Membrane-anchored β2-microglobtilin stabilizes a highly receptive state of MHC class I molecules

The magnitude of response elicited by CTL-inducing vaccines correlates with the density of MHC class I (MHC-I)-peptide complexes formed on the APC membrane. The MHC-I L chain, beta(2)-microglobulin (beta(2)m), governs complex stability. We reasoned that genetically converting beta(2)m into an integral membrane protein should exert a marked stabilizing effect on the resulting MHC-I molecules and enhance vaccine efficacy. In the present study we show that, expression of membranal human beta(2)m (hbeta(2)m) in mouse RMA-S cells elevates MHC-I thermal stability. RMA-S transfectants bind an exogenous peptide at concentrations 10(4)- to 10(6)-fold lower than parental RMA-S, as detected by complex-specific Abs and by T cell activation. Moreover, saturation of the transfectants' MHC-I by exogenous peptide occurs within 1 min, as compared with similar to1 h required for parental cells. At saturation, however, level of peptide bound by modified cells is only 3- to 5-fold higher. Expression of native hbeta(2)m only,results in marginal effect on the binding profile. Soluble beta(2)m has no effect on the accelerated kinetics, but the kinetics of transfectants parallel that of parental cells in the presence of Abs to hbeta(2)m. Ab inhibition and coimmunoprecipitation analyses suggest that both prolonged persistence of peptide-receptive H chain/beta(2)m heterodimers and fast heterodimer formation via lateral diffusion may contribute to stabilization. In vivo, peptide-loaded transfectants are considerably superior to parental cells in suppressing tumor growth. Our findings support the role of an allosteric mechanism in determining ternary MHC-I complex stability and propose membranal beta(2)m as a novel scaffold for CTL induction.