OBJECTIVE--To determine the distribution, activation, and tryptase/chymase phenotype of mast cells (MCs) in the rheumatoid lesion. METHODS--MC tryptase and chymase were studied by immunohistochemistry using monoclonal antibodies and examination by brightfield, interference, and fluorescent microscopy. Thirty four specimens of cartilage-pannus junction and 26 specimens of rheumatoid synovium, all derived from knee surgery, were examined. RESULTS--MCs were identified in all specimens examined, but their distribution and local concentrations varied, both within and between specimens. As a proportion of total synovial cells, there were more MCs in fibrous synovial tissues than in those with active inflammatory cell infiltrations; MCs usually showed a peripheral distribution around lymphocytic/mononuclear cell infiltrations. Most cartilage-pannus specimens demonstrated local concentrations of MCs at, or close to, sites of cartilage erosion, a significant proportion of which showed extracellular tryptase indicative of MC degranulation. MC degranulation was often associated with localised oedema and disruption of the stromal matrix. Two MC phenotypes were identified: one population contained tryptase alone (MCT) whilst another contained both tryptase and chymase (MCTC). The ratio MCT:MCTC approximated 8:1. CONCLUSIONS--This histological study demonstrated that local concentrations of MCs and their activation/degranulation are commonly observed in the rheumatoid lesion, and especially at sites of cartilage erosion. Such observations add weight to the concept that MCs contribute to the processes of inflammation, matrix degradation and tissue remodelling.
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