The relative contributions of infectious and mitotic spread to HTLV-1 persistence

Human T-lymphotropic virus type-1 (HTLV-1) persists within hosts via infectious spread (de novo infection) and mitotic spread (infected cell proliferation), creating a population structure of multiple clones (infected cell populations with identical genomic proviral integration sites). The relative contributions of infectious and mitotic spread to HTLV-1 persistence are unknown, and will determine the efficacy of different approaches to treatment. The prevailing view is that infectious spread is negligible in HTLV-1 persistence beyond early infection. However, in light of recent high-throughput data on the abundance of HTLV-1 clones, and recent estimates of HTLV-1 clonal diversity that are substantially higher than previously thought (typically between 10⁴ and 10⁵ HTLV-1⁺ T cell clones in the body of an asymptomatic carrier or patient with HTLV-1-associated myelopathy/tropical spastic paraparesis), ongoing infectious spread during chronic infection remains possible. We estimate the ratio of infectious to mitotic spread using a hybrid model of deterministic and stochastic processes, fitted to previously published HTLV-1 clonal diversity estimates. We investigate the robustness of our estimates using three alternative estimators. We find that, contrary to previous belief, infectious spread persists during chronic infection, even after HTLV-1 proviral load has reached its set point, and we estimate that between 100 and 200 new HTLV-1 clones are created and killed every day. We find broad agreement between all estimators. The risk of HTLV-1-associated malignancy and inflammatory disease is strongly correlated with proviral load, which in turn is correlated with the number of HTLV-1-infected clones, which are created by de novo infection. Our results therefore imply that suppression of de novo infection may reduce the risk of malignant transformation.

There is no effective antiretroviral treatment for infection with Human T-lymphotropic virus type-1 (HTLV-1), which causes a range of inflammatory diseases and the aggressive malignancy Adult T-cell Leukaemia/Lymphoma (ATL) in approximately 10% of infected people. Within hosts the virus spreads via infectious spread (de novo infection) and mitotic spread (infected cell division). The relative contributions of each mechanism are unknown, and have major implications for drug development and clinical management of infection. We estimate the ratio of infectious to mitotic spread during the infection’s chronic phase using three methods. Each method indicates infectious spread at low but persistent levels after proviral load has reached set point, contrary to the prevailing view that infectious spread features in early infection only. Risk of disease in HTLV-1 infection is known to increase with proviral load, via mutations accrued from repeated infected cell division. Our analyses suggest that ongoing infectious spread may provide an additional mechanism whereby chronic infection becomes malignant. Further, because antiretroviral drugs against Human Immunodeficiency Virus (HIV) inhibit HTLV-1 infectious spread, they may reduce the risk of HTLV-1 malignancy.

Interferon Lambda Family along with HTLV-1 Proviral Load, Tax, and HBZ Implicated in the Pathogenesis of Myelopathy/Tropical Spastic Paraparesis

HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) is a chronic neuroinflammatory disease related to human T lymphotropic virus type 1 (HTLV-1) infection. Interferon type III (IFN-λ), which includes IL28, IL29, and IL28R, and affects the outcome of viral infections, might be complicated in the progression of HAM/TSP. Here, we investigated the host-virus interactions in the manifestation of HAM/TSP, using IL28B, IL29, IL28R, HTLV-1 Tax, HTLV-1 basic leucine zipper factor (HBZ), and proviral load (PVL). The study groups consisted of 20 patients with HAM/TSP, 20 asymptomatic HTLV-1 carriers (ACs), and 20 healthy controls (HCs). The means of PVL, Tax, and HBZ gene expressions in the HAM/TSP group (p = 0.004, 0.006, and < 0.0001, respectively) were significantly higher than in the AC group. The comparison of IL28B, IL29, and IL28R expression in the HAM/TSP, AC, and HC groups revealed no significant difference between the first 2, but lower concentrations in the HCs (IL28B: p = 0.03, 0.01; IL29: p = 0.07, 0.01; and IL28R: p < 0.0001, respectively). In the HAM/TSP group, correlations were seen between Tax and HBZ (R = 0.61, p = 0.004) and between Tax and IL29 (R = 0.45, p = 0.04). Negative correlations were observed between Tax and IL28B (R = -0.49, p = 0.02) and between HBZ and IL28R (R = -0.43, p = 0.06). In the ACs, an inverse correlation was found between Tax and IL28B (R = -0.42, p = 0.06). These findings suggest that IL29, IL28B, and IL28R interfere in the infection of HAM/TSP, mainly via Tax activation.

Risk factors associated with HTLV-1 vertical transmission in Brazil: longer breastfeeding, higher maternal proviral load and previous HTLV-1-infected offspring

HTLV-1 is transmitted primarily either through sexual intercourse or from mother to child. The mother/child pairs were classified as seroconcordant or serodiscordant. We analyzed mother to child transmission (MTCT) according to sociodemographic, clinical and epidemiological characteristics of the mother, child's gender and duration of breastfeeding. Between June 2006 and August 2016 we followed 192 mothers with HTLV-1 infection (mean age 41 years old), resulting in 499 exposed offspring, 288 (57.7%) of whom were tested for HTLV-1, making up the final sample for the study, along with their 134 respective mothers. Among the tested mother/child pairs, 41 (14.2%) were HTLV-1 positive, highlighted that seven of 134 family clusters concentrated 48.8% of positive cases. Variables associated with a positive child: breastfeeding duration ≥12 months, maternal PVL ≥100 copies/104 PBMC, mother's age at delivery >26 years old, and HTLV-1 in more than one child of the same mother. In a multiple logistic regression, breastfeeding ≥12 months, higher maternal PVL and ≥2 previous HTLV-1-infected children remained independently associated with the outcome. Thus, high maternal PVL and breastfeeding beyond 12 months were independently associated with MTCT of the HTLV-1 infection. Our results reinforce the need for both prenatal HTLV screening in endemic areas and for advising mothers on breastfeeding.

Evaluation of HTLV-1 HBZ and proviral load, together with host IFN λ3, in pathogenesis of HAM/TSP

Human T-cell lymphotropic virus 1 (HTLV-1) is associated with two progressive diseases: HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP) and adult T-cell leukemia/lymphoma (ATLL). Although HTLV-1 proviral load (PVL) has been introduced as a risk factor for these diseases' progression, it is not sufficient on its own to yield an accurate estimation of the outcome of the infection. In the present study, PVL and HTLV-1 basic leucine zipper factor (HBZ) expression level as viral factors, and IFN λ3 as a host factor, were evaluated in HAM/TSP patients and HTLV-1 asymptomatic carriers (ACs). During 2014-2015, 12 HAM/TSP patients and 18 ACs who had been referred to the HTLV-1 Clinic, Ghaem Hospital, Mashhad University of Medical Sciences (MUMS), Mashhad, Iran, were enrolled in this study. Peripheral blood mononuclear cells (PBMCs) were isolated and the DNA and mRNA were extracted for quantification of HBZ, IFN λ3 expression, and PVL using real-time PCR (TaqMan method). Although the PVL was higher in the HAM/TSP group, with a 94% confidence interval, there were no considerable differences in terms of HBZ mRNA and PVL between ACs and HAM patients. IFN λ3 expression in the HAM/TSP group was significantly higher than in the ACs (P = 0.02). To the best of our knowledge, no study has evaluated the expression level of IFN λ3 in HTLV-1 positive patients. The immune response against HTLV-1 viral antigens and virulent factors will therefore further refine our knowledge of interactions between the virus and host in the pathogenesis of HTLV-1-related disorders. The virus PVL and the host IFN λ3 can be used as pathogenic factors of HTLV-1 infected patients at risk of HAM/TSP manifestation. J. Med. Virol. 89:1102-1107, 2017. © 2016 Wiley Periodicals, Inc.

Structure and possible function of a G-quadruplex in the long terminal repeat of the proviral HIV-1 genome

The long terminal repeat (LTR) of the proviral human immunodeficiency virus (HIV)-1 genome is integral to virus transcription and host cell infection. The guanine-rich U3 region within the LTR promoter, previously shown to form G-quadruplex structures, represents an attractive target to inhibit HIV transcription and replication. In this work, we report the structure of a biologically relevant G-quadruplex within the LTR promoter region of HIV-1. The guanine-rich sequence designated LTR-IV forms a well-defined structure in physiological cationic solution. The nuclear magnetic resonance (NMR) structure of this sequence reveals a parallel-stranded G-quadruplex containing a single-nucleotide thymine bulge, which participates in a conserved stacking interaction with a neighboring single-nucleotide adenine loop. Transcription analysis in a HIV-1 replication competent cell indicates that the LTR-IV region may act as a modulator of G-quadruplex formation in the LTR promoter. Consequently, the LTR-IV G-quadruplex structure presented within this work could represent a valuable target for the design of HIV therapeutics.

