Hepatitis A virus capsid protein VP1 has a heterogeneous C terminus

Hijacking host extracellular vesicle machinery by hepatotropic viruses: current understandings and future prospects

Recent advances in studies exploring the roles of extracellular vesicles (EVs) in viral transmission and replication have illuminated hepatotropic viruses, such as hepatitis A (HAV), hepatitis B (HBV), hepatitis C (HCV), hepatitis D (HDV), and hepatitis E (HEV). While previous investigations have uncovered these viruses' ability to exploit cellular EV pathways for replication and transmission, most have focused on the impacts of exosomal pathways. With an improved understanding of EVs, four main subtypes, including exosomes, microvesicles, large oncosomes, and apoptotic bodies, have been categorized based on size and biogenic pathways. However, there remains a noticeable gap in comprehensive reviews summarizing recent findings and outlining future perspectives for EV studies related to hepatotropic viruses. This review aims to consolidate insights into EV pathways utilized by hepatotropic viruses, offering guidance for the future research direction in this field. By comprehending the diverse range of hepatotropic virus-associated EVs and their role in cellular communication during productive viral infections, this review may offer valuable insights for targeting therapeutics and devising strategies to combat virulent hepatotropic virus infections and the associated incidence of liver cancer.

Hepatitis A virus infection

Hepatitis A is a vaccine-preventable infection caused by the hepatitis A virus (HAV). Over 150 million new infections of hepatitis A occur annually. HAV causes an acute inflammatory reaction in the liver that usually resolves spontaneously without chronic sequelae. However, up to 20% of patients experience a prolonged or relapsed course and <1% experience acute liver failure. Host factors, such as immunological status, age, pregnancy and underlying hepatic diseases, can affect the severity of disease. Anti-HAV IgG antibodies produced in response to HAV infection persist for life and protect against re-infection; vaccine-induced antibodies against hepatitis A confer long-term protection. The WHO recommends vaccination for individuals at higher risk of infection and/or severe disease in countries with very low and low hepatitis A virus endemicity, and universal childhood vaccination in intermediate endemicity countries. To date, >25 countries worldwide have implemented such programmes, resulting in a reduction in the incidence of HAV infection. Improving hygiene and sanitation, rapid identification of outbreaks and fast and accurate intervention in outbreak control are essential to reducing HAV transmission.

Pathogenicity and virulence of hepatitis A virus

Hepatitis A is an acute infection of the liver, which is mostly asymptomatic in children and increases the severity with age. Although in most patients the infection resolves completely, in a few of them it may follow a prolonged or relapsed course or even a fulminant form. The reason for these different outcomes is unknown, but it is generally accepted that host factors such as the immunological status, age and the occurrence of underlaying hepatic diseases are the main determinants of the severity. However, it cannot be ruled out that some virus traits may also contribute to the severe clinical outcomes. In this review, we will analyze which genetic determinants of the virus may determine virulence, in the context of a paradigmatic virus in terms of its genomic, molecular, replicative, and evolutionary features.

Quasi-enveloped hepatitis virus assembly and release

Hepatitis A virus (HAV) and hepatitis E virus (HEV) infections are the main causes for acute hepatitis worldwide. Both viruses had long been considered as nonenveloped viruses. However, recent work has uncovered that both viruses circulate in the bloodstream as membrane-cloaked, "quasi-enveloped" particles that are, surprisingly, infectious and likely the only form mediating virus spread within the host. The discovery of quasi-enveloped HAV and HEV particles has fundamentally changed the traditional view on the life cycle and pathogenesis of these viruses. However, because HAV and HEV are phylogenetically unrelated and their capsid assembly processes are quite distinct, it is not clear whether they use similar or different mechanisms for envelopment and exit. This review provides an overview of the current knowledge about the assembly and exit processes of HAV and HEV and perspectives for future studies.

Hepatitis A virus structural protein pX interacts with ALIX and promotes the secretion of virions and foreign proteins through exosome-like vesicles

Hepatitis A virus (HAV), a classic nonenveloped virus, has recently been found to be released mainly in the form of quasi-enveloped HAV (eHAV) by hijacking host endosomal sorting complexes required for transport (ESCRT) complexes. Unlike the nonenveloped virion, eHAV contains the viral protein pX on the surface of the HAV capsid as an extension of VP1. How HAV capsids acquire the host envelope and whether the pX protein is involved in this process were previously unknown. Here, we analyse the role of pX in foreign protein secretion in exosome-like extracellular vesicles (EVs) and the formation of eHAV. Fusion of pX to eGFP guided eGFP into exosome-like EVs through directing eGFP into multivesicular bodies (MVBs), and apoptosis-linked gene 2-interacting protein X (ALIX) release was significantly enhanced. Coimmunoprecipitation (co-IP) demonstrated the interaction between pX and the ALIX V domain. Removal of the C-terminal half of pX abolished eHAV release and reduced the interaction between the HAV virion and ALIX. Finally, the C-terminal half of pX alone was sufficient for loading eGFP into EVs by interacting with ALIX. In conclusion, the C-terminal part of pX is important for eHAV production and may have potential for large protein complex loading into exosome-like EVs for therapeutic purposes.

