THE SOLUBLE POOL OF BETA-2-MICROGLOBULIN FREE HLA CLASS-I ALPHA-CHAINS - QUALITATIVE AND QUANTITATIVE CHARACTERIZATION

Previous studies have demonstrated that HLA class I heterodimers are p resent in plasma and cell culture supernatants. They can be precipitat ed by mAb the binding of which is dependent on the proper association of the polymorphic alpha-chain with beta2-Microglobulin (beta2-m). The molecular mass of the alpha-chain ranges from 45 to 35 kDa with a num ber of intermediate products. We report on the identification of 35-kD a soluble beta2-M free HLA class I H chains immunoprecipitated by mAb LA45 from cell culture media of activated B and T cells. Furthermore, a peptide-based competitive immunosorbent assay was established to det ermine the amounts of soluble HLA class I alpha-chains. By means of th is assay, we formally proved the specificity of mAb LA45 for a linear epitope on HLA class I H chains centered on residues arginine-asparagi ne at positions 62 and 63 of the alpha1-domain. PHA or rIL-2 were iden tified as efficient stimuli for PBMC leading to the generation of solu ble beta2-m free HLA class I H chains. Testing of cell lines represent ing distinct stages of hematopoietic differentiation demonstrated a si gnificant correlation between cell surface expression of beta2-m free HLA class I H chains and amounts of soluble LA45 reactive molecules. H owever, three of six human T lymphotropic virus type I transfected cel l lines, although expressing beta2-m free H chains, do not generate so luble molecules. Finally, human sera were found to contain considerabl e amounts of beta2-M free HLA class I H chains. The average amount of these molecules in sera of individuals with one positive LA45 allele w as determined to be 46.9 +/- 38.6 nM/liter.