Cell Association in Rous Sarcoma Virus (RSV) Rescue and Cell Infection

In my article I tried to present the results of early experiments suggesting a significant role for cell association in Rous sarcoma virus transformation of non-permissive cells and revealing that infectious virus can be efficiently rescued from such cells by their fusion with permissive chicken fibroblasts.

Lower HIV provirus levels are associated with more APOBEC3G protein in blood resting memory CD4+ T lymphocytes of controllers in vivo

Immunodeficiency does not progress for prolonged periods in some HLA B57- and/or B27-positive subjects with human immunodeficiency virus type 1 (HIV) infection, even in the absence of antiretroviral therapy (ART). These "controllers" have fewer HIV provirus-containing peripheral blood mononuclear cells than "non-controller" subjects, but lymphocytes that harbor latent proviruses were not specifically examined in studies to date. Provirus levels in resting memory cells that can serve as latent reservoirs of HIV in blood were compared here between controllers and ART-suppressed non-controllers. APOBEC3G (A3G), a cellular factor that blocks provirus formation at multiple steps if not antagonized by HIV virion infectivity factor (Vif), was also studied. HLA-linked HIV control was associated with less provirus and more A3G protein in resting CD4+ T central memory (Tcm) and effector memory (Tem) lymphocytes (provirus: p = 0.01 for Tcm and p = 0.02 for Tem; A3G: p = 0.02 for Tcm and p = 0.02 for Tem). Resting memory T cells with the highest A3G protein levels (>0.5 RLU per unit of actin) had the lowest levels of provirus (<1,000 copies of DNA per million cells) in vivo (p = 0.03, Fisher's exact test). Using two different experimental approaches, Vif-positive viruses with more A3G were found to have decreased virion infectivity ex vivo. These results raise the hypothesis that HIV control is associated with increased cellular A3G that may be packaged into Vif-positive virions to add that mode of inhibition of provirus formation to previously described adaptive immune mechanisms for HIV control.

Expression of APOBEC3G/3F and G-to-A hypermutation levels in HIV-1-infected children with different profiles of disease progression

Objective

Increasing evidence has accumulated showing the role of APOBEC3G (A3G) and 3F (A3F) in the control of HIV-1 replication and disease progression in humans. However, very few studies have been conducted in HIV-infected children. Here, we analyzed the levels of A3G and A3F expression and induced G-to-A hypermutation in a group of children with distinct profiles of disease progression.

Methodology/Principal Findings

Perinatally HIV-infected children were classified as progressors or long-term non-progressors according to criteria based on HIV viral load and CD4 T-cell counts over time. A group of uninfected control children were also enrolled in the study. PBMC proviral DNA was assessed for G-to-A hypermutation, whereas A3G and A3F mRNA were isolated and quantified through TaqMan® real-time PCR. No correlation was observed between disease progression and A3G/A3F expression or hypermutation levels. Although all children analyzed showed higher expression levels of A3G compared to A3F (an average fold of 5 times), a surprisingly high A3F-related hypermutation rate was evidenced in the cohort, irrespective of the child's disease progression profile.

Conclusion

Our results contribute to the current controversy as to whether HIV disease progression is related to A3G/A3F enzymatic activity. To our knowledge, this is the first study analyzing A3G/F expression in HIV-infected children, and it may pave the way to a better understanding of the host factors governing HIV disease in the pediatric setting.

Transcription of preintegrated HIV-1 cDNA modulates cell surface expression of major histocompatibility complex class I via Nef

Although transcription from unintegrated human immunodeficiency virus type 1 (HIV-1) DNA can occur inside infected cells, yielding all classes of viral mRNA transcripts, the translation of viral proteins is very limited. One of the proteins made is Nef, but it is unclear whether Nef produced in this way is able to play a role in immune evasion as occurs with integrated virus. We therefore asked whether transcription from preintegrated HIV-1 cDNAs could result in Nef-mediated modulation of cell surface major histocompatibility complex class I (MHC-I) expression. We infected a Rev-CEM green fluorescent protein (GFP) reporter cell line with virus and blocked integration though use of either an inactive integrase or the integrase inhibitor raltegravir. Infected cells were assayed by flow cytometry for cell surface expression of the HLA-A, HLA-B, and HLA-C allotypes (HLA-ABC), HLA-A31, and HLA-E. Viral RNA and DNA products were assayed via quantitative PCR (qPCR). The prevention of integration had no effect, relative to productively infected cells, on levels of expression of multiply spliced viral mRNA transcripts and Nef protein. Downregulation of HLA-ABC and HLA-A31 also occurred at levels similar to those seen in cells in which integration had occurred. Parallel experiments assaying cell surface HLA-ABC expression in infected activated primary CD4(+) T cells produced a similar pattern of results. Hence, the capacity of HIV-1 to modulate MHC-I is not linked to its ability to integrate. Thus, Nef-mediated evasion of host immune responsiveness might be attributable, in part at least, to transcription from unintegrated viral DNA.

Confronting proviral HIV infection

The course of HIV infection is arrested by antiretroviral therapy (ART). However, life-long ART is undesirable. To eradicate infection, strategies are needed to deplete the rare population of proviral genomes that persist and reemerge if ART is interrupted. Proviral HIV persists due to the simultaneous deficiency of factors required to allow proviral expression and virion production, and a predominance of factors that obstruct proviral expression. Combining ART with global inducers of T-cell activation has so far failed to eradicate HIV infection. One approach to the selective removal of obstacles to proviral expression, inhibition of the chromatin remodeling enzyme histone deacetylase, has entered clinical testing. Additional approaches may be needed. Ultimately, therapies that eliminate rare cells that persistently express HIV and interrupt low levels of viremia that persist in some patients may be required to render depletion of proviral HIV infection clinically relevant, and lead to the clearance of HIV infection.

Superinfection of human immunodeficiency virus type 1 (HIV-1) to cell clone persistently infected with defective virus induces production of highly cytopathogenic HIV-1

Superinfection with human immunodeficiency virus type 1 (HIV-1) in human subjects, defined as reinfection with a heterologous strain of HIV-1, has become a topic of great interest. To illustrate the significance of this occurrence, we performed HIV-1 superinfection of L-2 cells, which were isolated from MT-4 cells persistently infected with subtype B HIV-1 as a cell clone continuously producing defective HIV-1 particles. L-2 cells carrying provirus with a one-base insertion in the pol protease were superinfected with HIV-1 derived from primary isolates of subtype B or CRF01_AE. The kinetics of the superinfection in L-2 were very slow compared with those of primary infections in MT-4. Interestingly, L-2 shifted after superinfection to become a producer of highly cytopathogenic HIV-1. Molecular characterization revealed that superinfection occurred in only about 10% of the CRF01_AE-superinfected L-2, which carried provirus of both subtypes and produced viral particles containing genomic RNA of both subtypes. Surprisingly, such cytopathogenic HIV-1 showed predominantly the original subtype B phenotype. Thus, the mechanism of the production of cytopathic HIV-1 seemed to be mediated by trans complementation with pol products of superinfected CRF01_AE. These findings suggest the significance of long-lived infected cells as recipients for superinfection that could result in the generation of new HIV-1 variants with high virulence in patients who are off therapy or do not adhere to treatment, and may indicate the need for precautions against such superinfection.

Conversion of mouse mammary tumor virus to a lymphomagenic virus

Type B leukemogenic virus is a variant of mouse mammary tumor virus (MMTV) that causes thymic lymphomas rather than mammary tumors in mice. We demonstrate that conversion of a mammotropic MMTV to a T-cell-tropic virus requires two alterations in the long terminal repeat: (i) acquisition of a T-cell-specific enhancer and (ii) loss of transcriptional repression through deletion of negative regulatory elements (NREs) or by suppression of NRE activity after appropriate positioning of the enhancer.

Mouse mammary tumor virus infects human cells

Mouse mammary tumor virus (MMTV) has long been speculated to be involved in human breast cancer and more recently in human primary biliary cirrhosis. Despite complete proviral sequences markedly homologous to MMTV being identified in human breast cancer tissue, no convincing evidence has been presented to date that MMTV can infect human cells. Using both wild-type and a genetically marked virus (MMTV-EGFP), we show here the successful infection of a number of different human cells by MMTV. Furthermore, infection of human cells is shown to be almost as efficient as the infection of murine mammary epithelial cells. Sequencing of PCR products from integrated proviruses reveals that reverse transcription and integration of the viral genome has occurred as expected. Furthermore, sequencing of two independent MMTV proviral integration sites reveal them to be present only in the human and not in the mouse genome. Infection requires an intact MMTV envelope protein and is blocked either by heat inactivation of the virus or by specific neutralizing anti-MMTV serum, ruling out a nonspecific mechanism of viral transfer. Thus, MMTV can infect human cells and this finding provides a possible explanation for the detection by others of MMTV sequences in human breast cancer patients.

