Analysis of polypeptides synthesized in bovine respiratory syncytial virus-infected cells

A novel protein expression strategy using recombinant bovine respiratory syncytial virus (BRSV): modifications of the peptide sequence between the two furin cleavage sites of the BRSV fusion protein yield secreted proteins, but affect processing and function of the BRSV fusion protein

The bovine respiratory syncytial virus (BRSV) fusion (F) protein is cleaved at two furin cleavage sites, which results in generation of the disulfide-linked F(1) and F(2) subunits and release of an intervening peptide of 27 aa (pep27). A series of mutated open reading frames encoding F proteins that lacked the entire pep27, that contained an arbitrarily chosen 23 aa sequence instead of pep27 or in which pep27 was replaced by the amino acid sequences for the bovine cytokines interleukin 2 (boIL2), interleukin 4 (boIL4) or gamma interferon (boIFN-gamma) was constructed. Transient expression experiments revealed that the sequence of the intervening peptide influenced intracellular transport, maturation of the F protein and F-mediated syncytium formation. Expression of boIL2, boIL4 or boIFN-gamma in place of pep27 resulted in secretion of the cytokines into the culture medium. All mutated F proteins except the boIFN-gamma-containing variant could be expressed by and were functional for recombinant BRSV. Characterization of the cell culture properties of the recombinants demonstrated that the amino acid sequence between the two furin cleavage sites affected entry into target cells, direct spreading of virions from cell to cell and virus growth. Secretion of boIL2 and boIL4 into the medium of cells infected with the respective recombinants demonstrated that the F protein can be used to express secreted heterologous bioactive peptides or (glyco)proteins, which might be of interest for the development of novel RSV vaccines.

Characterization of bovine respiratory syncytial virus isolated in Brazil

This paper presents the first isolation of bovine respiratory syncytial virus in Brazil and its physicochemical, morphological and molecular characterization. The virus was isolated from 33 samples of nasotracheal secretions, successively inoculated into a Madin-Darby bovine kidney cell culture, which was characterized by physicochemical tests and morphological observation by electron microscopy. The Brazilian sample is an RNA pleomorphic, enveloped, thermolabile and non-hemagglutinating spicular virus. Reverse transcription, followed by nested polymerase chain reaction (nRT-PCR) assay was carried out using oligonucleotides B1, B2A, B3 and B4 for the fusion proteins (F) and B5A, B6A, B7A and B8 for the attachment protein (G). The nRT-PCR-F amplified a fragment of 481 bp corresponding to part of the gene that codes for protein F, whereas nRT-PCR-G amplified a fragment of 371 bp, in agreement with part of the G gene. The virus isolated from Brazilian samples in this study corresponded to the bovine respiratory syncytial virus, and RT-PCR proved to be useful for the diagnosis of bovine clinical samples.

Deletion and substitution analysis defines regions and residues within the phosphoprotein of bovine respiratory syncytial virus that affect transcription, RNA replication, and interaction with the nucleoprotein

The phosphoprotein (P) of bovine respiratory syncytial virus (BRSV) is a multifunctional protein that plays a central role in transcription and replication of the viral genomic RNA. To investigate the domains and specific residues involved in different activities of the P protein, we generated a total of 22 deletion and 17 point mutants of the P protein. These mutants were characterized using an intracellular BRSV-CAT minigenome replication system for the ability to (1) direct minigenome transcription, (2) direct minigenome replication, and (3) form complexes with nucleocapsid protein (N) and large polymerase protein (L). These studies revealed that all the regions of P protein except amino acids 41-80 are essential for minigenome transcription and replication. Interestingly, amino acids 41-60 appeared to contain sequences that negatively regulate transcription and replication. Analysis of the N- or C-terminal ends indicated that deletion of up to 3 amino acids from the N- or C-terminus completely ablated the replication, while leaving substantial residual transcription. Single amino acid substitutions within the N-terminal 4 or C-terminal 13 amino acids showed that substitution at position 2, 4, 234, 236, 238, 240, or 241 was highly inhibitory to both transcription and replication, whereas substitution at position 3 was highly inhibitory to replication while leaving substantial residual transcription. Substitution of serine residues at the C-terminus indicated that loss of phosphorylation sites did not appear to have any effect on transcription and replication. Coimmunoprecipitation of P-N and P-L complexes with P-specific antiserum revealed that substitution mutations at the N- or C-terminus did not affect binding to N and L proteins, except that substitution mutation at C-terminus position 234, 236, 238, 240, or 241 affected binding to N protein by 10-fold.

