Buoyant density of the hepatitis A virus-like particle in cesium chloride

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.

History of the Discovery of Hepatitis A Virus

Disease outbreaks resembling hepatitis A have been known since antiquity. However, it was not until World War II when two forms of viral hepatitis were clearly differentiated. After the discovery of Australia antigen and its association with hepatitis B, similar methodologies were used to find the hepatitis A virus. The virus was ultimately identified when investigators changed the focus of their search from serum to feces and applied appropriate technology.

Large-scale purification of inactivated hepatitis A virus by centrifugation in non-ionic gradients

Formalin-inactivated hepatitis A virus (HAV) can be purified for vaccine preparation by centrifugation in Renografin-76 (diatrizoate meglumine and diatrizoate sodium) gradients. Both continuous-flow rate-zonal and isopycnic methods were used for the separation of a major antigen component from minor antigen and host protein. The major antigen component, which appeared to contain complete virions by electron microscopy, could be recovered from gradients and accounted for approximately one third of the total antigen in the starting material. The HAV-specific purified antigen could be enriched 200-300-fold by either centrifugation procedure. The purified HAV antigen, when adsorbed to alum and inoculated into mice, was found to be highly immunogenic.

An electron microscopical investigation of faecal small round viruses

A retrospective study of small round featureless viruses (SRVs) initially identified by negative-staining electron microscopy of stool samples was performed. A variety of technique, including immunoelectron microscopy and caesium chloride gradient centrifugation, was applied in an attempt to classify further these viruses. Over a four-year period, 64 SRV-positive samples were reported (1.8% of the stool samples sent for electron microscopy and 6.2% of the total number of positive samples), of which 53 were available for further study. A significant degree of misclassification was found. Viruses previously identified as SRVs were shown to be astrovirus (n = 14), calicivirus (n = 2), and "Norwalk-like" virus (n = 1). The majority of the 36 remaining samples were identified as parvovirus-like (n = 27) (75%), 14 of which were associated with the presence of adenovirus particles. Enteroviruses (n = 3) and hepatitis A virus (n = 1) were infrequently detected. The remaining viruses (n = 5) could not be adequately classified. Parvovirus may be the predominant SRV associated with acute diarrhoeal disease in childhood.

Use of immunosorbent electron microscopy for detection of rota- and hepatitis A virus in sucrose solutions

Immunosorbent electron microscopy was used to demonstrate rotavirus in solutions of varying sucrose concentrations after 18, 42 and 66 h of incubation. About 50% of adsorption of virus particles to the grid was achieved after 18 h incubation and nearly 100% after 42 h when compared to trapping of virus from sucrose free solutions. Hepatitis A virus was purified in a 10-30% sucrose gradient and each fraction was examined by immunosorbent electron microscopy, direct electron microscopy, immune electron microscopy and radioimmunoassay. The sensitivities of immunosorbent electron microscopy and radioimmunoassay were essentially similar and considerably greater than direct electron microscopy and conventional immune electron microscopy.

Infectious hepatitis A virus particles produced in cell culture consist of three distinct types with different buoyant densities in CsCl

Although hepatitis A virus (HAV) released by infected BS-C-1 cells banded predominantly at 1.325 g/cm3 (major component) in CsCl, smaller proportions of infectious virions banded at 1.42 g/cm3 (dense HAV particles) and at 1.27 g/cm3 (previously unrecognized light HAV particles). cDNA-RNA hybridization confirmed the banding of viral RNA at each density, and immune electron microscopy demonstrated apparently complete viral particles in each peak fraction. The ratio of the infectivity (radioimmunofocus assay) titer to the antigen (radioimmunoassay) titer of the major component was approximately 15-fold greater than that of dense HAV particles and 4-fold that of light HAV particles. After extraction with chloroform, the buoyant density of light and major component HAV particles remained unchanged, indicating that the lower density of the light particles was not due to association with lipids. Light particles also banded at a lower density (1.21 g/cm3) in metrizamide than did the major component (1.31 g/cm3). Dense HAV particles, detected by subsequent centrifugation in CsCl, were indistinguishable from the major component when first banded in metrizamide (1.31 g/cm3). However, dense HAV particles recovered from CsCl subsequently banded at 1.37 g/cm3 in metrizamide. Electrophoresis of virion RNA under denaturing conditions demonstrated that dense, major-component, and light HAV particles all contained RNA of similar length. Thus, infectious HAV particles released by BS-C-1 cells in vitro consist of three distinct types which band at substantially different densities in CsC1, suggesting different capsid structures with varied permeability to cesium or different degrees of hydration.