Hepatitis A Virus Genome Organization and Replication Strategy

Hepatitis A virus (HAV) is a positive-strand RNA virus classified in the genus Hepatovirus of the family Picornaviridae It is an ancient virus with a long evolutionary history and multiple features of its capsid structure, genome organization, and replication cycle that distinguish it from other mammalian picornaviruses. HAV proteins are produced by cap-independent translation of a single, long open reading frame under direction of an inefficient, upstream internal ribosome entry site (IRES). Genome replication occurs slowly and is noncytopathic, with transcription likely primed by a uridylated protein primer as in other picornaviruses. Newly produced quasi-enveloped virions (eHAV) are released from cells in a nonlytic fashion in a unique process mediated by interactions of capsid proteins with components of the host cell endosomal sorting complexes required for transport (ESCRT) system.

Structural view of the 2A protease from human rhinovirus C15

The majority of outbreaks of the common cold are caused by rhinoviruses. The 2A protease (2Apro) of human rhinoviruses (HRVs) is known to play important roles in the propagation of the virus and the modulation of host signal pathways to facilitate viral replication. The 2Apro from human rhinovirus C15 (HRV-C15) has been expressed in Escherichia coli and purified by affinity chromatography, ion-exchange chromatography and gel-filtration chromatography. The crystals diffracted to 2.6 Å resolution. The structure was solved by molecular replacement using the structure of 2Apro from coxsackievirus A16 (CVA16) as the search model. The structure contains a conserved His-Asp-Cys catalytic triad and a Zn2+-binding site. Comparison with other 2Apro structures from enteroviruses reveals that the substrate-binding cleft of 2Apro from HRV-C15 exhibits a more open conformation, which presumably favours substrate binding.

Cellular localization and effects of ectopically expressed hepatitis A virus proteins 2B, 2C, 3A and their intermediates 2BC, 3AB and 3ABC

In the course of hepatitis A virus (HAV) infections, the seven nonstructural proteins and their intermediates are barely detectable. Therefore, little is known about their functions and mechanisms of action. Ectopic expression of the presumably membrane-associated proteins 2B, 2C, 3A and their intermediates 2BC, 3AB and 3ABC allowed the intracellular localization of these proteins and their possible function during the replication cycle of HAV to be investigated. In this study, we used rhesus monkey kidney cells, which are commonly used for cell culture experiments, and human liver cells, which are the natural target cells. We detected specific associations of these proteins with distinct membrane compartments and the cytoskeleton, different morphological alterations of the respective structures, and specific effects on cellular functions. Besides comparable findings in both cell lines used with regard to localization and effects of the proteins examined, we also found distinct differences. The data obtained identify so far undocumented interactions with and effects of the HAV proteins investigated on cellular components, which may reflect unknown aspects of the interaction of HAV with the host cell, for example the modification of the ERGIC (ER-Golgi intermediate compartment) structure, an interaction with lipid droplets and lysosomes, and inhibition of the classical secretory pathway.

Synthetic peptides for the immunodiagnosis of hepatitis A virus infection

VP1, VP2 and VP3 molecules of hepatitis A virus are exposed capsid proteins that have shown to be antigenic and are used for diagnosis in recombinant-antigen commercial kits. In this study, we developed a sequence analysis in order to predict diagnostic peptide epitopes, followed by their spot synthesis on functionalized cellulose paper (Pepscan). This paper with synthetic peptides was tested against a sera pool of hepatitis A patients. Two peptide sequences, that have shown an antigenic recognition, were selected for greater scale synthesis on resin. A dimeric form of one of these peptides (IMT-1996), located in the C-Terminus region of protein VP1, was antigenic with a recognition frequency of 87-100% of anti-IgG antibodies and 100% of anti-IgM antibodies employing the immunological assays MABA and ELISA. We propose peptide IMT-1996, with less than twenty residues, as a cheaper alternative for prevalence studies and diagnosis of hepatitis A infection.

Structural basis for host membrane remodeling induced by protein 2B of hepatitis A virus

The complexity of viral RNA synthesis and the numerous participating factors require a mechanism to topologically coordinate and concentrate these multiple viral and cellular components, ensuring a concerted function. Similarly to all other positive-strand RNA viruses, picornaviruses induce rearrangements of host intracellular membranes to create structures that act as functional scaffolds for genome replication. The membrane-targeting proteins 2B and 2C, their precursor 2BC, and protein 3A appear to be primarily involved in membrane remodeling. Little is known about the structure of these proteins and the mechanisms by which they induce massive membrane remodeling. Here we report the crystal structure of the soluble region of hepatitis A virus (HAV) protein 2B, consisting of two domains: a C-terminal helical bundle preceded by an N-terminally curved five-stranded antiparallel β-sheet that displays striking structural similarity to the β-barrel domain of enteroviral 2A proteins. Moreover, the helicoidal arrangement of the protein molecules in the crystal provides a model for 2B-induced host membrane remodeling during HAV infection.

IMPORTANCE

No structural information is currently available for the 2B protein of any picornavirus despite it being involved in a critical process in viral factory formation: the rearrangement of host intracellular membranes. Here we present the structure of the soluble domain of the 2B protein of hepatitis A virus (HAV). Its arrangement, both in crystals and in solution under physiological conditions, can help to understand its function and sheds some light on the membrane rearrangement process, a putative target of future antiviral drugs. Moreover, this first structure of a picornaviral 2B protein also unveils a closer evolutionary relationship between the hepatovirus and enterovirus genera within the Picornaviridae family.

