The reduced virulence of the thymotropic Moloney murine leukemia virus derivative MoMuLV-TB is mapped to 11 mutations within the U3 region of the long terminal repeat

Recombinant Origins of Pathogenic and Nonpathogenic Mouse Gammaretroviruses with Polytropic Host Range

Ecotropic, xenotropic, and polytropic mouse leukemia viruses (E-, X-, and P-MLVs) exist in mice as infectious viruses and endogenous retroviruses (ERVs) inserted into mouse chromosomes. All three MLV subgroups are linked to leukemogenesis, which involves generation of recombinants with polytropic host range. Although P-MLVs are deemed to be the proximal agents of disease induction, few biologically characterized infectious P-MLVs have been sequenced for comparative analysis. We analyzed the complete genomes of 16 naturally occurring infectious P-MLVs, 12 of which were typed for pathogenic potential. We sought to identify ERV progenitors, recombinational hot spots, and segments that are always replaced, never replaced, or linked to pathogenesis or host range. Each P-MLV has an E-MLV backbone with P- or X-ERV replacements that together cover 100% of the recombinant genomes, with different substitution patterns for X- and P-ERVs. Two segments are always replaced, both coding for envelope (Env) protein segments: the N terminus of the surface subunit and the cytoplasmic tail R peptide. Viral gag gene replacements are influenced by host restriction genes Fv1 and Apobec3 Pathogenic potential maps to the env transmembrane subunit segment encoding the N-heptad repeat (HR1). Molecular dynamics simulations identified three novel interdomain salt bridges in the lymphomagenic virus HR1 that could affect structural stability, entry or sensitivity to host immune responses. The long terminal repeats of lymphomagenic P-MLVs are differentially altered by recombinations, duplications, or mutations. This analysis of the naturally occurring, sometimes pathogenic P-MLV recombinants defines the limits and extent of intersubgroup recombination and identifies specific sequence changes linked to pathogenesis and host interactions.IMPORTANCE During virus-induced leukemogenesis, ecotropic mouse leukemia viruses (MLVs) recombine with nonecotropic endogenous retroviruses (ERVs) to produce polytropic MLVs (P-MLVs). Analysis of 16 P-MLV genomes identified two segments consistently replaced: one at the envelope N terminus that alters receptor choice and one in the R peptide at the envelope C terminus, which is removed during virus assembly. Genome-wide analysis shows that nonecotropic replacements in the progenitor ecotropic MLV genome are more extensive than previously appreciated, covering 100% of the genome; contributions from xenotropic and polytropic ERVs differentially alter the regions responsible for receptor determination or subject to APOBEC3 and Fv1 restriction. All pathogenic viruses had modifications in the regulatory elements in their long terminal repeats and differed in a helical segment of envelope involved in entry and targeted by the host immune system. Virus-induced leukemogenesis thus involves generation of complex recombinants, and specific replacements are linked to pathogenesis and host restrictions.

MoMuLV-ts-1: A Unique Mouse Model of Retrovirus-Induced Lymphoma Transmitted by Breast Milk

Our laboratory has developed a murine model of lymphoma via breast milk transmission of MoMuLV-ts-1 (Moloney murine leukemia virus-temperature sensitive mutant-1). Uninfected offspring suckled from infected surrogate mothers become infected and develop lymphoma. Multiple gene integration sites of ts-1 into the infected mouse genome including tacc3, aurka, ndel1, tpx2, p53, and rhamm were identified, and mRNA expressions were quantitated. These genes produce centrosomal proteins, which may be involved in abnormal chromosomal segregation leading to aneuploidy or multiploidy, thus causing lymphoma. Since there is no report to date on this retroviral model leading to centrosomal abnormality, and causing lymphoma development, this is a valuable and unique model to study the centrosomal involvement in lymphomagenesis.

Down-regulation of Jab1, HIF-1alpha, and VEGF by Moloney murine leukemia virus-ts1 infection: a possible cause of neurodegeneration

Moloney murine leukemia virus-temperature sensitive (MoMuLV-ts1)-mediated neuronal death is a result of both loss of glial support and release of cytokines and neurotoxins from ts1-infected glial cells. Here the authors propose vascular endothelial growth factor (VEGF) down-regulation as another contributory factor in neuronal degeneration induced by ts1 infection. To determine how ts1 affects VEGF expression in ts1-infected brain, the authors examined the expression of several proteins that are important in regulating the expression of VEGF. The authors found significant decreases in Jun-activating domain-binding protein 1 (Jab1), hypoxia-inducible factor (HIF)-1alpha, and VEGF levels and increases in p53 protein levels in ts1-infected brains compared to noninfected control brains. The authors suggest that a decrease Jab1 expression in ts1 infection leads to accumulation of p53, which binds to HIF-1alpha to accelerate its degradation. A rapid degradation of HIF-1alpha leads to decreased VEGF production and secretion. Considering that endothelial cells are the most conspicuous in virus replication and production in ts1 infection, but are not killed by the infection, the authors examined the expression of these proteins using infected and noninfected mouse cerebrovascular endothelial (CVE) cells. The ts1- infected CVE cells showed decreased Jab1, HIF-1alpha, and VEGF mRNA and protein levels and increased p53 protein levels compared with noninfected cells, consistent with the results found in vivo. These results confirm that ts1 infection results in insufficient secretion of VEGF from endothelial cells and may result in decreased neuroprotection. This study suggested that ts1-mediated neuropathology in mice may result from changes in expression and activity of Jab1, p53, and HIF-1alpha, with a final target on VEGF expression and neuronal degeneration.

