Comparative In Vitro and In Vivo Analysis of H1N1 and H1N2 Variant Influenza Viruses Isolated from Humans between 2011 and 2016

Abstract

Influenza A virus pandemics are rare events caused by novel viruses which have the ability to spread in susceptible human populations. With respect to H1 subtype viruses, swine H1N1 and H1N2 viruses occasionally cross the species barrier to cause human infection. Recently isolated from humans (termed variants), swine viruses were shown to display great genetic and antigenic diversity, hence posing considerable public health risk. Here, we utilized and approaches to provide characterization of H1 subtype variant viruses isolated since the 2009 pandemic and discuss the findings in context with previously studied H1 subtype human isolates. The variant viruses were well adapted to replicate in the human respiratory cell line Calu-3 and the respiratory tracts of mice and ferrets. However, with respect to hemagglutinin (HA) activation pH, the variant viruses had fusion pH thresholds closer to that of most classical swine and triple-reassortant H1 isolates rather than viruses that had adapted to humans. Consistent with previous observations for swine isolates, the tested variant viruses were capable of efficient transmission between cohoused ferrets but could transmit via respiratory droplets to differing degrees. Overall, this investigation demonstrates that swine H1 viruses that infected humans possess adaptations required for robust replication and, in some cases, efficient respiratory droplet transmission in a mammalian model and therefore need to be closely monitored for additional molecular changes that could facilitate transmission among humans. This work highlights the need for risk assessments of emerging H1 viruses as they continue to evolve and cause human infections. Influenza A virus is a continuously evolving respiratory pathogen. Endemic in swine, H1 and H3 subtype viruses sporadically cause human infections. As each zoonotic infection represents an opportunity for human adaptation, the emergence of a transmissible influenza virus to which there is little or no preexisting immunity is an ongoing threat to public health. Recently isolated variant H1 subtype viruses were shown to display extensive genetic diversity and in many instances were antigenically distinct from seasonal vaccine strains. In this study, we provide characterization of representative H1N1v and H1N2v viruses isolated since the 2009 pandemic. Our results show that although recent variant H1 viruses possess some adaptation markers of concern, these viruses have not fully adapted to humans and require further adaptation to present a pandemic threat. This investigation highlights the need for close monitoring of emerging variant influenza viruses for molecular changes that could facilitate efficient transmission among humans.

Interspecies Transmission of Reassortant Swine Influenza A Virus Containing Genes from Swine Influenza A(H1N1)pdm09 and A(H1N2) Viruses

“…However, in comparison with ferrets, titers in pigs were lower and, in some contact pigs, were below the positive threshold. These findings are comparable with outcomes reported for pH1N1 virus infection of pigs and ferrets (36,37,39,40) and a comparison of swine H1 viruses, including H1N2v strains, when infecting ferrets (41). A North American reassortant H1N1 swine IAV incorporating pH1N1-origin polymerase acidic, NP, and M genes also reportedly showed similar replication kinetics to a swine IAV pH1N1 isolate when infecting ferrets (14).…”

Aerosol Transmission from Infected Swine to Ferrets of an H3N2 Virus Collected from an Agricultural Fair and Associated with Human Variant Infections

“…Previous studies examining the pathogenesis of variant IAV have shown these viruses to be well adapted to replication in the respiratory tissue of ferrets (24,45,46). Virus titers and kinetics of virus detection in the nasal swabs from ferrets infected with sw/OH/2017 were similar to previously reported data with H1N1v, H1N2v, and H3N2v.…”

“…Analysis of variant influenza viruses has shown H1N1v and H3N2v to be identical to IAV isolated from pigs at the same fair, suggesting that a subpopulation of swine lineage IAV are capable of replication in humans in situations of high viral titers and close contact (19,20,22,44). Previous risk assessment studies focused on direct inoculation of ferrets with variant IAV isolated from humans (23,24,45,46). In this study, we sought to recapitulate the primary step of interspecies transmission by modeling the swine-human transmission potential using a ferret surrogate.…”

“…This may be attributed to the presence of molecular markers in PB2 (271A, 590S, 591R, and 661A) and PA (669V) that are present in variant IAV previously shown to replicate in ferrets. The major genetic difference between sw/OH/ 2017 and variant HxN2 IAV in prior ferret infection studies is the lineage of the NA gene segment (23,24,26,45). The NA of sw/OH/2017 is from the N2-2002B lineage that resulted from the spillover of human seasonal IAV into the commercial swine population in 2002 (4, 5).…”

Identification, Genetic Analysis, and Pathogenicity of Classical Swine H1N1 and Human-Swine Reassortant H1N1 Influenza Viruses from Pigs in China

“…To evaluate the pathogenicity of the two H1N1 viruses in mammalian species, we inoculated BALB/c mice with 10 6 EID 50 /50µL of each virus and determined virus replication, mortality, and pathology. Previous studies showed that the H1N1 virus and its variant viruses isolated from different hosts and cases displayed a different pathogenicity in mice [39][40][41][42][43][44][45]. Those isolates showed effective replication in lungs but varied in their ability to cause morbidity.…”