Cancer Immune Therapy Edited by G. Stuhler and P. Walden ...

34 3 Processing and Presentation of Tumor-associated Antigens
take of substrates or the release of peptide products [42]. A structural explanation
was given by Whitby et al. [43]. In their studies it was shown that PA26, the trypanosome
homolog of PA28, induces conformational changes in the a subunits of the
20S proteasome core complex. Based on this and the observation that PA28 facilitates
product exit, it was proposed that that such an open conformation may allow
the release of slightly longer peptides. Thus these peptides would be rescued from
further degradation, which may support the generation of certain CTL epitope precursor
peptides in particular of epitopes that are produced as N-terminally extended
epitope precursor peptides. Nevertheless, since not only PA28 but also the 19S regulator
can induce the opening of the central gate, it remains unclear whether the
above observations indeed fully explain the effects of PA28 on substrate cleavage.
Detailedanalysis ofthe effectsof PA28onproteasomalcleavageusagehave shownthat
PA28 does not confer new cleavage specificities nor has it a major impact on the rate of
substrate degradation. Instead, PA28 markedly enhances the frequency of usage of
specific, already preferred or minor cleavage sites resulting in an immediate liberation
of the intervening peptide fragments [44 and unpublished observation]. These data
and also a recent mutational analysis of PA28 [45] suggest that PA28 binding may induce
conformational changes within the 20S complex which changes the accessibility
of the active sites. Thus the dramatic effects of PA28 on antigenic peptide liberation are
unlikely to be explained by the opening of the 20S catalytic channel alone.
3.3
The Proteasome System and Tumor Antigen Presentation
Despite its central role in the generation of tumor epitopes which bind to HLA class
I molecules, and therefore its potential role in tumor surveillance and control of tumor
growth, the analysis of proteasome function in tumor cells is still at its beginning.
So far the experimental analysis of proteasomes in tumors has mainly concentrated
on the constitutive expression of IFN-g induced proteasome subunits LMP2
(ib1) and LMP7 (ib7), which both are encoded within the HLA region in the direct
neighborhood to the TAPs [46, 47]. More recent studies also included the analysis of
MECL-1 (ib2) and the PA28 subunits, both of which are encoded on different chromosomes.
Reverse transcription-polymerase chain reaction (RT-PCR) analysis of a
large number of different tumors shows that in many instances LMP2 and LMP7 expression
is down-regulated, where the extent of their down-regulation is variable and
does not appear to be specific for a given type of tumor nor follow a specific pattern
[48]. In these studies the expression of the MECL-1 subunit and PA28 appeared not
be affected. Concomitant with the down-regulation of LMP expression, one often
also observes an impaired expression of TAP mRNAs, suggesting that these neighboring
genes possess common regulators whose activity is affected in tumor cells.
With regard to LMP subunit expression, a similar observation was also made in
Adeno type 5-induced tumors [49]. However, it is important to mention that expression
of LMP2 and LMP7 (and TAPs) in all these cases was restored upon treatment
with IFN-g. Thus, while the reversible down-regulation of components of the protea-