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

42 4 T Cells In Tumor Immunity
cultureº (MLTC). This consisted of the repeated periodical in vitro stimulation of peripheral
blood lymphocytes with the autologous tumor cell line in medium supplemented
with interleukin (IL)-2. Under these conditions, tumor-reactive T lymphocytes
were observed in 50 % of melanoma patients [20]. Thus, since this in vitro procedure
presupposes the availability of autologous tumor cell lines, the majority of tumor-reactive
T cells were initially isolated from melanoma. Because this is the tumor
type among human tumors allowing a relatively high efficiency isolation of in vitro
adapted, continuously growing, tumor cell lines. Indeed, the proportion of tumors
from which it is possible to establish a cell line varies from 30 to 40 % in different laboratories.
The only other human tumor type from which cell lines can be isolated
with a relatively high rate of success is the renal cell carcinoma. This rate ranges
from 10 %, [21] to even 60 % of attempts [22], and may attain 80 % for short-term tumor
cell lines [23]. As mentioned above, another rich and reliable source of tumor-reactive
T cells were the TILs mainly from melanoma lesions but also from other tumors
such as colon or renal cell carcinomas readily expandable in in vitro culture
with high-dose IL-2 [24].
The advent of techniques for both cloning antigen-specific T cells and measuring
their cytolytic activity facilitated the isolation of tumor-reactive T cell clones from
both MLTC and TIL cultures. Stable in vitro growing CTL clones were the critical
tools which allowed the molecular identification of the first tumor antigens. Several
strategies were successfully used to achieve this goal. The first involved the cloning
of the genes encoding the tumor antigens from genomic or cDNA libraries created
from the corresponding autologous melanoma tumor cells [25, 26]. The second strategy
was the direct isolation and sequencing of the active antigenic peptide species
bound to MHC class I molecules on the surface of the tumor cells [27]. In both cases,
tumor-reactive CTL clones are used to monitor each of the multiple steps leading to
cloning of the gene or to biochemically identifying the antigenic peptide. It was later
realized that expression cloning in Escherichia coli of B cell-defined tumor antigens
was also feasible using autologous sera from cancer patients [28]. This technology
has been given the name of SEREX, for Serological Identification of Antigens by Recombinant
Expression cloning. More than 1200 genes cloned to date have been listed
[29]. Interestingly, several genes were independently cloned by both approaches indicating
the existence of both T and B cell responses directed against the same target
gene product naturally induced in the course of tumor progression. Examples of
these genes are the MAGE-1, tyrosinase and NY-ESO-1. Other variants of molecular
cloning of tumor antigens have been successfully used such as for instance representational
difference analysis [30]. Thus, relatively large numbers of CTL-defined
tumor antigens have been identified in the last 10 years. Several recent reviews contain
complete listings of the tumor antigens [31, 32].
As expected from the understanding of molecular events involved in antigen recognition
by CTL [33], tumor antigens are non-covalently associated tri-molecular complexes.
The three molecular components are a polymorphic heavy chain encoded by
sets of three pairs of alleles inherited co-dominantly in the class I MHC, a non-polymorphic
b2-microglobulin and a small antigenic peptide. The latter is generated by a
dedicated antigen-processing machinery that involves proteolytic degradation of pro-