An evaluation of immune electron microscopy as a method for the quantitation of virus particles

A comparison is made between poliovirus titres obtained by immune electron microscopy and those obtained by plaque assay. The former method yields smaller values which cannot be explained by inefficient sedimentation of virus-antibody clumps or incomplete settling on the grid. Using this comparison and immune electron microscopy the theoretical titre of hepatitis A virus can be estimated.

Diagnostic virology using electron microscopic techniques

This chapter illustrates the development of the use of electron microscopy in viral diagnosis. The field covered is confined to medical viral diagnosis, but parallel developments have taken place in both veterinary and botanical fields and techniques derived from both these sources are also included where relevant. It is reported that the scanning transmission mode of operation, which can induce image contrast changes electronically, may enhance studies with unstained sections and perhaps facilitate thin section immune electron microscopy (IEM). The application of negative stain IEM has been particularly useful for the study of the antigenic nature of some of the newly discovered noncultivable viruses. Viral antigens can also be detected in thin sections of infected cells by IEM with suitably labeled specific antibodies. Confirmation of viral infection by electron microscopy on tissues originally processed for light microscopy is also frequently useful.

Hepatitis A virus infection: pathogenesis and serodiagnosis of acute disease

The early development of immune electron microscopic (IEM) methods for the detection of HAV in acute-phase stool suspensions and antibody to HAV (anti-HAV) in serum made it possible to serologically identify cases of hepatitis A using paired acute and convalescent phase sera. Introduction of less cumbersome and time-consuming serologic test methods, including complement fixation (CF) and immune adherence hemagglutination (IAHA), made it feasible to rapidly assay larger numbers of specimens for HAV or anti-HAV. Subsequent development of sensitive immunofluorescence (IF) assays, solid-phase radioimmunoassays (RIA), and enzyme immunoassays (EIA) for HAV and anti-HAV heralded intensive laboratory studies of the biophysical and biochemical properties of the virus as well as efforts to define the pathogenesis and clinical course of disease. Results of the latter studies showed that the bulk of HAV was usually excreted in stool before the onset of clinical symptoms. Other serologic studies demonstrated that all acutely ill patients had circulating anti-HAV IgM, while all convalescent patients were positive for anti-HAV IgG. The development of sensitive serologic tests (RIA and EIA) that could differentiate between anti-HAV IgM and IgG made it possible to serodiagnose an acute case of hepatitis A using a single-phase serum specimen.

Characterization and classification of virus particles associated with hepatitis A. II. Type and configuration of nucleic acid

Virus particles banding at 1.34 g/ml in CsCl and sedimenting at 160S in sucrose gradients were isolated from fecal specimens of patients suffering from hepatitis. In the presence of 4 M urea and about 90% formamide, these particles released linear nucleic acid molecules of the kinked appearance characteristic of single-stranded RNA or single-stranded DNA. They could be distinguished from the nucleic acid of phage lambda added to the preparation as a marker for double-stranded configuration. Experiments in which the virus particles under investigation were incubated at pH 12.9 at 50 degrees C for 30 min revealed that their nucleic acid molecules were hydrolyzed as readily as the RNA genome of poliovirus type 2 analyzed in parallel. Both the single-stranded DNA of phage phiX174 and that of parvovirus LuIII, however, proved unaffected by this treatment, and the double-stranded DNA of phage lambda was denatured to single-stranded molecules. It was concluded, therefore, that the virus of human hepatitis A contains a linear genome of single-stranded RNA and has to be classified with the picornaviruses.

