Hemagglutinin of swine influenza virus: a single amino acid change pleiotropically affects viral antigenicity and replication

Host cell-mediated selection of a mutant influenza A virus that has lost a complex oligosaccharide from the tip of the hemagglutinin

During serial passage in Madin-Darby bovine kidney (MDBK) cells, a substrain of influenza virus A/WSN is lost from the population and is replaced by a mutant virus with altered host cell binding properties. This selection does not occur during growth in chicken embryo fibroblasts (CEF). It occurs during growth in MDBK cells because the parental virus produced by these cells has a dramatically reduced affinity for cellular receptors [Crecelius, D.M., Deom, C. M. & Schulze, I. T. (1984) Virology 139, 164-177]. We have now compared the hemagglutinin (HA) subunits, HA1 and HA2, of the parent and mutant viruses by NaDodSO4/PAGE and have found that when the viruses are grown in either host cell the HA1 subunit of the mutant is smaller than that of the parent virus. The nonglycosylated HAs, made in the presence of tunicamycin, have the same apparent molecular weight, indicating that the HA1 subunit of the mutant virus contains less carbohydrate than that of the parent. This reduction in carbohydrate content was observed with 11 independently derived mutants that had been selected by growth in MDBK cells. The nucleotide sequence of the HA gene of the parent and mutant viruses indicates that there are five potential glycosylation sites on the parent HA1 subunit and four on the mutant and that the mutation responsible for this difference is a single base change that eliminates the glycosylation site at amino acid 125 of the parent HA1 subunit. Treatment of the parent and mutant HAs from both cell sources with endo-beta-N-acetylglucosaminidases F and H showed that the HA1 of the parent virus has four complex and one high-mannose oligosaccharides, whereas that of the mutant virus has three complex and one high-mannose oligosaccharides. Thus, all of the potential sites on both HA1 subunits are glycosylated. We conclude that the oligosaccharide attached to amino acid 125 of the parent HA by MDBK cells can reduce the affinity of the virus for cellular receptors and that the mutant virus has a higher affinity than the parent because the mutant HA is not glycosylated at that site. Since amino acid 125 of the parent HA is glycosylated by both CEF and MDBK cells, we further conclude that the host-determined structure of the oligosaccharide at that site affects the affinity of the parent virus for cellular receptors and, thereby, determines whether the mutant virus will have a growth advantage.

Antigenic differences between virulent and avirulent strains of Semliki Forest viruses detected with monoclonal antibodies. Brief report

Eleven monoclonal antibodies (MAs) reacted strongly in an enzyme immunoassay with virulent Semliki Forest virus (SFV) replicating in L cell monolayers. Three MAs showed a considerably diminished reaction with an avirulent strain of SFV both in enzyme immunoassays and plaque reduction tests.

Receptor binding characteristics of strains of the influenza Hong Kong subtype, using a periodate sensitivity test

Members of the Hong Kong influenza subtype were analysed for cell-receptor binding activity. This was done by a periodate sensitivity test, involving treatment of fowl erythrocytes with varying concentrations of potassium periodate, before their use in a virus haemagglutination assay. Three classes of receptor binding activity were clearly separated by this test. A 'standard' group comprised early members of the Hong Kong subtype. Two variant groups included later epidemic strains and some lateral terminal strains. Departures from the standard group were correlated with particular amino acid substitutions in the area of the haemagglutinin close to the proposed cell-receptor pocket. In particular, HA1 residue 226 was directly implicated. These amino acid changes were also detectable immunologically. The biological significance of this correlation is discussed. The periodate test is reproducible and could prove to be a useful tool in identifying particular residues in the haemagglutinin molecule which influence binding to the cellular receptor.

Small peptides induce antibodies with a sequence and structural requirement for binding antigen comparable to antibodies raised against the native protein

Antisera were raised against the chemically synthesized peptide corresponding to each epitope of three foot-and-mouth disease virus strains. Peptide synthesis was further used to determine which amino acid residues in each epitope are important for the specificity of antisera raised against the whole virus. The specificity of the antibody paratope for its epitope was shown to depend on structure as well as sequence. Anti-virus sera demonstrated a greater specificity for the homologous peptide than did the anti-peptide sera. Two of the three peptides were able to induce neutralizing antibodies against the homologous virus. The specificities of the antibodies present in the anti-peptide sera were also inferred from the reactions of each with related sets of peptides. The cross-reactions observed for the anti-peptide sera were readily explained in terms of the antibody specificities determined to be present. The findings also suggest that the diversity of antibodies raised against small peptides is limited and is determined by the immune system. A similar limited response to the native protein was observed, which may account for the high frequency with which anti-peptide sera react with the native homologous protein.

Mutations in the hemagglutinin receptor-binding site can change the biological properties of an influenza virus

Avian influenza virus reassortants containing human influenza virus hemagglutinins do not replicate in ducks. Two mutations in the receptor-binding site of a human hemagglutinin at residues 226 and 228 allowed replication in ducks. The mutations resulted in a receptor-binding-site sequence identical to the known avian influenza virus sequences.