Nonproducer malignant tumor cells with rescuable sarcoma virus genome isolated from a recurrent Moloney sarcoma

Functional and genetic analysis of coronavirus replicase-transcriptase proteins

The coronavirus replicase-transcriptase complex is an assembly of viral and cellular proteins that mediate the synthesis of genome and subgenome-sized mRNAs in the virus-infected cell. Here, we report a genetic and functional analysis of 19 temperature-sensitive (ts) mutants of Murine hepatitis virus MHV-A59 that are unable to synthesize viral RNA when the infection is initiated and maintained at the non-permissive temperature. Both classical and biochemical complementation analysis leads us to predict that the majority of MHV-A59 ORF1a replicase gene products (non-structural proteins nsp1-nsp11) form a single complementation group (cistron1) while the replicase gene products encoded in ORF1b (non-structural proteins nsp12-nsp16) are able to function in trans and comprise at least three, and possibly five, further complementation groups (cistrons II-VI). Also, we have identified mutations in the non-structural proteins nsp 4, nsp5, nsp10, nsp12, nsp14, and nsp16 that are responsible for the ts phenotype of eight MHV-A59 mutants, which allows us to conclude that these proteins are essential for the assembly of a functional replicase-transcriptase complex. Finally, our analysis of viral RNA synthesis in ts mutant virus-infected cells allows us to discriminate three phenotypes with regard to the inability of specific mutants to synthesize viral RNA at the non-permissive temperature. Mutant LA ts6 appeared to be defective in continuing negative-strand synthesis, mutant Alb ts16 appeared to form negative strands but these were not utilized for positive-strand RNA synthesis, and mutant Alb ts22 was defective in the elongation of both positive- and negative-strand RNA. On the basis of these results, we propose a model that describes a pathway for viral RNA synthesis in MHV-A59-infected cells. Further biochemical analysis of these mutants should allow us to identify intermediates in this pathway and elucidate the precise function(s) of the viral replicase proteins involved.

Replication of enterotropic and polytropic murine coronaviruses in cultured cell lines of mouse origin

To understand the virus-cell interactions that occur during murine coronavirus infection, six murine cell lines (A3-1M, B16, CMT-93, DBT, IC-21 and J774A.1) were inoculated with eight murine coronaviruses, including prototype strains of both polytropic and enterotropic biotypes, and new isolates. All virus strains produced a cytopathic effect (CPE) with cell-to-cell fusion in B16, DBT, IC-21 and J774A.1 cells. The CPE was induced most rapidly in IC-21 cells and was visible microscopically in all cell lines tested. In contrast, the coronaviruses produced little CPE in A3-1M and CMT-93 cells. Although most virus-infected cells, except KQ3E-infected A3-1M, CMT-93 and J774A.1 cells, produced progeny viruses in the supernatants when assayed by plaque formation on DBT cells, the kinetics of viral replication were dependent on both the cell line and virus strain; replication of prototype strains was higher than that of new isolates. There was no significant difference in replication of enterotropic and polytropic strains. B16 cells supported the highest level of viral replication. To determine the sensitivity of the cell lines to murine coronaviruses, the 50% tissue culture infectious dose of the coronaviruses was determined with B16, DBT, IC-21 and J774A.1 cells, and compared to that with DBT cells. The results indicate that IC-21 cells were the most sensitive to murine coronaviruses. These data suggest that B16 and IC-21 cells are suitable for large-scale preparation and isolation of murine coronaviruses, respectively.

