilm formation, and role in pathogenesis.

The front line of enteric host defense against unwelcome intrusion of harmful microorganisms: mucins, antimicrobial peptides, and microbiota

The intestinal tract is a complex ecosystem that combines resident microbiota and the cells of various phenotypes with complex metabolic activities that line the epithelial wall. The intestinal cells that make up the epithelium provide physical and chemical barriers that protect the host against the unwanted intrusion of microorganisms that hijack the cellular molecules and signaling pathways of the host and become pathogenic. Some of the organisms making up the intestinal microbiota also have microbicidal effects that contribute to the barrier against enteric pathogens. This review describes the two cell lineages present in the intestinal epithelium: the goblet cells and the Paneth cells, both of which play a pivotal role in the first line of enteric defense by producing mucus and antimicrobial peptides, respectively. We also analyze recent insights into the intestinal microbiota and the mechanisms by which some resident species act as a barrier to enteric pathogens. Moreover, this review examines whether the cells producing mucins or antimicrobial peptides and the resident microbiota act in partnership and whether they function individually and/or synergistically to provide the host with an effective front line of defense against harmful enteric pathogens.