Soluble histocompatibility antigens in synovial fluids of patients with rheumatoid arthritis

Soluble histocompatibility antigens of the class II region have been detected in synovial fluids obtained from patients with rheumatoid arthritis. A capture immunoassay involving two monoclonal antibodies was used; interference by rheumatoid factor, which is a feature of such assays, was overcome by mild pretreatment of fluids with 2-mercaptoethanol. No HLA class II antigen could be detected in matched sera from patients, even when levels were high in synovial fluids. Released HLA-class II material was of high molecular weight (greater than 1000 kD) and was linked to HLA-class I antigen. However, no significant amounts of other common cell surface antigens were detected in the complex, suggesting a preferential release of MHC antigens from cells of the inflamed synovium. Attempts to induce production of similar material from a cell line which expresses HLA class II strongly at the cell surface, by stressing the cells in various ways did not succeed, indicating that release is an active process.