Rat C-Type Virus induced in Rat Sarcoma Cells by 5-Bromodeoxyuridine

HALOGENATED derivatives of uridine are highly effective inducers of latent C-type RNA viruses^1,2 and have been successfully used to induce viruses identical to, or similar to, the C-type RNA tumour viruses in mouse, rat and human cells^3–6. In previous experiments we used 5-bromodeoxyuridine (BrUdR) for induction of focus-forming virus in non-productive rat cells that have been transformed by mouse sarcoma virus². We describe here the induction of a C-type RNA virus in the cells of the rat tumour cell line XC, which contains the Rous sarcoma virus genome⁷. The induced virus possesses the group specific (gs) antigens of rat C-type viruses but not those of chicken C-type viruses.     

Oncogenicity of an Endogenous C-Type Virus Chemically Activated from Mouse Cells in Culture

RNA C-type viruses are known to exist in an unexpressed form in all mouse cells. These endogenous viruses can be activated from cells in tissue culture under certain experimental conditions. The biologic activity in vivo of one class of chemically-activated C-type virus has been studied. This virus is shown to induce a specific tumor, lymphatic leukemia, in mice of a low leukemic incidence strain.     

C-type lectins do not act as functional receptors for filovirus entry into cells

Cellular C-type lectins have been reported to facilitate filovirus infection by binding to glycans on filovirus glycoprotein (GP). However, it is not clearly known whether interaction between C-type lectins and GP mediates all the steps of virus entry (i.e., attachment, internalization, and membrane fusion). In this study, we generated vesicular stomatitis viruses pseudotyped with mutant GPs that have impaired structures of the putative receptor binding regions and thus reduced ability to infect the monkey kidney cells that are routinely used for virus propagation. We found that infectivities of viruses with the mutant GPs dropped in C-type lectin-expressing cells, parallel with those in the monkey kidney cells, whereas binding activities of these GPs to the C-type lectins were not correlated with the reduced infectivities. These results suggest that C-type lectin-mediated entry of filoviruses requires other cellular molecule(s) that may be involved in virion internalization or membrane fusion.     

Coexistence of Intraspecies and Interspecies Specific Antigenic Determinants on the Major Structural Polypeptide of Mammalian C-type Viruses

THE report of a shared viral antigen (termed gs-3) among mammalian C-type viruses from four species¹, extending an earlier report of cross reactivity between mouse and cat viral antigens², has far reaching implications in the search for human cancer viruses or their gene products. The report is confirmed both by the data presented here and also by the data obtained by another laboratory³. Our gel diffusion assays using various selected sera against mouse, hamster and cat crude and purified C-type viral antigens indicate that the cross reactive antigenic determinants are specifically present on the major structural polypeptide of C-type viruses. The polypeptide also carries species specific determinants. These conclusions are drawn from complement fixation and gel diffusion tests using six types of antisera (either individual sera or pools) prepared as described in Table 1.     

C-Type Virus Released from Cultured Human Rhabdomyosarcoma Cells

RD-114 virus, released from human rhabdomyosarcoma cells, has all the characteristics of a mammalian C-type virus. Immunological tests indicate that it differs from all known C-type viruses and is the most likely candidate for a human C-type virus yet described.     

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.     

Retrovirus expression in normal and pathogenic processes of man.

C-type viruses have been isolated from primates only on rare occasions. To date, an endogenous C-type virus of man has not been isolated. Nevertheless, the evidence for C-type viral expression in human tissue increases. Evidence for viral antigen expression during human gestation and the pathogenesis of systemic lupus erythematosus (SLE) is summarized. The data are discussed in connection with the current hypotheses that retroviruses may participate in both normal and abnormal biologic processes.     

C-Type Virus associated with Gibbon Lymphosarcoma

C-TYPE viruses have been established as the causal agents of leukaemia in murine and feline species and have been characterized^1,2. C-type virus is also probably associated with fibrosarcoma in non-human primates^3–6. To determine whether viruses with identical characteristics are associated with other neoplasms in simian species, we looked for C-type viruses in cases of leukaemia. A gibbon (Hylobates lar) with a disseminated tumour (later confirmed as lymphosarcoma) was made available to the Comparative Oncology Laboratory by Dr Malcolm Jones of the University of California, San Francisco Medical Center. The principal sites of involvement (lymph node, liver and bone marrow) were extensively overrun with massive neoplastic cells, which were predominantly prolymphocytic forms. Electron microscopy revealed C-type particles identical to those observed in vitro in sections from lymph nodes, liver, spleen and bone marrow.     

Purification of Large Amounts of Murine Ribonucleic Acid Tumor Viruses Produced in Roller Bottle Cultures

Murine ribonucleic acid tumor viruses and C-type virus particles are produced in relatively large quantities in roller bottle cultures. The viruses present in large volumes of culture fluids can be purified by a simple two-step procedure involving polyethylene glycol precipitation and equilibrium centrifugation in sucrose density gradients.     

