Abstract
ABSTRACTHepatitis A virus (HAV) is an unusual picornavirus that is released from cells cloaked in host-derived membranes. These quasi-enveloped virions (eHAV) are the only particle type circulating in blood during infection, whereas only nonenveloped virions are shed in feces. The reason for this is uncertain. Hepatocytes, the only cell type known to support HAV replication in vivo, are highly polarized epithelial cells with basolateral membranes facing onto hepatic (blood) sinusoids and apical membranes abutting biliary canaliculi from which bile is secreted to the gut. To assess whether eHAV and nonenveloped virus egress from cells via vectorially distinct pathways, we studied infected polarized cultures of Caco-2 and HepG2-N6 cells. Most (>99%) progeny virions were released apically from Caco-2 cells, whereas basolateral (64%) versus apical (36%) release was more balanced with HepG2-N6 cells. Both apically and basolaterally released virions were predominantly enveloped, with no suggestion of differential vectorial release of eHAV versus naked virions. Basolateral to apical transcytosis of either particle type was minimal (<0.02%/h) in HepG2-N6 cells, arguing against this as a mechanism for differences in membrane envelopment of serum versus fecal virus. High concentrations of human bile acids converted eHAV to nonenveloped virions, whereas virus present in bile from HAV-infected Ifnar1−/− Ifngr1−/− and Mavs−/− mice banded over a range of densities extending from that of eHAV to that of nonenveloped virions. We conclude that nonenveloped virions shed in feces are derived from eHAV released across the canalicular membrane and stripped of membranes by the detergent action of bile acids within the proximal biliary canaliculus.
Highlights
Hepatitis A virus (HAV) is an unusual member of the Picornaviridae family
Hepatocytes are highly polarized cells of epithelial origin with distinct basolateral and apical membranes and very specialized protein export pathways [10, 11]. Their basolateral membrane faces onto the space of Disse, which communicates with blood flowing through the hepatic sinusoids, whereas the smaller apical membrane abuts the lumen of the biliary canaliculus (Fig. 1A)
We show that high concentrations of human bile acids effectively convert quasi-enveloped eHAV virions to naked, nonenveloped particles and demonstrate the presence of virions undergoing this transition in bile collected from infected mice
Summary
Hepatitis A virus (HAV) is an unusual member of the Picornaviridae family. Its capsid differs structurally from that of other mammalian picornaviruses, with a VP2 domain swap found only in insect-resident members of the Picornavirales [1]. While the capsid of virus shed in feces of infected individuals is naked and nonenveloped, virions circulating in the blood during acute infection are completely enveloped in host-derived membranes that provide protection from neutralizing antibodies directed against the capsid [5]. These quasienveloped virions (eHAV) are infectious and are similar to exosomes in both size and buoyant density. Bile acid concentrations are greater and their detergent action is much stronger in the proximal biliary canaliculus where bile originates, making this hypothesis a continuing possibility [11]
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