Characterization and functional studies of rheumatoid synovial mast cells: Activation by secretagogues, anti‐IgE, and a histamine‐releasing lymphokine

Microscopic analysis of synovial specimens from 35 patients with rheumatoid arthritis (RA) and 7 patients with osteoarthritis revealed mast cell hyperplasia in perivascular regions, in fibrous interstitial areas, and clustered around the periphery of lymphoid aggregates. Metachromatic staining, immunofluorescence studies, and ultrastructural analysis revealed a single population of connective tissue‐type mast cells with surface IgE receptors. Total extractable histamine of synovial tissue was 4.15 ± 2.30 μg/gm (n = 8) for RA synovium and 0.53 ± 0.23 μg/gm (n = 7) for OA synovium. Mast cell secretion was assessed and specific release of histamine from RA synovial mast cells was observed following stimulation with anti‐IgE (32.3%), compound 48/80 (40.1%), calcium ionophore A23187 (25.2%), and a partially purified lymphokine with histamine‐releasing activity (23.9%).