Rapid thymomas induced by Abelson murine leukemia virus

Role of Infectious Virus Expression and Immune Response in Retrovirus-Induced Oncogenesis

Some of the parameters involved in retrovirus-induced oncogenesis were analyzed in newborn mice injected with Moloney-murine leukemia virus (M-MuLV) as well as in adults who received the virus by the intraperitoneal (i.p.) or intrathymic (i.t.) route. The neonatally injected mice were permissive for both viral replication and virus-induced cell-surface antigen expression on thymus cells, peripheral T and B lymphocytes, and macrophages, whereas the M-MuLV was not present in the adult i.p. injected mice. Instead, in i.t. injected mice, the virus was expressed in thymus and peripheral T cells only, but was not detected in tail extracts as assessed by means of the UV-XC plaque assay. Lack of virus spread in adult-treated animals correlated with a prompt humoral and cellular immune response, whereas the highly viremic newborn inoculated mice showed negligible virus-specific antibody production and an extremely low frequency of splenic cytotoxic T lymphocyte precursors. Moreover, immune response in both groups of adult-treated mice efficiently prevented tumor induction by Moloney-murine sarcoma virus (M-MuSV), which has the same antigenic determinants as M-MuLV, its natural helper. In contrast, M-MuSV sarcomas grew progressively in newborn inoculated mice and killed the host. Finally, 80 % of neonatally injected mice developed lymphomas, whereas all treated adults remained free of disease for more than 15 months. These findings imply that the immune response may, in fact, prevent retrovirus-induced oncogenesis through restriction of virus replication and/or destruction of virus-infected cells.

Novel endogenous type D retroviral particles expressed at high levels in a SCID mouse thymic lymphoma

A xenograft model of the human disease Langerhans cell histiocytosis (LCH) was investigated with severe combined immunodeficiency (SCID) mice. Transplantation of human LCH biopsy material into SCID mice resulted in the generation of mouse tumors resembling lymphomas. A thymoma cell line (ThyE1M6) was generated from one of these mice and found to display significant levels of Mg2+-dependent reverse transcriptase activity. Electron microscopy revealed particles with type D retroviral morphology budding from ThyE1M6 cells at a high frequency, whereas control cultures were negative. Reverse transcription-PCR of virion RNA with degenerate primers for conserved regions of various mouse, human, and primate retroviruses amplified novel sequences related to primate type D retroviruses, murine intracisternal A particles, Jaagsiekte sheep retrovirus, and murine long interspersed nuclear elements but not other retroviral classes. We demonstrate that these sequences represent a novel group of endogenous retroviruses expressed at low levels in mice but expressed at high levels in the ThyE1M6 cell line. Furthermore, we propose that the activation of endogenous retroviral elements may be associated with a high incidence of thymomas in SCID mice.

v-Abl activates c-myc transcription through the E2F site

The v-abl oncogene of Abelson murine leukemia virus encodes a deregulated form of the cellular nonreceptor tyrosine kinase. v-Abl activates c-myc transcription, and c-Myc is an essential downstream component in the v-Abl transformation program. To explore the mechanism by which v-Abl activates c-myc transcription, a cotransfection assay was developed. We show that transactivation of a c-myc promoter by v-Abl requires the SH1 (tyrosine kinase) and SH2 domains of v-Abl; the C-terminal domains are not required for transactivation. The assay also identified the E2F site in the c-myc promoter as a v-Abl-responsive element. In addition, multimerized E2F sites were shown to be sufficient to confer v-Abl-dependent activation on a minimal promoter. This is the first identification of a v-Abl response element for transcriptional activation. v-Abl tyrosine kinase-dependent changes in proteins binding the c-myc E2F site were also demonstrated, including induction of a complex containing DP1, p107, cyclin A, and cdk2. Identification of v-Abl-dependent changes in E2F-binding proteins provides an important link between v-Abl, transcription, cell cycle regulation, and control of cellular growth.

BCR-ABL and v-abl oncogenes induce distinct patterns of thymic lymphoma involving different lymphocyte subsets

The human BCR-ABL oncogenes encoded by the Philadelphia chromosome (Ph) affect the pathogenesis of diverse types of leukemia and yet are rarely associated with T-lymphoid leukemia. To determine whether BCR-ABL kinases are inefficient in transforming T lymphocytes, BCR-ABL-expressing retroviruses were injected intrathymically into mice. Thymomas that expressed BCR-ABL kinase developed after a relatively long latent period. In most thymomas, deletion of 3' proviral sequences resulted in loss of tk-neo and occasionally caused expression of kinase-active carboxy-terminally truncated BCR-ABL oncoprotein. In contrast, deletion of 3' proviral sequences was not observed in thymomas induced with Abelson murine leukemia virus (A-MuLV). BCR-ABL viruses induced distinct patterns of disease and involved different thymocyte subsets than A-MuLV and Moloney murine leukemia virus (Mo-MuLV). While Mo-MuLV only induced Thy-1+ thymomas, v-abl- and BCR-ABL-induced thymomas often contained mixed populations of B220+ and Thy-1+ lymphocytes in the same tumor. In most v-abl and BCR-ABL tumors, Thy-1+ lymphoid cells expressed CD8 and a continuum of CD4 ranging from negative to positive. Conversely, Mo-MuLV thymomas contained distinct populations of CD4+ cells that were either CD8+ or CD8-. A-MuLV-transformed T-lymphoid cells did not express the CD3/T-cell receptor complex, while BCR-ABL tumors were CD3+. Thus, BCR-ABL viruses preferentially induce somewhat more differentiated T lymphocytes than are transformed by A-MuLV. Furthermore, rare B220+ lymphocytes may represent preferred v-abl and BCR-ABL transformation targets in the thymus.

