Pandemic Potential of Reassortant Swine Influenza A Viruses

Influenza A viruses are capable of causing disease in several species, including birds, humans and swine. Host specificity of the viruses is not absolute, and is influenced by a range of factors. Swine play a pivotal role in the interspecies transmission of influenza A viruses, as they are susceptible to infection with both human and avian strains and have been implicated as a “mixing vessel” for the reassortment of influenza A viruses from different species. The reassortment of influenza A viruses of human and avian origin led to human influenza pandemics in 1957 and 1968. The dynamics of swine influenza viruses in North America changed drastically with the introduction of the avian-origin PA and PB2 and human-origin HA, NA, and PB1 gene segments and the creation of the triple reassortant swine virus lineage in 1998. While the previously circulating classical swine H1N1 influenza virus lineage was very stable in the swine population, triple reassortant lineage viruses have supplanted the classical H1N1 lineage and undergone repeated reassortment events, acquiring HA and NA genes from human, swine, and avian influenza viruses, while maintaining triple reassortant internal gene (TRIG) cassette. Viruses of the triple reassortant lineage have been very successful in the swine population, yet the mechanisms underlying their unique characteristics and increased fitness have not been elucidated. Here we address the pandemic potential of triple reassortant swine influenza A viruses, their transmissibility, and their relative fitness compared to classical and double reassortant swine influenza viruses. Several triple reassortant viruses, including one with avian-origin HA and NA, were characterized in the ferret, which is a commonly used model for human influenza infection. The effect of the TRIG cassette on the reassortment potential and temperature sensitivity of swine influenza viruses was determined in cell culture, and the replication and transmission of a classical and a reassortant swine virus were compared in pigs. We found that triple reassortant swine viruses replicated efficiently in the ferret model, although there was some variation in transmission efficiencies. An H2N3 virus with avian-origin HA and NA was transmissible in the ferret model, and this transmissibility could be abolished with a single amino acid change in the HA protein that altered its receptor binding specificity. Avian H2N3 viruses were also capable of replicating in ferrets without adaptation and could acquire transmissibility through a change in the receptor binding specificity of the HA protein. Both double and triple reassortant swine viruses had an advantage over the classical H1N1 swine virus at early timepoints in cell culture. Reassortant viruses also demonstrated less temperature sensitivity than the classical H1N1 swine virus. The triple reassortant H1N1 virus had an increased reassortment potential in cell culture compared to the classical swine H1N1 virus as determined by acquisition of a human HA gene. Triple reassortant swine viruses have an increased ability to establish infection, and an increased potential for reassortment, potentially introducing novel HA genes into a host population. This indicates that triple reassortant swine viruses may have an increased potential to cause human pandemics. In April 2009, a novel H1N1 pandemic virus containing five of the six genes of the TRIG cassette emerged in the human population, emphasizing the importance of reassortant swine influenza A viruses in the generation of human pandemics

Infection with 2009 H1N1 influenza virus primes for immunological memory in human nose-associated lymphoid tissue, offering cross-reactive immunity to H1N1 and avian H5N1 viruses

Influenza is a highly contagious mucosal infection in the respiratory tract. 2009 pandemic H1N1 (pH1N1) virus infection resulted in substantial morbidity and mortality in humans. Little is known on whether immunological memory develops following pH1N1 infection and whether it provides protection against other virus subtypes. Enzyme-linked immunosorbent spot assay was used to analyze hemagglutinin (HA)-specific memory B cell responses after virus antigen stimulation in nasal-associated lymphoid tissues (NALT) from children and adults. Individuals with serological evidence of previous exposure to pH1N1 showed significant cross-reactive HA-specific memory B responses to pH1N1, seasonal H1N1(sH1N1) and avian H5N1(aH5N1) viruses upon pH1N1 virus stimulation. pH1N1 virus antigen elicited stronger cross-reactive memory B cell responses than sH1N1 virus. Intriguingly, aH5N1 virus also activated cross-reactive memory responses to sH1N1 and pH1N1 HAs in those who had previous pH1N1 exposure, and that correlated well with the memory response stimulated by pH1N1 virus antigen. These memory B cell responses resulted in cross-reactive neutralizing antibodies against sH1N1, 1918 H1N1 and aH5N1viruses. 2009 pH1N1 infection appeared to have primed human host with B cell memory in NALT that offers cross-protective mucosal immunity against not only H1N1 but also aH5N1 viruses. These findings may have important implications to future vaccination strategies against influenza. It will be important to induce and/or enhance such cross-protective mucosal memory B cells

From where did the 2009 'swine-origin' influenza A virus (H1N1) emerge?