Up-regulation of pro-nerve growth factor, neurotrophin receptor p75, and sortilin is associated with retrovirus-induced spongiform encephalomyelopathy

The progressive spongiform encephalomyelopathy caused by ts1, a neuropathogenic temperature-sensitive mutant of Moloney murine leukemia virus (MoMuLV-ts1), results in motor neuronal loss without direct neuronal infection. We have previously reported that ts1-mediated neuronal degeneration in mice has a multifactorial pathogenesis. Here, we report that in the ts1-infected central nervous system (CNS) activated neural cells showed intense immunoreactivity for pro-nerve growth factor (proNGF), neurotrophin receptor p75 (p75(NTR)), and sortilin in the areas showing spongiform changes. Since recent studies suggested that proNGF is more active than mature NGF in inducing neuronal death after binding to co-receptors p75(NTR)/sortilin, we hypothesized that overexpression of proNGF, sortilin and p75(NTR) play a role in ts1-induced neurodegeneration. We found that proNGF and p75(NTR), but not sortilin, mRNA and protein were significantly elevated in ts1-infected brainstem compared to non-infected control tissue. There was extensive tyrosine phosphorylation of p75(NTR), a marker for its activation, in ts1-infected brainstem with abundance in degenerating neurons. We explored whether the increase in the in vivo proNGF expression also occurs in cultured immortalized C1 astrocytes infected by ts1 virus. The proNGF level was significantly increased in infected C1 cells compared to control cells only after addition of fibroblast growth factor (FGF-1). We also showed increased expression of FGF-1 in the CNS of ts1-infected mice. Our findings suggest that the FGF-1 signaling pathway may be responsible for the overexpression of proNGF in neural cells during pathogenesis of ts1-induced neurodegeneration. This study provides new in vivo insights into the possible role of proNGF and its receptors in ts1-induced neurodegeneration.

Up-regulation of astrocyte cyclooxygenase-2, CCAAT/enhancer-binding protein-homology protein, glucose-related protein 78, eukaryotic initiation factor 2 alpha, and c-Jun N-terminal kinase by a neurovirulent murine retrovirus

In susceptible strains of mice, infection with the mutant retrovirus MoMuLV-ts1 causes a neurodegeneration and immunodeficiency syndrome that resembles human human immunodeficiency virus-acquired immunodeficiency syndrome (HIV-AIDS). In this study the authors show increased expression of cyclooxygenase-2 (COX-2) in the brainstem tissues of ts1-infected mice. Up-regulated central nervous system (CNS) levels of this enzyme are associated with HIV-associated dementia and other inflammatory and neurodegenerative diseases such as amyotrophic lateral sclerosis, Alzheimer's disease, and Parkinson's disease. In brainstem sections, the authors find that astrocytes surrounding spongiform lesions contain increased amounts of immunoreactive COX-2. COX-2 is also up-regulated in cultured ts1-infected cells from the C1 astrocytic cell line, and activation of c-Jun N-terminal kinase, or JNK, pathway. Markers of endoplasmic reticulum (ER) stress, specifically the CCAAT/enhancer-binding protein (CHOP), the glucose-related protein 78 (GRP78), and phosphorylated eukaryotic initiation factor 2 alpha (eIF2 alpha), were also up-regulated in ts1-infected C1 astrocytes. Up-regulation of COX-2 and the above ER signaling factors was reversed by treatment of the infected cells with curcumin which specifically inhibits the JNK/c-Jun pathway. These findings indicate that the JNK/c-Jun pathway is most likely responsible for COX-2 expression induced by ts1 in astrocytes, and that ts1 infection in astrocytes may lead to up-regulation of both inflammatory and ER stress pathways in the central nervous system. Because COX-2 inhibitors are now widely used to treat inflammatory conditions in animals and humans, this finding suggests that these drugs may be useful for therapeutic intervention in neurodegenerative syndromes as well.

