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

16.2 Treatment of Tumor Metastases with Immunocytokines
16.2.3
Carcinoembryonic Antigen (CEA)-based DNA Vaccines for Colon Carcinoma Boosted by
IL-2 Immunocytokine
A new area of cancer immunotherapy emerged during the last decade based on the recognition
that some tumors encode rejection antigens capable of inducing a tumorprotective
immune response primarily by activating the cellular arm of the immune
response [18]. This approach was based on the finding that CTLs are particularly effective
in recognizing tumor cells as foreign and eradicating them [19]. Recent efforts focused
on the development of DNA-based cancer vaccines designed to overcome peripheral
T cell tolerance against tumor-self antigens, such as human CEA [20]. This
well-characterized oncofetal glycoprotein of Mr 180 000±200 000 which is encoded by
29 genes and part of the immunoglobulin supergene family [21±23] has already served
as a useful target for CEA-specific radioimmunoconjugates in clinical detection and
therapy protocols [24±26], and was used more recently to construct DNA vaccines for
cancer immunotherapy [27]. A real advantage for these immunotherapy studies was
the availability of a mouse line that carriers the genomic DNA transgene for human
CEA [28]. This CEA transgenic mouse also expresses CEA in a tissue-specific manner,
similar to that observed in humans, in which the colon is the major site for CEA production.
Importantly for our purposes, anti-CEA CD8 + T cells were elicited in these
CEA-transgenic mice after in vivo priming with CEA-transfected fibroblasts [29]. However,
anti-CEA antibody responses were not detected in such transgenic mice unless
an independent carrier was used, suggesting tolerance to CEA in the CD4 + Tcell compartment
[29]. Importantly, recent studies indicated that CEA-based vaccines can induce
CEA-specific immune responses in transgenic mice and in humans [30]. In fact,
a wide range of CEA immunogens, including cDNA encoding the entire CEA gene or
a variety of CEA peptides with dominant MHC class I antigen anchor residues, carried
by a replication-deficient vaccinia virus, induced CEA-specific MHC-restricted CTL responses,
T cell proliferation, as well as CD4 + T cell and antibody responses in both
CEA-transgenic mice [31, 32] and cancer patients [30].
We determined in a series of experiments whether peripheral T cell tolerance to
CEA, a human tumor self-antigen, could be overcome by a DNA vaccine containing
the entire gene encoding CEA delivered per os by an attenuated Salmonella typhimurium
carrier to CEA-transgenic mice [33]. It was of particular interest to assess
whether this vaccine could induce a T cell-mediated, tumor-protective immune response
that was effective in rejecting a lethal challenge of murine MC38 colon carcinoma
cells, stably transduced to express CEA and the human epithelial cell adhesion
molecule (Ep-CAM/KSA). In view of our encouraging results obtained in effectively
boosting tumor-protective immunity against murine colon carcinoma induced by
the huKS1/4±IL-2 immunocytokines with small, non-curative doses of huKS1/4±
IL-2 [12], it was important to determine whether the protective immunity induced by
the CEA-based DNA vaccine could also be further improved by such immunocytokine
boosts.
In initial experiments, scanning confocal microscopy and Western blotting demonstrated
that we could selectively obtain subcellular localization of CEA by designing
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