Characterization of molecular species carrying gross cell surface antigen

Natural Antilymphoma Antibodies in C3Hf Mice Serum: Lack of Identity with Autoimmune and anti Murine Leukemia Virus Antibodies

Absorption experiments on C3Hf normal mouse sera followed by cytotoxic tests on EL4 lymphoma cells were done to investigate a possible identity between natural antilymphoma antibodies (NAA) and various types of autoantibodies known to be present in normal mouse sera. Single C3Hf normal sera were also tested both by cytotoxicity on EL4 cells and by radioimmune precipitation assay (RIP) on 125I-labelled AKR ecotropic virus to ascertain whether or not viral antigen are the target structures of the NAA activity. The study provides evidence that NAA coexist with autoanticorpal and antiviral activities although they are distinct entities.

A nonstructural gag-encoded glycoprotein precursor is necessary for efficient spreading and pathogenesis of murine leukemia viruses

In addition to the Gag-Pol and Env precursors whose translation initiates at AUG codons, murine, feline, and simian type C oncoviruses also express glycosylated Gag-Pol precursors (glycoGag), glycoGag translation is initiated at CUG codons located upstream of the Gag AUG initiation codon. In contrast to Gag, glycoGag is translocated into the endoplasmic reticulum and is absent from virions. Since glycoGag has been described to be dispensable ex vivo, we investigated the in vivo effects of a glycoGag- mutation in the Friend murine leukemia virus (F-MuLV). F-MuLV induces severe early hemolytic anemia and subsequent erythroleukemia within 2 months after inoculation of newborn mice. We obtained a glycoGag- F-MuLV, strain H5, by inserting an octanucleotide linker downstream of the CUG codon leading to the reading of a stop codon in all reading frames upstream of the Gag AUG. F-MuLV H5 did not induce severe early hemolytic anemia, and latency of erythroleukemia was significantly increased most likely because of an approximately 1-week delay in the in vivo spreading. Accordingly, induction of recombinant polytropic viruses was also significantly delayed. Close examination of ex vivo spreading kinetics also showed a slower dissemination of F-MuLV H5. Western blot (immunoblot) performed after inoculation of newborn mice with this glycoGag- virus indicated the emergence of new glycoGag+ viruses. PCR analyses with F-MuLV-specific primers demonstrated in vivo pseudoreversions restoring the glycoGag reading frame. Our results demonstrated that glycoGag expression is positively selected and essential for full spreading and pathogenic abilities.

Characterization of monoclonal antibodies against the human immunodeficiency virus matrix protein, p17gag: identification of epitopes exposed at the surfaces of infected cells

Eight monoclonal antibodies reactive with the matrix protein of human immunodeficiency virus type 1 (HIV-1), p17gag, were isolated from rats which had been immunized with solubilized HIV-1 lysate. The epitope specificities of these antibodies were determined with a series of synthetic peptides representing overlapping regions of p17. Six of the antibodies were mapped to three distinct regions of p17, while two antibodies (G11g1 and G11h3) reacted only with intact recombinant p17, suggesting that they were directed against conformational or discontinuous epitopes. All the antibodies bound to HIV-infected cells which had been permeabilized with acetone, but only G11g1 and G11h3 reacted with live HIV-infected cells. Specificity studies with diverse virus strains demonstrated that these two antibodies recognized distinct epitopes, one which was group specific for HIV-1, and one which was shared with HIV type 2 and simian immunodeficiency virus. Binding competition studies indicated that these epitopes were proximal in native p17. Despite their reactivity with intact cells, these two antibodies did not possess appreciable virus-neutralizing activity. These results indicate that a form of p17 is expressed on the surfaces of live HIV-infected cells which is accessible to some, but not all, antibodies against p17. These cell surface molecules may play a role in the generation of antibodies against p17gag that are characteristic of early stages of HIV infection, and they may act as natural targets for the immune system and as potential targets for immunotherapy of HIV-infected cells.

Impairment of immunogenicity by antigen presentation in liposomes made from dimyristoylphosphatidylethanolamine linked to the secretion of prostaglandins by macrophages

The induction of antibody response in syngeneic rats by the Gross virus cell surface antigen (GCSAa) was dependent on the presentation of GCSAa into liposomes made from distearoylphosphatidylcholine (DSPC). GCSAa liposomes made from dimyristoylphosphatidylethanolamine (DMPE) were nonimmunogenic, even when used as anamnestic immunogens. Spleen cells, from rats twice immunized with GCSAa-DSPC-liposomes and used to transfer the anti-GCSAa immune response into naive recipients after a tertiary immunostimulation in vitro in the presence of naive peritoneal exudate cells (PEC), responded to soluble GCSAa only after irradiation at 500 rds and to GCSAa-DMPE-liposomes only when indomethacin was added during the in vitro stimulation. The preincubation of these cells with empty DMPE liposomes or the addition of supernatant from PEC fed with DMPE liposomes abrogated the response to GCSAa-DSPC liposomes. Using a specific radioimmunoassay, prostaglandin E2 was demonstrated to be produced by PEC when fed with DMPE liposomes, and not when fed with DSPC liposomes. This prostaglandin E2 secretion by PEC induced by DMPE liposomes was inhibited by indomethacin.

