Proteins of the murine C-type RNA tumour viruses: isolation of a group-specific antigen by isoelectric focusing

Modified radioimmunoassay for murine sarcoma-leukemia virus group-specific antigen

Iodination of disrupted Moloney strain murine sarcoma-leukemia virus resulted in labeled group-specific (gs) protein which was subsequently purified on an isoelectrofocusing column. This iodinated purified gs antigen, prepared from a relatively small quantity of purified virus, was used in a radioimmunoassay. A radioimmunoassay inhibition method was developed so that antibody specific for mammalian C-type gs antigen could be measured in undiluted or low dilutions of test serum without altering the known reagents of the test. The gs antigen isolated from purified Moloney strain murine sarcoma-leukemia virus has an isoelectric point (pH 5.95) which is significantly lower than that reported for other murine leukemia viruses.

Structural proteins of mammalian oncogenic RNA viruses: multiple antigenic determinants of the major internal protein and envelope glycoprotein

The antigenic determinants of two purified protein constituents of mammalian C-type RNA viruses, the major structural protein of about 30,000 daltons, and the membrane glycopeptides of about 70,000 daltons were examined by competition radioimmunoassay. By the appropriate choice of antiserum and competing proteins, it was possible to distinguish type-specific, group-specific, and interspecies determinants. Both of the viral constituents were found to contain each of these three classes of antigens. The results suggested that the majority of the determinants of the major structural protein were group specific, 5% to 30% were interspecies, and a small fraction were type specific. In the case of the envelope glycopeptides, the chief determinants were type and group specific, and a small fraction were interspecies.

Properties of noninfectious and transforming viruses released by Murine sarcoma virus-induced hamster tumor cells

The cell culture lines HTG2 and HTG3 were established from a transplantable hamster tumor induced by a murine sarcoma virus (strain Gz-MSV) after 17 and 60 in vivo passages, respectively. The viruses released by these two cell lines markedly differ in morphology, antigenic composition, infectivity, transforming ability, and enzymatic activity. HTG2 virions contained the sarcoma genome but were noninfectious for mouse and hamster cells (S+H-virus). HTG3 virions transformed hamster but not mouse cells. Whereas HTG2 cells and its virus contained murine type C virus gs-1 antigen, all HTG3 clonal lines expressed both murine and hamster type C virus gs-1 antigens. The RNA-dependent DNA polymerase activity of HTG2 virus was very low, whereas that of HTG3 virus was relatively high. HTG2 virions contained electron-lucent centers only. HTG3 virus consisted of the expected mixture of virions with electron-dense and electron-lucent centers. Many broken or incomplete virions were present in both viruses. HTG2 virus is a noninfectious "defective" sarcoma virus without detectable helper virus. Data obtained in these experiments suggest that HTG3 virus is a hamster type C virus pseudotype of Gz-MSV (Gz-MSV [HaLV]). The genome of Gz-MSV is capable of antigenic expression in heterologous cells and in the presence of heterologous viruses. Attempts to chemically activate hamster type C virus (HaLV) from HTG2 cells were unsuccessful. The HTG1 cell culture line, established from another Gz-MSV-induced hamster tumor, initially released a virus indistinguishable from the HTG2 virus. After in vitro passage, spontaneous activation of HaLV occurred in HTG1 cells, and the resultant Gz-MSV (HaLV) had properties similar to those of the HTG3 virus.

Feline leukemia and RD-114 virus group-specific proteins: comparison of amino terminal sequence

The major internal virion polypeptide from feline and RD-114 type C viruses has been subjected to amino terminal sequence analyses with the Beckman automated sequencer. These proteins, as well as their homologs in rat and mouse viruses, begin with the sequence prolylleucylarginyl (Pro-Leu-Arg). Virus RD-114 differs from conventional feline type C viruses that show about 80 percent relatedness based on calculation of the minimum number of base changes to give equivalent coding for the protein segments analyzed. In addition, insertion of a gap in the RD-114 sequence is necessary to maintain positional homology. The difference between RD-114 and feline leukemia virus appears as great as the difference between mouse type C viruses and either of these two viruses. Thus, even though current evidence suggests that RD-114 is of feline origin, the sequence differences between RD-114 and conventional feline virus group-specific proteins is well beyond that based on one or a few point mutations.

