Replication of murine leukemia virus in heterologous cells: interaction between ecotropic and xenotropic viruses

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.

Activation of transcription factor Nrf-2 and its downstream targets in response to moloney murine leukemia virus ts1-induced thiol depletion and oxidative stress in astrocytes

The neuroimmunodegenerative syndrome that develops in mice infected with ts1, a mutant of Moloney murine leukemia virus, resembles human AIDS. Both ts1 and human immunodeficiency virus type 1 infect astrocytes, microglia, and oligodendrocytes but do not infect neurons. Oxidative stress has been implicated in the neuropathology of AIDS dementia and other neurodegenerative diseases. We report here that ts1 infection of astrocytes (both transformed C1 cells and primary cultures) also induces thiol (i.e., glutathione and cysteine) depletion and reactive oxygen species (ROS) accumulation, events occurring in parallel with viral envelope precursor gPr80(env) accumulation and upregulated expression of endoplasmic reticulum chaperones GRP78 and GRP94. Furthermore, ts1-infected astrocytes mobilize their thiol redox defenses by upregulating levels of the Nrf-2 transcription factor, as well its targets, the xCT cystine/glutamate antiporter, gamma-glutamylcysteine ligase, and glutathione peroxidase. Depleting intracellular thiols by treating uninfected astrocytes with buthionine sulfoximine (BSO), a glutathione synthesis inhibitor, or by culturing in cystine-deficient medium, also induces ROS accumulation, activates Nrf-2, and upregulates Nrf-2 target gene expression in these astrocytes. Overexpression of Nrf-2 in astrocytes specifically increases expression of the above thiol synthesis-related proteins. Further treatment with BSO or N-acetylcysteine in transfected cells modulates this expression. Thiol depletion also accelerates cell death, while thiol supplementation promotes survival of ts1-infected cells. Together, our results indicate that ts1 infection of astrocytes, along with ts1-induced gPr80(env) accumulation, endoplasmic reticulum stress, thiol depletion, and oxidative stress, accelerates cell death; in response to the thiol depletion and oxidative stress, astrocytes activate their Nrf-2-mediated thiol antioxidant defenses, promoting cell survival.

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.

Effects of mutant ubiquitin on ts1 retrovirus-mediated neuropathology

ts1 is a temperature-sensitive mutant of Moloney murine leukemia virus that induces a rapid spongiform encephalopathy in mice infected as newborns. The pathological features include the formation of ubiquitinated inclusions resembling Lewy bodies. To determine how perturbation of the ubiquitin-proteasome pathway might affect ts1-mediated neurodegeneration, the virus was introduced into transgenic mice in which the assembly of ubiquitin chains was compromised by the expression of dominant-negative mutant ubiquitin. The onset of symptoms was greatly delayed in a transgenic mouse line expressing K48R mutant ubiquitin; no such delay was observed in mice expressing a wild-type ubiquitin transgene or K63R mutant ubiquitin. The extended latency was found to correlate with a delayed increase in viral titers. Pathological findings in K48R transgenic mice at 60 days were found to be similar to those in the other strains at 30 days, suggesting that while delayed, the neurodegenerative process in K48R mice was otherwise similar. These data demonstrate the sensitivity of retroviral replication to the partial disruption of ubiquitin-mediated proteolysis in vivo, a finding that may have therapeutic potential.

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.

Enhanced proteolysis of IkappaBalpha and IkappaBbeta proteins in astrocytes by Moloney murine leukemia virus (MoMuLV)-ts1 infection: a potential mechanism of NF-kappaB activation

