Study On The Mechamism Of Human Infection By Low Pathogenic Avian Influenza Virus

Influenza is an acute, highly contagious disease caused by Influenza virus. Influenza viruses are members of the Orthomyxoviridae, a family of enveloped negative sense, single-stranded ribonucleic acid(RNA) viruses with segmented genomes. Influenza viruses are classified into three types, A, B, and C, according to antigenic differences between their NP(Nucleoprotein) and M1(Matrix1) protein. In the past decades most influenza pandemics were caused by influenza A viruses.The haemagglutinin(HA) exist on the surface of the virus is responsible for attachment and also plays a great important part in the process of membrane fusion. The receptor binding specificity of influenza A viruses becomes a major determinant for the host tropism of the virus, which enables interspecies transmission. To date, only the H1, H2, and H3subtypes of influenza A viruses have adapted to humans, causing annual seasonal flu worldwide. Furthermore, the H5, H9subtypes have been reported to cause sporadic infections in humans. H5N1HPAIV has been paid great attention to since the first human infection caused by H5N1influenza virus occurred in HongKong in1997, It is known that some low pathogenic avian influenza viruses(H7N9, H6N1, H10N8occurred in China in2013) have caused human infection, so it is urgent to detect the receptor binding properties of the low pathogenic avian influenza viruses.In February2013, a novel reassortant influenza A(H7N9) virus was identified in eastern China. This is the first time reported human infection by H7N9influenza virus worldwide. Two influenza viruses(A/Shanghai/1/2013, A/Anhui/1/2013) were isolated in the early pandemic. Sequence alignment of HA proteins from the two H7N9influenza virus revealed that there are8amino acids difference between the two proteins,4(S138A, G186V, T221P, Q226L) of which located in the RBS of HA protein. It is noteworthy that H7N9HA has a naturally occurring Q226L substitution. These findings have led to the assumption that the AH-H7N9lineage virus might have acquired high-affinity human receptor-binding properties. In this study, we expressed the SH H7N9HA, AH H7N9HA, and AH H7N9226Q HA proteins and detected the receptor binding preference of the three proteins, we found that SH H7N9HA preferentially binds to the avian receptor analog, whereas AH H7N9HA binds to both avian and human receptor analogs, AH H7N9226Q HA was found to exhibit dual receptor-binding property same with AH H7N9HA. Together with the complex structures of the three proteins with receptor anologs, all the results indicat that H7N9influenza virus caused human infection due to its ability to binding to human receptor, whereas high binding properties to avian receptor led to its limited human-to-human transmission, Q226L substitution is not solely responsible for the receptor-binding change, other amino acid substitutions contribute to the receptor-binding switch.On June21,2013, the first case of human infection by H6N1influenza virus was reported in Taiwan. In this study, by sequence alignment of HA proteins of H6N1influenza viruses in Taiwan province, we selected three influenza viruses(A/duck/Taiwan/0526/72, A/chicken/Taiwan/A2837/2013, A/Taiwan/2/2013), expressed their HA proteins to detect their receptor binding preference. All the results showed that Duck H6N1HA can only bind to avian receptor anolog, whereas Chicken H6N1HA and Human H6N1HA can bind to both receptor analogs, but Chicken H6N1HA showed a avian receptor anolog binding preference, Human H6N1HA showed a human receptor anolog binding preference, we proposed that the evolution process of receptor binding properties of Taiwan-isolated H6N1have undergone two main stages:initially avian receptor binding preference, secondarily dual receptor binding ability. the result of the binding activity of the two mutant HA proteins of Chicken H6N1HA showed that Chicken H6N1190E228G HA can only bind to avian receptor anolog, whereas Chicken H6N1186L HA showed a same binding activity with Human H6N1HA indicating that amino acids at the positions186,190,228played an important role in the receptor binding preference switch, E190V and G228S can coferred H6N1influenza virus aquire the ability to bind to human receptor, whereas P186L can contribute to the receptor binding preference change.In order to understand the receptor binding preference of H15N8influenza virus, we expressed the HA protein of A/duck/Australia/341/83. Result showed that H15N8HA preferentially binds to the avian receptor analog, with no binding to the human receptor analog.In conclusion, compared with HPAIV, LPAIV poses a great threat to human health, it will cause new pandemic once it acquires the human receptor preference(such as H6N1influenza virus). In this research we illustrated the molecular basis of influenza interspecies transmission via solving2types of influenza viruses HAs complexed with its receptors, which has important significance in the prevention and prediction of another influenza pademic.