Background and objectives Seronegative joint diseases are characterised by the lack of autoantibodies and rheumatoid factor, potent biomarkers for disease activity in seropositive arthritis. Promising alternative biomarkers in seronegative arthritis are the Damage Associated Molecular Patterns (DAMPs), S100A8 and S100A9, proteins specifically expressed by infiltrating phagocytes. In this study we explore the biomarker potential of S100A8/S100A9 to asses joint inflammation and damage in IL-1Ra-/- mice, a mouse model for seronegative arthritis in which serum autoantibodies are not correlated to disease activity.
Materials and methods Serum of IL-1Ra-/- and BALB/c mice was collected every two weeks and swelling in the hind paws was macroscopically scored. Serum levels of IL-1β, IL-6 and TNFα were measured by Luminex technology and S100A8/A9 complex levels by ELISA. Hind paws were isolated and histologically scored for cell influx, cartilage damage and bone erosion, sections were also stained for S100A9 expression. Synovial S100A8 was imaged by injection of anti-S100A8-Cy7 antibody and 24 h p.i. fluorescent images were acquired in the IVIS Lumina optical imaging system.
Results Starting at week 8 up to week 16, serum levels of S100A8/A9 in IL-1Ra-/- mice were significantly increased (1640 ± 1008 ng/ml) compared to BALB/c mice (429 ± 191 ng/ml, P = 0.005) and strongly correlated to joint swelling (r = 0.740, P < 0.0001) and microscopic parameters for joint pathology (r = 0.672–0.770, P < 0.0001), in contrast to levels of IL-1β, IL-6, and TNFα. Serum S100A8/A9 levels already correlated to joint swelling (r = 0.410, P = 0.047) as early as week 8 and high serum levels at week 10 were predictive for increased joint swelling at week 16 (r = 0.563, P = 0.004). Local expression of S100A9 within the synovium correlated to joint damage and serum S100A8/A9 levels and imaging of S100A8 using specific anti-S100A8-Cy7 antibody showed a specific targeting in inflamed joints of IL-1Ra-/- mice when compared to a control isotype (P = 0.044).
Conclusions High levels of serum S100A8/A9 correlate to joint inflammation and destruction in experimental seronegative arthritis and local synovial expression of S100-DAMPs can be monitored in vivo by molecular imaging. These findings underline the potential of S100-DAMPs as a serum and imaging biomarker for disease severity in seronegative arthritis.