Background: About 30% of cutaneous psoriasis (PsC) patients develop psoriatic arthritis (PsA) in the joint, which is under-recognized by dermatologists. Biomarkers for PsA are needed so that early referral to a rheumatologist is made. Kallikreins (KLKs) are secreted serine proteases implicated in skin desquamation and inflammation. This study examined KLK potential as serum biomarkers of PsA in cutaneous psoriasis patients.

Methods: KLKs were measured by ELISAs in synovial fluids of three PsA patients and three control early osteoarthritis (OA) patients, as well as in a cohort of 152 serum samples collected from age-and sex-matched PsC patients, with (n= 76) or without PsA (n= 76). KLK expression in psoriatic plaques was examined by immunohistochemistry. Univariate and multivariate logistic regression analyses were conducted to analyze the association between serum KLK levels and disease class (PsC, PsA). Serum KLKs that associated with PsA were correlated with clinical parameters of skin and joint activity.

Results: Among the seven KLKs tested, KLK6 and KLK8 were elevated in both PsA synovial fluids and psoriatic plaques, but only serum KLK8 levels were associated with psoriatic disease (odds ratio= 2.56, p= 0.03). Although significantly elevated in PsC and PsA sera compared to healthy controls, KLK8 did not discriminate PsA from PsC patients. KLK8 correlated positively with the psoriasis area and severity index (PASI)(r= 0.43, p= 0.001) independent of age, sex and psoriasis duration (β= 1.153, p= 0.0003) and exhibited no correlations with tender or swollen joint counts.

Conclusions: Increased KLK8 serum level in PsA patients reflects disease activity in the skin but not in the joints. Serum KLK levels are not useful for screening psoriasis patients for PsA.