Download File - JOHN J. HADDAD, Ph.D.

6 Lévy et al.
some instances, inhibition of some of the peptidases has resulted in a more
efficient presentation of antigenic peptides (39). Taken together, the large
number of N-terminal exopeptidases and their broad enzymatic activities predict
that the majority of peptides produced by the proteasomes are trimmed to sizes
too short to bind MHC class I molecules long before they have had a chance of
crossing the endoplasmic reticulum membrane.
Among the cytosolic peptidases described above, TPP II is the only
peptidase to date that may also produce the C-termini of a limited number of
MHC class I–restricted peptide antigens, albeit inefficiently. TPP II is a peptidase
that habitually removes N-terminal tripeptides. However, it has also
been shown to cleave polypeptides internally (40). This activity is essential
for the production of an HIV-Nef-derived peptide presented by HLA-A3 and
HLA-A11 (41). However, it is unclear if the latter activity of TPP II is
capable of cleaving full-length proteins or rather cleaves long peptide fragments
produced by other proteases. Given that Lys residues are the preferred
residues for these intra-protein cleavages, MHC class I ligands containing
C-terminal Lys (typically HLA-A3 and HLA-A11 ligands) may be in part
processed by TPP II.
In the endoplasmic reticulum, additional aminopeptidases have been
identified. These peptidases are termed ERAP or ERAAP in mice and ERAP1
and 2 in humans (42–44). Human ERAP1 and 2 form heterodimers. ERAAP was
originally shown to influence the presentation of antigenic peptides, particularly
those containing Pro as anchor amino acid at position 2 (45). This is explained by
the inefficient translocation of Pro2-containing peptides through the TAP (46).
Those peptides are probably transported as N-terminally elongated precursor
and trimmed in the ER before or after binding to MHC class I molecules. Thus,
human MHC alleles such as HLA-B7, -B35, -B51, -B54, -B55, -B56, and -B67
and murine H-2L d , which preferentially select ligands with Pro2 as anchor
residues, may depend more on the activity of this peptidase than other alleles.
Silencing of ERAAP expression in L d -transfected murine L cells led to a
reduction of cell surface expression of L d , confirming the importance of this
peptidase in the trimming of L d ligands (42). However, ERAAP / mice also
displayed reduced H-2K b or D b expression at the surface of splenocytes (but not
of embryonic fibroblasts) (47,48). In those mice, ERAAP was shown to affect
the presentation of some H-2 b -restricted peptides but not others, while it had no
impact on the cross-presentation of soluble antigens (48,49). Thus, the proteolytic
activities of ERAAP are required for the processing of a larger number
of antigenic peptides than just those containing Pro at position 2. As with the
cytosolic exopeptidases, ERAAP activities have negative effects on antigen
processing in that the trimming of N-terminally extended peptide intermediates
may continue beyond the size limit for binding to MHC class I molecules (49).
Recent results indicated that MHC class I molecules may bind N-terminally
extended intermediates in the endoplasmic reticulum, thereby limiting
the destructive effects of progressive ERAAP trimming (50). Surprisingly,