Polymorphisms in the most oncolytic reovirus strain confer enhanced cell attachment, transcription and single-step replication kinetics.

Reovirus serotype 3 Dearing (T3D) replicates preferentially in transformed cells and is in clinical trials as a cancer therapy. Laboratory strains of T3D, however, exhibit differences in plaque size on cancer cells and differences in oncolytic activity in vivo. This study aimed to determine why the most oncolytic T3D reovirus lab strain, the Patrick Lee laboratory strain (T3D(PL)), replicates more efficiently in cancer cells than other commonly used laboratory strains, the Kevin Coombs laboratory strain (T3DKC) and Terence Dermody laboratory (T3D(TD)) strain. In single-step growth curves, T3D(PL) titers increased at higher rates and produced similar to 9-fold higher burst size. Furthermore, the number of reovirus antigen-positive cells increased more rapidly for T3D(PL) than for T3D(TD). In conclusion, the most oncolytic T3D(PL) possesses replication advantages in a single round of infection. Two specific mechanisms for enhanced infection by T3D(PL) were identified. First, T3D(PL) exhibited higher cell attachment, which was attributed to a higher proportion of virus particles with insufficient (<= 3) sigma 1 cell attachment proteins. Second, T3D(PL) transcribed RNA at rates superior to those of the less oncolytic T3D strains, which is attributed to polymorphisms in M1-encoding mu 2 protein, as confirmed in an in vitro transcription assay, and which thus demonstrates that T3D(PL) has an inherent transcription advantage that is cell type independent. Accordingly, T3D(PL) established rapid onset of viral RNA and protein synthesis, leading to more rapid kinetics of progeny virus production, larger virus burst size, and higher levels of cell death. Together, these results emphasize the importance of paying close attention to genomic divergence between virus laboratory strains and, mechanistically, reveal the importance of the rapid onset of infection for reovirus oncolysis. IMPORTANCE Reovirus serotype 3 Dearing (T3D) is in clinical trials for cancer therapy. Recently, it was discovered that highly 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 murine model of melanoma. The current study reveals two mechanisms for the enhanced efficiency of T3D(PL) in cancer cells. Due to polymorphisms in two viral genes, within the first round of reovirus infection, T3D(PL) binds to cells more efficiency and more rapidly produces viral RNAs; this increased rate of infection relative to that of the less oncolytic strains gives T3D(PL) a strong inherent advantage that culminates in higher virus production, more cell death, and higher virus spread.