Lack of correlation between rejection of tumor cells co‐expressing interleukin‐2 and B7.1 and vaccine efficiency

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Abstract
Genetically modifying tumor cells to express a variety of cytokines such as interleukin‐2 (IL‐2) or the co‐stimulatory molecule B7.1 leads to increased immunogenicity and reduced tumorigenicity of tumors in several models with T cells involved in the process. We have previously reported decreased tumorigenicity of the murine plasmacytoma J558L [major histocompatibility complex (MHC) class I+ and class II−] expressing IL‐2 or B7.1. When systemic immunity was analyzed, immunization with either J558‐IL2 or J558‐B7.1 cells generated moderate protection against unmodified J558L tumor cells, comparable to immunization with a tumor cells/adjuvant Corynebacterium parvum mixture. In this study, we asked whether the co‐expression of IL‐2 and B7.1 in tumor cells would augment vaccine potency, cytotoxic T lymphocyte (CTL) activity and protective immunity. Rejection of single IL‐2 or B7.1 or co‐transfected IL‐2/B7.1 cells occurred in most syngeneic animals but not in T cell‐deficient nude mice, thus confirming that T cells were required for tumor rejection. We knew from previous experiments that CD8+ T cells were responsible for rejection. Surprisingly, immunization with J558‐IL2/B7.1 cells followed by challenge with parental J558L caused a reduction in systemic protection as compared to J558‐B7.1 or J558‐IL2 alone. We examined the mechanism underlying this unexpected result: 6 days after injection of J558‐IL2/B7.1 cells, tumor were nearly completely destroyed and were almost devoid of CD8+ cells, while CD8+ cells were increased in both IL‐2‐ and B7.1‐transfected tumors. In addition, immunization with J558‐IL2/B7.1 tumors had an adverse effect on the generation of CTL. Mice immunized with J558‐B7.1 and to a lesser extent J558‐IL2 cells mounted a CTL response against J558L cells while, in contrast, no CTL activity could be detected in mice immunized with J558‐IL2/B7.1, thus showing a correlation between the absence of CTL activity and the lack of in vivo protection. We demonstrate that “hyperstimulation” of the immune response by genetically modified cancer vaccines can have adverse effects on tumor immunity, even though the mechanism is not yet completely understood.