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

316 16 Immunocytokines: Versatile Molecules for Biotherapy of Malignant Disease
class I antibodies (H-2K d /H-2D d ) completely suppressed CD8 + T cell-mediated cytotoxicity
in vitro. In fact, MHC class I antigen restriction was demonstrated for both
CT26-KSA and CT26 target cells, emphasizing that the cytotoxic response occurs independent
of the KS antigen which clearly is not the antigen recognized by the T
cells. Actually the origin of the T cell antigen in this system remains to be elucidated.
In this regard, it is well known that T cells recognize peptides presented in the binding
groove of MHC class I antigens. Since both CT26-KSA and CT26 cells were killed
by CD8 + T cells in vitro, these cells apparently share an antigen peptide present on
both related cell lines. Interestingly, it was reported previously that CT26 colon carcinoma
cells engineered to secrete granulocyte macrophage colony stimulating factor
(GM-CSF) generated several tumor-specific cytotoxic T cell lines that recognized a
single immunodominant tumor-associated, MHC class I-restricted peptide antigen
(SPSYVYHQF) [13]. This particular antigen was derived as a non-mutated nonamer
from the gp70 envelope protein of an endogenous ectotropic murine leukemia provirus.
However, this same immunodominant peptide was apparently not recognized
by CD8 + T cells in our experiments. In fact, our data indicated that SPSYVYHQFpulsed
RENCA tumor cells, syngeneic with BALB/c mice, could not be lysed in vitro
by CT26-KSA-specific cytotoxic T lymphocytes (CTLs) [11, 12].
A third line of evidence for a cellular immune response mediated by CD8 + T lymphocytes
was provided by histological and immunohistochemical analyses. These
studies were performed on liver tissues of tumor-bearing mice following treatment
with the huKS1/4±IL-2 immunocytokine and compared to that of control animals receiving
either phosphate-buffered saline (PBS) injections or an equivalent mixture of
antibody and cytokine. Only those mice that received immunocytokine treatment revealed
a strong, predominantly lymphocytic infiltrate in micrometastasis-bearing livers,
in contrast to all control mice. Subsequent analysis of these infiltrates by immunohistochemical
methods indicated strong positive staining for both, CD8 + and
CD4 + T cells. These findings clearly illustrated the presence of an intense local immune
response largely consisting of CD8 + and CD4 + T lymphocytes in tumor-bearing
livers of only immunocytokine-treated mice [14].
16.2.2
Long-lived Tumor-Protective Immunity is Boosted by Non-curative Doses of huKS1/4±
IL-2 Immunocytokine
Further proof for an induction of CD8 + T cell-mediated immunity was provided by
demonstrating a persistent tumor-protective memory immune response. In fact,
mice cured of established pulmonary micrometastases following treatment with tumor-specific
huKS1/4±IL-2 immunocytokine revealed a complete absence of hepatic
metastases in 50 % of the animals upon a lethal challenge with colon carcinoma cells
up to 20 weeks after initial vaccination. The remaining 50 % showed a decrease in
metastases. Since there were always several animals that completely failed to reject
the tumor cells, we tested the hypothesis that boosting with non-curative doses of
huKS1/4±IL-2 will result in tumor rejection in all experimental animals after tumor
challenge. The data shown in Tab. 16.2 indicate that this was indeed the case when