Expression and detection of endogenous mouse C-type RNA viruses

Summary Many naturally occurring C-type RNA viruses are of endogenous origin. The genetic information for synthesizing these RNA viruses is present in the DNA of normal mouse cells, probably as part of their chromosomal DNA. Some C-type viruses infect mouse cells (homotropic virus), while others infect certain tissue culture cells from other species but not mouse fibroblasts (xenotropic virus). All mouse strains studied appear to contain endogenous xenotropic viral genomes. However, based on the regularity with which homotropic virus is detected, inbred mice can be divided into high, low, and nonvirus-yielding strains. Nucleic acid hybridization studies have shown that DNA from high virus strains contains several copies of the homotropic virus genome, while that from low virus strains contains fewer copies, and DNA from nonvirus strains lacks a significant portion of the homotropic virus genome. In vivo and in vitro genetic studies support the nucleic acid hybridization results. In addition, high virus mouse strains are more likely than low virus strains to release virus that will replicate efficiently in their own cells. Methods for the activation and detection of endogenous C-type virus in tissue culture are discussed. Presented at the Session in Depth on Endogenous Viruses in Cell Culture at the Twenty-fifth Annual Meeting of the Tissue Culture Association, June 1974.     

Endogenous C-Type Viruses of BALB/c Cells: Frequencies of Spontaneous and Chemical Induction

A sensitive biological assay has been developed for studying the frequency of virus induction from mouse cells containing poorly infectious endogenous C-type viruses. A low spontaneous frequency of virus activation of one but not a second endogenous virus of BALB/c cells has been demonstrated. The resistance of BALB/c cells to superinfection by the more inducible virus appears to be absolute, whereas resistance to the other virus is only relative. Thus, the evidence indicates that the BALB/c cell exerts differential effects at two levels of regulation on the expression of its biologically distinguishable endogenous C-type viruses.     

Counter-Immunoelectrophoresis: Rapid Method for Detecting Group-Specific Antigen and Antibodies Associated with Oncogenic Ribonucleic Acid Viruses

Counter-immunoelectrophoresis was used for the quantitation of group-specific antigen and antibodies of C-type ribonucleic acid leukemia and sarcoma viruses.     

Inhibitor of the DNA-dependent DNA Polymerase of Some RNA Tumour Viruses in Feline Sera

AN inhibitor of the RNA-dependent DNA polymerases^1,2 of mammalian C-type viruses was found in sera from rats bearing transplantable tumours, induced by murine C-type RNA tumour viruses^3,4. Partially purified polymerases of murine leukaemia virus³ and feline leukaemia virus (FeLV)⁴ were shown to be antigenic in rabbits and a rat, respectively. We have detected an inhibitor of the DNA-dependent DNA polymerase^5,6 of feline and murine C-type viruses in the sera of cats inoculated in utero and/or postnatally with the Gardner-Arnstein strain of feline sarcoma virus (FSV)⁷ and in the sera of cats bearing spontaneous sarcomas, lymphomas or carcinomas.     

Relationship of Reticuloendotheliosis Virus to the Avian Tumor Viruses: Nucleic Acid and Polypeptide Composition

The RNA content and polypeptide composition of reticuloendotheliosis virus (REV) was compared to that of C-type RNA tumor viruses. Two RNA species with approximate sedimentation values of 64S and 4S were observed after sucrose gradient centrifugation of RNA extracted from purified REV. The high-molecular-weight RNA species of REV sedimented slightly faster than that of the Bryan strain of Rous sarcoma virus (RSV). Although these characteristics were consistent with those of other C-type RNA tumor viruses, significant differences were observed when the polypeptide composition of REV was compared with that of RSV possessing envelope determinants of Rous-associated virus RAV-2 and RAV-3. Five polypeptides of which two were glycosylated were resolved by polyacrylamide gel electrophoresis. The major nonglycosylated polypeptide of REV did not comigrate with that of RSV (RAV-2)-RSV(RAV-3). The majority of the group-specific antigen reactivity resides in this major nonglycosylated polypeptide of avian tumor viruses and comigrates when proteins of several avian tumor viruses are subjected to coelectrophoresis. This difference in the migration of the major polypeptide of REV and RSV(RAV-2)-RSV(RAV-3) may explain the absence of avian tumor virus group-specific antigen in REV.     

Human macrophage C-type lectin specific for galactose and N-acetylgalactosamine promotes filovirus entry

Filoviruses cause lethal hemorrhagic disease in humans and nonhuman primates. An initial target of filovirus infection is the mononuclear phagocytic cell. Calcium-dependent (C-type) lectins such as dendritic cell- or liver/lymph node-specific ICAM-3 grabbing nonintegrin (DC-SIGN or L-SIGN, respectively), as well as the hepatic asialoglycoprotein receptor, bind to Ebola or Marburg virus glycoprotein (GP) and enhance the infectivity of these viruses in vitro. Here, we demonstrate that a recently identified human macrophage galactose- and N-acetylgalactosamine-specific C-type lectin (hMGL), whose ligand specificity differs from DC-SIGN and L-SIGN, also enhances the infectivity of filoviruses. This enhancement was substantially weaker for the Reston and Marburg viruses than for the highly pathogenic Zaire virus. We also show that the heavily glycosylated, mucin-like domain on the filovirus GP is required for efficient interaction with this lectin. Furthermore, hMGL, like DC-SIGN and L-SIGN, is present on cells known to be major targets of filoviruses (i.e., macrophages and dendritic cells), suggesting a role for these C-type lectins in viral replication in vivo. We propose that filoviruses use different C-type lectins to gain cellular entry, depending on the cell type, and promote efficient viral replication.     

