EXOGENOUS GAMMA-2-MICROGLOBULIN IS REQUIRED FOR ANTIGENIC PEPTIDE BINDING TO ISOLATED CLASS-I MAJOR HISTOCOMPATIBILITY COMPLEX-MOLECULES

Binding of antigenic peptides to purified class I major histocompatibility complex (MHC) molecules, as measured by antigen-specific cytolytic T lymphocyte (CTL) degranulation, was found to occur in the presence of serum but not in its absence. The role of soluble beta-2-microglobulin (beta-2m), a normal component of serum, in class I-peptide complex formation was therefore examined. Sera depleted of beta-2m did not support effective peptide binding to class I, but binding was restored in the presence of low concentrations of purified human beta-2m. Sequential incubation of immobilized class I with human beta-2m first, followed by peptide, resulted in antigenic complex formation, while reversing the order of pulsing could not. Similar results were obtained in experiments examining H-2D(b), K(b) and K(d) with appropriate peptides and CTL. These results demonstrate that mature class I proteins are not able to directly bind peptide, but that interaction with exogenous beta-2m results in a structure that will subsequently bind peptide. Binding of exogenous beta-2m appears to results in "empty" class I molecules, possibly by exchange for endogenous beta-2m, with a concomitant loss of endogenous peptide.