Moloney murine leukemia virus (MoMuLV)-ts1-mediated neuronal degeneration in mice is likely due to loss of glial support and release of inflammatory cytokines and neurotoxins from surrounding ts1-infected glial cells including astrocytes. NF-kappaB is a transcription factor that participates in the transcriptional activation of a variety of immune and inflammatory genes. We investigated whether ts1 activates NF-kappaB in astrocytes and examined the mechanism(s) responsible for the activation of NF-kappaB by ts1 infection in vitro. Here we present evidence that ts1 infection of astrocytes in vitro activates NF-kappaB by enhanced proteolysis of the NF-kappaB inhibitors, IkappaBalpha and IkappaBbeta. In in vitro studies using protease inhibitors, IkappaBalpha proteolysis in ts1-infected astrocytes was significantly blocked by a specific calpain inhibitor calpeptin but not by MG-132, a specific proteasome inhibitor, whereas rapid IkappaBbeta proteolysis was blocked by MG-132. Furthermore, treatment with MG-132 increased levels of multiubiquitinated IkappaBbeta protein in ts1-infected astrocytes. These results indicate that the calpain proteolysis is a major mechanism of IkappaBalpha proteolysis in ts1-infected astrocytes. Additionally, ts1 infection of astrocytes in vitro increased expression of inducible nitric oxide synthase (iNOS), a NF-kappaB-dependent gene product. Our results suggest that NF-kappaB activation in ts1-infected astrocytes is mediated by enhanced proteolysis of IkappaBalpha and IkappaBbeta through two different proteolytic pathways, the calpain and ubiquitin-proteasome pathways, resulting in increased expression of iNOS, a NF-kappaB-dependent gene.

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.

Neurodegeneration induced by MoMuLV-ts1 and increased expression of Fas and TNF-alpha in the central nervous system

Infection of neonatal mice with ts1, the neuropathogenic mutant of the Moloney murine leukemia virus, results in motor neuronal death in the brainstem and the spinal cord, with gliosis and demyelination, but no inflammatory cell infiltration into the CNS. To evaluate the possible mechanism(s) of ts1-induced neuropathogenesis, we measured CNS expression of cytokines and cell death-related genes in ts1-infected mice with neurological signs and compared with control uninfected mice. In the brainstem, the expression of Fas and tumor necrosis factor alpha (TNF-alpha) was increased in the ts1-infected mice. Both TNF-alpha and Fas were detected in astrocytes, and Fas was also detected in neurons in the brainstem. Some TNF-alpha-immunolabeled cells also appeared to be microglial cells. Most Fas-positive cells, including astrocytes and neurons, showed cytoplasmic vacuolization and other degenerative changes. In addition, Fas ligand-immunolabeled cells were also detected in sites where spongiform degeneration occurred. This study suggests that neural cell death in ts1-induced neurodegeneration is likely due to Fas- and TNF-alpha-mediated cell death mechanisms.

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.

Protein-energy malnutrition delays small-intestinal recovery in neonatal pigs infected with rotavirus

Infectious diarrheal diseases and protein-energy malnutrition (PEM) are major causes of child morbidity and mortality worldwide. In the present study, PEM was superimposed on rotavirus infection in neonatal pigs to simulate chronic small intestinal stress in malnourished infants with viral gastroenteritis. Two-day-old cesarean-derived pigs (n = 39) were allotted to three treatment groups: 1) noninfected, full-fed; 2) infected, full-fed; and 3) infected, malnourished. Two days postinfection, severe diarrhea and weight loss (11%) were accompanied by reductions in villus height (60%) and lactase activity (78%) and increased crypt depth (32%) in infected full-fed compared with noninfected pigs (P < 0.05). Malnutrition blunted (P < 0.05) increases in crypt depth elicited by rotavirus. By 9 d postinfection, body weight was 59% less, villus height and lactase activity remained lower (50%), and crypt depth remained greater (62%) in infected full-fed compared with noninfected pigs (P < 0.05). However, diarrhea began to clear in infected full-fed, but not in infected malnourished pigs. Plasma insulin-like growth factor-I (IGF-I) was reduced 68% and crypt depth was reduced 19% in infected-malnourished compared with infected full-fed pigs (P < 0.05). Sixteen days postinfection, full-fed pigs had recovered from rotaviral infection; however, in infected-malnourished pigs, diarrhea and growth stasis persisted, and plasma IGF-I, villus height and alkaline phosphatase activity remained reduced compared with infected full-fed pigs (P < 0.05). Overall, PEM prolonged diarrhea and delayed small-intestinal recovery, indicating that nutritional status during diarrhea is essential for recovery from rotaviral enteritis.

