Structures of Two Human Astrovirus Capsid/Neutralizing Antibody Complexes Reveal Distinct Epitopes and Inhibition of Virus Attachment to Cells.

Human astrovirus is an important cause of viral gastroenteritis worldwide. Young children, the elderly, and the immunocompromised are especially at risk for contracting severe disease. However, no vaccines exist to combat human astrovirus infection. Evidence points to the importance of antibodies in protecting healthy adults from reinfection. To develop an effective subunit vaccine that broadly protects against diverse astrovirus serotypes, we must understand how neutralizing antibodies target the capsid surface at the molecular level. Here, we report the structures of the human astrovirus capsid spike domain bound to two neutralizing monoclonal antibodies. These antibodies bind two distinct conformational epitopes on the spike surface. We add to existing evidence that the human astrovirus capsid spike contains a receptor-binding domain and demonstrate that both antibodies neutralize human astrovirus by blocking virus attachment to host cells. We identify patches of conserved amino acids which overlap or border the antibody epitopes and may constitute a receptor-binding site. Our findings provide a basis for developing therapies to prevent and treat human astrovirus gastroenteritis. IMPORTANCE Human astroviruses 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 block astrovirus infection. Here, we determined the crystal structures of the astrovirus capsid protein in complex with two virus-neutralizing antibodies. We show that the antibodies bind to two distinct sites on the capsid spike domain, however, both antibodies block virus attachment to human cells. Importantly, our findings support the use of the human astrovirus capsid spike as an antigen in a subunit-based vaccine to prevent astrovirus disease.

Discovery of three novel neutralizing antibody epitopes on the human astrovirus capsid spike and mechanistic insights into virus neutralization.

Human astroviruses (HAstVs) are a leading cause of viral childhood diarrhea that infect nearly every individual during their lifetime. Although human astroviruses are highly prevalent, no approved vaccine currently exists. Antibody responses appear to play an important role in protection from HAstV infection, however knowledge about the neutralizing epitope landscape is lacking, as only 3 neutralizing antibody epitopes have previously been determined. Here, we structurally define the epitopes of 3 uncharacterized HAstV-neutralizing monoclonal antibodies: antibody 4B6 with X-ray crystallography to 2.67 Å, and antibodies 3H4 and 3B4 simultaneously with single-particle cryogenic-electron microscopy to 3.33 Å. We assess the epitope locations relative to conserved regions on the capsid spike and find that while antibodies 4B6 and 3B4 target the upper variable loop regions of the HAstV spike protein, antibody 3H4 targets a novel region near the base of the spike that is more conserved. Additionally, we found that all 3 antibodies bind with high affinity, and they compete with receptor FcRn binding to the capsid spike. These studies inform which regions of the HAstV capsid can be targeted by monoclonal antibody therapies and could aid in rational vaccine design.