Synovial tissue macrophage as a source of the chemotactic cytokine IL-8

Cells of the synovial microenvironment may recruit neutrophils (PMN) and lymphocytes into synovial fluid, as well as lymphocytes into the synovial tissues, of arthritic patients. We have investigated the production of the chemotactic cytokine IL-8 by using sera, synovial fluid, synovial tissue, and macrophages and fibroblasts isolated from synovial tissues from 75 arthritic patients. IL-8 levels were higher in synovial fluid from rheumatoid (RA) patients (mean ± SE, 14.37 ± 5.8 ng/ml), compared with synovial fluid from osteoarthritis patients (0.135 ± 17 ng/ml) (p < 0.05) or from patients with other arthritides (5.52 ± 5.11 ng/ml). IL-8 from RA sera was 8.44 ± 2.33 ng/ml, compared with nondetectable levels found in normal sera. IL-8 levels from RA sera and synovial fluid were strongly positively correlated (r = 0.96, p < 0.05). Moreover, RA synovial fluid chemotactic activity for PMN in these fluids was inhibited 40 ± 5% upon incubation with neutralizing polyclonal antibody to IL-8. Synovial tissue fibroblasts released only small amounts of constitutive IL-8 but could be induced to produce IL-8 by stimulation with either IL-1β, TNF-α, or LPS. In contrast, unlike normal PBMC or alveolar macrophages, macrophages isolated from RA synovial tissue constitutively expressed both IL-8 mRNA and antigenic IL-8. RA synovial macrophage IL-8 expression was not augmented by incubation with either LPS, TNF-α, or IL-1β. Immunohistochemical analysis of synovial tissue showed that a greater percentage of RA macrophages than osteoarthritis macrophages reacted with anti-IL-8. Whereas macrophages were the predominant cell for immunolocalization of IL-8, <5% of synovial tissue fibroblasts were positive for immunolocalized IL-8. These results suggest that macrophage-derived IL-8 may play an important role in the recruitment of PMN in synovial inflammation associated with RA.