We have evaluated the capacity of a novel, nanoparticle-based
tumor vaccine to eradicate established
tumors in mice. C57BL/6 mice were intradermally (i.d.) inoculated with
ovalbumin (OVA)-expressing EG-7
tumor cells. When the
tumor size reached 7-8 mm, the
tumor-bearing mice were i.d. injected near the
tumor-draining lymph node (DLN) with
liposomes encapsulated with unmethylated
cytosine-phosphorothioate-guanine containing
oligodeoxynucleotides (
CpG-ODN) (CpG-
liposomes) co-encapsulated with OVA. This vaccination protocol markedly prevented the growth of the established
tumor mass and approximately 50% of
tumor-bearing mice became completely cured.
Tumor eradication correlated with the generation of OVA/H-2K(b)-tetramer(+) CTLs in the
tumor DLN and at the
tumor site with specific cytotoxicity toward EG-7 cells. Interestingly, tetramer(+) CTLs failed to be induced in lymph node-deficient Aly/Aly mice. Thus, tetramer(+) CTLs appeared to be generated in the
tumor DLN and subsequently migrated into the
tumor site. In vivo antibody blocking experiments revealed that CD8(+) T cells, but not CD4(+) T, NK or NKT cells, were the major effector cells mediating
tumor eradication. CTL induction was also inhibited when vaccinated
tumor-bearing mice were treated with both anti-IFN-alpha and anti-IFN-beta mAbs but not with anti-IFN-alpha or anti-IFN-beta mAb alone. Neither IFN-gamma(-/-) nor
IL-12(-/-) mice showed impaired induction of tetramer(+) CTLs. Thus, these findings revealed that
CpG-ODN-induced IFN-alpha/beta, but not
IL-12 or IFN-gamma, is critical for the generation of
tumor-specific CTLs in response to vaccination. These results highlight the potential utility of CpG-
liposomes co-encapsulated with
protein tumor antigens as therapeutic
vaccines in
cancer patients.