Murine type-C virus group-specific antigens: interstrain immunochemical, biophysical, and amino acid sequence differences

Downstream processing of cell culture-derived virus particles

Manufacturing of cell culture-derived virus particles for vaccination and gene therapy is a rapidly growing field in the biopharmaceutical industry. The process involves a number of complex tasks and unit operations ranging from selection of host cells and virus strains for the cultivation in bioreactors to the purification and formulation of the final product. For the majority of cell culture-derived products, efforts focused on maximization of bioreactor yields, whereas design and optimization of downstream processes were often neglected. Owing to this biased focus, downstream procedures today often constitute a bottleneck in various manufacturing processes and account for the majority of the overall production costs. For efficient production methods, particularly in sight of constantly increasing economic pressure within human healthcare systems, highly productive downstream schemes have to be developed. Here, we discuss unit operations and downstream trains to purify virus particles for use as vaccines and vectors for gene therapy.

Effect of chemical modification and fragmentation on antigenic determinants of internal protein p30 and surface glycoprotein gp70 of type C retroviruses

The effects of protein modification on the antigenic determinants of p30 and gp70 of type C retroviruses were investigated by using solid-phase competition radioimmunoassays. Proteins were modified by reduction with 2-mercaptoethanol and subsequent carboxymethylation of SH groups with iodoacetamide or by amidination of alpha and epsilon amino groups with methylacetimidate. The type-specific determinants of gp70 were found to be conformational in nature, as they were destroyed by these chemical modifications. Group- and interspecies-specific determinants of gp70 antigens, however, appear to be sequential and do not involve residues susceptible to these chemical reagents. Conformation-dependent type-specific determinants of p30 were affected only by methylacetimidate. Group- and interspecies-specific determinants of p30 are similar to those of gp70 in that they also appear to be sequential antigenic sites. Therefore, the broadly reactive group- and interspecies-specific determinants of gp70 and p30 can be followed into small peptides. Accordingly, a cyanogen bromide cleavage fragment derived from the carboxyl-terminal one-third of Rauscher leukemia virus p30 was found to carry group-specific determinants but no detectable interspecies-specific determinants. In contrast, a peptide obtained by limited trypsin cleavage of p30, which was derived from the NH(2)-terminal region of the protein, contained at least one of the interspecies determinants shared with feline leukemia virus p27.

The quantitation of cell surface antigens by antibody-complement mediated cytotoxicity: application to murine leukemia virus-infected mouse cells

51Cr-antibody-complement mediated cytotoxicity (ACC) can be used to quantify nanogram amounts of cell surface antigens. When pure antigen is available for calibration, inhibition of ACC by known amounts of antigen and by whole cells yields an estimate of the number of antigenic equivalents per cell. ACC is more suitable for this purpose than radioimmunoprecipitation assay. By ACC the number of antigenic equivalents of the oncornaviral proteins gp70 and p30 has been determined on the surface of various cells infected with Rauscher and Gross murine leukemia viruses.

Rauscher leukemia virus-induced tumor antigens: complete separation from gp70, p30 and H-2

The possibility that some or all of the viral proteins, gp70, p30, and the histocompatibility antigen, H-2, function as the tumor-specific transplantation antigen (TSTA) of the R-MuLV-induced leukemia, RBL-5, and also in the secondary in vitro induction of cytotoxic T lymphocytes (CTL), was investigated. The antigen was obtained by isolating the plasma membranes of RBL-5 cells and solubilizing with sodium deoxycholate (DOC) followed by gel filtration chromatography. A fraction containing excellent tumor-rejection activity but low amounts of gp70, p30 and H-2 was chromatographed on goat anti-gp 70 goat anti-p 30 and sheep anti-H-2b immunoaffinity columns. The data obtained indicate that gp 70, p 30 or H-2 do not function as TSTA of RBL-5 leukemia, individually or as a complex. Similarly, the antigen responsible for the specific secondary induction of CTL in vitro is distinct from these three proteins.

