Lymphocyte surface marker expression in rheumatic diseases: evidence for prior activation of lymphocytes in vivo.

Expression of major histocompatibility complex (MHC) class II and other lymphocyte activation markers on peripheral blood and synovial fluid T lymphocytes from patients with rheumatoid arthritis (RA), psoriatic arthritis, and Reiter's syndrome were measured and the mean fluorescence intensities of these antigens were assessed. Increased expression of MHC class II antigens of synovial fluid T lymphocytes is not unique to RA, though it is quantitatively greater on RA synovial fluid T cells. There was less expression of other lymphocyte activation markers (4F2, transferin receptor) and a marked discordance between the expression of these markers and the interleukin 2 receptor (IL2r). Synovial fluid T lymphocytes contain a subpopulation of larger cells expressing MHC class II and other lymphocyte activation antigens with the exception of the IL2r. Mean fluorescence intensity of CD3 and CD4 antigens on synovial fluid T lymphocytes was decreased in all three patient groups, suggesting prior in vivo exposure of synovial fluid T lymphocytes to an unknown antigen.