Immunohistochemical characterization of synovial cells in arthritic MRL-lpr/lpr mice

MRL-lpr/lpr (MRL/l) mice spontaneously develop an autoimmune disease associated with arthritic manifestations. We used a recently developed mild demineralization procedure, followed by immunohistochemical staining of frozen sections, to investigate cell patterns in the hindlimbs of MRL/l mice at various stages of arthritic disease. Large numbers of Mac-1 (Mas 034)-positive, macrophage-like cells were seen both within the thickened synovial lining layer and in the deeper layers of the synovial tissue in all stages of arthritis. Ia-expressing cells were scarce in the lining layer, but occurred in moderate numbers in the deeper layers of synovial tissue. Lymphocytes were totally absent in MRL/l joints in all stages of arthritis, as demonstrated by lack of staining with Ly-1, Lyt-2, GK 1,5, and antiimmunoglobulin antibodies. Our findings are discussed and related to other types of experimental arthritis and to rheumatoid arthritis in humans.