Viral load and its relationship to quinolinic acid, TNF alpha, and IL-6 levels in the CNS of retroviral infected mice

Mouse models of infection of the central nervous system (CNS) have been used to study retroviral-induced neurologic disease. Ecotropic-neurotropic murine leukemia virus (MuLV) infection of susceptible neonatal mice causes a neurologic disease characterized by progressive hindlimb paralysis. The lesions consist of chronic noninflammatory spongiform change predominantly involving brainstem and spinal cord. Two molecularly cloned strains of MuLV, ts-1, a temperature-sensitive mutant of Moloney MuLV, and pNE-8, derived from a feral mouse isolate Cas-Br-E, were used in this study. Infected mice were sacrificed at regular intervals postinoculation throughout the time-course of disease. The neuropathology was evaluated using standard histological and immunohistopathological techniques. Tissue concentrations of viral proteins and potentially cytotoxic factors were compared with the histopathology in select regions of the CNS. Areas of extensive vacuolation with neuronal and oligodendroglial infection were observed in spinal cord, brainstem, and cerebellum. High titers of infectious virus were observed within CNS lesions, whereas low titers were observed in morphologically uninvolved areas. Western blot analysis revealed abundant production of viral envelope proteins, which correlated well with infectious virus titers. Serum quinolinic acid (QUIN) concentrations in both groups of noninfected and infected mice were similar. However, CNS tissue concentrations of QUIN, TNF alpha, and IL-6 in ts-1 infected mice were significantly higher than in pNE-8 infected or noninfected mice. The difference in concentration of these factors may be the result of greater activation of macrophages/microglia in ts-1 infected mice. During murine retroviral encephalitis, CNS damage may be mediated by direct infection of CNS cells and may be enhanced by indirect effects of neurotoxic factors possibly secreted by infected/activated macrophages.

Murine retrovirus-induced depletion of T cells is mediated through activation-induced death by apoptosis

ts1, a mutant of Moloney murine leukemia virus, causes neurologic disorders and acute immunodeficiency associated with the destruction of thymocytes and helper T cells. In this study, we examined whether apoptosis was involved in ts1-induced killings of T cells. Neonatal mice were inoculated with ts1, and 20 to 23 days postinoculation, when cytopathic effects on T cells normally appear, thymocytes and splenic lymphocytes were isolated and examined. Our results showed that several features of apoptosis were present in ts1-infected thymocytes and splenic lymphocytes. Apoptotic fragmented DNA, condensation of the chromatin, and enhanced cell death after stimulation with mitogens which was preventable with protein synthesis inhibitors, all of which are common features of apoptotic cell death, were observed in ts1-infected cells. Several other viruses, including human immunodeficiency virus, have been shown to cause apoptotic death of T cells. Here we show for the first time that a murine retrovirus which also induces immunodeficiency can cause apoptotic T-cell death. Future studies with this murine retrovirus may provide important results to help us better understand the mechanisms of retrovirus-induced apoptosis of T cells.

A simple method for obtaining highly viable virus from culture supernatant

Traditionally density-gradient methods are used to purify viruses. However, these procedures are not only time consuming and cumbersome, recovery of viable viruses are often quite low. In this report, a single-step concentration technique was used to concentrate a mutant of Moloney murine leukemia virus (ts1) virus from culture supernatants by ultrafiltration. A special ultrafiltration unit with a 100,000 mol wt cut-off was able to concentrate viruses about 30-fold without losing any infectivity. In comparison, traditional sucrose density gradient purified viruses lost a significant portion of their infectivity. This technique could be used for concentrating other viruses for many useful purposes where more viable viruses are needed, e.g., study of virus-cell binding.

Correlation of specific virus-astrocyte interactions and cytopathic effects induced by ts1, a neurovirulent mutant of Moloney murine leukemia virus

