A simple method for clonal selection of hepatitis A virus based on recovery of virus from radioimmunofocus overlays

A cytopathogenic, apoptosis-inducing variant of hepatitis A virus

A cytopathogenic variant of hepatitis A virus (HAV(cyt/HB1.1)) was isolated from persistently infected BS-C-1 cells by serial passages in FRhK-4 cells. This virus shows a rapid replication pattern and high final titers are obtained, which are main characteristics of cytopathogenic HAVs. Sequencing of the nontranslated regions and the coding regions for 2ABC and 3AB revealed that mutations are distributed all over these regions and that certain mutated sites correspond to those in other cytopathogenic HAV variants. Investigating the mechanisms causing the cytopathic effect in FRhK-4 cells infected with this variant, we found that an apoptotic reaction takes place.

Identification of amino acids located in the antibody binding sites of human hepatitis A virus

Antigenic mutants of human hepatitis A virus (human-HAV) were isolated by their resistance to neutralizing monoclonal antibodies raised to human-HAV. The nucleotide sequence determined for the capsid regions of 12 mutants identified amino acid changes that clustered in three non-overlapping sites; one in VP3 and two in VP1. All mutants had a change at amino acid residue 70 in VP3, indicating its primary importance for antibody binding. Ten mutants had two amino acid changes occurring in the VP3 site as well as one in one of the two VP1 sites. These data suggest that both sites in VP1 interact with the single VP3 site to form the immunodominant epitope of HAV. The amino acid changes found in the antigenic mutants of human-HAV selected in this study were located in the same positions as changes found in strains of HAV isolated from Old World monkeys. These simian strains of HAV are not recognized by most monoclonal antibodies raised to human-HAV, suggesting that the observed amino acid changes are part of the antibody binding site.

Antigenic structure of human hepatitis A virus defined by analysis of escape mutants selected against murine monoclonal antibodies

We examined the antigenic structure of human hepatitis A virus (HAV) by characterizing a series of 21 murine monoclonal-antibody-resistant neutralization escape mutants derived from the HM175 virus strain. The escape phenotype of each mutant was associated with reduced antibody binding in radioimmunofocus assays. Neutralization escape mutations were identified at the Asp-70 and Gln-74 residues of the capsid protein VP3, as well as at Ser-102, Val-171, Ala-176, and Lys-221 of VP1. With the exception of the Lys-221 mutants, substantial cross-resistance was evident among escape mutants tested against a panel of 22 neutralizing monoclonal antibodies, suggesting that the involved residues contribute to epitopes composing a single antigenic site. As mutations at one or more of these residues conferred resistance to 20 of 22 murine antibodies, this site appears to be immunodominant in the mouse. However, multiple mutants selected independently against any one monoclonal antibody had mutations at only one or, at the most, two amino acid residues within the capsid proteins, confirming that there are multiple epitopes within this antigenic site and suggesting that single-amino-acid residues contributing to these epitopes may play key roles in the binding of individual antibodies. A second, potentially independent antigenic site was identified by three escape mutants with different substitutions at Lys-221 of VP1. These mutants were resistant only to antibody H7C27, while H7C27 effectively neutralized all other escape mutants. These data support the existence of an immunodominant neutralization site in the antigenic structure of hepatitis A virus which involves residues of VP3 and VP1 and a second, potentially independent site involving residue 221 of VP1.

Antigenic and genetic variation in cytopathic hepatitis A virus variants arising during persistent infection: evidence for genetic recombination

Variants of hepatitis A virus (pHM175 virus) recovered from persistently infected green monkey kidney (BS-C-1) cells induced a cytopathic effect during serial passage in BS-C-1 or fetal rhesus kidney (FRhK-4) cells. Epitope-specific radioimmunofocus assays showed that this virus comprised two virion populations, one with altered antigenicity including neutralization resistance to monoclonal antibody K24F2, and the other with normal antigenic characteristics. Replication of the antigenic variant was favored over that of virus with the normal antigenic phenotype during persistent infection, while virus with the normal antigenic phenotype was selected during serial passage. Viruses of each type were clonally isolated; both were cytopathic in cell cultures and displayed a rapid replication phenotype when compared with the noncytopathic passage 16 (p16) HM175 virus which was used to establish the original persistent infection. The two cytopathic virus clones contained 31 and 34 nucleotide changes from the sequence of p16 HM175. Both shared a common 5' sequence (bases 30 to 1677), as well as sequence identity in the P2-P3 region (bases 3249 to 5303 and 6462 to 6781) and 3' terminus (bases 7272 to 7478). VP3, VP1, and 3Cpro contained different mutations in the two virus clones, with amino acid substitutions at residues 70 of VP3 and 197 and 276 of VP1 of the antigenic variant. These capsid mutations did not affect virion thermal stability. A comparison of the nearly complete genomic sequences of three clonally isolated cytopathic variants was suggestive of genetic recombination between these viruses during persistent infection and indicated that mutations in both 5' and 3' nontranslated regions and in the nonstructural proteins 2A, 2B, 2C, 3A, and 3Dpol may be related to the cytopathic phenotype.

