Programmed death-1 (PD-1) is an immunoregulatory molecule functioning by down-regulating immune responses. PD-1 is present on follicular helper T cells (TFH) and is important in the formation of plasma cells. PD-1 exists in a bioactive soluble form (sPD-1) and is thought to be implicated in disease activity in chronic rheumatoid arthritis (RA).
We measured sPD-1 at baseline and 9 months after treatment initiation in plasma from early RA patients (n = 34). We tested for correlations with the Disease Activity Score using 28 joint counts (DAS28), the Health Assessment Questionnaire (HAQ) score, immunoglobulin M rheumatoid factor (IgM-RF), anti-cyclic citrullinated peptide (anti-CCP) antibodies, C-reactive protein (CRP), interleukin-21 (IL-21), and total Sharp score (TSS). We also measured sPD-1 in plasma from healthy volunteers (HV) (n = 20) and in plasma and synovial fluid (SF) from patients with chronic RA (> 8 years of disease, n = 30). We further investigated the cellular expression of PD-1 and its ligand PD-L1.
sPD-1 concentrations in early [median 0.421 ng/mL, interquartile range (IQR) 0.04-2.560 ng/mL] and chronic (median 0.239 ng/mL, IQR 0.184-0.584 ng/mL) RA were increased compared with HV (median 0.04 ng/mL, IQR 0.04-0.04 ng/mL) (all p < 0.005). In early RA the change in sPD-1 was associated with DAS28 (r = 0.363, p < 0.05) and HAQ score (r = 0.554, p < 0.05) and inversely with TSS at 3-5 years (r = -0.468, p < 0.05). sPD-1 concentration correlated with IgM-RF, anti-CCP antibodies, and IL-21 (all p < 0.05). PD-1 was primarily expressed by synovial memory T cells whereas PD-L1 was mainly expressed by synovial monocytes.
The significantly elevated plasma levels of sPD-1 in early RA, the association with core disease parameters, and the inverse correlation with TSS suggest that sPD-1 is an important mediator in inflammatory and radiographic disease progression.