Oluwafemi Adu, University of Alberta

Oluwafemi F. Adu1, Zoe Turner2, Ross A. Edwards1, Dana Kocincova1, Egor P Tchesnokov1, Andrei Drabovich2, Nathan Zelyas3, Kalyan Das1, Matthias Götte1 (1) Department of Medical Microbiology and Immunology, College of Medicine and Dentistry, University of Alberta, CA. (2) Division of Analytical and Environmental Toxicology, Department of Laboratory Medicine & Pathology, University of Alberta (3) Public Health Laboratory, Alberta Precision Laboratories, Edmonton, Canada

Fighting & Responding

Highly pathogenic avian influenza (HPAI) H5 viruses, particularly those within clade 2.3.4.4b, continue to pose substantial global threats. Recent human infections, including the 2024 case in British Columbia (BC), highlights the need to define pre-existing and population level immunity to emerging H5 strains. We evaluated the breadth and functional efficiency of cross recognizing antibody responses in sera or plasma from 22 individuals infected with seasonal H1N1 or H3N2 viruses against a panel of H5N1 HA and NA antigens, including those from the 2024 BC strain. Immunoglobulin subtype profiling by mass spectrometry identified IgG1 and IgA1 as the dominant HA specific responses. IgG1 binding titers positively correlated with cross neutralization efficiency measured using lentiviral H5N1 pseudoviruses. Neutralization activity varied across individuals, with some samples exhibiting limited or no detectable activity against the H5 pseudoviruses tested. Structural analysis by cryogenic electron microscopy (cryo-EM) combined with competition ELISA defined the antigenic landscape of the polyclonal cross-reactive responses and demonstrated that most antibodies were directed toward the conserved HA stem rather than the head domain. Collectively, these findings indicate that recent seasonal H1N1 or H3N2 infection can elicit stem focused antibody responses with measurable cross neutralizing activity against heterologous H5N1 strains. While the magnitude of protection and its clinical relevance remain to be determined, the mapped polyclonal HA epitopes provide a framework for the rational design of medical countermeasures against seasonal influenza and HPAI viruses.