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

32 3 Processing and Presentation of Tumor-associated Antigens
b1i and b5 i expression did not appear to be essential. On the other hand, it was
found that LMP7 expression was essential for the processing of an influenza-derived
cytotoxic T lymphocyte (CTL) epitope [27]. These experiments suggested that immuno-subunits,
while not being essential of antigen presentation, may be of importance
for the generation of specific CTL epitopes.
More detailed investigations over the recent years have changed this initial skeptical
view concerning the hypothetical function of immuno-subunits in antigen processing.
The analysis of by now almost 20 different MHC class I epitopes (mostly viral) shows
that the liberation of some epitopes is greatly improved by, or absolutely dependent
on, the function of immuno-proteasomes, but the generation of others is not affected
at all [28±31] (Tab. 3.1).
In vitro digestion experiments with purified 20S proteasomes and synthetic polypeptides
encompassing the antigenic peptide and their natural flanking sequences as
substrates confirmed the observations made in intact cellular systems. Quantitative
analysis of in vitro produced peptide fragments by mass spectrometry showed that
that immuno-subunit incorporation changed the cleavage site preference of proteasomes
[28±30].
Interestingly, in none of the above analysis was a negative effect of immuno-subunit
expression on viral CTL epitope production observed. Remarkably, recent studies
have indicated that that this may be different in the case of self-antigen-derived CTL
epitopes (Tab. 3.1) [32]. Morel et al. reported that the incorporation of immuno-subunits
into the proteasome abrogated the production of two antigenic peptides, one derived
from the self protein RU1 and one derived from the melanoma differentiation
antigen Melan-A. On the other hand, we recently observed that such a down-regulation
of tumor epitope generation upon exposure to IFN-g can also occur in the absence
of induction of immuno-subunits expression [33].
Tab. 3.1 Effects of immuno-proteasomes and PA28 on MHC class I antigen processing
Antigen Epitope Effect of immuno-subunits Effect of PA28
Ovalbumin 257±264 no effect ND
HY antigen ND + ND
Influenza A nucleoprotein 366±374 + ND
Murine cytomegalovirus pp89 168±176 ND +
JAK1 tyrosine kinase 355±363 ND +
Influenza A/PR/8 nucleoprotein 146±154 ND +
Hepatitis B virus core antigen 141±151 + ND
Adenovirus E1A 234±243 no effect no effect
Adenovirus E1B 192±200 + ND
LCMV nucleoprotein 118±126 + no effect
Moloney murine leukemia virus gag pr75 75±83 + +
Moloney murine leukemia virus env gp70 189±196 no effect no effect
RU1 tumor antigen 34±42 ± ND
Melan-A tumor antigen 26±35 ± ND