Abstract
Host defense peptides (HDPs) are an important first line of defense with antimicrobial and immunomoduatory properties. Because they act on the microbial membranes or host immune cells, HDPs pose a low risk of triggering microbial resistance and therefore, are being actively investigated as a novel class of antimicrobials and vaccine adjuvants. Cathelicidins and β-defensins are two major families of HDPs in avian species. More than a dozen HDPs exist in birds, with the genes in each HDP family clustered in a single chromosomal segment, apparently as a result of gene duplication and diversification. In contrast to their mammalian counterparts that adopt various spatial conformations, mature avian cathelicidins are mostly α-helical. Avian β-defensins, on the other hand, adopt triple-stranded β-sheet structures similar to their mammalian relatives. Besides classical β-defensins, a group of avian-specific β-defensin-related peptides, namely ovodefensins, exist with a different six-cysteine motif. Like their mammalian counterparts, avian cathelicidins and defensins are derived from either myeloid or epithelial origin expressed in a majority of tissues with broad-spectrum antibacterial and immune regulatory activities. Structure-function relationship studies with several avian HDPs have led to identification of the peptide analogs with potential for use as antimicrobials and vaccine adjuvants. Dietary modulation of endogenous HDP synthesis has also emerged as a promising alternative approach to disease control and prevention in chickens.
Highlights
Host defense peptides (HDPs), known as antimicrobial peptides, constitute a large group of small peptides that have been discovered in virtually all forms of life [1,2,3]
C-terminal segments are highly variable across species and proteolytically cleaved from the cathelicidin-like domain to become biologically active, Neutrophilic granule proteins (NGPs) are conserved throughout the entire sequence and functionally active without being processed. (B) The defensin family includes classical α, β, and θ-defensins with indicated disulfide bonds as well as four subfamilies of defensin-related peptides with unknown disulfide bonding patterns
Phylogenetic analysis of all publically available avian cathelicidins revealed that they are clustered into three distinct clades, namely CATH1/3, CATH2, and CATH-B1 (Figure 4), suggesting that these three clades of cathelicidin genes have evolved before divergence of these bird species from each other, unlike mammalian cathelicidin genes, a majority of which were duplicated after species separation [18]
Summary
Host defense peptides (HDPs), known as antimicrobial peptides, constitute a large group of small peptides that have been discovered in virtually all forms of life [1,2,3]. HDPs represent an important first-line of defense in those species whose adaptive immune system is lacking or primitive. Synthesized initially as precursors, HDPs are generally processed by host proteases to release mature peptides upon infection and inflammation [1,2]. Mature HDPs are broadly active against Gram-negative and Gram-positive bacteria, mycobacteria, fungi, viruses, and even cancerous cells [1,2]. HDPs were recently found to interact with several membrane-bound or intracellular receptors with a profound ability to modulate the host response to inflammation and infection [5,6,7]. A number of HDPs have been found with preferential expression in the male reproductive tract and several are linked to sperm maturation and might have potential for infertility treatment [9,10]
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