Hemagglutination mediated by the spike protein of cell-adapted bovine torovirus

Abstract

Bovine torovirus (BToV)-Aichi, recently isolated in cultured cells, showed hemagglutination (HA) activity, although the virus has a truncated hemagglutinin-esterase (HE) protein, judging from its gene structure, indicating the existence of another viral protein with HA activity. We examined whether the spike (S) protein possesses HA activity. A BToV antiserum used in this study, reactive to S but not to HE, inhibited HA activity. Furthermore, cells infected with BToV and those expressing S showed hemadsorption (HAD) activity, which was inhibited by the anti-BToV serum; however, HAD activity by expressed HE was not blocked. These data indicate that the S protein of BToV-Aichi is responsible for its HA activity.

Reverse Genetics with a Full-length Infectious cDNA Clone of Bovine Torovirus

“…Characterization of recombinant BToV with manipulating HE gene. Since cell-adapted BToV generally lost the full-length HE gene, HE protein is not essential for viral growth in cells or could suppress growth (34)(35)(36)(37)69). To study this, we generated rHEf with full-length HE gene and rHEf/HA, rHEs/HA with HA-tagged HEf, or HEs gene.…”

“…ToVs are a causative agent of diarrheic and respiratory diseases, and are detected in many species including bovine, equine, swine, goat, and human species (2-7). Although ToV infections are generally asymptomatic or do not have severe symptoms, the bovine torovirus (BToV) (8- 16) ToVs were difficult to propagate in cultured cells, with the exception of equine ToV (6); however, in the last 13 years, a new cell line, human rectal tumor-18 (HRT18) cells, has been shown to be susceptible to BToV, and several BToVs in Japan were successfully isolated and propagated using HRT18 cells (33)(34)(35)(36)(37). BToV with the full length HE gene was initially isolated from diarrheal feces, whereas cell-adapted BToV, after several passages in HRT18 cells, generally lost a full-length HE protein as a result of HE gene mutation (34,35).…”

“…3A). This suggested that HE protein is dispensable for virus replication in cells and may have a negative effect on it (34)(35)(36)(37). However, the precise functions of BToV HE protein in cellular viral growth still remains elusive.…”

“…2B). Cell-adapted wtBToV in our laboratory lost full-length HE (HEf) due to a stop codon at nt 481 (the base number of HE gene), CAG (Q)→TAG (stop), resulting in the soluble form HE 160 aa in length (HEs) (36,37) (Fig. 3A).…”

Lineage specific antigenic differences in porcine torovirus hemagglutinin-esterase (PToV-HE) protein

“…Despite the great amount of knowledge acquired on this protein family, it is not yet clear what is the exact role of HE during the torovirus infectious cycle. The EToV, the only cell culture adapted torovirus, as well as different BToV strains that could be isolated in cell culture, all lack a functional HE protein [ 13 - 16 ] due to deletions or mutations in the HE gene, acquired during their adaptation to in vitro growth conditions. Thus, HE expression seems to be detrimental for in vitro culture of these viruses, as it occurs with the coronavirus murine hepatitis virus (MHV) [ 17 ].…”