Glycosylation of the influenza A virus hemagglutinin (HA) is a key determinant of viral fitness and a major mechanism for antibody-based immune evasion. Contemporary human A(H3N2) viruses possess many putative N-linked glycosylation sites on HA—up to 13 sites have been identified. Glycan type and the degree to which each putative site is occupied are not uniform across HA: most sites are nearly completely glycosylated, whereas a subset of sites are only partially occupied. The glycosylation sites at amino acid residues 45 and 144 are highlighted as examples of low-occupancy locations that are also evolutionarily unstable. Prior work has characterized effects of highly occupied glycosylation sites on HA function and antigenicity; however, the biological and immunological consequences of low-occupancy glycosylation sites have been less well defined.
To investigate the role of low-occupancy glycosylation sites, the investigators used reverse-genetics to generate A(H3N2) influenza A viruses that lacked the glycans at HA residue 45, residue 144, or both. The engineered viruses were examined across several functional assays to determine effects on receptor binding, thermal stability, membrane fusion, replication in cell culture, and immune responses in an animal model. Growth was measured in Madin-Darby canine kidney (MDCK) epithelial cell culture, a standard in vitro system for influenza replication. Immune effects were assessed in mice, focusing on antibody responses that correlate with virus neutralization and protection from disease.
Removal of glycans at the two low-occupancy sites produced measurable effects on multiple viral phenotypes. Specifically, viruses lacking glycosylation at residue 45 and/or residue 144 showed reduced receptor binding, decreased thermal stability, and impaired fusion activity compared with viruses retaining those glycan sites. Despite these reductions in biophysical and functional properties associated with HA, the low level of glycosylation at residues 45 and 144 did not translate into diminished replication in MDCK epithelial cell culture: viral growth in this in vitro system remained unaffected by the absence of these glycans.
These findings indicate that although low-occupancy glycans at residues 45 and 144 contribute to optimal receptor interaction, HA stability, and membrane fusion, their limited presence is not essential for efficient replication in commonly used cell culture conditions.
In vivo, the low-occupancy glycosylation sites influenced immune recognition. The presence of even a minimal level of glycosylation at residues 45 and 144 reduced titers of antibodies in mice that correlate with virus neutralization and protection against influenza disease. In other words, low-level glycan occupancy at these HA positions attenuated antibody responses that would otherwise contribute to protection.
This immunomodulatory effect occurred despite the modest negative impact of these glycans on receptor binding and fusion identified in vitro. The data presented in the source article demonstrate a dissociation between measurable reductions in some aspects of viral fitness and a meaningful capacity to blunt host antibody responses.
The authors propose a model in which the temporary or low-level introduction of N-linked glycosylation sites at specific HA positions allows A(H3N2) viruses to reduce immune pressure directed at antigenic and receptor binding sites while minimizing impairment of replication and virulence. Low-occupancy glycosylation therefore represents a strategic compromise: it provides sufficient masking of antigenic determinants to lower neutralizing antibody titers, yet imposes only limited functional cost to the virus.
Because residues 45 and 144 have been used by A(H3N2) viruses only during limited periods of evolution, the authors suggest that even partial glycan occupancy can exert selective pressure. The balance between immune escape and fitness cost afforded by low-occupancy glycans may help explain how A(H3N2) viruses maintain sustained circulation in human populations over time.
From the experiments summarized in the source, low-occupancy N-linked glycosylation sites on HA—specifically at residues 45 and 144—can reduce antibody titers that correlate with protection without substantially reducing viral replication in MDCK cell culture, despite measurable reductions in receptor binding, thermal stability, and fusion. The study supports the hypothesis that transient or partial glycosylation at particular HA residues is an evolutionary strategy that contributes to immune evasion and continued circulation of A(H3N2) influenza A virus in humans.
glycosylation; hemagglutinin; immune evasion; influenza; viral evolution