Myxoma Virus Induces Ligand Independent Extrinsic Apoptosis in Human Myeloma Cells

Abstract

Introduction Multiple myeloma is a clonal malignancy of plasma B cells. While recent advances have improved overall prognosis, virtually all myeloma patients still succumb to relapsing disease. Therefore, novel therapies to treat this disease remain urgently needed. We have recently demonstrated that treatment of human multiple myeloma cells with an oncolytic virus known as myxoma results in rapid cell death even in the absence of viral replication; however, the specific mechanisms and pathways involved remain unknown. Materials and Methods To determine how Myxoma virus eliminates human multiple myeloma cells, we queried the apoptotic pathways which were activated following viral infection using immunoblot and other cell biology approaches. Results Our results indicate that myxoma virus infection initiates apoptosis in multiple myeloma cells through activation of the extrinsic initiator caspase-8. Caspase-8 activation subsequently results in cleavage of Bid and loss of mitochondrial membrane potential causing secondary activation of caspase-9. Activation of caspase-8 appears to be independent of extrinsic death ligands and instead correlates with depletion of cellular inhibitors of apoptosis. We hypothesize that this depletion results from virally mediated host-protein shutoff since a myxoma construct which overexpresses the viral decapping enzymes displays improved oncolytic potential. Conclusion Taken together, these results suggest that myxoma virus eliminates human multiple myeloma cells through a pathway unique to oncolytic poxviruses making it an excellent therapeutic option for the treatment of relapsed or refractory patients.

Systemic therapy with oncolytic myxoma virus cures established residual multiple myeloma in mice

“…To facilitate studies into how MYXV treatment impacts MRD, we therefore sought to identify a murine MM cell which recapitulated the previously described response of human MM to MYXV treatment. 7,8 Four established murine MM cell lines: P3.6.2.8.1, MOPC-31C, MOPC-315, and MOPC-315.BM were either mock treated or infected with vMYX-GFP. The rate of infection (Figure 1a) and lytic potential (Figure 1b) of MYXV in each cell line was then compared with that seen in the human U266 MM cell line using flowcytometry and MTT assay.…”

“…6 Unfortunately, while this trial demonstrates that OV has tremendous promise for relapsed MM patients, application of the particular oncolytic measles virus tested is heavily restricted due to the high sero-prevalence of antimeasles immunoglobulin. In order to overcome this obstacle, our lab had previously investigated the antiMM potential of a nonhuman oncolytic agent, known as myxoma virus (MYXV), 5,7,8 whose natural tropism is tightly restricted to lagomorphs (rabbits). 9,10 This work demonstrated that, despite its nonhuman tropism, MYXV infection efficiently killed human MM cells by inducing a rapid apoptotic response.…”

“…9,10 This work demonstrated that, despite its nonhuman tropism, MYXV infection efficiently killed human MM cells by inducing a rapid apoptotic response. 7,8 This response was not observed in normal hematopoietic cells allowing ex vivo treatment with MYXV to prevent MM relapse in the context of autologous stem cell transplant (auto-SCT). 7 Unfortunately, while reinfusion of malignant cells is thought to play some role in MM recurrence following auto-SCT, 11,12 the primary cause of relapse in most patients is likely expansion of minimal residual disease (MRD) which escapes chemotherapeutic eradication within protected bone marrow (BM) niches.…”

Ex Vivo Oncolytic Virotherapy with Myxoma Virus Arms Multiple Allogeneic Bone Marrow Transplant Leukocytes to Enhance Graft versus Tumor

“…Work to date on MYXV-mediated oncolysis of human MM has well characterized its ability to infect and kill human MM via a rapid induction of caspase-8-mediated apoptosis 20 . In stark contrast to this MYXV sensitivity of human MM, we show here that the murine MM cell line MOPC315.BM is uniquely resistant to infection by free MYXV because of its inability to bind MYXV virions, indicating that these cells are highly resistant to direct virus oncolysis, at least as assessed in vitro.…”

“…Notably, MYXV selectively deletes MM cells from PBMCs or BM from patients ex vivo without harming CD34 + HSCs because normal stem cells are incapable of binding MYXV 15, 16, 17, 18, 19. The fate of MYXV-infected human MM cells is a rapidly induced apoptosis that is triggered through activation of the extrinsic caspase-8 pathway 20 . Previous studies investigating MYXV purging of human MM engrafted into irradiated NOD/Scid/IL2Rγ −/− (NSG) mice demonstrated that ex vivo pre-treatment of an allo-HSC transplant with MYXV maintains GVT effects in vivo without altering the engraftment of normal human HSCs into the recipients 15 .…”