Spontaneous expression of C-type virus in DBA/2 mice is associated with an increased rate of mortality, independent of neoplastic disease

Ecotropic C-type retroviruses isolated from both normal and dimethylbenzanthracene-treated DBA/2 mice could be classified into three major groups, Ea, Eb, and Ec, that differed in structure and biology. Weanling DBA/2 mice were generally free of viruses, but a fraction of adult individuals became virus positive and were apparently selectively associated with a high expression of the Eb viruses. Some of the ecotropic viruses from DBA/2 mice acted as exogenous pathogens. They caused viremia and a moderate incidence of leukemia when injected into C3H and ST/a mice. In addition, they caused an appreciable number of early deaths without signs of malignancy. To evaluate the natural role of the viruses, we studied the survival of spontaneously viremic and nonviremic DBA/2 mice. The viremic animals as a group were characterized by a significantly reduced life-span that was not related to neoplasia. These observations indicated that endogenous C-type retroviruses can be pathogenic without preselection of the host for disease. They also emphasize that endogenous viruses, like their exogenous counterparts, can have a broader pathogenic spectrum than normally appreciated.     

Retention of Antigen Specificity of Murine Sarcoma Viruses grown in Hamster Cells <Emphasis Type="Italic">in vitro</Emphasis>

TUMOURS can be induced in hamsters by the various strains of murine sarcoma virus (MSV)^1–6. Tumours differ, however, in the antigens which are expressed. Whereas the cell line HT-1, derived from early passages of a hamster tumour induced by the Moloney strain of MSV (M-MSV), contains no trace of infectious virus or virion antigen^2,7, tumours induced by the Harvey (H), Kirsten (Ki) and later passages of the M-MSV-(GLV) viruses have yielded sarcoma viruses with a hamster-specific host range^3–6,8 which do not share envelope^4–6,9 or group specific¹⁰ antigens with murine viruses. The HT-1 cell does retain the MSV genome which can be rescued by murine leukaemia viruses². Such rescued viruses are termed pseudo-types and contain the envelope and group-specific antigens of the rescuing virus. The virus preparation from tumours induced by M-MSV(GLV) differed from the other hamster-specific viruses in that a non-sarcomagenic C-type virus could be isolated from cultures infected beyond the cell transformation end point⁶. This virus was also hamster-specific in host range and antigenic properties and specifically interfered with cell transformation by the various hamster-specific virus strains⁹. This virus also shared an ether-stable virion-antigen with a C-type virus found in a lymphoma which occurred spontaneously in a hamster¹⁰. This shared antigen seems to be the principal structural polypeptide of hamster C-type viruses and is structurally similar but antigenically distinct from its mouse homologue (unpublished work of S. O., C. Foreman, G. K. and R. V. G.). These findings led us to propose that the hamster-specific non-sarcomagenic C-type virus was a hamster leukaemia virus (in the generic but not necessarily the pathological sense) and the virus is therefore designated HaLV^9,10. The hamster-specific sarcoma viruses are considered to be pseudotypes of MSV rescued in vivo by HaLV and are abbreviated accordingly; for example, M-MSV(HaLV) represents the hamster-specific sarcoma virus rescued from M-MSV induced tumours. This is plausible because HaLV is able to rescue the MSV genome from HT-1 cells⁶. (This change in the nomenclature has been made in order to reflect the antigenic composition of the hamster-specific virus more accurately. In addition, to indicate the virus rescued from M-MSV(GLV)-induced hamster tumours, a terminal G is added after the parentheses. This has been done only to distinguish it from the virus obtained from M-MSV induced hamster tumours, for there is no evidence of residual activity from GLV.)     

DC-SIGN: escape mechanism for pathogens

Dendritic cells (DCs) are crucial in the defence against pathogens. Invading pathogens are recognized by Toll-like receptors (TLRs) and receptors such as C-type lectins expressed on the surface of DCs. However, it is becoming evident that some pathogens, including viruses, such as HIV-1, and non-viral pathogens, such as Mycobacterium tuberculosis, subvert DC functions to escape immune surveillance by targeting the C-type lectin DC-SIGN (DC-specific intercellular adhesion molecule-grabbing nonintegrin). Notably, these pathogens misuse DC-SIGN by distinct mechanisms that either circumvent antigen processing or alter TLR-mediated signalling, skewing T-cell responses. This implies that adaptation of pathogens to target DC-SIGN might support pathogen survival.