Resistance to cellular immune response in AKR leukemias

Spontaneous AKR leukemias express murine leukemia virus (MuLV) gag and env gene-encoded structural proteins on their cell surface. Cytotoxic T lymphocytes (CTL) induced in AKR mice by syngeneic leukemia 369 which expressed high amounts of H-2 antigens recognized viral gag polyproteins in association with H-2K antigens as target antigens. H-2K-negative leukemias were resistant to lysis by AKR/Gross MuLV-specific CTL and did not induce a cellular immune response. However, they became susceptible after stimulation with interferon. H-2K-positive leukemias induced CTL which were cytotoxic for 369 cells. However, the majority of H-2K-positive leukemias was not lysed by CTL induced by autologous immunization. These leukemias were also resistant to lysis by anti-369 CTL, but could restimulate AKR/Gross-specific CTL in vitro, and were susceptible to lysis by H-2Kk-restricted CTL against AKR minor histocompatibility antigens. Thus, there could be specific defects of the H-2Kk antigens of these tumors. However, there were also qualitative and quantitative differences in antigenic determinants of the gag target antigens in these leukemias. Therefore, in addition to quantitative reduction of the H-2K restriction elements, qualitative alterations of H-2 antigens or of the viral target antigens may impair T cell cytotoxicity and thus influence leukemogenesis of AKR spontaneous leukemia.

Enhancement of immunogenicity of tumour virus antigen by liposomes: the effect of lipid composition

The secondary humoral response evoked in W/Fu rats by the weakly immunogenic soluble Gross cell surface antigen (GCSAa) extracted from the syngeneic (C58NT)D lymphoma can be enhanced when GCSAa is presented in liposomes, and this requires the antigen to be strongly associated with the phospholipid bilayers. In order to investigate further the role of the phospholipid microenvironment in the membrane presentation of this antigen, the relationship between the phospholipid composition and the immunogenic potency of GCSAa liposomes was explored. For a given neutral phospholipid component, optimal immunogenicity was obtained when 20% cholesterol was present and when a negatively charged phospholipid was included as a minor component. When phosphatidylcholines (PC) were used as the major neutral component, the immunogenicity of GCSAa liposomes from optimal for distearoyl PC (DSPC) decreased when decreasing the PC acyl chain length down to the background level for dilauroyl PC (DLPC). Similarly, the use of PC with acyl chain of increasing unsaturation was followed by a decline in the immunogenicity of the GCSAa liposomes up to the background level for dilinoleoyl PC (DLiPC). Replacing PC headgroups by phosphatidylethanolamine (PE) headgroups abolished the enhancing effect of the liposome presentation on GCSAa immunogenicity. Three groups of phospholipids unable to promote the expression of the GCSAa immunogenicity could be distinguished: the DLPC, DLiPC group, unable to prime the animals but, contrary to the soluble antigen, able to boost the animal after an appropriate priming with GCSAa-DSPC-liposomes, the dimyristoyl PE group, unable to prime or to boost and non-toxic in vitro for the macrophages, and the dipalmitoyl PE group, acutely toxic for macrophages.

Monoclonal antibody to the amino-terminal L sequence of murine leukemia virus glycosylated gag polyproteins demonstrates their unusual orientation in the cell membrane

To analyze cell surface murine leukemia virus gag protein expression, we have prepared monoclonal antibodies against the spontaneous AKR T lymphoma KKT-2. One of these antibodies, 43-13, detects an AKR-specific viral p12 determinant. A second monoclonal antibody, 43-17, detects a novel murine leukemia virus-related antigen found on glycosylated gag polyproteins (gp95gag, gp85gag, and gp55gag) on the surface of cells infected with and producing ecotropic endogenous viruses, but does not detect antigens within these virions. The 43-17 antibody immunoprecipitates the precursor of the cell surface gag protein whether in its glycosylated or unglycosylated state, but does not detect the cytoplasmic precursor of the virion gag proteins (Pr65gag). Based on these findings, we have localized the 43-17 determinant to the unique amino-terminal part of the glycosylated gag polyprotein (the L domain). We have determined that gp95gag contains L-p15-p12-p30-p10 determinants, whereas gp85gag lacks the carboxyterminal p10 determinant, and gp55gag lacks both p30 and p10 carboxy terminal determinants. Analysis of cell surface gag expression with the 43-17 antibody leads us to propose that the L domain plays a crucial role in (i) the insertion and orientation of murine leukemia virus gag polyproteins in the cell membrane and (ii) the relative abundance of expression of AKR leukemia virus versus Moloney murine leukemia virus glycosylated gag polyproteins in infected cells.

Presentation of an MuLV-related tumour antigen in liposomes as a potent tertiary immunogen after adoptive transfer

The immunogenicity of Gross virus cell surface antigen (GCSAa) extracted from rat lymphoma cells can be dramatically increased by its presentation into liposomes, probably by mimicking the cell membrane presentation. Induction of an anti-GCSAa secondary antibody response has been found to require the use of liposomes as GCSAa vehicle for both the priming and the boosting immunizations. In order to investigate the sensitivity of highly immune cells to the liposome presentation, immune spleen cells were stimulated in vitro with either soluble GCSAa or GCSAa-liposomes and transferred together with the immunogen into syngeneic animals. Only spleen cells from high responders, which have been immunized twice with GCSAa-liposomes, were able to generate an antibody response in naive recipients after their restimulation with the GCSAa-liposome preparation. Their restimulation with soluble antigen was ineffective unless 10% peritoneal exudate cells (PEC) from naive rats were added during the in vitro incubation. Stimulation with GCSAa-liposomes was further improved by the addition of 10% PEC. Macrophages were found to play a central role in the induction of antibody response in the recipients after stimulation with GCSAa-liposomes. Treatment of immune spleen cells with the macrophage-killing agent leucine methyl ester prior their restimulation in vitro with GCSA-liposomes in the absence of PEC, or depletion of macrophage after their in vitro incubation with this immunogen, completely abolished the induction of anti-GCSAa antibodies in the recipients.

