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HLA genes have been identified as key genetic factors contributing to many chronic diseases characterized by autoimmune features. The role of HLA encoded molecules in the pathogenesis of these diseases is unresolved. We have now analysed soluble HLA-DR molecules circulating in the serum of patients with different autoimmune diseases and have defined parameters controlling serum levels. Patients with HLA-DR associated diseases were characterized by elevated serum concentrations of HLA-DR molecules and were clearly distinct from patients with HLA-B27 associated disorders. We did not find evidence for a correlation between
disease activity, laboratory abnormalities and elevated serum concentrations of soluble HLA-DR molecules. Studies in normal individuals indicated that soluble HLA-DR molecules are at least partially regulated by the HLA haplotype. Highest serum concentrations were found in individuals carrying the HLA-DR3 or HLA-DR4 haplotype raising the possibility that the phenomenon of HLA-
disease association reflects differences in the genetic control of soluble HLA-DR molecules. Interferon-gamma treatment caused an increase in serum concentrations of soluble HLA-DR molecules, whereas a decrease of circulating HLA-DR molecules was associated with an immunosuppressive with cyclosporine A. These data suggest that the patient’s immunoresponsiveness represents a second important mechanism controlling circulating HLA-DR molecules.