Biosynthesis and chemical and immunological characterization of avian reticuloendotheliosis virus env gene-encoded proteins

First report on molecular characterization and phylogenetic analysis of Reticuloendotheliosis virus in Sudan

The reticuloendotheliosis virus (REV) group of retroviruses infects a wide range of avian species, including chickens, turkeys, ducks, geese, quail, and prairie chickens. The infection can result in immunosuppression, runting syndrome, high mortality, acute reticular cell neoplasia, or T- and/or B-cell lymphoma. One PCR positive chicken spleen sample obtained in a previous study in addition to one Marek's disease and three fowl pox (FP) vaccine samples were investigated in this study. A PCR assay was performed to detect the presence of REV provirus DNA in these samples. The results indicated the contamination of fowl pox virus and Marek's disease vaccines with REV. In addition, detection of integration of REV inside the genome of fowl pox vaccine was confirmed using primers corresponding to the FPV DNA regions flanking the REV integration site. Alignments of two sequences, one from the spleen tissue and the other from contaminated FP vaccine with REV, with other REV (env) gene sequences obtained from GenBank indicated their high similarity. Furthermore, phylogenetic analysis indicated that the partial part of (env) gene of our two isolates was closely related to variants from India, USA, Taiwan, and China. These results confirmed the contamination of commercial fowl pox and Marek's disease vaccines used in Sudan with REV. Phylogenetic analysis indicated that the partial part of (env) gene sequences from Sudan was closely related to variants from India, USA, Taiwan, and China.

First isolation of reticuloendotheliosis virus from mallards in China

Reticuloendotheliosis virus (REV) causes an oncogenic, immunosuppressive and runting syndrome in many avian hosts worldwide. REV infection has never been reported in mallard ducks, however. To identify REV infection in mallards, we collected 40 mallard duck samples from Jilin Province of China. In this study, the REV strain, DBYR1102, was first isolated from a mallard in China and identified by PCR, indirect immunofluorescence assay and electron microscopy. The gp90 gene and complete LTR of DBYR1102 were amplified and sequenced. Phylogenetic analysis based on gp90 genes of REV indicated that the REV strain DBYR1102 is closely related to strain HLJR0901 from northeastern China, the prairie chicken isolate APC-566, and REV subtype III, represented by chick syncytial virus. This new strain is distantly related to two other subtypes of REV, 170A and SNV. Phylogenetic analysis based on the LTR yielded information similar to that obtained with the gp90 genes. The results of this study not only expand our epidemiological understanding of REV in the wild birds of China but also demonstrate the potential role of wild waterfowl in REV transmission.

Identification of a conserved B-cell epitope on reticuloendotheliosis virus envelope protein by screening a phage-displayed random peptide library

Background

The gp90 protein of avian reticuloendotheliosis-associated virus (REV-A) is an important envelope glycoprotein, which is responsible for inducing protective antibody immune responses in animals. B-cell epitopes on the gp90 protein of REV have not been well studied and reported.

Methods and Results

This study describes the identification of a linear B-cell epitope on the gp90 protein by screening a phage-displayed 12-mer random peptide library with the neutralizing monoclonal antibody (mAb) A9E8 directed against the gp90. The mAb A9E8 recognized phages displaying peptides with the consensus motif SVQYHPL. Amino acid sequence of the motif exactly matched ²¹³SVQYHPL²¹⁹ of the gp90. Further identification of the displayed B cell epitope was conducted using a set of truncated peptides expressed as GST fusion proteins and the Western blot results indicated that ²¹³SVQYHPL²¹⁹ was the minimal determinant of the linear B cell epitope recognized by the mAb A9E8. Moreover, an eight amino acid peptide SVQYHPLA was proven to be the minimal unit of the epitope with the maximal binding activity to mAb A9E8. The REV-A-positive chicken serum reacted with the minimal linear epitopes in Western blot, revealing the importance of the eight amino acids of the epitope in antibody-epitope binding activity. Furthermore, we found that the epitope is a common motif shared among REV-A and other members of REV group.