The use of retroviral vectors for gene therapy-what are the risks? A review of retroviral pathogenesis and its relevance to retroviral vector-mediated gene delivery

Retroviral vector-mediated gene transfer has been central to the development of gene therapy. Retroviruses have several distinct advantages over other vectors, especially when permanent gene transfer is the preferred outcome. The most important advantage that retroviral vectors offer is their ability to transform their single stranded RNA genome into a double stranded DNA molecule that stably integrates into the target cell genome. This means that retroviral vectors can be used to permanently modify the host cell nuclear genome. Recently, retroviral vector-mediated gene transfer, as well as the broader gene therapy field, has been re-invigorated with the development of a new class of retroviral vectors which are derived from lentiviruses. These have the unique ability amongst retroviruses of being able to infect non-cycling cells. Vectors derived from lentiviruses have provided a quantum leap in technology and seemingly offer the means to achieve significant levels of gene transfer in vivo.The ability of retroviruses to integrate into the host cell chromosome also raises the possibility of insertional mutagenesis and oncogene activation. Both these phenomena are well known in the interactions of certain types of wild-type retroviruses with their hosts. However, until recently they had not been observed in replication defective retroviral vector-mediated gene transfer, either in animal models or in clinical trials. This has meant the potential disadvantages of retroviral mediated gene therapy have, until recently, been seen as largely, if not entirely, hypothetical. The recent clinical trial of gammac mediated gene therapy for X-linked severe combined immunodeficiency (X-SCID) has proven the potential of retroviral mediated gene transfer for the treatment of inherited metabolic disease. However, it has also illustrated the potential dangers involved, with 2 out of 10 patients developing T cell leukemia as a consequence of the treatment. A considered review of retroviral induced pathogenesis suggests these events were qualitatively, if not quantitatively, predictable. In addition, it is clear that the probability of such events can be greatly reduced by relatively simple vector modifications, such as the use of self-inactivating vectors and vectors derived from non-oncogenic retroviruses. However, these approaches remain to be fully developed and validated. This review also suggests that, in all likelihood, there are no other major retroviral pathogenetic mechanisms that are of general relevance to replication defective retroviral vectors. These are important conclusions as they suggest that, by careful design and engineering of retroviral vectors, we can continue to use this gene transfer technology with confidence.

Activation of endoplasmic reticulum stress signaling pathway is associated with neuronal degeneration in MoMuLV-ts1-induced spongiform encephalomyelopathy

Temperature-sensitive mutant of Moloney murine leukemia virus-TB (MoMuLV-ts1)-mediated neuronal death in mice is likely due to both loss of glial support and release of cytokines and neurotoxins from ts1-infected glial cells. Cytotoxic mediators present in ts1-induced spongiform lesions may generate endoplasmic reticulum (ER) stress, which has been implicated in the pathogenesis of a variety of neurodegenerative diseases. We investigated whether ER stress signaling is involved in ts1-mediated neuronal loss in the brain of infected mice. ts1-infected brainstems were found to show significant increases in phosphorylation of the double-stranded RNA-dependent protein kinase-like ER kinase and eukaryotic initiation factor 2-alpha. In addition, increased expression of growth arrest DNA damage 153 (GADD153), glucose-regulated protein 78, and caspase-12 were accompanied by increases in processing of caspase-12 and its downstream target, caspase-3. All of these events are markers of ER stress. We observed that GADD153 and cleaved caspase-3 were present in degenerative neurons in the lesions of infected mice, but not in uninfected controls. Phosphorylated calmodulin-dependent protein kinase II-alpha was significantly increased, and was coexpressed with GADD153 in a large proportion of neurons undergoing early and advanced degenerative changes. Finally, neuronal degeneration in spongiform lesions was associated with increase in calcium (Ca(2+)) accumulation in mitochondria. Together, these results suggest that ts1 infection-mediated neuronal degeneration in mice may result from activation of ER stress signaling pathways, presumably initiated by perturbation of Ca(2+) homeostasis. Our findings highlight the importance of the ER stress signaling pathway in ts1 infection-induced neuronal degeneration and death.

Sequence analysis of a neuropathogenic variant of Moloney murine leukemia virus ts1: evidence for recombination

The nucleotide sequence of the complete genome of ts1, a neuropathogenic variant of Moloney murine leukemia virus, has been determined. Although it is possible that the genome of ts1 accumulated point mutations during reverse transcription, our results now indicate that some of these mutations may have resulted from recombination.