A tumor antigen in tissue cultures derived from patients with Hodgkin's disease

Pellets obtained from supernatant fluids of monolayer cultures of cells from patients with Hodgkin's disease were fractionated by isopycnic density sedimentation. Material in a peak of specific gravity 1.15-1.21 g/ml from two Hodgkin's disease cultures was used to immunize rabbits, and the antisera obtained in this manner were reacted by agar-gel diffusion and immunoelectrophoresis with antigens from the purified peaks and the unfractionated pellets of centrifuged culture medium from all the cultures. The antisera reacted with material from 9 of 10 lines derived from spleens of patients with Hodgkin's disease, 2 of 8 cell lines from histologically negative spleens from patients with Hodgkin's disease and with 3 of 6 lymphoma cell lines not diagnosed as Hodgkin's disease. The antisera did not react with 12 cell cultures prepared from normal adult and fetal spleen and thymus. The antigen from cultures from patients with Hodgkin's disease was not found in material sedimenting at lower specific gravities; it resisted Tweenether solubilization, and migrated as a single band by immunoelectrophoresis. The antigen was not found in disrupted, noncultured tumor cells from patients with Hodgkin's disease, and an antiserum against noncultured, minced tumor tissue did not react with the Hodgkin's disease tissue-culture material. No immunological relationship was found between the tissue culture antigen and Epstein-Barr, RD-114, or Rauscher murine leukemia viruses. The Hodgkin's disease antigen may be a tumorrelated antigen or a component of an oncogenic virus.

Analysis of high-molecular-weight ribonucleic acid associated with intracisternal A particles

Intracisternal A particles, known primarily for their association with various tumors, have been shown to contain high-molecular-weight (HMW) ribonucleic acid (RNA) by velocity centrifugation, using linear glycerol gradients. This HMW RNA is sensitive to ribonuclease digestion and alkali treatment but is resistant to Pronase treatment. By a double-labeling experiment, HMW RNA was shown to be intrinsic to intracisternal A particles and not to have resulted from cytoplasmic polysomal RNA aggregation. By a reconstitution experiment, it was determined that the results were not due to C-type virus contamination. The synthesis of HMW RNA in intracisternal A particles is inhibited by actinomycin D and ethidium bromide. These observations emphasize that there are probably some taxonomic relationships between intracisternal A particles and oncogenic RNA viruses.

In situ hybridization of RD114-virus RNA with human metaphase chromosomes

In situ hybridization was used to detect regions of human metaphase chromosomes exhibiting complementarity with radioactively-labeled RNA isolated from RD114-virus. The region of homology, as identified by autoradiography, was confined to one D-group chromosome.

Wild-type gross leukemia virus: classification of soluble antigens (GSA)

By inhibiting techniques using indirect immunofluorescence tests and indirect immunoelectron microscopy, the G(Gross) soluble antigens (GSA) in the body fluids of AKR and C58 mice, which have a high incidence of spontaneous leukemia, were classified according to the known specificity of G antigens in the murine Gross leukemia system. GSA existing in the plasma of nonleukemic and leukemic AKR mice and in the ascitic fluid of transplanted AKR spontaneous leukemia K36 showed the several specificities corresponding to G cell surface antigens, GCSAa, b, and c, and type-specific and group-specific viral envelope antigens, tsVEA and gsVEA, respectively. However, the plasma of nonleukemic C58 mice lacks GSAc, which can be recognized by the G-typing mouse serum. GSA corresponding to G(IX) antigen was not detected in the body fluids.

Major group-specific protein of rat type C viruses

The major internal protein of a rat type C virus pseudotype of murine sarcoma virus, MSV(RaLV), was purified by isoelectric focusing (pI = 8.6) and used to prepare antibody in guinea pigs. The protein was identified by its reaction with antisera reactive with the mammalian type C virus group-specific (gs) antigenic determinant, gs-3. The guinea pig antisera mainly contained species-specific (gs-1) antibody for reactions in gel diffusion with other type C viruses were limited to those of rat origin, whereas in complement fixation tests heterologous reactions could be eliminated by use of appropriate antiserum concentrations without affecting homologous reactions. Guinea pig antisera against mouse, hamster, or cat gs-1 determinants did not react with MSV(RaLV) purified gs protein or with any of several other rat type C viruses.

