Analysis of the Contributions of the Equine Herpesvirus 1 Glycoprotein gB Homolog to Virus Entry and Direct Cell-to-Cell Spread

Experiments to analyze the functions of the equine herpesvirus 1 (EHV-1) glycoprotein gB were performed. Cell lines which stably expressed either the full-length EHV-1 gB or only the extracellular portion of gB (amino acids 1 to 844) were constructed and were termed TCgBf and TCgBΔ, respectively. Using the cell line TCgBf, a gB-negative viral mutant, L11ΔgB, was generated by replacing a 2.1-kb BglII-NruI fragment in the EHV-1 strain RacL11 gB with the Escherichia coli LacZ gene. EHV-1 strain RacL11, the modified live vaccine strain RacH, and L11ΔgB were used for functional studies. It was shown that: (i) EHV-1 gB is essential for virus growth in vitro since gB-negative L11ΔgB exhibited titers of <10 PFU/ml when grown and titrated on noncomplementing cells. (ii) The cell line expressing truncated gB (TCgBΔ) did not complement for the growth of L11ΔgB, but the RacH virus grew to titers comparable to those of RacL11 in all cell lines tested. Since RacH had amino acids 944-980 of gB replaced by 7 missense amino acids as determined by nucleotide sequence analysis, the extreme carboxyterminus but not a domain between amino acid residues 845 and 943, probably the transmembrane domain, of EHV-1 gB is dispensable for virus growth in cultured cells. (iii) Single infected cells but no plaque formation were observed after infection of noncomplementing cells with L11ΔgB, demonstrating the requirement of EHV-1 gB for direct cell-to-cell spread of infection. (iv) The attachment of gB-negative L11ΔgB virions to target cells was similar to both phenotypically complemented L11/gB and parent RacL11 virus. (v) L11ΔgB viral titers could be enhanced by using the fusogen polyethylene glycol (PEG). The increase of L11ΔgB titers by PEG treatment, however, was considerably lower compared to gB-negative pseudorabies virus, suggesting that EHV-1 gB might not be as stringently required for virus penetration as are its homologs in other Alphaherpesvirinae.