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.

The skin actively contributes to host defense by mounting an innate immune response that includes the production of antimicrobial peptides. These peptides, which include but are not limited to the cathelicidin and defensin gene families, provide rapid, broad-spectrum defense against infection by acting as natural antibiotics and by participating in host cell processes involved in immune defense. This review discusses the biology and clinical relevance of antimicrobial peptides expressed in the skin. The importance of the epithelial contribution to host immunity is evident, as alterations in antimicrobial peptide expression have been associated with various pathologic processes.

Innate immune defense against microbial pathogens occurs by physical barriers, by recruitment of cells such as neutrophils, NK cells, and macrophages, and by secretion of molecules with antimicrobial activity. Such molecules are produced by various epithelia including skin. The importance of antimicrobial peptides has been shown in cathelicidin-deficient mice, which have increased susceptibility to skin infection by Streptococcus. Although keratinocytes increase cathelicidin expression upon injury, their role relative to neutrophil cathelicidin and their sites of peptide storage and activation have not been elucidated. Herein, it is reported that cathelicidin predominantly resides in granules of the superficial epidermis and partially localizes in lamellar bodies as determined by immunogold electron microscopy and immunoblot of lamellar bodies isolated from mice. In cultured keratinocytes, cathelicidin displays a granular distribution and partially localizes within the Golgi apparatus. Cathelicidin processing can be observed by western blot analysis in keratinocyte extracts but not in conditioned media. Further, fluorescent bacteria colocalize with cathelicidin in granules both intracellularly and at the cell surface. These observations illustrate the immune defense potential of keratinocytes acting directly through storage and processing of antimicrobial peptides.