Unveiling serine protease activity profiles in Netherton syndrome skin across clinical subtypes by noninvasive analysis.

Uncontrolled kallikrein-related peptidase (KLK) activity underlies Netherton syndrome (NS). However, whether KLK expression and activity vary between lesional and nonlesional skin, or across NS clinical subtypes, remains unclear, which could be crucial for treatment optimization. Using noninvasive skin sampling and skin biopsies, we profiled the expression and activity of five NS-relevant KLKs (the trypsin-like KLK5, KLK6, KLK13, and KLK14, and the chymotrypsin-like KLK7) in lesional and nonlesional skin from 20 patients with NS with the two clinical subtypes. We found that KLK5 and KLK7 were strongly expressed in the upper epidermis of healthy controls and patients with NS, regardless of lesion status. In contrast, KLK6 and KLK13 were predominantly increased in NS lesional skin. KLK14 showed weak expression in NS epidermis and healthy control skin, but was more strongly expressed in nonepithelial cells, including neutrophils and mast cells. Regarding protease activity, total trypsin-like serine protease activity (comprising KLK5, KLK6, KLK13, and KLK14 activities) and KLK7-like activity in lesional and nonlesional NS skin, regardless of clinical subtype, were higher as compared with healthy controls. Notably, within each clinical subtype, lesional and nonlesional skin displayed similar protease activity levels. However, patients with scaly erythroderma (SE) subtype showed higher KLK7-like protease activity in lesional superficial skin than patients with ichthyosis linearis circumflexa subtype. In one patient with NS-SE undergoing partially effective secukinumab therapy, superficial skin protease activity showed no significant change. Measurement of KLK activity from tape strips revealed distinct proteolytic patterns among patients with NS and represents a noninvasive tool for evaluating treatment efficacy in NS clinical trials. This study identifies differences in kallikrein-related peptidase (KLK) expression and activity between healthy skin and Netherton syndrome (NS) skin, distinguishing lesional and nonlesional areas in the two major clinical subtypes. Increased expression of KLK6 and KLK13 is characteristic of the epidermis in NS lesional skin. KLK14 can be expressed by both keratinocytes and immune cells. Patients with scaly erythroderma subtype NS show higher KLK7-like activity in lesional skin, highlighting subtype-specific differences.