Post-translational modification of Rauscher leukemia virus precursor polyproteins encoded by the gag gene

Post-translational modifications of retrovirus gag gene-encoded polyproteins include proteolytic cleavage, phosphorylation, and glycosylation. To study the sequence of these events, we labeled JLS-V9 cells chronically infected with Rauscher murine leukemia virus in pulse-chase experiments with the radioactive precursors [35S]methionine, [14C]mannose, [3H]glucosamine, and [32P]phosphate. Newly synthesized gag polyproteins which incorporated label, and the modified products derived from them, were identified by immunoprecipitation of cell lysates with anti-p30 rabbit serum, followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography. Pulse-chase experiments were carried out in the presence as well as in the absence of tunicamycin, an inhibitor of glycosylation. Among the three major polyproteins synthesized in the absence of tunicamycin, two were found to be glycosylated but not phosphorylated. These were designated gPr80gag and gP94gag. Both shared identical [35S]methionine peptides with Pr65gag and p30. Of the two nonglycosylated precursors, Pr65gag and Pr75gag, only Pr65gag was found to be detectably phosphorylated, and Pr75gag could be readily identified only when glycosylation was inhibited. On the basis of these results, a scheme for the post-translational modification of gag polyproteins is proposed. According to this scheme the gag gene-encoded polyproteins are processed from a common precursor, Pr75gag, by two divergent pathways: one leading through the intermediate Pr65gag to internal virion components via cleavage and phosphorylation and the other via tunicamycin-sensitive mannosylation to the intermediate gPr80gag, which is further glycosylated to yield cell surface polyprotein gP94gag.

Specificity of response to viral proteins in horses infected with equine infectious anemia virus

Three structural proteins of equine infectious anemia virus were purified, labeled with 125I, and utilized in radioimmunoassays with horse sera and antisera to heterologous retroviruses. Whereas radioimmunoassay titers for the major protein, p25, were 500- to 1,000-fold higher than titers in immunodiffusion, for clinical purposes these two procedures were equivalent. Antibodies to two low-molecular-weight proteins, p12 and p10, were also found in infected horses, but with a lower frequency and lower titers. As a rule, only sera positive for p25 also contained antibody to p12 and p10. Antisera to the major structural protein of other retroviruses did not precipitate equine infectious anemia virus p25. These sera include antibody to mammalian type C viruses, bovine leukemia virus, visna virus, mouse mammary tumor virus, squirrel monkey retrovirus, and Mason-Pfizer monkey virus.

Tryptic peptide analysis of gag gene proteins of endogenous mouse type C viruses

Tryptic digests of the internal proteins p30, p15, p12, and p10 of mouse xenotropic, ecotropic, and amphotropic type C viruses were subjected to cation-exchange chromatography. Analysis of these maps revealed that the p30 proteins from representative isolates of all three viral subgroups were distinguishable. The p15 proteins were all unique. The p12 proteins of NZB xenotropic and wild-mouse amphotropic viruses were not identical and yielded peptide maps remarkably different from that of the ecotropic virus. The p10 proteins of xenotropic and ecotropic viruses were identical and were dissimilar to that of the wild-mouse amphotropic virus.

Molecular diversity among five different endogenous primate retroviruses

Genetically transmitted retroviruses of Old and New World monkeys include type C viruses isolated from baboons (M7), macaque (MAC-1), and owl monkeys (OMC-1) and type D viruses from langurs (PO-1-Lu) and squirrel monkeys (SMRV, M534). Each of these isolates is unrelated to the others by nucleic acid hybridization criteria and contains a unique array of virion-associated proteins which can be resolved by agarose gel filtration and polyacrylamide gel electrophoresis under denaturing conditions. The major structural protein of each virus has a distinct primary structure, as determined by two-dimensional tryptic peptide analysis, and is antigenically different from the others. The major virion phosphoproteins of endogenous primate type C viruses (pp15) are also different from those of type D viruses (pp13-pp14). Immunological and structural analyses show that the endogenous langur virus and the horizontally transmitted Mason-Pfizer virus of rhesus monkeys are closely related to one another, consistent with the sequence homology detected in their RNA genomes. Although certain radioimmunoassays detect interspecies antigenic determinants common to either the p30 or gp70 proteins of some of these viruses, no one assay has yet been designed which can detect all groups of endogenous primate retroviridae. The data lead to the conclusion that primates contain a minimum of three different sets of genetically transmitted type C and type D retroviral genes.

