Unveiling serine protease activity profiles in Netherton syndrome skin across clinical subtypes by non-invasive analysis

Uncontrolled kallikrein-related peptidase (KLK) activity underlies Netherton syndrome (NS). However, whether KLK expression and activity vary between lesional and non-lesional skin, or across NS clinical subtypes, remains unclear, which could be crucial for treatment optimization. Using non-invasive 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 non-lesional skin from twenty NS patients with the two clinical subtypes. We found that KLK5 and KLK7 were strongly expressed in the upper epidermis of healthy controls and NS patients, 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 non-epithelial 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 non-lesional NS skin, regardless of clinical subtype, were higher as compared to healthy controls. Notably, within each clinical subtype, lesional and non-lesional skin displayed similar protease activity levels. However, scaly erythroderma (SE) subtype patients showed higher KLK7-like protease activity in lesional superficial skin than ichthyosis linearis circumflexa subtype patients. In one NS-SE patient 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 NS patients and represents a non-invasive tool for evaluating treatment efficacy in NS clinical trials.