Induction of p53 accumulation by Moloney murine leukemia virus-ts1 infection in astrocytes via activation of extracellular signal-regulated kinases 1/2

SUMMARY

We previously reported that Moloney murine leukemia virus-ts1-mediated neuronal degeneration in mice is likely a result of both loss of glial support and release of cytokines and neurotoxins from ts1-infected glial cells. Viral infection in some cell types regulates expression of p53 protein, a key regulator of cell proliferation and death. Therefore, we hypothesized that p53 and its dependent genes may be linked with ts1-mediated neuropathology. We examined the presence of p53 and its dependent gene product, a proapoptotic protein bax-alpha, in ts1-induced spongiform encephalomyelopathy. Compared with controls, the lesions of infected animals contained increased levels of p53 and bax-alpha in astrocytes, as shown by strong nuclear p53 and cytoplasmic bax-alpha immunoreactivity in astrocytes. To determine how ts1 affects p53 expression in astrocytes, we then assessed the expression of p53 and its dependent genes, such as bax-alpha and p21, in infected and uninfected immortalized C1 astrocytes and studied possible pathways responsible for p53 accumulation in infected astrocytes. In these studies using mitogen-activated protein kinase inhibitors, infection-induced increases in the p53 level were partially blocked by PD98059, a synthetic inhibitor of MEK1 that is the immediate upstream kinase of extracellular signal-regulated kinases 1/2 (ERK1/2), but not by SB202190, a potent p38 kinase inhibitor. Furthermore, treatment with PD98059 significantly decreased the level of p21 protein, a p53-dependent gene product. These results suggest that ts1 infection may stabilize p53 protein through activation of ERKs in C1 astrocytes, leading to increased expression of the p21 and bax-alpha proteins, both of which induce cell cycle arrest and apoptosis. Our studies suggest that ts1 neuropathology in mice may result from changes in expression and activity of p53, brought about in part by ts1 activation of ERK.

A single Glu(62)-to-Lys(62) mutation in the Mos residues of the R7Delta447Gag-tMos protein causes the mutant virus to induce brain lesions

We previously reported that R7Delta447, a 2954-base-pair (bp) laboratory-generated Moloney murine sarcoma virus, induced subcutaneous tumors in about 14% of infected mice but did not induce brain lesions. We now report that R7Delta447K, a spontaneous mutant of R7Delta447, induced brain lesions as well as subcutaneous tumors in all injected mice. The genomes of the two viruses differ in a single base pair: the deduced Glu(62) of the Mos residue of the R7Delta447 Gag-tMos protein is changed to Lys(62). More R7Delta447 than R7Delta447K focus-forming units were detected in both NIH3T3 and mouse cerebral vascular endothelial (MCVE) cells. However, R7Delta447K transformed NIH3T3 and MCVE cells more acutely than did R7Delta447. A distinctive feature that distinguished the morphologic transformation of R7Delta447- and R7Delta447K-infected MCVE cells is the markedly prolonged spindle-shaped phase exhibited by R7Delta447-infected MCVE cells. In addition, R7Delta447K was more efficient in inducing the phosphorylation of ERK1/2 than R7Delta447 in both MCVE and NIH3T3 cells. Moreover morphologic transformation was inhibited, and levels of phosphorylated ERK1/2 were reduced when R7Delta447- or R7Delta447K-infected NIH3T3 or MCVE cells were grown in the presence of the MEK1/2-specific inhibitor PD98095. Thus, we have identified a key residue in the Gag-tMos protein that profoundly affects activation of the Mos/MEK/ERK pathway, virus and cell replication, morphologic transformation in vitro and pathogenicity in vivo.

Members of the GATA family of transcription factors bind to the U3 region of Cas-Br-E and graffi retroviruses and transactivate their expression

Cas-Br-E and Graffi are two murine viruses that induce myeloid leukemia in mice: while Cas-Br-E induces mostly non-T, non-B leukemia composed of very immature cells, Graffi causes exclusively a granulocytic leukemia (E. Rassart, J. Houde, C. Denicourt, M. Ru, C. Barat, E. Edouard, L. Poliquin, and D. Bergeron, Curr. Top. Microbiol. Immunol. 211:201-210, 1995). In an attempt to understand the basis of the myeloid specificity of these two retroviruses, we used DNase I footprinting analysis and gel mobility shift assays to identify a number of protein binding sites within the Cas-Br-E and Graffi U3 regions. Two protected regions include potential GATA binding sites. Methylation interference analysis with different hematopoietic nuclear extracts showed the importance of the G residues in these GATA sites, and supershift assays clearly identified the binding factors as GATA-1, GATA-2, and GATA-3. Transient assays with long terminal repeat (LTR)-chloramphenicol acetyltransferase constructs showed that these three GATA family members are indeed able to transactivate Cas-Br-E and Graffi LTRs. Thus, the availability and relative abundance of the various members of the GATA family of transcription factors in a given cell type could influence the transcriptional tissue specificity of murine leukemia viruses and hence their disease specificity.