Immunochemical behaviour of a tumour-associated antigen obtained from an AKR mouse lymphoma

A soluble tumour-associated antigen was prepared by homogenizing AKR lymphoma cells (L15) followed by treatment with acetone and glycine-HCl buffer pH 3. It was analyzed by immunoelectrophoresis, rocket electrophoresis in the presence of Concanavalin A and SDS-PAGE. It showed rapid electrophoretic mobility, and the major component had an estimated molecular weight of 70,000 daltons. The molecular composition of the antigen included a glycoprotein. Specific antibodies against this antigen were demonstrated in BALB/c mice in which the AKR lymphoma was conditioned to grow by introducing the L15 cells into a subcutaneously implanted glass cylinder. In this model antibodies were found in both tumour-bearing (progressor) and tumour-rejecting (regressor) animals. In vivo experiments showed that pretreatment of BALB/c mice with L15 acellular extracts, 10 days before tumour challenge, led to a significant increase in allogeneic tumour growth. A rabbit anti-tumour extract serum was prepared and was rendered tumour -specific by exhaustive absorption with normal AKR serum and tissues. It shared the same epitopic specificity with tumour progressor or regressor mouse sera since indirect immunofluorescence using L15 cells could be reciprocally inhibited.

Feline leukemia virus-and feline sarcoma virus-related polypeptides released by virus producer and nonproducer cells

Polypeptides specific for feline leukemia virus (FeLV) have been identified in the media of cells that produce FeLV as well as in nonproducer cells transformed by feline sarcoma viruses (FeSV). Cat fibroblasts that were persistently infected with FELV release the major virus envelope glycoprotein, whereas cultured cat lymphoma cells shed both glycopeptides related to the virus core gene (gag) and glycopeptides related to the virus envelope gene (env). Mink cells and cat cells transformed by FeSV secrete polypeptides of a wide range of sizes that cross-react with the major virus core protein p27. Differences in the classes of p27-related proteins produced may be related to the strain of virus and the cell type. Cat cells transformed by FeSV release a glycopeptide that appears to be processed differently from those identified in the media of FeSV-transformed mink cells. The possibility that such FeLV-related secretory proteins may interfere with the immune response of the host is discussed.

Use of an automatic cell harvester in a cellular radioimmunoassay

An automatic cell harvester was used in the final step of a cellular radioimmunoassay to collect cell bound anti-rat IgG 125I-F(ab')2. Studies on the reliability of this collection method were performed with antibodies directed against cell surface antigens induced by the Gross murine leukemia virus and produced by immunization of W/Fu rats with the syngeneic (C58NT)D lymphoma. Glutaraldehyde-fixed as well as untreated Gross virus induced lymphoma cells could be used. Similar and specific antibody binding curves were obtained when the cells were incubated with the anti-(C58NT)D serum and anti-rat IgG 125I-F(ab')2 in the presence of 0.1% NaN3. Background levels of non-specific binding of anti-rat 125I-F(ab')2 to mouse lymphoma cells or rat thymocytes were only a few cpm above the background of the gamma-counter. This allowed detection of surface immunoglobulin positive lymphocytes among as few as 30,000 rat splenocytes. In addition, this cellular radioimmunoassay was found to be suitable for the measurement of solubilized cell surface antigen by its capacity to inhibit the binding of the specific antibodies to the target cells.

Nucleotide sequence of AKV murine leukemia virus

AKV is an endogenous, ecotropic murine leukemia virus that serves as one of the parents of the recombinant; oncogenic mink cell focus-forming viruses that arise in preleukemic AKR mice. I report the 8,374-nucleotide-long sequence of AKV, as determined from the infectious molecular clone AKR-623. The 5'-leader sequence of AKV extends to nucleotide 639, after which lies a long open reading frame encoding the gag and pol gene products. The reading frame is interrupted by a single amber codon separating the gag and pol genes. The pol gene overlaps the env gene within the 3' region of the AKV genome. The nucleotide sequence of the 5' region of AKV reveals the following features. (i) The 5'-leader sequence lacks any AUG codon to initiate translation of gPr80gag, suggesting that gPr80gag is not required for the replication of AKV. (ii) A short portion of the leader region diverges in sequence from the closely related Moloney murine leukemia virus and appears to be related to a sequence highly repeated in eucaryotic genomes. (iii) As in Moloney murine leukemia virus, there is a potential RNA secondary structure flanking the amber codon that separates the gag and pol genes. This structure might function as a regulatory protein binding site that controls the relative levels of synthesis of the gag and pol precursors. The nucleotide sequence of the 3' region of AKV is compared with sequences reported previously from both infectious and noninfectious molecular clones of AKV.