Immunological studies on viral polypeptide synthesis in cells replicating murine sarcoma-leukemia virus

Antibodies to disrupted murine sarcoma-leukemia virus (MSV[MLV]) were used to study the synthesis of viral polypeptides in the transformed, virus-producing rat cell line 78A1. When cultures were labeled for 10 min with radioactive amino acids, about 9% of the total labeled proteins were precipitated with antiserum against purified MSV(MLV), and 3 to 4% were precipitated with the same antiserum after it had been absorbed with an extract from uninfected rat cells. The difference is due to the presence in the unabsorbed antiserum of antibodies to cellular proteins that are present in purified virus preparations. Intracellular viral proteins labeled with radioactive amino acids were isolated by immunoprecipitation and analyzed by electrophoresis in sodium dodecyl sulfate-polyacrylamide gels. The mobilities of intracellular viral polypeptides were identical to those of the purified virion. However, labeled polypeptides having electrophoretic mobilities lower than that of the major virion polypeptide, the group-specific antigen of molecular weight 31,000, were present in higher proportion in the total cell extract and in the membrane fraction than in the virion. These polypeptides appear to be of cellular origin for they were present only in minute amounts in the immunoprecipitates obtained with the absorbed serum. After a 10-min labeling period, radioactive proteins were assembled into extracellular virions rapidly for the first 4 hr followed by a slower rate. More than 2% of the total proteins of the cell labeled in a 10-min pulse were assembled into virions at the completion of a 24-hr chase. The high-molecular-weight polypeptides with the same mobilities as those detected in the immunoprecipitate of intracellular proteins were found in virions released from cells after a 10-min pulse. A larger proportion of these high-molecular-weight proteins was detected in virions released after short chase periods (30-120 min) than after longer chase periods (6-24 hr). Two possible interpretations of these data are that the high-molecular-weight cell-derived polypeptides (i) have a turnover rate higher than that of the major virion polypeptides or (ii) are cleaved proteolytically from the virions during long incubation in the culture media.

Purification and immunological characterization of the major internal protein of the RD-114 virus

The major internal protein of the RD-114 virus that appeared in a human tumor cell line, RD, after passage through fetal cats was purified by isoelectric focusing and used to prepare antibody in guinea pigs. The protein has a molecular weight of 33,500 in sodium dodecyl sulfate-polyacrylamide gels, and an isoelectric point of 9.1. This contrasts with a molecular weight of 25,000 and an isoelectric point of 8.3 obtained previously for the major protein of cat C-type viruses. The RD-114 protein carries determinants common to all mammalian C-type viruses but not species-specific determinants of cat, mouse, rat, hamster, chicken, and Russell's viper C-type viruses. In addition, two other proteins found in cat virus preparations were not detected in the RD-114 virus. Antiserum to the RD-114 viral protein proved highly specific by complement fixation and gel diffusion assays, and, in addition to the above mentioned mammalian viruses, the antiserum did not react with concentrates of the Woolly monkey or Gibbon ape C-type viruses. These data support the conclusion that the RD-114 virus is a human virus activated by passage through fetal cats. Until additional isolates are made from human cells, the identity of the RD-114 virus as a human C-type tumor virus cannot be fully established; however, this would be highly probable if the species-specific group-specific antigen of RD-114 could be demonstrated in human tumor and/or embryonic tissues.

Interactions of immunoglobulins G and M in the detection of the mammalian C-type virus cross-reactive antigen

The mammalian C-type tumor viruses share an antigenic determinant, gs-3, located on the major internal polypeptide of the virion. Detection of this determined in gel diffusion assays by antiserums prepared in rats by immunization with rat tumor homogenates carrying murine virus and serums prepared in a rabbit by immunization with purified murine gs antigen depended on antibodies present in the fractions containing immunoglobulins M and G. The immunoglobulin G fraction by itself precipitated only the homologous murine antigen. Neither fraction alone precipitated heterologous (cat, rat, or hamster) antigen (definition of the gs-3 reaction), while a mixture of the two fractions did. The gs-3 reaction was eliminated by treatment of the serums with beta-mercaptoethanol, also indicating a requirement for immunoglobulin M antibodies.

