Expression of Nonpolymorphic HLA-E: Evidence for Inefficient Peptide Binding

The assembly of the classical, polymorphic major histocompatibility complex class I molecules in the endoplasmic reticulum requires the presence of peptide ligands and ~2-microglobulin (~2m). Formation of this trimolecular complex is a prerequisite for e~cient transport to the cell surface, where presented peptides are scanned by T lymphocytes. The function of the other class I molecules i in dispute. The human, nonclassical c ass I gene, HLA-E, was found to be ubiquitously transcribed, whereas cell surface xpression was dif~cult o detect upon transfection. Pulse chase experiments revealed that the HLA-E heavy chain in transfectants, obtained with the murine myeloma cell line P3X63-Ag8.653 (X63), displays a significant reduction in oligosaccharide maturation and intracellular t ansport compared with HLA-B27 in corresponding transfectants. The accordingly low HLA-E cell surface xpression could be significantly enhanced by either educing the culture temperature or by supplementing the medium with human ~2m, suggesting inefficient binding of endogenous peptides to HLA-E. To analyze whether HLA-E binds peptides and to identify the corresponding li ands, fractions of acid-extracted material from HLA-E/X63 transfectants were separated by reverse phase HPLC and were tested for their abilit