ts1 is a highly neuropathogenic and lymphocytopathic mutant of Moloney murine leukemia virus TB (MoMuLV-TB). We previously reported that the primary neuropathogenic determinant of ts1 maps to a single amino acid substitution, Val-25-->Ile, in precursor envelope protein gPr80env. This Val-25-->Ile substitution apparently renders gPr80env inefficient for transport from the endoplasmic reticulum to the Golgi apparatus. These findings suggest that the cytopathic effect of ts1 in neural cells might be due to the accumulation of gPr80env in the endoplasmic reticulum. Since endothelial and glial cells are targets of ts1 infection in the central nervous system, we established primary endothelial and astrocyte cultures to investigate the mechanism of cell killing caused by ts1. A continuous cell line, TB, was used as a control. Our results showed that both ts1 and MoMuLV-TB replicated and induced a cytopathic effect in astrocyte cultures, albeit to different degrees; ts1 appeared to be more lethal than MoMuLV-TB. On the other hand, ts1 and MoMuLV-TB infections of endothelial or TB cells were not cytopathic. The cytopathic effect in infected astrocytes correlated with the inefficiency of gPr80env transport and the intracellular accumulation of gPr80env as well as aberrant virus particles.

Protective role of cytotoxic lymphocytes against murine leukemia virus-induced neurologic disease and immunodeficiency is enhanced by the presence of helper T cells

We examined the role of T cells and their separated subsets in providing immunity against ts1 (a mutant of the Moloney murine leukemia virus) induced paralysis and immunodeficiency. Adoptive transfer of syngeneic total T cells from immunized mice protected newborn mice, at least partially, from ts1-induced disease syndrome. In infected mice who received total immune T cells, virus replication was reduced and the mice survived longer. When only separated immune CD8+ T cells were transferred to infected mice, similar protection, albeit to a lesser extent, was observed. Transfer of separated immune CD4+ T cells alone gave no protection. However, when recombined CD4+ and CD8+ cells were transferred together, an immune response similar to that when total T cells were transferred was observed. Cytotoxic assays from ts1-immunized mice revealed the presence of virus-specific CD8+ cytotoxic T lymphocytes that could lyse virus-expressing cells at a high effector/target ratio. We conclude that CD8+ T cells alone can provide immunity against ts1-induced paralysis and immunodeficiency and that the simultaneous presence of CD4+ T cells can also significantly enhance the immune response.

ts1, a temperature-sensitive mutant of Moloney murine leukemia virus TB, can infect both CD4+ and CD8+ T cells but requires CD4+ T cells in order to cause paralysis and immunodeficiency

When neonatal FVB/N mice were inoculated with ts1, a temperature-sensitive mutant of Moloney murine leukemia virus TB, they developed a progressive bilateral hindlimb paralysis and immunodeficiency leading to death 4 to 6 weeks after inoculation. T lymphocytes have been shown to be primarily responsible for this ts1-induced syndrome. Here we compare the role played by each subset of T lymphocytes, i.e., CD4+ and CD8+ T cells, in disease development. Mice were depleted of a specific subset for the first 10 days of their lives by using either anti-CD4 or anti-CD8 monoclonal antibodies in vivo. Disease development in these mice was then monitored. Depletion of CD4+ T cells significantly attenuated the ts1-induced syndrome: virus replication was decreased, disease latency was extended, and death was prevented in 60% of the mice. Similar treatment with anti-CD8 antibody had almost no effect on disease progression. However, when depletion was begun 2 weeks after neonatal ts1 inoculation, CD4+ T cell depletion did not affect disease development. ts1 infected CD4+ and CD8+ T lymphocytes equally well in vivo, as shown by flow cytometric analysis, but virus replication was restricted primarily to the CD4+ subset of T cells, as found by in vitro assay. Hence, CD4+ T lymphocytes play an important role in the development of ts1-induced paralysis and immunodeficiency. The mechanism of this CD4+ T-cell-mediated disease production by ts1 is not clear; however, increased replication of ts1 in the CD4+ T cells, especially in the early stages of the disease, seems to play a crucial role.

Oligomerization and transport of the envelope protein of Moloney murine leukemia virus-TB and of ts1, a neurovirulent temperature-sensitive mutant of MoMuLV-TB

