Serum interleukin 40: an innovative diagnostic biomarker for patients with systemic lupus erythematosus

Interleukin (IL)-40 is a recently identified cytokine with a novel role in the pathogenesis of inflammatory diseases. Since systemic lupus erythematosus (SLE) is an autoimmune disease characterised by a pro-inflammatory response, it is likely that IL-40 contributes to the underlying disease processes of this disorder. The aim of the current study was to evaluate the potential of IL-40 to act as a diagnostic biomarker for SLE.

The study included 99 patients with SLE who attended the Rheumatology Unit at Baghdad Teaching Hospital. These subjects were divided into three subgroups according to disease status: inactive, n = 33; active moderate, n = 33; and active severe, n = 33. Additionally, 33 matched controls were studied. Full medical histories, body mass index, gender and clinical disease activity, the latter evaluated with the SLE disease activity index, were collected. Laboratory parameters measured included anti-dsDNA antibodies, C3 and C4 levels, erythrocyte sedimentation rate and C-reactive protein titres. Serum IL-40 levels were quantified using an enzyme-linked immunosorbent assay.

IL-40 levels were significantly higher in patients (12.5420 ± 3.00575 ng/L) than in controls (6.1138 ± 0.59452 ng/L; p < 0.01). Mean serum IL-40 concentration was highest in the active severe group (15.2291 ± 2.26540 ng/L) and decreased, in order of disease severity, in the remaining cohorts: active moderate, 13.0643 ± 1.23927 ng/L; inactive, 9.3325 ± 1.62807 ng/L (P < 0.01); controls, 6.1138 ± 0.59452 ng/L. Serum IL-40 levels showed excellent validity for the diagnosis of SLE with a cut-off value of ≥ 9.3 ng/ml and area under the curve of 0.987. Sensitivity, specificity and accuracy were 99%, 90.9% and 96.97%, respectively (P < 0.001).

Serum IL-40 levels were elevated in SLE patients. It is therefore proposed that IL-40 is a novel cytokine which is associated with SLE and positively linked with disease severity.