Identification of immunologically cross-reactive proteins of Sindbis virus: evidence for unique conformation of E1 glycoprotein from infected cells

A duplex real-time reverse transcriptase polymerase chain reaction assay for the detection of St. Louis encephalitis and eastern equine encephalitis viruses

A duplex TaqMan real-time reverse transcriptase polymerase chain reaction (PCR) assay was developed for the detection of St. Louis encephalitis virus (SLEV) and eastern equine encephalitis virus (EEEV), for use in human and vector surveillance. The respective targets selected for the assay were the conserved NS5 and E1 genes of the 2 viruses. Because of the insufficient number of NS5 sequences from SLEV strains in the GenBank database, we determined the sequence of an approximately 1-kb region for each of 25 strains of SLEV to select primers and probes in a conserved region. Our assay has a sensitivity of 5 gene copies (gc)/reaction for EEEV and 10 gc/reaction for SLEV, and its performance is linear for at least 6 log(10) gc. The assay is specific and detected all strains of SLEV (69) and EEEV (12) that were tested. An internal control ensures detection of efficient nucleic acid extraction and possible PCR inhibition.

Monoclonal antibodies that cross-react with the E1 glycoprotein of different alphavirus serogroups: characterization including passive protection in vivo

A panel of four monoclonal antibodies (mAb) were produced that cross-react with representatives of two different togavirus serogroups, namely sindbis (SIN) and Semliki Forest (SF) viruses, by ELISA and ADCMC assays. Three of these mAb, IgG2a and IgG2b isotypes, passively protected C3H/Hej mice against 10 and 100 LD50 of SF challenge and one, IgM, did not protect against either challenge dose, or even at 1 LD50. All these mAb were cross-reactive with the E1 glycoprotein of the viruses by immunoblotting in which three different patterns of reactivity were evident, suggesting that three epitopes were involved. The patterns depended upon whether the mAb recognized E1 extracted from purified virions or infected cells and whether SDS-PAGE and immunoblotting were done in the presence or absence of beta-mercaptoethanol. One mAb (IgM) reacted with nonreduced or reduced E1 from either virions or cells suggesting recognition of a linear epitope. The other three mAb reacted with nonreduced but not reduced E1 from virions suggesting that recognition depends upon conformational epitopes. These three mAb reacted also with nonreduced E1 extracted from SF-infected cells whereas only one reacted with nonreduced E1 extracted from SIN-infected cells.

Molecular pathogenesis of Sindbis virus encephalitis in experimental animals

In general, the analysis of a number of strains of Sindbis virus has revealed amino acid differences of potential importance for virulence at relatively few positions in the E2-glycoprotein. Only 10 amino acid changes are potentially implicated, and 9 of these 10 lie in the N-terminal half of the protein (Fig. 1.). Currently, there is strong evidence to implicate 3 of these positions (E2-55, -114, and -172) in virulence (Table V). As more recombinant viruses are prepared and analyzed, the evidence for or against the relevance of other changes should become apparent. As is generally true in alphaviruses, the E1 gene is more invariant than E2 and analysis of several strains has revealed amino acid changes at only four positions (Fig. 2). Two, 72, and 75, are just N-terminal to the hydrophobic segment postulated to be the site of fusion activity, suggesting the possibility that virus entry into the host cell could be affected by amino acid differences at these locations. The other two changes (at 237 and 313) are distant from the fusion site on the linear molecule, but changes at 313 do affect the pH fusion suggesting participation of this site in providing stability to the glycoprotein trimers. The mechanism of altered virulence associated with any amino acid change in the E1- or E2-glycoproteins has yet to be determined. The change at E2-114 associated with reduced virulence in mice shows reduced latency and increased virulence in BHK-21 cells in vitro. This suggests that some changes result in enhanced replication that is host cell-specific. There are several points in the replication cycle of Sindbis virus where the glycoproteins and their ability to undergo conformational changes play an important role in efficiency of replication. These include attachment, fusion, transport through the Golgi, assembly, and budding from the cell surface. Some steps in replication involve host cell proteins (Baric et al., 1983), so that there may be unique, unexplored interactions with neurons or ependymal cells leading to increased neurovirulence for mice that are not represented in the typical BHK, Vero, or chick embryo fibroblast cell culture system. The task now will be to determine why specific amino changes in the proteins of Sindbis virus cause such dramatic changes in the biological properties of the virus.