Conclusions and Significance

We identified ²¹³SVQYHPL²¹⁹ as a gp90-specific linear B-cell epitope recognized by the neutralizing mAb A9E8. The results in this study may have potential applications in development of diagnostic techniques and epitope-based marker vaccines against REV-A and other viruses of the REV group.

The secondary structure of the R region of a murine leukemia virus is important for stimulation of long terminal repeat-driven gene expression

In addition to their role in reverse transcription, the R-region sequences of some retroviruses affect viral transcription. The first 28 nucleotides of the R region within the long terminal repeat (LTR) of the murine type C retrovirus SL3 were predicted to form a stem-loop structure. We tested whether this structure affected the transcriptional activity of the viral LTR. Mutations that altered either side of the stem and thus disrupted base pairing were generated. These decreased the level of expression of a reporter gene under the control of viral LTR sequences about 5-fold in transient expression assays and 10-fold in cells stably transformed with the LTR-reporter plasmids. We also generated a compensatory mutant in which both the ascending and descending sides of the stem were mutated such that the nucleotide sequence was different but the predicted secondary structure was maintained. Most of the activity of the wild-type SL3 element was restored in this mutant. Thus, the stem-loop structure was important for the maximum activity of the SL3 LTR. Primer extension analysis indicated that the stem-loop structure affected the levels of cytoplasmic RNA. Nuclear run-on assays indicated that deletion of the R region had a small effect on transcriptional initiation and no effect on RNA polymerase processivity. Thus, the main effect of the R-region element was on one or more steps that occurred after the template was transcribed by RNA polymerase. This finding implied that the main function of the R-region element involved RNA processing. R-region sequences of human immunodeficiency virus type 1 or mouse mammary tumor virus could not replace the SL3 element. R-region sequences from an avian reticuloendotheliosis virus partially substituted for the SL3 sequences. R-region sequences from Moloney murine leukemia virus or feline leukemia virus did function in place of the SL3 element. Thus, the R region element appears to be a general feature of the mammalian type C genus of retroviruses.

Nonreciprocal pseudotyping: murine leukemia virus proteins cannot efficiently package spleen necrosis virus-based vector RNA

It has been documented that spleen necrosis virus (SNV) can package murine leukemia virus (MLV) RNA efficiently and propagate MLV vectors to the same titers as it propagates SNV-based vectors. Although the SNV packaging signal (E) and MLV packaging signal (Psi) have little sequence homology, similar double-hairpin RNA structures were predicted and supported by experimental evidence. To test whether SNV RNA can be packaged by MLV proteins, we modified an SNV vector to be expressed in an MLV-based murine helper cell line. Surprisingly, we found that MLV proteins could not support the replication of SNV vectors. The decrease in titer was approximately 2,000- to 20,000-fold in one round of retroviral replication. RNA analysis revealed that SNV RNA was not efficiently packaged by MLV proteins. RNA hybridization of the cellular and viral RNAs indicated that SNV RNA was packaged at least 25-fold less efficiently than MLV RNA, which was the sensitivity limit of the hybridization assay. The contrast between the MLV and SNV packaging specificity is striking. SNV proteins can recognize both SNV E and MLV Psi, but MLV can recognize only MLV Psi. This is the first demonstration of two retroviruses with nonreciprocal packaging specificities.

