Antimicrobial peptides: Effector molecules of the cutaneous defense system

Human skin is always in contact with the environment and is covered with a characteristic microflora, but becomes surprisingly rarely infected. One reason for this natural resistance might be the existence of a “chemical barrier” consisting in constitutively and inducibly produced antimicrobial peptides and proteins (AMPs), which include some β-defensins, RNase 7, the S100-protein psoriasin and the cathelicidin LL-37. Most of these AMPs can be induced in vitro in epithelial cells by proinflammatory cytokines or bacteria. In vivo AMPs are mainly expressed in uppermost and differentiated parts of inflammatory lesions and wounds, but some are also focally expressed in healthy skin in the absence of inflammation, suggesting that also non-inflammatory stimuli of endogenous and/or exogenous origin can stimulate AMP synthesis.Increased levels of AMPs in psoriatic lesions may explain why psoriasis patients rarely suffer from skin infections. Furthermore, an increased infection rate in atopic dermatitis patients could be the consequence of decreased levels of AMPs in atopic lesions. These observations may indicate an important role of the “chemical skin barrier” in prevention of skin infection and suggest that artificial stimulation of this system, without inflammation, would be beneficial as “immune stimulus”.