Characterization and classification of virus particles associated with hepatitis A. I. Size, density, and sedimentation

Virus-like particles were purified from stools of patients in an epidemic of hepatitis A in Germany. When reference MS-1 chimpanzee pre-inoculation and convalescent sera were used, the close serological relationship of the purified particles to well-known isolates of hepatitis A could be established. On the other hand, the physicochemical characteristics of the particles were determined in parallel to the characteristics of a marker parvovirus (LuIII) and a marker picornavirus (poliovirus type 2). It could be shown that the majority of the hepatitis A-associated particles band at 1.34 g/ml in CsCl and, like poliovirus, sediment at about 160S. In addition, a distinct hepatitis A antigen was observed, which banded at 1.305 g/ml and sedimented between 50 and 90S. A further component accumulated in the density range of between 1.38 and 1.44 g/ml. However, it seemed to be rather labile. Upon reisolation from CsCl and sedimentation in sucrose, it resolved into a 160S, a 90 to 100S, and a 50S form. The size of the 160S particles (27 to 29 nm) could be readily distinguished from that of the parvovirus (22 to 24 nm). It is concluded, therefore, that hepatitis A-associated virus particles are more likely to be classified with the picornaviruses than with the parvoviruses.

Biochemical and biophysical characterization of light and heavy density hepatitis A virus particles: evidence HAV is an RNA virus

Heavy density HAV was also shown to be sensitive to low concentrations of RNase. The results of these biophysical and biochemical studies strongly support the notion HAV is an enterovirus.

Studies on human hepatitis A virus in chimpanzees

Several chimpanzees found to be seronegative for hepatitis A by immune electron microscopy and by radioimmunoassay were inoculated with known infective faecal extracts from several sources, including the MS-1 strain of hepatitis A virus, first passage material from chimpanzee George and a strain of hepatitis A virus obtained during a naturally occurring outbreak of infection in Germany. Elevated serum transaminase levels were found in the chimpanzees about 19 days after inoculation. Virus particles were found in faeces as early as 9 days after infection in one chimpanzee and by days 12 and 14 in the others. Excretion of virus continued for 9 to 19 days, and the maximum numbers of virus particles were found on the 17th to the 19th day after inoculation. The buoyant density in caesium chloride of virus particles separated from faeces was 1.31 to 1.43 g/ml. Most of the particles were found in the fraction with a density of 1.34 g/ml.

Hepatitis A antibody was detected in the serum by immune electron microscopy and by radioimmunoassay during the period of incubation while virus particles were still being excreted in the faeces and coinciding approximately with the onset of biochemical evidence of liver damage.

A very close or identical morphological and serological relationship was demonstrated between hepatitis A virus isolated from man and from the experimentally infected chimpanzees.

Recent advances in the identification of hepatitis viruses

Resulting directly from the discovery of virus-related antigens, rapid progress has marked the last decade of viral hepatitis research. The hepatitis B virion has been tentatively identified as a DNA virus with an endogenous DNA polymerase, and new serological markers for type B hepatitis have been discovered. Hepatitis A antigen has been identified on a virus-like particle thought to be the hepatitis A virion. Progressively more sophisticated assays for hepatitis antigens and antibodies have been applied to the study of viral hepatitis epidemiology and biochemical-biophysical characterization of the agents. Most recently, knowledge learned from such studies has been exploited to develop a prototype non-infectious but immunogenic hepatitis B vaccine using hepatitis B surface antigen (HBsAg) purified in large quantities from chronic HBsAg carriers. Especially exciting is the prospect, suggested by serological studies of viral hepatitis, that hepatitis viruses besides hepatitis A and B viruses will be identified.