Structural markers on core protein p30 of murine leukemia virus: functional correlation with Fv-1 tropism

Tryptic peptide maps from more than 50 isolates of murine leukemia virus (MuLV) have shown that, in general, the structure of core protein p30 is highly conserved. However, a structurally variable region of p30 has been identified that is functionally associated with Fv-1 tropism. On the basis of this structural variability, MuLV strains can be classified as B-tropic, N-tropic, xenotropic, and/or as being derived from wild mice. Certain xenotropic viruses have a p30 like that of B-tropic MuLV and presumably would be subject to restriction in cells containing an Fv-In allele. Other p30 structural markers serve to distinguish the exogenous Friend, Moloney, and Rauscher viruses from endogenous MuLV. Furthermore, some MuLV strains have structural differences in their p30s that are useful as strain-specific markers. Finally, a possible sarcoma-associated alteration in the structure of p30 has been noted in the ml clone of Moloney murine sarcoma virus.

Amino- and carboxyl-terminal amino acid sequences of proteins coded by gag gene of murine leukemia virus

The amino- and carboxyl-terminal amino acid sequences of proteins (p10, p12, p15, and p30) coded by the gag gene of Rauscher and AKR murine leukemia viruses were determined. Among these proteins, p15 from both viruses appears to have a blocked amino end. Proline was found to be the common NH(2) terminus of both p30s and both p12s, and alanine of both p10s. The amino-terminal sequences of p30s are identical, as are those of p10s, while the p12 sequences are clearly distinctive but also show substantial homology. The carboxyl-terminal amino acids of both viral p30s and p12s are leucine and phenylalanine, respectively. Rauscher leukemia virus p15 has tyrosine as the carboxyl terminus while AKR virus p15 has phenylalanine in this position. The compositional and sequence data provide definite chemical criteria for the identification of analogous gag gene products and for the comparison of viral proteins isolated in different laboratories. On the basis of amino acid sequences and the previously proposed H-p15-p12-p30-p10-COOH peptide sequence in the precursor polyprotein, a model for cleavage sites involved in the post-translational processing of the precursor coded for by the gag gene is proposed.

Quantitative immunoelectrophoretic assay for murine oncornavirus p30: noncovalent facilitation by sodium dodecyl sulfate

Treatment of Rauscher murine leukemia virus lysates with the anionic detergent sodium dodecyl sulfate (SDS) at concentrations between 0.2 to 2.0% SDS per mg of viral protein greatly increased the anodal electrophoretic mobility of p30, the major internal polypeptide. SDS treatment did not reduce p30 antigenicity or cause nonspecific precipitation of normal serum proteins during subsequent immunoanalysis. The increased anodal electrophoretic mobility allowed assay of Rauscher murine leukemia virus p30 by Laurell rocket immunoelectrophoresis. An SDS-facilitated rocket immunoelectrophoresis assay is described that was highly reproducible (coefficient of variability, less than 3.0%) and capable of detecting 125 ng of viral protein. To our knowledge, this is the first report of a quantitative immunoelectrophoretic assay for an oncornavirus antigen. Since SDS binding is a general property of proteins, this method of noncovalently altering electrophoretic mobility appears to be applicable to other antigen-antibody systems.

Solubilized TSTA and the major viral structural proteins, gp70 and p30, in the immune response to murine leukemias induced by Friend and Rauscher virus

Antigens present in gp70 and p30 purified from Rauscher virus, were tested for immunogenicity in various assays measuring the anti-tumor immune response against lymphocytic leukemias of Friend (FBL-3) or Rauscher (RBL-5) virus origin. p30 had no effect on in vitro cytotoxicity against tumor cell targets mediated by either an anti RBL-5 serum or lymphocytes from animals immunized with FBL-3 cells. gp70 had had no effect on serum-mediated cytotoxicity but used at high concentrations it inhibited cell-mediated cytotoxicity. When used to immunize mice directly against subsequent challenge with the RBL-5 and FBL-3 leukemias, p30 had no discernible effect, while gp70 afforded partial protection against RBL-5 but only at high concentrations. Cell-free preparations of tumor membranes containing negligible amounts of gp70 were antigenically superior to gp70 in both the in vitro and in vivo assays. It is concluded that antigens on these purified proteins that are also expressed on tumor cells are not major targets of the anti-tumor immune response in this system.