Identification of an immunodominant antigenic site involving the capsid protein VP3 of hepatitis A virus

Hepatitis A virus, an hepatotropic picornavirus, is a common cause of acute hepatitis in man for which there is no available vaccine. Competitive binding studies carried out in solid phase suggest that neutralizing monoclonal antibodies to hepatitis A virus recognize a limited number of epitopes on the capsid surface, although the polypeptide locations of these epitopes are not well defined. Neutralization-escape mutants, selected for resistance to monoclonal antibodies, demonstrate broad cross-resistance to other monoclonal antibodies. Sequencing of virion RNA from several of these mutants demonstrated that replacement of aspartic acid residue 70 of capsid protein VP3 (residue 3070) with histidine or alanine confers resistance to neutralization by monoclonal antibody K2-4F2 and prevents binding of this antibody and other antibodies with similar solid-phase competition profiles. These results indicate that residue 3070 contributes to an immunodominant antigenic site. Mutation at residue 102 of VP1 (residue 1102) confers partial resistance against antibody B5-B3 and several other antibodies but does not prevent antibody attachment. Both VP3 and VP1 sites align closely in the linear peptide sequences with sites of neutralization-escape mutations in poliovirus and human rhinovirus, suggesting conservation of structure among these diverse picornaviruses. However, because partial neutralization resistance to several monoclonal antibodies (2D2, 3E1, and B5-B3) was associated with mutation at either residue 3070 or residue 1102, these sites appear more closely related functionally in hepatitis A virus than in these other picornaviruses.

Complete nucleotide sequence of a cell culture-adapted variant of hepatitis A virus: comparison with wild-type virus with restricted capacity for in vitro replication

To determine the molecular changes associated with adaptation of hepatitis A virus (HAV) to growth in cell culture, the genome of a cell culture-adapted variant of HM175 strain HAV (p16 HM175, 16th in vitro passage level) was molecularly cloned and the complete nucleotide sequence of the virus was determined. Compared with wild-type virus, p16 HM175 replicates efficiently in monkey kidney (BS-C-1) cells (approximately 58 RNA-containing particles per one infectious unit, compared with 2.4 x 10(5) for wild-type HM175). The nucleotide sequence of p16 HM175 revealed a total of 19 mutations from the wild-type genome, including 5 mutations in the 5' nontranslated region, 1 mutation in the 3' nontranslated region, and 13 mutations predicting 8 changes in the amino acid sequences of HAV proteins. Only one amino acid substitution occurred among the capsid proteins (VP2), while others involved proteins 2A, 2B, 2C, VPg, and 3Dpol. When the sequence of p16 virus was compared with that reported previously for an independently isolated, cell culture-adapted variant of HM175 virus (J.I. Cohen et al., (1987). Proc. Natl. Acad. Sci. USA 84, 2497-2501), there were three identical mutations in nontranslated regions of the RNA, and four mutations involving identical amino acids in proteins VP2, 2B, and 3Dpol. The distribution of these mutations within the genome suggests that changes in RNA replication may be of primary importance in adaptation of the virus to growth in vitro. These data are thus helpful in understanding the molecular basis of adaptation of HAV to cell culture and, since attenuation frequently accompanies adaptation of virus to growth in cell culture, may be of benefit in planning for attenuated vaccine development.

Development of a plaque assay for a cytopathic, rapidly replicating isolate of hepatitis A virus

Most hepatitis A virus (HAV) replication in cell culture has been reported to be nonlytic and relatively slow. A rapidly replicating isolate of strain HM-175 from persistently infected, serially passed cell cultures (pHM-175) was found to induce a cytopathic effect. This observation allowed the development of a classic plaque assay for pHM-175 in FRhK-4 cells. The plaques were neutralized by polyclonal and monoclonal antisera to HAV.

Cytopathology, plaque assay, and heat inactivation of hepatitis A virus strain HM175

Hepatitis A virus (HAV) strain HM175 derived after repeated subculture of persistently infected B-SC-1 cells caused a specific cytopathic effect upon acute infection of B-SC-1 cells. The virus formed visible plaques on B-SC-1 cell monolayers after 9 to 14 days of incubation at 34 degrees C, and virus can therefore be titrated by plaque assay. Virus could be re-isolated from plaques of infected cells, which allows the clonal isolation of HAV variants. The stability of a plaque-purified variant of HAV at elevated temperatures exceeded that of poliovirus type 1, but this variant is less stable than previously reported strains of HAV.

Neutralization escape mutants define a dominant immunogenic neutralization site on hepatitis A virus

Hepatitis A virus is an hepatotrophic human picornavirus which demonstrates little antigenic variability. To topologically map immunogenic sites on hepatitis A virus which elicit neutralizing antibodies, eight neutralizing monoclonal antibodies were evaluated in competition immunoassays employing radiolabeled monoclonal antibodies and HM-175 virus. Whereas two antibodies (K3-4C8 and K3-2F2) bound to intimately overlapping epitopes, the epitope bound by a third antibody (B5-B3) was distinctly different as evidenced by a lack of competition between antibodies for binding to the virus. The other five antibodies variably blocked the binding of both K3-4C8-K3-2F2 and B5-B3, suggesting that these epitopes are closely spaced and perhaps part of a single neutralization immunogenic site. Several combinations of monoclonal antibodies blocked the binding of polyclonal human convalescent antibody by greater than 96%, indicating that the neutralization epitopes bound by these antibodies are immunodominant in humans. Spontaneously arising HM-175 mutants were selected for resistance to monoclonal antibody-mediated neutralization. Fourteen clonally isolated mutants demonstrated substantial resistance to multiple monoclonal antibodies, including K3-4C8-K3-2F2 and B5-B3. In addition, 13 mutants demonstrated a 10-fold or greater reduction in neutraliztion mediated by polyclonal human antibody. Neutralization resistance was associated with reduced antibody binding. These results suggest that hepatitis A virus may differ from poliovirus in possessing a single, dominant neutralization immunogenic site and therefore may be a better candidate for synthetic peptide or antiidiotype vaccine development.