Dissociation and exchange of the beta 2-microglobulin subunit of HLA-A and HLA-B antigens.

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Abstract
Human histocompatibility antigens HLA-A, HLA-B, and HLA-C are a complex of 2 noncovalently associated subunits, a H chain glycoprotein (.alpha.) carrying the genetic polymorphism and an invariant L chain, .beta.2-microglobulin (.beta.2m). Upon incubation of papain-solubilized HLA with radiolabeled urinary .beta.2m, the latter is incorporated into HLA, where it substitutes for the preexisting .beta.2m that has dissociated from the complex. The association-dissociation equilibrium that governs this .beta.2m exchange reaction was investigated and was characterized by a long lifetime of the complex (half-life of 80 min at 37.degree. C) and relatively low Kd (4 nM). The .beta.2m exchange was used as the basis of a radioimmunoassay for HLA antigens with radiolabeled .beta.2m as a unique label for all HLA specificities. In a similar fashion, radiolabeled .beta.2m can be incorporated into HLA at the cell surface. Although the process is slower and less extensive than in solution, it can be used as a means to tag cells with specific probes for HLA antigens.