Multiplicity of human immunodeficiency virus infections in lymphoid tissue

Human immunodeficiency virus type 1 (HIV-1)-infected splenocytes in humans were recently shown to harbor three to four proviruses per cell on average (A. Jung et al., Nature 418:144, 2002). However, the mechanisms that lead to such extensive multiple infections are not understood. Here, we find by using mathematical analysis that two mechanisms quantitatively capture the distribution of proviral genomes in HIV-1-infected splenocytes, one where multiple genomes are acquired one at a time in a series of sequential infectious contacts of a target cell with free virions and infected cells, and the other where cell-to-cell transmission of multiple virions or genomes results from a single infectious contact of a target cell with an infected cell. The two mechanisms imply different genetic diversities of proviruses within an infected cell and therefore different rates of emergence of drug resistance via recombination.

The immune control and cell-to-cell spread of human T-lymphotropic virus type 1

Human T-lymphotropic virus type 1 (HTLV-1) varies little in sequence compared with human immunodeficiency virus type 1 (HIV) and it is difficult to detect HTLV-1 mRNA, proteins or virions in fresh blood. But the strong and chronically activated T cell response to the virus indicates that HTLV-1 proteins are expressed persistently. It now appears that the efficiency of an individual's cytotoxic T cell (CTL) response to HTLV-1 is the chief single determinant of that person's provirus load, which can differ between HTLV-1-infected people by more than 10 000-fold. Progress is now being made towards defining this CTL ‘efficiency’ in terms of host genetics, T cell function, T cell gene expression and mathematical dynamics. Lymphocytes that are naturally infected with HTLV-1 do not produce enveloped extracellular virions in short-term culture and this has reinforced the erroneous conclusion that the virus is latent. But recent evidence shows that HTLV-1 can spread directly between lymphocytes across a specialized, virus-induced cell–cell contact – a ‘viral synapse’. Instead of making extracellular virions, HTLV-1 uses the mobility of the host cell to spread within and between hosts. In this review the evidence is summarized on the persistent gene expression of HTLV-1 in vivo, the role of the immune system in protection and pathogenesis in HTLV-1 infection, and the mechanism of cell-to-cell spread of HTLV-1.

Microglia from mice transgenic for a provirus encoding a monocyte-tropic HIV type 1 isolate produce infectious virus and display in vitro and in vivo upregulation of lipopolysaccharide-induced chemokine gene expression

A large body of evidence has indicated that microglia are the predominant cellular location for HIV-1 in the brains of HIV-1-infected individuals and play a direct role in the development of HIV-1-associated dementia (HAD). Therefore, investigation of the mechanism by which HIV-1-infected microglia contribute to the development of HIV-associated dementia should be facilitated by the creation of a mouse model wherein microglia carry replication-competent HIV-1. To circumvent the inability of HIV-1 to infect mouse cells, we developed a mouse line that is transgenic for a full-length proviral clone of a monocyte-tropic HIV-1 isolate, HIV-1(JR-CSF) (JR-CSF mice), whose T cells and monocytes produce infectious HIV-1. We detected expression of the long terminal repeat-regulated proviral transgene in the microglia of these transgenic mice and demonstrated that it was increased by in vitro and in vivo stimulation with lipopolysaccharide. Furthermore, microglia isolated from JR-CSF mouse brains produced HIV-1 that was infectious in vitro and in vivo. We examined the effect that carriage of the HIV-1 provirus had on chemokine gene regulation in the brains of these mice and demonstrated that MCP-1 gene expression by JR-CSF mouse microglia and brains was more responsive to in vitro and in vivo stimulation with lipopolysaccharide than were microglia and brains from control mice. Thus, this study indicates that the JR-CSF mice may represent a new mouse model to study the effect of HIV-1 replication on microglia function and its contribution to HIV-1-associated neurological disease.

PCR-based cloning and immunocytological titration of infectious porcine endogenous retrovirus subgroup A and B

Two pig endogenous retroviruses (PERV), PERV-A and -B, productively infect human cells and are therefore considered to constitute a potential risk in pig-to-human xenotransplantation. A PCR-based cloning technique to isolate infectious PERV proviruses was established. Overlapping 3' half and 5' halves of PERV proviral genomes were amplified using DNA extracted from human 293 cells infected with PERV-A or -B. These clones were fused at a unique restriction site in the overlapping region and tested for their infectivity. Representative constructs possessed the same infectious properties as their parent isolates. We also developed a polyclonal anti-PERV serum by using recombinant PERV capsid protein derived from one of the infectious constructs as immunogen and established an immunocytological method for detection and titration of PERV infection. This detection method proved to be more sensitive than the current method of choice (transfer of MLV-lacZ vectors) for infectivity assessment of PERV. These findings should be considered for future characterization of PERV isolates.

Insertional polymorphisms of full-length endogenous retroviruses in humans

Human endogenous retrovirus K (HERV-K) is distinctive among the retroviruses in the human genome in that many HERV-K proviruses were inserted into the human germline after the human and chimpanzee lineages evolutionarily diverged [1, 2]. However, all full-length endogenous retroviruses described to date in humans are sufficiently old that all humans examined were homozygous for their presence [1]. Moreover, none are intact; all have lethal mutations [1, 3, 4]. Here, we describe the first endogenous retroviruses in humans for which both the full-length provirus and the preintegration site alleles are shown to be present in the human population today. One provirus, called HERV-K113, was present in about 30% of tested individuals, while a second, called HERV-K115, was found in about 15%. HERV-K113 has full-length open reading frames (ORFs) for all viral proteins and lacks any nonsynonymous substitutions in amino acid motifs that are well conserved among retroviruses. This is the first such endogenous retrovirus identified in humans. These findings indicate that HERV-K remained capable of reinfecting humans through very recent evolutionary times and that HERV-K113 is an excellent candidate for an endogenous retrovirus that is capable of reinfecting humans today.

trans-acting inhibition of genomic RNA dimerization by Rous sarcoma virus matrix mutants

The genomic RNA of retroviruses exists within the virion as a noncovalently linked dimer. Previously, we identified a mutant of the viral matrix (MA) protein of Rous sarcoma virus that disrupts viral RNA dimerization. This mutant, Myr1E, is modified at the N terminus of MA by the addition of 10 amino acids from the Src protein, resulting in the production of particles containing monomeric RNA. Dimerization is reestablished by a single amino acid substitution that abolishes myristylation (Myr1E-). To distinguish between cis and trans effects involving Myr1E, additional mutations were generated. In Myr1E.cc and Myr1E-.cc, different nucleotides were utilized to encode the same protein as Myr1E and Myr1E-, respectively. The alterations in RNA sequence did not change the properties of the viral mutants. Myr1E.ATG- was constructed so that translation began at the gag AUG, resulting in synthesis of the wild-type Gag protein but maintenance of the src RNA sequence. This mutant had normal infectivity and dimeric RNA, indicating that the src sequence did not prevent dimer formation. All of the src-containing RNA sequences formed dimers in vitro. Examination of MA-green fluorescent protein fusion proteins revealed that the wild-type and mutant MA proteins Myr1E.ATG-, Myr1E-, and Myr1E-.cc had distinctly different patterns of subcellular localization compared with Myr1E and Myr1E.cc MA proteins. This finding suggests that proper localization of the MA protein may be required for RNA dimer formation and infectivity. Taken together, these results provide compelling evidence that the genomic RNA dimerization defect is due to a trans-acting effect of the mutant MA proteins.

Clonality analysis of various hematopoietic disorders in cats naturally infected with feline leukemia virus

The clonality analysis of the bone marrow cells was carried out by detecting the integrated proviruses of feline leukemia virus (FeLV) to understand the pathogenesis of FeLV-associated hematopoietic disorders in cats. Bone marrow cells from 4 cases with acute myeloid leukemia (AML), 9 cases with myelodysplastic syndromes (MDS), 2 cases with pure red cell aplasia (PRCA) and 3 healthy carriers infected with FeLV were subjected to Southern blot analyses using an exogenous FeLV probe. Clonal hematopoiesis was found in all the cases with AML and in 6 of the 9 cases with MDS, but not in the cases with both PRCA and healthy carriers infected with FeLV. In the 2 cases with MDS, it was thought that the same clones of the hematopoietic cells might proliferate before and after the progression of the disease irrespective of the changes of the hematological diagnoses by cytological examination. This study indicates that MDS in cats is a disease manifestation as a result of clonal proliferation of hematopoietic cells and can be recognized as a pre-leukemic state of AML.

