Alphavirus infection

Griffin DE, Hardwick JM (1997) Regulators of apoptosis on the road to persistent alphavirus infection. Annu Rev Microbiol 51 565-592. 

Other nonspecific or immune responses that occur following alphavirus infection include the induction of secretion of interferon138-141 and the activation of cytotoxic macrophages.142 There have also been reports of virus-specific cytotoxic T cell responses induced against alphaviruses,143-146 although it has proven difficult to show that these T cell responses play a significant role in protection. 

Levine V, Huang Q, Isaacs JT, Reed JC, Griffin DE, Hardwick JM. Conversion of lytic to persistent alphavirus infection by the bcl-2 cellular oncogene. Nature. 1993 361 739-741. 

Mullbacher A, Blanden RV. H-2-linked control of cytotoxic T-cell responsiveness to alphavirus infection Presence of H-2Dk during differentiation and stimulation converts stem cells of low responder genotype to T cells of responder phenotype. J Exp Med. 1979 149 786-790. 

Tesh RB, McLean RG, Shroyer DA et al. Ross River virus (To viridae Alphavirus) infection (epidemic polyarthritis) in American Samoa. Trans R Soc Trop Med Hyg 1981 75 426-431. 

The replication of alphaviruses is relatively well understood at the molecular level and extensive review articles on these viruses have recently appeared (Strauss and Strauss, 1977, 1983 Kaariainen and Soderlund, 1978 Schlesinger and Kaariainen, 1980 Garoff et al., 1982). In particular, an excellent review concerning the effects of alphavirus infection on host cell macromolecular synthesis has been written by Wengler (1980), and I will concentrate on recent progress in this field. 

It is well known that overall protein synthesis of permissive vertebrate cells is inhibited by alphavirus infections (Wengler, 1980). When incorporation of radioactive amino acids into proteins is analyzed using SDS-polyacrylamide gel electrophoresis (PAGE), the decrease in host protein synthesis is concomitant with the increase in the relative amounts of virus-specific polypeptides. At the end of the exponential phase of virus growth, the majority of the newly synthesized proteins are virus specific (Pfefferkorn and Shapiro, 1974 Strauss and Strauss, 1977). In general, the rate of the inhibition of host protein synthesis depends upon the multiplicity of infection as well as the time after infection. 

Alphavirus infection also inhibits cellular RNA synthesis without a concomitant increase in the rate or extent of mRNA degradation. The rate of this inhibition depends on the multiplicity of infection. 

Inhibition of protein synthesis leads to a cessation of the appearance of newly synthesized ribosomes in the cytoplasm of eukaryotic cells (Hadjiolov and Nikolaev, 1976 Wengler, 1980). This phenomenon has not been characterized in detail, but is has been shown that synthesis and/or processing of the precursors to ribosomal RNAs is inhibited. Since alphavirus infection strongly inhibits host cell pro-... 

These data suggest that the alphavirus-induced inhibition of host cell RNA synthesis occurs at the transcriptional level. To test this possibility, the cell-free transcription system (Manley et al., 1980) which has been developed for studying an inhibitory factor present in poliovirus-infected cells (Baron and Baltimore, 1982) would be useful. With such a system, it would be possible to determine whether inhibitory factor(s) are induced in alphavirus-infected cells or whether the inhibition results from a competition for the available pool of RNA precursors. The genetic approach is not possible at this time since no conditional lethal alphavirus mutant has been described which affects the regulation of host cell RNA synthesis. 

Infection of A. albopictus cells with alphaviruses leads to the synthesis of maximum titers of infectious virus by approximately 24 hr postinfection (acute phase). At the acute phase, up to 85% of the cells released infectious virus (Davey and Dalgarno, 1974), and the rate of synthesis of intracellular 42 S and 26 S viral RNA and structural proteins is maximal. By 48 hr postinfection, viral 26 S RNA and protein syntheses are inhibited, and the proportion of cells releasing virus is dramatically reduced (Davey and Dalgarno, 1974 Eaton, 1979). Inhibition of 42 S RNA synthesis occurs at 3 days after infection (start of chronic phase see Fig. 2). The nature of the factor(s) responsible for inhibiting viral replication in infected mosquito cells at the chronic phase is not known. The fact that large amounts of 42 S RNA are made in infected mosquito cells 48 hr after infection, at a time when viral structural protein synthesis is inhibited, suggests that the replication inhibition factor may act initially at the level of viral protein synthesis (Eaton, 1979). Eaton demonstrated that viral structural protein synthesis is inhibited before a decrease in 26 S RNA synthesis is detected in SIN-infected A. albopictus cells. Thus, the biphasic nature of alphavirus infection is also observed in the case of flavivirus infection in mosquito cells (Paul et al., 1969). 

