Cytotoxic T lymphocyte response against non-immunoselected tumor antigens predicts the outcome of gene therapy with IL-12-transduced tumor cell vaccine

The colon-adenocarcinoma C26, carrying two endogenous tumor-associated anti gens (TAA) recognized by CTL, has been transduced with the gene coding for the human folate receptor alpha (FR alpha) as an additional antigen in orde r to study efficacy of vaccination against a tumor expressing multiple anti gens. A dicistronic vector was used to transduce the IL-12 genes to create C26/IL-12/FR alpha that has been used as a cellular vaccine to treat mice b earing lung metastases of C26/FR alpha. After vaccination mice were partial ly splenectomized and splenic lymphocytes frozen and used retrospectively t o study in vitro CD8 T cell response related to the treatment outcome. Vacc ination cured 50% of mice and the effect was CD8 T cell dependent. Mice eit her cured (responders) or not cured (nonresponders) by vaccination develope d tumor-specific CTL. However, analysis of CTL specificity and pCTL frequen cies revealed thai responders had predominant CTL activity against endogeno us C26-related tumor antigens, whereas nonresponders had CTL that recognize d preferentially the FR alpha antigen. CD8 from responder mice were charact erized to release high levels of granulocyte-macrophage (GM)-CSF upon antig en stimulation. Tumors obtained from mice that died despite vaccination los t expression of the FR alpha transgene but maintained expression of endogen ous C26 antigens. Immuno-selection against FR alpha antigen was not observe d in tumors from non-vaccinated controls and from CD8-depleted vaccinated m ice. Down-regulation of FR alpha antigen expression was due, at least in pa rt, to methylation of retroviral vector long terminal repeat promoter since FRa: expression was partially restored, ex vivo, by treatment with 5-aza-2 '-deoxy-cytidine (aza). These results indicate that T cell-mediated immunos election and production of GM-CSF are determining factors for the efficacy of tumor vaccines.