Isolation of a new feline sarcoma virus (HZ1-FeSV): biochemical and immunological characterization of its translation product

A new strain of feline sarcoma virus, designated HZ1-FeSV, was isolated from a 4-year-old domestic cat with multicentric fibrosarcoma. A primary tumor cell line was established and virus produced from that line was found to induce foci in feline embryonic lung fibroblasts (FLF3) and mink lung fibroblasts (CCL64) in tissue culture and fibrosarcomas in inoculated 10-week-old kittens. The derivation of transformed nonproducer clones of FLF3 and CCL64 cells containing helper virus-rescuable, focus-forming activity indicated that HZ1-FeSV was defective for replication. The only discernible translation product of the HZ1-FeSV genome in cultured cells was a 100,000-Da polyprotein (P100) which contained amino-terminal sequences of the FeLV gag gene precursor protein covalently linked to a sarcoma virus-specific domain. Immunoprecipitates containing P100 exhibited a protein kinase activity capable of phosphorylating tyrosine residues of P100. Immunologically, P100 was highly cross-reactive with gag-fes polyproteins encoded by two previously characterized strains of FeSV, the GA- and the ST-FeSV. By comparison of methionine-containing tryptic peptides, the HZ1-FeSV protein was shown to be more closely related to the GA-FeSV protein than to the ST-FeSV protein, but to be distinguishable from both other proteins.

The specificity of H-2-restricted cytotoxic T lymphocytes directed to AKR/Gross leukemia virus-induced tumors. II. Altered gp70 display and production of noninfectious virus particles by an insusceptible variant tumor

Derived from the susceptible AKR.H-2bSL1 tumor cell line, a variant tumor subclone, cl.18-5, was selectively insusceptible to H-2-restricted anti-AKR/Gross virus cytotoxic T lymphocytes (CTL) due to its failure to be recognized. In this study, the expression of virus-related products by variant cl.18-5 cells was compared to that of AKR.H-2bSL1 cells and a susceptible clone, as an approach towards defining the virus-associated antigens recognized by anti-AKR/Gross virus CTL. Despite the type specificity of the CTL, cl.18-5 displayed normal levels of the group-specific antigen (gag) encoded proteins p30, p15, p12 and p10, and the gag-associated Gross cell surface antigen. These results were confirmed by fluorescence-activated cell sorter analysis employing monoclonal antibodies specific for either AKR p12 or the cell surface glycosylated form of AKR ecotropic gag product. In contrast, cl.18-5 was variably less sensitive than AKR.H-2bSL1 to the action of complement and xenogeneic antisera directed against the envelope (env) product gp70. In addition, a panel of five monoclonal antibodies to gp70, which detect distinct endogenous ecotropic viral determinants, lysed AKR.H-2bSL1, but not cl.18-5 cells. However, absorption experiments indicated that cl.18-5 did express near normal levels of these specificities, suggesting an alteration in the orientation or topographical distribution of these determinants. Consistent with an inappropriate display of env products, cl.18-5 was found to be deficient in the production of infectious ecotropic leukemia virus. The particulate fraction of the cell-free supernatant of cl.18-5 contained normal levels of reverse transcriptase activity, indicating that noninfectious viral particles were being produced. Collectively, these results point to an association between recognition by anti-AKR/Gross virus CTL and the expression of ecotropic gp70 required for infectivity of virus.

Structure of glycosylated and unglycosylated gag and gag-pol precursor proteins of Moloney murine leukemia virus

Precursor polyproteins containing translational products of the gag gene of Moloney murine leukemia virus were purified by gel electrophoresis and cleaved into large fragments by hydroxylamine, mild acid hydrolysis, or cyanogen bromide. The hydroxylamine cleavage method (specific for asparagine-glycine bonds) was adapted especially for this study. The electrophoretic mobility and antigenicity of the fragments and, in some cases, the presence or absence of [35S]methionine revealed detailed information on the structure of Pr65gag, gPr80gag, and Pr75gag (the unglycosylated variant of gPr80gag formed in vivo in the presence of tunicamycin or in vitro in a reticulocyte cell-free system). When compared with Pr65gag, gPr80gag contains 7,000 daltons of additional amino acids, presumably as, or as part of, a leader sequence at or very close to its N terminus. We present evidence that this leader may have replaced part of the p15 sequence. Furthermore, gPr80gag contains three separate carbohydrate groups. One is attached to the presumed leader sequence or to the p15 domain, and two are attached to the p30 domain. Each of the Moloney murine leukemia virus gag precursor proteins Pr65gag, gPr80gag, and Pr75gag corresponds with a read-through product into the pol gene. We designated these products Pr180gag-pol, gPr200gag-pol, and Pr190gag-pol (the unglycosylated variant of gPr200gag-pol), respectively. gPr200gag-pol contains all of the extra amino acids and carbohydrate groups present in gPr80gag and at least one carbohydrate group in its pol sequences.

Nucleotide sequences of feline sarcoma virus long terminal repeats and 5' leaders show extensive homology to those of other mammalian retroviruses

The nucleotide sequences of the Gardner-Arnstein feline sarcoma virus (FeSV) long terminal repeat and the adjacent leader sequences 5' to the viral gag gene were determined. These were compared with homologous portions of Synder-Theilen FeSV and with previously published sequences for Moloney murine sarcoma virus and simian sarcoma virus proviral DNA. More than 75% of the residues in the FeSV R and U5 regions were homologous to sequences within the same regions of the other viral long terminal repeats. Unexpectedly, alignment of the FeSV sequences with those of the Moloney murine sarcoma and simian sarcoma viruses showed similar extents of homology within U3. The homologous U3 regions included the inverted repeats, a single set of putative enhancer sequences, corresponding to a "72-base-pair" repeat, and sequences, including the CAT and TATA boxes, characteristic of eucaryotic promotors. The 5' leader sequences of both FeSV strains included a binding site for prolyl tRNA and a putative splice donor sequence. In addition, the FeSV leader contained a long open reading frame which was adjacent to and in phase with the ATG codon at the 5' end of the FeSV gag gene. The open reading frame could code for a signal peptide of about 7.4 kilodaltons. Our results support the concept that the virogenic portions of both FeSV and simian sarcoma virus were ancestrally derived from viruses of rodent origin, with conservation of regulatory sequences as well as the viral structural genes.

