External glycopeptide binding to MHC class-I in relation to expression of TAP transporters, beta 2-microglobulin and to pH.

MHC class-I binding glycopeptides are easily visualized on the cell surface by carbohydrate specific monoclonal antibodies. By comparing the staining intensity between anti-carbohydrate and anti-MHC class-I specific monoclonal antibodies, an estimation of the fraction of peptide accessible 'empty' sites on the cell surface of MHC class-I molecules can be made. This system was used to analyze glycopeptide binding to MHC class-I molecules in relation to transporter associated with antigen processing (TAP) peptide transporters and beta 2-M expression, using gene targeted mice, and in relation to pH. Approximately 15, 40, and 95% 'empty' Db molecules were found on activated T cells from normal, beta 2-M-/- and TAP -/- mice, respectively. The ASN9-6h-Gal2 glycopeptide also bound to transfected 'empty' Db molecules on T1-Db, T2-Db and T3-Db cells with a preference for T2-Db cells, lacking TAP peptide transporters. The stability of glycopeptide binding to H-2Db is also highest on T2-Db cells. pH was found to influence binding either positively or negatively, using four different glycopeptides, binding either to Db or Kb. We conclude that external glycopeptide binding may reflect important functional properties in the MHC class-I system and that pH in different processing compartments might influence the expressed peptide repertoire.