Many
tumors evade host immunity by lowering expression of major histocompatibility complex (MHC) molecules. Theoretically, low MHC expression should activate natural killer (NK) cells and in some cases suppress
tumor growth; nevertheless, some
tumors also produce high concentrations of immunosuppressive
cytokines, such as
transforming growth factor (
TGF)-beta, to inhibit the activity of NK cells. Using a canine
transmissible venereal tumor (CTVT) model, we have previously demonstrated that
IL-6 is a strong antagonist for
TGF-beta. Herein, we found that
IL-6 alone was unable to significantly promote
TGF-beta-inhibited NK activities. Conversely,
IL-15 alone strongly promoted NK activities; however, NK activities were inhibited to baseline levels following the addition of
TGF-beta. Therefore, a new strategy using combined immunogene
therapy of both
IL-6 and
IL-15 mediated by electroporation was used in this study. This combined
IL-6 and
IL-15 treatment effectively relieved the inhibitory effect of
TGF-beta and activated NK cell cytotoxicity of lymphokine-activated killer (LAK) cells. Similarly, in isolated DX5+ NK cells, only
IL-6 and
IL-15 in combination significantly overcame the inhibitory effect of
TGF-beta and promoted NK cytotoxicity. The group of BALB/c mice injected with plasmids with
IL-6 and
IL-15 genes (pIL-6/pIL-15) had the highest percentages of DX5+ NK cells as compared with either the pIL-6 or pIL-15 groups. Further, in SCID mice inoculated with CTVT, electroporation-mediated delivery of pIL-6/pIL-15 was significantly more efficient in suppressing both
tumor establishment and
tumor growth as compared with pIL-6 or pIL-15 inoculation alone. In addition, the anti-asialo GM-1 antibody abolished NK activities in SCID mice and resulted in outgrowth of the
tumors. Together, these results suggest that the
TGF-beta-associated inhibition of NK cytotoxicity cannot be adequately restored by simply antagonizing
TGF-beta with IL-6: the co-existence of NK activating factors such as
IL-15 is also important in restoring
TGF-beta-inhibited cytotoxicity. This study highlights the therapeutic potential of the pIL-6/pIL-15 combination by inhibiting
TGF-beta activity and enhancing NK cytotoxicity.