Mapping the domains on the phosphoprotein of bovine respiratory syncytial virus required for N-P and P-L interactions using a minigenome system

The interaction of bovine respiratory syncytial virus (BRSV) phosphoprotein (P) with nucleocapsid (N) and large polymerase (L) proteins was investigated using an intracellular BRSV-CAT minigenome replication system. Coimmunoprecipitation assays using P-specific antiserum revealed that the P protein can form complexes with N and L proteins. Deletion mutant analysis of the P protein was performed to identify the regions of P protein that interact with N and L proteins. The results indicate that two independent N-binding sites exist on the P protein: an internal region of 161-180 amino acids and a C-terminal region of 221-241 amino acids. The L-binding site was mapped to a region of P protein encompassing amino acids 121-160. The data suggest that N and L protein binding domains on the P protein do not overlap.

Bovine respiratory syncytial virus nonstructural proteins NS1 and NS2 cooperatively antagonize alpha/beta interferon-induced antiviral response

The functions of bovine respiratory syncytial virus (BRSV) nonstructural proteins NS1 and NS2 were studied by generation and analysis of recombinant BRSV carrying single and double gene deletions. Whereas in MDBK cells the lack of either or both NS genes resulted in a 5,000- to 10,000-fold reduction of virus titers, in Vero cells a moderate (10-fold) reduction was observed. Interestingly, cell culture supernatants from infected MDBK cells were able to restrain the growth of NS deletion mutants in Vero cells, suggesting the involvement of NS proteins in escape from cytokine-mediated host cell responses. The responsible factors in MDBK supernatants were identified as type I interferons by neutralization of the inhibitory effect with antibodies blocking the alpha interferon (IFN-alpha) receptor. Treatment of cells with recombinant universal IFN-alpha A/D or IFN-beta revealed severe inhibition of single and double deletion mutants, whereas growth of full-length BRSV was not greatly affected. Surprisingly, all NS deletion mutants were equally repressed, indicating an obligatory cooperation of NS1 and NS2 in antagonizing IFN-mediated antiviral mechanisms. To verify this finding, we generated recombinant rabies virus (rRV) expressing either NS1 or NS2 and determined their IFN sensitivity. In cells coinfected with NS1- and NS2-expressing rRVs, virus replication was resistant to doses of IFN which caused a 1,000-fold reduction of replication in cells infected with wild-type RV or with each of the NS-expressing rRVs alone. Thus, BRSV NS proteins have the potential to cooperatively protect an unrelated virus from IFN-alpha/beta mediated antiviral responses. Interestingly, BRSV NS proteins provided a more pronounced resistance to IFN in the bovine cell line MDBK than in cell lines of other origins, suggesting adaptation to host-specific antiviral responses. The findings described have a major impact on the design of live recombinant BRSV and HRSV vaccines.

Bovine respiratory syncytial virus (BRSV): a review

Bovine respiratory syncytial virus (BRSV) infection is the major cause of respiratory disease in calves during the first year of life. The study of the virus has been difficult because of its lability and very poor growth in cell culture. However, during the last decade, the introduction of new immunological and biotechnological techniques has facilitated a more extensive study of BRSV as illustrated by the increasing number of papers published. Despite this growing focus, many aspects of the pathogenesis, epidemiology, immunology etc. remain obscure. The course and outcome of the infection is very complex and unpredictable which makes the diagnosis and subsequent therapy very difficult. BRSV is closely related to human respiratory syncytial virus (HRSV) which is an important cause of respiratory disease in young children. In contrast to BRSV, the recent knowledge of HRSV is regularly extensively reviewed in several books and journals. The present paper contains an updated review on BRSV covering most aspects of the structure, molecular biology, pathogenesis, pathology, clinical features, epidemiology, diagnosis and immunology based on approximately 140 references from international research journals.

