Human synovial dendritic cells. Direct observation of transition to fibroblasts.

A subpopulation of human synovial cells develop a stellate morphology during in vitro culture. These cells, called stellate cells or dendritic cells, appear capable of collagenase production. In other tissues, particularly mouse spleen and human peripheral blood, cells with a similar shape express immune region associated (Ia) antigens, stimulate a primary mixed leukocyte reaction, and present antigen to lymphocytes. To characterize synovial dendritic cells, we observed changes in their morphology by time-lapse videomicroscopy. Parallel cultures of cells from the same specimens were observed periodically for HLA-DR surface antigens by immunofluorescence microscopy. During culture, synovial dendritic cells gradually lost their distinct morphologic appearance and became indistinguishable from fibroblasts. Moreover, fibroblasts occasionally assumed a dendritic morphology. When cells were examined at 24-48 h or later, we could not detect HLA-DR antigens on synovial dendritic cells, defined as cells with 4 or more stellate, branching projections. Cells with only 1-3 projections often had HLA-DR antigens. Because human synovial dendritic cells may lose their characteristic morphology upon culture and because they appear to lack Ia antigens in the unstimulated state, we suggest that synovial dendritic cells may be closely related to fibroblasts.