Rubella virus replication: effect of interferons and actinomycin D

The pathogenesis of Rift Valley fever

Rift Valley fever (RVF) is an emerging zoonotic disease distributed in sub-Saharan African countries and the Arabian Peninsula. The disease is caused by the Rift Valley fever virus (RVFV) of the family Bunyaviridae and the genus Phlebovirus. The virus is transmitted by mosquitoes, and virus replication in domestic ruminant results in high rates of mortality and abortion. RVFV infection in humans usually causes a self-limiting, acute and febrile illness; however, a small number of cases progress to neurological disorders, partial or complete blindness, hemorrhagic fever, or thrombosis. This review describes the pathology of RVF in human patients and several animal models, and summarizes the role of viral virulence factors and host factors that affect RVFV pathogenesis.

Correlation between rubella antibody levels and cytokine measures of cell-mediated immunity

Despite a safe and effective vaccine, endemic rubella remains a problem in developing countries. Isolated cases and outbreaks can occur in areas with high vaccine coverage. Individuals, especially pregnant women who remain unimmunized or do not seroconvert, are susceptible to infection and their infants are at risk for congenital rubella syndrome (CRS). Both humoral and cellular immune responses contribute to immune protection. Classically, immunity to rubella has been assessed through the detection of rubella-specific antibody titers. In this study we examined correlates of both humoral and cellular immunity in a large population of immunized young adults in Olmsted County, MN. We were unable to find any significant correlation between cytokine production after in-vitro rubella stimulation and serum antibody titers.

Rubella virus replication and links to teratogenicity

Rubella virus (RV) is the causative agent of the disease known more popularly as German measles. Rubella is predominantly a childhood disease and is endemic throughout the world. Natural infections of rubella occur only in humans and are generally mild. Complications of rubella infection, most commonly polyarthralgia in adult women, do exist; occasionally more serious sequelae occur. However, the primary public health concern of RV infection is its teratogenicity. RV infection of women during the first trimester of pregnancy can induce a spectrum of congenital defects in the newborn, known as congenital rubella syndrome (CRS). The development of vaccines and implementation of vaccination strategies have substantially reduced the incidence of disease and in turn of CRS in developed countries. The pathway whereby RV infection leads to teratogenesis has not been elucidated, but the cytopathology in infected fetal tissues suggests necrosis and/or apoptosis as well as inhibition of cell division of critical precursor cells involved in organogenesis. In cell culture, a number of unusual features of RV replication have been observed, including mitochondrial abnormalities, and disruption of the cytoskeleton; these manifestations are most probably linked and play some role in RV teratogenesis. Further understanding of the mechanism of RV teratogenesis will be brought about by the investigation of RV replication and virus-host interactions.

Rubella virus replication and links to teratogenicity

Rubella virus (RV) is the causative agent of the disease known more popularly as German measles. Rubella is predominantly a childhood disease and is endemic throughout the world. Natural infections of rubella occur only in humans and are generally mild. Complications of rubella infection, most commonly polyarthralgia in adult women, do exist; occasionally more serious sequelae occur. However, the primary public health concern of RV infection is its teratogenicity. RV infection of women during the first trimester of pregnancy can induce a spectrum of congenital defects in the newborn, known as congenital rubella syndrome (CRS). The development of vaccines and implementation of vaccination strategies have substantially reduced the incidence of disease and in turn of CRS in developed countries. The pathway whereby RV infection leads to teratogenesis has not been elucidated, but the cytopathology in infected fetal tissues suggests necrosis and/or apoptosis as well as inhibition of cell division of critical precursor cells involved in organogenesis. In cell culture, a number of unusual features of RV replication have been observed, including mitochondrial abnormalities, and disruption of the cytoskeleton; these manifestations are most probably linked and play some role in RV teratogenesis. Further understanding of the mechanism of RV teratogenesis will be brought about by the investigation of RV replication and virus-host interactions.

Rubella virus capsid protein induces apoptosis in transfected RK13 cells

Rubella virus is an enveloped positive-strand RNA virus that can cause mild to severe birth defects or death in an infected fetus. RV induction of programmed cell death, demonstrated in cell culture, has been implicated in the pathogenesis. The timing of apoptosis, 48 h p.i., suggested that accumulation of RV structural proteins might induce cell death in infected cells. Expression of RV structural proteins, capsid, envelope glycoproteins E1 and E2, in transiently transfected RK13 cells was as potent an inducer of cell death as RV infection. Immunofluorescence microscopy revealed that RV structural protein transfected cells exhibited the condensed nuclei typical of apoptotic cell death. Transfection with the capsid protein construct, but not E2 and E1, resulted in as much cell death as joint expression of all three RV structural proteins. Capsid required a membrane-anchoring domain to induce cell death, but a heterologous polypeptide fused to the capsid membrane anchor did not cause apoptosis. Deletion mutants demonstrated that the apoptosis-inducing activity resides in the N-terminal 170 amino acids of capsid. Though apoptosis-inducing capsid constructs appear to have an ER sub-cellular localization, disruption of the ER calcium storage capacity does not correlate with cell death. Mechanisms consistent with these results are discussed.

