Evaluation of IL-34 in psoriasis and psoriatic arthritis patients: Correlation with disease activity and severity

Interleukin-34 (IL-34) is a newly discovered cytokine essential for skin homoeostasis. It is involved in macrophage differentiation, osteoclastogenesis and inflammation. This could suggest a potential role in the pathogenesis of psoriasis and psoriatic arthritis (PsA). To assess serum IL-34 level in psoriatic patients with and without arthritis and to correlate it with disease activity and severity. Serum IL-34 level was measured in 45 psoriasis patients (21 had PsA) and 20 healthy controls. Patients were clinically assessed using psoriasis skin area severity index (PASI), composite psoriasis disease activity index (CPDAI) and peripheral joint score (PJS). Radiological assessment of hands and feet was done using modified Sharp-van der Heijde (mSvH) scoring method for PsA. The mean age of the PsA patients (47.4 ± 10.2 years) was comparable with the psoriasis only patients (42.5 ± 7.5 years) and control (43.4 ± 7 years). Serum IL-34 was significantly higher among PsA patients compared to those without arthritis and controls (median = 2500 ng/L, 512 ng/L and 325 ng/L, respectively). CPDAI was significantly higher in PsA compared to patients without arthritis. PASI scores were comparable between patients. Serum IL-34 level correlated significantly with each of PASI, CPDAI and PJS but not with the mSvH score. Receiver operator characteristic curve analysis revealed that serum IL-34 testing showed excellent diagnostic performance for PsA in psoriasis patients. The markedly elevated IL-34 serum level in PsA patients compared to non-arthritic ones and its remarkable correlation with PsA disease activity, suggest its importance as a marker for arthritis in psoriasis patients.