Theiler's murine encephalomyelitis virus contrasts with encephalomyocarditis and foot-and-mouth disease viruses in its functional utilization of the StopGo non-standard translation mechanism.

The picornaviruses' genome consists of a positive-sense ssRNA. Like many picornaviruses, cardioviruses synthesize two distinct polyprotein precursors from adjacent but non-overlapping genome segments. Both the [L-1ABCD-2A] and the [2BC-3ABCD] polyproteins are proteolytically processed to yield mature capsid and non-structural proteins, respectively. An unusual translational event, known as 'StopGo' or 'Stop-Carry on', is responsible for the release of the [L-1ABCD-2A] polyprotein from the ribosome and synthesis of the N-terminal amino acid of the [2BC-3ABCD] polyprotein. A common feature of these viruses is the presence of a highly conserved signature sequence for StopGo: -D(V/I)ExNPG(↓)P-, where -D(V/I)ExNPG are the last 7 aa of 2A, and the last P- is the first amino acid of 2B. Here, we report that, in contrast to encephalomyocarditis virus and foot-and-mouth disease virus, a functional StopGo does not appear to be essential for Theiler's murine encephalomyelitis virus viability when tested in vitro and in vivo.

Virus infection induces neuronal apoptosis: A comparison with trophic factor withdrawal.

Multicellular organisms can employ a number of defences to combat viral replication, the most dramatic being implementation of a cell autonomous apoptotic process. The overall cost to the viability of an organism of losing infected cells by apoptosis may be small if the dying cells can be substituted. In contrast, suicide of irreplaceable cells such as highly specialised neurons may have a more dramatic, even fatal consequence. Previous in vitro approaches to understanding whether neurotropic viruses cause neurons to apoptose have utilised transformed cell lines. These are not in the appropriate state of differentiation to provide an accurate indication of events in vivo. We have chosen to characterise the ability of a model CNS disease-causing virus, Semliki Forest virus (SFV), to infect and trigger apoptosis in primary cultures of nerve growth factor (NGF)-dependent sensory neurons. These cells are known to die when deprived of NGF and constitute a useful indicator of apoptosis. We observe that infection causes cell death which bears the morphological hallmarks of apoptosis, this occurs even in the present of survival promoting NGF and is concomitant with new virus production. Using the TUNEL (transferase dUTP nick end labelling) technique we show that SFV-induced apoptosis involves DNA fragmentation and requires caspase (CED-3/ICE cysteine protease) activation, as does apoptosis induced by NGF-deprivation. Extensive areas of apoptosis, as defined using a combination of ultrastructural analysis and TUNEL occur in infected neonatal mouse brains. The novel evidence that infection of primary neurons with SFV induces apoptosis with activation of one or more caspases defines a system for the further anlaysis of apoptosis regulation in physiologically relevant neurons.

Aurothiolates enhance the replication of Semliki Forest virus in the CNS and the exocrine pancreas.

The A7(74) strain of Semliki Forest virus (SFV) is avirulent and the L10 strain virulent in adult mice. A7(74) infection of adult mouse brain gives rise to small discrete foci of infection which, in immunocompetent animals, are cleared within 10 days. In contrast L10 infection results in a widespread and fatal central nervous system infection. Aurothiolates are linear, 2-coordinate complexes in which two ligands are covalently bound on either side of a gold nucleus in a +1 oxidation state (gold (I)). Pretreatment of A7(74) infected mice with two distinct aurothiolates (sodium aurothiomalate and aurothioglucose) resulted in significantly increased brain virus titers, and large confluent areas of infection in the brain similar to the pattern of infection seen with the L10 strain. The gold (I) moiety of aurothiolates was demonstrated to be the active component, since thiomalic acid when administered alone had no potentiating effect on the infection. Although both aurothiolates allowed productive replication and spread of A7(74) within the nature mouse brain, enhanced neuronal destruction was not apparent. There were no significant changes in virus distribution in any other tissue except for the exocrine pancreas and the myocardium where widespread infection of the acinar cells and occasional infected myocytes were observed.

Defective RNAs in mosquito cells persistently infected with Bunyamwera virus.

Viral protein and RNA synthesis were compared in BHK and Aedes albopictus C6/36 (mosquito) cells infected with Bunyamwera virus. In BHK cells host protein synthesis was inhibited and viral proteins were detected until the cells died; in C6/36 cells there was little inhibition of host proteins and viral proteins could not be detected after 36 h post-infection. Relatively more S segment RNA than L or M segment RNA was produced in infected C6/36 cells compared to BHK cells. A persistent infection of C6/36 cells was established and the cells were passaged at weekly intervals for over a year. The titre of virus released from the cells and the level of viral RNA in the cells at different passages fluctuated markedly, but there was no simple relationship between virus titre and the amount of viral RNA. Northern blot analysis of viral RNA extracted from persistently infected cells revealed the presence of subgenomic RNAs derived from the L RNA segment. These defective RNAs were not packaged into nucleocapsids. The presence of the defective RNAs did not correlate with resistance of cells cloned from the persistently infected population to superinfection with homologous virus. Hence the role of these defective RNAs in the maintenance of the persistent state remains to be elucidated.

bcl-2 acts early to restrict Semliki Forest virus replication and delays virus-induced programmed cell death.

