Sang Eun Lee, et al. pathogenesis of NS. Several studies reported that the poly- morphisms in the SPINK5 gene are also associated with AD.84,89 Glu420Lys single-nucleotide polymorphism in the SPINK5 gene was observed to be associated with AD and six other SPINK5 gene polymorphisms were also found in Japanese patients with AD.84,89 In addition, another protease inhibitor, cystatin A-encoding gene (CSTA gene) poly- morphism (+344c variant) has been reported to be asso- ciated with AD.90 Cystatin A is a cysteine protease inhibi- tor and is known to inhibit the endogenous cathepsins B, -H, and -L and the exogenous proteases from house dust mites, such as dermatophagoides pteronyssinus (Der p) 1 and dermatophagoides farinae (Der f) 1. This +344c variant in CSTA gene results in the reduced levels of cystatin-A in the skin surface and sweat, allowing exogenous proteases to break down the integrity of the SC, which in turn enhances the penetration of allergens, triggering an aggravation of AD. These genetic variants in protease and protease inhi- bitor genes can be combined in some patients with AD, resulting in a more severe defect in the skin barrier. Elevated pH The skin has a distinct pH gradient across the SC, with a progressively more acidic pH from the deeper layer to the outer layer of the SC.91 Studies have demonstrated the ‘acid mantle’ within the SC plays a critical role in regulating epi- dermal permeability barrier homeostasis, the epidermal antimicrobial barrier, and SC integrity/cohesion by regulat- ing the activity of various enzymes including serine pro- teases and lipid-processing enzymes.91 Skin pH is known to be generated by the endogenous processes, including trans- urocanic acid production from histidine, free fatty acid pro- duction via secretory phospholipase A (sPLA2), and the sodium-proton antiporter-1 (NHE-1)-mediated H+ secre- tion.92 Numerous endogenous factors, such as moisture, sweat, sebum, anatomic site, genetic predisposition and age as well as environmental factors, such as soap/detergents, topical irritant, antibiotics, and cosmetic products, and occlusive dressings can affect skin pH.93 In the lesions of patients with AD, the skin pH has been reported to be significantly elevated.94 The genetically determined barrier deficiency and various environmental factors are thought to be associated with the increased skin surface pH in AD lesions. Since Palmer, et al.95 first identified the loss-of-function mutation in the gene encoding filaggrin (FLG) as a strong predisposing factor for AD, a number of additional studies have reported other mutations of FLG genes, including 20 mutations in European populations and 17 in Asian popula- tions.96 From these reports, it is now conceived that muta- tions of the FLG gene are the most significant genetic factors in AD and the resultant genetically determined epidermal barrier defect is the primary event triggering immunologic pathogenesis in AD. Filaggrin itself contributes to the epi- dermal barrier integrity by cross-linking the keratin fila- ments and the being degraded into a combined pool of highly hygroscopic amino acids, called the NMF, thereby contri- buting to the SC hydration. It is also postulated that filag- grin contributes to the formation of acid mantle within SC through the generation of urocanic acid via filaggrin-histi- dine-urocanic acid cascade.97 Therefore, filaggrin deficiency in AD lesion leads to defects in the formation of cornified envelope and a decreased ability of maintaining SC hydra- tion and a concomitant elevation of pH. The elevated pH enhances the activity of KLK5 and KLK7, which have opti- mal activity at neutral pH, resulting in the over-degradation of corneodesmosomes and a decrease in SC integrity and cohesion. In addition, the abnormally neutral skin pH inhi- bits the activity of lipid-processing enzymes, including β- GlcCer’ase, aSMase, and sPLA2, which are essential in the production of ceramide and free fatty acids and causes impaired lipid processing and defects in the lipid barrier. SC acidity is also important in epidermal antimicrobial barrier function, inhibiting the growth of pathogens. Growth of Staphylococcus aureus, which colonizes the lesional skin of AD, is normally inhibited at low skin pH; therefore the elevated pH in AD lesional skin leads to bacterial growth, resulting in allergic inflammation and aggravation of AD.94 In addition to these genetic factors, various environmental factors can increase skin surface pH in AD patients. For instance, the frequent use of soaps/detergents seems not only to aggravate AD through directly irritating the skin by removing skin surface lipids, but also raises skin pH because most detergents are alkaline; but even at neutral, overuse of soap and detergent can have a negative effect on epider- mal barrier function. Exogenous proteases Various contact allergens and aeroallergens are considered important factors in the initiation and aggravation of AD. Proteolytic activity from allergens has been known to play a role in the pathogenesis of allergic diseases including allergic rhinitis, asthma, and AD through inducing Th2 allergic inflammation and directly affecting the structure and function of epidermal barrier, thereby facilitating further penetration of allergens through the defective skin barrier.98 House dust mite and cockroach allergens, the most important environmental factors in the pathogenesis of AD, have been shown to have proteolytic activity. House dust mite allergens have been reported to contain several cysteine and serine proteases. Serine proteases from mite allergens, Der p 3 and Der p 9, are known to activate PAR-2 on keratinocytes to produce cytokines and contribute to the pathogenesis of AD, whereas Der p 1 with cystein protease 816 Yonsei Med J http://www.eymj.org Volume 51 Number 6 November 2010