Antigenic diversity of human sapoviruses

Abstract

Sapovirus (SaV) is a causative agent of gastroenteritis. On the basis of capsid protein (VP1) nucleotide sequences, SaV can be divided into 5 genogroups (GI-GV), of which the GI, GII, GIV, and GV strains infect humans. SaV is uncultivable, but expression of recombinant VP1 in insect cells results in formation of viruslike particles (VLPs) that are antigenically similar to native SaV. In this study, we newly expressed SaV GII and GIV VLPs to compare genetic and antigenic relationships among all human SaV genogroups. Hyperimmune antiserum samples against VLPs reacted strongly with homologous VLPs. However, several antiserum samples weakly cross-reacted against heterologous VLPs in an antibody ELISA. Conversely, an antigen ELISA showed that VLPs of SaV in all human genogroups were antigenically distinct. These findings indicate a likely correspondence between SaV antigenicity and VP1 genogrouping and genotyping.

Figure 1

Phylogenetic tree of sapoviruses on the basis of entire nucleotide sequences of the capsid protein. Different genogroups and genotypes are indicated. The numbers on each branch indicate the bootstrap values for the genotype. The scale bar at the lower left represents nucleotide substitutions per site. GenBank accession nos. for reference strains: Arg39, AY289803; Bristol, HCA249939; C12, AY603425; Chiba000496, AJ412800; Chiba010658, AJ606696; Cruise ship, AY289804; Dresden, AY694184; Ehime643, DQ366345; Ehime1107, DQ058829; Houston27, U95644; Manchester, X86560; Mc2, AY237419; Mc10, AY237420; Mc114, AY237422; Mex340, AF435812; NK24, AY646856; Parkville, U73124; PEC, AF182760; Potsdam, AF294739; Sapporo, U65427; SK15, AY646855; Stockholm, AF194182; SW278, DQ125333; Syd3, DQ104357; Syd53, DQ104360; and Yokote1, AB253740.

Figure 2

Electron micrographs of A) Syd53 and B) Syd3 viruslike particles of sapovirus. Scale bars = 100 nm.

Figure 3

Antibody ELISA absorbances at 492 nm of 6 viruslike particles (VLPs) of sapovirus. A) Mc114 GI/1; B) Yokote1 GI/5; C) Syd53 GII/3; D) Syd3 GIV/1; E) NK24 GV/1; F) NoV Osaka659. Wells were coated with ≈100 ng of purified VLPs. Antiserum was used in 2-fold dilutions from 1:500 to 1,024,000 in phosphate-buffered saline, 0.1% Tween 20, 5% skim milk shown as 1–12 along the x-axis. OD, optical density; NoV, Norwalk virus.

Figure 4

Amino acid alignment of capsid (VP1) sequences of sapoviruses GI/1 Mc114, GI/5 Yokote1, GII/3 Syd53, GIV/1 Syd3, and GV/1 NK24. Sequences in rectangular boxes represent predicted P2 domains (13). Asterisks indicate conserved amino acids among these 3 VP1 sequences.