Ts1, a temperature-sensitive mutant of Moloney murine leukemia virus-TB (MoMuLV-TB), causes a progressive hindlimb paralytic disease in susceptible strains of mice. Previously, it has been shown that a single amino acid substitution, Val-25----Ile in gPr80env, is responsible for the temperature sensitivity, inefficient transport, and processing of gPr80env at the restrictive temperature and the neurovirulence of ts1. Since the neurovirulence of ts1 is associated with inefficient transport and processing of gPr80env and since in other systems involving viral envelope proteins it has been shown that correct folding and oligomerization of envelope monomers are required for efficient transport, we have investigated the ability of gPr80env derived from either wild-type MoMuLV-TB or ts1 to associate into oligomeric complexes. In these experiments, we establish that at both the restrictive and the nonrestrictive temperatures gPr80env molecules derived from MoMuLV-TB associate to form oligomeric complexes and these oligomers are most likely trimers. gPr80env molecules derived from ts1 also oligomerize at both temperatures; however, at the restrictive temperature, most of the molecules within the trimeric complexes remain as gPr80env and are not processed to gp70 and Prp15E. These results indicate that lack of oligomerization of gPr80env is not responsible for the transport defect of ts1. Therefore, by interacting specifically with critical sites within target cells, oligomers of mutant gPr80env rather than "tangles" of monomeric viral envelope proteins may be involved in the neurodegenerative disorder produced by ts1.

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.

High susceptibility of FVB/N mice to the paralytic disease induced by ts1, a mutant of Moloney murine leukemia virus TB

The ts1 mutant of Moloney murine leukemia virus TB causes a degenerative neurologic and immunologic disease in susceptible strains of mice. This disease syndrome is characterized by development of spongiform encephalomyelopathy resulting in hindlimb paralysis, generalized bodywasting, and marked thymic atrophy associated with immune deficiency. The viral genetic determinants responsible for hindlimb paralysis in BALB/c and CFW/D mice have been localized to two point mutations in the env gene: one results in a Val-25----IIe substitution in the envelope precursor polyprotein gPr80env and the other, in an Arg-430----Lys substitution in the gp70. In this report we present studies showing that FVB/N mice were highly susceptible to ts1 and exhibited the shortest and most uniform latency period of all the murine strains tested. In addition, we have found that, unlike in CFW/D and BALB/c mice, only the Val-25----IIe substitution in the gPr80env is required to induce hindlimb paralysis in FVB/N mice. Our studies show that there was enhanced replication of ts1 in all tissues of FVB/N mice and that the virus titer in the spinal cord was more than 10-fold higher in FVB/N than in BALB/c mice by 30 days postinoculation, when the clinical signs of paralysis became evident in FVB/N mice. Apparently, other host factors that do not require the Arg-430----Lys substitution allowed high levels of viral replication within the central nervous system of FVB/N mice. These results, together with the finding that 100% of FVB/N mice that were inoculated with ts1 at 5 days of age developed hindlimb paralysis at 30-60 days postinoculation, whereas only 33% of 5-day-old BALB/c mice developed hindlimb paralysis with a much longer latency period, suggest that subtle virus-host interactions determine the incidence, the latency period, and the severity of the disease caused by ts1.

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.

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.

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.

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

Chimeric constructs were generated by exchanging genomic fragments between the potent T-cell lymphoma inducer Moloney murine leukemia virus (MoMuLV) and its derivative MoMuLV-TB, which induces T-cell lymphoma after a relatively longer latent period. Analysis of the T-cell lymphoma-inducing potential of the hybrid viruses that were obtained localized the primary determinant critical to efficient T-cell lymphoma induction to the MoMuLV ClaI-XbaI fragment which comprises 48 nucleotides (nt) of p15E, p2E, the 3'-noncoding sequence, and 298 nt of U3. The 438-base-pair ClaI-XbaI fragments of MoMuLV and MoMuLV-TB differed in only 11 nt. Nine mutations were found within the enhancer. These mutations occurred within the two CORE, the two GRE-LVa, and two of the four NF1 nuclear factor-binding motifs. MoMuLV-TB replicated better than MoMuLV in thymus-bone marrow (TB) cells, a cultured cell line of lymphoid origin. In addition, MoMuLV-TB and NwtTB-2, a recombinant virus with the ClaI-SmaI fragment of MoMuLV-TB in a MoMuLV background, replicated in thymocytes as efficiently as did MoMuLV or TBNwt-2, the reciprocal recombinant virus, with the ClaI-SmaI fragment of MoMuLV in a MoMuLV-TB background. Like NwtTB-4, a recombinant virus with the ClaI-XbaI fragment of MoMuLV-TB in a MoMuLV background, NwtTB-2 induced lymphoma after a long latent period. The finding given above suggests that thymotropism is not the only factor that determines the T-cell lymphoma-inducing potential of MoMuLV. It appears likely that mutations in one or more of the MoMuLV-TB nuclear factor-binding motifs may have altered the interaction of the enhancer with specific nuclear factors; this, in turn, may affect the T-cell lymphoma-inducing potential of MoMuLV-TB.

