Receptor binding, structure, and tissue tropism of cattle-infecting H5N1 avian influenza virus hemagglutinin.

The ongoing circulation of highly pathogenic avian influenza (HPAI) A (H5N1) viruses, particularly clade 2.3.4.4b strains, poses a significant threat to animal and public health. Recent outbreaks in cattle highlight concerns about cross-species transmission and zoonotic spillover. Here, we found that the hemagglutinin (HA) protein from a cattle-infecting H5N1 virus has acquired slight binding to human-like α2-6-linked receptors while still exhibiting a strong preference for avian-like α2-3-linked sialic acid receptors. Immunohistochemical staining revealed HA binding to bovine pulmonary and mammary tissues, aligning with clinical observations. HA also binds effectively to human conjunctival, tracheal, and mammary tissues, indicating a risk for human transmission, notably in cases of conjunctivitis. High-resolution cryo-electron microscopy (cryo-EM) structures of this H5 HA in complex with either α2-3 or α2-6 receptors elucidate the molecular mechanisms underlying its receptor-binding properties. These findings provide critical insights into the tropism and transmission potential of this emerging pathogen.