A pathogenic picornavirus acquires an envelope by hijacking cellular membranes

Animal viruses are broadly categorized structurally by the presence or absence of an envelope composed of a lipid-bilayer membrane(1), attributes that profoundly affect stability, transmission, and immune recognition. Among those lacking an envelope, the Picornaviridae are a large and diverse family of positive-strand RNA viruses that includes hepatitis A virus (HAV), an ancient human pathogen that remains a common cause of enterically-transmitted hepatitis(2–4). HAV infects in a stealth-like manner and replicates efficiently in the liver(5). Virus-specific antibodies appear only after 3–4 weeks of infection, and typically herald its resolution(3,4). Although unexplained mechanistically, both anti-HAV antibody and inactivated whole-virus vaccines prevent disease when administered as late as 2 weeks after exposure(6), when virus replication is well established in the liver(5). Here, we show that HAV released from cells is cloaked in host-derived membranes, thereby protecting the virion from antibody-mediated neutralization. These enveloped viruses (“eHAV”) resemble exosomes(7), small vesicles that are increasingly recognized to play important roles in intercellular communications. They are fully infectious, sensitive to chloroform extraction, and circulate in the blood of infected humans. Their biogenesis is dependent upon host proteins associated with endosomal-sorting complexes required for transport (ESCRT)(8), VPS4B and ALIX. While the hijacking of membranes by HAV facilitates escape from neutralizing antibodies and likely promotes virus spread within the liver, anti-capsid antibodies restrict replication following infection with eHAV, suggesting a possible explanation for post-exposure prophylaxis. Membrane hijacking by HAV blurs the classic distinction between “enveloped” and “nonenveloped” viruses, and has broad implications for mechanisms of viral egress from infected cells as well as host immune responses.