Cloning of an infectious milk-borne mouse mammary tumor virus (MMTV) DNA from a mammary tumor that developed in an endogenous MMTV-free wild mouse

Molecular characterization of infectious mouse mammary tumor viruses (MMTVs) has been hampered due to the problem of cloning a full-length exogenous virus into a plasmid. The present report describes our strategy for obtaining a full-length clone of an exogenous MMTV from a mouse mammary tumor that arose spontaneously in a wild Chinese mouse free of endogenous MMTV and shows that the cloned virus (JYG-MMTV) is expressed in rat RBA cells. Four-week-old C58/J x CBA/CaJ female mice, free of both endogenous and exogenous MMTVs, were injected with virus-secreting RBA cells. The progeny of these mice were bred, and their offspring were tested for the presence of MMTV. These third-generation mice were found to actively produce MMTV that was shed in their milk and transmitted to their offspring. The virus was detected not only in the mammary glands of these young mice, but also in their spleens and bone marrow. These results suggest that our plasmid-cloned exogenous JYG-MMTV is infectious. This virus can now be used effectively in manipulating the various genes of JYG-MMTV and other MMTV strains to understand their structure/function relationships.

Mouse mammary tumor virus carrying a bacterial supF gene has wild-type pathogenicity and enables rapid isolation of proviral integration sites

Mouse mammary tumor virus (MMTV) has frequently been used as an insertional mutagen to identify provirally activated mammary proto-oncogenes. To expedite and facilitate the process of cloning MMTV insertion sites, we have introduced a bacterial supF suppressor tRNA gene into the long terminal repeat (LTR) of MMTV, thus allowing selection of clones containing it in lambda vectors bearing amber mutations. The presence of supF in the LTR should circumvent the screening process for proviral insertion sites, since only those lambda clones with supF-containing proviral-cellular junction fragments should be able to form plaques on a lawn of wild-type Escherichia coli (i.e., lacking supF). The resulting virus (MMTVsupF) induced mammary tumors at the expected rate in infected mice, deleted the appropriate T-cell population by virtue of its superantigen gene, and stably retained the supF gene after passage via the milk to female offspring. To test the selective function of the system, size-selected DNA containing two proviral-cellular junction fragments from an MMTV supF-induced mammary tumor was ligated into lambdagtWES.lambdaB, packaged, and plated on a supF-deficient bacterial host for selection of supF-containing clones. All plaques tested contained the desired cloned fragments, thus demonstrating the utility of this modified provirus for the rapid cloning of MMTV insertion sites.

Murine leukemia provirus-mediated activation of the Notch1 gene leads to induction of HES-1 in a mouse T lymphoma cell line, DL-3

Constitutive activation of Notch signaling is known to be associated with tumorigenesis. In a mouse T lymphoma cell line, DL-3, we found that a murine leukemia provirus was inserted in the Notch1 locus, which led to marked expression of a virus-Notch1 fusion mRNA encoding an intracellular portion of the Notch1 protein. Furthermore, expression and nuclear localization of this constitutively active form of Notch1 protein were confirmed. Corresponding to this finding, the transcription of the hairy/enhancer of split (HES-1) gene, a known target of Notch1 signaling, was elevated in this cell line. A potential role for overexpressed HES-1 in the development of the lymphoma was discussed.

In vivo footprinting of the enhancer sequences in the upstream long terminal repeat of Moloney murine leukemia virus: differential binding of nuclear factors in different cell types

The enhancer sequences in the Moloney murine leukemia virus (M-MuLV) long terminal repeat (LTR) are of considerable interest since they are crucial for virus replication and the ability of the virus to induce T lymphomas. While extensive studies have identified numerous nuclear factors that can potentially bind to M-MuLV enhancer DNA in vitro, it has not been made clear which of these factors are bound in vivo. To address this problem, we carried out in vivo footprinting of the M-MuLV enhancer in infected cells by in vivo treatment with dimethyl sulfate (DMS) followed by visualization through ligation-mediated PCR (LMPCR) and gel electrophoresis. In vivo DMS-LMPCR footprinting of the upstream LTR revealed evidence for factor binding at several previously characterized motifs. In particular, protection of guanines in the central LVb/Ets and Core sites within the 75-bp repeats was detected in infected NIH 3T3 fibroblasts, Ti-6 lymphoid cells, and thymic tumor cells. In contrast, factor binding at the NF-1 sites was found in infected fibroblasts but not in T-lymphoid cells. These results are consistent with the results of previous experiments indicating the importance of the LVb/Ets and Core sequences for many retroviruses and the biological importance especially of the NF-1 sites in fibroblasts and T-lymphoid cells. No evidence for factor binding to the glucocorticoid responsive element and LVa sites was found. Additional sites of protein binding included a region in the GC-rich sequences downstream of the 75-bp repeats (only in fibroblasts), a hypersensitive guanine on the minus strand in the LVc site (only in T-lymphoid cells), and a region upstream of the 75-bp repeats. These experiments provide concrete evidence for the differential in vivo binding of nuclear factors to the M-MuLV enhancers in different cell types.

Establishment of MAIDS-defective virus-infected B cell lines and their characterization

Mice inoculated with the murine AIDS (MAIDS)-defective virus develop severe B and T cell dysfunctions. The primary event in the development of this disease is the infection and polyclonal expansion of the target cells of this defective virus, which have been reported to belong to the B cell lineage. To further study the central role that these cells play in the development of MAIDS, we attempted to establish MAIDS-defective virus-infected B cell lines in vitro. We succeeded in establishing two cell lines, SD1 and CSTB5, from the enlarged organs of C57BL/6 mice inoculated with helper-free stocks of the MAIDS-defective virus. Both cell lines are not transplantable in syngeneic C57BL/6 mice or in nude or CD8-/- mice and are apparently not malignant. They both belong to the B lineage, as their immunoglobulin (Ig) genes, but not the T cell receptor (TcR) beta locus, are rearranged, suggesting that they are relatively mature B cells. However, analysis of cell surface marker expression by FACS revealed a surface phenotype similar to that of pre-B cells (MHC I+, MHC II+, B7.2+, sIgM-, sIgG-, kappa-, B220-, CD5-, Thy1.2-, TcR-, CD3-, CD4-, CD8-, Mac-1-, 33D1-). Additionally, the CSTB5 cells express CD40 and the SD1 cells express CD43. Both cell lines contain the MAIDS-defective provirus and express the expected 4.2-kb viral RNA and the corresponding Pr60gag protein. The CSTB5 cells are nonproducer, while the SD1 cell line produces what appears to be an endogenous MuLV. The phenotype of these cell lines is very similar to what is known about the target B cells of this virus in vivo. These new established cell lines are likely to be useful in elucidating the mechanism(s) by which the MAIDS-defective virus causes its target B cells to proliferate and induce T cell anergy in infected animals.

Comparison of in vitro and in vivo infectivity of different clade B HIV-1 envelope chimeric simian/human immunodeficiency viruses in Macaca mulatta

The use of HIV-1 env/SIVmac chimeric viruses expressing divergent HIV-1 envelopes of clinical isolates, facilitates homologous and heterologous evaluation of various recombinant HIV-1 envelope vaccine candidates in lower primates. In this study we compare the in vitro and in vivo infectivity, via intravenous (IV) and intravaginal (IVAG) routes of infection, of stocks of chimeric viruses expressing env from four different clade B HIV-1 isolates. The TCID50/ml was 7.1 x 10(4), 1.0 x 10(4), 6.3 x 10(4), and 1.2 x 10(3) for SHIVsf13, SHIVHan2, SHIVNM-3rn, and SHIVW6.1D, respectively, with a MID50/ml upon IV inoculation of 3.2 x 10(3), 3.2 x 10(4), 3.2 x 10(4), and 3.2 x 10(3), respectively. The same SHIVsf13 stock was infectious after IVAG administration, requiring a 300-fold higher virus dose. Plasma antigenemia and cell-associated viremia were generally highest at weeks 2 or 4 after infection and decreased to subdetectable levels after 8-12 weeks. All infected animals tested developed anti-HIV-1 gp120 antibodies. Inoculated virus dose showed no (linear) quantitative correlation with cellular virus load, duration of viremia, plasma antigenemia, and anti-gp120 antibody titers. No significant changes in peripheral blood CD4 cell levels were observed and none of the animals has shown evidence of disease progression to date (i.e., 13 months postinfection). Four in vivo passages of cell-associated SHIVW6.1D did not result in increased virulence. Vaccine development studies in macaques monkeys have become feasible with the use of various clade B HIV-1 env SHIV chimeras.

