Cytokine-enhanced intravenous oncolytic virotherapy

Abstract Background Oncolytic viruses are emerging as promising cancer therapies; a key challenge in their use is delivery of virus to tumour targets in the face of host immunity. In a recent translational clinical study we found that intravenously administered reovirus successfully reaches liver tumours, despite patients all having high concentrations of neutralising anti-reovirus antibodies. Functional virus was recoverable from several white-cell subsets, suggesting that systemically administered virus evades neutralisation by close cell-association (hitch-hiking). We hypothesised that this occurrence could be manipulated to enhance virotherapy by expanding the numbers of carrier cells in the circulation before viral administration. Methods We used immune competent mice (C57BL/6) that had been vaccinated with reovirus, or phosphate buffered saline (PBS) control, before establishing syngeneic melanoma (B16-Ova) tumours on their flank. Mice were then treated with one of three cytokines—interleukin 2, granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage colony-stimulating factor (GM-CSF)—or PBS as control, for 3 days, either before or after virotherapy (reovirus) or PBS as control therapy. Unvaccinated mice were also tested with this regimen. Tumours were then measured daily or harvested for correlative analyses. Mice were humanely killed when tumours reached 1 cm in any dimension. Eight mice were used in each treatment group and experiments were repeated at least twice to ensure consistency of the findings. Findings Treatment with GM-CSF enhanced the number of circulating granulocytes, and enhanced macrophage infiltration into tumours. When reovirus-immune mice were treated with GM-CSF before virotherapy with reovirus, tumours regressed in all mice and were undetectable 60 days later; but reovirus-immune mice treated with either interleukin 2 or G-CSF before virotherapy, or treated with GM-CSF alone, all developed tumours that necessitated euthanasia. Moreover the combination of GM-CSF and reovirus was only effective if GM-CSF was administered before reovirus. The efficacy of GM-CSF plus reovirus was considerably diminished in unvaccinated reovirus-naive mice, but was restored if three cycles of GM-CSF plus reovirus were administered, suggesting that in this regimen anti-Rep antibodies contributed to the efficacy of therapy. Interpretation We found that oncolytic virotherapy with reovirus is enhanced by vaccinating mice against the virus and pretreatment with GM-CSF. Further in-vivo experiments, and associated in-vitro assays, are underway to characterise the mechanism involved. Pragmatically these data suggest a novel method for enhancing oncolytic virotherapy that can be readily implemented because both reovirus and GM-CSF are used clinically; accordingly a translational clinical trial protocol is being developed to explore this possibility further. Funding UK Medical Research Council, Royal Society of Medicine.