To analyze S‐100 protein expression, in the form of myeloid‐related protein 8 (MRP8), MRP14, and the heterodimer MRP8/MRP14, in psoriatic arthritis (PsA) patients compared with rheumatoid arthritis (RA) and spondylarthropathy (SpA) patients, and to determine the effect of methotrexate (MTX) on the MRP antigen expression in PsA patients.

Serum, synovial fluid (SF), and synovium (taken at arthroscopy) samples were obtained from PsA (before and after MTX treatment), RA, and SpA patients. Concentrations of MRP8/MRP14 in serum and SF were measured by enzyme‐linked immunosorbent assay. Expression of MRP8, MRP14, and MRP8/MRP14 in synovium was determined by immunohistochemistry.

MRP8, MRP14, and MRP8/MRP14 levels were increased in serum, SF, and synovium from PsA, RA, and SpA patients. In all 3 groups, paired samples of serum and SF showed significantly higher MRP8/MRP14 levels in SF (mean ± SD 15,310 ± 16,999 ng/ml [median 11,400]) than in serum (908 ± 679 ng/ml [median 695]) (P = 0.0001). MRP8/MRP14 levels in serum correlated with systemic parameters of disease activity (erythrocyte sedimentation rate [ESR] r = 0.55, P = 0.005; C‐reactive protein [CRP] level r = 0.55, P = 0.005), whereas levels in SF correlated with local parameters of disease activity (white blood cell count r = 0.45, P = 0.01; acute‐phase serum amyloid A level r = 0.32, P = 0.03). MRP expression was significantly higher in the synovial sublining layer (SLL) of PsA patients compared with RA and SpA patients. MRP antigens were predominantly expressed in perivascular areas of the SLL in PsA patients. Following MTX treatment, MRP expression in serum and synovium from PsA patients was significantly reduced. Serum levels of MRP were more sensitive to the effects of MTX than were the ESR, CRP, or clinical joint scores.

MRP levels in serum and SF correlate with local and systemic inflammation and are equally increased in PsA, RA, and SpA patients. In contrast, MRP8, MRP14, and MRP8/MRP14 expression in the SLL of PsA patients is increased, particularly in perivascular regions, compared with that in RA and SpA patients, suggesting a central role of MRP proteins in transendothelial migration of leukocytes in PsA. Moreover, MRP expression is reduced following MTX treatment. MRP proteins may represent a novel therapeutic target in inflammatory arthritis.