The immunodominant proteins of reticuloendotheliosis virus

The antigenic profiles of three REV prototype strains, CSV, SNV and REV-T and eight Israeli isolates were analysed by SDS-PAGE and immunoblotting with convalescent chicken serum, three mAbs, 11A25, 11C237 and 11C100, a rabbit antiserum to REV-T whole virus (Cui et al., 1986) and a rabbit antiserum to REV-A p30 gag protein (Tsai et al., 1985). Under both reducing (+DTT) and non-reducing conditions of SDS-PAGE, a major immunodominant 75-100 kDa band was shared by all strains examined. In contrast to the chicken serum that recognized both continuous and discontinuous epitopes on the 75-100 kDa band of all the isolates, the mAbs and the two rabbit sera behaved otherwise. Only the DTT-resistant epitopes on the 75-100 kDa band of REV-T were recognized by the rabbit antisera and the mAb 11C237, and only the DTT-labile epitopes of REV-T 75-100 kDa antigen were detected by mAb 11C100. The two mAbs 11A25 and 11C237 detected discontinuous epitopes of all the strains except SNV, while the rabbit antisera recognized the discontinuous epitopes on the 75-100 kDa band of all the 11 strains. The rabbit antisera and mAb 11C237 detected additional lower molecular weight proteins and the mAb 11C237 also detected three proteins of high molecular weight under non-reducing conditions only. The p30 antiserum detected the low molecular weight proteins demonstrating their gag gene-encoded identity. From these results we conclude that the major immunogen of REV is the 75-100 kDa protein that contains both continuous and discontinuous epitopes. With this panel of antibodies the eight new isolates appeared to belong antigenically to REV subtype 3 (Chen et al., 1987).

Detection of contamination of vaccines with the reticuloendotheliosis virus by reverse transcriptase polymerase chain reaction (RT-PCR)

The reverse transcriptase polymerase chain reaction (RT-PCR) was applied to detect contamination of Marek's disease (MD) vaccine with reticuloendotheliosis virus (REV). The env primers were used for the 1st RT-PCR to amplify the DNA fragments of REV-A and -T. The rel and env primers were used for nested-PCR to confirm the sites deleted from REV-T and REV-A. Specific amplification products were detected in the 1st RT-PCR with these primers. By nested PCR with the env and the rel primer pairs, the products originating from REV-A and -T were identified. This system, using the env primer pairs, showed a specific amplification with several REV strains (REV-T, DE, CE, KI and 0202), but no amplified product was detected with MDV, NDV, IBV or ILTV. The 1st RT-PCR detected the virus in a concentration of 10(3) in 50% fluorescent antibody infectious dose per ml (FAID50/ml) and the nested PCR detected 10(1) FAID50/ml virus. The sensitivity of the RT-PCR system was found to be higher than that of the FA assay. This system provides a rapid, sensitive and specific method for detection of contamination of MD vaccines with REV-RNA, and it may be applied for quality control of live vaccines.

Fowlpox virus recombinants expressing the envelope glycoprotein of an avian reticuloendotheliosis retrovirus induce neutralizing antibodies and reduce viremia in chickens

Eight stable fowlpox virus (FPV) recombinants which express the envelope glycoprotein of the spleen necrosis virus (SNV) strain of reticuloendotheliosis virus (REV), an avian retrovirus, were constructed. These recombinants differ in the genomic location of the inserted genes, in the orientation of the insert relative to flanking viral sequences, and in the promoter used to drive expression of the env gene. Of these variables, promoter strength seems to be the most crucial. The P7.5 promoter of vaccinia virus, which is commonly used in the construction of both vaccinia virus and FPV recombinants, resulted in lower levels of expression of the envelope antigen in infected chicken cells compared with a strong synthetic promoter, as determined by immunofluorescence and enzyme-linked immunosorbent assay. Two peptides encoded by the env gene, the 21-kDa transmembrane peptide and a 62-kDa precursor, were detected by immunoprecipitation of labeled proteins from cells infected with recombinant FPVs, using monoclonal antibodies against REV. These peptides comigrated with those precipitated from REV-infected cells. One of the recombinants (f29R-SNenv) was used for vaccination of 1-day-old chickens. Vaccinated chicks developed neutralizing antibodies to SNV more rapidly than did unvaccinated controls following SNV challenge and were protected against both viremia and the SNV-induced runting syndrome.

Reticuloendotheliosis type C and primate type D oncoretroviruses are members of the same receptor interference group

The reticuloendotheliosis viruses (REVs), originally isolated from avian species, constitute a group of retroviruses which are more closely related to mammalian retroviruses than to other avian retroviruses. The envelope glycoproteins of members of the REV group display a striking amino acid sequence identity with a group of primate oncoretroviruses which belong to a single receptor interference group and include all of the type D and some type C primate oncoretroviruses. Members of the REV group also have a broad host range which covers most avian cells and some mammalian cells, including those of simian and human origin. In view of this broad host range and the envelope sequence similarities, we investigated the cross-interference pattern between REV and primate virus groups to determine whether they utilized the same receptor. Superinfection experiments using a vector virus containing an Escherichia coli lacZ gene showed that reticuloendotheliosis and simian oncoretroviruses constitute a single receptor interference group on both human and canine cells and indicate that the viruses bind to the same receptor to initiate infection. These results suggest that this receptor binding specificity has been maintained over a wide range of retroviruses and may be responsible for the broad spread of these retroviruses between different orders of vertebrates.

