Multiple myeloma (MM) is a disseminated malignancy of terminally differentiated plasma cells residing primarily in the bone marrow. The disease is associated with bone destruction, anemia and compromised immune function(1
). Current treatments for MM, including stem cell transplantation and high-dose chemotherapy, have significantly prolonged median survival to 6 years, but still fall short as a cure. There is a clear need for the development and testing of novel treatment options that can be delivered intravenously to reach and destroy disseminated disease sites(2
Vesicular stomatitis virus (VSV) is a non-segmented, negative strand RNA virus of the Rhabdoviridae family that selectively kills malignant cells and has demonstrated oncolytic activity in various preclinical cancer models (3
). VSV infections normally occur in livestock, causing only mild symptoms during naturally occurring human infections (4
). Lethal infection of neural tissue in animal models prompted the use of viral engineering to attenuate VSV and minimize toxicity (5
). Detection of viral infection in cells results in activation of Interferon regulatory factors (IRFs), and expression of Type I Interferons (IFN) that can bind the α/β interferon receptor (IFNAR). IFNAR activation induces cellular innate immune responses by stimulating production of proteins that facilitate viral clearance by degrading viral intermediates, inhibiting translation, inducing apoptosis and activating release of pro-inflammatory cytokines(7
). The Type I Interferons include several IFNα subtypes encoded by 14 IFNα genes and four IFNα pseudogenes, and IFNβ, which is encoded by a single gene(9
). While there have been differences described in receptor activation by IFNα and IFNβ(10
), IFNβ has been described to be expressed early upon viral detection, inducing IRF7 expression that activates further expression of the various IFNα genes(9
). Of the five genes encoded by the VSV genome, the VSV Matrix protein (M) facilitates evasion of innate immunity by blocking nucleocytoplasmic transport of mRNA preventing synthesis of IFN and other antiviral or pro-inflammatory proteins (11
). Aberrations leading to tumorigenesis also diminish innate immune response pathways thereby rendering cancer cells susceptible to VSV replication and oncolysis(8
). VSV attenuation can be achieved by deleting (or mutating) residue 51 of the M protein (VSV-MΔ51) to reverse viral suppression of IFN synthesis(13
) or by incorporation of the IFNβ gene into the viral genome(14
), inducing IFNβ expression and activation of downstream antiviral genes including IFNα, to promote viral clearance from non-cancerous tissues. Cancer cells that are weakly responsive to IFN remain permissive to viral propagation and oncolysis.
The potential of VSV as a novel myeloma therapy was assessed in the immune competent 5TGM1 model of MM. The 5TGM1 murine myeloma cell-line can be implanted in C57Bl/KaLwRij syngeneic mice to form rapidly growing subcutaneous tumors or injected intravenously to induce orthotopic myeloma that can be monitored by measuring IgG2b paraprotein secreted by myeloma cells (15
). This allows novel MM therapies to be tested in model that closely resembles disseminated myeloma in patients in the presence of intact anti-viral innate and adaptive immune responses.
While VSV-MΔ51 has enhanced tumor specificity by inducing IFN production, viral replication is significantly compromised even in IFN-resistant cells (16
). Having previously demonstrated that recombinant VSV-MΔ51 coding for the sodium iodide symporter (NIS) has weak anti-myeloma activity(17
), we hypothesized VSV coding for Interferon-β (VSV-IFNβ) would specifically destroy cancer cells in vivo
while maintaining viral potency. IFNα has historically been used as therapy against myeloma, albeit with weak and variable efficacy and has reported to have various anti-tumor properties including direct pro-apoptotic activity, promotion of long-lasting anti-tumor immunity and inhibition of tumor blood vessel formation (18
). IFNβ however, has been utilized extensively in various oncolytic viral vectors including adenovirus(21
) and measles viruses (23
), demonstrating that viral vectors expressing IFNβ can be safely and successfully utilized for the treatment of cancer. VSV expressing IFNβ has also previously shown antitumor efficacy in preclinical cancer models including renal cell carcinoma(14
) and malignant pleural mesothelioma(24
), while safety studies indicates viral IFNβ expression successfully alleviates VSV neurotoxicity(5
), making VSV-IFNβ a potentially potent and safe vector for systemic treatment of Multiple myeloma.
Here we report the potent oncolytic activity of intravenously administered VSV-IFNβ in an immune competent model of MM. VSV is able to specifically target tumor sites in vivo inducing destruction of myeloma cells in subcutaneous tumors and within the bone marrow. Destruction of disseminated orthotopic myeloma resulted in transiently reduced and subsequently delayed disease burden and prolonged survival. Therapeutic efficacy was achieved in the presence of robust antiviral antibody response with no detectable toxicity. We further demonstrate that VSV expressing murine IFNβ shows significantly enhanced therapy in mice bearing disseminated myeloma making VSV-IFNβ a strong candidate as a potential new therapeutic vector for the treatment of myeloma.