Chronic myeloproliferative disease induced by site-specific integration of Abelson murine leukemia virus-infected hemopoietic stem cells

We recently showed that hemopoietic stem cells expressing the v-abl oncogene can cause leukemia when injected into lethally irradiated recipient mice. Progenitor cells expressing v-abl did not significantly contribute to disease development, and the leukemia was monoclonal in origin. By serially transplanting v-abl-transduced hemopoietic stem cells into normal, nonirradiated syngeneic recipients, we showed that multiple stem-cell clones do exist in some recipients. These cells fluctuated as normal stem cells do and could home to normal bone marrow. Based on the time course of disease, the recipients developed either an acute or a chronic phase of disorder. All recipients with the acute disease had stem-cell clones with random Abelson murine leukemia virus integration sites. All recipients with the chronic disorder had a specific Abelson murine leukemia virus integration site. We believe this abl-specific integration site, termed ASI, is important in abl-mediated stem-cell leukemogenesis.

v-abl causes hematopoietic disease distinct from that caused by bcr-abl

v-abl, the oncogene transduced by Abelson murine leukemia virus, was first characterized by its ability to transform lymphoid cells. bcr-abl, the oncogene formed by a t(9;22) translocation thought to occur in human hematopoietic stem cells, is detectable in almost all cases of chronic myelogenous leukemia (CML), a malignancy of granulocytic cells. bcr-abl also causes a CML-like syndrome in mice whose bone-marrow cells are infected with a retrovirus transducing the gene. More recent reports have suggested that v-abl can, however, cause a disease similar to CML. We demonstrate here that v-abl, when transduced in a helper virus-containing system, causes disease similar to, but distinct from, the CML-like syndrome induced by bcr-abl. Animals whose bone marrow has been infected by v-abl virus develop modest splenomegaly, marked granulocytosis, and malignant disease of several hematopoietic cell types. Unlike animals with CML-like disease resulting from bcr-abl, the polymorphonuclear leukocytes from animals infected with a v-abl construct do not contain the v-abl provirus at a significant frequency. Histopathologic analysis also shows significant differences between the diseases caused by v-abl and bcr-abl.

Thymic targets for Abelson murine leukemia virus are early gamma/delta T lymphocytes

Molecular analysis has shown that the majority of Abelson murine leukemia virus (Ab-MuLV)-induced primary thymomas represent transformed gamma/delta thymocytes. Many of these thymomas are of monoclonal origin as judged by provirus integration pattern and contain rearranged genes encoding T-cell receptor (TCR) gamma and delta chains but germ-line immunoglobulin heavy-chain genes. Some of the monoclonal tumors contain multiple rearranged alleles encoding TCR gamma, delta, and beta chains. Further, one Ab-MuLV thymoma cell line contained germ-line-configuration TCR gamma- and delta-chain genes, which became rearranged after in vitro propagation. Clones of this cell line were observed to rearrange these genes after intrathymic passage. Also, some subclones of this cell line underwent rearrangement of their immunoglobulin heavy-chain genes in culture. These observations suggest that the thymic targets for Ab-MuLV transformation are early gamma/delta thymocytes, some of which continue to rearrange their TCR gamma- and delta-chain genes.

Leukemia initiated by hemopoietic stem cells expressing the v-abl oncogene

We report a mouse model with which to study leukemogenesis initiated by a specific genetic change introduced into a primary lymphoid-myeloid pluripotent stem cell. Fetal liver hemopoietic cells were infected with a high titer of helper-free Abelson murine leukemia virus (A-MuLV) and were used to reconstitute lethally irradiated mice. Two weeks later, progenies of a single primitive hemopoietic stem cell carrying a specifically integrated A-MuLV proviral DNA could be detected in both colony-forming units in spleen and myeloid colony-forming cells in the bone marrow. Beginning at 3 weeks after transplantation, the recipients developed elevated leukocyte counts, splenomegaly, and increase of blast cells in the peripheral blood. Multiple clones of A-MuLV-infected cells were infused into each recipient. However, in the same animal, DNA extracted from various affected organs and from factor-independent lymphoid and myeloid immortalized cells all contained an identical, specifically integrated proviral genome. The A-MuLV-infected stem cells differentiated into various lineages of hemopoietic cells. Our data show that the expression of the v-abl oncogene in a primary lymphoid-myeloid hemopoietic stem cell directly initiates leukemogenesis by stimulating factor-independent growth. The monoclonal-type disease development seen in these animals may require the occurrence of an additional genetic event.

Fucose and galactose receptor and liver recognition by lymphoma cells

A syngeneic model system for the study of metastases is described. The system consisted of 2 lymphoma clones (A/63-I and A/63-2) derived from a single thymoma (A/63) induced by a wild-type Abelson-Moloney viral complex. Phenotype and genotype analyses revealed that both clones were derived from transformation of early T-cell precursors. An in vivo study of the colonizing potential following intravenous (i.v.) injection of clones showed that only the A/63-I cell clone colonized the liver. This observation was confirmed by quantitative analysis of organ distribution of both cell clones consecutive to i.v. injection of 125IUdR-labelled cells. In the same way, an in vitro study of the invasive potential of both clones was performed on frozen liver sections and showed that only the A/63-I cell clone had the ability to attach to liver. This specific adhesion was inhibited by L-fucose, D-galactose, N-acetyl-D-galactosamine (D-GalNAc) and with D-galactose- and L-fucose-containing neoglycoproteins. Differences in cell surface carbohydrates of the 2 cell clones were detected using various lectins: peanut agglutinin (PNA), Dolichos biflorus (DBA), Aleuria aurantia (AAA) and Galactia tenuiflora agglutinins (GTA). A/63-I was found to react strongly with PNA, DBA and GTA, and the removal of sialic acid by neuraminidase treatment increased DBA and PNA receptor sites of A/63-2 as compared to A/63-I. The present data suggest that cell-surface GalNAc, galactosyl and fucosyl residues are responsible for the ability of the A/63-I cell clone to recognize liver tissue probably through binding to a Kupffer-cell-associated lectin.