Mutational analysis of the bovine respiratory syncytial virus nucleocapsid protein using a minigenome system: mutations that affect encapsidation, RNA synthesis, and interaction with the phosphoprotein

The nucleocapsid (N) protein of bovine respiratory syncytial virus (BRSV) is a multifunctional protein that plays a central role in transcription and replication of viral genomic RNA. To investigate the domains and specific residues involved in different N activities, we generated a total of 27 deletion and 12 point mutants of the N protein. These mutants were characterized using an intracellular BRSV-CAT minigenome replication system for the ability to (1) direct minigenome RNA synthesis, (2) direct minigenome encapsidation, and (3) form a complex with the phosphoprotein (P). The mutations tested were defective in synthesis of RNA from the BRSV-CAT minigenome template with the exception of the following: a deletion involving the first N-terminal amino acid and mutations involving conservative substitution at the second amino acid and at certain internal cysteine residues. Micrococcal nuclease enzyme protection assays showed that mutations involving amino acids 1-364 of the 391-amino-acid N protein prevented minigenome encapsidation. Thus the BRSV N protein has a C-terminal, 27-amino-acid tail that is not required for encapsidation. Interestingly, two of the mutations that ablated encapsidation did not greatly affect RNA synthesis; the mutant involving deletion of the N-terminal amino acid and the mutant involving a substitution at position 2. This finding indicates that the formation of a nucleocapsid sufficient to protect the RNA from nuclease is not required for template function. Coimmunoprecipitation of N and P using N- or P-specific antiserum revealed two regions of the N protein that are important for association with the P protein: a central portion of 244-290 amino acids and a C-terminal portion of 338-364 amino acids.

Generation of bovine respiratory syncytial virus (BRSV) from cDNA: BRSV NS2 is not essential for virus replication in tissue culture, and the human RSV leader region acts as a functional BRSV genome promoter

In order to generate recombinant bovine respiratory syncytial virus (BRSV), the genome of BRSV strain A51908, variant ATue51908, was cloned as cDNA. We provide here the sequence of the BRSV genome ends and of the entire L gene. This completes the sequence of the BRSV genome, which comprises a total of 15,140 nucleotides. To establish a vaccinia virus-free recovery system, a BHK-derived cell line stably expressing T7 RNA polymerase was generated (BSR T7/5). Recombinant BRSV was reproducibly recovered from cDNA constructs after T7 RNA polymerase-driven expression of antigenome sense RNA and of BRSV N, P, M2, and L proteins from transfected plasmids. Chimeric viruses in which the BRSV leader region was replaced by the human respiratory syncytial virus (HRSV) leader region replicated in cell culture as efficiently as their nonchimeric counterparts, demonstrating that all cis-acting sequences of the HRSV promoter are faithfully recognized by the BRSV polymerase complex. In addition, we report the successful recovery of a BRSV mutant lacking the complete NS2 gene, which encodes a nonstructural protein of unknown function. The NS2-deficient BRSV replicated autonomously and could be passaged, demonstrating that NS2 is not essential for virus replication in cell culture. However, growth of the mutant was considerably slower than and final infectious titers were reduced by a factor of at least 10 compared to wild-type BRSV, indicating that NS2 provides a supporting factor required for full replication capacity.

Genetic analysis of the G and P genes in ungulate respiratory syncytial viruses by RNase A mismatch cleavage method

The G and P genes of bovine, ovine and caprine respiratory syncytial (RS) viruses were analyzed by RNase A one-dimensional fingerprinting, using A 51908 as the reference strain. Antisense G or P RNA probes of bovine RS virus strain A 51908 were hybridized to total RNA extracted from bovine turbinate cells infected with bovine, ovine or caprine RS virus strains. The RNA:RNA heteroduplexes were digested with RNase A and the resistant products were analyzed by gel electrophoresis. Comparative analysis of the cleavage patterns revealed heterogeneity among bovine, ovine and caprine RS virus isolates. Ovine RS virus strains generated RNA cleavage patterns more distantly related to the bovine or caprine RS virus strains, particularly in the G gene. Statistical analysis of the results obtained indicated that genetic differences between bovine and ovine viruses were larger, compared with the ones among bovine strains themselves. The same analysis also revealed a close genetic relation among bovine and caprine strains. These results are discussed in terms of ungulate RS virus genetic variation and vaccine development.