Entry of mouse hepatitis virus into cells by endosomal and nonendosomal pathways

OBLV60 is an acid-dependent syncytium-forming variant isolated from OBL21 cells persistently infected with the pH-independent mouse hepatitis virus (MHV)-4 strain. The fusion activity of OBLV60 can be strictly regulated by controlling pH and thus provides the means to definitively examine the entry of MHV into cells by endosomal and nonendosomal pathways. Shortly after high multiplicity infection, both MHV-4 and OBLV60 were detected by electron microscopy in endosomal vesicles and were recovered from lysates of cells treated with proteinase K to remove extracellular virus. For OBLV60, but not MHV-4, exposure to lysosomotropic compounds early in infection prevented viral penetration and significantly reduced viral yields. These results suggested that both MHV-4 and OBLV60 utilized the endosomal route of entry into cells, but that MHV-4 did not require acidification of endosomal vesicles. Studies on the entry of virus through fusion at the cell surface were performed by briefly exposing surface-bound OBLV60 to a fusion-permissive pH under conditions that prevent endocytic entry. Acid treatment of surface-bound OBLV60 caused a significant increase in the yields of virus produced in cultures of fusion-sensitive Sac- or DBT cells, demonstrating entry of virus by fusion at the cell surface. No measurable increase in virus production was detected with acid treatment of OBLV60 bound to OBL21 cells, suggesting that entry at the cell surface does not occur in these cells, which are resistant to MHV-induced syncytia formation. These results raise interesting questions concerning how mechanisms of MHV entry influence the selection of fusion variants.

Alteration of the pH dependence of coronavirus-induced cell fusion: effect of mutations in the spike glycoprotein

Infection of susceptible murine cells with the coronavirus mouse hepatitis virus type 4 (MHV4) results in extensive cell-cell fusion at pHs from 5.5 to 8.5. The endosomotropic weak bases chloroquine and ammonium chloride do not prevent MHV4 infection. In marked contrast, we have selected variants from a neural cell line persistently infected with MHV4 which are entirely dependent on acid pH to fuse host cells and are strongly inhibited by endosomotropic weak bases. Wild-type and variant viruses were compared at the level of the fusion-active surface (S) glycoprotein gene. Cloning and sequencing of each 4,131-base open reading frame predicted a total of eight amino acid differences which fell into three distinct clusters. Each S glycoprotein, when expressed from cDNA, was synthesized in equivalent amounts, and similar proportions were transported to the cell surface. Wild-type S induced cell-cell fusion at neutral pH, whereas variant S required prolonged exposure to acidic pH to induce fusion. Expression of hybrid S genes prepared by exchange of restriction fragments between wild-type and variant cDNAs revealed that elimination of neutral pH fusion was solely dependent on amino acid alterations at positions 1067 (Q to H), 1094 (Q to H), and 1114 (L to R). These changes lie within a predicted heptad repeat region of the transmembrane cleavage fragment of S (S2). These findings demonstrate that the pH dependence of coronavirus fusion is highly variable and that this variability can be determined by as few as three amino acid residues.

Analysis of murine coronavirus surface glycoprotein functions by using monoclonal antibodies

The murine coronavirus surface glycoprotein gene was expressed as a fusion protein in bacteria, and the expressed protein was used to generate S protein-specific monoclonal antibodies (MAbs). Three of the MAbs, 11F, 30B, and 10G, were able to neutralize virus infectivity, and two of them, 11F and 10G, were able to block virus-induced, cell-to-cell fusion. The binding sites of the 11F, 30B, and 10G MAbs were determined by Western immunoblotting and epitope mapping. The 11F and 30B MAbs bound to sites located, respectively, between amino acids 33 to 40 and 395 to 406 in the amino-terminal (S1) subunit of the S protein, and the 10G MAb bound to a site located between amino acids 1123 and 1137 in the carboxy-terminal (S2) subunit. These data define more precisely the interactions between the S1 and S2 subunits of the murine coronavirus S protein and provide further insights into its structure and function.

Murine coronavirus nonstructural protein ns2 is not essential for virus replication in transformed cells

Two isolates of the murine hepatitis virus (MHV) strain JHM, which differed in their ability to express the nonstructural gene product ns2, were characterized. The MHV Wb3 isolate encodes a 30,000-molecular-weight ns2 protein that can be readily detected in infected cells by using a specific monoclonal antibody, MAb 2A. The MHV Wb1 isolate is a deletion mutant that lacks a functional ns2 gene and the transcriptional signals required for the synthesis of an ns2 mRNA. However, there are no obviously significant differences in the growth of the MHV Wb1 and MHV Wb3 isolates in continuous cell lines or in the synthesis of viral mRNAs or proteins in infected cells. These results demonstrate that the ns2 gene product is not essential for MHV replication in transformed murine cells and suggests that the function of the ns2 gene may only be manifest in vivo.