Lymphoid and mesenchymal tumors in transgenic mice expressing the v-fps protein-tyrosine kinase

src, abl, and fps/fes are prototypes for a family of genes encoding nonreceptor protein-tyrosine kinases. The oncogenic potential of the v-fps protein-tyrosine kinase was investigated by introduction of the gag-fps coding sequence of Fujinami sarcoma virus into the mouse germ line. Transgenic mice with v-fps under the transcriptional control of a 5' human beta-globin promoter (GF) or with both 5' and 3' beta-globin regulatory sequences (GEF) were viable. Unexpectedly, both GF and GEF transgenes were expressed in a wide variety of tissues and induced a spectrum of benign and malignant tumors. These tumors, which included lymphomas, thymomas, fibrosarcomas, angiosarcomas, hemangiomas, and neurofibrosarcomas, developed with various frequencies after latent periods of 2 to 12 months. The majority of lymphoid neoplasms appeared to be of T-cell origin and were monoclonal, as judged by rearrangements of the T-cell receptor beta or immunoglobulin genes. Some tissues that expressed the v-fps oncogene, such as heart, brain, lung, and testes, developed no malignant tumors. The v-fps protein-tyrosine kinase therefore has a broad but not unrestricted range of oncogenic activity in cells of lymphoid and mesenchymal origin. The incomplete penetrance of the neoplastic phenotype and the monoclonality of lymphoid tumors suggest that tumor formation in v-fps mice requires genetic or epigenetic events in addition to expression of the P130gag-fps protein-tyrosine kinase.

Lymphoid and mesenchymal tumors in transgenic mice expressing the v-fps protein-tyrosine kinase

src, abl, and fps/fes are prototypes for a family of genes encoding nonreceptor protein-tyrosine kinases. The oncogenic potential of the v-fps protein-tyrosine kinase was investigated by introduction of the gag-fps coding sequence of Fujinami sarcoma virus into the mouse germ line. Transgenic mice with v-fps under the transcriptional control of a 5' human beta-globin promoter (GF) or with both 5' and 3' beta-globin regulatory sequences (GEF) were viable. Unexpectedly, both GF and GEF transgenes were expressed in a wide variety of tissues and induced a spectrum of benign and malignant tumors. These tumors, which included lymphomas, thymomas, fibrosarcomas, angiosarcomas, hemangiomas, and neurofibrosarcomas, developed with various frequencies after latent periods of 2 to 12 months. The majority of lymphoid neoplasms appeared to be of T-cell origin and were monoclonal, as judged by rearrangements of the T-cell receptor beta or immunoglobulin genes. Some tissues that expressed the v-fps oncogene, such as heart, brain, lung, and testes, developed no malignant tumors. The v-fps protein-tyrosine kinase therefore has a broad but not unrestricted range of oncogenic activity in cells of lymphoid and mesenchymal origin. The incomplete penetrance of the neoplastic phenotype and the monoclonality of lymphoid tumors suggest that tumor formation in v-fps mice requires genetic or epigenetic events in addition to expression of the P130gag-fps protein-tyrosine kinase.

Early pre-B-cell transformation induced by the v-fms oncogene in long-term mouse bone marrow cultures

Murine long-term bone marrow cultures that support B-lymphoid-cell development were infected with a helper-free retrovirus containing the v-fms oncogene. Infection of B-lymphoid cultures resulted in the rapid clonal outgrowth of early pre-B cells, which grew to high cell densities on stromal cell feeder layers, expressed v-fms-coded glycoproteins, and underwent immunoglobulin heavy-chain gene rearrangements. Late-passage cultures gave rise to factor-independent variants that proliferated in the absence of feeder layers, developed resistance to hydrocortisone, and became tumorigenic in syngeneic mice. The v-fms oncogene therefore recapitulates known effects of the v-abl and bcr-abl oncogenes on B-lineage cells. The ability of v-fms to induce transformation of early pre-B cells in vitro underscores the capacity of oncogenic mutants of the colony-stimulating factor-1 receptor to function outside the mononuclear phagocyte lineage.

A bcr-v-abl oncogene induces lymphomas in transgenic mice

In chronic myeloid leukemia and some cases of acute lymphoblastic leukemia, a 9;22 chromosome translocation has fused most of the c-abl oncogene to a gene designated bcr. To explore in vivo the biological effects of the chimeric gene, we introduced a facsimile of the translocation product, a bcr-v-abl gene, into the mouse germ line under the control of the immunoglobulin heavy-chain enhancer or a retroviral long terminal repeat. Some transgenic mice bearing either construct developed clonal lymphoid tumors. T lymphomas predominated, but some pre-B lymphomas developed. The transgenes were expressed in the tumors but not detectably in the lymphoid tissues of nontumorous transgenic animals, implying that transcription is activated by a low-frequency somatic event. These results demonstrate that bcr-v-abl is tumorigenic in vivo and provide a new animal model for lymphomagenesis.

Identification of a novel bone marrow-derived B-cell progenitor population that coexpresses B220 and Thy-1 and is highly enriched for Abelson leukemia virus targets

A novel stage in early B-lymphocyte differentiation has been identified in normal mouse bone marrow cells. Earlier work had demonstrated that bone marrow cells characterized by low levels of Thy-1 and lack of a panel of lineage markers (Thy-1lo Lin- cells) were highly enriched for pluripotent hematopoietic stem cells. In this paper, we present evidence that another bone marrow population, which expressed low levels of Thy-1 and coexpressed B220, a B-lineage-specific form of the leukocyte common antigen, contained early and potent precursors for B lymphocytes upon in vivo transfer to irradiated hosts. These Thy-1lo B220+ cells, comprising 1 to 2% of bone marrow cells, were enriched for large cells in the mitotic cycle; the population lacked significant pluripotent hematopoietic stem cell activity and myeloid-erythroid progenitors. Most strikingly, Thy-1lo B220+ cells represented a highly enriched population of bone marrow cells that could be targets of Abelson murine leukemia virus transformation. We propose that Thy-1lo B220+ bone marrow cells represent the earliest stage of committed lymphocyte progenitors, intermediate in differentiation between Thy-1lo Lin- pluripotent stem cells and, in the B lineage, Thy-1- B220+ pre-B cells.

Early pre-B-cell transformation induced by the v-fms oncogene in long-term mouse bone marrow cultures

Murine long-term bone marrow cultures that support B-lymphoid-cell development were infected with a helper-free retrovirus containing the v-fms oncogene. Infection of B-lymphoid cultures resulted in the rapid clonal outgrowth of early pre-B cells, which grew to high cell densities on stromal cell feeder layers, expressed v-fms-coded glycoproteins, and underwent immunoglobulin heavy-chain gene rearrangements. Late-passage cultures gave rise to factor-independent variants that proliferated in the absence of feeder layers, developed resistance to hydrocortisone, and became tumorigenic in syngeneic mice. The v-fms oncogene therefore recapitulates known effects of the v-abl and bcr-abl oncogenes on B-lineage cells. The ability of v-fms to induce transformation of early pre-B cells in vitro underscores the capacity of oncogenic mutants of the colony-stimulating factor-1 receptor to function outside the mononuclear phagocyte lineage.