A rapid bioassay to monitor murine leukemia virus infection in mice using cellular gp 71 binding

A rapid and sensitive bioassay based on the availability of cell surface receptors for the binding of purified envelope glycoprotein, gp71, of Rauscher murine leukemia virus (R-MuLV) was developed to serially monitor viral-induced leukemogenesis in individual BALB/cAnN mice. The specificity of the bioassay was demonstrated by the competition of [125I]gp71 cellular binding with murine ecotropic viruses, purified unlabelled R-MuLV envelope glycoprotein and by antiserum to R-MuLV gp71. In contrast, there was no effect on the [125I]gp71 binding level with the addition of murine xenotropic viruses, R-MuLV p30, or several other proteins. The [125I]gp71 binding level of circulating leukocytes was significantly (P less than 0.05) reduced in mice after R-MuLV infection. The reduction of cellular gp71 binding developed in two stages and the latter stage was highly dependent (P less than 0.05) on circulating infectious virus titer. Using this technique, the gp71 cellular binding levels of 48-60 individual mice can be assayed in a 4 h period. The advantages of this bioassay compared to standard immunological and tissue culture techniques used in studying retrovirus expression and viral-cell interactions are discussed.

Independence of H-2 and viral antigens on the cell surface and absence of H-2 antigens on murine leukemia virus and mouse mammary tumor virus particles

By indirect immunoelectron microscopy we tested for the presence of H-2 antigens on murine mammary tumor virus (MMTV) and murine leukemia virus (MuLV) particles. The association of H-2 antigens and viral antigens on the virus-infected cell surface was investigated with antibody-induced redistribution. Mammary tumor cells and leukemia cell lines with different H-2 genotypes and carrying different MuMTV or MuLV were used. No H-2 antigens could be demonstrated on the envelope of MMTV and MuLV particles, even after the permeabilization of their envelopes with saponin. On the surface of virus-infected cells antibody-induced patching or capping of the viral antigens did not result in copatching or cocapping of the H-2 antigens. In the reciprocal tests no co-redistribution of viral antigens with H-2 antigens was seen. Our experiments failed to show any physical association between H-2 antigens and MMTV or MuLV antigens on the cell surface.

A cell surface antigen of the mouse related to xenotropic MuLv defined by naturally occurring antibody and monoclonal antibody. Relation to Gix G(rada1), G(aksl2) systems of MuLV-related antigens

A new cell surface antigen of the mouse related to xenotropic murine leukemia virus (MuLV) is described. The antigen, designated G(erld), is defined by cytotoxic tests with the B6-x-ray-induced ERLD and naturally occurring antibody. G(erld) is distinct from all previously defined cell surface antigens. Monoclonal antibody with the same specificity has been developed. Inbred mouse strains are classified as G(erld)+ or G(erld)- according to the presence of absence of the antigen on lymphoid cells. G(erld)+ strains differ with regard to quantitative expression of G(erld) on normal thymocytes. The emergence of G(erld)+ tumors in G(erld)- strains indicates the presence of genes coding for the antigen even in strains not normally expressing the antigen. G(erld) has the characteristic of a differentiation antigen in normal mice. In G(erld)+ strains, high levels of the antigen are found on thymocytes with lower levels being detected on cells of spleen, lymph nodes and bone marrow. No G(erld) was detected in brain or kidney or on erythrocytes. The segregation ratios for G(erld) expression on thymocytes in backcross and F2 mice of crosses between G(erld)+ (B6, 129, and B6-Gix+) and G(erld)- (BALB/c) strains suggest that G(erld) expression is controlled by a single locus in B6, by two unlinked loci in 129, and by three unlinked loci in B6-Gix+ mice. Induction of the antigen by MuLV infection of permissive cells in vitro indicates that G(erld) is closely related to xenotropic and dualtropic MuLV; all xenotropic and dualtropic MuLV tested induced the antigen, whereas the majority of ecotropic and the two amphotropic MuLV failed to do so. As dualtropic MuLV are thought to be recombinants between ecotropic and xenotropic MuLV sequences, G(erld) coding by dualtropic MuLV may signify the contribution of the xenotropic part in the recombinational event. Serological and biochemical characterization indicates that G(erld) is related to the gp 70 component of the MuLV envelope. The relation of G(erld) to the previously defined gp 70-related cell surface antigens (Gix, G(rada), and G(aksl2) is discussed, particularly with regard to their characteristics as differentiation antigens, the genetic origin of dualtropic MuLV, and the leukemogenicity of MuLV.