Purification of hepatitis A virus from chimpanzee stools

Hepatitis A virus antigen was purified from early acute-phase chimpanzee stools by a rapid three-step procedure using 7% polyethylene glycol precipitation, CsCl banding, and Sepharose 2B column chromatography. Electron microscopic examination of the hepatitis A virus entigen preparation revealed highly purified hepatitis A virus particles.

Multiple buoyant densities of hepatitis A virus in cesium chloride gradients

Hepatitis A virus (HAV) recovered from stools of human cases of hepatitis A and from stools of chimpanzees experimentally infected with HAV was shown to possess multiple buoyant densities in CsCl gradients. The greatest proportion of HAV was most frequently found at a buoyant density of 1.32-1.34 g/cm3, however, large proportions of HAV were also frequently found at higher densities, including 1.36-1.37, 1.40-1.42, and 1.45-1.48 g/cm3. These findings are consistent with the notion that HAV may be a parvovirus.

Purification of hepatitis A antigen from feces and detection of antigen and antibody by immune adherence hemagglutination

Hepatitis A antigen (HA Ag) was purified from feces collected during acute illness from patients with naturally occurring viral hepatitis, type A. Positive fecal specimens were identified by immune electron microscopy, but for detection of HA Agduring purification immune adherence hemagglutination (IAHA) and microtiter solid-phase radioimmunoassay were used. Isopycnic banding in cesium chloride, rate-zonal separation in sucrose, and preparative zonal electrophoresis were used in various combinations for successive purification, and the purified antigen was successfully used in a test for antibody by IAHA. Seronconversions to HA Ag were demonstrated by IAHA in 20 instances of hepatitis A virus infection, but in none of six cases of type B hepatitis or three cases of post-transfusion hepatitis unrelated to heaptitis A or B viruses, nor in two individuals without hepatitis. In addition, the temporal pattern of antibody development during type A hepatitis was studied in serial sera from an experimentally infected chimpanzee. Antibody titers by IAHA correlated well with antibody ratings determined by immune electron microscopy.

Hepatitis A. Perspectives and recent advances

The basis for the epidemiologic and etiologic differentiation of two major forms of viral hepatitis, hepatitis A and B, was established in a series of studies undertaken between 1930 and 1970. Final recovery and visualization of the presumed etiologic agent of hepatitis A was not, however, accomplished until the technique of immune electron microscopy was applied to the examination of specimen materials collected from individuals in the early acute stages of infection. Morphologically homogeneous virus-like particles of 27 nm diameter have now been recovered from stools of patients with hepatitis A ill from a variety of sources. Antibody to these particles has been shown to develop during the course of infection with hepatitis A but not with hepatitis B and disease has been induced in nonhuman primates inoculated with purified particle containing fractions. The classification of hepatitis A virus has not been conclusively established, but it would appear to be either a parvovirus or an enterovirus.

Faecal shedding of hepatitis-A antigen

Serial stool specimens from two individuals with experimental hepatitis-A infection were examined and the shedding pattern of hepatitis-A antigen (HAAg) was determined by immune electron microscopy. HAAg particles were detected at least 5 days before the development of abnormal transaminase levels and jaundice, but not later than the day of peak transaminase levels. The pattern of early faecal shedding of HAAg particles correlated well with the early infectivity of faeces and accorded with the suggestion that HAAg is involved in the aetiology of hepatitis-A infection.

Reoviruslike agent in stools: association with infantile diarrhea and development of serologic tests

Reoviruslike particles were visualized by electron microscopy in stool filtrates prepared from stools of infants and young children with severe acute gastroenteritis. Patients who had such particles in their stools and whose paired acute and convalescent serums were tested developed an antibody response to the reoviruslike agent, which was measured by immune electron microscopy and by complement fixation. The reoviruslike agent was antigenically related to the epizootic diarrhea of infant mice virus and the Nebraska calf diarrhea virus.