Role of cell cytoskeleton in Mo-MuLV env transport and processing: implications in ts1 neuropathology

Treatment of Mo-MuLV-infected cells with cytochalasin B (CB), a microfilament disrupting drug, caused a reduction in virus yield as judged by infectivity assay and reverse transcriptase activity. Pulse-chase experiments with [3H]leucine showed that the env precursor, gPr80env, was inefficiently processed in cells treated with CB. In the presence of monensin, an inhibitor of glycoprotein transport, gPr80env accumulated intracellularly and no gp70 was observed on the cell surface, indicating a complete block in the processing of gPr80env. Pulse-chase studies also showed that gPr80env was not processed in the presence of monensin. SDS-PAGE analysis of TX-100-extracted cell cytoskeletons (TX-insoluble fraction) iodinated and immunoprecipitated with goat anti-gp70 antiserum showed that CB or monensin treatment caused a marked increase of gPr80env in the cytoskeleton-rich fraction. However, the amount of gPr80env associated with the TX-soluble fraction in both CB or monensin-treated and untreated cells labeled with [3H]leucine was about the same. The gPr80env in the TX-100-soluble fraction of the cell was the endoglycosidase H (Endo-H) sensitive mannose-rich form, whereas the cytoskeleton-associated gPr80env was the partially Endo-H-resistant complex carbohydrate form. In the presence of CB or monensin, the complex carbohydrate form of gPr80env accumulated in the cytoskeleton-rich cell fraction. Examination of Mo-MuLV ts1 mutant, which is defective in the processing of env precursor polyprotein, also revealed an accumulation of the complex carbohydrate form of gPr80env in the cytoskeleton-rich fraction and an absence of gp70 on the surface of the cell at the restrictive temperature (39 degrees C). These studies suggest that the cytoskeleton plays a role in the transport and processing of MuLV gPr80env and that oligosaccharide conversion is an important factor in this process. Further, the accumulation of gPr80env on the cytoskeleton of ts1 infected cells at restrictive temperature may play a role in the neurological disorder caused by Mo-MuLV ts1 mutant.

The neurovirulent determinants of ts1, a paralytogenic mutant of Moloney murine leukemia virus TB, are localized in at least two functionally distinct regions of the genome

To better understand the molecular mechanism involved in retrovirus ts1-induced paralytic disease in mice, we constructed a panel of recombinant viruses between ts1 and the wild-type viruses Moloney murine leukemia virus (MoMuLV) and MoMuLV-TB, a strain of MoMuLV. These recombinant viruses were constructed in an attempt to identify the sequence(s) in the genome of ts1 which contains the critical mutation(s) responsible for the neurovirulence of ts1. Two functionally distinct sequences in the genome of ts1 were found to be responsible for its paralytogenic ability. One of these sequences, the 0.77-kilobase-pair XbaI-BamHI (nucleotides 5765 to 6537) fragment which encodes the 5' half of gp70 and 11 base pairs upstream of the env gene coding sequence, determines the inability of ts1 to process Pr80env. The other sequence, the 2.30-kilobase-pair BamHI-PstI (nucleotides 538 to 8264 and 1 to 567) fragment, which comprises nearly two-thirds of the env gene, the long terminal repeat, and the 5' noncoding sequence, determines the enhanced neurotropism of ts1. Replacement of any one of these two regions with the homologous region from either one of the two wild-type viruses resulted in recombinant viruses which either totally failed to induce paralysis or induced a greatly attenuated form of paresis in some of the infected mice.

ts1, a Paralytogenic mutant of Moloney murine leukemia virus TB, has an enhanced ability to replicate in the central nervous system and primary nerve cell culture