Mutant cells that abnormally process plasma membrane glycoproteins encoded by murine leukemia virus

Wild-type normal rat kidney fibroblasts infected with the Friend strain of murine leukemia virus (MuLV) contain two virus-encoded glycoproteins on the outer surfaces of their plasma membranes: an envelope glycoprotein with an apparent molecular weight of 70,000 (gp70), and a glycoprotein that reacts with antisera to the major virion internal core proteins p30, p15, p12 and p10 and has an apparent molecular weight of 93,000 (gp93gag). To analyze the functions of these glycoproteins and to develop a model system for studying genetics of membrane synthesis, we used an immunoselection method to isolate variant cell clones defective in processing these glycoproteins into their plasma membranes. Several lines of evidence, including complementation of glycoprotein processing defects by fusion with uninfected wild-type cells, indicate that the immunoselected variants have stably inherited membrane synthesis abnormalities that are encoded by cellular rather than by viral genes. The H-4 cell line, which was selected by use of antiserum to gp70, has metabolic defects that interfere with processing of both gp70 and gp93gag into its plasma membranes. Nevertheless, this cell line releases noninfectious MuLV. Furthermore, two cell lines (2 and 5), which were selected by use of antiserum to the virion core protein p30, specifically lack detectable cell surface or intracellular gp93gag but contain cell surface gp70 and release infectious MuLV. These results suggest that MuLV particles can bud efficiently from cells that lack known virus-encoded plasma membrane constituents.

Measurement of Gross cell-surface antigen and p30 level in murine retrovirus-infected cell lines

The level of Gross cell-surface antigen (GCSAa) expression at the surface of murine retrovirus-infected fibroblasts was determined by quantitative absorption of the anti-GCSAa activity of a serum produced in syngeneic W/Fu rats immunized against (C58NT)D lymphoma, and tested in a cytotoxicity assay against E male G2 lymphoma cells. While GCSAa was specifically expressed on Gross-type virus (G-MuLV)-induced lymphoma cells, and while G-MuLV and G-related MuLV induced a high level of GCSAa expression on murine fibroblasts, the Friend-Moloney-Rauscher (FMR) group viruses (FMR MuLV) and xenotropic isolates were also able to induce a high or intermediate level of GCSAa. Since GCSAa has been shown to be borne by glycosylated precursors of the viral nucleocapside (gp95gag and gp85gag), the amount of GCSAa expressed on these cells was compared to the level of cytoplasmic p30. In G- and G-related MuLV-infected cell lines, a significant relationship was found between the amount of GCSAa and the level of p30, whereas in FMR-MuLV or xenotropic virus-infected cells the amount of GCSAa varied independently of the p30 level. These results could explain the discrepancy in the specificity of expression of GCSAa in vivo and in vitro.

Effect of tunicamycin on cell fusion induced by Mason-Pfizer monkey virus

Mason-Pfizer monkey virus, a D-type retrovirus, has been shown to induce multinucleate cell (syncytium) formation or cell fusion in several normal primate cells. A series of experiments has been carried out to examine whether a glycosylated "fusion-inducing" product is responsible for this biological property of Mason-Pfizer monkey virus. Treatment of rhesus monkey fetal lung cells with different concentrations of tunicamycin, a potent inhibitor of glycosylation, during infection with Mason-Pfizer monkey virus had no effect on cell fusion even though up to 5 micrograms of the drug per ml was tested. Furthermore, no significant effect on the extent of syncytium formation in rhesus monkey fetal lung cells was observed when the time of addition or duration of treatment with this inhibitor was varied. Nevertheless, tunicamycin was very effective in blocking glycosylation in rhesus cells since virions produced in the presence of this drug completely lacked gp70 and gp20, the two structural glycoproteins of Mason-Pfizer monkey virus. These non-glycosylated virus particles produced in the presence of tunicamycin were noninfectious as determined by a protein A binding assay and were unable to induce syncytium formation when assayed on rhesus cells. These results indicate that glycosylation of the fusion-inducing product is not required for multinucleate cell formation induced by Mason Pfizer monkey virus.

Structure of glycosylated and unglycosylated gag polyproteins of Rauscher murine leukemia virus: carbohydrate attachment sites

The structural relationships among the gag polyproteins Pr65gag, Pr75gag, and gPr80gag of Rauscher murine leukemia virus were studied by endoglycosidase H digestion and formic acid cleavage. Fragments were identified by precipitation with specific antisera to constituent virion structural proteins followed by one-dimensional mapping. Endoglycosidase H reduced the size of gPr80gag to that of Pr75gag. By comparing fragments of gPr80gag and the apoprotein Pr75gag, the former was shown to contain two mannose-rich oligosaccharide units. By comparing fragments of Pr65gag and Pr75gag, the latter was shown to differ from Pr65gag at the amino terminus by the presence of a leader peptide approximately 7,000 daltons in size. The internal and carboxyl-terminal peptides of the two unglycosylated polyproteins were not detectably different. The location of the two N-linked carbohydrate chains in gPr80gag has been specified. One occurs in the carboxyl-terminal half of the polyprotein at asparagine177 of the p30 sequence and the other is found in a 23,000-dalton fragment located in the amino-terminal region of gPr80gag and containing the additional amino acid sequences not found in Pr65gag plus a substantial portion of p15.

Amplified env and gag products on AKR cells. Origin from different murine leukemia virus genomes

Thymocytes of AKR mice express two species of gp70, the envelope glycoprotein of murine leukemia virus (MuLV), encoded by the env gene. One is denoted Ec+ gp70 in reference to the type-antigen Ec and association with ecotropic virus. The other, Ec- gp70, resembles gp70 found also on thymocytes of mouse strains that are not overt producers of MuLV, and has no evident relation to ecotropic virus. Expression of Ec- gp70 type, but not of Ec+ gp70 type, is amplified with age on AKR thymocytes. In contrast, viral core polyproteins, encoded by the gag gene and simultaneously amplified with age, appear to be related to ecotropic virus. These observations imply selective amplification of products of env and gag genes from two sorts of provirus, a phenomenon which may be connected to the dual genetic origin of recombinant mink-cell-focus inducing viruses in AKR mice.