The swine-origin influenza A (H1N1) virus that appeared in 2009 and was first found in human beings in Mexico, is a reassortant with at least three parents. Six of the genes are closest in sequence to those of H1N2 'triple-reassortant' influenza viruses isolated from pigs in North America around 1999-2000. Its other two genes are from different Eurasian 'avian-like' viruses of pigs; the NA gene is closest to H1N1 viruses isolated in Europe in 1991-1993, and the MP gene is closest to H3N2 viruses isolated in Asia in 1999-2000. The sequences of these genes do not directly reveal the immediate source of the virus as the closest were from isolates collected more than a decade before the human pandemic started. The three parents of the virus may have been assembled in one place by natural means, such as by migrating birds, however the consistent link with pig viruses suggests that human activity was involved. We discuss a published suggestion that unsampled pig herds, the intercontinental live pig trade, together with porous quarantine barriers, generated the reassortant. We contrast that suggestion with the possibility that laboratory errors involving the sharing of virus isolates and cultured cells, or perhaps vaccine production, may have been involved. Gene sequences from isolates that bridge the time and phylogenetic gap between the new virus and its parents will distinguish between these possibilities, and we suggest where they should be sought. It is important that the source of the new virus be found if we wish to avoid future pandemics rather than just trying to minimize the consequences after they have emerged. Influenza virus is a very significant zoonotic pathogen. Public confidence in influenza research, and the agribusinesses that are based on influenza's many hosts, has been eroded by several recent events involving the virus. Measures that might restore confidence include establishing a unified international administrative framework coordinating surveillance, research and commercial work with this virus, and maintaining a registry of all influenza isolates

Broadly cross-reactive antibodies dominate the human B cell response against 2009 pandemic H1N1 influenza virus infection

Although scarce after annual influenza vaccination, B cells producing antibodies capable of neutralizing multiple influenza strains are abundant in humans infected with pandemic 2009 H1N1 influenza

Multiple reassortment between pandemic (H1N1) 2009 and endemic influenza viruses in pigs, United States.

As a result of human-to-pig transmission, pandemic influenza A (H1N1) 2009 virus was detected in pigs soon after it emerged in humans. In the United States, this transmission was quickly followed by multiple reassortment between the pandemic virus and endemic swine viruses. Nine reassortant viruses representing 7 genotypes were detected in commercial pig farms in the United States. Field observations suggested that the newly described reassortant viruses did not differ substantially from pandemic (H1N1) 2009 or endemic strains in their ability to cause disease. Comparable growth properties of reassortant and endemic viruses in vitro supported these observations; similarly, a representative reassortant virus replicated in ferrets to the same extent as did pandemic (H1N1) 2009 and endemic swine virus. These novel reassortant viruses highlight the increasing complexity of influenza viruses within pig populations and the frequency at which viral diversification occurs in this ecologically important viral reservoir.As a result of human-to-pig transmission, pandemic influenza A (H1N1) 2009 virus was detected in pigs soon after it emerged in humans. In the United States, this transmission was quickly followed by multiple reassortment between the pandemic virus and endemic swine viruses. Nine reassortant viruses representing 7 genotypes were detected in commercial pig farms in the United States. Field observations suggested that the newly described reassortant viruses did not differ substantially from pandemic (H1N1) 2009 or endemic strains in their ability to cause disease. Comparable growth properties of reassortant and endemic viruses in vitro supported these observations; similarly, a representative reassortant virus replicated in ferrets to the same extent as did pandemic (H1N1) 2009 and endemic swine virus. These novel reassortant viruses highlight the increasing complexity of influenza viruses within pig populations and the frequency at which viral diversification occurs in this ecologically important viral reservoir