Ulbrecht, Matthias; Kellermann, Josef; Johnson, Judith P.; Weiss, Elisabeth H.
Impaired Intracellular Transport and Cell Surface Expression of Nonpolymorphic HLA-E. Evidence for Inefficient Peptide Binding.
In: Journal of Experimental Medicine, Vol. 176: pp. 1083-1090
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 is in dispute. The human, nonclassical class I gene, HLA-E, was found to be
ubiquitously transcribed, whereas cell surface expression was dif~cult to 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 transport compared with HLA-B27 in corresponding
transfectants. The accordingly low HLA-E cell surface expression could be significantly enhanced
by either reducing 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 ligands, fractions of acid-extracted material from
HLA-E/X63 transfectants were separated by reverse phase HPLC and were tested for their ability
to enhance HLA-E cell surface expression. Two fractions specifically increased the HLA class
I expression on the HLA-E transfectant clone.