Structure of a Human Astrovirus Capsid-Antibody Complex and Mechanistic Insights into Virus Neutralization

Abstract

Human astroviruses (HAstVs) are a leading cause of viral diarrhea in young children, the immunocompromised, and the elderly. There are no vaccines or antiviral therapies against HAstV disease. Several lines of evidence point to the presence of protective antibodies in healthy adults as a mechanism governing protection against reinfection by HAstV. However, development of anti-HAstV therapies is hampered by the gap in knowledge of protective antibody epitopes on the HAstV capsid surface. Here, we report the structure of the HAstV capsid spike domain bound to the neutralizing monoclonal antibody PL-2. The antibody uses all six complementarity-determining regions to bind to a quaternary epitope on each side of the dimeric capsid spike. We provide evidence that the HAstV capsid spike is a receptor-binding domain and that the antibody neutralizes HAstV by blocking virus attachment to cells. We identify patches of conserved amino acids that overlap the antibody epitope and may comprise a receptor-binding site. Our studies provide a foundation for the development of therapies to prevent and treat HAstV diarrheal disease.IMPORTANCE Human astroviruses (HAstVs) infect nearly every person in the world during childhood and cause diarrhea, vomiting, and fever. Despite the prevalence of this virus, little is known about how antibodies in healthy adults protect them against reinfection. Here, we determined the crystal structure of a complex of the HAstV capsid protein and a virus-neutralizing antibody. We show that the antibody binds to the outermost spike domain of the capsid, and we provide evidence that the antibody blocks virus attachment to human cells. Importantly, our findings suggest that a subunit-based vaccine focusing the immune system on the HAstV capsid spike domain could be effective in protecting children against HAstV disease.KEYWORDS antibody, astrovirus, capsid, receptor binding, structure, vaccines, virus neutralization T he Astroviridae family is comprised of two genera, Mamastrovirus and Avastrovirus, which infect mammalian and avian species, respectively (1). Members of the Avastrovirus genus cause a variety of disease manifestations, growth defects, and mortality in poultry (2). Members of the Mamastrovirus genus cause infections in humans and a wide range of mammals (3). Human astroviruses (HAstVs) are classified into eight canonical serotypes (HAstV-1 to ) within the Mamastrovirus genogroup 1 (4), where HAstV-1 is the predominant serotype worldwide (5, 6). HAstV is a leading cause of viral diarrhea in children, immunocompromised individuals, and the elderly (7). There are approximately 3.9 million cases of viral diarrhea due to HAstV in the United States every year (8). In addition, highly divergent strains of HAstV have recently been attributed to encephalitis in immunocompromised individuals (9-11).

Isolation of Neutralizing Monoclonal Antibodies to Human Astrovirus and Characterization of Virus Variants That Escape Neutralization

“…To identify the neutralization epitopes on the HAstV capsid, we first evaluated the capacity of the core and spike domains to induce total (determined by enzyme-linked immunosorbent assay [ELISA]) and neutralizing antibodies. These two structural domains of the virus can be produced in Escherichia coli in a soluble and correctly folded form (10)(11)(12)(13)(14). Rabbits and mice were immunized with either the purified, recombinant core or the spike domain of HAstV-1 (core1 or spike1).…”

“…Cryo-electron microscopy of mature HAstV virions reveals particles of 44 nm comprised of a T ϭ 3 icosahedral shell and 30 globular spikes (9). The crystal structures of both VP34 (10,11) and VP25 (11)(12)(13)(14) have been recently determined; VP34 folds into a domain that constitutes the shell of the virus capsid (core domain), while VP27/VP25 form dimeric capsid spikes (spike domain). VP34 is derived from the highly conserved N-terminal region of VP70, while VP27/VP25, which differ only at their amino termini, are derived from the hypervariable region of VP70 (15).…”

“…Despite important advances in the structural characterization of the HAstV particle, the functional sites on the virus capsid, including the location of the receptor-binding site and the antigenic determinants where neutralizing antibodies bind, have been poorly characterized. Only one neutralizing antibody epitope, located on the capsid spike domain, has previously been defined by X-ray crystallography (12).…”

Identification of a Type-Specific Epitope in the ORF2 Protein of Duck Astrovirus Type 1

“…Both the ORF1a and ORF1b encode the nonstructural proteins (NSPs), containing enzymes and participating in viral replication, whereas the ORF2 encodes the viral capsid polyprotein [20,21]. ORF2, containing antigenic determinant, can induce the production of neutralizing antibody interacting with the host [22][23][24].…”

Astrovirus Gastroenteritis in Children Younger than 5 Years in Ouagadougou (Burkina Faso): Prevalence and Risks Factors Influencing Severity

“…Human astroviruses are recognized as an endemic cause of acute gastroenteritis globally [10]. Serological studies have shown that up to 90% of individuals in certain populations have antibodies against HAstVs [11,12]. These viruses are transmitted by the fecal-oral route, from person to person and from animals to humans [10].…”