Hepatitis A virus and the origins of picornaviruses

Hepatitis A virus (HAV) remains enigmatic, despite 1.4 million cases worldwide annually. It differs radically from other picornaviruses, existing in an enveloped form and being unusually stable, both genetically and physically, but has proved difficult to study. Here we report high-resolution X-ray structures for the mature virus and the empty particle. The structures of the two particles are indistinguishable, apart from some disorder on the inside of the empty particle. The full virus contains the small viral protein VP4, whereas the empty particle harbours only the uncleaved precursor, VP0. The smooth particle surface is devoid of depressions that might correspond to receptor-binding sites. Peptide scanning data extend the previously reported VP3 antigenic site, while structure-based predictions suggest further epitopes. HAV contains no pocket factor and can withstand remarkably high temperature and low pH, and empty particles are even more robust than full particles. The virus probably uncoats via a novel mechanism, being assembled differently to other picornaviruses. It utilizes a VP2 'domain swap' characteristic of insect picorna-like viruses, and structure-based phylogenetic analysis places HAV between typical picornaviruses and the insect viruses. The enigmatic properties of HAV may reflect its position as a link between 'modern' picornaviruses and the more 'primitive' precursor insect viruses; for instance, HAV retains the ability to move from cell-to-cell by transcytosis.

Naked Viruses That Aren't Always Naked: Quasi-Enveloped Agents of Acute Hepatitis

Historically, viruses were considered to be either enveloped or nonenveloped. However, recent work on hepatitis A virus and hepatitis E virus challenges this long-held tenet. Whereas these human pathogens are shed in feces as naked nonenveloped virions, recent studies indicate that both circulate in the blood completely masked in membranes during acute infection. These membrane-wrapped virions are as infectious as their naked counterparts, although they do not express a virally encoded protein on their surface, thus distinguishing them from conventional enveloped viruses. The absence of a viral fusion protein implies that these quasi-enveloped virions have unique mechanisms for entry into cells. Like true enveloped viruses, however, these phylogenetically distinct viruses usurp components of the host ESCRT system to hijack host cell membranes and noncytolytically exit infected cells. The membrane protects these viruses from neutralizing antibodies, facilitating dissemination within the host, whereas nonenveloped virions shed in feces are stable in the environment, allowing for epidemic transmission.

Virus isolate from carp: genetic characterization reveals a novel picornavirus with two aphthovirus 2A-like sequences

Picornaviruses have been isolated from a variety of hosts, mainly mammals and birds. Here, we describe the sequence analysis of carp picornavirus 1 (CPV-1) F37/06 that was isolated from an organ pool (heart, brain, liver) of a common carp (Cyprinus carpio). This carp perished after an accidental discharge of liquid manure into a fish pond and presented without obvious clinical symptoms. Experimental intraperitoneal infection of young carp with CPV-1 revealed no clinical signs, but the virus was re-isolated from various organs. Sequence analysis of almost the complete genome (7632 nt excluding the poly-A tract) revealed a novel picornavirus clade. In phylogenetic trees, the polymerase sequence clusters with parechoviruses, duck hepatitis A virus, eel picornavirus and aquamavirus A. The ORF includes 6807 nt and encodes a polyprotein of 2269 amino acids. CPV-1 has a genome layout like that of picornaviruses except for the presence of two aphthovirus 2A-like NPGP sequence motifs: VPg+5'UTR[1AB-1C-1D-2A1(npgp)/2A2(npgp)-2B-2C(ATPase)/3A-3B(VPg)-3C(pro)-3D(pol)]3'UTR-poly-A. 2A1(npgp) and 2A2(npgp) are separated by 133 amino acids. The proteins 2A2(npgp), 2B, 3A and 3B(VPg) have no significant similarity to the corresponding proteins of other picornaviruses. Amino acid identities of the orthologous proteins P1, 2C, 3C(pro) and 3D(pol) range from 16.4 to 40.8 % in the eel picornavirus/CPV-1 comparison. 3D(pol) shows the closest similarity to eel picornavirus, with an amino acid identity of 40.8 %, followed by human parechovirus (36.5 %), duck hepatitis A virus (32.7 %) and swine pasivirus (29.3 %). Both the unique genome organization and low sequence similarity support the assignment of CPV-1 to a novel picornavirus species within a novel genus.

Peek-a-boo: membrane hijacking and the pathogenesis of viral hepatitis

Historically, animal viruses have been classified on the basis of the presence or absence of an envelope - an external lipid bilayer membrane typically carrying one or more viral glycoproteins. However, growing evidence indicates that some 'non-enveloped' viruses circulate in the blood of infected individuals enveloped in host-derived membranes that provide protection from neutralizing antibodies. In this opinion article, we discuss this novel strategy for virus survival and consider how it contributes to the pathogenesis of acute viral hepatitis. The acquisition of an envelope by non-enveloped viruses profoundly influences their interaction with the host at both the cellular and system level and challenges how we think about vaccine protection against these infections.

