Date Published: March 28, 2013
Publisher: Public Library of Science
Author(s): Benjamin Mänz, Dominik Dornfeld, Veronika Götz, Roland Zell, Petra Zimmermann, Otto Haller, Georg Kochs, Martin Schwemmle, Juan Ortin.
The interferon-induced dynamin-like MxA GTPase restricts the replication of influenza A viruses. We identified adaptive mutations in the nucleoprotein (NP) of pandemic strains A/Brevig Mission/1/1918 (1918) and A/Hamburg/4/2009 (pH1N1) that confer MxA resistance. These resistance-associated amino acids in NP differ between the two strains but form a similar discrete surface-exposed cluster in the body domain of NP, indicating that MxA resistance evolved independently. The 1918 cluster was conserved in all descendent strains of seasonal influenza viruses. Introduction of this cluster into the NP of the MxA-sensitive influenza virus A/Thailand/1(KAN-1)/04 (H5N1) resulted in a gain of MxA resistance coupled with a decrease in viral replication fitness. Conversely, introduction of MxA-sensitive amino acids into pH1N1 NP enhanced viral growth in Mx-negative cells. We conclude that human MxA represents a barrier against zoonotic introduction of avian influenza viruses and that adaptive mutations in the viral NP should be carefully monitored.
Avian influenza A viruses sporadically transmit from waterfowl, their natural reservoir, into the human population –. These zoonotic viruses usually cannot propagate in the new human host, nor do they readily transmit between humans –. In rare cases, however, influenza A viruses of avian origin break the species barrier and establish new virus lineages in humans. In the last 100 years, the introduction of an influenza A virus with a novel nucleoprotein (NP) gene segment occurred only on two occasions, both of which led to pandemics: in 1918 (“Spanish” H1N1) an avian virus and in 2009 (pH1N1) a reassortant virus (comprising gene segments of two swine influenza viruses) established a stable lineage in humans , . In contrast, the 1957 (“Asian” H2N2) and the 1968 (“Hong-Kong” H3N2) pandemics were caused by genetic reassortment events, whereby the circulating human strains acquired some gene segments from avian sources but kept, among others, their 1918-derived NP .
Influenza A viruses sporadically transmit from the avian reservoir into the human population. Here we describe specific mutations found in the NP of the 1918 and 2009 pandemic viruses that confer resistance to the IFN-induced human MxA GTPase, a major restriction factor for influenza and other orthomyxoviruses. As MxA strongly inhibits transcription and replication of the viral genome early in infection, its antiviral activity can be readily analyzed in polymerase reconstitution (minireplicon) assays , . Using this assay, we identified a cluster of surface-exposed amino acids in the body domain of NP crucial for Mx resistance. Interestingly, different amino acid positions were identified in 1918 and pH1N1 NP, yet all were located in the same domain. All resistance-associated amino acids are conserved in previous and current human influenza A viruses (Table 1), and the continuing acquisition of resistance-enhancing mutations (Figure 7) suggests strong positive selection pressure by MxA. Of note, mutations conferring MxA resistance are absent in avian influenza A viruses, although we did observe the emergence of adaptive NP mutations in avian-derived viruses circulating in swine (Figure 4 and Figure 7). These substitutions in NP not only increased resistance to swine Mx1 but also to human MxA, supporting the theory that swine are an excellent intermediate host for the generation of viruses with pandemic potential.