Proliferative activity of cells in the synovium as demonstrated by a monoclonal antibody, Ki67

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Abstract
The presence of proliferating cells has been sought in the synovium of rheumatoid arthritic (RA) and osteoarthritic (OA) joints using the monoclonal antibody Ki67, which marks a nuclear antigen present in all stages of the cell cycle apart from Go. Synovia were studied from 21 RA and 14 OA cases using the indirect immunoperoxidase technique. Double-staining was performed on 18 RA and 17 OA synovia with the simultaneous labelling of lysozyme (muramidase) by the immuno-alkaline phosphatase method and with Ki67 by the indirect immunoperoxidase method. Most of the RA and OA synovia showed an absence of Ki67-positive cells in the intimal cell layer. Three RA and four OA synovia showed no more than ten proliferating cells in the whole of the intimal layer examined. Similar results were obtained when double-labelling was performed. Eight RA and six OA synovia showed the presence of occasional Ki67-positive cells in the intimal layer. The total number of intimal cells was measured for each histological section, and the proliferation index calculated as the percentage of total cells in the intimal layer showing Ki67-positive staining. This varied between 0.03% and 0.0033% (between 1 : 2800 and 1 : 30 000 cells). In contrast, there were plentiful Ki67-positive cells present in the lymphocytic infiltrate and around blood vessels in the RA synovia and in the synovial infiltrate, where present, in the OA cases. The results provide further support for the suggestion that the intimal cell layer of the synovium is predominantly composed of macrophages, with some fibroblasts, and that the macrophages have not arisen locally by cell division (hyperplasia) but have migrated from the underlying synovial blood vessels.