A pathogenic picornavirus acquires an envelope by hijacking cellular membranes

Animal viruses are broadly categorized structurally by the presence or absence of an envelope composed of a lipid-bilayer membrane, attributes that profoundly affect stability, transmission and immune recognition. Among those lacking an envelope, the Picornaviridae are a large and diverse family of positive-strand RNA viruses that includes hepatitis A virus (HAV), an ancient human pathogen that remains a common cause of enterically transmitted hepatitis. HAV infects in a stealth-like manner and replicates efficiently in the liver. Virus-specific antibodies appear only after 3-4 weeks of infection, and typically herald its resolution. Although unexplained mechanistically, both anti-HAV antibody and inactivated whole-virus vaccines prevent disease when administered as late as 2 weeks after exposure, when virus replication is well established in the liver. Here we show that HAV released from cells is cloaked in host-derived membranes, thereby protecting the virion from antibody-mediated neutralization. These enveloped viruses ('eHAV') resemble exosomes, small vesicles that are increasingly recognized to be important in intercellular communications. They are fully infectious, sensitive to extraction with chloroform, and circulate in the blood of infected humans. Their biogenesis is dependent on host proteins associated with endosomal-sorting complexes required for transport (ESCRT), namely VPS4B and ALIX. Whereas the hijacking of membranes by HAV facilitates escape from neutralizing antibodies and probably promotes virus spread within the liver, anti-capsid antibodies restrict replication after infection with eHAV, suggesting a possible explanation for prophylaxis after exposure. Membrane hijacking by HAV blurs the classic distinction between 'enveloped' and 'non-enveloped' viruses and has broad implications for mechanisms of viral egress from infected cells as well as host immune responses.

Hepatitis A virus evolution and the potential emergence of new variants escaping the presently available vaccines

Hepatitis A is the most common infection of the liver worldwide and is fecal-orally transmitted. Its incidence tends to decrease with improvements in hygiene conditions but at the same time its severity increases. Hepatitis A virus is the causative agent of acute hepatitis in humans and belongs to the Hepatovirus genus in the Picornaviridae family, and it has very unique characteristics. This article reviews some molecular and biological properties that allow the virus to live in a very quiescent way and to build an extremely stable capsid that is able to persist in and out of the body. Additionally, the relationship between the genomic composition and the structural and antigenic properties of the capsid is discussed, and the potential emergence of antigenic variants is evaluated from an evolutionary perspective.

Determinants in 3Dpol modulate the rate of growth of hepatitis A virus

Hepatitis A virus (HAV), an atypical member of the Picornaviridae, grows poorly in cell culture. To define determinants of HAV growth, we introduced a blasticidin (Bsd) resistance gene into the virus genome and selected variants that grew at high concentrations of Bsd. The mutants grew fast and had increased rates of RNA replication and translation but did not produce significantly higher virus yields. Nucleotide sequence analysis and reverse genetic studies revealed that a T6069G change resulting in a F42L amino acid substitution in the viral polymerase (3D(pol)) was required for growth at high Bsd concentrations whereas a silent C7027T mutation enhanced the growth rate. Here, we identified a novel determinant(s) in 3D(pol) that controls the kinetics of HAV growth.

Hepatitis A virus (HAV) packaging size limit

Background

Hepatitis A virus (HAV), an atypical Picornaviridae that causes acute hepatitis in humans, grows poorly in cell culture and in general does not cause cytopathic effect. Foreign sequences have been inserted into different parts of the HAV genome. However, the packaging size limit of HAV has not been determined. The purpose of the present study is to investigate the maximum size of additional sequences that the HAV genome can tolerate without loosing infectivity.

Results

In vitro T7 polymerase transcripts of HAV constructs containing a 456-nt fragment coding for a blasticidin (Bsd) resistance gene, a 1,098-nt fragment coding for the same gene fused to GFP (GFP-Bsd), or a 1,032-nt fragment containing a hygromycin (Hyg) resistance gene cloned into the 2A-2B junction of the HAV genome were transfected into fetal Rhesus monkey kidney (FRhK4) cells. After antibiotic selection, cells transfected with the HAV construct containing the resistance gene for Bsd but not the GFP-Bsd or Hyg survived and formed colonies. To determine whether this size limitation was due to the position of the insertion, a 606 bp fragment coding for the Encephalomyocarditis virus (EMCV) internal ribosome entry site (IRES) sequence was cloned into the 5' nontranslated (NTR) region of HAV. The resulting HAV-IRES retained the EMCV IRES insertion for 1-2 passages. HAV constructs containing both the EMCV IRES at the 5' NTR and the Bsd-resistance gene at the 2A-2B junction could not be rescued in the presence of Bsd but, in the absence of antibiotic, the rescued viruses contained deletions in both inserted sequences.

Conclusion

HAV constructs containing insertions of approximately 500-600 nt but not 1,000 nt produced viable viruses, which indicated that the HAV particles can successfully package approximately 600 nt of additional sequences and maintain infectivity.

