Figure 4. Regulatory T cells depletion with interleukin-2 (IL-2) enhances viral delivery to systemic metastatic disease.

(a and b) C57Bl/6 mice were left uninjected (nontumor bearing) or were injected intravenously (IV) with B16 cells to seed metastases in the lung. After 9 days, the mice received an intraperitoneal injection of PC61 or a control immunoglobulin G. One group received the natural killer (NK) cell-depleting asialo GM-1 antibody 24 hours before PC61 (no NK cell). After a further 24 hours, the mice were injected intraperitoneally with phosphate-buffered saline (PBS) or recombinant human IL-2 (75,000 U/injection for 10 injections). Two hours after this last injection of IL-2/PBS, the mice received 10⁸ plaque forming units (pfu) of VSV-GFP IV. After a further 36 hours the mice were killed, the lungs were removed, and viral titers were measured from freeze–thaw lysates of (a) the lungs or (b) hearts (two mice/group). One group of mice, that had been injected IV with 10⁸ pfu of VSV-GFP 3 weeks before the seeding of the B16 lung metastases, was treated with PC61/IL-2 and VSV (VSV, PC61/IL-2, preimmune). (c) Dissociated heart tissues and (d) spleen tissues from the mice described in were analyzed using flow cytometry for GFP expression. GFP, green fluorescent protein; VSV, vesicular stomatitis virus.