Host-gene control of C-type RNA tumor virus: inheritance of the group-specific antigen of murine leukemia virus

Genetic studies were aimed at elucidation of the mechanism of inheritance of the group-specific antigen of the murine leukemia virus. Two approaches have been used. First, a classical Mendelian hybridization experiment was performed with mice of the high-leukemia AKR strain and the low-leukemia C57L strain; the first filial (F(1)) generation hybrids, the second filial (F(2)) generation hybrids, and the backcrosses to the two parental strains. Second, a number of partially inbred lines derived from the F(2) generation of the same cross were used. The results of these studies demonstrate a specific genetic regulation of expression of group-specific antigens. Genes permissive to the expression of the antigen are dominant or semidominant to their nonpermissive alleles. It appears that two dominant genes carried by the AKR strain, but not by the C57L strain, are necessary for the presence of a complete virus; one of these controls the expression of group-specific antigen.

Host-gene control of C-type tumor virus-expression and tumorigenesis: relevance of studies in inbred mice to cancer in man and other species

C-type RNA viruses are tumorigenic in mice and other species. They are widespread and transmitted in an intimate relationship with host-cell genetic material. Tumorigenesis is no doubt a complex biological phenomenon, and involves both genetic and environmental factors. Among the genetic factors, single-locus determination is probably exceptional; yet, a number of genes are known in inbred mice and other species, including man, that can exert a major influence. Clearly, studies with inbred mice apply outside the murine model and justify generalizations on the etiology of cancer. The types of cancer observed in man are similar to those of inbred strains of mice, and their stochastic and often familial occurrences suggest that genetic factors play important roles in both man and mouse. A number of heritable disorders predipsose to tumorigenesis. Investigations of the relationship between various polymorphic phenotypes and human tumors should be intensified. For instance, a study of high-risk human-tumor groups could aid considerably not only in the recognition of etiological agents but also of underlying host-gene controlled mechanisms. Indeed epidemiological studies have already led to the identification of mutant genes that influence the incidence of human cancer. Such studies may be guided by findings in inbred strains of mice. There is good evidence in inbred mice that mutations in host chromosomes determine whether or not genes coding for RNA oncogene viral "functions" become expressed as cancer. Although an etiological role of C-type RNA virus has not yet been demonstrated in human cancer, studies of the interaction of viral and host-cell genomes in mice and other species, and the visualization of C-type virus in human tumors have provided good reasons for suspecting the presence in man of oncogenic genes (oncogenes) homologous to those of the C-type viral genomes, which are well studied in mice, hamsters, cats, rats, and chickens.

Specific inhibition of mammalian ribonucleic acid C-type virus deoxyribonucleic acid polymerases by rat antisera

Inhibition of the ribonucleic acid (RNA)- and deoxyribonucleic acid (DNA)-dependent DNA polymerase activities of mammalian C-type viruses was obtained with sera from rats bearing murine leukemia virus-induced transplant tumors. Polymerase activities of nonmammalian (viper) C-type virus and murine mammary tumor virus were not inhibited by such sera nor by serum from a rat immunized with the DNA polymerase of feline leukemia virus purified by isoelectric focusing. The latter serum appeared to inhibit preferentially the DNA-dependent DNA polymerase activity of mammalian C-type viruses showing no inhibition of RNA-dependent DNA synthesis.

