Acute myeloid leukemia targeting by myxoma virus in vivo depends on cell binding but not permissiveness to infection in vitro

Abstract

Some oncolytic viruses, such as myxoma virus (MYXV), can selectively target malignant hematopoietic cells, while sparing normal hematopoietic cells. This capacity for discrimination creates an opportunity to use oncolytic viruses as ex vivo purging agents of autologous hematopoietic cell grafts in patients with hematologic malignancies. However, the mechanisms by which oncolytic viruses select malignant hematopoietic cells are poorly understood. In this study, we investigated how MYXV specifically targets human AML cells. MYXV prevented chloroma formation and bone marrow engraftment of two human AML cell lines, KG-1 and THP-1. The reduction in human leukemia engraftment after ex vivo MYXV treatment was dose-dependent and required a minimum MOI of 3. Both AML cell lines demonstrated MYXV binding to leukemia cell membranes following co-incubation: however, evidence of productive MYXV infection was observed only in THP-1 cells. This observation, that KG-1 can be targeted in vivo even in the absence of in vitro permissive viral infection, contrasts with the current understanding of oncolytic virotherapy, which assumes that virus infection and productive replication is a requirement. Preventing MYXV binding to AML cells with heparin abrogated the purging capacity of MYXV, indicating that binding of infectious virus particles is a necessary step for effective viral oncolysis. Our results challenge the current dogma of oncolytic virotherapy and show that in vitro permissiveness to an oncolytic virus is not necessarily an accurate predictor of oncolytic potency in vivo.

Myxoma virus suppresses proliferation of activated T lymphocytes yet permits oncolytic virus transfer to cancer cells

“…14 The safety of using MYXV with human hematopoietic stem/progenitor cells has been correlated to the virus' inability to bind or infect normal human CD34 1 hematopoietic cells. 9,10 Of the hundreds of immunocompromised mice that we have transplanted with MYXV-treated cells, none have shown pox ulcerations or any semblance of viral infection whatsoever. Moreover, MYXV has been widely distributed in the Australian environment as a means to control feral populations of European rabbits, and no human infections have ever been reported.…”

“…[8][9][10] MYXV is a viral oncolytic agent that is nonpathogenic to humans and mice but has natural tropism for a variety of human cancers. [11][12][13] In the course of developing MYXV as an ex vivo purging agent for transplant, we serendipitously discovered that NSG mice receiving human HCT xenografts treated ex vivo with MYXV developed no GVHD, lived longer, and yet still exhibited robust human hematopoietic engraftment in the recipient bone marrow.…”

Gene expression profiling of hematologic malignant cell lines resistant to oncolytic virus treatment

“…Several oncolytic viruses are in development, including coxsackievirus A21 for multiple myeloma [9], reovirus for lymphoma [10], and myxoma for acute myeloid leukemia (AML) [11]. Vaccine-strain measles and mumps virus combinations has also led to a synergistic increase in cytotoxicity in myeloid leukemia cells [12].…”

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

“…For some target cell lines, the susceptibility to MYXV oncolysis is dependent on cell stimulation 21, 28. To test whether MOPC315.BM might display a stimulation-dependent permissiveness, we either mock-treated or infected MOPC315.BM cells with vMyx-M135KO-GFP with or without stimulation with phorbol 12-myristate13-acetate (PMA) and ionomycin.…”

“…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. In previous reports investigating MYXV oncolysis of primary human acute myeloid leukemia (AML) cells, cell surface binding by virus was shown to be a major determinant of therapeutic benefit when preventing tumor engraftment into irradiated NSG mice 28 . However, here we demonstrate that ex vivo virotherapy with MYXV arms cells within a donor allotransplant resulting in a dramatic positive enhancement of GVT in a model where the pre-seeded residual target tumor cell is highly resistant to direct infection by free virus.…”