Infection of a human leukemia K-562 cell line with Semliki Forest virus

Infection of the human leukemia hematopoietic stem cell line, K-562, with Semliki Forest virus (SFV) can be characterized by three stages: (1) an early virus-proliferating stage lasting 1 to 4 days post-infection (pi) in which infectious virus is produced in high titers (10(3)pfu/cell) but in which there is minimal cytopathic effect. All cells appear viable by trypan blue dye exclusion, although they do not proliferate, and DNA and cell protein synthesis decrease to less than 3% of uninfected controls within 24 hours; (2) an intermediate stage extending from day 5 to about day 24-30 pi in which the amount of infectious virus declines to low levels. During this stage, viral protein synthesis decreases to undetectable levels, although viral gylcoproteins are readily demonstrated by immunofluorescence and by immunoblot; however, capsid protein appears to degrade within 21 days pi. Cell numbers remain constant but the viability of the non-proliferating cells determined by trypan blue exclusion could not be determined with confidence; (3) a final long-term stage in which viral glycoproteins, E1 and E2, are detectable by immunoblots and immunofluorescence for many months but the cells are metabolically inactive and do not synthesize viral proteins. These non-viable cells do not lyse for as long as 2 years.

Fusion of Semliki Forest virus infected Aedes albopictus cells at low pH is a fusion from within

Herein, it is shown for the first time that the mechanism of fusion followed in Aedes albopictus cells infected with Semliki Forest virus induced by low pH exposure is a "fusion from within". Several parameters were studied disclosing that the development of the fusion capacity of the cells is directly related to the synthesis of viral specific products. These findings were further substantiated by utilizing various chemicals to inhibit viral specific events during infection, protein synthesis and maturation. Removal of exogenous virions produced at 16 hours post infection by proteinase K digestion clearly revealed that the viral proteins located at the cell surface and not the exogenous virions were responsible for the fusogenic activity. The presence of these viral proteins at the cell surface was disclosed by immunofluorescence employing anti-SFV antibodies elicited in rabbits. Additional evidence for the participation of the viral proteins at the cell surface in the fusion reaction was obtained by Bromelaine digestion which inhibited the fusion and tunicamycin treatment which only partially inhibited the fusion but revealed the inevitable presence of the E1 protein.

Detection of immunologically cross-reacting capsid protein of alphaviruses on the surfaces of infected L929 cells

Hyperimmune, but not normal immune, monospecific antiserum made to capsid protein of Sindbis virus (SIN) was found to cause cytolysis equally well of both SIN- and Semliki Forest virus-infected L929 cells in antibody-dependent, complement-mediated cytotoxicity assays. The cell surface reactivity of the hyperimmune antiserum was also demonstrated by solid-phase radioimmune assays with unfixed infected cells or infected cells fixed with low concentrations of glutaraldehyde (0.025%) before reactivity with antisera. Higher concentrations of glutaraldehyde lowered the sensitivity of detection. Purified SIN capsid protein specifically inhibited antibody-dependent, complement-mediated cytotoxicity by the monospecific anti-capsid protein serum on SIN- and Semliki Forest virus-infected target cells. That hyperimmune anti-SIN serum also cross-reacts with capsid protein on the surface of Semliki Forest virus-infected cells was suggested by the fact that capsid protein inhibited cross-cytolysis in the antibody-dependent, complement-mediated cytotoxicity assay. The latter antiserum was collected after repeated injections of purified virions over a 9-month period. The results suggest that hyperimmune monospecific antisera made to SIN capsid protein or hyperimmune antisera to SIN or Semliki Forest virions detect homologous and cross-reacting capsid protein determinants on the surface of infected cells.

Detection of Semliki Forest virus in cell culture by use of an enzyme immunoassay with peroxidase-labeled monoclonal antibodies specific for glycoproteins E1 and E2

Four noncompeting monoclonal antibodies (MA) directed against either the E1 (UM 8.64 and 8.139) or E2 (UM 8.55 and 8.73) glycoprotein of Semliki Forest virus were purified and labeled with horseradish peroxidase. Each enzyme-labeled MA was tested alone and in combination with others for its sensitivity to detect virus-infected cells. Semliki Forest virus-infected L cells seeded as monolayers in 96-well plates were screened for the virus after incubation with enzyme-labeled MA and a substrate. In this system single enzyme-labeled MA even at high dilution (10(3.0) to 10(4.5] were able to detect virus-infected cells. The sensitivity of the test could be enhanced by combining two noncompeting MA (10(4.5) to 10(5.0]. Combinations of three and four MA were less effective, due to high absorbance values for noninfected cells. The threshold of virus defection was between 10(5) and 10(6) PFU/ml. This test is sensitive and specific and therefore may be useful for diagnostic purposes.