Studies on the pathology, especially brain lesions, induced by R7, a spontaneous mutant of Moloney murine sarcoma virus 124

We have recently isolated R7, a spontaneous Moloney murine sarcoma virus (MoMuSV) 124 variant. Molecular cloning and sequence analysis showed that, relative to MoMuSV 124, R7 has an extra repeat in each enhancer and a truncated mos gene in frame with the truncated gag coding sequence. This report presents a detailed study on the pathology induced by R7. R7 induced not only sarcomas with well developed angiomatous components but also brain lesions. Brain lesions were observed in all less-than-48-hour-old BALB/c mice inoculated with greater than 2 x 10(5) R7 focus-forming units (FFUs). R7 was detected in all brains examined by day 9 after inoculation, and brain lesions were observed in two of four mice examined by day 14 after inoculation. Light microscopy of brains revealed that approximately 15% of the lesions were unenclosed blood pools of varying sizes containing red blood cells and inflammatory cells spreading into surrounding brain tissues. The remainder of the brain lesions had tumor cells. These lesions ranged from a few enlarged vascular endothelial cells intermixed with blood cells to large circumscribed lesions consisting of well developed tangled masses of vessels surrounded by blood pools. Activated astrocytes surrounded and infiltrated the tumors. In addition, the thymus of R7-infected mice regressed significantly and precipitously due to apoptosis (especially of cortical thymocytes) at the end stage of the disease.

R7, a spontaneous mutant of Moloney murine sarcoma virus 124 with three direct repeats and an in-frame truncated gag-mos gene, induces brain lesions

We have isolated Recombinant 7 (R7), a spontaneous mutant of SV7, a molecular clone of MoMuSV124. Like SV7, R7 induces subcutaneous fibrosarcomas, spleen tumors, and mesentery tumors infiltrated by proliferating vessels lined by transformed endothelial cells. However, it also induces brain lesions. We have molecularly cloned and sequenced the R7 proviral DNA and shown that the R7 genome consists of 3401 bp. It has three direct repeats in each enhancer. Its coding sequence consists of only 176 bp of p15, 263 bp of p30, a 7-bp insertion, and 853 bp of an N-terminally truncated mos gene. From the sequence of R7 we have deduced that the truncated mos sequence is in-frame with all of the gag sequence and the 7-bp insertion. The incorporation of the 3' end of the p15 sequence further suggests that the R7 Gag-Mos is myristylated. We have also shown that the molecularly cloned R7 virus transformed NIH/3T3 fibroblasts about sevenfold better than the parental SV7. We have also confirmed that molecularly cloned R7 induces the same disease phenotype as that induced by the nonmolecularly cloned R7.

Long terminal repeat enhancer core sequences in proviruses adjacent to c-myc in T-cell lymphomas induced by a murine retrovirus

The transcriptional enhancer in the long terminal repeat (LTR) of the T-lymphomagenic retrovirus SL3-3 differs from that of the nonleukemogenic virus Akv at several sites, including a single base pair difference in an element termed the enhancer core. Mutation of this T-A base pair to the C-G C-G sequence found in Akv significantly attenuated the leukemogenicity of SL3-3. Thus, this difference is important for viral leukemogenicity. Since Akv is an endogenous virus, this suggests that the C-G in its core is an adaptation to being minimally pathogenic. Most tumors that occurred in mice inoculated with the mutant virus, called SAA, contained proviruses with reversion or potential suppressor mutations in the enhancer core. We also found that the 72-bp tandem repeats constituting the viral enhancer could vary in number. Most tumors contained mixtures of proviruses with various numbers of 72-bp units, usually between one and four. Variation in repeat number was most likely due to recombination events involving template misalignment during viral replication. Thus, two processes during viral replication, misincorporation and recombination, combined to alter LTR enhancer structure and generate more pathogenic variants from the mutant virus. In SAA-induced tumors, enhancers of proviruses adjacent to c-myc had the largest number of core reversion or suppressor mutations of all of the viral enhancers in those tumors. This observation was consistent with the hypothesis that one function of the LTR enhancers in leukemogenesis is to activate proto-oncogenes such as c-myc.

Development of physical forms of unintegrated retroviral DNA in mouse spinal cord tissue during ts1-induced spongiform encephalomyelopathy: elevated levels of a novel single-stranded form in paralyzed mice

ts1 is a murine leukemia virus that causes rapidly evolving hindlimb paralysis in susceptible strains of mice. Following perinatal infection, three physical forms of unintegrated viral DNA were detected in the spinal cord by Southern blot hybridization. Linear and supercoiled closed-circle viral double-stranded DNAs were detected in both the central nervous system and non-central nervous system tissues. An elevated level of a novel minus-sense single-stranded form of viral DNA, which had a very high mobility in agarose gels, was correlated with the onset of symptoms of paralysis. As the severity of paralysis progressed, the level of this single-stranded form increased rapidly, with the highest level in the spinal cords of moribund mice. Since the virulence of a number of cytopathic retroviruses has been associated with the presence of increased amounts of unintegrated viral DNA in the tissues of the infected hosts, this novel form of highly mobile unintegrated single-stranded DNA may have a role in the neuropathogenesis of ts1.