Immunogenic epitopes on the surface of the hepatitis A virus capsid: Impact of secondary structure and/or isoelectric point on chimeric virus assembly

Hepatitis A virus (HAV) protein 2A has the capacity to harbor and expose a short foreign epitope. The chimeric virus, HAV-gp41, bearing seven amino acids of the 2F5 epitope of the HIV glycoprotein gp41, was shown to replicate in cell culture and laboratory animals and to induce a humoral immune response. As an extension of this work, we now investigated the possibility to insert longer epitopes, their impact on genetic stability, and the production of chimeric HAV. Twenty-seven amino acid residues of either HIV gp41, comprising the 2F5 epitope, or of a mimotope (F78) of the hypervariable region 1 of the hepatitis C virus (HCV) envelope protein E2 were inserted near the C-terminus of HAV 2A and viral capsid formation and replication were studied. The genome of the chimeric virus (HAV-F78) had reduced replication ability, yet the sedimentation profile of the chimeric particles was unchanged and the HCV sequence was maintained over serial viral passages. In contrast, no capsids were formed when an extended HIV epitope of 27 residues was inserted, precluding the rescue of infectious chimeric virus. Based on structural analyses, the data suggest that the isoelectric point (pI) and/or the secondary structure of the chimeric proteins are essential determinants that affect HAV particle formation for which protein 2A serves as an assembly signal.

Genetic analysis of HAV strains in Tunisia reveals two new antigenic variants

To evaluate the genetic variability of hepatitis A virus (HAV) isolates in Tunisia, serum samples were collected from 99 patients in different Tunisian areas in 2003 containing 92 cases with acute hepatitis, five with severe acute hepatitis and two with fulminant hepatitis. The entire VP1 gene was amplified and sequenced. Sequences were then aligned and a phylogenetic analysis was performed. Additionally, the amino acid (aa) sequence of the VP1 was determined. The analysis of Tunisian HAV isolates revealed that all the isolates were sub-genotype IA with 96.4%-99.8% of identity and showed the emergence of two novel antigenic variants. The Tun31-03 antigenic variant, with a 38 aa deletion containing Met156, Val171, Leu174 and Ala176 and located between 150 and 187 aa of the VP1 protein where neutralization escape mutations, was found. The second antigenic variant, Tun36-03, was isolated from a patient with fulminant hepatitis and presented a substitution of Thr by Pro at position 10 of the VP1 protein. This amino acid is located in a peptide presenting an antigenically reactive epitope of the VP1 protein. This substitution has never been described previously.

Genetic variability and molecular evolution of hepatitis A virus

Hepatitis A virus (HAV), the causative agent of type A viral hepatitis, was first identified about three decades ago. Recent findings have shown that HAV possess several characteristics that make it unique among the family Picornaviridae, particularly in terms of its mechanisms of polyprotein processing and virion morphogenesis. HAV circulates in vivo as distributions of closely genetically related variants referred to as quasispecies. HAV exploits all known mechanisms of genetic variation to ensure its survival, including mutation and recombination. Only one serotype and six different genetic groups (three humans and three simian) have been described. HAV mutation rate is significantly lower as compared to other members of the family Picornaviridae. The mode of evolution appears, at least in part, to contribute to the presence of only one known serotype.

Stable growth of wild-type hepatitis A virus in cell culture

Human wild-type (wt) hepatitis A virus (HAV), the causative agent of acute hepatitis, barely grows in cell culture and in the process accumulates attenuating and cell culture-adapting mutations. This genetic instability of wt HAV in cell culture is a major roadblock to studying HAV pathogenesis and producing live vaccines that are not overly attenuated for humans. To develop a robust cell culture system capable of supporting the efficient growth of wt HAV, we transfected different cell lines with in vitro RNA transcripts of wt HAV containing the blasticidin resistance gene. Blasticidin-resistant colonies grew only in transfected Huh7 cells and produced infectious virus. HAV was genetically stable in Huh7 cells for at least nine serial passages and did not accumulate attenuating or cell culture-adapting mutations. Treatment with alpha interferon A/D cured the blasticidin-resistant Huh7 cells of the HAV infection. The cured cells, termed Huh7-A-I cells, did not contain virus or HAV antigens and were sensitive to blasticidin. Huh7-A-I cells were more permissive than parental cells for wt HAV infection, including a natural isolate from a human stool sample, and produced 10-fold-more infectious particles. This is the first report of a cell line that allows the genetically stable growth of human wt HAV. The viral vectors and cells described here should allow better insight into the pathogenesis of HAV and the development of attenuated vaccines. The cell lines susceptible to wt HAV growth may also be used to detect and isolate infectious virus from patient and environmental samples.

Hepatitis A virus: from discovery to vaccines

Hepatitis A virus (HAV), the causative agent of type A viral hepatitis, is an ancient human virus that was first identified almost 35 years ago. It has several characteristics that make it unique among the Picornaviridae, particularly in terms of its mechanisms of polyprotein processing and virion morphogenesis, and which likely contribute to its pathobiology. Although efficacious vaccines containing formalin-inactivated virus produced in cell culture have been licensed in multiple countries, their use has been limited by cost considerations. Changes in public health sanitation and generally increasing standards of living are leading to a decreasing incidence of acute hepatitis A worldwide, with the result that the prevalence of preexisting immunity among adults is declining in many regions. These changes in the epidemiology of HAV may paradoxically enhance the disease burden, as greater numbers of individuals become infected at older ages when disease is more likely to be clinically evident, thus providing greater incentives for vaccine utilization.

