CD69 expression on lymphocytes and interleukin-15 levels in synovial fluids from different inflammatory arthropathies

Objective. The aim was to study a possible relationship between CD69 expression on lymphocytes and interleukin-15 (IL-15) levels in synovial fluid (SF) from patients with different inflammatory arthropathies. Methods. CD69 expression was assessed by two-color flow cytometry on different subsets of synovial fluid lymphocytes (SFL) obtained from patients with diagnoses of rheumatoid arthritis (RA), seronegative spondyloarthropathy (SSd), and crystal-associated arthritis (CAA). The IL-15 levels in synovial fluid supernatants were measured by enzyme-linked immunoassay (ELISA). Results. No significant differences between the three groups of arthropathies were observed in the distribution of synovial fluid lymphocyte subsets. CD69-positive SFL were mainly CD3-, CD45RO-, and CCR5-positive cells. Although no significant differences in the percentage of CD69-positive lymphocytes were observed between the three groups of patients, the highest level of CD69 expression on lymphocytes was observed in RA patients (mean fluorescence intensity 37.9 +/- 5.2 compared to 17.5 +/- 3.3 in SSd and 12.4 +/- 2.6 in CAA, P = 0.007 and P < 0.001, respectively). In addition, the expression of CD69 on SFL from RA correlated with their respective IL-15 levels measured in SF supernatants. Conclusion. Our data provide in vivo evidence of the putative role that IL-15 can play in the high expression of CD69 on SFL from RA patients.