No abstract

Antimicrobial peptides (AMPs) are multi-functional peptides whose fundamental biological role in vivo has been proposed to be the elimination of pathogenic microorganisms, including Gram-positive and -negative bacteria, fungi, and viruses. Genes encoding these peptides are expressed in a variety of cells in the host, including circulating phagocytic cells and mucosal epithelial cells, demonstrating a wide range of utility in the innate immune system. Expression of these genes is tightly regulated; they are induced by pathogens and cytokines as part of the host defense response, and they can be suppressed by bacterial virulence factors and environmental factors which can lead to increased susceptibility to infection. New research has also cast light on alternative functionalities, including immunomodulatory activities, which are related to their unique structural characteristics. These peptides represent not only an important component of innate host defense against microbial colonization and a link between innate and adaptive immunity, but also form a foundation for the development of new therapeutic agents.

Antimicrobial peptides are found in both myeloid cells and mucosal tissues of many vertebrates and invertebrates. These peptides are predicted to operate as a first-line host defense mechanism exerting broad-spectrum activity against pathogenic bacteria, fungi, parasites, and enveloped viruses. We report the characterization of a novel 25-residue linear antimicrobial peptide found in the skin mucous secretions of the winter flounder (Pleuronectes americanus). This peptide was purified through multiple chromatographic methods to obtain a single peak by reversed-phase high performance liquid chromatography. This purified peptide, which we named pleurocidin, exhibited antimicrobial activity against Escherichia coli in a bacterial cell lysis plate assay. Mass spectrometry and amino acid sequence analysis indicated that it is 25 amino acids in length. Pleurocidin is predicted to assume an amphipathic ␣-helical conformation similar to many other linear antimicrobial peptides. There is a high degree of homology between pleurocidin and two antimicrobial peptides, ceratotoxin from the Mediterranean fruit fly and dermaseptin from the skin of a hylid frog. The minimal inhibitory concentration and minimal bactericidal concentration of pleurocidin were determined against 11 different Gram-negative and Gram-positive bacteria. Immunohistochemistry locates pleurocidin in the epithelial mucous cells of flounder skin. Pleurocidin represents a novel antimicrobial peptide found in fish and may play a role in innate host defense.

The respiratory epithelium maintains an effective antimicrobial environment to prevent colonization by microorganisms in inspired air. In addition to constitutively present host defenses which include antimicrobial peptides and proteins, the epithelial cells respond to the presence of microbes by the induction two complementary parts of an innate immune response. The first response is the increased production of antimicrobial agents, and the second is the induction of a signal network to recruit phagocytic cells to contain the infection. Inflammatory mediators released by the recruited cells as well as from the epithelium itself further induce the expression of the antimicrobial agents. The result is an effective prevention of microbial colonization. The epithelial cells recognize the pathogen-associated patterns on microbes by surface receptors such as CD14 and Toll-like receptors. Subsequent signal transduction pathways have been identified which result in the increased transcription of host defense response genes. Diseases such as cystic fibrosis, or environmental exposures such as the inhalation of air pollution particles, may create an environment that impairs the expression or activity of the host defenses in the airway. This can lead to increased susceptibility to airway infections.

Extracts of the bovine tracheal mucosa have an abundant peptide with potent antimicrobial activity. The 38-amino acid peptide, which we have named tracheal antimicrobial peptide (TAP), was isolated by a sequential use of size-exclusion, ion-exchange, and reverse-phase chromatographic fractionations using antimicrobial activity as a functional assay. The yield was -2 ,ug/g of wet mucosa. The complete peptide sequence was determined by a combination of peptide and cDNA analysis. The amino acid sequence of TAP is