Cellular changes associated with persistent hepatitis A infection in vitro

Internal ribosomal entry-site activities of clinical isolate-derived hepatitis A virus and inhibitory effects of amantadine

AIM

Little is known about specific naturally-occurring internal ribosomal entry site (IRES) activities of hepatitis A virus (HAV). We examined these activities using the bicistronic reporter assay and the effects of antiviral amantadine against their activities.

METHODS

Six HAV IRES clones from three patients with fulminant hepatitis and three with self-limited acute hepatitis were obtained. The activities of their IRES were analyzed using bicistronic reporter assay in hepatocyte- and non-hepatocyte-derived cell lines, and the potential efficaciousness of the amantadine was examined.

RESULTS

One clone from fulminant hepatitis had a deletion in domains III-IV of HAV IRES had higher IRES activities than HM175 in HLE and Huh-7 cells. In Huh-7 cells, amantadine is effective for inhibiting HAV IRES activities, and especially fulminant hepatitis-derived ones.

CONCLUSION

HAV IRES derived from clinical isolates have various activities. Bicistronic reporter assay using clinical isolates may be another useful tool for testing antiviral activities like those of amantadine and the new acridines and hydrazones recently reported.

A cytopathic and a cell culture adapted hepatitis A virus strain differ in cell killing but not in intracellular membrane rearrangements

Aside from a common gene organization shared with other picornaviruses, hepatitis A virus (HAV) is characterized by its slow-growth phenotype, the inability to shut off host macromolecular synthesis, and, in general, lack of cytopathic (cp) effects in permissive cell cultures. Nevertheless, several cp HAV strains have been isolated during the past decade. In FRhK-4 cells infected with HM175/24a, a fast-growing cp strain, increasing amounts of viral RNA, detected by fluorescence in situ hybridization, indicated viral RNA replication. An ultrastructural analysis of the infected cells revealed a tubular-vesicular network in close proximity to the rough endoplasmic reticulum. Infection of the same cell type with a cell culture adapted (cc) strain, HM175/P35, divulged membrane alterations indistinguishable from the network induced by the cp strain. The overall appearance of the tubular-vesicular network resembles membrane alterations induced by other picornaviruses. However, the shape of the vesicle-like structures is rather oblong and tubular and, thus, seems to be specific for HAV. By electron microscopic immunocytochemistry (IEM), proteins 2B and 2C were found exclusively on the membranes of the network. Proteins expressed from the open reading frame of the cc HAV variant or 2B proteins originating from HM175 cp, cc, or the wt strain expressed in the absence of other HAV proteins induced membrane alterations resembling those seen in HAV-infected cells. The induction of similar structures suggests that protein 2B is involved in the rearrangement of cellular membranes. In all cases, IEM demonstrated that the 2B protein was closely associated with altered membranes. The extent of membrane changes did not seem to increase for both the cp strain and the cc strain during the infectious cycle. Late in the infection and shortly before the culture died off, a large number of cells infected with HM175/24a showed typical signs of apoptosis, whereas the cc strain did not induce cell killing in the same type of cells. Therefore, we conclude that cell death in HM175/24a-infected cells is induced by apoptosis rather than by cytopathology.

Biologic properties of human herpesvirus 7 strain SB

The growth characteristics of human herpesvirus 7 strain SB (HHV-7 (SB)) were studied in human umbilical cord blood lymphocyte (CBL) cultures. The virus has approximately a 4-day growth cycle, as measured by immunofluorescence analysis, quantitation of the relative viral DNA concentration, and examination of infected cells by electron microscopy on consecutive days post-infection. By systematically varying the culture media components, improved culturing conditions were established. Activated lymphocytes were required for virus growth. HHV-7(SB) grew best in phytohemagglutinin-stimulated CBL cultured in media containing 0.01 mg/ml hydrocortisone. Addition of recombinant human interleukin 2 (IL-2) at concentrations exceeding 1-10 U/ml inhibited virus growth in most CBL cultures. Addition of exogenous IL-2 to the culture media had no effect on viral DNA production. However, the percentage of virus antigen-positive cells was highest when 0.1-1 U/ml was added to the media. Differences in the ability of individual CBL cultures to replicate HHV-7(SB) was not explained by differing CD4+ cell concentrations. However, individual cultures varied in the level of endogenous IL-2 production, which may contribute to the virus growth variability in CBL. HHV-7(SB) grew in the CD4-positive T-cell line SupT1, but not in a variety of other lymphocyte, fibroblast, or epithelial cell lines. Nine compounds were tested for antiviral activity against HHV-7 in vitro. Phosphonoformic acid inhibited virus growth with a 50% effective concentration of 4.8 microM. Ganciclovir (200 microM) and phosphonoacetic acid (100 microM) inhibited more than 90% of virus production. None of the compounds were cytotoxic at concentrations which inhibited the virus. A generalized increase in host cell protein synthesis was also observed in virus-infected cells similar to that seen in CBL infected with human herpesvirus 6.