Autoimmunity induced by lactate dehydrogenase-elevating virus: monoclonal autoantibodies against Golgi antigens and other subcellular elements

Immunocompetent mice infected with LDVAGIA developed autoantibodies against Golgi antigen as early as 6-7 days postinfection preceding anti-viral antibodies for nearly a week. The anti-Golgi antibody titres peaked between two and three weeks postinfection independent of the applied virus dose. Already one week postinfection anti-Golgi autoantibodies were prominent in IgG subclasses IgG2a and b. Spleen cells from these mice were fused with myeloma cells and the culture fluids were screened by indirect immunofluorescence for antibodies reactive with the Golgi antigen of normal cultured cells. A panel of cloned stable antibody-producing hybridomas has been obtained. Some antibodies showed broad cross-species reactivity, recognizing similar antigenic determinants in the Golgi region of mouse, rat and other mammalian cells and also in avian and piscine cells, whereas others recognized determinants in this cell compartment only in mammalian cells and one recognized Golgi antigen only in particular murine tumor cells. From 19 Golgi antibody producing hybridomas 3 secreted IgM-antibodies, 10 synthesized autoantibodies of the subclass IgG2a and 6 of IgG2b. Applied in immunoelectron microscopy mABs decorated the Golgi organelle. A considerable amount of LDVAGIA-induced hybridomas produced antibodies against conserved antigens associated with the cytoskeleton, mitochondria, and the nucleus, LDV-induced monoclonal anti-Golgi antibodies decorated the Golgi area in LDV- and mock-infected macrophages. However, cytoplasmic fluorescence characteristic for LDV-infected cells was not observed indicating that the anti-Golgi autoantibodies do not cross-react with the virus.

Vulnerability of rat and mouse brain cells to murine hepatitis virus (JHM-strain): studies in vivo and in vitro

The pathogenicity and cell tropism of mouse hepatitis virus (MHV-JHM-strain) in the developing mouse (Balb/c) and rat (Wistar and Lewis) brain were analysed. Intracranial infection of Balb/c mice at postnatal day 5 induced a lethal encephalitis in all animals. Of Wistar rats infected at day 2 or 5 after birth, 30 to 70%, respectively, survived. The distribution of viral antigen was studied in frozen brain sections of animals that died after infection; astrocytes were found to be the major virus-infected cell type throughout the central nervous system. More than 75% of the surviving rat pups developed paralysis, but viral antigen was detected in only few brain cells and not in astrocytes. The cell tropism of MHV-JHM was examined further in virus-infected glial cell cultures derived from brains of rats or mice. In the glial cultures derived from Wistar rats, only oligodendrocytes were infected, whereas in cultures derived from mouse or Lewis rat brain viral antigen was detected in both astrocytes and oligodendrocytes. Infection of astrocytes led to the formation of syncytia and degradation of the cytoskeleton. Infected rat oligodendrocytes gradually disappeared from the cultures because of cell death. These phenomena indicate that, besides an indirect autoimmune response triggered by infected astrocytes, direct virus-induced injury to astrocytes or to oligodendrocytes can have a dominant role in the neuropathogenicity of mouse hepatitis virus. The present results underscore the importance of species and developmental stage of experimental animals in the neurotropism and pathogenicity of MHV-JHM.

Identification of two epitopes in the carboxyterminal 15 amino acids of the E1 glycoprotein of mouse hepatitis virus A59 by using hybrid proteins

cDNA fragments coding for the carboxy terminus of the E1 envelope glycoprotein from mouse hepatitis virus A59, a coronavirus, were cloned into the bacterial expression vector pEX. Clones expressing E1 antigenic determinants were selected with a polyclonal anti-E1 antibody and used for immunization of rabbits and for affinity purification of existing polyclonal antisera. Immunofluorescence testing and immunoperoxidase labeling of coronavirus-infected cells showed that these reagents were monospecific for E1. In addition, by using hybrid proteins containing different lengths of the E1 carboxy terminus to affinity-purify a polyclonal antiserum against E1, we have been able to define two epitopes within the last 15 amino acid residues of the protein. These epitope-specific antibodies bind to E1 in Golgi and perinuclear membranes as well as to budding viruses; they do not, however, label the plasma membrane or the membranes of post-Golgi vesicles transporting virions to the cell surface.

