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

Abstract

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 enabling protection of 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 that overlap or border the antibody epitopes and may constitute a receptor-binding site. Our findings provide a basis to develop therapies that 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 virus infection. Here, we determined crystal structures of the astrovirus capsid protein in complex with two virus-neutralizing antibodies. We show that the antibodies bind 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.

Antigenic determinants of HAstV-VA1 neutralization and their relevance in the human immune response

“…The characterization of the neutralization mechanisms of the reported anti-VA1 Nt-MAbs, together with the identification of the actual epitopes of the antibodies on the virus spike by structural biology approaches, as has been previously shown for classical HAstV (17), will aid in identifying virus functional sites required for astrovirus cell entry. This information will be relevant for the development of prophylactic and therapeutic approaches to address this neurotropic astrovirus strain.…”

“…These two polypeptides are derived from a capsid precursor protein of 86 kDa (VP86) processed intracellularly by an undefined protease (14). Previously, the antigenic sites of neutralizing monoclonal antibodies (Nt-MAbs) to the capsid spike domain of classical HAstVs have been mapped (15)(16)(17). However, despite the medical relevance of the non-classical HAstV-VA1, there is limited information about its antigenic sites.…”

Structure and immunogenicity of the murine astrovirus capsid spike

“…The SJ001 strain of MuAstV has a unique two-amino-acid deletion in relation to similar MuAstV strains between residues 535 and 536 (TTXXGAE) within the extended tip of the loop region of the β8-β9 hairpin, which suggests that this loop may also be under particular selective pressure. Interestingly, all previous structurally characterized neutralizing antibodies that target the HAstV spike have been found to predominantly target loop regions, and have also been shown to prevent the spike from attaching to cells [30], likely by preventing receptor binding. The functional significance of the loops is not known, but given their high variability, their immunodominance could potentially aid the virus by drawing antibodies towards regions of the spike in which the virus can readily mutate without significant consequence to escape neutralization, unlike a functional receptor-binding domain, which faces selective pressure to remain conserved to maintain viral fitness.…”

“…These data, in conjunction with previous mouse vaccinations with recombinant HAstV spike, show that recombinant astrovirus spikes are able to induce anti-spike IgG antibody responses. Although it is currently unknown if the anti-MuAstV spike IgG antibodies are neutralizing (there is not yet an in vitro MuAstV infection model to test them), it has previously been shown that antibodies elicited by vaccination with recombinant HAstV spike are capable of neutralizing HAstV in cell culture and can also block HAstV spike attachment to human Caco2 cells [30,31]. Notably, we show that recombinant MuAstV spike vaccination can induce higher levels of anti-spike IgG antibody than MuAstV infection in vivo, as was seen in three of the four mice tested, which presents promising potential to immunize mice against MuAstV using a recombinant spike-based vaccine.…”

“…The mature infectious form of HAstV contains an icosahedral shell formed by the capsid core domains with 30 homodimeric capsid spike domains protruding from the 2-fold icosahedral symmetry axes [29]. The HAstV capsid spike domain is thought to be responsible for attachment and entry of the virus [30]. Antibodies targeting the spike domain have been found to neutralize HAstV in cell culture [31][32][33].…”