Analysis of the bovine respiratory syncytial virus fusion protein (F) using monoclonal antibodies

Seven monoclonal antibodies (MAbs) directed against bovine respiratory syncytial virus (BRSV) fusion (F) protein were produced and characterized by radioimmunoprecipitation and immunofluorescence assays. These seven MAbs together with the previously described MAbs (Beeler and Van Wyke Coelingh, 1989) to the F protein of human respiratory syncytial virus (HRSV) were used to study the antigenic variation of 12 strains of ungulate RSV. All except one MAbs specific for the HRSV-F protein reacted with ungulate RSV strains less efficiently, indicating that some epitopes are conserved, and others are not conserved on the F proteins of HRSV and BRSV strains. Three MAbs specific to the BRSV-F protein neutralized virus infectivity and reacted with all the ungulate RSV strains, suggesting that these epitopes are well conserved. Based on the reactivity of three other MAbs specific to the BRSV-F protein, ungulate RSVs could be grouped into two subgroups. The results indicated that there are antigenic variations in the F protein among ungulate RSV strains.

Development of nested PCR assays for detection of bovine respiratory syncytial virus in clinical samples

Two nested PCR assays were developed for the detection of bovine respiratory syncytial virus (BRSV). Primers were selected from the gene encoding the F fusion protein (PCR-F) and the gene encoding the G attachment protein (PCR-G). Biotinylated oligonucleotide probes, termed F and G, were selected for the hybridization of the respective PCR products. The sensitivities of the PCR-F and PCR-G assays were similar, both detecting 0.1 tissue culture infective dose of the virus. The PCR-F assay amplified all bovine strains and one human strain (RS32) tested. No cross-reactions were observed with nine heterologous respiratory viruses. PCR-F products of bovine and human RSV strains were discriminated by using endonuclease restriction enzyme ScaI, which specifically cleaved, products of BRSV. Oligonucleotide probe F was also specific for products of BRSV. The PCR-G assay detected all bovine strains and none of the human strains tested. A faint electrophoretic band was also observed with products of Sendai virus. However, probe G did not hybridize with this product, only with products of BRSV. Nasal swabs collected from cattle with no symptoms and cattle in the acute stage of respiratory disease were analyzed for BRSV by the immunofluorescence (IF) method and by the PCR-F and PCR-G assays. The virus was detected by the PCR assays in 31 of 35 (89%) samples tested. Only 23 samples (66%) were positive by the IF method, and these samples were also positive by both the PCR-F and PCR-G assays. The 31 samples detected as positive by PCR originated from cattle presenting clinical signs of acute respiratory disease; the four PCR-negative samples originated from clinically asymptomatic neighboring cattle. All sampled animals subsequently seroconverted and became reactive to BRSV. Thus, the detection of BRSV by PCR correlated with clinical observations and was considerably more sensitive (66 versus 89%) than IF. These results indicate that both nested PCR assays provide rapid and sensitive means for the detection of BRSV infection in cattle. Considering its higher specificity, the PCR-F assay can be recommended as the method of choice in the analysis of clinical specimens of BRSV.

Reliable confirmation of antibodies to bovine respiratory syncytial virus (BRSV) by enzyme-linked immunosorbent assay using BRSV nucleocapsid protein expressed in insect cells

The nucleocapsid (N) protein of bovine respiratory syncytial virus (BRSV) in the baculovirus expression system was evaluated as a source of antigen in an enzyme-linked immunosorbent assay (ELISA) for the detection of respiratory syncytial virus (RSV) antibodies. The recombinant N protein was purified from infected-cell extracts by sucrose gradient centrifugation and used in the ELISA for the detection of antibodies to various RSV strains. The ELISA was compared with the virus neutralization (VN) test for determining BRSV antibodies in 10 consecutive serum samples from four calves vaccinated with a live modified BRSV vaccine and from two nonvaccinated control calves. The ELISA compared favorably with the VN test for detecting serological responses. All serum samples which were positive in the VN test were also positive in the ELISA. None of the serum samples collected from the two nonvaccinated calves reacted in the ELISA. To determine the usefulness of the ELISA for epidemiological studies, 58 cattle serum samples were tested in the ELISA and the VN test. Approximately 94% (42 of 45) of field serum samples which were positive in the ELISA were also positive in the VN test. No case was found in which the ELISA result was negative and the VN test result was positive. Thirteen of the serum samples were negative in both methods. Our results indicate that the ELISA with the baculovirus-expressed N protein as an antigen is an efficient, sensitive, and specific method for detecting serum antibodies to RSV.

