Angiostatin enhances B7.1-mediated cancer immunotherapy independently of effects on vascular endothelial growth factor expression

Tumors must develop an adequate vascular network to meet their increasing d emands for nutrition and oxygen. Angiostatin, a multiple kringle (1-4)-cont aining fragment of plasminogen, is an effective natural inhibitor of tumor angiogenesis. Here we show that gene transfer of angiostatin into small (0. 1 cm in diameter) solid EL-4 lymphomas established in syngeneic C57BL/6 mic e led to reduced tumor angiogenesis and weak inhibition of tumor growth. In contrast, when angiostatin gene therapy was preceded by in situ gene trans fer of the T-cell costimulator B7.1, large (0.4 cm in diameter) tumors were rapidly and completely eradicated, whereas B7.1 and angiostatin monotherap ies were ineffective. Combined gene transfer of B7.1 and angiostatin genera ted potent systemic antitumor immunity that was effective in eradicating a systemic challenge of 10(7) EL-4 cells. Gene transfer of angiostatin expres sion plasmids led to overexpression of angiostatin in tumors, increased apo ptosis of tumor cells, and decreased density of tumor blood vessels, which may allow the immune system to overcome tumor immune resistance. The latter effects were not the result of a decrease in vascular endothelial growth f actor expression, as tumoral vascular endothelial growth factor expression increased slightly after angiostatin gene transfer, presumably in response to increasing hypoxia. These results suggest that combining immunogene ther apy with a vascular attack by angiostatin is a particularly effective appro ach for eliciting antitumor immunity.