Activation of TLRs, expressed by epidermal keratinocytes, is directly involved in the induction of antimicrobial peptides [21, 55]. This diverse family of small, mostly cationic polypeptides exerts a broad spectrum of cytotoxic activity against bacteria, fungi, parasites, and enveloped viruses. During the inflammatory processes of the skin, keratinocytes are the main cellular sources of antimicrobial peptides and their expression levels correlate with the susceptibility of the skin to infections. The local accumulation of antimicrobial proteins offers a fast and very efficient way to prevent microbes from establishing an infection. Expression of antimicrobial peptides is induced upon encounter with pathogens and during wound healing [17, 26, 44]. Activation of antimicrobial genes by PAMPs can be further increased by proinflammatory cytokines produced at sites of inflammation by either keratinocytes or other cell types [17, 26, 28, 29, 44, 64]. Most keratinocyte-derived antimicrobial peptides belong to defensin, cathelici-din or RNase gene families and are able to kill or inactivate a wide spectrum of microorganisms mainly by forming pores and permeabiliz-ing microbial membranes.
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