Homogeneous hepatitis A virus particles. Proteolytic release of the assembly signal 2A from procapsids by factor Xa

Among the picornaviridae, hepatitis A virus (HAV) is unique in that its assembly is driven by domain 2A of P1-2A, the precursor of the structural proteins (Probst, C., Jecht, M., and Gauss-Müller, V. (1999) J. Biol. Chem. 274, 4527-4531). Whereas infected individuals excrete in stool mature HAV capsids with VP1 as the major structural protein, its C-terminal extended form VP1-2A is the main component of immature procapsids produced in HAV-infected cells in culture. Obviously, a postassembly proteolytic step is required to remove the primary assembly signal 2A from VP1-2A of procapsids. Mutants of VP1-2A were expressed in COS7 cells to determine the cleavage site in VP1-2A and to test for the cleavage potential of viral and host proteinases (factor Xa and thrombin). Site-specific in vitro cleavage by factor Xa and thrombin occurred in procapsids that contained VP1-2A with engineered cognate cleavage sites for these proteinases. Interestingly, factor Xa but not thrombin liberated mature VP1 also from native procapsids in an assembly-dependent manner. The data show that domain 2A, which is required for pentamerization of its precursor polypeptides and thus for the primary step of HAV assembly, is removed from the surface of immature procapsid by a host proteinase. Moreover, our data open a novel avenue to produce homogeneous HAV particles from recombinant intermediates by in vitro treatment with exogenously added proteases such as factor Xa or thrombin.

Molecular evolution of hepatitis A virus: a new classification based on the complete VP1 protein

Hepatitis A virus (HAV) is a positive-stranded RNA virus in the genus Hepatovirus in the family Picornaviridae So far, analysis of the genetic variability of HAV has been based on two discrete regions, the VP1/2A junction and the VP1 N terminus. In this report, we determined the nucleotide and deduced amino acid sequences of the complete VP1 gene of 81 strains from France, Kosovo, Mexico, Argentina, Chile, and Uruguay and compared them with the sequences of seven strains of HAV isolated elsewhere. Overall strain variation in the complete VP1 gene was found to be as high as 23.7% at the nucleotide level and 10.5% at the amino acid level. Different phylogenetic methods revealed that HAV sequences form five distinct and well-supported genetic lineages. Within these lineages, HAV sequences clustered by geographical origin only for European strains. The analysis of the complete VP1 gene allowed insight into the mode of evolution of HAV and revealed the emergence of a novel variant with a 15-amino-acid deletion located on the VP1 region where neutralization escape mutations were found. This could be the first antigenic variant of HAV so far identified.

Identification of VP1/2A and 2C as virulence genes of hepatitis A virus and demonstration of genetic instability of 2C

Fourteen different chimeric virus genomes were constructed from two infectious cDNA clones encoding a virulent and an attenuated isolate, respectively, of the HM175 strain of hepatitis A virus. The ability of each recombinant virus to infect tamarins and to cause acute hepatitis was determined. Comparisons of the genotype and phenotype of each virus suggested that VP1/2A and 2C genes were responsible for virulence. The 2C gene derived from the attenuated parent virus was unstable, and one or more mutations arose in this gene during the first passage in tamarins.

Analysis of deletion mutants indicates that the 2A polypeptide of hepatitis A virus participates in virion morphogenesis

Unlike all other picornaviruses, the primary cleavage of the hepatitis A virus (HAV) polyprotein occurs at the 2A/2B junction and is carried out by the only proteinase encoded by the virus, 3C(pro). The resulting P1-2A capsid protein precursor is subsequently cleaved by 3C(pro) to generate VP0, VP3, and VP1-2A, which associate as pentamers. An unidentified cellular proteinase acting at the VP1/2A junction releases the mature capsid protein VP1 from VP1-2A later in the morphogenesis process. Although these aspects of polyprotein processing are well characterized, the function of 2A is unknown. To study its role in the viral life cycle, we assessed the infectivity of synthetic, genome-length RNAs containing 11 different in-frame deletions in the 2A region. Deletions in the N-terminal 40% of 2A abolished infectivity, whereas deletions in the C-terminal 60% resulted in viruses with a small-focus replication phenotype. C-terminal deletions in 2A had no effect on RNA replication kinetics under one-step growth conditions, nor did they have an effect on capsid protein synthesis and 3C(pro)-mediated processing. However, C-terminal deletions in 2A altered the VP1/2A cleavage, resulting in accumulation of uncleaved VP1-2A precursor in virions and possibly accounting for a delay in the appearance of infectious particles with these mutants, as well as a fourfold decrease in specific infectivity of the virus particles. When the capsid proteins were expressed from recombinant vaccinia viruses, the N-terminal part of 2A was required for efficient cleavage of the P1-2A precursor by 3C(pro) and assembly of structural precursors into pentamers. These data indicate that the N-terminal domain of 2A must be present as a C-terminal extension of P1 for folding of the capsid protein precursor to allow efficient 3C(pro)-mediated cleavages and to promote pentamer assembly, after which cleavage at the VP1/2A junction releases the mature VP1 protein, a process that appears to be necessary to produce highly infectious particles.