Defective HIV-1 provirus encoding a multitarget-ribozyme inhibits accumulation of spliced and unspliced HIV-1 mRNAs, reduces infectivity of viral progeny, and protects the cells from pathogenesis

A HeLa T4 cell line containing a defective human immunodeficiency virus type 1 (HIV-1) DNA (HD4) was isolated. After transactivation with Tat, the HD4 DNA was transcribed into a single 3.7-kb mRNA that encodes a chimeric CD4/Env protein and a multitarget-ribozyme directed against multiple sites within the gp120 coding region of HIV-1 RNA (Chen et al., 1992). Early steps in HIV infection such as entry, reverse transcription, and proviral DNA formation were not affected in HD4 cells, and HD4 was efficiently transactivated after either HIV-1 or HIV-2 infections. HIV-2, which lacks all of the HIV-1-specific ribozyme target sites, replicated to high levels in HD4 cells whereas HIV-1 replication was selectively inhibited. Despite a reduced accumulation of all HIV-1 transcripts, transactivation of HD4 was efficient. Surprisingly, the most abundant, multiply spliced mRNAs were reduced even though they lack all of the ribozyme target sites. These results strongly suggest that the ribozyme co-localizes with unspliced HIV-1 pre-mRNA and/or genomic HIV-1 RNA in the nucleus. Cleavage of these precursor RNAs explains the reduction of all spliced and unspliced HIV-1 RNAs. Cleavage of genomic RNA probably contributed to the three-fold reduction in the infectivity of viral progeny. Thus, the HD4 ribozyme RNA functioned as a ribozyme in the nucleus and as a mRNA for a chimeric CD4/Env protein in the cytoplasm. Its unusual large size for a ribozyme (3.7 kb) indicates that, in the future, other antiviral proteins, like negative transdominant mutant HIV-1 proteins, may also be encoded to increase its antiviral potential in a gene therapy approach.

Characterization of molecularly cloned simian-human immunodeficiency viruses causing rapid CD4+ lymphocyte depletion in rhesus monkeys

In vivo passage of a chimeric simian-human immunodeficiency virus (SHIV-89.6) expressing the human immunodeficiency virus type 1 (HIV-1) tat, rev, vpu, and env genes generated pathogenic viruses (SHIV-89.6P) inducing rapid CD4+ lymphocyte depletion and AIDS-like illness in rhesus monkeys (K. Reimann, J. T. Li, R. Veazey, M. Halloran, I.-W. Park, G. B. Karlsson, J. Sodroski, and N. L. Letvin, J. Virol. 70:6922-6928, 1996). To characterize the molecular changes responsible for this increase in virulence, infectious proviral clones of SHIV-89.6P isolates were derived. Viruses generated from some of these clones caused a rapid and profound decline of CD4+ lymphocytes in a high percentage of inoculated monkeys. Nucleotide changes potentially responsible for the increased virulence of SHIV-89.6P were limited to the env, tat, or long terminal repeat sequences, with most of the observed changes in env. Nucleotide changes in env altered 12 amino acids in the gp120 and gp41 exterior domains, and a 140-bp deletion in env resulted in the substitution of the carboxyl terminus of the SIVmac gp41 glycoprotein for that of the HIV-1 gp41 glycoprotein. The availability of pathogenic proviral clones should facilitate dissection of the molecular determinants of SHIV-89.6P virulence.

A point mutation in the HIV-1 Tat responsive element is associated with postintegration latency

Study of the mechanism of HIV-1 postintegration latency in the ACH2 cell line demonstrates that these cells failed to increase HIV-1 production following treatment with exogenous Tat. Reasoning that the defect in ACH2 cells involves the Tat response, we analyzed the sequence of tat cDNA and Tat responsive element (TAR) from the virus integrated in ACH2. Tat cDNA sequence is closely related to that of HIV LAI, and the encoded protein is fully functional in terms of long terminal repeat (LTR) transactivation. Cloning of a region corresponding to the 5'-LTR from ACH2, however, identified a point mutation (C37 -> T) in TAR. This mutation impaired Tat responsiveness of the LTR in transient transfection assays, and the measured defect was complemented in cells that had been treated with tetradecanoyl phorbol acetate or tumor necrosis factor type alpha (TNF-alpha). A compensatory mutation in TAR (G28 -> A), designed to reestablish base pairing in the TAR hairpin, restored wild-type Tat responsiveness. When the (C37 -> T) mutation was introduced in an infectious clone of HIV-1, no viral production was measured in the absence of TNF-alpha, whereas full complementation was observed when the infection was conducted in the presence of TNF-alpha or when a compensatory mutation (G28 -> A) was introduced into TAR. These experiments identify a novel mutation associated with HIV-1 latency and suggest that alterations in the Tat-TAR axis can be a crucial determinant of the latent phenotype in infected individuals.

Infectious proviral clones of chimpanzee foamy virus (SFVcpz) generated by long PCR reveal close functional relatedness to human foamy virus

Infectious proviral clones of simian foamy virus isolated from chimpanzee (SFVcpz) were generated by long PCR. Two overlapping fragments representing the complete provirus were amplified from genomic DNA of infected cells. Four 8.8-kbp amplimers extending from base 1 of the provirus into the env gene and five 4.45-kbp amplimers reaching from env to the end of the 3'-LTR were cloned into pCR II. Subsequently, the proviral fragments were combined in a chessboard manner to generate 20 plasmids containing full-length proviral DNA. Four plasmids produced infectious virus after transfection of susceptible cells. A distinct proviral form bearing a deletion in the transactivator gene joining both exons of a second regulatory gene present in wild-type foamy virus-infected cells started to emerge 48 hr after transfection of BHK cells with infectious SFVcpz DNA. This observation supports a novel hypothesis to explain establishment of foamy virus latency. The transactivator protein Taf of SFVcpz transcomplemented for the homologous protein Bel-1 of the unique human foamy virus isolate (HFV) and Bel-1 exhibited the reciprocal activity, suggesting that HFV could represent a variant of chimpanzee foamy virus.

Nef stimulates human immunodeficiency virus type 1 proviral DNA synthesis

The Nef protein of human immunodeficiency virus type 1 (HIV-1) stimulates viral infectivity. The mechanism of this phenotype was investigated. Viruses containing disrupted nef genes were 4 to 40 times less infectious than wild-type HIV-1 in a single-round infection. The Nef-mediated stimulation HIV-1 infectivity was dependent on the association of Nef with the plasma membrane and could be observed when Nef was provided in trans in the virus producer but not target cells. The impaired infectiousness of nef-defective (delta Nef) virions was observed whether or not CD4 was present in either of these cells. Furthermore, it was independent of the mode of viral entry, since it was not rescued by pseudotyping Env- HIV-1 virions with the amphotropic murine leukemia virus envelope glycoproteins. As predicted from this result, wild-type and delta Nef virions entered cells with equal efficiencies. However, despite their normal content in viral genomic RNA and reverse transcriptase activity, delta Nef viruses were limited in their ability to perform reverse transcription once internalized in several cell types, including peripheral blood lymphocytes. Since Nef does not appear to be abundant in virions, these results suggest that Nef acts in producer cells to allow the generation of particles fully competent for completing steps that follow entry, leading to efficient reverse transcription of the HIV-1 genome. Using a trans complementation assay, we found that Nef proteins from a number of primary HIV-1 isolates as well as, to a milder degree, those from HIV-2ST and SIVMAC239 could enhance the infectivity of delta Nef HIV-1. This indicates that the Nef-mediated stimulation of proviral DNA synthesis is highly conserved and likely plays an important role in vivo.

Increase in sensitivity to soluble CD4 by primary HIV type 1 isolates after passage through C8166 cells: association with sequence differences in the first constant (C1) region of glycoprotein 120

Primary isolates of human immunodeficiency virus type 1 (HIV-1) were obtained by coculture of peripheral blood lymphocytes (PBLs) from HIV-1-infected people with PBLs from uninfected donors. These viral stocks tend to be resistant to neutralization/inactivation by soluble CD4 (sCD4). When these stocks were passed through the T cell line C8166, virus stocks emerged that were sensitive to sCD4. Pre- and post-C8166 stocks maintained their sCD4-resistant and -sensitive phenotypes, respectively, with further passage in PBLs. Pre- and post-C8166 stocks were biologically cloned by two cycles of limiting dilution. The majority (14 of 17) of pre-C8166 clones were sCD4 resistant, and, conversely, the majority of post-C8166 clones (11 of 12) were sensitive to sCD4. Nucleotide and predicted amino acid sequence analysis in the env (gp120) region revealed a limited number of differences between the clones. The only differences that sorted with biological phenotype were in the first constant (C1) region of gp120. Adaptation to growth in C8166 cells and conversion from the sCD4-resistant to the sCD4-sensitive phenotype represent the emergence to prominence of viral species in the pre-C8166 stock that have a replication advantage in C8166 coincident with increased sensitivity to sCD4.

