The Avian-Origin PB1 Gene Segment Facilitated Replication and Transmissibility of the H3N2/1968 Pandemic Influenza Virus

Abstract

IMPORTANCEDespite the high impact of influenza pandemics on human health, some mechanisms underlying the emergence of pandemic influenza viruses still are poorly understood. Thus, it was unclear why both H2N2/1957 and H3N2/1968 reassortant pandemic viruses contained, in addition to the avian HA, the PB1 gene segment of the avian parent. Here, we addressed this long-standing question by modeling the emergence of the H3N2/1968 virus from its putative human and avian precursors. We show that the avian PB1 segment increased activity of the viral polymerase and facilitated viral replication. Our results suggest that in addition to the acquisition of antigenically novel HA (i.e., antigenic shift), enhanced viral polymerase activity is required for the emergence of pandemic influenza viruses from their seasonal human precursors. Influenza A viruses are single-stranded negative-sense RNA viruses. Their genome consists of 8 RNA segments, each of which encodes 1 to 3 proteins (1). Pandemic influenza has been occurring at irregular intervals for at least 500 years, causing significant, occasionally catastrophic mortality in the human population (2, 3). Genetic data available for the last four pandemic viruses (H1N1/1918, H2N2/1957, H3N2/1968, and H1N1/2009) indicate that they all contained an antigenically novel hemagglutinin (HA) which was derived from animal influenza A viruses (reviewed in references 2-5). The origin of the other seven gene segments of the pandemic viruses varied (3,4,6). The source of the non-HA gene segments of the H1N1/1918 virus is not clear due to the lack of sequence data for coexistent avian and mammalian influenza A viruses. The precursor of the H1N1/2009 virus emerged and circulated in pigs; this virus was transmitted and adapted to humans as a whole. Two other pandemic viruses emerged via gene mixing (reassortment) between animal viruses and contemporary human viruses and contained either 5 gene segments (H2N2/1957) or 6 gene segments (H3N2/1968) of the human virus parent.Avian influenza viruses do not replicate efficiently in humans. It is believed that exchange via reassortment of nonadapted gene segments from an avian virus by segments from a human-adapted parent might allow the emerging pandemic virus to overcome host range restriction (2,7,8). Surprisingly, both 1957 and 1968 pandemic viruses contained the avian PB1 segment (gene segment 2) (9). This segment encodes three proteins, polymerase basic protein 1 (PB1), PB1-F2, and PB1-N40, which are translated from the start codons 1, 4, and 5, respectively (1, 10). PB1 is the core subunit of the trimeric viral RNA-dependent RNA polymerase complex (PB1, PB2, and PA), which is responsible for transcription and replication of the viral genome (1, 11). PB1-F2 and PB1-N40 are not essential for virus replication, but they can affect replication efficiency and contribute to pathogenicity (10, 12).

Recombinant influenza virus with a pandemic H2N2 polymerase complex has a higher adaptive potential than one with seasonal H2N2 polymerase complex

“…Recently, Wendel et al (2015) demonstrated that the viral polymerase activity of a seasonal H2N2 vRNP can be enhanced by replacing its PB1 with an HK/68 PB1. Our findings also support this, but our results further suggest that there are some mutations in the PB2 of the HK/68 virus that also might contribute to the enhanced viral polymerase activity of pandemic H3N2 virus (Fig.…”

“…Both 1957 and 1968 pandemic viruses had an avian PB1 segment, whereas the 2009 pandemic virus had avian PB2 and PA gene segments circulating in swine influenza viruses (Kawaoka et al, 1989;Smith et al, 2009;Webby et al, 2000). Although there is some evidence suggesting that reassorted polymerase complexes with avian PB1 might have enhanced polymerase activity in human cells (Chen et al, 2008;Li et al, 2009;Naffakh et al, 2000;Wendel et al, 2015), the evolutionary advantages of having a reassorted polymerase complex in a pandemic virus are not clear. For example, it is not known whether a reassorted polymerase might have a role in facilitating the genesis or transmission of pandemic influenza virus.…”

Acquisition of Avian-Origin PB1 Facilitates Viral RNA Synthesis by the 2009 Pandemic H1N1 Virus Polymerase

“…Moreover, the reassortment between a human H3N2 and an avian H5N1 virus led to the generation of an avian PB1-containing reassortant with increased virulence in vivo, although the reassorted polymerase activity remained unchanged in vitro [15]. Swapping the PB1 segment of a human seasonal H2N2 virus with that of the 1968 pandemic strain, which is of avian-origin, increased viral polymerase activity and facilitated replication and transmission of the reassortant virus in guinea pigs [16].The 2009 pandemic H1N1 (pH1N1) is a "quadruple reassortant" virus resulted from the reassortment between the "triple reassortant" North American swine H1N2 viruses and the Eurasian "avian-like" swine H1N1 viruses. It contains the PB2 and PA gene segments of avian-origin, and the PB1 segment originated from human seasonal H3N2 viruses.…”

The PB1 segment of an influenza A virus H1N1 2009pdm isolate enhances the replication efficiency of specific influenza vaccine strains in cell culture and embryonated eggs

“…Notably, a recent study revealed that the avian-origin PB1 segment that represents the ancestor of the PB1 of the pandemic 2009 H1N1 influenza virus further facilitated the transmission and replication of the pandemic human H3N2 influenza virus in 1968 (Wendel et al, 2015).…”