SV7, a molecular clone of Moloney murine sarcoma virus 349, transforms vascular endothelial cells

SV7, a progeny of Moloney murine sarcoma virus 349 cells, was molecularly cloned. SV7 induced sarcomas consisting of vascular and fibrous components. The large blood-filled vascular dilatations appeared grossly as dark red spots in the tumors and constituted up to 50% of the tumor volume. These vascular structures, ranging from small capillaries to cavernous vascular dilatations, were lined by one to several layers of neoplastic endothelial cells. Thick papillary outgrowths of the neoplastic endothelium extended into and often occluded the vessel lumens. The fibrous component consisted mostly of spindle cells and granulocytes, which provided the stroma for the vascular structures. The vascular and fibrous components appeared to have arisen independently. Lymphopenia accompanied by myeloid metaplasia was observed in the spleen of both SV7- and myeloproliferative sarcoma virus (MPSV)-infected mice. The blood of SV7-infected mice had a much higher level of circulating granulocytes than did that of MPSV-infected mice. The latter manifested a more advanced myeloid metaplasia, characterized by aggregates of myelomonocytic blast cells in the spleen.

Deletion of a GC-rich region flanking the enhancer element within the long terminal repeat sequences alters the disease specificity of Moloney murine leukemia virus

Moloney murine leukemia virus (M-MuLV) is a replication-competent retrovirus which induces T-lymphoblastic lymphoma 2 to 4 months after inoculation. Enhancer sequences in the U3 region of the M-MuLV long terminal repeat, primarily the 75-bp tandem repeats, strongly influence the disease specificity and latency of M-MuLV. We investigated the role of GC-rich sequences downstream of the tandem repeats in the disease specificity of M-MuLV. A recombinant M-MuLV lacking 23 bases of a GC-rich sequence (-174 to -151), Delta 27A M-MuLV, was tested for pathogenesis in neonatal NIH Swiss mice. Delta 27A M-MuLV induced disease with a longer latency than did M-MuLV (7 versus 3 months) in greater than 85% of inoculated mice. More interestingly, this virus showed an expanded repertoire of hematopoietic diseases. Molecular analyses and histopathologic examinations indicated that while 39% of mice inoculated with Delta 27A M-MuLV developed T-cell lymphoblastic lymphoma typical of wild-type M-MuLV, the majority developed acute myeloid leukemia, erythroleukemia, or B-cell lymphoma. Viral DNA corresponding to Delta 27A M-MuLV was detectable in most of the tumors analyzed. These findings indicate that the GC-rich region significantly influences the disease specificity and latency of M-MuLV.

T, not B, lymphocytes are required for immunodeficiency and paralysis induced by ts1, a mutant of Moloney murine leukemia virus-TB

BALB/c mice when injected as newborn with ts1, a temperature-sensitive mutant of Moloney murine leukemia virus-TB, developed a fatal hindlimb paralysis and immunodeficiency. This disease induction was prevented, to a great extent, by transient depletion of the peripheral T lymphocytes during the early course of infection by using anti-Thy 1.2 antibody. FVB/N mice, which are highly susceptible to ts1, but express Thy 1.1 instead of Thy 1.2 on their T lymphocytes, did not show any difference in the disease profile when treated similarly with anti-Thy 1.2 antibody. Transient depletion of the peripheral B lymphocytes in BALB/c mice in the early course of ts1 infection had no effect on the disease induction. In the T cell depleted BALB/c mice, virus replication was reduced, survival of the mice was increased and viral specific antibodies were produced, whereas, in the B cell depleted mice the disease process went on in a fashion similar to untreated mice infected with ts1. Thus, this study demonstrates that the disease syndrome induced by ts1 in BALB/c mice is dependent upon the presence of T lymphocytes during the early course of infection, and that presence of B lymphocytes have little or no effect on the disease outcome.

Analysis of the significance of two single-base-pair differences in the SL3-3 and Akv virus long terminal repeats

Two single-base-pair differences between the long terminal repeats (LTRs) of the T-lymphomagenic murine retrovirus SL3-3 and nonleukemogenic Akv virus were tested for effects on activity of the LTRs. Evidence was obtained from electrophoretic mobility shift assays for the presence of at least one factor in both T and non-T cells that bound to the region of the viral enhancers that contained the differences. However, no significant differences in activity in expression assays were detected when the two base-pair differences were exchanged between the two LTRs. Therefore, they do not contribute to the higher activity of the SL3-3 LTR in T-lymphoma cell lines.

