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

fic CTL responses. DC vaccination may be able to boost the frequency of an existing
CTL precursor pool, but maintaining this as a memory response appears to be difficult
[141, 142]. Again, it is hoped that the crucial high-affinity responses [166] will be
developed by DC vaccination. Chromium-release assays using tumor cell targets is a
laudable goal; however, this is clearly not widely achievable. Nonetheless, the ability
to isolate tumor RNA and amplifying it allows autologous tumor target substitutes
to be created from patients` Epstein±Barr virus-transformed B cells or other cell
sources [118, 120]. The ELISPOT technique is now being widely applied and with observer-independent
counting methodology now available this is providing useful information
regarding IFN-g-producing CD8 T lymphocyte responses; providing that
sensitization in vitro is done first, flow cytometry detection of cytokine-producing
cells may also be as sensitive. This technique can also be combined with specific T
cell receptor analysis. Analysis of specific T cell receptors for HLA class I-restricted
peptides is now possible using biotinylated soluble HLA-A2 (i. e. locus allele specific)
TAA peptide-loaded tetramers and these detected by flow cytometry. Alternative technology
uses different recombinant soluble (Fc/HLA-A2) dimers loaded with TAA
peptide. Data in melanoma, chronic myeloid leukemia and bowel cancer suggests
that the pre-existing TAA-specific T cell frequency is of the order of 0.1±2.0 % [167,
168] and this can be expanded. However, it is very clear that TAA-specific T lymphocytes
do not necessarily have antitumor activity. Thus, the limited correlation to date
suggests cytotoxic assays will not. The functional contribution of the tetramer-positive
cells may even be a negative one. Most studies necessarily sample the blood, but
it is a valid point that direct tumor sampling may be required as effective CTLs may
traffic from the blood to the tumor site.
There is good evidence that CD4 + helper T lymphocyte responses driven by DCs contribute
to CTL development. CD4 + TAA-specific responses have also be shown to
contribute to antitumor responses, not only as direct cytotoxic (class II-restricted) effectors),
but also indirectly via recruitment and activation of other cells types. Classically,
this is considered to be measured as part of the DTH response. An increase in
the DTH response has been documented after DC vaccination and there may be
some correlation with clinical outcome [91, 132]. DCs are also capable of activating
NK responses [58] and Flt-3L effects have been shown to be at least in part due to a
direct NK-activating effect [169].
9.12
Tumor Escape
9.12 Tumor Escape
Cancers evolve other methods of immune evasion apart from avoiding DC initiation
and presentation of the immune response. In patients who disease has progressed
through conventional therapy there has been extensive opportunity for selective pressure
to evolve tumor variants. The malignant cells may produce a host of factors
which directly compromise immune cell function, e. g. transforming growth factorb,
VEGF and even products such as PSA, which has only recently been shown to
have immunosuppressive qualities [170]. More importantly in terms of CTL effec-
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