Structural Basis for Escape of Human Astrovirus from Antibody Neutralization: Broad Implications for Rational Vaccine Design

Abstract

Human astroviruses are recognized as a leading cause of viral diarrhea worldwide in children, immunocompromised patients, and the elderly. There are currently no vaccines available to prevent astrovirus infection; however, antibodies developed by healthy individuals during previous infection correlate with protection from reinfection, suggesting that an effective vaccine could be developed. In this study, we investigated the molecular mechanism by which several strains of human astrovirus serotype 2 (HAstV-2) are resistant to the potent HAstV-2-neutralizing monoclonal antibody PL-2 (MAb PL-2). Sequencing of the HAstV-2 capsid genes reveals mutations in the PL-2 epitope within the capsid's spike domain. To understand the molecular basis for resistance from MAb PL-2 neutralization, we determined the 1.35-Å-resolution crystal structure of the capsid spike from one of these HAstV-2 strains. Our structure reveals a dramatic conformational change in a loop within the PL-2 epitope due to a serine-to-proline mutation, locking the loop in a conformation that sterically blocks binding and neutralization by MAb PL-2. We show that mutation to serine permits loop flexibility and recovers MAb PL-2 binding. Importantly, we find that HAstV-2 capsid spike containing a serine in this loop is immunogenic and elicits antibodies that neutralize all HAstV-2 strains. Taken together, our results have broad implications for rational selection of vaccine strains that do not contain prolines in antigenic loops, so as to elicit antibodies against diverse loop conformations. IMPORTANCE Human astroviruses (HAstVs) infect nearly every person in the world during childhood and cause diarrhea, vomiting, and fever. In this study, we investigated how several strains of HAstV are resistant to a virus-neutralizing monoclonal antibody. We determined the crystal structure of the capsid protein spike domain from one of these HAstV strains and found that a single amino acid mutation induces a structural change in a loop that is responsible for antibody binding. Our findings reveal how viruses can escape antibody neutralization and provide insight for the rational design of vaccines to elicit diverse antibodies that provide broader protection from infection.

Conformational Flexibility in Respiratory Syncytial Virus G Neutralizing Epitopes

“…Likewise, residues after K187 in the C-terminal region of the CCD also appear to be flexible. Thus, RSV G is part of a growing list of antigens with flexible or intrinsically disordered regions (IDRs) that are targeted by antibodies (48)(49)(50)(51)(52)(53)(54)(55)(56). The observation of different conformations of the RSV G CCD raises several important questions.…”

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

“…HAstV serotype 8 strain Yuc8 was isolated in our laboratory. All viral strains were activated with trypsin and grown as described previously (19).…”

Immunogenicity and Efficacy Evaluation of Subunit Astrovirus Vaccines

“…Humoral immunity plays an important role in protecting against AstVs since the presence of protective antibodies against HAstV in healthy adults provides a mechanism of protection against reinfection [46,47]. The development of anti-AstV therapies is hampered by the gap in knowledge of protective antibody epitopes on the AstV capsid surface [48] and the immunosuppressing peptides along AstV CP [46,49,50]. Most of AstV vaccine research studies, including our previous study, have been focused on the full-length CP vaccine candidates, which all showed partial unsatisfactory immunity [36,37,51].…”