Murine leukemia virus proteins expressed on the surface of infected cells in culture

Infection of JLS-V9 cells in culture with Rauscher murine leukemia virus induced the appearance on the cell surface of two classes of viral proteins: Rauscher murine leukemia virus gp70, and glycoproteins related to the viral core (gag) proteins with apparent molecular weights in sodium dodecyl sulfate polyacrylamide gels of 80 x 10(3) and 95 x 10(3). The latter proteins were identified by lactoperoxidase-catalyzed iodination of the cell surface and by metabolic labeling with [(3)H]mannose followed by immunoprecipitation with an antiserum directed against the major viral core protein, p30. Tryptic peptide maps of chloramine T-iodinated proteins indicated that 80 x 10(3) - and 95 x 10(3)-molecular-weight proteins were closely related. The 95 x 10(3)-molecular-weight protein from Rauscher murine leukemia virus-infected cells had a tyrosine fingerprint which was identical to that of the 95 x 10(3)-molecular-weight gag surface polyprotein of endogenous virus-producing AKR-A cells, suggesting that expression on the cell surface of glycosylated forms of gag precursor polyproteins may not be an exclusive property of leukemic thymocytes, but a more general phenomenon in murine leukemia virus infection. Tryptic fingerprint analysis of iodinated viral and cell-bound gp70's before and after desialylation indicated a lower level of glycosylation in the cell-bound gp70 population than in virions. Analysis of only surface-iodinated gp70 showed a simple pattern of exposed tryptic peptides which was very similar in Rauscher murine leukemia virus-infected cells and in AKR-A cells.

Specificity of human antibodies to oncovirus glycoproteins: recognition of antigen by natural antibodies directed against carbohydrate structures

Antibodies in human sera from healthy individuals were shown to be reactive with highly purified 70,000-dalton envelope glycoprotein (gp70) of the simian sarcoma virus-simian sarcoma-associated virus (SSV-SSAV) complex in radioimmunoprecipitation assays under certain conditions. The specificity of the reaction was analyzed in absorption tests with normal human serum proteins, assays of viral gp70 antigenicity after exposure to exo- and endoglycosidases or trypsin, and carbohydrate hapten inhibition studies. On the basis of the results obtained in these experiments we have concluded that immune recognition of SSV-SSAV gp70 can be mediated by naturally occurring heterophil antibodies in human sera that are reactive by virtue of binding to the carbohydrate moiety of the viral gp70 molecules. The results are consistent with the idea that the antibodies in question are elicited as a result of exposure to many natural substances possessing widely crossreacting antigens and are not a result of widespread infection of man with replication-competent oncoviruses.

Serological and immunochemical studies of H-2 allospecificities on K36, a syngeneic tumour of AKR

Expression of H-2 antigenic specificities on K36, a spontaneous leukaemia originating from AKR (H-2k) mice, was studied by serology and immunochemistry. Two ascites lines of the tumour, as well as a tissue culture adjusted and cloned tumour line, were used in these studies with similar results being obtained. K36 expresses on its cell surface D-region encoded H-2K antigens but does not express K-region encoded H-2K alloantigens. It also expresses on its cell membrane, H-2 specificities of foreign haplotypes not present on normal AKR lymphoid cells. The molecular basis of the H-2Dd specificity on K36 (H-2k) was analysed by immunoprecipitation and polyacrylamide gel electrophoresis. The specificity was shown to be present on a glycoprotein of apparent molecular weight 45,000. However, antisera against the H-2Dd private specificity (H-2.4) precipitate additional glycoprotein of 45,000D and also 70,000D. In tryptic peptide maps of the isolated 45,000D fraction precipitated by anti-H-2.4 serum from radiolabelled K36 glycoprotein, all H-2Dd specific peptides were present in the same quantitative ratio. This is consistent with the structural identity of the foreign H-2Dd from the K36 tumour with normal H-2Dd and supports the hypothesis of a regulator system controlling the H-2 allelism. Under certain circumstances such a system could cause suppression of one and derepression of the other H-2 gene products.

Association of endogenous retroviruses with radiation-induced leukemias of BALB/c mice

X-irradiation of BALB/c mice in the second month of life induced a high incidence of generalized lymphatic leukemia of T-cell origin, beginning at 7 months of age. Infectious ecotropic murine leukemia virus (B-tropic predominant over N-tropic) was isolable from all tumor extracts but exhibited a wide titer range among individual leukemias. Detection of infectious xenotropic virus usually required extensive amplification on indicator cells. Dual-tropic (mink cell focus-forming) virus has not been found in the leukemias. Expression of ecotropic virus in tail extracts prepared at 6.5 months of age, although greatly enhanced compared with unirradiated controls, was not found to be prognostic of tumor development in individual mice. We conclude that leukemogenesis does not show a simple dependence on infectious murine leukemia virus expression in these mice.

