Early synthesis of Semliki Forest virus-specific proteins in infected chicken cells

Neurogenic spread of Semliki Forest virus in mice

We present evidence that in SFV-infected mice the virus invades the brain via nervous tracts, and that in the early stage of infection of the brain itself dissemination is restricted to neurons. After intranasal (i.n.) application, migration along the neuronal route appeared to be the principle mechanism of spread irrespective of the virulence of the strain used. After subcutaneous (s.c.) infection, neuronal spread could also be demonstrated, however the pattern and time course of brain invasion were different. The LD50 of strain Hd depended significantly on the route of virus application: compared with the i.n. route, its value increased after s.c. infection. This increase was correlated with high plasma interferon induction and natural killer cell activation. In contrast, these activities were virtually absent during the early course after i.n. infection.

Semliki Forest virus: a probe for membrane traffic in the animal cell

The traffic among the cellular compartments is thought to be mediated by membrane vesicles, which bud from one compartment and fuse with the next. Despite the continuous exchange of membrane components among them, the organelles maintain their characteristic protein and lipid compositions such that the traffic remains selective, thus, avoiding intermixing of components. This membrane traffic recycles components from the cell surface to the interior of the cell and back to the cell surface again. The membrane traffic between the ER and the cell surface involves a major sorting problem. Little is known of how the animal cell has solved this problem in molecular terms. One experimental tool in this direction is provided by some enveloped animal viruses, which mature at the cell surface of infected cells. Such viruses include influenza virus, Semliki Forest virus (SFV), Sindbis virus, and vesicular stomatitis virus (VSV). They are extremely simple in makeup and hence are very well characterized. The purpose of this article is to illustrate the use of the enveloped viruses as tools in the study of membrane traffic in the animal cell. This is done in the context of the life cycle of the virus in the host cell. The article will be concerned mainly with Semliki Forest virus (SFV), which is the virus that has been worked upon in the chapter. SFV belongs to the alphaviruses, a genus of the togavirus family.

Comparison of specific and cross-reactive antigens of alphaviruses on virions and infected cells

Rabbit hyperimmune antisera against Sindbis (SIN) or Semliki Forest (SF) virus were absorbed with purified SIN virus or SIN virus-infected cells, or with SF virus or SF virus-infected cells. Residual antibody titers were determined by hemagglutination inhibition (HAI) and antibody-dependent, complement-mediated cytolysis (ADCMC) assays. It appeared that absorption with virus-infected cells removed ADCMC-detectable cross-reactive antibody much more efficiently than did absorption with either virus. HAI assays with the same absorbed antisera indicated that both virus and virus-infected cells removed HAI-detectable cross-reactive antibody. On the basis of these and other data, there appeared to be a cross-reactive antigen present on virus-infected cells which was detectable by ADCMC and was distinct from the cross-reactive antigen assayed by HAI.

Polypeptide synthesis in alphavirus-infected Aedes albopictus cells during the establishment of persistent infection

Polypeptide synthesis was examined in mosquito cells during the establishment of a persistent infection with two alphaviruses, Ross River virus (RRV) and Semliki Forest virus (SFV), and in vertebrate cells cytopathically-infected with the same viruses. In Aedes albopictus cell, RRV reached peak titres at 34--48 hours p.i. At 12 hours 85 per cent of cells assayed as infected by infective centre assay; by 48 hours when persistence was established, virus production was reduced and less than 5 per cent of cells assayed as infected. There was no shut-down of host polypeptide synthesis during infection. Viral polypeptide synthesis was maximal between 10 and 24 hours p.i. The major viral polypeptides labelled were nucleocapsid protein and envelope protein(s). The precursor polypeptide p95 which was prominent in infected BHK cells was not detected in mosquito cells. Similar results were obtained on SFV infection. During the establishment of persistence there was a coordinate decline in the synthesis of RRV polypeptides, reaching undetectable levels by 72 hours p.i. Subculturing persitently-infected cells led to a small increase in viral polypeptide synthesis and virus titre. In contrast, during RRV growth in BHK celos host protein synthesis was severly inhibited and by 9--11 hours p.i. virus-specific polypeptide synthesis represented more than 90 per cent of total protein synthetic activity.

