Expression Levels of the Attachment Protein G Differ Between Strains of a Murine Pneumovirus and Determine the Virulence

Pneumonia virus of mice (PVM), the mouse homolog to respiratory syncytial virus (RSV), is increasingly used as surrogate model to study pneumovirus pathogenesis in a more natural pathogen-host relation. Two major strains of PVM, strain 15 and J3666 are currently used in laboratories, with preferences for either one or the other based on the well-documented isolation history of strain 15 or the suggested higher virulence of strain J3666. Using conventional and long read sequencing, we found that the PVM strain J3666 represents two distinct virus populations, which are defined by sequence and structure of the G and SH genes encoding the putative attachment and small hydrophobic proteins, in addition to further nucleotide polymorphisms. Specifically, a nucleotide polymorphism at position 65 in the G gene results in either an upstream open reading frame (uORF) preceding the main ORF in frame, or an extension of the major G ORF by 18 codons. The impact of the different forms of the J3666-G genes on PVM was examined by generating recombinant PVMs differing exclusively in the distinctive 5 portion of the respective G gene. This revealed that the population expressing a G protein with an extended main G ORF was more virulent, whereas the presence of a uORF attenuated virulence. The virulence of PVM correlated with increased expression levels of G, whereas attenuation was rather associated with downregulated expression of G due to the presence of a uORF. Thus, modulation of G protein levels may be an important mechanism by which pneumoviruses modulate virulence. ImportanceThe pneumonia virus of mice strain J3666 is considered a more virulent and more suitable model for severe lower respiratory tract infections. The organization of the gene for the attachment protein G is reported to contain a small upstream open reading frame (uORF) preceding the main G ORF in frame. The translated G protein is predicted to comprise 396 amino acids. We report that this virus strain may be a mixture of two different populations, each with differing virulence. The more virulent population encodes a G protein of potentially 414 amino acids instead of a small uORF. This G gene organization is associated with an increased G protein expression. Importantly, this organization of the G gene is in line with that of several newly identified pneumoviruses, i.e., canine and swine pneumoviruses. These viruses may comprise a distinct group within the Pneumoviridae family.