Immunologic studies of the low molecular weight DNA binding protein of murine oncornaviruses

A low molecular weight, highly basic DNA-binding protein was purified from several oncornaviruses by the sequential procedures of gel filtration in guanidine-hydrochloride, DEAE-cellulose chromatography and affinity chromatography on single-stranded DNA sepharose. The binding protein from Rauscher and woolly monkey type-C viruses was the fastest migrating of the virion proteins in SDS-polyacrylamide gels and thus is designated p10 according to previous convention although our estimates of molecular weight were 8-9,000 daltons. The binding protein from these two viruses was resolved into two bands by acid-urea electrophoresis although only a single NH2 terminal amino acid was detected (S. Oroszlan, personal communication), thus indicating charge heterogeneity. Antibody to Rauscher virus p10 species-specific in gel diffusion and complement-fixation tests and did not exhibit cross-reactivity with other virion proteins. A DNA-binding protein was also detected in preparations of mouse mammary tumor virus. This purified protein had an apparent molecular weight of 12,500, was the second fastest migrating component in the virus preparations, and was antigenically unrelated to the mouse type-C virus p10.

Interspecies homology of RNA tumor virus proteins

We report the application of a highly sensitive column chromatographic technique to the comparison of tryptic peptide maps of some RNA tumor virus proteins. By combining microbore ion-exchange chromatography with a sensitive fluorescent assay using o-phthalaldehyde, we obtained high-resolution peptide maps starting with only microgram amounts of protein. Our discovery of coincident peptides from the 15,000 and 30,000 molecular weight proteins from murine and feline leukemia viruses supports serological evidence for interspecies antigenic determinants; coincident peptides were also found for the 10,000 molecular weight proteins from these viruses, although immunochemical data did not reveal interspecies determinants. The relatively large number of coeluting peptides found in the 15,000 and 10,000 molecular weight proteins is strong evidence for the existence of homology.

Equine infectious anemia virus: evidence favoring classification as a retravirus

Equine infectious anemia virus (EIAV) has a density of 1.154 g/cm3 in sucrose a high-molecular-weight RNA similar in size to Rauscher murine leukemia virus, and an internal virion reverse transcriptase that utilizes the synthetic RNA template poly(rA) but not the synthetic DNA template poly(dA), both with (dT)12 as primer. Although capable of utilizing manganese at low concentrations (approximately 0.1 mM), EIAV reverse transcriptase showed highest activity in the presence of 9 mM magnesium. The major protein of EIAV has a slightly lower molecular weight than the comparable protein of type C viruses and co-electrophoresed with 125I-labeled p25 of Mason-Pfizer monkey virus. A reference horse serum with antibodies to the major EIAV protein reacted only with EIAV and not with other type C or non-type C retraviruses. Reciprocally, a broadly reactive serum to type C virus p30s and specific sera to a variety of non-type C retraviruses did not react with EIAV. We recommend the inclusion of EIAV in the family Retraviridae.

Low-molecular- weight Rauscher leukemia virus protein with preferential binding for single-stranded RNA and DNA

A sensitive nitrocellulose filter assay that measures the retention of 125I single-stranded calf thymus DNA has been used to detect and purify DNA-binding proteins that retain a biological function from Rauscher murine leukemia virus. By consecutive purification on oligo (dT)- cellulose and DEAE-Bio-Gel columns and centrifugation in 10 to 30% glycerol gradients, RNA-dependent DNA polymerase has been separated from a second virion DNA-binding protein. The binding of this protein to DNA was strongly affected by NaCl concentration but showed little change in activity over a wide range of temperature or pH. After glycerol gradient purification, polyacrylamide gel electrophoresis of this protein showed one major band with a molecular weight of approximately 9,800. This protein binds about as well as to single-stranded Escherichia coli or calf thymus DNA or 70S type C viral RNA. The binding to 125I single-stranded calf thymus DNA is very efficiently inhibited by unlabeled single-stranded DNA from either E. coli or calf thymus and by 70S murine or feline viral RNA. Much larger amounts of double-stranded DNA are required to produce an equivalent percentage of inhibition. This protein, therefore, shows preferential binding to single-stranded DNA or viral RNA.