Replication of subgenomic hepatitis A virus RNAs expressing firefly luciferase is enhanced by mutations associated with adaptation of virus to growth in cultured cells

Replication of hepatitis A virus (HAV) in cultured cells is inefficient and difficult to study due to its protracted and generally noncytopathic cycle. To gain a better understanding of the mechanisms involved, we constructed a subgenomic HAV replicon by replacing most of the P1 capsid-coding sequence from an infectious cDNA copy of the cell culture-adapted HM175/18f virus genome with sequence encoding firefly luciferase. Replication of this RNA in transfected Huh-7 cells (derived from a human hepatocellular carcinoma) led to increased expression of luciferase relative to that in cells transfected with similar RNA transcripts containing a lethal premature termination mutation in 3D(pol) (RNA polymerase). However, replication could not be confirmed in either FrhK4 cells or BSC-1 cells, cells that are typically used for propagation of HAV. Replication was substantially slower than that observed with replicons derived from other picornaviruses, as the basal luciferase activity produced by translation of input RNA did not begin to increase until 24 to 48 h after transfection. Replication of the RNA was reversibly inhibited by guanidine. The inclusion of VP4 sequence downstream of the viral internal ribosomal entry site had no effect on the basal level of luciferase or subsequent increases in luciferase related to its amplification. Thus, in this system this sequence does not contribute to viral translation or replication, as suggested previously. Amplification of the replicon RNA was profoundly enhanced by the inclusion of P2 (but not 5' noncoding sequence or P3) segment mutations associated with adaptation of wild-type virus to growth in cell culture. These results provide a simple reporter system for monitoring the translation and replication of HAV RNA and show that critical mutations that enhance the growth of virus in cultured cells do so by promoting replication of viral RNA in the absence of encapsidation, packaging, and cellular export of the viral genome.

Hepatitis A virus polyprotein processing by Escherichia coli proteases

Hepatitis A virus (HAV) encodes a single polyprotein, which is post-translationally processed. This processing represents an essential step in capsid formation. The virus possesses only one protease, 3C, responsible for all cleavages, except for that at the VP1/2A junction region, which is processed by cellular proteases. In this study, data demonstrates that HAV polyprotein processing by Escherichia coli protease(s) leads to the formation of particulate structures. P3 polyprotein processing in E. coli is not dependent on an active 3C protease: the same processing pattern is observed with wild-type 3C or with several 3C mutants. However, this processing pattern is temperature-dependent, since it differs at 37 or 42 degrees C. The bacterial protease(s) cleave scissile bonds other than those of HAV; this contributes to the low efficiency of particle formation.

Analysis of full-length hepatitis A virus genome in sera from patients with fulminant and self-limited acute type A hepatitis

BACKGROUND/AIMS

Type A hepatitis still poses a considerable problem worldwide. Why some patients progress to fulminant type A hepatitis and others do not is still unknown. To examine whether genomic differences of hepatitis A virus (HAV) are responsible for the severity of the disease, we analyzed the whole HAV genomes from patients with fulminant and self-limited acute type A hepatitis.

METHODS

Sera from three patients with sporadic type A fulminant hepatitis (FH) and three patients with acute hepatitis (AH) were examined for HAV RNA. Full-length nucleotide sequences were determined using long reverse transcription polymerase chain reaction, 5' and 3' rapid amplification of cDNA ends methods, and direct sequencing. The amino acid sequences were deduced from the nucleotide sequences.

RESULTS

HAV RNA was detected in all six patients examined. From the sequence of viral protein 1/2A, all cases were revealed to be genotype IA. By comparing with genotype IA, wild-type HAV strain GBM, the analysis of whole genomes from the six cases showed no specific substitutions between FH and AH. Completely identical nucleotide sequences were observed at 3' non-translated region (NTR) in all six cases. In 5'NTR, less nucleotide substitutions were found in FH than in AH, and in the non-structural protein 2B region, a little more amino acid substitutions seemed to be found in FH than in AH.

CONCLUSIONS

This study showed that full-length HAV could be analyzed from serum samples. Although there were no unique nucleotide or amino acid substitutions, possible associations were suggested between the severity of type A hepatitis and the nucleotide substitutions in 5'NTR and the amino acid substitutions in 2B.

Characterization of recombinant hepatitis A virus genomes containing exogenous sequences at the 2A/2B junction