In Vitro Immune-Receptor Gene Rearrangemets in Clonal Thymic Lymphomas Induced by Abelson Murine Leukemia Virus

The Abelson and Moloney murine leukemia virus complex (A-MuLV/M-MuLV) induces rapidly growing thymic lymphomas following direct injection into the thymus of newborn BALB/c and C57BL/6 mice. Southern blot analysis with a v-abl specific probe not only demonstrated that primary tumors are clonal, but also that the pattern of A-MuLV provirus integration is quite stable in primary tumor cells, as well as in their derived cell lines and clones. Most of the cell samples were able to rearrange the immunoglobulin heavy chain genes in culture, whereas in two cases the T cell receptor gamma chain genes also underwent rearrangement. Since the recombination mechanism is operative only in very immature lymphoid cells, these data provide indirect evidence for the lack of differentiation of A-MuLV cell targets in the thymus.

Regulation of T lymphocyte proliferation. Interleukin 2-mediated induction of c-myb gene expression is dependent on T lymphocyte activation state

We previously reported that with time, after antigenic stimulation of antigen-regulated murine T lymphocyte clones, total IL-2-R expression decayed 10-50-fold, commensurate with a decline in the ability of the cells to proliferate to IL-2. However, late after antigenic stimulation, when the cells were refractory to the IL-2-proliferative stimulus, high levels of high affinity IL-2-R remained. In this report we further explore the basis of unresponsiveness to IL-2 in the quiescent clones. We show that the proto-oncogene c-myc is induced in the late cell population by IL-2 to comparable levels observed early after antigen stimulation. IL-2-dependent c-myb induction, however, is seen only early after activation but not in the late-activated population. Analysis of the IL-2-dependent expression of c-myb mRNA with time after antigenic stimulation showed that steadystate c-myb expression declines dramatically with kinetics closely paralleling a decay in IL-2-dependent proliferative ability. In contrast, steadystate c-myc expression remains high throughout this period. Expression of c-myb is critical for proliferation of these cells since antisense oligodeoxy-nucleotide to c-myb can inhibit their IL-2-dependent proliferation. We present evidence for a pathway of c-myb induction via the TCR that is independent of the IL-2/IL-2-R interaction. In addition, the inhibition of IL-2-R-induced c-myb expression by 2-aminopurine and enhanced induction of c-myb via the TCR demonstrate that TCR activation and IL-2-R activation lead to induction of c-myb by different mechanisms.

A bcr-v-abl oncogene induces lymphomas in transgenic mice

In chronic myeloid leukemia and some cases of acute lymphoblastic leukemia, a 9;22 chromosome translocation has fused most of the c-abl oncogene to a gene designated bcr. To explore in vivo the biological effects of the chimeric gene, we introduced a facsimile of the translocation product, a bcr-v-abl gene, into the mouse germ line under the control of the immunoglobulin heavy-chain enhancer or a retroviral long terminal repeat. Some transgenic mice bearing either construct developed clonal lymphoid tumors. T lymphomas predominated, but some pre-B lymphomas developed. The transgenes were expressed in the tumors but not detectably in the lymphoid tissues of nontumorous transgenic animals, implying that transcription is activated by a low-frequency somatic event. These results demonstrate that bcr-v-abl is tumorigenic in vivo and provide a new animal model for lymphomagenesis.

Identification of a novel bone marrow-derived B-cell progenitor population that coexpresses B220 and Thy-1 and is highly enriched for Abelson leukemia virus targets

A novel stage in early B-lymphocyte differentiation has been identified in normal mouse bone marrow cells. Earlier work had demonstrated that bone marrow cells characterized by low levels of Thy-1 and lack of a panel of lineage markers (Thy-1lo Lin- cells) were highly enriched for pluripotent hematopoietic stem cells. In this paper, we present evidence that another bone marrow population, which expressed low levels of Thy-1 and coexpressed B220, a B-lineage-specific form of the leukocyte common antigen, contained early and potent precursors for B lymphocytes upon in vivo transfer to irradiated hosts. These Thy-1lo B220+ cells, comprising 1 to 2% of bone marrow cells, were enriched for large cells in the mitotic cycle; the population lacked significant pluripotent hematopoietic stem cell activity and myeloid-erythroid progenitors. Most strikingly, Thy-1lo B220+ cells represented a highly enriched population of bone marrow cells that could be targets of Abelson murine leukemia virus transformation. We propose that Thy-1lo B220+ bone marrow cells represent the earliest stage of committed lymphocyte progenitors, intermediate in differentiation between Thy-1lo Lin- pluripotent stem cells and, in the B lineage, Thy-1- B220+ pre-B cells.

Distinct helper virus requirements for Abelson murine leukemia virus-induced pre-B- and T-cell lymphomas

Abelson murine leukemia virus (A-MuLV) can induce pre-B- or T-cell lymphomas (thymomas) in mice depending on the route and time of injection. Previous studies have shown that the choice of the helper virus used to rescue A-MuLV greatly influences its ability to induce pre-B-cell lymphomas. In this study, we investigated the role of the helper virus in A-MuLV-induced thymomas. A-MuLV rescued with the helper Moloney MuLV, BALB/c endogenous N-tropic MuLV, and two chimeric MuLVs derived from these two parents were injected intrathymically in young adult NIH Swiss mice. All four A-MuLV pseudotypes were found to be equally efficient in the induction of thymomas, whereas drastic differences were observed in their pre-B-cell lymphomagenic potential. Thymoma induction by A-MuLV was independent of the replication potential of the helper virus in the thymus, and no helper proviral sequences could be detected in the majority of thymomas induced by A-MuLV rescued with parental BALB/c endogenous or chimeric MuLVs. In the thymomas in which helper proviruses were present, none of them were found integrated in the Ahi-1 region, a common proviral integration site found in A-MuLV-induced pre-B-cell lymphomas (Y. Poirer, C. Kozak, and P. Jolicoeur, J. Virol. 62:3985-3992, 1988). In addition, helper-free stocks of A-MuLV were found to be as lymphomagneic as other pseudotypes in inducing thymomas after intrathymic inoculation, in contrast to their inability to induce pre-B-cell lymphomas when injected intraperitoneally in newborn mice. Restriction enzyme analysis revealed one to three A-MuLV proviruses in each thymoma, indicating the oligoclonality of these tumors. Analysis of the immunoglobulin and T-cell receptor loci confirmed that the major population of cells of these primary thymomas belongs to the T-cell lineage. Together, these results indicate that the helper virus has no effect in the induction of A-MuLV-induced T-cell lymphomas, in contrast to its important role in the induction of A-MuLV-induced pre-B-cell lymphomas. Our data also revealed distinct biological requirements for transformation of these two target cells by v-abl.