Structural difference in the fusion protein among strains of bovine respiratory syncytial virus

The polypeptides of different strains of bovine respiratory syncytial virus (RSV) were compared. Altered electrophoretic migrations were observed in the G, F, P, M and 22 kDa polypeptides. The molecular weight of the F2 fragment in human RSV (Long strain) and bovine RSV (A51908 and Md-X strains) was approximately 20 kDa whereas it was approximately 15.5 kDa in caprine RSV and bovine RSV (FS-1 and VC-464 strains). The size difference of the F2 subunit was due to difference in the extent of glycosylation.

Structure and sequence comparison of bovine respiratory syncytial virus fusion protein

The fusion (F) proteins of 10 strains of bovine respiratory syncytial virus (BRSV) were compared by radioimmunoprecipitation with fractionation on SDS-polyacrylamide gels. Two different molecular weights (15 kDa and 20 kDa) of the F2 proteins were demonstrated among the BRSV strains tested. To delineate the molecular basis for differences in the molecular weights of F2 subunits among the BRSV strains, the nucleotide sequences of the F genes of FS1 and VC464 strains were determined from cDNA clones. The deduced amino acid sequences were then compared to those of BRSV strains RB94, 391-2 and A51908. The F gene was highly conserved (> 95%) among BRSV strains. Comparison of the deduced F2 amino acid sequences showed that the strain with F2 subunits of 20 kDa had three N-linked glycosylation sites, whereas the strains with F2 subunits of 15 kDa had two N-linked glycosylation sites. Analysis of F2 subunits in their deglycosylated forms indicated that the difference in the molecular weights of the F2 subunits was due to the difference in the extent of glycosylation.

Gene junction sequences of bovine respiratory syncytial virus

The nucleotide sequences of seven gene junctions (N-P, P-M, M-SH, SH-G, G-F, F-M2 and M2-L) of bovine respiratory syncytial virus (BRSV) strain A51908 were determined by dideoxynucleotide sequencing of cDNAs from polytranscript mRNAs and from genomic RNA. By comparison with the consensus sequences derived from human respiratory syncytial virus (HRSV) mRNAs, gene-start and gene-end sequences were found in all BRSV mRNAs. There was a perfect match between the BRSV and HRSV in all gene-start sequences, except for the sequence of the SH gene which contained one nucleotide difference compared to HRSV A2; and the gene-start sequence of the L gene, which was one nucleotide shorter than the corresponding sequence of HRSV. Analysis of the intergenic regions showed a high degree of divergence in the nucleotide sequence between BRSV and HRSV. However, the length of the nucleotides in the intergenic sequences was similar for a given gene junction. As in the case of HRSV, the M2 and L genes of BRSV overlap by 68 nucleotides, suggesting a similar transcription attenuation mechanism. The sequences of the overlap, corresponding to the 3' end of the L gene, were almost identical between BRSV and HRSV.

Identification of subgroups of bovine respiratory syncytial virus

The occurrence of antigenic variation among nine isolates of bovine respiratory syncytial virus (BRSV) was determined by examining their reaction patterns to human respiratory syncytial virus (HRSV) subgroup A and B monoclonal antibodies (MAbs) by enzyme immunoassay and radioimmunoprecipitation with fractionation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis by using MAbs and polyclonal antisera to HRSV and BRSV. Shared epitopes were demonstrated on four of five structural proteins between BRSV and both subgroups A and B of HRSV. The nine isolates of BRSV showed different patterns of reactivity in enzyme immunoassays with panels of MAbs to HRSV subgroups A and B. Major variations in the molecular weights of the P (phosphoprotein) and F (fusion protein) proteins were demonstrated among the BRSV isolates tested. These results suggest that BRSV belongs to a different antigenic grouping than HRSV and that BRSV is composed of two distinct subgroups.