Effect of a single amino acid substitution in the V3 domain of the human immunodeficiency virus type 1: generation of revertant viruses to overcome defects in infectivity in specific cell types

Proviral clones of human immunodeficiency virus type 1 which contained single amino acid changes in the envelope V3 region were constructed. PCR amplification of Sup-T1 T cells transfected with one such mutant, G312T, revealed low levels of virus that resulted in the generation of a revertant virus, in which an alanine replaced the threonine residue at amino acid 312. The revertant virus (rA312) was fully infectious in Sup-T1 cells but lacked the ability to infect AA5 cells. The presence of a second mutation in a subsequent revertant virus (rR306), in which arginine was substituted for serine at amino acid 306 within the V3 loop, restored the ability of the mutated virus to infect AA5 cells. Our data highlight the importance of the V3 loop in defining virus tropism for specific cell types in culture and further suggest that a degree of interplay exists among V3 loop residues that helps maintain or control its biological function of the virus.

Antibodies of symptomatic human immunodeficiency virus type 1-infected individuals are directed to the V3 domain of noninfectious and not of infectious virions present in autologous serum

The present study was designed to determine the antibody specificity for the human immunodeficiency virus type 1 (HIV-1) V3 domains of infectious and noninfectious virions present in the serum of AIDS patients. To accomplish this, HIV-1 was isolated in the presence of autologous antibodies from the serum samples of six AIDS patients in HIV-1-negative donor peripheral blood mononuclear cells by short-term cultivation. The isolated virus, defined as the infectious cell-free virus (iCFV), was characterized by sequence analysis of the proviral DNA coding for the third hypervariable (V3) region of the external glycoprotein gp120. This was carried out by amplifying and cloning the V3 region. In all six cases studied, 20 randomly selected V3 clones derived from the proviral DNA of the iCFV, 20 clones from patient cell-free virus, and 20 clones from cell-integrated virus were sequenced to study the distribution and frequency of the intrapatient virus population. The number of major virus variants in the six patients ranged from three to nine. The various V3 sequences found in the AIDS patients showed the typical amino acid pattern of the syncytium-inducing and non-syncytium-inducing viral phenotypes characteristic for the late stage of infection. However, only one patient-specific iCFV variant was detected within the 20 V3 clones analyzed per virus isolation. For the six patients a total of 34 V3-loop variants, either iCFV or non-iCFV, was observed. All 34 V3-loop sequences were expressed as glutathione-S-transferase fusion proteins (V3-GST). The autologous antibody response to the V3-GST fusion proteins was studied by Western immunoblot analysis. A strong antibody response to almost all non-iCFV V3-GST proteins was found in the sera of the six patients. In contrast, the autologous antibody response to the six iCFV V3 loops was undetectable (in four patients) or very faint (in two patients) compared with that to the non-iCFV V3 loops. Five of the six iCFV loops showed positively charged amino acids at positions strongly associated with the syncytium-inducing phenotype. These findings suggest that our in vitro isolation system selects for virions which are not recognized by V3-specific antibodies and are infectious both in vitro and in vivo.

Fusogenicity of mutant and chimeric proviruses derived from molecular clones of cytopathic and noncytopathic human immunodeficiency virus type 2

Analysis of the phenotype of the molecular clones of cytopathic and fusogenic, noncytopathic and nonfusogenic, and chimeric proviruses of human immunodeficiency virus type 2 (HIV-2) suggests that the major determinant of the attenuated fusogenicity and cytopathicity of HIV-2 was located in the 3'-half of the genome, with envelope playing the more dominant role. However, no single linear domain within the envelope, including the major CD4 binding domain and fusogenic domain, was sufficient by itself for syncytia induction and cytopathic effects. Truncation of the transmembrane envelope glycoprotein downstream of the transmembrane region was not a major factor in this regard. However, truncation within the transmembrane region rendered the provirus replication incompetent. The regulatory genes (tat, rev) and auxiliary gene (nef) did not seem to play a critical role in determining HIV-2 fusogenicity in vitro. The results suggest the importance of the overall conformation of the envelope in the divergent phenotypes of HIV-2.

Long-range mapping of Mis-2, a common provirus integration site identified in murine leukemia virus-induced thymomas and located 160 kilobase pairs downstream of Myb

The nondefective Moloney murine leukemia virus (MuLV) induces clonal or oligoclonal T-cell tumors in mice or rats. The proviruses of these nondefective MuLVs have been shown to act as insertion mutagens most frequently activating an adjacent cellular gene involved in cell growth control. Mutations by provirus insertions, recognized as common provirus integration sites, have been instrumental in identifying novel cellular genes involved in tumor formation. We have searched for new common provirus integration sites in Moloney MuLV-induced thymomas. Using cellular sequences flanking a provirus cloned from one of these tumors, we found one region, designated Mis-2, which was the target of provirus integration in a low (3%) percentage of these tumors. Mis-2 was mapped on mouse chromosome 10, approximately 160 kbp downstream of myb. The Mis-2 region may contain a novel gene involved in tumor development.

The RRE of human immunodeficiency virus type 1 contributes to cell-type-specific viral tropism

As part of a general program investigating the mechanism of the Rev axis of human immunodeficiency virus type 1 (HIV-1) autoregulation, a series of proviral HIV-1 mutants which differ from the parental HXB2 strain at selected positions within the RRE were constructed. All of the mutations were designed to perturb the RRE by introducing local helix disruptions without altering the coding potential of the overlapping envelope open reading frame. Viral replication in various cell types was monitored by a cell supernatant reverse transcriptase assay and Northern (RNA blot) analysis. All proviral RRE mutants displayed at least some impairment in replication. However, the relative impairment varied drastically among the various cell types tested. This suggests that the RRE may contribute to cell-type-specific viral tropism.

Nucleotide sequence and biological properties of a pathogenic proviral molecular clone of neurovirulent visna virus

Intracerebral serial passage of visna virus KV1514 through three Icelandic sheep was used to select for strains with increased neurovirulence. A strain (KV1772) with increased neuropathogenicity was obtained. We isolated several proviral molecular clones from a plaque-purified biological clone of KV1772 that induced typical visna virus pathology in young sheep. One of the clones (kv72) was infectious, while others contained mutations or were permuted and required gene recombination with other proviral clones to generate infectious virus after transfection. Stable plasmids containing functional, full-length, visna virus KV1772 genomes were constructed from the proviral molecular clones. The in vitro cytopathic effects of virus derived from these clones varied depending upon the tissue origin of the infected cells. A goat cell line became persistently infected with molecularly cloned KV1772 virus; these cells resisted the cell-killing effects and continuously shed high levels of infectious virus. We determined the complete nucleotide sequence of a KV1772 provirus; it contains open reading frames for all structural and accessory genes previously identified in the visna virus genome and is highly homologous to other published visna virus sequences. Progeny of molecularly cloned KV1772 virus rapidly induced both a pronounced neuropathology and an unexpected, strong, neutralizing antibody response in experimentally infected young Icelandic sheep. The availability of stable plasmids of replication-competent and pathogenic proviral molecular clones of visna virus should now enable the study of the genetic determinants of neurovirulence and their interaction with the host immune system in visna virus pathogenesis.

Aurothiolates inhibit HIV-1 infectivity by gold(I) ligand exchange with a component of the virion surface

Aurothioglucose and aurothiomalate have anti-HIV-1 activity in vitro. Antiviral activity requires the formation of a reactive intermediate with a molar equivalent amount of a thiol ligand. This activates gold(I) ligand exchange between the reactive species bis(thiolato)gold(I) and acidic thiol groups exposed on the surface of proteins. Bis(thioglucose)gold(I) (bisAuTG) which is formed by the reaction of molar equivalent amounts of aurothioglucose and 1-thio-beta-D-glucose completely protected MT-4 and CEM cells against HIV-1NL4-3-induced cytopathogenicity. Although bisAuTG is an inhibitor of human immunodeficiency virus-1 (HIV-1) reverse transcriptase in a cell-free assay, its antiviral effect is due to modification of a surface component of the virion. The HIV-1 strain NL4-3 is 200-fold more sensitive to inhibition of infectivity by bisAuTG than are the strains MN, RF, and SF-2. HIV-1NL4-3 has a unique cysteine residue close to the amino terminus of its gp41 envelope glycoprotein (residue 532 of gp160) which we hypothesize is the target of bisAuTG binding. Mutation of that residue alters HIV-1NL4-3 infectivity and dominantly suppresses virus assembly when coexpressed with the wild-type NL4-3 genome. We show that bisAuTG treatment releases gp120 from the surface of cells expressing wild-type HIV-1NL4-3 envelope glycoprotein, but it does not release gp120 if Cys532 is mutationally altered to Ala. Thus, the antiviral effect of bisAuTG on HIV-1NL4-3 is due to an effect on the association of gp120 with gp41.