B cell precursors are present in the thymus during early development

An in vitro system for transforming immature lymphoid cells present in the thymus at early development has been established. By phenotype analysis of the transformants obtained, we observed that B cell precursors, susceptible to Abelson murine leukemia virus (A-MuLV)- or Harvey murine sarcoma virus (H-MuSV)-induced lymphogenesis, were present at high frequency in the fetal thymus of BALB/c mice. These precursors recolonized alymphoid thymus lobes in vitro, as do T cell precursors. It was further observed that B precursors in the fetal liver were also capable of recolonizing alymphoid thymus lobes and were stored in a thymic environment. These results suggest that stroma cells of the fetal thymus may possess the capacity to support the growth of B precursors. On the other hand, B cell precursors sensitive to the viral transformation were undetectable in the fetal thymus of C57BL/6, although immunohistochemical analysis suggested their presence. However, in the fetal liver of the same strain, B precursors recolonizing alymphoid thymus in vitro were sensitive to the viral transformation. Based on these results, we will discuss both the role and fate of thymic B precursors. In addition, we also obtained T cell lymphomas at different stages of differentiation from the fetal thymus of C57BL/6 infected with A-MuLV or H-MuSV. These data indicate the usefulness of our system in establishing cell lines derived from intrathymic lymphogenesis at early development.

In vitro malignant progression of cells derived from Abelson murine leukaemia virus-induced thymic lymphomas

Cell lines derived from A-MuLV induced thymic lymphomas in BALB/c and C57BL/6 mice were analysed for their in vivo and in vitro potential of growth. Despite their immunogenicity, cell lines of BALB/c origin readily grew in syngeneic recipients. On the contrary, all cell lines of C57BL/6 origin failed to grow in immunocompetent hosts even though they were able to form tumours in immunosuppressed syngeneic mice. Among C57BL/6 lymphoma cells progression toward a more malignant phenotype was observed in TB6-3 cells, and in their derived clones, after several in vitro passages. This event was accompanied by the in vitro loss of requirement for exogenous growth factor(s) when tumorigenic TB6-3 cells were plated at high density. Moreover, culture medium from fully malignant TB-3 cells was mitogenic for mature T-lymphoma cells suggesting the involvement of an autocrine mechanism in the control of cell proliferation. Apparently, the viral oncogene (v-abl) is not directly involved in malignant progression since no differences between nontumorigenic and tumorigenic cells could be detected in A-MuLV integration patterns, v-abl specific mRNA expression, and P160gag-abl production.

Proliferation and differentiation of Abelson virus-infected murine myeloid leukemia cell lines does not involve an autocrine mechanism

Culture in agar of cloned promonocytic leukemia cell lines derived from Abelson virus-infected mice produced colonies of both a compact and diffuse morphology. Diffuse colonies contained fewer cells capable of forming colonies when recultured in agar than did compact colonies. Serial subcloning of cells from diffuse, but not compact, colonies ultimately led to the complete loss of colony-forming cells, i.e., to clonal extinction. The production of both compact and diffuse agar colonies was independent of the cell density of either the static liquid culture from which cells were taken for culture in agar, or the number of cells per agar culture. Furthermore, bioassays of culture supernatants indicated the leukemia cells did not secrete hemopoietic growth factors active on normal hemopoietic cells, transforming growth factors active on adherent cell lines, or factors that influenced the growth of the leukemic cells themselves. Collectively, these data suggest neither growth-factor independent replication nor the spontaneous differentiation of Abelson virus-infected myeloid cells involves autocrine secretion of growth regulators.

The viral and cellular forms of the Abelson (abl) oncogene

The precision of molecular biology has allowed a better definition of the components of the Abelson system. We know the gene structures and gene products for the cellular and viral forms of this family of related tyrosine kinases. However, many basic issues first identified in the early biological observations of Abelson, Rabstein, and others remain unanswered. The precise pathway for transformation in biochemical terms remains unknown for Ab-MLV and all of its relatives. Relatively little can be said to explain the preferential growth stimulation for certain hematopoietic cell types by the viral and other altered forms of the oncogene, and no clear insights into the function of the normal cellular forms of the abl oncogene are available. Future progress will certainly depend on the intensive efforts by many workers in the broader field of cellular growth control mechanisms.

Important role of the long terminal repeat of the helper Moloney murine leukemia virus in Abelson virus-induced lymphoma

The helper virus has been shown to play a critical role in the development of lymphoma induced by the defective Abelson murine leukemia virus (A-MuLV). Indeed, A-MuLV pseudotyped with some viruses, such as the Moloney MuLV, has been shown to be highly lymphogenic, whereas A-MuLV pseudotyped with other viruses, such as the BALB/c endogenous N-tropic MuLV, has been shown to be devoid of lymphogenic potential (N. Rosenberg and D. Baltimore, J. Exp. Med. 147:1126-1141, 1978; C. D. Scher, J. Exp. Med. 147: 1044-1053, 1978). To map the viral DNA sequences encoding the determinant of the lymphogenic potential of Moloney MuLV when complexed with A-MuLV, we constructed chimeric helper viral DNA genomes in vitro between parental cloned infectious viral DNA genomes from Moloney MuLV and from BALB/c endogenous N-tropic MuLV. Chimeric helper MuLVs, recovered after transfection of NIH 3T3 cells were used to rescue A-MuLV, and the pseudotypes were inoculated into newborn NIH Swiss, CD-1, and SWR/J mice to test their lymphogenic potential. We found that a 0.44-kilobase-pair PstI-KpnI long terminal repeat-containing fragment from the Moloney MuLV was sufficient to confer some, but not complete, lymphogenic potential to a chimeric virus (p7M2) in NIH Swiss and SWR/J mice, but not in CD-1 mice. The addition of the 3'-end env sequences (comprising the carboxy terminus of gp70 and all p15E) to the U3 long terminal repeat sequences restored the full lymphogenic potential of the Moloney MuLV. Our data indicate that the 3'-end sequences of the helper Moloney MuLV are somehow involved in the development of lymphoma induced by A-MuLV. The same sequences have previously been found to harbor the determinant of leukemogenicity and of disease specificity of Moloney MuLV when inoculated alone.

