Christensen, S. R. | bioRxiv Science

bioRxiv2022

The 2009 pandemic H1N1 hemagglutinin stalk remained antigenically stable after circulating in humans for a decade

An H1N1 influenza virus caused a pandemic in 2009 and descendants of this virus continue to circulate seasonally in humans. Upon infection with the 2009 H1N1 pandemic strain (pH1N1), many humans produced antibodies against epitopes in the hemagglutinin (HA) stalk. HA stalk-focused antibody responses were common among pH1N1-infected individuals because HA stalk epitopes were conserved between the pH1N1 strain and previously circulating H1N1 strains. Here, we completed a series of experiments to determine if the pH1N1 HA stalk has acquired substitutions since 2009 that prevent the binding of human antibodies. We identified several amino acid substitutions that have accrued in the pH1N1 HA stalk from 2009-2019. We completed enzyme-linked immunosorbent assays, absorption-based binding assays, and surface plasmon resonance experiments to determine if these substitutions affect antibody binding. Using sera collected from 230 humans (aged 21-80 years), we found that pH1N1 HA stalk substitutions that have emerged since 2009 do not affect antibody binding. Our data suggest that the HA stalk domain of pH1N1 viruses remained antigenically stable after circulating in humans for a decade. ImportanceIn 2009, a new pandemic H1N1 (pH1N1) virus began circulating in humans. Many individuals mounted hemagglutinin (HA) stalk-focused antibody responses upon infection with the 2009 pH1N1, since the HA stalk of this virus was relatively conserved with other seasonal H1N1 strains. Here, we completed a series of studies to determine if the 2009 pH1N1 strain has undergone antigenic drift in the HA stalk domain over the past decade. We found that serum antibodies from 230 humans could not antigenically distinguish the 2009 and 2019 HA stalk. These data suggest that the HA stalk of pH1N1 has remained antigenically stable, despite the presence of high levels of HA stalk antibodies within the human population.

microbiology↗

bioRxiv2018

Assessing the protective potential of H1N1 influenza virus hemagglutinin head and stalk antibodies in humans

Seasonal influenza viruses are a major cause of human disease worldwide. Most neutralizing antibodies (Abs) elicited by influenza viruses target the head domain of the hemagglutinin (HA) protein. Anti-HA head Abs can be highly potent, but they have limited breadth since the HA head is variable. There is great interest in developing new universal immunization strategies that elicit broadly neutralizing Abs against conserved regions of HA, such as the stalk domain. Although HA stalk Abs can provide protection in animal models, it is unknown if they are present at sufficient levels in humans to provide protection against naturally-acquired influenza virus infections. Here, we quantified H1N1 HA head and stalk-specific Abs in 179 adults hospitalized during the 2015-2016 influenza virus season. We found that HA head Abs, as measured by hemagglutinin-inhibition (HAI) assays, were associated with protection against naturally-acquired H1N1 infection. HA stalk-specific serum total IgG titers were also associated with protection, but this association was slightly attenuated and not statistically significant after adjustment for HA head-specific Ab titers. We found higher titers of HA stalk-specific IgG1 and IgA Abs in sera from uninfected participants than from infected participants; however, we found no difference in sera in vitro antibody dependent cellular cytotoxicity activity. In passive transfer experiments, sera from participants with high HAI activity efficiently protected mice, while sera with low HAI activity protected mice to a lower extent. Our data suggest that human HA head and stalk Abs both contribute to protection against H1N1 infection.\n\nImportanceAbs targeting the HA head of influenza viruses are often associated with protection from influenza virus infections. These Abs typically have limited breadth since mutations frequently arise in HA head epitopes. New vaccines targeting the more conserved HA stalk domain are being developed. Abs that target the HA stalk are protective in animal models, but it is unknown if these Abs exist at protective levels in humans. Here, we found that Abs against both the HA head and HA stalk were associated with protection from naturally-acquired human influenza virus infections during the 2015-2016 influenza season.

microbiology↗