Eurasian Avian-Like Swine Influenza A Viruses Escape Human MxA Restriction through Distinct Mutations in Their Nucleoprotein

Abstract

To cross the human species barrier, influenza A viruses (IAV) of avian origin have to overcome the interferon-induced host restriction factor MxA by acquiring distinct mutations in their nucleoprotein (NP). We recently demonstrated that North American classical swine IAV are able to partially escape MxA restriction. Here we investigated whether the Eurasian avian-like swine IAV lineage currently circulating in European swine would likewise evade restriction by human MxA. We found that the NP of the influenza virus isolate A/Swine/Belzig/2/2001 (Belzig-NP) exhibits increased MxA escape, similar in extent to that with human IAV NPs. Mutational analysis revealed that the MxA escape mutations in Belzig-NP differ from the known MxA resistance cluster of the North American classical swine lineage and human-derived IAV NPs. A mouse-adapted avian IAV of the H7N7 subtype encoding Belzig-NP showed significantly greater viral growth in both MxA-expressing cells and MxA-transgenic mice than control viruses lacking the MxA escape mutations. Similarly, the growth of the recombinant Belzig virus was only marginally affected in MxA-expressing cells and MxA-transgenic mice, in contrast to that of Belzig mutant viruses lacking MxA escape mutations in the NP. Phylogenetic analysis of the Eurasian avian-like swine IAV revealed that the NP amino acids required for MxA escape were acquired successively and were maintained after their introduction. Our results suggest that the circulation of IAV in the swine population can result in the selection of NP variants with a high degree of MxA resistance, thereby increasing the zoonotic potential of these viruses. IMPORTANCE The human MxA protein efficiently blocks the replication of IAV from nonhuman species. In rare cases, however, these IAV overcome the species barrier and become pandemic. All known pandemic viruses have acquired and maintained MxA escape mutations in the viral NP and thus are not efficiently controlled by MxA. Intriguingly, partial MxA resistance can also be acquired in other hosts that express antivirally active Mx proteins, such as swine. To perform a risk assessment of IAV circulating in the European swine population, we analyzed the degree of MxA resistance of Eurasian avian-like swine IAV. Our data demonstrate that these viruses carry formerly undescribed Mx resistance mutations in the NP that mediate efficient escape from human MxA. We conclude that Eurasian avian-like swine IAV possess substantial zoonotic potential.

Mx genes: host determinants controlling influenza virus infection and trans-species transmission

“…In fact, the 2009 pandemic H1N1 virus features an MxA escape signature that is suggestive of porcine MX1 evasion (Manz et al 2013). Recent phylogenetic analyses revealed that the viral NP of the Eurasian avian-like swine lineage successively gained MxA escape mutations that increase the zoonotic potential of these viruses (Dornfeld et al 2019). It is conceivable that new MxA escape mutations in NP may arise in the future, be they located in the well-defined MxA sensitivity region (Manz et al 2013) or at novel sites as recently suggested by a deep mutational scanning approach (Ashenberg et al 2017).…”

Bat-Borne Influenza A Viruses: An Awakening

“…In fact, all known human-adapted IAV strains encode specific adaptive mutations in NP, facilitating MxA escape (Mänz et al 2013;Ashenberg et al 2017). Importantly, the 2009 pandemic virus as well as the recently isolated Eurasian avian-like swine IAV highlight that these adaptive mutations can be acquired in functional Mx1-expressing intermediate hosts like swine (Mänz et al 2013;Dornfeld et al 2018). Intriguingly, the Mx1 proteins from Sturnira lilium and Carollia perspicillata, both bat influenza host species (Tong et al 2012(Tong et al , 2013, exert antiviral activity (Fuchs et al 2017).…”

Equine Mx1 Restricts Influenza A Virus Replication by Targeting at Distinct Site of its Nucleoprotein

“…MxA represents a considerable barrier against the zoonotic introduction of avian influenza viruses into the human population [7]. Interestingly, several investigations reported the antiviral properties of MxA from a human and mouse [10,20,21], porcine [22,23], and bovine [14,[24][25][26] origin. The interspecies transmission would have occurred when avian-origin IAV acquired certain mutations in the NP which overcome the MxA restriction [20,22,27].…”

“…Interestingly, several investigations reported the antiviral properties of MxA from a human and mouse [10,20,21], porcine [22,23], and bovine [14,[24][25][26] origin. The interspecies transmission would have occurred when avian-origin IAV acquired certain mutations in the NP which overcome the MxA restriction [20,22,27]. A few of these mutations were already found in circulating IAV strains before they were even reported to cross the species barrier [20,22], but in many cases, the adaptational mutations occurred subsequent to infection and transmission.…”

“…The interspecies transmission would have occurred when avian-origin IAV acquired certain mutations in the NP which overcome the MxA restriction [20,22,27]. A few of these mutations were already found in circulating IAV strains before they were even reported to cross the species barrier [20,22], but in many cases, the adaptational mutations occurred subsequent to infection and transmission. Indeed, recent studies indicated that IAVs which successfully established stable lineages in humans acquired adaptive mutations in the NP [20,28].…”