From these data, one may infer that viral mRNAs replace the majority of cellular mRNAs on the polysomes in CPE-susceptible cells, a situation analogous to that described above for alphavirus-infected vertebrate cells. Two questions remain to be answered (1) how do acutely infected cells recover quickly without any treatment. 

Antiviral therapy has no proven benefit in alphavirus infections. Recovery... 

An additional virus that has more recently gained some attention as a possible vector is that of the sindbis virus. A member of the alphavirus family, this ssRNA virus can infect a broad range of both insect and vertebrate cells. The mature virion particles consist of the RNA genome com-plexed with a capsid protein C. This, in turn, is enveloped by a lipid bilayer in which two additional viral proteins (El and E2) are embedded. The E2 polypeptide appears to mediate viral binding to the surface receptors of susceptible cells. The major mammalian cell surface receptor it targets appears to be the highly conserved, widely distributed laminin receptor. 

The lipids in the viral envelope are taken from the host cell. Pfef-ferkorn and Hunter (1963) had already shown that the viral phospholipids are largely derived from cellular phospholipids synthesized before infection. Subsequent studies of the phospholipid, glycolipid, and cholesterol content of the alphaviruses have shown that the lipid composi-... 

The structure of an alphavirus particle is simpler than that of all known cellular organelles, but it is built according to the same principles. This is because the viral genome is small and the virus must use for its construction those cellular components normally engaged in the biogenesis of host cell membranes. This means that studies of viral replication can be exploited to study cellular functions at the molecular level. Naturally viral infections also perturb cellular physiology, but there is usually enough time early in infection for studies to be carried out before cellular malfunction becomes a source of error. 

Welsh, J. K., Skurrie, 1. J., and May, J. T. (1978). Use of Semliki forest virus to identify lipid-mediated antiviral activity and anti-alphavirus immunoglobulin A in human milk. Infect. Immun. 19, 395 01. 

Sedative. An agent that allays excitement. Semliki Forest virus. A species of viruses of the genus Alphavirus, originally isolated from Aedes mosquitoes in Western Uganda it is widespread in Africa, where it appears to be nonpathogenic, although infections of laboratory workers have occurred. 

Mastrangelo AJ, Hardwick JM, Bex F, Betenbaugh MJ (2000a), Part I. Bcl-2 and Bcl-XL limit apoptosis upon infection with alphavirus vectors, Biotechnol. Bioeng. 67 544-554. 

Unfortunately, there are no structures available for either the flaviviruses or alphaviruses under conditions approximating the fusion state. For both groups of viruses, entry is believed to occur following attachment of the virus to the cellular receptor and internalization of the particle into an endosome (Kielian, 1995 Heinz and Allison, 2001). Acidification of the endosome results in rearrangement of envelope proteins and subsequent insertion of the fusion peptide into the endosomal membrane (Levy-Mintz and Kielian, 1991 Allison et al., 2001). Ultimately this results in fusion of cellular and viral membranes and release of the nucleocapsid core and genome RNA into the cytoplasm of the infected cell. In vitro experiments... 

Although natural infections with the encephalitic alphaviruses are acquired by mosquito bite, the viruses are also highly infectious by aerosol. VEE... 

Maximal amounts of virus are typically produced from mammalian cells within 8 to 10 hours after infection, and disintegration of the infected cell is likely due to programmed cell death (apoptosis) rather than direct effects of the virus on cellular function.87 In contrast, alphaviruses initially replicate to high titer in arthropod cells with little or no evidence of cytopathology. The surviving cells continue to produce lesser amounts of virus, often for weeks or months. The ability of the virus to replicate without causing cell death in arthropod cells may be critical for maintenance of the virus in the mosquito vector in nature. 

The effect of giving immune serum to animals after the establishment of intracerebral infections has also been evaluated. Several studies, employing different alphaviruses, have demonstrated at least partial protection if the immune serum was administered within 24 hours of infection.149 150 152-154 Other studies have suggested that postinfection serum transfer may also cause a more severe pathology, or may merely delay the onset of disease symptoms. 152 155 Aggressive serotherapy following infections of two laboratory workers who developed acute WEE encephalitis resulted in the survival of one patient156 but was ineffective in the second patient.157... 

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