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

338 16 Immunocytokines: Versatile Molecules for Biotherapy of Malignant Disease
equivalent mixture of antibody and rhIL-2 or PBS. In fact at least a doubling in lifespan
was seen following induction of experimental liver metastasis only in mice treated
with the immunocytokine [92]. The specificity of ch14.18±IL-2 immunotherapy
was demonstrated by ineffective treatment of liver metastases of GD2-negative TBJ
cells with the ch14.18±IL-2 immunocytokine [92]. Taken together, these data indicated
that therapy with an immunocytokine directed against ganglioside GD2, that is
a naturally occurring and heterogeneously expressed antigen in this tumor model,
can effectively eradicate disseminated metastasis in at least two different metastatic
sites. It is noteworthy that similar situations are to be expected in humans, where
TAAs are distributed heterogeneously and metastasis is usually not limited to a single
organ system. Due to the various similarities of this murine neuroblastoma
model with human neuroblastoma, the therapeutic effect observed with the
ch14.18±IL-2 immunocytokine could thus be indicative of a potentially successful
clinical application.
16.6.3
Mechanism of Immunocytokine-mediated Immune Responses
In contrast to previously reported T cell-mediated immune responses following therapy
with immunocytokines in animal models for murine colorectal carcinoma [11,
12] and melanoma [61], several lines of evidence indicated that the effector mechanism
in the syngeneic neuroblastoma model was exclusively mediated by NK cells.
First, the treatment of liver and bone marrow metastases with ch14.18±IL-2 immunocytokine
proved completely effective in T cell-deficient scid/scid mice. This treatment
effect was only abrogated in scid/beige mice that lack both Tand NK cells. However,
reconstitution of NK cells in this mouse strain by injection of 3 × 10 7 NK cells
per mouse re-established the therapeutic efficacy of the immunocytokine. Second,
depletion of NK cells and CD8 + T cells in immunocompetent A/J mice eliminated
the therapeutic effect only in NK cell depleted mice [92]. A role for CD8 + T cells was
further excluded by s.c. injection of 1 × 10 6 NXS2 cells into mice with established liver
and bone marrow metastases that had been successfully treated with ch14.18±
IL-2 immunocytokine. Thus, s.c. tumor growth 11 days after this challenge of immunocytokine-treated
animals was the same as that of naive mice, untreated tumorbearing
mice or mice that received an equivalent mixture of antibody and cytokine.
These results indicated the absence of a memory immune response, a typical feature
of NK cells. Third, immunohistochemical analyses of inflammatory infiltrates in
livers of mice successfully treated with ch14.18±IL-2 indicated positive staining for
NK cells, but not for CD8 + T cells. This was in contrast to livers of mice that received
PBS injections or the equivalent antibody/IL-2 mixture, and showed no detectable inflammatory
cells [92].
Based on these in vivo data, strong evidence for an NK cell-mediated mechanism
was established which was further supported by in vitro analyses of NK cell activation
and killing of NXS2 target cells. Specifically, treatment of tumor-bearing mice with
ch14.18±IL-2 increased NK cell activity of freshly isolated splenocytes in contrast to
PBS and antibody/IL-2 mixture controls. Only splenocytes of mice receiving the im-