Semliki forest virus: cause of a fatal case of human encephalitis

A fatal case of human encephalitis has been observed for which our results indicate that Semliki Forest virus (SFV) was the etiologic agent. This is surprising in view of the fact that this virus, which has been widely studied, was believed to be one of the arboviruses nonpathogenic for man. Described are the clinical course, the virological examinations performed, and the histopathological findings in the central nervous system.

Replication of standard and defective Ross River virus in BHK cells: patterns of viral RNA and polypeptide synthesis

Virus-specific macromolecule synthesis has been examined in BHK cells infected with Ross River virus. Unpassaged virus (R-0) and tenth-passage virus (R-10) have been compared. In infected cells R-0 generates i) 45S, 28S, 33S and 26S viral RNAs, ii) virus-specific precursor polypeptides of mol. wt. 127,000, 95,000 and 61,000 and iii) viral envelope proteins (mol. wts. 52,000 and 49,000) and nucleocapsid protein (mol. wt. 32,000). Thus in terms of virus-specific RNA and polypeptide synthesis, the replication of standard RRV is analogous to that of Semliki Forest virus and Sindbis virus. R-10 interferes with the replication of standard Ross River virus and generates large amounts of 19S and 24S defective RNA species; 45S and 26S RNA synthesis was not markedly affected. Defective RNAs are associated with RNAse-sensitive, 50S cytoplasmic particles which contain a variety of (mainly host) proteins but no nucleocapsid protein. No evidence for translation of defective RNAs was obtained. R-10 infection is also characterized by a relatively early shut down of host protein syntehsis and by a reduction in virus-specific polypeptide synthesis and nucleocapsid formation. The data suggest that defective Ross River virus interferes primarily at the translational level.

Delayed-type hypersensitivity against Semliki Forest virus in mice

After intracutaneous immunization with purified inactivated Semliki Forest virus, a delayed-type hypersensitivity without detectable antibodies in serum was obtained in BALB/c mice. Low doses of antigen given intraperitoneally induced antibodies. Intracutaneous immunization with much higher doses induced no specific antibodies, but a footpad swelling was observed after challenge with homolgous antigen. Pretreatment with cyclophosphamide before immunization enhanced footpad swelling. Microscopic examination of footpads from sensitized mice at 24 h after challenge showed a mononuclear infiltrate. The delayed-type reaction could be transferred to syngenic mice with lymph node cells, but not with spleen cells or serum. The biphasic character of the delayed-type hypersensitivity is discussed.

Envelopments of Sindbis virus: synthesis and organization of proteins in cells infected with wild type and maturation-defective mutants

The synthesis and organization of Sindbis virus structural proteins was investigated in BHK cells infected with wild-type virus (SVHR) or temperature-sensitive (ts) mutants defective in maturation. Cells infected with ts-23 or ts-20 (complementation groups D and E) were similar in the polypeptides synthesized at the nonpermissive temperature and differed from SVHR-infected cells in that the envelope protein E2 was not cleaved from the PE2 precursor. Data from experiments utilizing pulse-chase procedures or protein synthesis inhibitors indicated that although infectious virions were released from cells infected with these mutants in shift-down experiments, the particles were produced almost exclusively from proteins synthesized after the return to permissive temperature. This suggests that a stable complex may be formed among the structural proteins before budding. A membrane fraction isolated from cells infected with either ts mutants or SVHR contained the PE2, E1, and C polypeptides, whereas E2 was restricted to fractions obtained from SVHR-infected cells. Although equivalent amounts of virus-specific protein were synthesized in cells infected with either mutant and the cells contained qualitatively the same proteins in the isolated membranes, cells infected with ts-23 did not have virus-specific proteins exposed on their surface that could be detected by ferritin-conjugated antibody-labeling procedures or lactoperoxidase-mediated iodination. In contrast, ts-20-infected cells had significant amounts of viral protein, mainly E1, that could be detected on the plasma membrane by either procedure. Iodine was incorporated into E1 and E2 on the surface of SVHR-infected cells in the same relative amounts as seen in iodinated virions. PE2, however, although present in membranes, could not be iodinated on the surface of infected cells under any of the conditions used in this study. We also monitored the relative efficiency with which these viral proteins could be removed from intact cells by dilute solutions of nonionic detergents. The results indicated that E2 was most efficiently removed, followed by E1. PE2 (the precursor to E2) and C remained associated with the cell and could be subsequently isolated in the membrane fraction.

