Potent anti-tumor activity of systemically-administered IL2-expressing Salmonella correlates with decreased angiogenesis and enhanced tumor apoptosis

Salmonella enterica serovar Typhimurium (hereafter S. typhimurium) stains have been shown to exert a potent inhibitory effect on the growth of human and mouse tumors in experimental models. Our laboratory has previously demonstrated that an attenuated strain of S. typhimurium engineered to express IL2 (designated strain GIDIL2) has demonstrable immunopotentiating properties, particularly affecting the innate arm of the immune system. In the present study, we wished to explore the properties of IL2-expressing Salmonella as an oncolytic agent in the highly tumorigenic B16F1 melanoma mouse model and shed light on its mechanism of action. Our data demonstrate that the systemic administration of a single dose of GIDIL2, two weeks post B16F1 implantation, had a significantly superior effect than its parental, non cytokine-expressing, strain (known as BRD509E). The improved response, which was dependent on the bacterial dose used, was observed in terms of stronger inhibition of tumor growth as well as enhanced host survival. The GIDIL2-induced anti-tumor response was correlated with decreased angiogenesis and increased necrosis within the tumor tissue. A treatment regimen involving multiple low doses of GIDIL2 was more efficacious than a single high dose regimen, resulting in extension of animal survival well beyond the normal 30 day post implantation period typically observed in this aggressive melanoma tumor model. This supports the notion of using cytokine-expressing attenuated Salmonella organisms in cancer therapy.