The therapeutic efficacy of the oncolytic virus Delta24-RGD in a murine glioma model depends primarily on antitumor immunity

Abstract

Oncolytic viruses selectively lyse tumor cells, making these agents a promising treatment modality for glioma. Accumulating data suggest that the immune system plays an important role in the anti-glioma activity of oncolytic viruses. In an immune competent glioma model, the therapeutic effect of the oncolytic adenovirus Delta24-RGD was found to depend primarily on antitumor immune responses.

The oncolytic adenovirus Delta24-RGD specifically replicates in tumor cells harboring a mutated retinoblastoma (Rb)-pathway, which is the case in more than 80% of human gliomas.^1,2 The anti-glioma oncolytic efficacy of Delta24-RGD adenovirus has been independently demonstrated in various xenograft models.^1,3,4 Clinical trials for patients with recurrent glioblastoma have been initiated.⁵ However, little is known about the role of the immune system in oncolytic adenovirus therapy, especially in the context of brain tumors. On one hand, the antiviral immune response could hamper oncolytic efficacy,⁶ whereas on the other hand, a beneficial antitumor immune response could be elicited, as has been reported for other oncolytic virus therapies.⁷ We hypothesized that intratumoral replication of Delta24-RGD would drive a pro-inflammatory immune response remodeling the glioma-induced immunosuppressive tumor microenvironment to one that fosters antitumor immunity.⁸

To study this, we employed an established immunocompetent orthotopic glioma mouse model using murine GL261 tumor cells transplanted in syngeneic C57BL/6 mice.⁹ In our study, we investigated the ability of Delta24-RGD oncolytic virus to infect, replicate and induce cytotoxicity in GL261 cells, allowing this model to be used to map the role of the immune response to Delta24-RGD therapy.¹⁰ Treatment resulted in approximately 50% long-term survivors, an effect that was completely abolished when mice were co-treated with the immunosuppressive agent dexamethasone. Upon intratumoral injection of Delta24-RGD a local rapid release of acute-phase cytokines, including the proinflammatory interleukins IL-1β and IL-6 was detected within 6–12 hours. Furthermore, the antiviral cytokine interferon γ (IFNγ) was also rapidly induced. This induction was completely inhibited by dexamethasone treatment. Apart from the cytokines, also significantly upregulated were the chemokines (C-X-C) motif 10 (CXCL10, better known as IP-10) and (C-C) motif 3 (CCL3, also known as MIP-1α), signals eliciting the recruitment of lymphocytes and monocytes, respectively. Following these signals, macrophages and CD8⁺ T cells migrated into the tumor microenvironment by day 1 and 14, respectively, as revealed by immunohistochemistry. Again, dexamethasone treatment had an inhibitory effect, diminishing the intratumoral influx of macrophages and CD8+ T-cells. Analysis of the humoral arm of the immune system revealed the presence of adenovirus neutralizing antibodies in the blood starting 96 h after viral injection. The cellular response revealed specific GL261 cancer cell recognition by the splenocytic T cells of virus-treated mice at 14 d after treatment. This sequence of events, as summarized in Figure 1, led to the induction of a memory response, which prohibited tumor growth upon rechallenge in the long-term survivors of the Delta24-RGD treatment group.

Figure 1.

The antiglioma immune response during Delta24-RGD oncolytic virus therapy underlies therapeutic efficacy. The Delta24-RGD virus intratumorally injected, infects glioma cells (black) and induces cell lysis. As a result, inflammatory and immunostimulatory cytokines (IL-6, IFNγ) and chemokines (IP-10, MIP-1α) are upregulated locally, leading to an intratumoral influx of macrophages (mø, purple) and CD8⁺ T cells (blue) by 6 h and 14 d, respectively. Meanwhile, plasma cells (yellow) start producing adenoviral neutralizing antibodies. Splenocytic T cells recognize the cancer cells, leading to an antitumor immune response and long term memory .

Collectively, our results indicate that the therapeutic efficacy of the oncolytic adenovirus Delta24-RGD in a murine glioma model is mainly dependent on a virus-induced antitumor immune response.

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.