Exogenous β₂‐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 β₂‐microglobulin (β₂m), a normal component of serum, in class I‐peptide complex formation was therefore examined. Sera depleted of β₂m did not support effective peptide binding to class I, but binding was restored in the presence of low concentrations of purified human β₂m. Sequential incubation of immobilized class I with human β₂m 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 β₂m results in a structure that will subsequently bind peptide. Binding of exogenous β₂m appears to result in “empty” class I molecules, possibly by exchange for endogenous β₂m, with a concomitant loss of endogenous peptide.