The G-IX system. A cell surface allo-antigen associated with murine leukemia virus; implications regarding chromosomal integration of the viral genome

This report concerns a cell surface antigen (G(IX); G = Gross) which exhibits mendelian inheritance but which also appears de novo in cells that become productively infected with MuLV (Gross), the wild-type leukemia virus of the mouse. In normal mice, G(IX) is a cell surface allo-antigen confined to lymphoid cells and found in highest amount on thymocytes. Four categories of inbred mouse strains can be distinguished according to how much G(IX) antigen is expressed on their thymocytes. G(IX) (-) strains have none; in the three G(IX) (+) categories, G(IX) (3), G(IX) (2), and G(IX) (1), the amounts of G(IX) antigen present (per thymocyte) are approximately in the ratios 3:2:1. A study of segregating populations derived mainly from strain 129 (the prototype G(IX) (3) strain) and C57BL/6 (the prototype G(IX) (-) strain) revealed that two unlinked chromosomal genes are required for expression of G(IX) on normal lymphoid cells. The phenotype G(IX) (+) is expressed only when both genes are present, as in 129 mice. C57BL/6 carries neither of them. At one locus, expression of G(IX) is fully dominant over nonexpression (G(IX) fully expressed in heterozygotes). At the second locus, which is linked with H-2 (at a distance of 36.4 +/- 2.7 units) in group IX (locus symbol G(IX)), expression is semidominant (50% expression of G(IX) in heterozygotes); gene order T:H-2:Tla:G(IX). As a rule, when cells of G(IX) (-) mice or rats become overtly infected with MuLV (Gross), an event which occurs spontaneously in older mice of certain strains and which also commonly accompanies malignant transformation, their phenotype is converted to G(IX) (+). This invites comparison with the emergence of TL(+) leukemia cells in TL(-) mouse strains which has been observed in previous studies and which implies that TL(-) --> TL(+) conversion has accompanied leukemic transformation of such cells. So far the only example of G(IX) (-) --> G(IX) (+) conversion taking place without overt MuLV infection is represented by the occurrence of GCSA(-):G(IX) (+) myelomas in BALB/c (GCSA:G(IX) (-)) mice. Unlike the other Gross cell surface antigen described earlier, GCSA, which is invariably associated with MuLV (Gross) infection and never occurs in its absence, G(IX) antigen sometimes occurs independently of productive MuLV infection; for example, thymocytes and some leukemias of 129 mice are GCSA(-):G(IX) (+), and MuLV-producing sarcomas may be GCSA(+):G(IX) (-). The frequent emergence of cells of G(IX) (+) phenotype in all mouse strains implies that the structural gene coding for G(IX) antigen is common to all mice. There is precedent for this in the TL system, in which two of the Tla genes in linkage group IX appear to be ubiquitous among mice, but are normally expressed only in strains of mice carrying a second (expression) gene. It is not yet certain whether either of the two segregating genes belongs to the MuLV genome rather than to the cellular genome. This leaves the question whether MuLV may have a chromosomal integration site still debatable. But there is a good prospect that further genetic analysis will provide the answer and so elucidate the special relationship of leukemia viruses to the cells of their natural hosts.

Antibody to the RNA-dependent DNA polymerase of mammalian C-type RNA tumor viruses

Sera from rats bearing transplantable tumors induced by murine C-type tumor viruses contain an inhibitor of the activity of the viral RNA-dependent DNA polymerase. The inhibitor is shown to be an immunoglobulin (IgG) directed against the enzyme. Antiserum made in rabbits against partially purified murine leukemia virus polymerase also inhibits the polymerases of other mammalian C-type RNA-containing tumor viruses. Thus, the polymerases from different mammalian tumor viruses are antigenically related.

Species-specific and interspecific antigenic determinants associated with the structural protein of feline C-type virus

The group-specific protein of feline leukemia viruses has been isolated by isoelectric focusing. This protein has a molecular weight, calculated from sodium dodecyl sulfate-acrylamide gel electrophoresis, of 25,000 and forms a resolved band in such gels when disrupted virions are analyzed. This protein contains both species-specific antigenic determinants, as well as interspecific cross-reactive determinants, as an integral part of its structure.

Immune virolysis: effect of antibody and complement on C-type RNA virus

In the presence of envelope antibody and complement, the AKR strain of mouse leukemia virus was lysed, with the result that (i) the viral nucleic acid became susceptible to ribonuclease digestion and (ii) the internal group-specific antigen of the virus was released. The internal localization of the group-specific antigen is confirmed, the evidence being based on the failure of group-specific antibody to lyse virus in the presence of complement.