Abelson virus transformation of an interleukin 2-dependent antigen-specific T-cell line

Abelson murine leukemia virus (A-MuLV) carries the gene v-abl, one of a group of oncogenes with structural and functional (tyrosine kinase) homology to three growth factor receptors. Work in this and other laboratories has shown that A-MuLV infection can render myeloid and lymphoid cells independent of the growth factors interleukin 3 and granulocyte-macrophage colony-stimulating factor. We have now shown that v-abl can also relieve interleukin 2 (IL-2) dependence in T cells. We infected a cloned IL-2-dependent antigen-specific cell line. Transformed cells were generated which were factor independent and tumorigenic. The transformants each bore unique v-abl DNA inserts and expressed v-abl mRNA. No elevation of expression of either IL-2 or its receptor could be detected in these cells. Thus, A-MuLV can short-circuit the dependence of hematopoietic cells on IL-2, IL-3, and possibly granulocyte-macrophage colony-stimulating factor, none of whose receptors are known to be of the tyrosine kinase type.

Cell transformation and tumor induction by Abelson murine leukemia virus in the absence of helper virus

We investigated the role of the Moloney helper virus, Moloney murine leukemia virus (Mo-MuLV), in cell transformation and tumor induction by the defective Abelson murine leukemia virus (Ab-MuLV). A molecular clone of Ab-MuLV (P160 strain) was transfected into the psi 2 packaging cell line, and helper virus-free Ab-MuLV (psi 2) was harvested from the supernatant medium. Ab-MuLV (psi 2) was as efficient as helper virus-containing Ab-MuLV (Mo-MuLV) in the transformation of primary bone marrow cells in vitro. Inoculation of weanling BALB/c mice with Ab-MuLV (psi 2) induced nonthymic pre-B-cell lymphomas with high efficiency and short latency (28 days). Adult BALB/c mice were less sensitive to tumor induction by a factor of 100. Ab-MuLV (psi 2) did not induce tumors in weanling C57BL/6 mice, unlike Ab-MuLV (Mo-MuLV). Examination of the proviral integration pattern in Ab-MuLV (psi 2)-induced tumor cell DNA revealed that each of the tumors contained a single integrated provirus. Immunoprecipitation of viral-encoded proteins in helper virus-free tumor cell lines detected the P160 Ab-MuLV-transforming protein; however, no trace of the gag, pol, and env helper virus-encoded proteins was found. Our results indicate that integration and expression of a single Ab-MuLV genome is sufficient for efficient transformation of primary bone marrow cells by Ab-MuLV in vitro and tumor induction in susceptible mice. However, the helper virus may contribute to tumor induction in weanling resistant mice.

Abelson virus transformation of an interleukin 2-dependent antigen-specific T-cell line

Abelson murine leukemia virus (A-MuLV) carries the gene v-abl, one of a group of oncogenes with structural and functional (tyrosine kinase) homology to three growth factor receptors. Work in this and other laboratories has shown that A-MuLV infection can render myeloid and lymphoid cells independent of the growth factors interleukin 3 and granulocyte-macrophage colony-stimulating factor. We have now shown that v-abl can also relieve interleukin 2 (IL-2) dependence in T cells. We infected a cloned IL-2-dependent antigen-specific cell line. Transformed cells were generated which were factor independent and tumorigenic. The transformants each bore unique v-abl DNA inserts and expressed v-abl mRNA. No elevation of expression of either IL-2 or its receptor could be detected in these cells. Thus, A-MuLV can short-circuit the dependence of hematopoietic cells on IL-2, IL-3, and possibly granulocyte-macrophage colony-stimulating factor, none of whose receptors are known to be of the tyrosine kinase type.

T-cell receptor and immunoglobulin genes are rearranged together in Abelson virus-transformed pre-B and pre-T cells

We have assessed the state of rearrangement and expression of B- and T-cell antigen receptor genes in cells of Abelson murine leukemia virus-transformed thymomas and other tumors. We found that unrearranged TcR gamma genes are expressed, as are unrearranged C mu genes, in pre-T, pre-B, and myeloid cells. We also found TcR gamma genes rearranged and expressed in putative pre-T cells and in cells apparently committed to the B-cell lineage. This is in contrast to the data from more mature T- and B-cell tumors. We conclude that in immature lymphoid cells both immunoglobulin and TcR gamma genes are accessible for rearrangement. We discuss the implications of these observations for an understanding of the B-T lymphoid differentiation event.

Abelson virus potentiates long-term growth of mature B lymphocytes

Abelson murine leukemia virus (A-MuLV) infection of mouse bone marrow cells usually leads to transformation of pre-B cells. However, when the environment is modified by the continuous presence of lipopolysaccharide (LPS), two novel types of membrane immunoglobulin (mIg)-positive B cell lines are generated. Because the cells which give rise to these cell lines copurify with mIg-positive bone marrow cells, the cell lines arise as a result of A-MuLV interaction with a new type of in vitro target cell. The cell lines generated fall into two groups which differ in several phenotypic characteristics. Group 1 cells are more differentiated than the typical pre-B cell transformant in that they synthesize mIgM and appear to resemble virgin B cells. The group 1 cells do not secrete immunoglobulin and are independent of LPS for growth. In addition, these cell lines synthesize the Abelson P160 protein, contain integrated abl proviral DNA, and are highly tumorigenic in syngeneic animals. The group 2 cell lines differ markedly from both the group 1 cells and from typical, pre-B cell A-MuLV transformants. These cells are mIgG positive and secrete large amounts of immunoglobulin into the culture medium. The cell lines are comprised of both adherent and nonadherent cells and do not synthesize P160 or contain integrated v-abl sequences. The group 2 cells are nontumorigenic in syngeneic animals and require LPS for growth and viability. Both types of cells have remained in culture for over 2 years with no changes in their phenotypic characteristics. This A-MuLV infection system and the novel mIg-positive cell lines may serve as useful models for studying biochemical and molecular properties of mature B cells.