Requirement of active human immunodeficiency virus type 1 integrase enzyme for productive infection of human T-lymphoid cells

The human immunodeficiency virus type 1 (HIV-1) integrase enzyme exhibits significant amino acid sequence conservation with integrase proteins of other retroviruses. We introduced specific amino acid substitutions at a number of the conserved residue positions of recombinant HIV-1 integrase. Some of these substitutions resulted in proteins which were not able to be purified in the same manner as the wild-type enzyme, and these were not studied further. The remaining mutant enzymes were assessed for their abilities to perform functions characteristic of the integrase protein. These included specific removal of the terminal dinucleotides from oligonucleotide substrates representative of the viral U5-long terminal repeat, nonspecific cleavage of oligonucleotide substrates, and mediation of the strand transfer (integration) reaction. Substitution at position 43, within the protein's zinc finger motif region, resulted in an enzyme with reduced specificity for cleavage of the terminal dinucleotide. In addition, a double substitution of aspartic acid and glutamine for valine and glutamic acid, respectively, at positions 151 and 152 within the D,D(35)E motif region rendered the integrase protein inactive for all of its functions. The introduction of this double substitution into an infectious HIV-1 provirus yielded a mutant virus that was incapable of productively infecting human T-lymphoid cells in culture.

In vitro enzymatic activity of human immunodeficiency virus type 1 reverse transcriptase mutants in the highly conserved YMDD amino acid motif correlates with the infectious potential of the proviral genome

Reverse transcriptases contain a highly conserved YXDD amino acid motif believed to be important in enzyme function. The second amino acid is not strictly conserved, with a methionine, valine or alanine occupying the second position in reverse transcriptases from various retroviruses and retroelements. Recently, a 3.5-A (0.35-nm) resolution electron density map of human immunodeficiency virus type 1 (HIV-1) reverse transcriptase positioned the YMDD motif within an antiparallel beta-hairpin structure which forms a portion of its catalytic site. To further explore the role of methionine of the conserved YMDD motif in HIV-1 reverse transcriptase function, we have substituted methionine with a valine, alanine, serine, glycine, or proline, reflecting in some cases sequence motifs of other related reverse transcriptases. Wild-type and mutant enzymes were expressed in Escherichia coli, partially purified by phosphocellulose chromatography, and assayed for the capacity to polymerize TTP by using a homopolymeric template [poly(rA)] with either a DNA [oligo(dT)] or an RNA [oligo(U)] primer. With a poly(rA).oligo(dT) template-primer, reverse transcriptases with the methionine replaced by valine (YVDD), serine (YSDD), or alanine (YADD) were 70 to 100% as active as the wild type, while those with the glycine substitution (YGDD) were approximately 5 to 10% as active. A proline substitution (YPDD) completely inactivated the enzyme. With a poly(rA).oligo(U) template-primer, only the activity of mutants with YVDD was similar to that of the wild type, while mutants with YADD and YSDD were approximately 5 to 10% as active as the wild-type enzyme. The reverse transcriptases with the YGDD and YPDD mutations demonstrated no activity above background. Proviruses containing the reverse transcriptase with the valine mutation (YVDD) produced viruses with infectivities similar to that of the wild type, as determined by measurement of p24 antigen in culture supernatants and visual inspection of syncytium formation. In contrast, proviruses with reverse transcriptases containing the YADD and YSDD mutations were less infectious than wild-type virus. These results point to the critical role of methionine of the YMDD motif in the activity of HIV-1 reverse transcriptase and subsequent replication potential of the virus.

Genetic analysis of the effects of Mtv-2 on the T cell repertoire in the WXG-2 mouse strain

In this report, the T cell repertoire was studied in a natural genetic model system using a novel mouse strain (WXG-2) carrying a single pathogenic mouse mammary tumor virus (MMTV) provirus (Mtv-2) on an otherwise MMTV-free genetic background. The Mtv-2 provirus has complete biological activity, produces infectious milk-transmitted virus, and contributes to mammary carcinogenesis by an insertion mutation mechanism. In mice carrying the Mtv-2 provirus, T cells expressing V beta 14 were specifically deleted in mice with a functional MHC class II I-E gene but not in I-E- controls. The deletion of V beta 14+ T cells was more rapid in mice with the Mtv-2 provirus than in Mtv-2-free control mice infected with exogenous MMTV. In addition, the Mtv-2 deletion phenotype was age dependent. A slow depletion of V beta 14+ T cells was observed, and greater than 95% of the V beta 14+ T cells were eliminated by 6 months of age. These experiments indicate that (i) the Mtv-2 provirus encodes or regulates expression of a V beta 14-specific superantigen, (ii) interactions between Mtv-2 and other MMTV proviruses are not necessary for the V beta 14 deletion phenotype, (iii) the presence of a retroviral superantigen in all cells is not sufficient for T cell depletion during neonatal development in the thymus, and (iv) the Mtv-2 provirus and its associated exogenous provirus have the same V beta specificity.(ABSTRACT TRUNCATED AT 250 WORDS)

Dual regulation of silent and productive infection in monocytes by distinct human immunodeficiency virus type 1 determinants

The regulation of human immunodeficiency virus type 1 infection and replication in primary monocytes was investigated by mutagenesis of recombinant proviral clones containing an env determinant required for the infectivity of monocytes. Virus replication was assayed by determination of reverse transcriptase activity in culture fluids and by recovery of virus from monocytes following cocultivation with uninfected peripheral blood mononuclear cells. Three virus replication phenotypes were observed in monocytes: productive infection, silent infection, and no infection. Incorporation of the monocytetropic env determinant in a full-length clone incapable of infection or replication in primary monocytes (no infection) conferred the capacity for highly efficient virus replication in monocytes (productive infection). Clones with the env determinant but lacking either functional vpr or vpu genes generated lower replication levels in monocytes. Mutation of both vpr and vpu, however, resulted in nearly complete attenuation of virus replication in monocytes, despite subsequent virus recovery from infected monocytes by cocultivation with uninfected peripheral blood mononuclear cells (silent infection). These findings indicate a central role for the "accessory" genes vpu and vpr in productive human immunodeficiency virus type 1 replication in monocytes and indicate that vpu and vpr may be capable of functional complementation.

Pathogenesis of feline leukemia virus T17: contrasting fates of helper, v-myc, and v-tcr proviruses in secondary tumors

A naturally occurring feline thymic lymphosarcoma (T17) provided the unique observation of a T-cell antigen receptor beta-chain gene (v-tcr) transduced by a retrovirus. The primary tumor contained three classes of feline leukemia virus (FeLV) provirus, which have now been characterized in more detail as (i) v-tcr-containing recombinant proviruses, (ii) v-myc-containing recombinant proviruses, and (iii) apparently full-length helper FeLV proviruses. The two transductions appear to have been independent events, with distinct recombinational junctions and no sequence overlap in the host-derived inserts. The T17 tumor cell line releases large numbers of FeLV particles of low infectivity; all three genomes are encapsidated, but passage of FeLV-T17 on feline fibroblast and lymphoma cells led to selective loss of the recombinant viruses. The oncogenic potential of the T17 virus complex was, therefore, tested by infection of neonatal cats with virus harvested directly from the primary T17 tumor cell line. A single inoculation of FeLV-T17 caused persistent low-grade infection culminating in thymic lymphosarcoma and acute thymic atrophy, which was accelerated by coinfection with the weakly pathogenic FeLV subgroup A (FeLV-A)/Glasgow-1 helper. Molecularly cloned FeLV-tcr virus (T-31) rescued for replication by a weakly pathogenic FeLV-A/Glasgow-1 helper virus was similarly tested in vivo and induced thymic atrophy and thymic lymphosarcomas. Most FeLV-T17-induced tumors manifested either v-myc or an activated c-myc allele and had undergone rearrangement of endogenous T-cell antigen receptor beta-chain genes, supporting the proposition that the oncogenic effects of c-myc linked to the FeLV long terminal repeat are targeted to a specific window in T-cell differentiation. However, neither the FeLV-T17-induced tumors nor the T-31 + FeLV-A-induced tumors contained clonally represented v-tcr sequences. Only one of the FeLV-T17-induced tumors contained detectable v-tcr proviruses, at a low copy number. While v-tcr does not have a readily transmissible oncogenic function, a more restricted role is not excluded, perhaps involving antigenic peptide-major histocompatibility complex recognition by the T-cell receptor complex. Such a function could be obscured by the genetic diversity of the outbred domestic cat host.