Restricted replication of mouse hepatitis virus A59 in primary mouse brain astrocytes correlates with reduced pathogenicity

Temperature-sensitive (ts) mutants of mouse hepatitis virus A59 (MHV-A59) are drastically attenuated in their pathogenic properties. Intracerebral inoculation of mice with 10(5) PFU of mutant ts342 results in prolonged infection of the central nervous system, whereas 100 PFU of wild-type virus are lethal (M. J. M. Koolen, A. D. M. E. Osterhaus, G. van Steenis, M. C. Horzinek, and B. A. M. van der Zeijst, Virology 125:393-402, 1983). In the Sac(-) cell line ts342 grows as well at 37 degrees C (the body temperature of mice) as at 31 degrees C (the permissive temperature). There is, however, a difference in primary cultures of mouse brain astrocytes. After infection with ts342, astrocytes produced low levels of infectious virus (5.2 +/- 3.7%) compared with virus yields after infection with wild-type virus. The fraction of wild-type virus- and ts342-infected cells was similar. Electron microscopy showed in wild-type virus-infected cells abundant virions in smooth vesicles usually closely associated with a well-developed Golgi apparatus. In mutant-infected cells no mature ts342 virus particles were found. There was no difference between ts342 and wild-type virus regarding the intracellular virus-specific RNAs. In ts342-infected cells the viral glycoproteins E2 and E1 were not detectable or were barely detectable. Either the mRNAs for the glycoproteins are not translated or the proteins are rapidly broken down. Revertants of ts342 were isolated. They grew as well as wild-type virus in astrocytes, indicating that they apparently produced sufficient amounts of E2 and E1, the ts defect itself rather than a second site mutation is responsible for the defect in replication, and the ts defect acts in unison with host-cell factors. The revertants also regained the lethal properties of wild-type virus.

Detection and preliminary characterization of a transmissible agent inducing autoantibody against Golgi-antigen

A transmissible agent inducing autoantibody has been found in association with tumours induced in STU mice with the progressor strain of Moloney sarcoma virus (Mo-MSV). The activity of the agent detectable in serum from tumour-bearing hosts was expressed by development of autoantibody against Golgi-associated evolutionary conserved antigen (Weiland et al. 1984). This agent was tentatively designated 'AGIA', anti-Golgi inducing agent, on account of its most remarkable biological activity. Concomittant with autoantibodies were cytotoxic antibodies reactive with a Mo-MSV non-producer transformant (Sac). Both antibody activities were regularly detectable 2 weeks after inoculation of the agent. At that time the antibody containing serum possessed an infectivity titre of approximately 10(7.5) ID50/ml. Signs of illness were not observed during this period. The antibody-inducing agent was lost from progressor Mo-MSV transformants during their first passage in culture. Neither murine embryo fibroblasts nor murine tumour cells were permissive for propagation of the agent in vitro.

Anti-tumor immunity in B-lymphocyte-deprived mice. III. Immunity to primary Moloney sarcoma virus-induced tumors

Tumor induction and immunity to tumors were studied following the injection of Moloney sarcoma virus (MSV) into mice whose B-lymphocyte functions had been suppressed by the chronic administration of anti-IgM antibodies. Two preparations of MSV were used; one which gives rise to tumors which uniformly regress in normal adult mice, and another which elicits progressively growing tumors in the majority of recipients. The tumor incidence, mean tumor size, and tempo of regression were not modified by treatment with anti-IgM. However, whereas tumors induced by the regressor virus were all rejected in 19 NRG-treated and 29 untreated recipients, continued growth was obtained in 2 of 23 B-lymphocyte-deprived mice. Furthermore, in 9 additional mice from this group, apparent rejection was followed by tumor recurrence at the site of the initial tumor. Continued growth was accompanied by widespread metastasis. These tumors were freely transplantable to normal syngeneic recipients. Metastasis and transplantability were also detected in 7 of 24 anti-IgM-treated mice given progressor virus, but were not seen in the control animals. Recurrence and metastasis was obtained despite the presence of high levels of specific cytotoxic T lymphocytes in the spleen. It is concluded that B lymphocytes or their products play an essential role in host protection against MSV-induced tumors.