A temperature-sensitive mutant of Moloney murine leukemia virus TB (MoMuLV-TB), ts1, which is defective in intracellular processing of envelope precursor protein (Pr80env), also possesses the ability to induce hind-limb paralysis in infected mice. To investigate whether ts1 has acquired neurotropism and to determine to what extent it can replicate in the central nervous system, we compared viral titers in the spleen, plasma, spinal cord, and brain throughout the course of infection of mice infected with ts1 and parental wild-type (wt) MoMuLV-TB. In both the ts1- and wt-inoculated mice, the concentrations of infectious virus recovered from the plasma and spleen increased rapidly and reached a plateau by 10 days postinfection (p.i.). In contrast, virus concentrations in the spinal cord and brain of ts1-inoculated mice increased gradually and reached a titer comparable to that in the spleen and exceeding that in the plasma only at 25 to 30 days p.i. At this time, the virus titer was approximately 200X greater in ts1-infected spinal cord tissue and approximately 20X greater in ts1-infected brain tissue than in the same wt-infected tissues. Paralysis became evident at 25 to 30 days p.i. in ts1-inoculated mice, whereas the wt-inoculated mice were normal. In addition, a substantial amount of Pr80env was detected in the spinal cords of ts1-inoculated mice compared with that found in the spinal cords of wt-inoculated mice. The infectious virus isolated from ts1-infected nerve tissue was found to possess the characteristic phenotype of the ts1 virus. Microscopic lesions of ts1-inoculated mice at 30 days p.i. consisted of vacuolar degeneration of motor neurons and spongy change of white matter in the brain stem and spinal cord. Similar but less severe lesions were observed in wt-inoculated mice. With primary cultures of central nervous system tissue we showed that ts1 can infect and replicate in both neuron and glial cells. In contrast, although wt MoMuLV-TB replicated in glial cell-rich culture, viral replication was barely detectable in neuron-rich culture.

A 1.6-kilobase-pair fragment in the genome of the ts1 mutant of Moloney murine leukemia virus TB that is associated with temperature sensitivity, nonprocessing of Pr80env, and paralytogenesis

ts1 and ts7, two temperature-sensitive mutants of Moloney murine leukemia virus strain TB induce hind-limb paralysis in 100% of CFW/D mice injected. These two paralytogenic mutants also share a defect in their inability to process the env precursor protein, Pr80env, at the restrictive temperature. To identify the mutation(s) in the genomes of the paralytogenic mutants which cause the inability to process Pr80env efficiently and confer the ability to cause hind-limb paralysis instead of lymphoma, we constructed chimeric genomes between ts1 and Moloney murine leukemia virus or the TB strain of the virus. We identified a 3.9-kilobase-pair HindIII-PstI sequence from nucleotides 4895 through 8264 and 1 through 567 of ts1, comprising the 3' end of the pol and all of the env genes, the long terminal repeat, and the 5' noncoding sequence, as being responsible for the temperature sensitivity, the inefficiency in processing Pr80env, and the induction of paralysis. We extended these findings by demonstrating that the 1.6-kilobase-pair pol-gp70 HindIII-BamHI DNA sequence from nucleotides 4895 through 6537 of ts1 within the 3.9-kilobase-pair HindIII-PstI fragment is necessary for ts1 to induce paralysis. In addition, we showed that this 1.6-kilobase-pair fragment also controls the processing of Pr80env and the temperature sensitivity of ts1.

Molecular cloning of two paralytogenic, temperature-sensitive mutants, ts1 and ts7, and the parental wild-type Moloney murine leukemia virus

ts1 and ts7, the paralytogenic, temperature-sensitive mutants of Moloney murine leukemia virus (MoMuLV), together with their wild-type parent, MoMuLV-TB, were molecularly cloned. ts1-19, ts7-22, and wt-25, the infectious viruses obtained on transfection to NIH/3T3 cells of the lambda Charon 21A recombinants of ts1, ts7, and wt, were found to have retained the characteristics of their non-molecularly cloned parents. In contrast to the wt virus, ts1-19 and ts7-22 are temperature-sensitive, inefficient in the intracellular processing of Pr80env at the restrictive temperature, and able to induce paralysis in CFW/D mice. Like the non-molecularly cloned ts7, the ts7-22 virion was also shown to be heat labile. The heat lability of the ts7 virion distinguishes it from ts1. Endonuclease restriction mapping with 11 endonucleases demonstrated that the base composition of MoMuLV-TB differs from that of the standard MoMuLV, but no difference was detected between the molecularly cloned ts1 and ts7 genomes. However, ts1 and ts7 differ from MoMuLV in the loss or acquisition of four different restriction sites, whereas they differ from MoMuLV-TB in the loss or acquisition of three different restriction sites.

