Monocyte activation in rheumatoid arthritis: evidence for in situ activation and differentiation in joints

Monocytes, defined by staining with the Mo-2 and Leu-M3 monoclonal antibodies which both detect the CD14 antigen, were studied in peripheral blood of control subjects and patients with rheumatoid arthritis as well as in rheumatoid synovial fluid of the latter for expression of activation/differentiation markers. The monocytes in the rheumatoid synovial fluids showed increased expression of class II major histocompatibility antigens (HLA-DR and -DQ) and decreased positivity for the peanut agglutinin receptor as compared with those from patient and control peripheral blood. There were no differences in phenotype between the control and patient peripheral blood monocytes. These findings are consistent with the hypothesis that monocytes are activated and differentiate into more mature macrophage-like cells after entry into the rheumatoid joint.