Structure and immunogenicity of the murine astrovirus capsid spike

Abstract

Human astroviruses (HAstVs) are small, non-enveloped icosahedral RNA viruses that are a significant cause of diarrhoea in young children. Despite their worldwide prevalence, HAstV pathogenesis studies and vaccine development remain challenging due to the lack of an animal model for HAstV infection. The recent development of a murine astrovirus (MuAstV) infection model in mice provides the opportunity to test proof-of-concept vaccines based on MuAstV antigens. To help establish a system in which an astrovirus capsid spike-based vaccine could be tested in vivo, we designed and produced a recombinant MuAstV capsid spike protein based on predicted secondary structure homology to HAstV spike proteins. The recombinant MuAstV spike can be expressed with high efficiency in Escherichia coli and retains antigenicity to polyclonal antibodies elicited by MuAstV infection. We determined the crystal structure of the MuAstV spike to 1.75 Å and assessed its structural conservation with HAstV capsid spike. Despite low sequence identity between the MuAstV and HAstV spikes and differences in their overall shapes, they share related structural folds. Additionally, we found that vaccination with MuAstV spike induced anti-MuAstV-spike antibodies, highlighting that the recombinant spike is immunogenic. These studies lay a foundation for future in vivo MuAstV challenge studies to test whether MuAstV spike can be the basis of an effective vaccine.

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

“…Of interest, two sites conserved on the spike of the eight classical HAstVs have been proposed as potential binding sites for a cell receptor (24). One of them, the P site located on the side of the spike, is in a similar location to that of the conserved C site identified when 15 different mouse astrovirus spike sequences were compared (23). These topologically conserved sites in classical human and murine astroviruses seem to overlap with the antigenic site 2 proposed in this work (Fig.…”

“…Classical and VA viruses represent two different HAstV clades. However, although their spike proteins have low sequence identity and apparent structural differences (8), they share a folding topology and a dimeric structure, which is also maintained in the MLB1 spike (22) and the recently described spike structure of murine astrovirus (23). In this regard, the previously reported escape mutations to Nt-Abs directed to classical HAstV serotypes 1, 2, and 8 (16) seem to lay in regions similar to those described in this work for the HAstV-VA1 spike (Fig.…”