T-cell receptor and immunoglobulin genes are rearranged together in Abelson virus-transformed pre-B and pre-T cells

We have assessed the state of rearrangement and expression of B- and T-cell antigen receptor genes in cells of Abelson murine leukemia virus-transformed thymomas and other tumors. We found that unrearranged TcR gamma genes are expressed, as are unrearranged C mu genes, in pre-T, pre-B, and myeloid cells. We also found TcR gamma genes rearranged and expressed in putative pre-T cells and in cells apparently committed to the B-cell lineage. This is in contrast to the data from more mature T- and B-cell tumors. We conclude that in immature lymphoid cells both immunoglobulin and TcR gamma genes are accessible for rearrangement. We discuss the implications of these observations for an understanding of the B-T lymphoid differentiation event.

Transformation of T-lymphoid cells by Abelson murine leukemia virus

Abelson leukemia virus (A-MuLV) is an oncogenic murine retrovirus whose genome contains sequences homologous to those of a normal cellular gene, c-abl. It has been demonstrated to cause rapid transformation of several cell types, including pre-B lymphocytes, macrophages, and fibroblasts. More recently, A-MuLV has been reported to induce thymic tumors in a mouse strain (C57BL/Ka) previously thought to be resistant to disease induction. We showed that the masses occurring after intrathymic injection of the virus were composed of lymphocytes of a previously described immature T-cell phenotype. This phenotype has been defined here by flow cytometry of 10 primary tumor samples stained with antibodies to several thymocyte differentiation antigens. Hybridization of DNAs from these tumors with v-abl, immunoglobulin mu, and T-cell antigen receptor beta-chain probes confirmed the T-lymphoid, polyclonal nature of the primary tumor cells. The primary tumors were malignant, as clearly shown by reinjection into Thy-congenic host animals. Further, four Thy- in vitro cell lines derived from three tumors differed from the majority of primary tumor cells and were similar to previously described A-MuLV-transformed pre-B cells. The consistent T-lymphoid phenotype exhibited by primary A-MuLV thymomas may represent one stage of normal thymocyte differentiation.

Protein phosphorylation at tyrosine residues in v-abl transformed mouse lymphocytes and fibroblasts

Phosphotyrosine antibodies were employed to immunodecorate and immunoprecipitate proteins phosphorylated at tyrosine residues in cells transformed by Abelson murine leukemia virus (A-MuLV). In pre-B and pre-T lymphoma cells transformed by A-MuLV, the major phosphotyrosine-containing protein has an MW of 160 kDa and shares immunologically detectable sequences with the v-abl oncogene product. Moreover, two different proteins of approximately 100 and 68 kDa, heavily phosphorylated at tyrosine, were identified. Lack of immunological cross-reactivity with viral products and phosphopeptide mapping showed that the 100 and 68 kDa proteins are coded by cellular genes. Phosphoproteins were undetectable in control resting lymphocytes. The 68 and the 100 kDa proteins were phosphorylated to different extents in proliferating lymphocytes, either stimulated by the growth factor IL-2, or transformed by M-MuLV (lacking the oncogene coded kinase). In fibroblasts transformed by A-MuLV, phosphotyrosine antibodies identified 2 proteins of 120 and 70 kDa. By immunological cross-reaction and by phosphopeptide mapping, the first was identified as a 120 kDa form of the v-abl coded kinase. The 70 kDa protein is coded by a cellular gene, is not structurally related to the 120 kDa v-abl kinase, and is different from any phosphotyrosine-containing protein detected in A-MuLV-transformed lymphocytes. These data show that, upon v-abl-induced transformation, phosphorylation at tyrosine takes place also on proteins other than the 160 or 120-kDa oncogene products. In lymphocytes and fibroblasts these proteins are different, suggesting that the cascade of events triggered by the v-abl gene in different cell types involves tyrosine phosphorylation of different specific proteins.

Abelson virus potentiates long-term growth of mature B lymphocytes

Abelson murine leukemia virus (A-MuLV) infection of mouse bone marrow cells usually leads to transformation of pre-B cells. However, when the environment is modified by the continuous presence of lipopolysaccharide (LPS), two novel types of membrane immunoglobulin (mIg)-positive B cell lines are generated. Because the cells which give rise to these cell lines copurify with mIg-positive bone marrow cells, the cell lines arise as a result of A-MuLV interaction with a new type of in vitro target cell. The cell lines generated fall into two groups which differ in several phenotypic characteristics. Group 1 cells are more differentiated than the typical pre-B cell transformant in that they synthesize mIgM and appear to resemble virgin B cells. The group 1 cells do not secrete immunoglobulin and are independent of LPS for growth. In addition, these cell lines synthesize the Abelson P160 protein, contain integrated abl proviral DNA, and are highly tumorigenic in syngeneic animals. The group 2 cell lines differ markedly from both the group 1 cells and from typical, pre-B cell A-MuLV transformants. These cells are mIgG positive and secrete large amounts of immunoglobulin into the culture medium. The cell lines are comprised of both adherent and nonadherent cells and do not synthesize P160 or contain integrated v-abl sequences. The group 2 cells are nontumorigenic in syngeneic animals and require LPS for growth and viability. Both types of cells have remained in culture for over 2 years with no changes in their phenotypic characteristics. This A-MuLV infection system and the novel mIg-positive cell lines may serve as useful models for studying biochemical and molecular properties of mature B cells.

