271. Interactions between Replicating Virotherapy and Tumor Growth in the Presence of an Intact Immune System

Abstract

An intact immune system provides potential opportunities for, and barriers against, effective virotherapy for cancer using replication competent viruses. On one hand, anti-viral and anti-tumor immune responses may allow greater levels of immune-mediated tumor killing than viral oncolysis alone; on the other hand, the same immune responses may inhibit viral replication and spread, thereby diminishing the efficacy of the virotherapy itself. We have investigated the complex interactions between the immune system, viral replication and tumor growth by treating established subcutaneous (s.c.) B16ova murine melanomas with the Western Reserve strain of Vaccinia Virus (VV). In SCID mice, intratumoral (i.t.) injection of 1|[times]|108 pfu of VV into 8-day-old established B16.ova tumor enhanced survival (median 19 days) compared to inactivated virus (median 13.5 days) (p=0.003). The lack of a functional immune system permitted systemic spread of VV and the mice succumbed to treatment-associated disease 11 days following injection. In C57Bl/6 immunocompetent mice, a single i.t. injection of 1|[times]|108 pfu of VV led to a much smaller survival advantage of only 4 days compared to controls (p=0.01). The intact immune system did, however, protect the mice from systemic toxicity. We have analysed these two models to characterize the immune effectors involved in controlling the balance between anti-tumor efficacy and systemic toxicity. Eleven days following injection of VV, viral titres recovered from whole tumors from immunodeficient mice were significantly higher (6.9|[times]|108 pfu) than from immunocompetent mice (4.9|[times]|107 pfu) (p=0.01). In C57Bl/6 mice treated with VV, a VV-specific CD8 response was generated but C57Bl/6 mice depleted of CD8 cells did not develop systemic toxicity and there was no difference in titre of i.t. virus recovered compared to non-depleted C57Bl/6 mice. Following treatment with VV, there were elevated titres of neutralising antibody to VV in serum from C57Bl/6, but not from immunodeficient mice. Although additional daily sequential i.t. injections had no further therapeutic effects, interestingly, two i.t. injections of VV separated by 5 days, led to complete tumor regression in 30% of C57Bl/6 mice without any associated systemic toxicity. The cured mice were protected long term (>90 days) against rechallenge with 1|[times]|106 B16.ova cells. In summary, complex interactions are established in immunocompetent animals, between tumor, VV and the immune system. Neutralising Abs against VV may be a double-edged sword, which protect the host from toxicity but impede oncolysis. However, a prime-boost strategy may modulate immunity directed against viral and/or tumor antigens that can shift the equilibrium towards increased tumor cell clearance to achieve tumor regression and long-term protection. We are currently defining the immune mediators involved in this prime-boost strategy.