In vitro differentiation of F-9 teratocarcinoma cells does not induce expression of endogenous retroviral glycoprotein

Undifferentiated F-9 teratocarcinoma cells derived from 129/J mice are induced in vitro to express several differentiation markers. Neither undifferentiated nor differentiated F-9 cells express endogenous retroviral glycoprotein (gp70), although the latter can be productively infected with exogenous retroviruses. This is discussed in context with previous findings that all antigen-activated lymphocytes of all mice express endogenous retroviral gp70.

Isolation and identification of virus-specific mRNAs in cells infected with mouse hepatitis virus (MHV-A59)

We have determined the kinetics of virus production and virus-specific RNA synthesis in Sac(−) cells infected with mouse hepatitis virus strain A59 (MHV-A59). Immunofluorescence showed that all cells became infected at a multiplicity of 10 PFU/cell. The virus was concentrated and purified to obtain the high titered stocks needed for these one-step growth experiments. Release of virus into the culture medium started 4 hr after infection (pi) and was complete at 10 hr pi. Synthesis of virus-specific RNA, measured by the incorporation of [3H]uridine in the presence of 1 μg/ml actinomycin D, also started at 4 hr pi and its maximum rate occurred between 6 and 8 hr pi. RNA labeled during this period was isolated from infected cells. About 50% of this RNA bound to oligo(dT)-cellulose; this material was denatured with glyoxal-dimethyl sulfoxide and analyzed by electrophoresis in 1% agarose gels. Seven RNA species with the following molecular weights were present: 5.6 × 106 (RNA1), 4.0 × 106 (RNA2), 3.0 × 106 (RNA3), 1.4 × 106 (RNA4), 1.2 × 106 (RNA5), 0.9 × 106 (RNA6), and 0.6 × 106 (RNA7). RNA1 comigrated with the viral genome. Artifacts caused by defective interfering particles or breakdown of RNA were excluded. To determine whether these RNA species were functional as messengers in infected cells, virus-specific RNAs present in polyribosomes were analyzed. EDTA treatment was used to discriminate between RNA present in polyribosomes and in EDTA-resistant, presumably ribonucleoprotein, particles. Most (91%) of RNA1 was present in EDTA-resistant particles; the remainder and all other RNAs synthesized between 6 and 8 hr pi were present in polyribosomes. We conclude that MHV-A59 has six subgenomic mRNAs. Since the total molecular mass (11.1 × 106 daltons) of these messengers is about twice that of the viral genome, sequence homologies must exist between the mRNAs. The position of these homologous regions and the translation products of each of the mRNAs remain to be determined.

Characterisation of viral RNA in cells infected with the murine coronavirus JHM

The murine coronavirus JHM induces in Sac(-) cells seven major and two minor RNA species. These RNAs are polyadenylated and single stranded. Their sizes were estimated by electrophoresis in agarose gels containing methylmercury hydroxide. The mol. wts. for the major species are 6.67 x 10(6) for RNA of genome size, 3.42 x 10(6) for RNA 2, 2.76 x 10(6) for RNA 3, 1.35 x 10(6) for RNA 4, 1.19 x 10(6) for RNA 5, 0.93 x 10(6) for RNA 6 and 0.62 x 10(6) for RNA 7. The minor species have a size of 4.7 x 10(6) (RNA a) and 1.5 x 10(6) (RNA b). No essential difference in the number and proportion of each RNA species was found between total cytoplasmic RNA, polyadenylated cytoplasmic RNA and RNA extracted from pelleted polysomes, nor was any difference found during the infection cycle. The major RNA species are likely to be subgenomic mRNAs.

Tumors induced by progressor sarcoma virus (Moloney) in mice: growth in the presence of an immune response and isolation of autonomously growing cells

