Peptide binding to active class II MHC protein on the cell surface

Solution studies have demonstrated the existence of two functionally distin ct isomers of empty class II MHC: an active isomer that binds peptide and a n inactive isomer that does not. Empty MHC molecules on the surface of APCs can load antigenic peptides directly from the extracellular medium, facili tating the generation of a diverse peptide repertoire for T cell presentati on. In this report, we examine I-Ek on the surface of Chinese hamster ovary cells with respect to the active and inactive isomers. As in the case of p urified soluble active I-Ek, active I-Ek on the cell surface is unstable, d ecaying to the inactive form in similar to 14 min. Evidence is presented su ggesting that at steady state <1% of the total cell surface I-Ek is active and that a significant fraction of these active molecules originates from i ntracellular pools as well as reactivation of inactive cell surface I-Ek.