Fig. 1

Detection of soluble programmed cell death 1 (sPD-1) protein in synovial fluid (SF) and serum of patients with rheumatoid arthritis (RA) and controls. a Concentrations of sPD-1 were analyzed in SF or serum of patients with RA or controls [osteoarthritis (OA) or healthy controls (HC)] by enzyme-linked immunosorbent assay. Horizontal bars represent the mean concentration within a given group. b The levels of sPD-1 in serum specimens from patients with moderate rheumatoid arthritis activity (Mo-RA; 3.2 < 28-joint Disease Activity Score ≤ 5.1) (p = 0.017) and patients with high rheumatoid arthritis activity (Hi-RA; 28-joint Disease Activity Score >5.1) (p = 0.048) were higher than those in remission (Re-RA) or those with low rheumatoid arthritis activity (Lo-RA; 2.6 ≤ 28-joint Disease Activity Score ≤ 3.2). c The levels of sPD-1 in serum specimens of patients with RA did not differ significantly between those with late RA (Late-RA; >12 months duration) and patients with extraarticular manifestations (Extra-RA) or between those early in the disease course (Early-RA; ≤12 months duration) and patients with limited joint manifestations (Limited-RA). d and e Correlation of sPD-1 levels with disease activity [28-joint Disease Activity Score (DAS28)] (r = 0.545, p < 0.001) and rheumatoid factor (RF) content (r = 0.306, p = 0.005). f sPD-1 levels were significantly reduced after methotrexate therapy (p = 0.011). g–i Correlation between serum concentrations of sPD-1 and tumor necrosis factor (TNF)-α (r = 0.309, p = 0.046), interferon (IFN)-γ (r = 0.395, p = 0.010), and interleukin (IL)-17A (r = 0.433, p = 0.006) in patients with RA. The r value indicates the calculated regression coefficient