Rubella virus-induced apoptosis varies among cell lines and is modulated by Bcl-XL and caspase inhibitors

Rubella virus (RV) causes multisystem birth defects in the fetuses of infected women. To investigate the cellular basis of this pathology, we examined the cytopathic effect of RV in three permissive cell lines: Vero 76, RK13, and BHK21. Electron microscopy and the TUNEL assay showed that the cytopathic effect resulted from RV-induced programmed cell death (apoptosis) in all three cell lines, but the extent of apoptosis varied among these cells. At 48 h postinfection, the RK13 cell line showed the greatest number of apoptotic cells, the Vero 76 cell line was approximately 3-fold less, and BHK21 had very few. An increased multiplicity of infection and longer time postinfection were required for the BHK21 cell line to reach the level of apoptotic cells in Vero 76 at 48 h. Purified RV induced apoptosis in a dose-dependent fashion, but not UV-inactivated RV or virus-depleted culture supernatant. Specific inhibitors of the apoptosis-specific proteases caspases reduced RV-induced apoptosis and led to higher levels of RV components in infected cells. To address the role of regulatory proteins in RV-induced apoptosis, the antiapoptotic gene Bcl-2 or Bcl-XL was transfected into RK13 cells. Although a high level of Bcl-2 family proteins was expressed, no protection was observed from apoptosis induced by RV, Sindbis virus, or staurosporine in RK13 cells. In BHK21 cells, however, increased expression of Bcl-XL protected cells from apoptosis. The observed variability in apoptotic response to RV of these cell lines demonstrates that programmed cell death is dependent on the unique properties of each cell and may be indicative of how selective organ damage occurs in a congenital rubella syndrome fetus.

Proteolytic processing of rubella virus nonstructural proteins

The genomic RNA of rubella virus contains two long open reading frames (ORF), a 5'-proximal ORF that codes for the nonstructural proteins and a 3'-proximal ORF that encodes the structural proteins. The cDNA encoding the nonstructural protein ORF of the wild-type M33 strain of rubella virus has been obtained and sequenced. Comparison between the nonstructural proteins of the M33 and Therien strains of rubella virus revealed a 98% homology in nucleotide sequence and 98.1% in deduced amino acid sequence. To examine the processing of rubella virus nonstructural protein, the complete nonstructural protein ORF was expressed in BHK cells using a pSFV expression vector. Three nonstructural protein products (p200, p150, and p90) with molecular weights of 200, 150, and 90 kDa were identified using antisera raised against synthetic peptides corresponding to regions of the nonstructural proteins. p200 is the polyprotein precursor, while p150 and p90 are the cleavage products. Site-directed mutagenesis of the Cys-1151 residue (one of the catalytic dyad residues of the viral protease) and of the Gly-1300 residue (the viral protease cleavage site) abrogated protease activity and p200 precursor cleavage, respectively. Coexpression of mutant constructs in BHK cells indicated that rubella virus protease can function both in cis and in trans.

La autoantigen binding to a 5' cis-element of rubella virus RNA correlates with element function in vivo

Rubella virus genomic RNA contains a 5' stem-loop (5'(+) SL) which is required for efficient translation and replication. The La autoantigen previously was shown to bind this RNA sequence in vitro. Results reported here demonstrate that this cellular RNA-binding protein binds to the RV 5' SL RNA with sufficient specificity for the binding to occur in the presence of excess total cellular RNA. Further, the affinity of purified La for the RV sequence is similar to its affinity for known cellular substrates. To address the functional significance of La binding, mutant forms of the RV 5'(+) SL were analysed which bind La weaker or stronger than the native form. These three forms of the RV 5' SL were incorporated into RV-luciferase constructs which expressed luciferase activity in transient transfection. The level of expression from each construct correlated with the ability of its RV sequence to bind La. The detection of La/RV RNA complexes in infected cells further supported the physiological relevance of this interaction. Possible implications of autoantigen La interaction with RV RNA for rubella virus pathology and vaccine associated adverse reactions are discussed.

Autoantigens interact with cis-acting elements of rubella virus RNA

Rubella virus (RV) infections in adult women can be associated with acute and chronic arthritic symptoms. In many autoimmune individuals, antibodies are found targeting endogenous proteins, called autoantigens, contained in ribonucleoprotein complexes (RNPs). In order to understand the molecular mechanisms involved in the RV-associated pathology, we investigated the nature of cellular factors binding RV RNA and whether such RNPs were recognized by antibodies in infected individuals. Previously, we noted that cellular proteins associated with the RV 5'(+) stem-loop (SL) RNA are recognized by serum with Ro reactivity. To better understand the nature of the autoantigens binding RV cis-acting elements, serum samples from individuals with various autoimmune diseases were tested for their ability to immunoprecipitate RNPs containing labeled RV RNAs. A subset of serum samples recognizing autoantigen La, or Ro and La, immunoprecipitated both the RV 5'(+)SL and 3'(+)SL RNA-protein complexes. Autoantigens binding the RV 5'(+)SL and 3'(+)SL RNAs differed in molecular mass, specificities for respective RNA binding substrates, and sensitivity to alkaline phosphatase treatment. The La autoantigen was found to interact with the RV 5'(+)SL RNA as determined by immunological techniques and binding reactions with mixtures containing recombinant La protein. To test whether there is a correlation between La binding to an RV RNA element and the appearance of an anti-La response, we measured anti-La titers in RV-infected individuals. Significant anti-La activity was detected in approximately one-third of RV-infected individuals 2 years postinfection.