Hepatitis A virus (HAV) differs from other members of the family Picornaviridae in that the cleavage of the polyprotein at the 2A/2B junction, commonly considered to be the primary polyprotein cleavage by analogy with other picornaviruses, is mediated by 3C(pro), the only proteinase encoded by the virus. However, it has never been formally demonstrated that the 2A/2B junction is the site of primary cleavage, and the actual function of the 2A sequence, which lacks homology with sequence of other picornaviruses, remains unknown. To determine whether 2A functions in cis as a precursor with the nonstructural proteins, we constructed dicistronic HAV genomes in which a heterologous picornaviral internal ribosome entry site was inserted at the 2A/2B junction. Transfection of permissive FRhK-4 cells with these dicistronic RNAs failed to result in the rescue of infectious virus, indicating a possible cis replication function spanning the 2A/2B junction. However, infectious virus was recovered from recombinant HAV genomes containing exogenous protein-coding sequences inserted in-frame at the 2A/2B junction and flanked by consensus 3C(pro) cleavage sites. The replication of these recombinants was less efficient than that of the parent virus but was variable and not dependent upon the length of the inserted sequence. An HAV recombinant containing a 420-nt insertion encoding the bleomycin resistance protein Zeo was stable for up to five passages in cell culture. Inserted sequences were deleted from replicating viruses, but this did not result from homologous recombination at the flanking 3C(pro) cleavage sites, since the 5' and 3' segments of the inserted sequence were retained in the deletion mutants. These results indicate that the HAV polyprotein can tolerate an insertion at the 2A/2B junction and that the 2A polypeptide does not function in cis as a 2AB precursor. Recombinant HAV genomes containing foreign protein-coding sequences inserted at the 2A/2B junction are novel and potentially useful protein expression vectors.

Expression of an antigenic adenovirus epitope in a group B coxsackievirus

Group B coxsackieviruses (CVB) cause human myocarditis, while human adenovirus type 2 (Ad2) is implicated as an agent of this disease. The L1 loop of the Ad2 hexon protein has been demonstrated to be antigenic in rabbits. To evaluate the feasibility of a multivalent vaccine strain against the CVB and Ad2, we cloned the sequence encoding the Ad2 hexon L1 loop, flanked by dissimilar sequences encoding the protease 2A (2Apro) recognition sites, into the genome of an attenuated strain of CVB type 3 (CVB3/0) at the junction of 2Apro and the capsid protein 1D. Progeny virus (CVB3-PL2-Ad2L1) was obtained following transfection of the construct into HeLa cells. Replication of CVB3-PL2-Ad2L1 in diverse cell cultures demonstrated that the yield of the chimeric virus was between 0.5 to 2 log units less than the parental strain. Western blot analyses of the CVB3 capsid protein 1D in CVB3-PL2-Ad2L1-infected HeLa cells demonstrated production of the expected capsid protein. Viral proteins were detected earlier and in approximately fourfold greater amounts in CVB3-PL2-Ad2L1-infected HeLa cells than in CVB3/0-infected cells. Cleavage of the CVB3-PL2-Ad2L1 polyprotein by 2Apro was slowed, accompanied by an accumulation of the fusion 1D-L1 loop protein. Reverse transcription-PCR sequence analysis of CVB3-PL2-Ad2L1 RNA demonstrated that the Ad2 hexon polypeptide coding sequence was maintained in the chimeric viral genome through at least 10 passages in HeLa cells. Mice inoculated with CVB3-PL2-Ad2L1 demonstrated a brief viremia with no replication detectable in the heart but prolonged replication of virus in the pancreas in the absence of pathologic changes in either organ. CVB3-PL2-Ad2L1 induced binding and neutralizing anti-Ad2 antibodies, in addition to antibodies against CVB3 in mice. CVB3-PL2-Ad2L1 was used to challenge mice previously inoculated with CVB3/0 and with preexisting anti-CVB3 neutralizing-antibody titers; anti-Ad2 neutralizing and binding antibodies were induced in these mice at higher levels than in mice without anti-CVB3 immunity. The data demonstrate that a CVB vector can stably express an antigenic polypeptide of Ad2 from within the CVB open reading frame that results in the induction of protective immune responses against both viruses.

Crystal structure of an inhibitor complex of the 3C proteinase from hepatitis A virus (HAV) and implications for the polyprotein processing in HAV

The proteolytic processing of the viral polyprotein is an essential step during the life cycle of hepatitis A virus (HAV), as it is in all positive-sense, single-stranded RNA viruses of animals. In HAV the 3C proteinase is the only proteolytic activity involved in the polyprotein processing. The specific recognition of the cleavage sites by the 3C proteinase depends on the amino acid sequence of the cleavage site. The structure of the complex of the HAV 3C proteinase and a dipeptide inhibitor has been determined by X-ray crystallography. The double-mutant of HAV 3C (C24S, F82A) was inhibited with the specific inhibitor iodoacetyl-valyl-phenylalanyl-amide. The resulting complex had an acetyl-Val-Phe-amide group covalently attached to the S(gamma) atom of the nucleophilic Cys 172 of the enzyme. Crystals of the complex of HAV 3C (C24S, F82A) acetyl-Val-Phe-amide were found to be monoclinic, space group P2(1), having 4 molecules in the asymmetric unit and diffracting to 1.9-A resolution. The final refined structure consists of 4 molecules of HAV 3C (C24S,F82A) acetyl-Val-Phe-amide, 1 molecule of DMSO, 1 molecule of glycerol, and 514 water molecules. There are considerable conformational differences among the four molecules in the asymmetric unit. The final R-factor is 20.4% for all observed reflections between 15.0- and 1.9-A resolution and the corresponding R(free) is 29.8%. The dipeptide inhibitor is bound to the S(1)(') and S(2)(') specificity subsites of the proteinase. The crystal structure reveals that the HAV 3C proteinase possesses a well-defined S(2)(') specificity pocket and suggests that the P(2)(') residue could be an important determinant for the selection of the primary cleavage site during the polyprotein processing in HAV.