Species and interspecies radioimmunoassays for rat type C virus p30: interviral comparisons and assay of human tumor extracts

The major internal protein, p30, of rat type C virus (RaLV) was purified and utilized to establish intra- and interspecies radioimmunoassays. Three rat viruses were compared in homologous and heterologous intraspecies assays with no evidence of type specificity. The only heterologous viruses to give inhibition in these species assays were the feline (FeLV) and hamster (HaLV) type C viruses; these reactions were incomplete and required high virus concentrations. An interspecies assay using a goat antiserum prepared after sequentially immunizing with FeLV, RD 114, and woolly monkey virus p30's and labeled RaLV p30 was inhibited by all mammalian type C viruses, although preferentially by RaLV, FeLV, and HaLV. Thus, as in a previously reported assay developed with HaLV p30, rat, hamster, and cat p30's seem more closely related to each other than to mouse type C virus p30. High levels of specific antigen were found in all cell lines producing rat virus, whereas embryonic tissues from several rat strains and cell lines considered virus-free based on other tests were negative for p30. Rats bearing tumors containing Moloney murine sarcoma virus (RaLV) did not contain free circulating antibody to RaLV p30. Fifty-one human tumor extracts (including two tumor cell lines) were tested for activity in the RaLV species and 47 in the interspecies assays after Sephadex gel filtration and pooling of material in the 15,000- to 40,000-molecular-weight range. At a sensitivity level of 7 ng/ml (0.7 ng/assay) in the interspecies assay, all human tissues, with one exception, were negative. The one positive result is considered nonspecific based on proteolysis of the labeled antigen. Input tissue protein of the purified tumor extracts averaged 1.9 mg/ml with a range of less than 0.025 to 22 mg/ml. Tissues from NIH Swiss mice processed in the same manner were positive in the interspecies assay but negative in the intraspecies RaLV assay.

N-terminal amino acid sequences in the major capsid proteins of foot-and-mouth disease virus types A, O, and C

Sequences of amino acids at the N-termini of virus proteins VP1, VP2, and VP3 were determined for foot-and-mouth disease virus types A12 strain 119, O1Brugge and C3Resende. In the polyacrylamide gel electrophoresis system used to purify the proteins, VP3 migrated faster than VP1 or VP2; and in the virion, VP3 could be cleaved by trypsin into VP3a and VP3b. The N-terminal amino acids for each of the virus types were glycine in VP1, aspartic acid in VP2, and threonine in VP3. No divergences in sequence across the virus types were indicated until at least the fourth position in VP1, and the third in VP3. For virus types A12, O1 and C3, the sequences were, respectively: for VP1 (Gly-ile-phe,pro,val---), (Gly,ile,phe---) and Gly-ile-phe,ala---); for VP2 (Asp,X,met---), (Asp---) and Asp-leu---); and for VP3 (Thr-thr-ala-thr---), (Thr-thr-ser---) and (Thr-thr---). Unresolved mixtures of VP3a and VP3b, from either A12 or O1 viruses, appeared to have the N-terminal amino acids threonine, which is presumed to be the same threonine as in uncleaved VP3 and serine, which is generated by the tryptic cleavage.

Appearance of C-type virus-like particles after co-cultivation of a human tumor-cell line with rat (XC) cells