Nonrandom distribution of gp120 N-linked glycosylation sites important for infectivity of human immunodeficiency virus type 1

More than 20 consensus N-linked glycosylation sites occur in the gp120 coding sequence of most isolates of human immunodeficiency virus type 1. Based on the N-linked glycosylation pattern of a well-characterized recombinant gp120, it is likely that N-linked sugars are present at most, if not all, of the consensus glycosylation sites of the heavily glycosylated gp120. In this study, we evaluated the relative importance of each of the 24 N-linked glycosylation sites of gp120 in the molecular clone HXB2 to viral infectivity. The ability of HXB2-derived mutants, each having 1 of the 24 N-linked glycosylation sites mutated by site-directed mutagenesis, to infect CD4-positive SupT1 cells was compared with that of the wild-type virus. We found that most of the individual consensus N-linked glycosylation sites are dispensable for viral infectivity. The five consensus N-linked glycosylation sites that are likely to have important roles in infectivity are all located in the amino-terminal half of gp120, indicating that the N-linked glycosylation sites that are important for infectivity of human immunodeficiency virus type 1 are not randomly distributed in gp120. We predict that a partially glycosylated gp120 with most of the dispensable N-linked glycosylation sites removed may be a better vaccine candidate than the fully glycosylated gp120.

Perinatal infection by human immunodeficiency virus type 1 (HIV-1): relationship between proviral copy number in vivo, viral properties in vitro, and clinical outcome

Human immunodeficiency virus type 1 (HIV-1) isolates from 25 perinatally HIV-1 infected children were classified according to their capacity to replicate in vitro as rapid (R), intermediate (S/R) and slow (S) variants. R-type viruses replicated on peripheral blood mononuclear cells (PBMCs) and grew better in T-lymphoid cells, even though 9 out of 12 isolates also maintained tropism for monocytoid cells. The S/R-type isolates replicated efficiently after several days of culture, while the S-type viruses displayed only a low and transient replication activity; however, both S/R- and S-type isolates exerted viral transactivation activity in an indicator monocytoid cell line. Replication patterns in vitro were significantly associated in vivo with the number of HIV-1 copies in PBMCs as determined by polymerase chain reaction: in children with R-type isolates, the number of HIV-1 proviral DNA molecules/10(5) PBMCs ranged from 62 to 571, and in children with S/R and S isolates the range was 5-43. Seven children had severe symptomatic HIV-1 infection, and in all an R-type virus was identified; 18 children had no or only mild symptoms, and among these, S-, S/R-, and R-type isolates were found in 5, 8, and 5 cases, respectively. Besides demonstrating HIV-1 variability in perinatal infection, these findings suggest that R-type virus might be a prerequisite for disease progression.

Demonstration of two distinct cytopathic effects with syncytium formation-defective human immunodeficiency virus type 1 mutants

The mechanism of human immunodeficiency virus type 1 (HIV-1) cytopathicity is poorly understood and might involve formation of multinucleated giant cells (syncytia), single-cell lysis, or both. In order to determine the contributions of the fusion domain to syncytium formation, single-cell lysis, and viral infectivity and to clarify the molecular details of these events, insertion mutations were made in the portion of env encoding this sequence in the functional HIV-1 proviral clone HXB2. Viruses produced from these mutant clones were found to have a partial (F3) or complete (F6) loss of syncytium-forming ability in acutely infected CEM, Sup T1, and MT4 T-cell lines. During the early stage of acute infection by F6 virus, there was a loss of the syncytial cytopathic effect, which resulted in increased cell viability, and a 1.9- to 2.6-fold increase in virus yield in the cell lines tested. In the late stage of acute infection, the single-cell cytopathic effect of F6 virus was similar to that of the parental HXB2 virus. The F3 and F6 viruses were also found to have a 1.7- to 43-fold reduction in infectivity compared with the HXB2 virus. The mutant F3 and F6 and parental HXB2 envelope proteins were expressed in vaccinia virus, and the mutant envelope proteins were observed to be defective in their ability to form syncytia. BSC-40 cells infected with vaccinia virus recombinants revealed no differences in kinetics of cleavage, cell surface expression, or CD4 binding capacity of the mutant and parental envelope proteins. These results demonstrate that a loss of syncytium formation results in an attenuation of infectivity and a loss of the syncytial cytopathic effect without a loss of single-cell lysis. These mutants may reflect in tissue culture the changes observed in the HIV isolates in vivo during disease progression, which exhibit marked differences in syncytium production.

Virological events leading to spontaneous AKR thymomas

The spontaneous leukemias of AKR mice are caused by mink cell focus-forming (MCF) viruses. These viruses are generated by recombination between several endogenous murine retroviruses. The virological events leading to the generation of the leukemogenic agent were investigated by using an oligonucleotide specific for the U3 region of the leukemogenic virus and env-reactive oligonucleotide probes specific for the different classes of endogenous murine leukemia virus. It was shown that (i) the leukemogenic MCF virus is formed by recombination between at least three different endogenous sequences; (ii) the U3 donor for the leukemogenic virus is the inducible xenotropic virus Bxv-1; (iii) all spontaneous tumors contain viruses with duplicated enhancer regions in their long terminal repeats; (iv) enhancer duplication is a somatic event, since Bxv-1 contains only one copy; (v) the first recombinant virus detectable in mass populations of thymocytes by Southern hybridization analysis contains all structural features of the ultimate leukemogenic virus; and (vi) the multiple novel viruses in a given tumor represent progeny of the same unique recombination events. On the basis of these results, an analysis of the virological events leading to AKR thymomas is presented.

CD4-independent, productive human immunodeficiency virus type 1 infection of hepatoma cell lines in vitro

Five hepatoma cell lines, including CZHC/8571, PLC/PRF/5, Hep3B, HepG2, and HUH7, were inoculated with three diverse isolates of human immunodeficiency virus type 1 (HIV-1). Productive infection was noted in all hepatoma cell lines, and expression of viral p24 antigen lasted for over 3 months, but its level decreased in proportion to the number of viable cells. HIV-1 antigens were also found in the cells by immunohistochemical staining and radioimmunoprecipitation assay, as were viral RNA by in situ hybridization and HIV-1-like particles by electron microscopy. Virus yield assays were also positive on supernatant fluids collected from hepatoma cultures inoculated with HIV-1. Despite their susceptibility to infection, all five hepatoma cell lines were negative for CD4 by immunofluorescence and for CD4 mRNA by slot-blot hybridization. In addition, HIV-1 infection of hepatoma cell lines was not blocked by anti-CD4 monoclonal antibody or soluble CD4. Together, these findings clearly demonstrate that all five hepatoma cell lines were susceptible to productive infection by HIV-1 in vitro via a CD4-independent mechanism.

Suppression of mouse mammary tumor proviral DNA and protooncogene expression: association with nutritional regulation of mammary tumor development

Chronic energy intake restriction (CEIR) reduces mouse mammary tumor virus (MMTV)-induced mammary tumors in C3H/Ou mice. Fewer than 10% of C3H/Ou mice developed mammary tumors during 88 wk of study when subjected to CEIR regardless of calorie source (fat vs. carbohydrate). By contrast, 100% of mice fed ad libitum diets relatively high in fat or carbohydrate or a commercial diet developed tumors by 35-40 wk. MMTV proviral DNA transcription was shown to be activated in spleen, liver, lung, kidney, small intestine, and mammary gland of mice consuming these diets ad libitum. By contrast, these messages were suppressed by CEIR in all tissues analyzed except spleen. MMTV proviral messages in liver and mammary gland increased with age in full-fed mice and were suppressed by CEIR. These findings suggest that the nutritional regulation of MMTV proviral DNA expression is tissue-specific. In CEIR mice the suppressed MMTV proviral DNA transcripts in mammary gland and liver increased with time in association with the delayed onset of mammary tumors. Mammary tumorigenesis in C3H mice is associated with integration of MMTV proviral DNA, which appears to activate a putative mammary tumor protooncogene, int-1. CEIR apparently decreases the frequency of viral reintegration adjacent to the int-1 gene and thus inhibits expression of int-1 and probably an initiation step in mammary tumorigenesis. Expression of other putative protooncogenes, int-2 and ras, in liver tissue was also reduced by CEIR. These findings indicate that both initiation and promotion of mammary tumorigenesis are influenced by CEIR in C3H/Ou mice.