Doxorubicin-enriched, ALDHbr mouse breast cancer stem cells are treatable to oncolytic herpes simplex virus type 1

Abstract

BackgroundThe primary objective of this study was to test whether oncolytic herpes simplex virus type 1 (HSV1) could eradicate chemoresistant cancer stem cells (CSCs).MethodsThe fluorescent aldefluor reagent-based technique was used to identify and isolate ALDHbr cells as CSCs from the 4T1 murine breast cancer cell line. The presence of ALDHbr 4T1 cells was also examined in 4T1 breast cancer transplanted in immune-competent syngeneic mice.ResultsCompared with ALDHlo cells, ALDHbr cells had a markedly higher ability to form tumor spheres in vitro and a higher tumorigenic potential in vivo. ALDHbr cells also exhibited increased doxorubicin resistance in vitro, which correlated with a selective increase in the percentage of ALDHbr cells after doxorubicin treatment and an increased expression of P-glycoprotein (P-gp), a known chemoresistance factor. In contrast, oncolytic HSV1 was able to kill ALDHbr cells in vitro and even more markedly in vivo. Furthermore, in in vivo studies, systemic administration of doxorubicin followed by intratumoral injection of oncolytic HSV1 resulted in much more significant suppression of tumor growth with increased median survival period compared with each treatment given alone (p<0.05). Though more CD8+ T lymphocytes were induced by oncolytic HSV1, no significant specific T cell response against CSCs was detected in vivo.ConclusionsThese results suggested that the use of oncolytic HSV1 following doxorubicin treatment may help eradicate residual chemoresistant CSCs in vivo.