Exogenous peptide ligand influences the expression and half‐life of free HLA class I heavy chains ubiquitously detected at the cell surface

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Abstract
A pool of free HLA class I chains has been detected at the plasma membrane of all cells concomitantly expressing folded and assembled class I molecules. To determine the origin of these free HLA heavy chains, we have examined the biosynthesis of a single HLA class I molecule, HLA-B27, expressed by a murine cell line (L-B27). In L-B27 cells, as previously shown in Epstein-Barr virus-transformed lymphoblastoid cell lines, a precursor/product relationship exists, early in biosynthesis, between free (HC10-reactive) and beta-2-microglobulin (β2m)-associated (W6/32-reactive) class I heavy chains as demonstrated by pulse/chase experiments. At later stages in class I biosynthesis, both HC10- and W6/32-reactive heavy chains display complex oligosaccharides and accumulate at the cell surface. HC10- and W6/32-reactive molecules are both very stable at the cell surface, with half-lifes (t1/2) of > 7 h and ∼ 4 h, respectively. Interestingly, cell surface expression and turnover of HC10- and W6/32-reactive molecules were affected by the addition of peptide ligands to the culture media. Culturing cells in the presence of HLA-B27 ligands resulted in the increased expression of W6/32-reactive molecules and the decreased expression of HC10-reactive molecules. Moreover, addition of exogenous peptide extended the t1/2 of W6/32-reactive molecules to > 7 h and reduced that of HC10-reactive molecules to 4 h. These results indicate that surface HC10-reactive molecules result largely from W6/32-reactive molecules following peptide and β2m dissociation. Therefore, HC10-reactive species are not only the precursors but also the end products in class I biosynthesis.