Single amino acid differences between closely related reovirus T3D lab strains alter oncolytic potency in vitro and in vivo .

Little is known about how genetic variations in viruses affect their success as therapeutic agents. The type 3 Dearing strain of Mammalian orthoreovirus (T3D) is undergoing clinical trials as an oncolytic virotherapy. Worldwide, studies on reovirus oncolysis use T3D stocks propagated in different laboratories. Here, we report that genetic diversification among T3D stocks from various sources extensively impacts oncolytic activity. The T3D strain from the Patrick Lee laboratory strain (TD3(PL)) showed significantly stronger oncolytic activities in a murine model of melanoma than the strain from the Terence Dermody laboratory (T3D(TD)). Overall in vitro replication and cytolytic properties of T3D laboratory strains were assessed by measuring virus plaque size on a panel of human and mouse tumor cells, and results were found to correlate with in vivo oncolytic potency in a melanoma model. T3D(PL) produced larger plaques than T3D(TD) and than the T3D strain from the ATCC (T3DATCC) and from the Kevin Coombs laboratory (T3DKC). Reassortant and reverse genetics analyses were used to decipher key genes and polymorphisms that govern enhanced plaque size of T3D(PL). Five single amino acid changes in the S4, M1, and L3 genome segments of reovirus were each partially correlated with plaque size and when combined were able to fully account for differences between T3D(PL) and T3D(TD). Moreover, polymorphisms were discovered in T3D(TD) that promoted virus replication and spread in tumors, and a new T3D(PL)/T3D(TD) hybrid was generated with enhanced plaque size compared to that of T3D(PL). Altogether, single amino acid changes acquired during laboratory virus propagation can have a large impact on reovirus therapeutic potency and warrant consideration as possible confounding variables between studies. IMPORTANCE The reovirus serotype 3 Dearing (T3D) strain is in clinical trials for cancer therapy. We find that closely related laboratory strains of T3D exhibit large differences in their abilities to replicate in cancer cells in vitro, which correlates with oncolytic activity in a in a murine model of melanoma. The study reveals that five single amino acid changes among three reovirus genes strongly impact reovirus therapeutic potency. In general, the findings suggest that attention should be given to genomic divergence of virus strains during research and optimization for cancer therapy.