Characterization of a new influenza virus type D

Influenza is caused by viruses belonging to the Orthomyxoviridae family. Currently three types of influenza virus are known: A (Influenza A virus, IAV), B (IBV) and C (ICV). Despite the fact that all these viruses are derived from a common ancestor they differ from each other by the number of segments, the size and sequence of RNA segments, antigenicity, pathogenicity and the spectrum of natural reservoirs. In 2011, a new influenza virus was isolated in the USA from pigs manifesting influenza-like symptoms. The virus was the most closely related to ICV. It was able to replicate in vitro in different cell cultures and displayed much broader cell tropism than human ICV. Moreover, in contrast to ICV, it was able to replicate at 370C. Electron microscopic studies demonstrated features characteristic of Orthomyxoviruses. Despite morphological and organizational similarities, the biological properties of the new virus, including biochemical activity, differ from that of other influenza viruses. Enzymatic assays revealed that the new virus had negligible neuraminidase but detectable O-acetyloesterase activity. Further studies evidenced that the new virus varied from ICV in receptor binding, despite its sharing a conserved array of functional domains in the viral RNA genome replication and viral entry machinery. Analysis conducted with the use of the model of crystal structure of the hemagglutinin-esterase fusion protein (HE) of the new virus and its receptor demonstrated that this protein was multifunctional. It catalyzes cellular receptor binding, receptor cleavage, as well as membrane fusion. Moreover, divergent receptor-binding sites than HE of ICV have been discovered in the new virus. These amino acid differences may alter the binding specificity and affinity of the HE protein to the receptor that in turn result in the observed differences in cellular tropism between the two viruses. It also possesses an open channel between the 230-helix and 270-loop in the receptor-binding site, which is a unique feature of this virus. This might explain why the new virus has a broad cell tropism. It is possible that the sequence variation in the fusion domain may influence the replication of this virus at a higher temperature when compared to ICV. Next-generation sequencing demonstrated that the genome of the new virus, similarly to ICV, had seven single-stranded negative-sense RNA segments coding 9 viral proteins. Deep RNA sequencing found a M1 protein expression strateg