Demonstration of Ia Antigens on Certain Dendritic Cells and on a Novel Elongate Cell Found in Human Synovial Tissue

Adherent cells from dissociated human synovial tissue obtained at surgery contain two types of distinctive cells with one or more elongated branching processes that strongly express Ia antigens. One type of cell with Ia antigens is non‐phagocytic and resembles the murine dendritic cell. It is primarily found in patients with rheumatoid arthritis and accounts for a considerable proportion of the identifiable cells with a stellate or dendritic morphology. The expression of Ia antigens progressively diminished in culture. The second type of novel cell with Ia antigens was highly elongate and fibroblastoid. It was readily obtained from patients with osteoarthritis. The cell was frequently characterized by a blunt‐ended filopodium‐like process at one pole of the cell, one or two tapering processes, and zones of microvilli, Evidence was obtained suggesting that this cell, which might otherwise be considered fibroblast‐like, is in the mononuclear phagocyte lineage.