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

16.6 Neuroblastoma
munocytokine showed a drastic increase in their lysis of NK cell-sensitive YAC-1 target
cells. In contrast, only a marginal increase in NK cell activation over the natural
NK cell activity was detected in splenocytes of mice treated with a combination of
antibody and IL-2. When cytotoxic activity of splenocytes obtained from mice treated
with the ch14.18±IL-2 immunocytokine was determined with NXS2 target cells, the
separation into NK cell and CD8 + T cell subpopulations indicated effective killing
only in the NK cell fraction whereas CD8 + T cells were completely ineffective. These
data provided further proof for an NK cell-mediated cellular immune response in
this model [92].
The dichotomy of immune mechanisms involved in ch14.18±IL-2 immunocytokinemediated
antitumor responses lead to the question as to whether low expression of
major MHC class I antigens could favor NK cells over T cells in the syngeneic neuroblastoma
tumor model. Therefore, MHC class I antigens were up-regulated by additional
s.c. application of recombinant murine IFN-g (mIFN-g). In this case, a combination
therapy of ch14.18±IL-2 immunocytokine and mIFN-g was more effective in
eradicating established bone marrow and liver metastasis. Although mIFN-g strongly
up-regulated MHC class I expression, splenocytes of mice successfully treated with
the immunocytokine/mIFN-g combination were incapable of MHC class I antigenrestricted
killing of NXS2 target cells. Thus, a switch from an NK- to a Tcell-mediated
mechanism did not occur. In fact, quite the opposite was observed as the addition of
anti-MHC class I antibodies increased the cytolytic activity of splenocytes obtained
from mice after completion of the immunocytokine/mIFN-g combination treatment.
This is typical for NK cell-mediated killing of tumor cells, since NK cells activity is
down-regulated by stimulated lectin type C inhibitory NK cell receptors of the LY49
family that are specific for MHC class I antigen molecules [93±95]Ç We did overcome
this down-regulation by addition of MHC class I antigen blocking antibodies. Consequently,
the increase achieved in antitumor activity of the ch14.18±IL-2 fusion protein
by addition of mIFN-g in vivo could be attributed to a further activation of NK cells.
This contention was further supported by the finding that splenocytes from mice
treated with the ch14.18±IL-2/mIFN-g combination achieved the highest lysis of
YAC1 cells, when compared with mIFN-g and ch14.18±IL-2 controls [92].
The absence of a Tcell-mediated immune response in the NSX-2 model and its concomitant
replacement by NK cells may be due to factors secreted by the tumor cells
that suppress T cells but stimulate NK cells. In fact, NXS2 cells were shown to produce
transforming growth factor (TGF)-b1 and IL-10 in vitro and in vivo, both immunomodulators
being associated with T cell anergy [96±101]. However, IL-10 was also
reported to stimulate NK cells [102] and to inhibit tumor metastasis via NK cell
mediated mechanisms [103]. Thus, it is feasible that the presence of TGF-b1 and
IL-10 in our model could favor NK cell dependent antitumor mechanisms by causing
NK cell stimulation and concomitant T cell-anergy.
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