A serially progagated cell line (L104) was established by co-cultivation of alung adenocarcinoma (L-1) from a patient with concurrent chronic lymphocytic leukemia and XC, a non-producer rat line, known to carry the Rous sarcoma virus (RSV) genome. Karyotype of the L104 cultures revealed predominantly rat-like patterns; however, about 5% of the cells reacted with HLA antibodies and demonstrated human isozyme patterns. Electron microscopy of L104 cells revealed the presence of C-type particles budding from the cell membranes and in cytoplasmic vacuoles. Virus was not detected in any of the other normal lung, lung tumor or XC cells examined after co-cultivation with XC cells. The particles isolated from tissue culture fluids had the biochemical and biophysical characteristics common to other known mammalian C-type particles and were serologically related to the woolly monkey virus (WMV)/gibbon ape leukemia virus (GaLV) complex. Cross-hybridization between viral 3H-DNA transcripts and cellular RNAs from virus-infected cells clearly show the presence of sequences in the L104 cellular RNA related to both the GaLV/WMV group of viruses and rat viruses. Hydroxyapatite chromatography reveals however that the primate-related sequences in the viral RNA are indistinguishable from WMV in thermal elution profile. The host range of L104 virus appears to vary greatly from WMV in being xenotropic and, in the cell lines thus far tested appears, to infect only rat cells. The virus gave positive KC but negative XC assays. Inoculation of whole cells or cell-free supernatants into weaning hamster did not result in either solid tumors or leukemia. Co-cultivation of appropriate cell lines may represent an approach to the detection of latent viruses in human neoplasia.

Serological studies with low-molecular-weight polypeptides from the Moloney strain of murine leukemia virus

Major virion low-molecular-weight polypeptides were isolated from the Moloney strain of murine leukemia virus (type C) by agarose chromatography in 6M guanidine hydrochloride and were shown to have molecular weights of 15,000 (p15), 12,000 (p12), and 10,000 (p10) by their elution volumes and by their relative mobilities in sodium dodecyl sulfate-polyacrylamide gels. Each polypeptide could be iodinated and employed in double antibody radioimmunoassay procedures. All three polypeptides demonstrated a high degree of type-specificity in serologic immunoprecipitation analysis and in corresponding competition immunoassays. The p15 was immunologically distinct from other viron polypeptides including p12 and p10; the p12 and p10 were highly related to each other but not to other virion polypeptides and were even more type-specific than the p15 in serologic tests. Competition immunoassays with p15 and p10 indicate that the Moloney strain of MuLV is only a distant relative of the Friend-Rauscher group. The combined use of the Kirsten and Moloney low-molecular-weight polypeptide immunoassays suggest that xenotropic viruses constitute yet another group(s) of murine leukemia virus with distinct type-specific antigens, further expanding an already heterogeneous group of mouse type C viruses.

Interspecies antigenic determinants of the reverse transcriptases and p30 proteins of mammalian type C viruses

The major internal structural proteins (p30) of type C viruses isolated from several mammalian species were studied by radioimmunoprecipitation and competitive radioimmunoassays. Three antigenically distinguishable sets of interspecies determinants could be demonstrated by both methods. One set of determinants shared by viruses of rodent origin (mouse and rat) can be detected readily in feline leukemia viruses but not in other type C viral groups. The p30 proteins of murine viruses also contain a second discrete set of antigenic determinants related to those in infectious primate viruses and endogenous porcine viruses, but not detected in the feline leukemia virus group. The p30 proteins of endogenous viruses of baboons and domestic cats share yet a third set of cross-reactive determinants not detected in type C viruses isolated from other species of animals. Enzyme inhibition studies performed with antisera raised toward the reverse transcriptases of these same groups of type C viruses showed the same patterns of immunological cross-reactions as observed with p30 proteins. The antigenic cross-reactions between the homologous proteins of type C virus isolated from genetically distant animals may reflect transmission of type C viruses across species barriers.

A p60 polypeptide in the feline leukemia virus pseudotype of Moloney sarcoma virus with murine leukemia virus p30 antigenic determinants

A 60,000-dalton polypeptide (p60) has been identified in the feline leukemia virus (FeLV) pseudotype of Moloney sarcoma virus [MSV(FeLV)]. This polypeptide is present in the purified virus complex in concentrations greater than either the murine p30 or the feline p27. Purified p60 crossreacts immunologically with murine p30 group antiserum and contains several interspecies determinants, whereas the group specific determinant of FeLV p27 is not detected. Comparison of peptide fingerprints of p60 and murine p30 show many peptides in common. Limited digestion of p60 with either trypsin or chymotrypsin produced p30-35 and p20 peptides which retain the MuLV p30 group and interspecies antigenic activities. The p30 produced by both enzymes comigrates in polyacrylamide gels with the murine p30 of MSV(FeLV), thus suggesting that p60 may be an uncleaved precursor to p30.