Antitumor efficacy of a human interleukin-12 expression plasmid demonstrated in a human peripheral blood leukocyte/human lung tumor xenograft SCID mouse model

Genes encoding the p35 and p40 subunits of human interleukin-12 (IL-12) and the bacterial aminoglycoside phosphotransferase were cloned into a mammalian expression plasmid. The resultant plasmid, pCMVIL-12neo, was used to transfect human lung tumor cell lines in vitro. Stably transfected subclones were generated and found to secrete human IL-12 for at least 10 days following a lethal dose of γ-radiation. The ability of the IL-12-producing tumor cells to promote an antitumor response in vivo was evaluated in SCID mice co-engrafted subcutaneously with human peripheral blood lymphocytes (PBLs) and viable human lung tumor cells (SCID-Winn assay). Using this model system, it was established that IL-12 released locally into tumors by irradiated IL-12-transfected cells activated the human PBL and promoted their ability to suppress tumor development in a dose-dependent fashion. PBL subset depletion studies revealed that the antitumor effect promoted by the IL-12-modified cells was dependent on the presence of human CD8⁺ T cells and, to a lesser extent, human CD56⁺ natural killer cells within the xenograft. We conclude that (a) irradiated human lung tumor cells genetically modified with pCMVIL-12neo secrete bioactive human IL-12 at concentrations sufficient to promote a human lymphocyte-mediated antitumor response in the microenvironment of the xenograft, and (b) that the SCID-Winn assay provides a useful model for the preclinical evaluation of cytokine-based human immunotherapy protocols.