As characterized by morphological assessment and terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling, Semliki Forest virus (SFV) infection of rat prostatic adenocarcinoma cells triggers an apoptotic cell response. Cell death proceeded more rapidly following infection with the neurovirulent L10 strain of SFV than with the avirulent A7 strain. Overexpression of the antiapoptotic proto-oncogene bcl-2 allowed survival of cultures infected with either strain of virus. bcl-2 overexpression drastically reduced the numbers of productively infected cells within the cultures. In situ hybridization for viral message-sense RNA coupled with immunostaining for viral protein indicated that bcl-2 functions at an early stage of the virus life cycle, at entry, pretranscriptional events or at transcription, to inhibit virus replication. Double-immunofluorescent labeling for bcl-2 and viral glycoproteins revealed double-positive cells, demonstrating that with time, this early block in replication can be overcome. These productively infected bcl-2-expressing cells do, with time, undergo apoptosis. As a result of changing the balance between cell death and cell division by restricting productive virus replication and delaying virus-induced cell death, bcl-2 expression led to the establishment of chronically infected cell lines which could be passaged.

Susceptibility to a neurotropic virus and its changing distribution in the developing brain is a function of CNS maturity.

Many major physiological changes occur within the rodent central nervous system (CNS) during the first few postnatal weeks. These include axonogenesis, synaptogenesis and myelination. Concomitant with CNS development over this period, there is a decrease in susceptibility to many neurotropic virus infections in that infection of suckling animals results in lethal encephalitis whereas infection of weanling animals is not lethal. The events underlying this dramatic change in susceptibility have been unclear. Here we demonstrate that age-related virulence of the neurotrophic alphavirus, Semliki Forest virus is dependent upon ability of the infection to spread in the CNS. This is not determined by maturity of interferon, or specific immune responses or the blood brain barrier, but by maturity of neuronal systems. Detailed study of the course of infection in the cortex, hippocampus and cerebellum during their postnatal development indicates that as these and other neuronal systems mature they become resistant to spread of the virus and the pattern of infection changes from widespread to focal.

Role of immune responses in protection and pathogenesis during Semliki Forest virus encephalitis.

The course of Semliki Forest virus (SFV) A7(74) infection in immunocompetent BALB/c, athymic nu/nu and severe combined immunodeficient (SCID) mice was compared. BALB/c mice remained healthy and exhibited transient viraemia and infectious virus in the brain from days 2 to 7. Antibodies were detectable by day 5. In comparison, SCID mice displayed a high incidence of paralysis and died: the average day of death was day 23. From infection until death, virus was present in blood and brain. No antibodies were detectable. Athymic mice were intermediate with a transient viraemia and a persistent (> 210 days) sub-clinical central nervous system (CNS) infection. These mice produced anti-viral IgM but not IgG. The pattern of infection in BALB/c or nu/nu mice could be recreated in infected SCID mice by transfer of immune serum from BALB/c or nu/nu mice with the important exception that although BALB/c immune serum could abolish infectivity titres in the CNS, scattered cells positive for viral RNA remained. Transfer of serum decreased mortality and delayed the onset of paralysis. Transfer to infected SCID mice of a non-neutralizing IgG anti-E2 monoclonal antibody did not affect the viraemia but could also reduce brain virus titres. Irrespective of specific immune responses, virus replication in CNS cells was restricted, was generally non-cytopathic and in the absence of specific immune responses could persist. From day 14 lesions of inflammatory, primary demyelination were observed throughout the CNS of BALB/c mice. In contrast, despite prolonged brain virus titres, no demyelinating lesions were observed in infected nu/nu or SCID mice. Lesions could be initiated in the latter by transfer of spleen cells but not antibody. In summary, the focal restricted infection in the CNS of adult mice infected with SFV A7(74) is independent of specific immune responses. IgM antibodies clear the viraemia. IgG antibodies including non-neutralizing antibodies reduce and clear infectious virus but cells positive for viral RNA remain. These may normally be cleared by T cell responses which are damaging and give rise to lesions of demyelination.

Sexual transmission of GB virus C/hepatitis G virus.

Although it is established that infection with GB virus C (GBV-C) or hepatitis G virus (HGV) can be transmitted parenterally, the prevalence of GBV-C/HGV viremia in the general population (2-5%) is relatively high compared with other parenterally borne viruses such as hepatitis C virus. To investigate the possibility of sexual transmission of GBV-C/HGV, we determined the frequency of viremia by the polymerase chain reaction and serological reactivity to the E2 protein by ELISA in samples collected from individuals at risk for sexually transmitted diseases attending a city genitourinary medicine clinic. GBV-C/HGV viremia was detected in 27 of 87 male homosexuals (31%) and 9 of 50 prostitutes (18%), frequencies significantly greater than those in matched controls (2/63) and local blood donors (2.3%). Among nonviremic individuals, a high frequency of serological reactivity to the E2 protein of GBV-C/HGV was also observed in the risk groups (male homosexuals: 14/60; prostitutes: 11/41), although these figures are likely to be underestimates of the frequency of past infection as detectable anti-E2 reactivity may attenuate rapidly over time following resolution of infection. Infection with GBV-C/HGV was more frequent among those coinfected with human immunodeficiency virus type 1. Among male homosexuals from whom retrospective samples were available, evidence for de novo infection was found in 9 of 22 individuals over a mean sampling time of 2.9 years, predicting an annualized incidence of GBV-C/HGV infection of approximately 11% in this group. The high prevalence and incidence of GBV-C/HGV infection in these individuals and prostitutes provides strong evidence for its spread by sexual contact. Further studies are required to investigate the mechanism of its transmission and the clinical significance of acute and persistent infection in these risk groups.