Abstract 18: Evaluating the contribution of anti-myeloma immunity for the efficacy of oncolytic reovirus therapy
Authors: Müller Louise M. E. , Parrish Christopher , Migneco Gemma , Scott Gina B. , Holmes Matthew , Melcher Alan A. , Lawson Michelle A. , Cook Gordon , Errington-Mais Fiona 
Published: August 2018

Abstract
Introduction: The aim of this study was to examine the contribution of antitumor immunity for the efficacy of oncolytic reovirus therapy against multiple myeloma (MM). Oncolytic viruses (OV) have two main mechanisms of action; direct lytic killing and potentiation of antitumor immunity. The direct oncolytic potential of reovirus in MM has previously been demonstrated both in vitro and in vivo, but the importance of an enhanced immunologic antitumor response remains underinvestigated. Reovirus-induced antitumor immunity has been demonstrated in other types of cancer such as melanoma and chronic leukemia, and is of major importance for the efficacy of OV treatment. Thus, it was hypothesized that reovirus-induced antimyeloma immunity would contribute significantly to the efficacy of reovirus treatment for MM.
Experimental Procedures: C57BL/KaLwRij mice were used in the 5TGM1 model system to establish MM in vivo. This model closely resembles human MM with induction of osteolytic bone disease and secretion of paraprotein. C57BL/KaLwRij mice have a fully functional immune system, comparable to C57BL/6 mice, and to our knowledge, this is the first immunocompetent model of MM for the study of reovirus efficacy. After establishment of MM in the bone by intravenous injection of bone-homing 5TGM1 cells, mice were treated with repeated injections of reovirus or PBS. Upon sacrifice, direct cytotoxicity and immune activation was examined using flow cytometry. All animal experiments were performed under an appropriate project license following approval by a local ethical review committee. In vivo findings were translated into human in vitro studies, using MM cell lines, healthy donor (HD) blood, and MM patient samples. Reovirus-induced Natural Killer (NK) cell activation and degranulation was examined using flow cytometry and priming of myeloma-specific T cells was performed using long-term priming cultures.
Results: Tumor burden was reduced by reovirus treatment both in the bone marrow (BM) and spleen of tumor-bearing mice. The immune cell populations, including NK cells, CD4+ T cells, and CD8+ T cells, were restored to levels of tumor-naïve mice in the BM. NK cells were activated in the BM following reovirus treatment, which indicates the onset of an innate immune response. In the spleen, an increase in CD4+ T cells, in combination with CD8+ activation, was indicative of an early adaptive immune response. These results translate into human in vitro findings, using both HD and MM patient samples, with activation of NK cells in response to reovirus treatment and subsequent enhancement of NK cell degranulation and killing of MM target cells. Encouragingly, reovirus-activated NK cells were able to kill OPM2 cells, which are resistant to direct lytic killing. Preliminary human in vitro studies suggest that reovirus treatment can prime antimyeloma CD8+ T cells for the induction of a long-term protective response.
Conclusions: Introducing a viral agent into the body requires a delicate immunologic balance to avoid neutralizing the virus by an antiviral response and simultaneously allowing the enhancement of antitumor immunity. The importance of enhanced antitumor immunity for OV therapy efficacy is becoming more widely recognized and antiviral immunity can in some circumstances contribute to tumor eradication. The findings in this study indicate that the antitumor immune response is also important in the MM setting, with activation of both innate and adaptive immune responses resulting in enhanced killing of MM cells, in particular those resistant to direct lytic killing. This suggests that both arms of OV therapy could play a role for MM eradication, including any minimal residual disease. Future work will explore whether the antimyeloma response can be further enhanced by combinatorial treatments, including current standard of care treatments.
Citation Format: Louise M. E. Müller, Christopher Parrish, Gemma Migneco, Gina B. Scott, Matthew Holmes, Alan A. Melcher, Michelle A. Lawson, Gordon Cook, Fiona Errington-Mais. Evaluating the contribution of anti-myeloma immunity for the efficacy of oncolytic reovirus therapy [abstract]. In: Proceedings of the Second AACR Conference on Hematologic Malignancies: Translating Discoveries to Novel Therapies; May 6-9, 2017; Boston, MA. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(24_Suppl):Abstract nr 18.