Levels of synovial fluid interleukin‐1 receptor antagonist in rheumatoid arthritis and other arthropathies. potential contribution from synovial fluid neutrophils

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Abstract
Objective. To measure synovial fluid (SF) levels of interleukin-1 receptor antagonist (IL-1ra) and to determine the capacity of SF neutrophils (PMN) to synthesize and release IL-1ra. Methods. A sensitive and specific enzyme-linked immunosorbent assay was used to measure SF IL-1ra protein concentrations and IL-1ra production by isolated SF PMN. Results. SF IL-1ra levels were elevated in 13 of 16 samples from patients with rheumatoid arthritis (RA) (mean 17.1 ng/ml), in 6 of 18 samples from patients with infectious or inflammatory, non-RA arthropathies (mean 10.6 ng/ml), and in none of 11 noninflammatory SF samples. SF IL-1ra levels correlated with SF PMN concentrations (r = 0.680, P < 0.00001). Isolated SF PMN contained preexisting IL-1ra protein in the absence of messenger RNA (mRNA). In addition, both lipopolysaccharide and granulocyte-macrophage colonystimulating factor induced modest increases in IL-1ra mRNA by cultured SF PMN. Conclusion. IL-1ra levels are increased in >80% of RA SF samples. SF PMN produce IL-1ra, possibly contributing to the levels of IL-1ra present within the SF.