B7.1 immunogene therapy effectively activates CD4+ tumor-infiltrating lymphocytes in the central nervous system in comparison with B7.2 gene therapy

The B7 gene utilizing immunogene therapy is one of the most common methods against tumor growth. However, there is no known study that investigated the difference between B7.1 and B7.2 with regard to B7 gene therapy in the central nervous system (CNS). Therefore, to clarify the difference, we established B7.1 or B7.2 gene transduced tumor cells originating from the murine T cell lymphoma cell line EL4 (EL4-B7.1 or EL4-B7.2). First, we observed the survival time after intracranial inoculation of parent (IC-wt) or genetically modified tumor cells. All mice in control groups (IC-wt or IC-mock) were dead within 16 days. While there was significant survival elongation in the B7.2 modified group (IC-B7.2, p=0.0002), all mice in this group were dead of tumor growth within 22 days. On the other hand, 60% of mice inoculated with EL4-B7.1 (IC-B7.1) survived more than 120 days (p<0.0001). Second, to shed light on the anti-tumor immune response in situ, we tried to analyze CD4+ tumor-infiltrating T lymphocytes (CD4+ TIL). To purify and analyze CD4+ TIL, we had to deplete F4/80+ microglia because of the CD4 expression. In terms of activation marker expression in CD4+ TIL, a small population was activated (CD25, 9.8%; CD69, 15.8%) in the control group (IC-wt). In contrast, the activation marker positive CD4+ TIL percentage both in IC-B7.1 (CD25, 25.1%; CD69, 40.1%) and IC-B7.2 (CD25, 16.2%; CD69, 28.3%) appeared to reflect the survival curve in both groups. These findings strongly suggest that, in the CNS, B7.1 gene therapy could effectively introduce CD4+ TIL activation compared with B7.2 gene therapy. This is the first study clearly describing the difference between B7.1 gene therapy and B7.2 gene therapy in the CNS in terms of the activation status of CD4+ TIL in situ.