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Factoring in Antigen Processing in Designing Antitumor T-Cell Vaccines 5
clear as another cap, the 19 S cap, is normally attached to the 20 S core and
contains subunits capable of binding and unfolding ubiquitylated substrates.
Irrespective of the exact biochemical function of PA28 in vivo, PA28 expression
has been shown to be important for the processing of some antigenic peptides in
cells, as the generation of the melanoma-associated peptide antigen TRP2360–368
depends solely on the expression of PA28 (23). It was recently shown that the
N-terminal region flanking the antigenic peptide TRP2 360–368 conferred sensitivity
to PA28 by promoting coordinated cleavages at the N- and C-termini of
that peptide antigen (24).
N-Terminal Exopeptidases
The size of peptides emerging from the proteasomes ranges from 3 to over 22 amino
acids in length (25). Over 60% of the proteasomal products are shorter than 8 to
9 amino acids and are therefore immunologically irrelevant as they are too short to
bind MHC molecules. Approximately 15% of the proteasomal products are peptides
of 8 to 9 amino acids displaying suitable anchor residues to be directly loaded onto
MHC class I molecules. Longer peptides with appropriate C-terminal anchor residues
have to be trimmed by N-terminal exopeptidases. Biochemical analyses of the
proteasomal degradation of antigenic peptide precursors have shown that some
antigenic peptides are produced only as N-terminally extended intermediates
(13,26–28), while others are produced both in their optimal sizes of 9 to 10 amino
acids and as N-terminally extended intermediates (16,28–30). In the latter case, it
appears that the peptide produced directly by the proteasome is preferentially
selected for presentation by MHC class I molecules (30).
Many cytosolic N-terminal exopeptidases have been shown to be capable
of trimming the extra amino acids at the N-termini of antigenic peptide intermediates
produced by the proteasomes. Tripeptidyl peptidase II (TPP II), bleomycin
hydrolase, Leu aminopeptidase, puromycin-sensitive aminopeptidase, and thimet
oligopeptidase have all been implicated in the trimming of antigenic peptide
intermediates (31). However, it is not yet clear if individual peptidases perform
unique, nonredundant functions in the trimming of particular MHC class I ligands as
the genetic deletion, the chemical inhibition or the overexpression of some of these
peptidases did not affect the presentation of selected CD8 þ T-cell epitopes (32–34).
Depending on the fragment released by the proteasomes, two of these peptidases
have been shown to act either sequentially or redundantly (35). It has been suggested
that most MHC class I–restricted peptide intermediates produced by the proteasome
as fragments longer than 15 amino acids are trimmed by TPP II (36). However,
recent studies have demonstrated that the presentation of several peptide antigens by
MHC class I remained unaffected in cells lacking TPP II activity, suggesting that
proteasomes only rarely produce fragments longer than 15 amino acids (37,38).
By virtue of their enzymatic activities, most N-terminal exopeptidases
have also a negative effect on antigen processing by trimming antigenic peptide
intermediates to sizes that are too short for binding to MHC class I molecules. In