Macrophage-like cells of the pannus area in rheumatoid arthritic joints

Frozen sections of pannus tissue taken from the joints of patients with rheumatoid arthritis have been investigated using immunohistological methods to determine the distribution of subsets of macrophage-like cells in this area. A panel of monoclonal antibodies including reagents specific in normal tissue for interdigitating cells (RFD1), macrophages (RFD7), epithelioid cells, (RFD9), monocytes (UCHMI), and osteoclasts (263C), were used. Indirect immunoperoxidase and combination indirect immunofluorescence procedures revealed the phenotypes of macrophage-like cells in four histologically distinct areas of the tissue: the synovial lining layers, the deeper stroma, areas of perivascular infiltration, and the articular cartilage junction where degeneration was occurring. It was discovered that 80% of the lining cells and a majority of macrophage-like cells of the stroma, express the phenotype RFD1+ RFD7+ UCHMI+. Cells with a typical 'dendritic cell' phenotype (RFD1+ RFD7-) were only present in the perivascular infiltrates, while 'classic macrophages' (RFD7+ RFD1-) were the cells accumulating at the cartilage junction. No significant numbers of RFD9+ epithelioid cells were seen. 263C+ osteoclasts were present in small numbers distributed throughout the stroma but did not appear to be involved in areas of cartilage degradation. This cellular distribution in the pannus is compared with previous studies on the rheumatoid synovium proper. It is concluded that a distinct inflammatory reaction occurs in the pannus and that classic activated macrophages are the cells involved in cartilage degradation.