Synthesis of Sindbis virus nonstructural polypeptides in chicken embryo fibroblasts

The identification of eight previously undescribed polypeptides in chicken embryo cells infected with Sindbis virus is reported. Seven of these polypeptides were distinguishable from the virus structural polypeptides and their precursors by their molecular weights and tryptic peptide maps. The eighth was closely related to pE2 (Schlesinger and Schlesinger, 1973), a precursor to one of the virus particle glycoproteins. Pulse-chase experiments and the use of an inhibitor of proteolytic cleavage allowed a division of the seven nonstructural (NS) polypeptides into three stable end products (NS p89, NS p82, and NS p60) and four precursors (p230, p215, p150, and p76). The labeling kinetics after synchronous initiation of translation indicated that synthesis of the NS polypeptides started at a single site and showed that the order of the genes coding for the NS polypeptides was (5' leads to 3') NS p60, NS p89, and NS p82. Short-pulse experiments under conditions of both synchronized and nonsynchronized translation suggested that cleavage of the primary translation product of the NS genes occurred only after its synthesis was completed and that the first cleavage removed the C-terminal polypeptide. From these and other experiments, we propose a detailed scheme for the synthesis and processing of Sindbis virus NS polypeptides.

Rabies virus protein synthesis in infected BHK-21 cells

Rabies virus specific polypeptide synthesis was examined under hypertonic conditions, which selectively inhibit cellular protein synthesis. The rabies virus proteins (L, G, N, M1, M2) were synthesized throughout the course of infection, with little change in their relative rates of synthesis. The rates of synthesis of the G and M1 polypeptides were more sensitive to increasing osmolarity than those of the L, N, and M2 polypeptides. Extrapolation to isotonicity of the results obtained under hypertonic conditions indicated that the molar ratios of the polypeptides synthesized under normal conditions were 0.4 (L), 64 (G), 100 (N), 75 (M1) and 35 (M2). A high-molecular-weight polypeptide (190,000), designated polypeptide L, was repeatedly detected both in infected cells and in extracellular virus. The estimated number of L polypeptide molecules per virion was 33. The synthesis of a viral glycoprotein precursor, designated gp78, , preceded the appearance of the mature viral glycoprotein in infected cells labeled with [3H]glucosamine under isotonic conditions. In cells labeled under hypertonic conditions, little or no mature viral glycoprotein was detected, but a virus-specific glycoprotein with an electrophoretic mobility similar to that of gp78 was observed. This glycoprotein could be chased into mature viral glycoprotein when the hypertonic conditions were made isotonic. These results suggest that a reversible block of viral glycoprotein synthesis occurs under hypertonic conditions.

Sialic acid uptake by BHK cells and subsequent incorporation into glycoproteins and glycolipids

BHK cells can be grown in the presence of growth medium to which radiolabeled sialic acid has been added. After 24 h, 85 percent of the radioactivity in the cells is covalently bound to glycoproteins and glycolipids. No metabolism of the radiolabeled sialic acid could be detected.