Retrovirus-induced spongiform encephalopathy: the 3'-end long terminal repeat-containing viral sequences influence the incidence of the disease and the specificity of the neurological syndrome

Using chimeric murine leukemia viruses (MuLVs) constructed in vitro with parental viral genomes from the neurotropic Cas-BR-E MuLV and the nonneurotropic amphotropic 4070-A MuLV, we previously mapped the paralysis-inducing determinant of Cas-BR-E MuLV within a pol-env region. To assess the role of the long terminal repeats (LTRs) in influencing the neurological disease, we constructed another chimeric MuLV (pNEMO-1)m harboring the gag-pol-env from Cas-BR-E MuLV and the LTR region from the strongly T-cell tropic Moloney MuLV. Although the Cas-BR-E MuLV induced mainly nonthymic leukemia, pNEMO-1 MuLV induced a thymic form of leukemia, as the parental Moloney MuLV. The pNEMO-1 MuLV induced neurological diseases less frequently than Cas-BR-E MuLV when inoculated intraperitoneally into NIH/Swiss, SIM.S, and SWR/J mice. However, it induced neurological disorders more frequently and with a shorter latency than Cas-BR-E MuLV when inoculated intrathymically. Most mice with a neurological disorder induced with pNEMO-1 MuLV showed a new clinical syndrome not usually seen with the parental Cas-BR-E MuLV: They had no lower limb paralysis but were excessively tremulous, spastic, and immobile. The topographical distribution of the spongiform degeneration in the brain of mice with this new syndrome was different from that seen in mice with lower limb paralysis induced by Cas-BR-E MuLV. These results indicate that the 1.0-kilobase-pair Cla I-Pvu I LTR-containing fragment harbors sequences influencing the incidence and the clinical manifestation of the neurological disease and suggest a specificity of LTR sequences for a new tissue (brain).

Isolation and characterization of Abelson murine leukemia virus-transformed mast cell lines from midgestation embryonic placenta

Abelson murine leukemia virus was used to transform cells of the midgestation embryonic placenta. The frequency of transformed foci in semisolid agarose was highest when cells were isolated at 10 days of gestation and cell lines could be established in liquid culture. The continuous cell lines express characteristics of cultured mast cells, including surface antigens which are shared with lymphocytes and mononuclear phagocytes. These results imply a relationship between the transforming gene product and the mast cell growth factor interleukin 3.

Different genes control the susceptibility of mice to Moloney or Abelson murine leukemia viruses

The susceptibility of mice to lymphoma induction by Moloney or Abelson murine leukemia virus has been compared in BALB/c, C57BL/6, and BALB/cXC57BL/6 recombinant inbred strains. BALB/c mice were found to be susceptible to lymphoma induction by either virus, and C57BL/6 mice were found to be relatively resistant to lymphoma induction by either virus. The genes that control these patterns of susceptibility to each virus are not the same because susceptibility to each virus segregated independently in CXB recombinant inbred strains. We also found, as reported by Cook (W. Cook, Proc. Natl. Acad. Sci. U.S.A. 79:2917-2921, 1982), when injected intrathymically that Abelson murine leukemia virus rapidly induced thymomas in weanling B6 mice. Examination of the cellular phenotypes of the tumors induced by Abelson murine leukemia virus or by Moloney murine leukemia virus indicated that different lymphocyte subpopulations were the targets for tumor induction by each virus.

Differentiation of Abelson murine leukemia virus-infected promonocytic leukemia cells

Virus-producing, tumorigenic, promonocytic leukemia cell lines were derived from Abelson murine leukemia virus-infected mice. This study shows that, of these 25 cloned lines, 22 were capable of extensive differentiation. It also shows that granulocyte-macrophage colony-stimulating activity increased the proportion of differentiating cells in 17/22 lines. Cells from agar colonies with a diffuse colony morphology had an increased expression of mature macrophage phenotypic characteristics, and a reduced proliferative capacity in vitro, compared to cells from agar colonies with a compact colony morphology. Cells from diffuse colonies also produced less Abelson virus, and were less tumorigenic in vivo than cells from compact colonies. Together, these results suggest some Abelson virus-producing leukemic cells are not blocked in their capacity to differentiate and are capable of reversing the transformed phenotype.

Thymocyte subsets transformed by Abelson murine leukemia virus

The infectious complex of Abelson murine leukemia virus was altered by replacing its usual helper virus, Moloney leukemia virus, with radiation leukemia virus (RadLV). After intrathymic injection of the Abelson-RadLV complex, thymomas arose rapidly, as described previously for injection of the Abelson-Moloney complex. Cell lines were derived from thymomas induced by each Abelson virus complex and were classified according to normal thymus cell phenotypes. Each virus complex induced some cell lines which were like a 0.7% subpopulation of murine thymocytes in that they failed to express the Thy-1 cell-surface antigen. These lines are thus far indistinguishable from some Abelson-derived bone marrow transformants classified as pre-B cells. However, the Abelson-Moloney complex induced some cell lines which expressed low levels of Thy-1 and which shared most markers with immature blast cells of the thymic medulla, whereas the Abelson-RadLV complex induced some lines which were clearly like thymic cortex blast cells. Thus, Abelson virus can induce thymoma cell lines of at least two, and possibly three, distinct phenotypes corresponding to normal thymocyte blast subsets, the determination of which can be influenced by helper virus sequences.

Thymocyte subsets transformed by Abelson murine leukemia virus

The infectious complex of Abelson murine leukemia virus was altered by replacing its usual helper virus, Moloney leukemia virus, with radiation leukemia virus (RadLV). After intrathymic injection of the Abelson-RadLV complex, thymomas arose rapidly, as described previously for injection of the Abelson-Moloney complex. Cell lines were derived from thymomas induced by each Abelson virus complex and were classified according to normal thymus cell phenotypes. Each virus complex induced some cell lines which were like a 0.7% subpopulation of murine thymocytes in that they failed to express the Thy-1 cell-surface antigen. These lines are thus far indistinguishable from some Abelson-derived bone marrow transformants classified as pre-B cells. However, the Abelson-Moloney complex induced some cell lines which expressed low levels of Thy-1 and which shared most markers with immature blast cells of the thymic medulla, whereas the Abelson-RadLV complex induced some lines which were clearly like thymic cortex blast cells. Thus, Abelson virus can induce thymoma cell lines of at least two, and possibly three, distinct phenotypes corresponding to normal thymocyte blast subsets, the determination of which can be influenced by helper virus sequences.