Ys. Hahn et al., ENDOGENOUS PRESENTATION OF A NASCENT ANTIGENIC EPITOPE TO CD8(+) CTL IS MORE EFFICIENT THAN EXOGENOUS PRESENTATION, Immunology and cell biology, 74(5), 1996, pp. 394-400
Cytotoxic T lymphocytes (CTL) recognize short antigenic peptides in as
sociation with class I MHC molecules at the cell surface. Newly synthe
sized viral polypeptides are processed in the cytoplasm and the fragme
nts of antigen are transported into the endoplasmic reticulum (ER) via
a peptide transporter where they complex with nascent class I molecul
es. The peptide-MHC complex is transported to the cell surface and pre
sented to CTL. Sequence analysis of endogenously expressed, MHC-associ
ated self or viral antigens indicates Chat the naturally processed pep
tides bound to class I MHC molecules are in general 9 +/- 1 residues l
ong. Peptides bound to specific class I MHC molecules have in common a
llele-specific motifs of conserved residues, The motif for the class I
K-d molecules has been shown to be nine or 10 residues with the seque
nce X-Tyr-(X)6-I/L or X-Tyr-(X)7-I/L. The Tyr residue at the second po
sition and the I/L residue at the ninth position are allele-specific a
nchor residues which appear to be required for binding of the peptide
to K-d. TO examine the stringency of the requirement for Tpr at the se
cond position, we have performed saturation mutagenesis of a minigene
encoding the class I K-d-restricted influenza HA210-219 site at the Ty
r residue 211. A series of 10 mutants was tested for effects on target
-cell sensitization. Most amino acid substitutions for the Tyr residue
resulted in a loss of endogenous peptide recognition by HA210-219 rea
ctive CTL, consistent with the critical rare of the Tyr at the second
position for interaction with K-d molecules. One mutant gene-product e
ncoding a His substitution for the Tyr residue was recognized by CTL.
However, the corresponding synthetic peptide containing a His substitu
tion at the dominant anchor position bound only weakly to K-d, and tar
get cells created with the peptide were poorly recognized by CTL. The
endogenous His-containing peptide was also less stably associated with
class I MHC K-d molecules al the cell surface than the wild-type Tyr
peptide, These data indicate that endogenous antigenic peptides may bi
nd newly-synthesized class I MHC molecules in the ER more efficiently
than fully formed class I molecules at the cell surface and that endog
enous peptides may dissociate from class I MHC molecules at different
rates. The implications of these findings for CTL recognition and epit
ope mapping an discussed.