Quantitation of soluble HLA class II molecules by an enzyme-linked immunosorbent assay

In order to quantify soluble HLA-DR,DQ,DP molecules (sHLA-RQP) an enzyme-linked immunosorbent assay was developed utilizing two monoclonal antibodies specific for HLA-DR,DP (Tü35) and HLA-DQ (Tü22) gene products, respectively. Highly purified HLA class II molecules isolated from a lymphoblastoid cell line were used for calculation of exact sHLA-RQP protein values. Circadian variations of sHLA-RQP plasma levels were studied in 7 healthy probands showing no significant deviations; measurements in 4 probands at intervals between 4 and 6 weeks revealed that sHLA-RQP levels remain relatively stable. The population analysis of 209 unrelated, HLA-typed healthy donors resulted in an average protein concentration of 1.53 +/- 2.44 micrograms/ml plasma for sHLA-RQP. Four out of 209 probands (= 1.9%) had no detectable sHLA-RQP. Significant associations of high or low sHLA-RQP levels to particular HLA-DR or -DQ specificities were not observed. However, plasma derived from HLA-DR9 positive had the highest and from HLA-DR8 positive donors the lowest mean sHLA-RQP values. By comparing HLA identical with two-haplotype-different siblings we found no evidence that sHLA-RQP plasma levels are under genetic control of the HLA complex or closely linked genes. Furthermore, soluble HLA class I plasma concentrations in 100 probands analyzed showed no correlation to those of sHLA-RQP.