Retrovirus-mediated gene transfer of B7-1 and MHC class II converts a poorly immunogenic neuroblastoma into a highly immunogenic one

The T cell co-stimulatory molecule B7-1 was transduced into a poorly immunogenic murine neuroblastoma cell line (Neuro-2a, N-2a) alone or in combination with MHC class II genes to test the ability of these genes to stimulate antitumor immunity. N-2a cells transduced with B7-1 exhibited reduced tumorigenicity, whereas N-2a cells overexpressing both MHC class II (syngeneic, I-A(k)) and B7-1 totally abrogated tumorigenicity. Rejection of I-A(k)/B7-1 cells was dependent on both CD4(+) and CD8(+) T cells. The ability of both vaccines to induce protection against parental N-2a was temporally dependent on the time of secondary N-2a challenge. To investigate the immunity generated by N-2a/B7-1 and N-2a/I-A(k)/B7-1 vaccines, we tested the ability of these modified cells to stimulate in vitro the proliferation of syngeneic splenocytes from naive mice. A significant increase in splenocyte proliferation was observed with N-2a/I-A(k)/B7-1 cells compared to N-2a cells. We also determined that vaccination with N-2a/I-A(k)/B7-1 cells was able to generate cytotoxic T cell responses to unmodified N-2a cells. The introduction of B9-1 and I-A(k) into N-2a was able to convert a pearly immunogenic tumor to a highly immunogenic one; however, mice bearing large established unmodified tumors had little response to vaccination with N-2a/I-A(k)/B7-1 cells. Our results emphasize the importance of tumor immunogenicity in the treatment of established tumors with MHC class II/B7-1 tumor cell vaccines.