Tricistronic viral vectors co-expressing interleukin-12 (1L-12) and CD80 (B7-1) for the immunotherapy of cancer: preclinical studies in myeloma.

Synergy between interleukin-12 (IL-12) and B7-1 (CD80) for cancer immunotherapy has previously been demonstrated in animal models of breast cancer, lymphoma, and multiple myeloma. With a view to human clinical application, tricistronic retroviral and adenovirus vectors co-expressing IL-12 (IL-12p40 plus IL-12p35) and CD80 were constructed by utilizing two internal ribosome entry site (IRES) sequences to link the three cDNAs. A murine stem cell virus (MSCV)-based retroviral vector (MSCV-hIL12.B7) utilized distinct IRES sequences from the encephalomyocarditis virus (EMCV) and the foot-and-mouth disease virus (FMCV), whereas Ad5-based adenovirus vectors contained transcriptional units with two EMCV IRES sequences under the control of murine (AdMh12.B7) or human (AdHh12.B7) cytomegalovirus promoters. AdMh12.B7 was found to consistently direct higher levels of IL-12 and CD80 expression than AdHh12.B7 following infection of a number of human tumor cell lines. In preclinical studies, the human myeloma cell line U266 was infected with MSCV-hIL12.B7 and a resulting clonal cell line, U/MSCV-h12.B7, was generated with stable expression of CD80 and secreting IL-12 at 1 ng/24 h/10(6) cells. By comparison, following AdMh12.B7 infection, 81% of infected U266 cells (U/AdMh12.B7) expressed CD80 and secreted IL-12 at 25-50 ng/24 h/10(6) cells. Both engineered myeloma cell lines stimulated enhanced allogeneic mixed lymphocyte proliferation and provoked increases in cytotoxic T-lymphocyte responses and gamma-interferon release from normal donor lymphocytes exposed to parental U266 cells. These results suggest potential clinical utility of AdMh12.B7 in immunotherapy strategies for the treatment of multiple myeloma and other cancers.