**RNase A superfamily**

Eight known functional RNase A ribonucleases genes are encoding small polypeptides of 15 kDa (Dyer & Rosenberg, 2006). Besides their well-documented ribonuclease activity, some of these proteins display unexpected antimicrobial activities unrelated to their primary function. Eosinophil-derived neurotoxin (EDN/RNase 2) and eosinophil cationic peptide (ECP/RNase 3) are proteins secreted by eosinophilic leukocytes and were primarily tested for their toxic role against parasites. *In vitro*, ECP has also an activity against Gram-positive and Gram-negative bacteria (Lehrer et al., 1989).

RNase A7 was identified as a major agent of the innate immune response of the skin acting on Gram-positive and Gram-negative bacteria and also on *C. albicans* (Harder & Schröder, 2002). RNase 7 transcripts were induced in keratinocyte culture by addition of TNF-alpha, interferon-gamma, interleukin-1 beta and in the presence of bacteria (Harder & Schröder, 2002). More recently, RNase 5 was added to the list of antimicrobial molecules present in skin (*stratum corneum*), and the same authors showed that skin proteases are involved in inhibition of RNases 5 and 7 (Abtin et al., 2009).

### **Psoriasin (S100A7)**

Psoriasin belongs to the S100 family of calcium-binding proteins. This family is composed of 21 genes and 11 proteins that have been found to be expressed in human epidermis or in cultured keratinocytes. Langerhans cells and melanocytes (Boni et al., 1997; Broome et al., 2003) express S100B and Meissner's corpuscules (sensorial receptors localized in the upper part of dermis) express S100P (Del Valle et al., 1994). These proteins possess two EF hands (helix-loop-helix calcium binding domains) and they act probably as calcium sensors. Several functions have been proposed for S100 proteins in keratinocytes, the main role being an implication in skin inflammatory processes (Jinquan et al., 1996). Another role could be keratinocytes membrane remodeling, that occurs during differentiation: psoriasin and another member of the S100 family, calgranulin-A (S100A9), have been shown to have their expression correlated with the degree of keratinocyte differentiation, suggesting that they are involved in this process (Martinsson et al., 2005]. A third role could be an involvement in the formation of calcium channels, in conjunction with annexins. Other postulated roles concern S100 proteins as substrate for transglutaminase, resulting in an incorporation of S100 in the cornified envelope; a last role could be a response to exogenous agents that modulate S100 proteins distribution and consequently their function (Eckert et al., 2004). Psoriasin has been found to be overexpressed in psoriasis. It is produced in *stratum corneum* by keratinocytes (Martinsson et al., 2005) and its basal expression is influenced by extracellular calcium level. Its expression in normal adult tissue is low, but high expression levels were detected in fetal skin, as for transferrin, suggesting a protective role in innate immunity. Psoriasin was found to be the main *E. coli*-cidal agent in the skin. It is a chemotactic agent for neutrophils and CD4+ T cells (Jinquan et al., 1996). Moreover, psoriasin mediates the production of several inflammatory cytokines and chemokines from neutrophils *via* MAPK p38 and ERK activation. It also induces reactive oxygen species production and the exocytosis of alpha-defensins from neutrophils (Zheng et al., 2008).
