Cathelicidins and other antimicrobial peptides are deployed at epithelial surfaces to defend against infection. These molecules have broad-spectrum killing activity against microbes and can have effects on specific mammalian cell types, potentially stimulating additional immune defense through direct chemotactic activity or induction of cytokine release. In humans, the cathelicidin hCAP18/LL-37 is processed to LL-37 in neutrophils, but on skin it can be further proteolytically processed to shorter forms. The influence of these cathelicidin peptides on keratinocyte function is not known. In the current study, DNA microarray analysis and confirmatory protein analysis showed that LL-37 affects the expression of several chemokines and cytokines by keratinocytes. Analysis of a synthetic peptide library derived from LL-37 showed that antimicrobial activity against bacterial, fungal, and viral skin pathogens resides within specific domains of the parent peptide, but antimicrobial activity does not directly correlate with the ability to stimulate IL-8 production in keratinocytes. IL-8 release was induced by d- and l-amino acid forms of cathelicidin and correlated with membrane permeability, suggesting that highly structure-specific binding to a cell surface receptor is not likely. However, this effect was inhibited by either pertussis toxin or AG1478, an epidermal growth factor receptor tyrosine kinase inhibitor, suggesting that cathelicidin may indirectly stimulate multiple signaling pathways associated with cell surface receptors. Taken together, these observations suggest that proteolytic processing may alter the balance between cathelicidin antimicrobial and host immunostimulatory functions.

The production of antimicrobial peptides and proteins is essential for defense against infection. Many of the known human antimicrobial peptides are multifunctional, with stimulatory activities such as chemotaxis while simultaneously acting as natural antibiotics. In humans, eccrine appendages express DCD and CAMP, genes encoding proteins processed into the antimicrobial peptides dermcidin and LL-37. In this study we show that after secretion onto the skin surface, the CAMP gene product is processed by a serine protease-dependent mechanism into multiple novel antimicrobial peptides distinct from the cathelicidin LL-37. These peptides show enhanced antimicrobial action, acquiring the ability to kill skin pathogens such as Staphylococcus aureus and Candida albicans. Furthermore, although LL-37 may influence the host inflammatory response by stimulating IL-8 release from keratinocytes, this activity is lost in subsequently processed peptides. Thus, a single gene product encoding an important defense molecule alters structure and function in the topical environment to shift the balance of activity toward direct inhibition of microbial colonization.

Antimicrobial peptides (AMPs) have been shown in animal and human systems to be effective natural antibiotics. However, it is unclear how they convey protection; they often appear inactive when assayed under culture conditions applied to synthetic antibiotics. This inactivation has been associated with loss of function in physiological concentrations of NaCl or serum. In this study we show that the balance of host ionic conditions dictate microbial sensitivity to AMPs. Carbonate is identified as the critical ionic factor present in mammalian tissues that imparts the ability of AMPs such as cathelicidins and defensins to kill at physiological NaCl concentrations. After adapting to carbonate-containing solutions, global changes occur in Staphylococcus aureus and Escherichia coli structure and gene expression despite no change in growth rate. Our findings show that changes in cell wall thickness and Sigma factor B expression correspond to the increased susceptibility to the AMP LL-37. These observations provide new insight into the factors involved in enabling function of innate immune effector molecules, and suggest that discovery of new antimicrobials should specifically target pathogens as they exist in the host and not the distinctly different phenotype of bacteria grown in culture broth.

Peptides and small proteins exhibiting antimicrobial activity have been isolated from many organisms ranging from insects to humans, including plants. Their role in defense is established, and their use in agriculture was already being proposed shortly after their discovery. However, some natural peptides have undesirable properties that complicate their application. Advances in peptide synthesis and high-throughput activity screening have made possible the de novo and rational design of novel peptides with improved properties. This review summarizes findings in the identification and design of short antimicrobial peptides with activity against plant pathogens, and will discuss alternatives for their heterologous production suited to plant disease control. Recent studies suggest that peptide antimicrobial action is not due solely to microbe permeation as previously described, but that more subtle factors might account for the specificity and absence of toxicity of some peptides. The elucidation of the mode of action and interaction with microbes will assist the improvement of peptide design with a view to targeting specific problems in agriculture and providing new tools for plant protection.