Immunohistological reassessment of accessory cell populations in normal and diseased human synovium

Accessory cell populations in normal and diseased synovial tissue have been reanalyzed following the development of the monoclonal antibody, EBM11, directed against the CD68 epitope which is expressed by macrophages in all locations so far examined. Previous studies used as a macrophage marker the monoclonal antibody RFD7 which binds only a proportion of (mature) macrophages. Using double indirect immunofluorescence, normal and rheumatoid synovial samples were examined for the presence of cells which bind the putative dendritic cell marker, RFD1, in conjunction with either RFD7 or EBM11. RFD1 positive cells were found in five of 40 normal synovia. Of these cells, 30-35% were negative for RFD7 or EBM11 and, when closely apposed to T-lymphocytes, showed a typical interdigitating morphology. In contrast, all of ten rheumatoid synovia contained RFD1 positive cells; the extent of double labelling with macrophage markers varied with the position of the cells in the tissue. As expected, EBM11 stained a larger number of cells with macrophage morphology than RFD7. The combination of RFD1 and EBM11 appears to be a useful method for identifying interdigitating dendritic cells in connective tissue, these cells being characterized by positive RFD1 and negative EBM11 binding. On this criterion, interdigitating dendritic cells were plentiful in rheumatoid synovium and present, albeit infrequently, in normal synovium.