N-linked glycosylation and reticuloendotheliosis retrovirus envelope glycoprotein function

Different properties of the spleen necrosis virus (SNV) envelope glycoprotein were analyzed following biosynthesis in the presence of glycosylation inhibitors. Tunicamycin, which inhibits all asparagine N-linked glycosylation, prevented intracellular processing and translocation to the cell surface of the envelope protein. In contrast, castanospermine or deoxymannojirimycin, which block glycosidase trimming of the early high-mannose chains and subsequent complex type N-glycosylation, did not inhibit proteolytic cleavage or cellular translocation. The ability of unglycosylated and partially glycosylated envelope protein to bind the viral receptor was assayed using an infection interference assay. Tunicamycin abrogated SNV envelope glycoprotein-induced receptor interference, whereas the trimming glycosidase inhibitors had no effect on interference. Similarly, tunicamycin but not the glycosidase inhibitors reduced the titers of released virus 100-fold. We conclude that carbohydrate trimming and complex N-glycosylation are not essential for envelope glycoprotein translocation, proteolytic cleavage, receptor binding, or infectivity, whereas cotranslational high-mannose N-glycosylation is essential for all of the SNV envelope glycoprotein properties tested. Syncytia formation can be induced following transfection into D17 cells of an envelope glycoprotein expression plasmid. Unlike virus particle infectivity, cell fusion is strongly inhibited by the glycosidase inhibitors.

Inhibition of human immunodeficiency virus infectivity by chloroquine

The effect of chloroquine, a drug known to affect intracellular exocytic pathways, was studied in two retroviral systems: human immunodeficiency virus (HIV-1) and avian reticuloendotheliosis virus (REV-A). With chloroquine treatment of virus-infected cells, significant size reduction of the cell- and virus-associated surface glycoproteins, gp90 of REV-A and gp120 of HIV-1, was observed. In the case of HIV-1, extracellular virions derived from treated cells contained very little gp120. Infectivity and reverse transcriptase assays of HIV-1 demonstrated that by chloroquine treatment the majority of the virions released was noninfectious and the total virus yield was also reduced. The data suggest that chloroquine inhibition of infectious virus production is most likely due to interference with terminal glycosylation in the trans-Golgi network.

Expression of avian reticuloendotheliosis virus envelope confers host resistance

We constructed two reticuloendotheliosis virus (REV) envelope gene expression plasmids, one containing the REV-A envelope gene, the other the spleen necrosis virus (SNV) envelope gene. Cell lines were generated by transfecting each of the REV envelope plasmids into D17 cells, a canine cell line. The levels of REV envelope glycoprotein in the cell lines were assayed by immunoprecipitating the envelope glycoproteins from lysates of cells that were labeled with [35S]methionine. Virological challenge assays determined the degree of resistance of each of the cell lines to REV-A or SNV infection. The expression of either envelope gene protected the cells from infection by either REV-A or SNV virus. Several cell lines were significantly more resistant to REV infection than the parental D17 cells, and two lines were 25,000-fold more resistant, approaching the resistance of REV-infected D17 cells to reinfection. The resistant cell lines were not able to confer resistance to susceptible cells by cocultivation. The level of resistance was correlated with the uniformity of expression of the REV envelope glycoproteins by the individual cells in a cell line and not with the absolute level of expression by the population of cells.

