High Concentration of Soluble HLA-DR in the Synovial Fluid: Generation and Significance in “Rheumatoid-like” Inflammatory Joint Diseases

In the search for its role in inflammatory joint diseases, soluble HLA-DR (sHLA-DR) was quantitated in 72 synovial fluids (SF) by a newly established immunoenzyme assay. Unlike other soluble receptors which accumulated only moderately (sCD25, sCD4) or negligibly (sHLA class I, sCD8) in the SF, SF sHLA-DR levels exceeded serum levels by up to 3 orders of magnitude and varied disease dependently from “control” values (traumatic synovitis and osteoarthritis: 9.9 ± 6.1 ng/ml). Clear-cut different SF sHLA-DR values in HLA-DR-associated “rheumatoid-like” (136.5 ± 130.0 ng/ml) vs HLA-B27-associated “spondylarthropathy-like” arthritic forms (28.4 ± 29.1 ng/ml) were most significant comparing oligoarticular juvenile chronic arthritis type I (147.6 ± 112.6 ng/ml) and type II (3.3 ± 1.1 ng/ml), thus offering a new classification marker. Also ex vivo, large amounts of sHLA-DR were released spontaneously by SF mononuclear cells and found to be related to the T-cell activation state. SF sHLA-DR may be shed in large complexes or micelles, as it eluted mainly at >450 kDa on gel filtration. Western blotting revealed that the majority of SF sHLA-DR consisted of full-length α- and β-chains. Minor fractions of smaller sized antigens seemed to be generated by proteolytic cleavage rather than by alternative splicing, since only minute amounts of HLA-DRB mRNA lacking the transmembrane exon could be amplified by RT–PCR. Distinct forms of high-dose sHLA-DR, able to provoke rather than to suppress T-cell responses, are discussed as contributing to some HLA-DR disease association.