Increased expression of CD23 in rheumatoid synovitis

OBJECTIVE: Soluble (s)CD23 is a potent macrophage stimulator. High levels of this molecule have been reported in rheumatoid arthritis (RA) serum. We investigated the expression of CD23 and its ligands in rheumatoid synovial fluid and cells. METHODS: Levels of sCD23, and cellular expression of CD23 and its ligands CD21, CD11b, and CD11c were measured in synovial fluid (SF) of RA patients and in blood of RA patients and controls. RESULTS: SF contained higher levels of sCD23 than either rheumatoid or normal sera (median 4.8, 3.16,and 1.13 ng/ml respectively, p <0.01). Synovial CD23 was found to be expressed principally on macrophages. While little CD21 expression was detected, CD11b and CD11c were both expressed at high levels, particularly on macrophages. CONCLUSIONS: Soluble CD23 is present in high levels in RA synovial fluid. Macrophages appear to be the principal source. Macrophages also express ligands for sCD23, and may therefore also be the targets of this potent pro-inflammatory molecule.