18The Dearing strain of Mammalian orthoreovirus (T3D) is undergoing clinical trials as an oncolytic 19 virotherapeutic agent. In this study, a comprehensive phenotypic and genetic comparison of T3D virus stocks 20 from various laboratories and commercial sources revealed that T3D laboratory strains differ substantially in their 21 oncolytic activities in vitro and in vivo. Superior replication of the most-oncolytic T3D lab strain was attributed to 22 virus will impact the immune-stimulatory landscape (e.g. cytokines), by interacting with specific cellular 64 signaling pathways. To test these predictions, we explored the relationships between oncolytic activity and virus 65 genetic and phenotypic features, among various highly related laboratory strains of T3D. 66In this study, we discovered that highly related laboratory strains of T3D exhibit significant differences in 67 oncolytic potency in vitro and in vivo. These findings reinforce that virus genomic and phenotypic features have a 68 strong impact on oncolytic proficiencies. Since the T3D strains were highly genetically related, they provided an 69 opportunity to discover novel determinants of reovirus oncolysis. A comprehensive genetic and phenotypic 70 comparison of T3D laboratory strains revealed that the most-oncolytic strain (obtained from the Patrick Lee 71 laboratory, T3D PL ) has 3 distinct and additive advantages over the less-oncolytic strains (obtained from the 72 Terence Dermody and Kevin Coombs laboratories, T3D TD and T3D KC , respectively). Relative to T3D TD and 73 T3D KC , T3D PL exhibited superior replication in transformed cells in a single round of infection, and induced less 74 IFN signaling which facilitated enhanced cell-cell spread. The genes that contribute to enhanced replication and 75 spread of T3D PL were then characterized, and found to be (1) the T3D PL S1-encoded σ1, which increased the 76 proportion of virions that harbour sufficient number of σ1 trimers for binding to cells, (2) T3D PL S4-encoded σ3, 77 which increased expression of NF-κB stimulated genes, (3) the M1-encoded µ2, which increased virus 78 transcription and filamentous factories, and (4) the L3-encoded λ1, which increased virus factory size. When 79 evaluating which specific polymorphisms in S4, M1, and L3 contribute to oncolysis, surprisingly specific amino 80 acid differences between T3D PL versus T3D TD had both positive and negative contributions, and ultimately a 81 hybrid between T3D PL and T3D TD exhibited superior oncolytic activity even relative to T3D PL . This finding 82 suggests that the T3D oncolytic vector may benefit from continued genetic development to fully thrive in tumors. 83Finally, important to the current excitement surrounding the immunotherapeutic value of oncolytic viruses, was 84 our discovery that T3D laboratory strains induced expression of different cytokines by infected tumor cells. 85Specifically, while T3D PL caused high induction of NF-κB-dependent cytokines but low induction of IFN-86 dependent cytokines, the reciprocal was tr...