Courses of tumors, which had been induced in adult STU mice with a regressor and with a progressor strain of Moloney sarcoma virus (MSV-M) were followed and compared. All 73 tumors induced by the regressor strain of MSV-M (R-MSV-M) regressed and 181 of 183 tumors induced by a progressor strain (P-MSV-M) grew progressively and killed their hosts between 16 and 171 days after infection. Even after inoculation of about 4 FFU of P-MSV-M tumor development may occur and lead to progressively growing tumors. Both strains of MSV-M induce strong immune responses in their host, namely cytotoxic effector cells, cytotoxic antibodies and tumor resistance. Simultaneous injection of mice on separate sites with P-MSV-M on one side and R-MSV-M producing ascitic cells on the other side did not induce mutual influences on the different tumor courses. It is therefore concluded that the immune response is obviously not decisive for the failure of P-MSV-M induced tumors to regress. From seven P-MSV-M induced primary tumors two producer and five nonproducer transformants could be isolated, maintained in culture and partly held as easily transplantable tumors in adult immunocompetent mice. Both producer transformants release sarcoma virus with the capacity to induce progressively growing tumors in adult mice. All transformants induce progressively growing tumors after transplantation independent of the applied cell dose. It is concluded that the ability of the P-MSV-M to induce stably transformed cells (in contrast to R-MSV-M, WEILAND et al., Brit. J. Cancer, 1979) favours clonal development and ultimately progressive tumor course, in spite of the presence of an immune response.

Coronavirus JHM: cell-free synthesis of structural protein p60

Sac(-) cells infected with murine coronavirus strain JHM shut off host cell protein synthesis and synthesized polypeptides with molecular weights of 150,000, 60,000, and 23,000. The 60,000- and 23,000-molecular-weight polypeptides comigrated with virion structural proteins p60 and p23, and the 60,000-molecular-weight protein was identified as p60 by tryptic peptide fingerprinting. Polyadenylate-containing RNA [poly(A) RNA] extracted from the cytoplasm of infected cells directed the synthesis of both 60,000- and 23,000-molecular-weight polypeptides in messenger-dependent cell-free systems derived from mouse L-cells and rabbit reticulocytes. The reticulocyte system also synthesized a 120,000-molecular-weight polypeptide that was specifically immunoprecipitated by antiserum raised against JHM virions. The identity of the 60,000- and 23,000-molecular-weight in vitro products was established by comigration with virion proteins, immunoprecipitation, and in the case of p60, tryptic peptide fingerprinting. The cytoplasmic poly(A) RNAs which encoded p60 and p23 sedimented in sucroseformamide gradients at 17S and 19S, respectively, and were clearly separable. These RNAs were among the major poly(A) RNA species synthesized in the cytoplasm of actinomycin D-treated cells late in infection, and the in vitro translation of size-fractionated RNA released from polysomes confirmed that they represent physiological mRNA's. These results suggest that the expression of the coronavirus JHM genome involves more than one subgenomic mRNA.

Growth pattern of tumours in mice induced by murine Moloney sarcoma-virus and sarcoma-virus-transformed cells

Transplantation of a Moloney sarcoma-virus (MSV-M)-transformed producer cell line (Sac(+)) induced progressively or regressively growing tumours in mice. Progressive growth always occurred after transplantation of an MSV-M non-producer transformant (Sac(-)), whereas the MSV-M released from the producer cells (Sac virus) always induced tumours which regressed. In contrast to the non-producer, the producer transformant Sac(+) as well as Sac virus induced a strong immune response, detected in vitro by cell- and antibody-mediated cytotoxicity assays, and in vivo by transplantation immunity. Implantation of Sac(-) cells led to solid, under-vascularized tumours, consisting histologically of uniform densely packed tumour cells. Sac-virus-induced tumours, however, were very well vascularized and arose by proliferation of different connective-tissue cells. After transplantation of Sac(+) cells, tumours were found to consist of typical tumour cells morphologically similar to Sac(-) cells intermingled with proliferated connective-tissue cells. Cultivation of tumour fragments from Sac(+) and Sac(-) tumours was followed by outgrowth of transformed tumour cells with the properties of the originally implanted cells. Tumour explant cultures from Sac-virus-induced tumours did not lead to growth of stably transformed cells. Co-culture of mouse embryo fibroblasts (MEF) with Sac(+) cells resulted in overgrowth of the transformed cells. Infection of MEF with Sac virus led to transiently transformed cells. It is concluded that Sac(+) cell tumours will resist the strong immune defence mechanisms they induce and grow progressively, if the inoculated cells are able to build up a solid, poorly vascularized nodule in the tissue. This always happens after implantation of 10(6) cells, but only occasionally when fewer cells are inoculated. Sac-virus-induced tumours will always regress owing to the strong immune response. The regression is furthered by the fact that MSV-M infection rarely if ever leads to a stable transformation.