MAST-CELL SUBPOPULATIONS IN THE SYNOVIAL TISSUE OF PATIENTS WITH OSTEOARTHRITIS - SELECTIVE INCREASE IN NUMBERS OF TRYPTASE-POSITIVE, CHYMASE-NEGATIVE MAST-CELLS

Although there is relatively little evidence of inflammation in osteoa rthritis (OA), increases in mast cell numbers and mast cell activation are prominent features of the synovial tissue, As little is known of the types of mast cells which may be involved, the numbers and distrib ution of mast cell subpopulations have been investigated as defined ac cording to their content of proteases. Tissue was obtained from patien ts with OA undergoing total knee replacement surgery (n=14) and from c ontrol subjects either post-mortem (n=11) or following leg amputation for peripheral vascular disease (n=3); a double-labelling immunocytoch emical procedure with monoclonal antibodies specific for tryptase and chymase mas applied to identify those mast cells which contain both tr yptase and chymase (MCTC) and those with tryptase but not chymase (MCT ). There was considerable variation between individual tissues and bet ween sites of tissue sampling, but cells of the MCTC subset were predo minant in the synovial layer of both groups of subjects without joint disease, accounting for some 60 per cent of all mast cells present, In tissue from OA patients, however, there appeared to have been a strik ing shift in the relative proportions of mast cells from the MCTC to t he MCT phenotype, with many more MCT cells present in the synovial tis sues of OA patients (median 53 MCT/mm(2)) than in tissue from post-mor tem (7.5 MCT/mm(2), P<0.0001) or amputation controls (12 MCT/mm(2)). I n contrast, numbers of synovial MCTC cells in the synovium of OA patie nts (20 MCTC/mm(2)) differed little from those in either of the contro l groups (both 12 MCTC/mm(2)). In several other conditions, the MCT ce lls have been linked with inflammatory events, but it seems that in OA , other factors may be operating to induce a selective expansion of th is subpopulation.