Feasibility of a Cell-Based Oncolytic Virotherapy Targeting Bone Marrow Lesions of Multiple Myeloma

Virotherapy is a potentially promising modality to treat intractable malignant tumors that are resistant to conventional chemotherapy and molecularly targeted therapy. From a viewpoint of clinical application, vaccinia virus (VV) has several advantages over other oncolytic viruses, such as a short life cycle, potent tissue penetration and robust lytic activity. We showed that intravenous injection of miRNA (let-7a)-regulated and thymidine kinase-deleted VV (MDVV) efficiently infects and kills multiple myeloma cells with much less adverse effects on normal cells and tissues in SCID mice carrying a subcutaneous xenograft (Blood 2014 124:2082). However, in another xenograft model carrying bone marrow (BM) myeloma lesions, MDVV failed to reduce tumor burden or prolong survival probably due to its poor penetrance to BM. Then, for an alternative strategy, we tested the feasibility of a cell-based virus delivery which targets BM lesions using myeloma cells infected ex vivo with MDVV.

A human myeloma cell line KPMM2 is absolutely dependent on interleukin-6 or a murine stromal cell line such as HESS-5 for its continuous growth and survival (Blood 1997 90:2437). It recapitulates BM myeloma lesions and IgG paraproteinemia when transplanted into SCID mice intravenously. Furthermore, KPMM2 cells are highly susceptible to and efficiently killed by MDVV. First, we tested whether KPMM2 cells can serve as MDVV carriers to cause secondary infection. MDVV was originally labeled by F-luc, and KPMM2 cells were transduced with R-luc (target: T), then seeded onto a HESS-5 monolayer. Twenty-four hrs later, MDVV-infected KPMM2 cells (effector: E) were added at various E/T ratios. Dual F/R-luc assay showed that target cell viability was significantly reduced at E/T ratio ≥ 1 along MDVV propagation. This lytic effect was dependent on time and a multiplicity of infection (MOI), but not perturbed by the co-culture with cord blood mononuclear cells (bystander). Next, we validated the possibility that MDVV-infected KPMM2 cells can deliver MDVV to BM after intravenous injection into SCID mice. Most of KPMM2 cells show high level expression of CXCR4, a critical regulator of myeloma cell homing to BM, and retain comparable CXCR4 expression even after MDVV infection. Actually, 48 to 72 hrs after 1x10⁷ MDVV-infected KPMM2 cells (labeled by F-luc) were injected into SCID mice, in vivo imaging analysis detected the accumulation of prominent F-luc signal at the BM following early phase trap in the lung (Figure). Taken together, the present cell-based strategy is a unique way to enable targeted delivery of MDVV to BM and application of oncolytic virotherapy to hematological malignancies, especially multiple myeloma. The therapeutic efficacy of MDVV-infected KPMM2 cells on BM myeloma lesions in SCID mice is currently under investigation.

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