![]() ![]() Circulating influenza viruses evade neutralization in their human hosts by acquiring escape mutations. To prevent them from being blocked by antibodies elicited due to vaccination and natural infection, changes in HA protein associated with antigenic drift happen continually over time as the virus replicates (Caton et al. Four main canonical epitopes, Ca (including Ca1 and Ca2), Cb, Sa, and Sb, on the globular HA head have been recognized by the host immune system. It contains a receptor-binding site (RBS) in a globular head domain which is composed of three domains, the 130-loop (residues 134–138), the 190-helix (residues 188–195), and the 220-loop (residues 221–228), and targets sialic acid residues on host cells (Skehel and Wiley 2000 Mair et al. ![]() Like other subtypes, pdm09 HA plays a pivotal role in binding host cell receptors and releasing the viral RNA into the cell (Beer et al. Most flu shots are designed to target HA surface proteins/antigens of influenza virus. However, antigenic drift, which continually happens over time as the virus replicates, might decrease the vaccine’s effectiveness. Vaccination reduces the risk of flu illness when most circulating flu viruses are well-matched to the vaccine. To date, the influenza A (H1N1) pdm09 virus has been circulating in the world for more than one decade and has become a global threat to public health. In April 2009, a novel swine-origin influenza A H1N1 (S-OIV) virus, classified as influenza A (H1N1) pdm09 or pdmH1N1, first emerged in Mexico and quickly replaced the traditional seasonal H1N1 to become the dominant H1N1 subtype (Novel Swine-Origin Influenza Novel Swine-Origin Influenza AVIT et al. There are 18 HA subtypes and 11 NA subtypes (H1–H18 and N1–N11, respectively), expressing various subtype combinations and circulating in nature (Latorre-Margalef et al. Influenza A viruses are classified into subtypes based on two surface proteins: hemagglutinin (HA) and neuraminidase (NA). Due to the difference in the selection of vaccine strains, people vaccinated in the southern hemisphere could still suffer a severe infection if infected with this antigenic drift pdm09 virus. The HI test showed that the binding activity of escape mutant to 2018 pdm09 sera was weaker than GLW/2018, suggesting that old vaccines might not effectively protect people from infection. Kinetic growth data revealed that the reassortant viruses, including the LCF/2019 with the PTIAAQE substitution, propagated faster than those rescued ones having the STTADQQ motif in the epitope Sb in Madin-Darby Canine Kidney (MDCK) cells. To characterize the pathogenicity of this novel substitution motif, a panel of reassortant viruses containing the LCF/2019 HA segment or the chimeric HA segment with the four substitutions were rescued. Phylogenetic analysis indicated that the epitope Sb started undergoing a rapid antigenic change in 2018. Compared with earlier isolates, this pdm09 virus's genetic sequence contains four substitutions, S186P, T188I, D190A, and Q192E, of the hemagglutinin (HA) segment at position 186–192 (H3 numbering) in the epitope Sb, and two of which are located at the 190-helix. This isolate, named A/Guangdong/LCF/2019 (LCF/19), was genetically sequenced, rescued by reverse genetics, and phylogenetically analyzed in the context of other relevant pdm09 isolates. Here, we analyzed a clinical influenza A (H1N1) pdm09-infected patient case hospitalized for two months in Guangdong (from Decemto February 15, 2020). The influenza A (H1N1) pdm09 virus emerged in 2009 and has been continuously circulating in humans for over ten years. ![]()
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