**4. Protease-activated receptors (PARs)**

PARs represent seven membrane-spanning G-protein coupled receptors, which are activated by serine proteases, which cleave a 'tethered' receptor-activating ligand at the Nterminus (Steinhoff et al. 2005). To date, four PARs (PAR1-4) have been characterized. PAR-1, PAR-3 and PAR-4 are activated by thrombine, and PAR-2 by trypsin and chymotrypsin. In human skin, PAR-2 is abundantly expressed by keratinocytes (Steinhoff et al. 1999), where it plays a role in regulating permeability barrier homeostasis, inflammation, pruritus, pigmentation, and wound healing upon activation by various endogenous and exogenous serine proteases. Whereas during skin-inflammation, PAR-2 is activated by neutrophil elastase and mast cell tryptase, upon infection or skin barrier defects proteases originating from certain bacteria, house dust mites, cockroaches or parasites can activate this receptor (Shpacovitch et al. 2007). Activation by *Propionibacterium acnes* protease causes induction of certain proinflammatory proteins, matrix metalloproteinases and antimicrobial peptides, including hCAP18/LL-37 (Lee et al. 2010).