Annulate lamellae and lytic HAV infection in vitro

The aim of this study was to examine the relationship between viral infection and annulate lamellae (AL) production by using quantitative and qualitative electron microscopy to document the size and numbers of AL in BS-C-1 cells infected with a lytic strain of hepatitis A virus (HAV). The progress of the HAV infection was found to occur in two phases. In phase 1, cell proliferation and cell death were roughly the same as that of the mock infected control, but there was an increase with time in the amount of hepatitis A antigen in the infected cells. In phase 2 cell division was minimal and cell death became manifest. AL were detected in both infected and control cells. Quantitative analysis indicated that the average number of AL was greater in infected cells compared to that in control cells in phase 1; in infected cells there were greater numbers of AL in phase 1 than in phase 2; the average number of membraneous leaves/AL was greater in infected cells than in control cells. Quantitative analysis also indicated that AL were very rare, with only about three AL per entire control cell and eight AL per entire infected cell. The study clearly establishes that viral infection can stimulate AL production. The data suggest stimulation of AL production in the virus infected cells was linked to the synthesis of viral antigen. Ultrastructural observations indicated that AL could be derived from either the rough endoplasmic reticulum or the nuclear membrane.

Morphological changes in HAV-infected Frp/3 cells and immunolocalization of HAAg

Electron and immunoelectron microscopic studies were carried out on HAV-infected Frp/3 cells. The infection led to a distinctive cytopathic effect (CPE) arising on day 3 up to the complete detachment of monolayers on day 7. Infected cells exhibited progressive modifications, beginning from the formation of long helical polyribosomes. Subsequently, hypertrophy, cisternal dilatation and degranulation of the RER could be observed. Furthermore, the formation of concentric membranous bodies (CMB), large myelin-like structures and annulate lamellae could be revealed at later times of infection. 24-27 nm virus-like particles were observed within cytoplasmic vesicles or outside extensively degenerated cells. Indirect immunoperoxidase staining were used to localize HAV antigen (HAAg) in thin sectioned infected Frp/3 cells. Vesicular inclusion bodies, often seen to contain electron-lucent particles, resulted darkly stained as well as tracts of the RER and myelin-like structures. Negatively stained preparations from cell lysates revealed small clusters of HAV particles which sometimes appeared to be still associated with residual membrane fragments. Our findings seem to suggest that HAV replication occurs in close association with cytoplasmic membranes and a direct involvement of the RER seems to be demonstrated.

Restricted replication of hepatitis A virus in cell culture: encapsidation of viral RNA depletes the pool of RNA available for replication

The replication of hepatitis A virus (HAV) in BS-C-1 cells was examined under single-cycle growth conditions by using strand-specific probes for detection of viral RNA species. No measurable lag phase was demonstrated between accumulation of positive-strand HAV RNA and production of infectious virions, indicating that replication of virion RNA is rate limiting for the production of infectious virus. Intracellular viral RNA was further analyzed by using 2 M LiCl to fractionate the insoluble nonvirion 35S RNA and replicative intermediates (RI) from the soluble virions and double-stranded replicative forms, in conjunction with sucrose density gradient ultracentrifugation to separate the different forms of viral RNA. Throughout the productive phase of HAV infection, 95 to 97% of positive-strand HAV RNA was soluble in 2 M LiCl and was shown to be contained in mature virions. Of the LiCl-insoluble HAV RNA, more than 99% was positive-stranded 35S RNA, whereas 0.4% was negative stranded and had the sedimentation and partial RNase resistance characteristics of RI. The pattern of RNA accumulation in HAV-infected cells is thus very different from that seen in poliovirus-infected cells, where large pools of RI and mRNA are produced before RNA is sequestered into mature virions. The results of this study suggest that encapsidation of positive-strand HAV RNA inhibits transcription at all times during the growth cycle, thereby reducing the pool of replicating RNA and the final yield of infectious HAV.