Restriction of Moloney murine leukemia virus replication in Moloney murine sarcoma virus-infected cells

MuSV349 is a TB-cell line which produces infectious MuSV with little or no MuLV detectable by the XC assay. The apparent restriction of MuLV replication in MuSV349 cells was investigated. A replication-competent helper virus, with protein composition nearly identical to that of Mo-MuLV was isolated from the viruses produced by MuSV349 cells. This helper MuLV after it was separated from MuSV and upon infection of TB cells produced viral titer similar to that of Mo-MuLV-infected TB cells indicating that its replication might have been restricted in the MuSV349 cells. To find out whether the suppression of the helper virus replication is due to the genetic peculiarities of the virus or MuSV349 cells, the relative amounts of MuLV and MuSV produced by several distinct clonal MuSV isolates (derived from a common progenitor) upon superinfection with Mo-MuLV were determined. The results of these experiments showed that while both SV7 and SV15F on coinfection with Mo-MuLV produced MuSV in excess over MuLV; and ts110 and tsSV13 on coinfection with Mo-MuLV produced MuLV in excess over MuSV. Since the same Mo-MuLV is used in these experiments and since upon transfer to a different cell, SV7, SV15F, and ts110 retain the property to restrict or not restrict MuLV replication it appears that the above property is determined by the genetics of the MuSV.

Isolation and characterization of a Mo-MuSV-transformed TB cell line that produces noninfectious MuSV particles with uncleaved gag protein which is processed in the presence of Mo-MuLV

A cell line, TBSV7, that produces noninfectious murine sarcoma virus (MuSV) in the absence of helper MuLV was isolated from TB cells infected with the supernatant of MuSV349 cells. These noninfectious MuSV particles with "immature" C-type virus morphology contain a 2.2 X 10(6)-Da genomic RNA and an uncleaved 62,000-Da gag precursor protein (Pr62). Neither viral envelope proteins (gp70, p15E, p12E) nor reverse transcriptase were detected in these virus particles. Pr62 was found to be phosphorylated in vivo and it could be phosphorylated in vitro with [gamma-32P]ATP, indicating that protein kinase was packaged in these noninfectious virions. In vitro processing of Pr62 to smaller molecular weight proteins could be achieved by the addition of Mo-MuLV and Nonidet P-40. The initial cleavage products were proteins with molecular weights of 38K (Pr38) and 27K (Pr27). Under optimum conditions Pr38 was cleaved to p30 and a protein band migrating with MuLV-p10, while Pr27 was cleaved to a 17,000-Da protein that migrated slower than MuLV-p15 and a protein band migrating with MuLV-p12. Pulse-chase experiments performed on TBSV7 cells superinfected with Mo-MuLV indicated that intracellular processing of Pr62 was much slower than that of Pr65. Cleavage protein products of Pr62 similar in size to the in vitro protein products were also detected in TBSV7 cells superinfected with MuLV.

A group of temperature-sensitive mutants of Moloney leukemia virus which is defective in cleavage of env precursor polypeptide in infected cells also induces hind-limb paralysis in newborn CFW/D mice

A group of temperature sensitive mutants of Moloney murine leukemia virus (MoMuLV) designated as ts1, ts7, and ts11 rapidly and invariably induce hind-limb paralysis ranging from 28 to 52 days postinjection of neonatal CFW/D mice. These temperature-sensitive mutants are defective in the processing of the precursor of the env protein, gPr80env in infected cells, resulting in the accumulation of gPr80env in the infected cell and production of virions with reduced amounts of gp70, p15E, and p12E when compared to that of the wild-type virion. In contrast, two nonparalytogenic ts mutants, ts3 and ts10, like the wild-type virus, show normal processing of gPr80env in infected cells and production of virions with a similar amount of env proteins to that of the wild-type virion.