Site-directed cytotoxic antibody against the C-terminal segment of the surface glycoprotein gp90 of avian reticuloendotheliosis virus

The major mature env-gene products of avian reticuloendotheliosis-associated virus (REV-A) are the surface glycoprotein (gp90) and the transmembrane protein (gp20). We have previously reported that gp90 was detected in the REV-A virus by Western blot analysis as well as in the REV-A-infected cells by radioimmunoprecipitation with antibodies raised in rabbits against the gp90 C-terminal tridecapeptide which was predicted from the nucleotide sequence (Wilhemsen et al., J. Virol., 52, 172, 1984). We have now shown that this antibody detected antigens on the REV-A-infected cells by fluorescence-activated cell sorter (FACS) analysis, and conferred specific cytotoxic effects on the infected cells in the presence of rabbit complement using the chromium release assay. These results clearly indicate that the C-terminal epitope of gp90 is situated on the surface of the REV-A-infected cells and accessible to site-directed antibodies which cause cytotoxicity by activating the complement system. The possible in vivo roles of this antibody are discussed.

Novel glycosylation pathways of retroviral envelope proteins identified with avian reticuloendotheliosis virus

Previously, we identified two mature glycoproteins, gp90, the surface glycoprotein, and gp20, the transmembrane protein, from avian reticuloendotheliosis virus and an avian reticuloendotheliosis virus env gene-encoded intracellular polyprotein gPr77env, but the precise relationship of gPr77env to the mature envelope proteins was not determined (W.-P. Tsai, T.D. Copeland, and S. Oroszlan, Virology 155:567-583, 1986). In the present study, using metabolic labeling of viral proteins with [35S]cysteine, radioimmunoprecipitation, and carbohydrate structure analysis, we have identified a higher-molecular-weight endo-H-resistant env gene-encoded polyprotein designated gPr115env in addition to the endo-H-sensitive gPr77env. It appears that gPr77env is the primary polyprotein precursor, modified with mannosyloligosaccharides that are processed into sialic-acid-rich extraordinarily large complex-type carbohydrates (up to 17 kilodaltons for each N-linked site) on the gp90 domain but not on the gPr22 domain. In this process, gPr77env is converted into the apparently endo-H-resistant secondary polyprotein, gPr115env, which is rapidly processed into gp90 and gPr22. The proteolytic processing which occurs only after the appearance of an endo-H resistant precursor is now clearly demonstrated for a retrovirus. Some important aspects of carbohydrate structure, including the site-specific glycosylation, as well as the intracellular location and nature of the potential enzyme involved in the proteolytic cleavage of gPr115env are discussed.

Transformation of avian lymphoid cells by reticuloendotheliosis virus

Avian reticuloendotheliosis virus (REV-T) is the most virulent of all retroviruses, inducing an invariably fatal leukemia in chickens with a latent period of 7-10 days. Unlike avian cells transformed by other acutely transforming viruses, lymphoid cells transformed by REV-T are immortalized. Furthermore, in vitro derived, REV-T transformed cells which do not produce virus are tumorigenic and induce lethal reticuloendotheliosis when injected into histocompatible birds. Thus REV-T transforms its target cell both in vitro and in vivo. In addition this transformation is independent of any helper virus functions. Like other acute leukemia viruses, REV-T is replication-defective and must co-replicate with a reticuloendotheliosis associated virus (REV-A). During evolution, a substantial portion of its genome has been deleted and replaced with a host-derived genetic sequence, designated v-rel. Presumably, the v-rel oncogene was transduced from a normal turkey DNA locus, c-rel. There are 9 regions of homology between c-rel and v-rel, however, several differences exist between these genes, suggesting that transformation by REV-T results from the production of an altered v-rel protein. The v-rel sequence is distinct from other known oncogenes and encodes a 57-kDa phosphoprotein. In REV-T transformed cells, this pp57v-rel protein is localized in the cytoplasm. The product of the v-rel oncogene is present at a low level, representing only about 0.003% of total methionine-labelled protein. In addition, pp57v-rel is relatively stable, having an estimated half-life of 4-10 h. The v-rel protein when purified close to homogeneity is complexed with a 40-kDa cellular phosphoprotein in transformed lymphoid cells and possesses serine kinase activity. This review discusses the molecular aspects of transformation by REV-T in the context of other oncogene-encoded proteins.