Oncolytic herpes simplex virus counteracts the hypoxia-induced modulation of glioblastoma stem-like cells.

Glioblastoma (GBM), a fatal malignant brain tumor, contains abundant hypoxic regions that provide a "niche" to promote both the maintenance and enrichment of glioblastoma stem-like cells (GSCs) and confer resistance to chemo- and radiotherapy. Since GSCs, with an ability to resist conventional therapies, may be responsible for tumor recurrence, targeting GSCs located in such a hypoxic environment may be critical to improving the therapeutic outcome for GBM patients. Oncolytic viral therapies have been tested in the clinic as a promising therapeutic approach for GBM. In this study, we analyzed and compared the therapeutic effects of oncolytic herpes simplex virus (oHSV) type 1 G47Δ (γ34.5(-)ICP6(-)LacZ(+)α47(-)) in patient-derived GSCs under normoxia (21% oxygen) and hypoxia (1% oxygen). GSCs cultured in hypoxia showed an increased ability to form neurospheres and expressed higher levels of the putative stem cell marker CD133 compared with GSCs cultured in normoxia. G47Δ exhibited a comparable ability to infect, replicate, and kill GSCs in normoxia and hypoxia in vitro. Importantly, G47Δ could counteract hypoxia-mediated enhancement of the stem-like properties of GSCs, inhibiting their self-renewal and stem cell marker expression. Using orthotopic human GSC xenografts in mice, we demonstrated that intratumoral injection of G47ΔUs11fluc, a newly developed G47Δ derivative that expresses firefly luciferase driven by a true late viral promoter, led to an equivalent frequency of viral infection and replication in hypoxic and nonhypoxic tumor areas. These findings suggest that oHSV G47Δ represents a promising therapeutic strategy to target and kill GSCs, not only in normoxic areas of GBM but also within the hypoxic niche.