Antigen-specific regression of established tumors induced by active immunization with irradiated IL-12- but not B7-1-transfected tumor cells

Transfection of modestly immunogenic tumors to express B7 family co-stimulator molecules results in their rejection by syngeneic mice, suggesting a possible clinical application in cancer patients. Immunization of naive mice with irradiated B7-1-transfected P1.HTR cells is sufficient to induce specific cytolytic T lymphocytes (CTL) and to protect against tumor challenge. However, patients to be treated will have an existing tumor burden; thus, preclinical models should examine therapeutic efficacy in an established tumor setting. Contrary to expectations, immunization of mice with irradiated B7-1-transfected P1.HTR cells had no impact on the growth of pre-established control-transfected tumors. Mice bearing control-transfected P1.HTR tumors successfully rejected living B7-1 transfectants on the contralateral flank, demonstrating the ability of tumor-bearing mice to respond to B7 co-stimulation. Inasmuch as IL-12 is another important factor for CTL maturation, P1.HTR transfectants expressing B7-1 and/or IL-12 were then constructed. Remarkably, regression of pre-established tumors was achieved following immunization with irradiated IL-12 transfectants, even without co-expression of B7-1. Rejection required a shared antigen with the tumor used for immunization, could not be reproduced with rIL-12 alone, depended on host T lymphocytes and correlated with a high IFN-gamma-producing T cell phenotype. In addition, IL-12-facilitated tumor rejection required co-operation with a CTLA-4 ligand provided by the host, and correlated with up-regulation of B7-1 and B7-2 on host antigen-presenting cells. Thus, active immunization in the established tumor setting is benefitted greatly by the provision of IL-12, which may recruit participation of sufficient B7 co-stimulation from the host that it need not be provided exogenously.