Association of gross virus-associated cell-surface antigen with liposomes

Gross Cell-Surface Antigen (GCSAa) was obtained from W/Fu (C58NT)D lymphoma cells by Nonidet P40(NP40) or 3M KCl extraction and further purified by Sephadex G200 filtration. GCSAa was associated with lipids (dipalmitoylphosphatidycholine, cholesterol and dicetylphosphate, in molar ratios of 7:2:1) to form multilamellar liposomes. The amount of protein associated with liposomes was found to be proportional to the protein concentration of the sensitizing cellular extract and to the amount of phospholipids used and, under defined conditions, 22-55% of the protein of the cellular extract could be associated with liposomes. Analysis of disrupted sensitized liposomes showed that the GCSAa-specific activity of the liposome-associated proteins was quite similar to that of the proteins of the sensitizing cellular extract. Ultracentrifugation of disrupted liposomes showed that about 75% of the liposome-associated GCSAa activity was firmly associated with lipids and that little GCSAa was trapped within aqueous compartments between lipidic lamellae. 1.8--8.0% of the liposome-associated GCSAa was expressed at the liposome surface. No striking differences in degree of GCSAa association were found between liposomes sensitized by NP40 or by 3M KCl extracts. Storage experiments at +4 degrees C showed that GCSAa-sensitized liposomes were fairly stable.

Induction of antibody response to liposome-associated Gross-virus cell-surface antigen (GCSAa)

The immunogenicity of a soluble fraction containing Gross-virus-associated cell-surface antigen (GCSAa) obtained from (C58NT)D lymphoma cells either by detergent (NP40) solubilization or by 3M KCl extraction, was studied in syngeneic W/Fu rats. Rats immunized by 2 s.c. injections of soluble antigen or soluble antigen mixed with empty liposomes and emulsified in complete Freund's adjuvant (CFA) failed to produce significant levels of cytotoxic antibodies to GCSAa. On the other hand, rats similarly immunized by negatively charged liposomes containing NP40-solubilized GCSAa, and emulsified in CFA, developed high and persistent levels of cytotoxic antibodies, and their response could even mimic that induced by viable (C58NT)D cells. A similar response could also be obtained in rats immunized with liposome-associated NP40-solubilized GCSAa, but without CFA. Rats immunized by comparable amounts of liposome-assocated 3M KCl-extracted GCSAa developed only low levels of cytotoxic antibodies, and their response was of shorter duration. These results strongly suggest that inclusion into liposomes of a solubilized proteic tumour-associated cell-surface antigen can provide an immunogen as potent as viable tumour cells in inducing an antibody response, and that the solubilization method may be critical.

Immunochemical characterization of tumor-associated surface antigens on a Moloney leukemia virus-lymphoma, MBL-2

Tumor-associated surface antigens (TASA) on a Moloney leukemia virus (M-MuLV)-induced lymphoma, MBL-2, in C57BL/6 mice (B6) were characterized. The surface proteins of MBL-2 cells were selectively radioiodinated and then extracted by Nonidet P40. The solubilized materials were then reacted with a variety of antisera: monospecific antisera to murine leukemia viral proteins (anti-gp69/71, anti-p30, anti-p15, anti-p12 and anti-p10), sera from B6 which regressed murine sarcoma tumors induced by murine sarcoma virus (anti-MSV) and a rabbit anti-MBL-2 antiserum. The resulting radioimmune precipitates were analyzed and compared in sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The following results were obtained. (1) Among all anti-viral protein antisera tested only anti-gp69/71 was active and detected a protein doublet of gp69/71 and its degradation fragments of 42,000 and 35,000 daltons. (2) Radioimmune precipitates prepared with anti-MSV showed a SDS-PAGE pattern similar to that seen with anti-gp69/71. This result indicated that the surface antigen detected by the anti-MSV serum on MBL-2 tumor cell was probably a viral envelope antigen. (3) The rabbit anti-MBL-2 serum detected on the cell membrane an antigen of approximately 95,000 daltons which was tumor-associated and did not appear to be related to virion components. The anti-MBL-2-serum still reacted with the 95,000 dalton antigen after absorption with disrupted M-MuLV virus and with gp69/71 and p30 purified from the virus.

Detection of polymorphism in BALB/c leukemia viruses with mouse antisera

Antisera produced in mice recognize primarily type-specific antigenic determinants on both the major core protein, p30, and the major envelope proteins, gp70 and p15(E), of the endogenous leukemia viruses (MuLV) of BALB/c mice. Three different mouse sera were investigated in detail. (i) Antisera prepared in C57BL/6 mice against the AKR leukemia K36 reacted with the gp70, p15(E), and p30 proteins of MuLV. Certain pools of the C57BL/6 anti-AKR K36 serum contained antibodies which serologically distinguished the p30 proteins of N-ecotropic, B-ecotropic, and xenotropic BALB/c MuLV. (ii) Antisera prepared in BALB/c mice against the BALB/c sarcoma 1315 contained antibodies that reacted with a type-specific antigen of the 1315 MuLV gp70 that is not found on other BALB/c MuLV. (iii) The normal sera of multiparous BALB/c mice contained antibodies that reacted with gp70 and p15(E) proteins of ecotropic MuLV. Sera from some of these mice contained antibodies that serologically distinguished the gp70 of N-ecotropic and B-ecotropic BALB/c viruses. These results emphasize the utility of mouse antisera in the serological typing of MuLV. Furthermore, the antigenic differences observed in the p30 and gp70 proteins should be of particular use in the future analysis of recombinant BALB/c MuLV.

Polymorphism of B-tropic leukemia viruses from BALB/c mice: association of a p30 antigen with N- versus B-tropism

Comparison of a number of murine leukemia virus clones by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed extensive protein polymorphism among B-tropic, but not N-tropic, isolates from BALB/c mice, particularly in migration of p30 proteins. A type-specific radioimmunoassay for p30 was developed which uniformly discriminated all B-tropic viruses from N-tropic viruses of BALB/c origin. N- and B-tropic viruses of C57BL/6 and AKR Fv-1b/b origin could also be distinguished by this assay.