The Moloney murine leukemia virus enhancer and its flanking sequences collaborate to determine virulence in T-cell lymphomagenesis

A panel of recombinant virus genomes was constructed by exchanging homologous genome fragments between the potent T-cell lymphoma inducer Moloney murine leukemia virus (MoMuLV) and its closely related but significantly less virulent relative MoMuLV-TB. Testing of these recombinant viruses in BALB/c mice established that only nucleotide changes within the Clal(-590)-Kpnl(36) fragment altered virulence. Fine analysis of this fragment showed that while mutations within the enhancer of MoMuLV-TB attenuated the latency period most, mutations within the MoMuLV-TB fragments flanking the enhancer also helped reduce the virulence of MoMuLV. The present study also suggests that the small difference in the relative number of lymphomas that developed primarily in the spleens of MoMuLV- or MoMuLV-TB-infected mice may correlate with nucleotide differences between the Clal-Kpnl fragments of the two viruses. However, the significantly greater proportion of premature death observed in MoMuLV-TB-relative to MoMuLV-infected mice could not be correlated with nucleotide differences in a specific genome fragment.

Site-directed mutagenesis of the codon for Ile-25 in gPr80env alters the neurovirulence of ts1, a mutant of Moloney murine leukemia virus TB

ts1, a spontaneous temperature-sensitive mutant of Moloney murine leukemia virus TB, causes hind-limb paralysis in mice. A Val-25----Ile substitution in gPr80env is responsible for temperature sensitivity, inefficient processing of gPr80env, and neurovirulence. In this study, the Ile-25 in gPr80env was replaced with Thr, Ala, Leu, Gly, and Glu by site-directed mutagenesis of the codon for Ile-25 to generate a new set of mutant viruses, i.e., ts1-T, -A, -L, -G, and -E, respectively. The phenotypic characteristics of these mutant viruses differed from those of ts1. For each mutant, the degree of temperature sensitivity was correlated with the degree of inefficient processing of gPr80env, and the following rank order was observed for both parameters: ts1-E greater than ts1-G greater than ts1-L greater than ts1-A greater than ts1 greater than ts1-T. In FVB/N mice, mutant viruses of low and intermediate temperature sensitivity and inefficiency in processing of gPr80env were neurovirulent and consistently caused mutant-specific disease profiles: ts1-T caused severe whole-body tremor, ts1-A generally caused hind-limb paralysis, and ts1-L generally caused a delayed-onset paraparesis. By 150 days postinfection, FVB/N mice that were infected with ts1-G and -E, mutants of high temperature sensitivity and inefficiency in processing of gPr80env, had lymphoid leukemia instead of a neurological disease. These results suggest that the dynamics of gPr80env processing are important in determining the neurovirulent phenotype in vivo.

Sequences in the U5-gag-pol region influence early and late pathogenic effects of Friend and Moloney murine leukemia viruses

Friend replication-competent murine leukemia virus (F-MuLV), clone 57, induces a severe early hemolytic anemia and a later erythroleukemia after inoculation of newborn IRW or ICFW mice, whereas Moloney MuLV (M-MuLV) induces only lymphoid leukemia. We have shown previously that the attenuated hemolytic and erythroleukemogenic abilities of an F-MuLV variant, clone B3, were due mostly to changes in the env gene and long terminal repeat, respectively. For the present study, we derived two constructs exchanging env fragments of F-MuLV 57 and M-MuLV and compared them with two constructs described by Chatis et al. (J. Virol. 52:248-254, 1984) exchanging the U3 region of the long terminal repeat of the same parental viruses. When comparing the hemolytic effect of these constructs with those of the parent, we found that the U5-gag-pol region of F-MuLV was required for development of severe early hemolytic anemia and that, unlike the env of F-MuLV B3, the env of M-MuLV was fully competent in inducing severe early hemolytic anemia when associated with the F-MuLV U5-gag-pol and U3 regions. As expected, induction of erythroleukemia depended on the presence of the F-MuLV U3 region; however, the presence of both the U3 and U5-gag-pol regions of F-MuLV appeared to be synergistic and was associated with a more rapid appearance of erythroleukemia.

Relative importance of elements within the SL3-3 virus enhancer for T-cell specificity

Elements within the enhancer of T-lymphomagenic SL3-3 virus were examined for their contributions to transcriptional activity in T lymphocytes and non-T cells. A region containing two sequences homologous to the enhancer core consensus sequence and a sequence homologous to the binding site for factor LVb was found to have the largest effect on activity. Evidence was obtained that suggests that the activity of this region was greater in T lymphocytes than in non-T cells and that multiple elements within it were necessary for activity. A second region, containing sequences homologous to the binding site of factor NF-I and the glucocorticoid response element, had about a twofold effect on transcription in both T lymphocytes and non-T cell lines. The twofold effect was seen whether the region containing the cores and LVb site was present or not. These results indicate that the most important region for the specificity of SL3-3 enhancer activity and, presumably, for viral leukemogenicity comprises the core elements and the LVb site. DNA-protein-binding studies demonstrated that one cellular factor, S/A-CBF, bound to both core elements, while a second cellular factor, S-CBF, bound to only one of them. In combination with earlier studies, this indicates that cells contain multiple factors that bind to the critical region.

