Baseline serum interleukin-34 levels independently predict radiographic progression in patients with rheumatoid arthritis.

The aim of our observational study was to investigate the clinical significance of interleukin (IL)-34, a novel osteoclastogenic cytokine, for predicting structural damage in patients with rheumatoid arthritis (RA). Serum IL-34 levels were measured in 100 RA patients, 36 patients with ankylosing spondylitis (AS), and 59 gender- and age-matched healthy individuals using an enzyme-linked immunosorbent assay. We also measured IL-34 concentrations in synovial fluid (SF) samples from 18 RA patients and 19 osteoarthritis (OA) patients. Progression of structural damage was assessed in 81 patients with RA by plain radiographs using the modified Sharp/van der Heijde score (SHS) at baseline and after an average 1.6-year follow-up period. Serum IL-34 levels were significantly higher in patients with RA (p < 0.001) or AS (p < 0.001) than in healthy controls. SF IL-34 levels were also significantly higher in RA patients than in OA patients (p < 0.001). In RA, serum IL-34 levels were associated with rheumatoid factor positivity (p = 0.01), current smoking (p < 0.01), erythrocyte sedimentation rate (p = 0.01), and C-reactive protein levels (p < 0.01), but not with disease activity score 28. ΔSHS/year was positively correlated with serum IL-34 levels (r = 0.443, p < 0.001). In multivariate logistic regression analyses, serum IL-34 level was an independent risk factor for radiographic progression. These results suggest that IL-34, a novel osteoclastogenic cytokine, plays a role in RA-associated joint damage and is a potential biomarker for predicting subsequent radiographic progression in patients with RA.