Sequence polymorphisms in the reovirus σ1 attachment protein modulate encapsidation efficiency and replication in mice.

Many functions of viral attachment proteins are established, but less is known about the biological importance of viral attachment protein encapsidation efficiency. The mammalian orthoreovirus (reovirus) σ1 attachment protein forms filamentous trimers that incorporate into pentamers of the λ2 capsid protein. Reovirus strains vary in the efficiency of σ1 encapsidation onto progeny virions, which influences viral stability during entry into cells and the efficacy of tumor cell lysis. While the role of σ1 encapsidation has been evaluated in studies using cultured cells, the contribution of attachment protein encapsidation efficiency to viral infection in animals is less clear. Polymorphisms in reovirus σ1 at residues 22 and 249 have been implicated in viral dissemination in mice and susceptibility to proteolysis in the murine intestine, respectively. To determine whether these residues contribute to σ1 encapsidation efficiency, we engineered σ1 mutant viruses with single- and double-residue substitutions at sites 22 and 249. We found that substitutions at these sites alter the encapsidation of σ1 and that reoviruses encapsidating higher amounts of σ1 bind cells more avidly and have a modest replication advantage in a cell-type-specific manner relative to low σ1-encapsidating reoviruses. Furthermore, we found that a high σ1-encapsidating reovirus replicates and disseminates more efficiently in mice relative to a low σ1-encapsidating reovirus. These findings provide evidence of a relationship between viral attachment protein encapsidation efficiency and viral replication in cell culture and animal hosts. IMPORTANCE: Viral attachment proteins can serve multiple functions during viral replication, including attachment to host cells, cell entry and disassembly, and modulation of host immune responses. The relationship between viral attachment protein encapsidation efficiency and viral replication in cells and animals is poorly understood. We engineered and characterized a panel of reoviruses that differ in the capacity to encapsidate the σ1 attachment protein. We found that strains encapsidating σ1 with higher efficiency bind cells more avidly and replicate and spread more efficiently in mice relative to those encapsidating σ1 with lower efficiency. These results highlight a function for σ1 attachment protein capsid abundance in viral replication in cells and animals, which may inform future use of reovirus as an oncolytic therapeutic.