Identification of calreticulin as a rubella virus RNA binding protein

Previously, we observed that sequences at the 3' end of rubella virus (RV) genomic RNA that form a stable stem-loop structure are necessary for initiation of RNA replication. A cytosolic protein found in Vero 76 cells (simian origin) specifically bound to the 3' (+)-stem-loop sequence. In the present study, we have purified the RNA binding protein and identified it as a simian homologue of human calreticulin. The purified calreticulin binds to the RV RNA with specificity similar to the protein present in cytosolic extracts. Human calreticulin antibodies recognize several forms of simian calreticulin, one of which is phosphorylated in vivo. A 2-fold increase in phosphorylation of this form of calreticulin is observed in RV-infected cells. Recombinant human calreticulin can bind RV 3' (+)-stem-loop RNA only after undergoing in vitro phosphorylation. This binding activity is abrogated by pretreatment of phosphorylated recombinant human calreticulin with alkaline phosphatase. The RV RNA was also immunoprecipitated from RV-infected UV-crosslinked Vero 76 cells by using calreticulin antibodies. Our results show that phosphorylated calreticulin is an RNA binding protein and phosphorylation is necessary for this activity. Specific binding of calreticulin to the cis-acting element of RV RNA in vivo suggests a possible role for this interaction in viral replication.

Molecular biology of rubella virus

This chapter summarizes the present medical significance of rubella virus. Rubella virus infection is systemic in nature and the accompanying symptoms are generally benign, the most pronounced being a mild rash of short duration. The most common complication of rubella virus infection is transient joint involvement such as polyarthralgia and arthritis. The primary health impact of rubella virus is that it is a teratogenic agent. The vaccination strategy is aimed at elimination of rubella and includes both universal vaccination of infants at 15 months of age with the trivalent measles, mumps, rubella (MMR) vaccine and specific targeting with the rubella vaccine of seronegative women planning pregnancy and seronegative adults who could come in contact with women of childbearing age, although it is recommended that any individual over the age of 12 months without evidence of natural infection or vaccination be vaccinated. Medically, the current challenge posed by rubella virus is to achieve complete vaccination coverage to prevent resurgences.

Nucleotide sequence of the 24S subgenomic messenger RNA of a vaccine strain (HPV77) of rubella virus: comparison with a wild-type strain (M33)

A full-length cDNA clone for the 24S subgenomic mRNA of the vaccine strain (HPV77) of rubella virus has been isolated from a cDNA library made from the RNAs of infected cells. Starting from the first Met start codon, the 24S mRNA codes for a precursor protein of 1063 amino acids (aa). This precursor encodes a capsid protein of 300 aa, and two envelope proteins, E1 (481 aa) and E2 (282 aa). Both the E1 and E2 proteins are preceded by a stretch of 21 hydrophobic aa, characteristic of a signal peptide, and each has three putative glycosylation sites in the polypeptide chains. Comparison between the structural proteins of the vaccine and the wild-type (wt; M33) strains of rubella virus, revealed that the E2 protein of the vaccine strain differs, in its apparent Mr, by approx. 3 kDa, from the wt strain. The difference could be due to decreased glycosylation of the vaccine strain E2 protein, as revealed by [3H]mannose incorporation studies. Five single-aa changes in the structural proteins occurred during the attenuation process, one each in the capsid and the E1 protein and three in the E2 protein. The change of Thr-412----Ile in the E2 protein results in the loss of a putative glycosylation site at Asn-410, which offers a plausible explanation for decreased glycosylation of the E2 protein from the vaccine strain of rubella virus.

Rubella virus: mechanism of attenuation in the vaccine strain (HPV77)

The vaccine type (HPV77 strain) of rubella virus replicates slower and manifests a delayed appearance of cytopathic effect in Vero-76 cells as compared to wild-type virus (M33). The change in cytopathic effect coincides with the delayed appearance of both genomic and subgenomic RNA as well as viral structural proteins in the cell. The delay in the appearance of the viral proteins in the cells was also evident when the cells infected with the vaccine-type virus were treated with the lysosomotropic agent such as chloroquine. Binding studies using [35S]methionine-labeled virus showed that the vaccine-type virus bound to the cells poorly and the binding was not completely competed out with the cold virus.