A Val-25-to-Ile substitution in the envelope precursor polyprotein, gPr80env, is responsible for the temperature sensitivity, inefficient processing of gPr80env, and neurovirulence of ts1, a mutant of Moloney murine leukemia virus TB

ts1 is a neurovirulent spontaneous temperature-sensitive mutant of Moloney murine leukemia virus TB which causes hindlimb paralysis in mice. Previously, it had been shown that the temperature-sensitive defect resided in the env gene. At the restrictive temperature, the envelope precursor polyprotein, gPr80env, is inefficiently processed intracellularly into two cleavage products, gp70 and Prp15E. This inefficient processing of gPr80env is correlated with neurovirulence. In this study, it was shown that a single amino acid substitution, Val-25----Ile in gPr80env, is responsible for the temperature sensitivity, inefficient processing of gPr80env at the restrictive temperature, and neurovirulence of ts1. At the restrictive temperature, a steady-state level of nonprocessed, endoglycosidase H-sensitive gPr80env remained in the endoplasmic reticulum of cells infected by ts1, but no endoglycosidase H-resistant gPr80env and only trace amounts of gp70 were detected in the infected cells. Since the host cell-encoded processing protease resides in the cis cisternae of the Golgi apparatus, inefficient processing of gPr80env at the restrictive temperature is most likely due to inefficient transport of gPr80env from the endoplasmic reticulum to the cis cisternae of the Golgi apparatus rather than due to misfolded gPr80env being a poor substrate for the processing protease at the restrictive temperature.

A single mutation in one of the CORE elements of Moloney murine leukemia virus reduced binding of a 42-kDa T lymphoma cell nuclear factor but did not affect lymphomagenesis

The genetic determinant responsible for virulence in Moloney murine leukemia virus (MoMuLV) induced T-cell lymphomagenesis has recently been mapped [J Virol 63:471-480, 1989] by homologous genomic fragment exchange between MoMuLV and MoMuLV-TB to the Clal/Xbal at the 3' end of the genome. This region of MoMuLV and MoMuLV-TB differs in 11 nucleotides. Of these 11 nucleotide differences, 9 are distributed within the two CORE, the two distal NF1, and the two GRE/LVa elements of the enhancer. Since both the CORE binding sites of MoMuLV-TB are mutated with respect to those of MoMuLV, we compared nuclear proteins of a thymus-bone marrow cell line and a T-lymphoma cell line (EMT), which bind to the wild-type and mutant CORE binding sites. Using both the bandshift assay and southwestern analysis with labeled synthetic deoxyoligonucleotides, we showed that a 42-kDa protein from TB and EMT cells bound specifically to the MoMuLV CORE element. The T----C transversion of nucleotide 6 of the CORE consensus, TGTGGT/CTAA, significantly reduced binding of the 42-kDa TB and EMT cell factors. However, the transversion of nucleotide 3 from T----C had little effect on the binding of the 42-kDa protein to the CORE element. In addition, the 42-kDa protein bound weakly to the CCAAT element of MoMuLV. A recombinant virus, NwtTB-6, was generated by introducing the two CORE mutations of MoMuLV-TB into the MoMuLV genome. Although the latency period of NwtTB-6 in the induction of lymphoma was not significantly different from that of MoMuLV, preliminary findings suggest that the lymphoma induced by NwtTB-6 may be more widely distributed.

ts1, a mutant of Moloney murine leukemia virus-TB, causes both immunodeficiency and neurologic disorders in BALB/c mice

BALB/c mice infected with ts1, a mutant of Moloney murine leukemia virus-TB, develop generalized body wasting, profound neurologic disorders, severe thymic atrophy and lymphopenia due to destruction of T lymphocytes and drastic immunodeficiency. ts1 was found not only able to infect T lymphocytes but also to impair their function. In addition, ts1 also infects and induces syncyntia formation in macrophages. The genetic determinant(s) responsible for ts1's ability to induce immunodeficiency has been localized to the env gene.

The role of the thymus in the pathogenesis of hind-limb paralysis induced by ts1, a mutant of Moloney murine leukemia virus-TB

Newborn homozygous BALB/c nude (nu/nu) mice, their heterozygous (+/nu) littermates, and normal BALB/c (+/+) mice were infected with ts1, a paralytogenic mutant of Moloney murine leukemia virus-TB (MoMuLV-TB). Our results indicate that while infection of +/nu and +/+ mice with ts1 results in severe pathological changes in the central nervous system (CNS) and paralysis, infection of nu/nu mice results in only mild to moderate pathology within the CNS and no paralysis. On the other hand, 50% of nude mice reconstituted with T cells when infected with ts1 developed paralysis and showed more pronounced degeneration of nervous tissue than nude mice infected with ts1 alone. These observations strongly suggest that the thymus, the functional T lymphocytes, or both play an important role in the ts1-induced neurologic disorders in infected mice.