Differentiation of human embryonal carcinoma cells induces human immunodeficiency virus permissiveness which is stimulated by human cytomegalovirus coinfection

A Novel cis Regulatory Element Regulates Human XIST in a CTCF-Dependent Manner

The long noncoding RNA XIST is the master regulator for the process of X chromosome inactivation (XCI) in mammalian females. Here, we report the existence of a hitherto-uncharacterized cis regulatory element (cRE) within the first exon of human XIST, which determines the transcriptional status of XIST during the initiation and maintenance phases of XCI. In the initiation phase, pluripotency factors bind to this cRE and keep XIST repressed. In the maintenance phase of XCI, the cRE is enriched for CTCF, which activates XIST transcription. By employing a CRISPR-dCas9-KRAB-based interference strategy, we demonstrate that binding of CTCF to the newly identified cRE is critical for regulating XIST in a YY1-dependent manner. Collectively, our study uncovers the combinatorial effect of multiple transcriptional regulators influencing XIST expression during the initiation and maintenance phases of XCI.

Variation in host susceptibility and infectiousness generated by co-infection: the myxoma-Trichostrongylus retortaeformis case in wild rabbits

One of the conditions that can affect host susceptibility and parasite transmission is the occurrence of concomitant infections. Parasites interact directly or indirectly within an individual host and often these interactions are modulated by the host immune response. We used a free-living rabbit population co-infected with the nematode Trichostrongylus retortaeformis, which appears to stimulate an acquired immune response, and the immunosuppressive poxvirus myxoma. Modelling was used to examine how myxoma infection alters the immune-mediated establishment and death/expulsion of T. retortaeformis, and consequently affects parasite intensity and duration of the infection. Simulations were based on the general T_H1–T_H2 immunological paradigm that proposes the polarization of the host immune response towards one of the two subsets of T helper cells. Our findings suggest that myxoma infections contribute to alter host susceptibility to the nematode, as co-infected rabbits showed higher worm intensity compared with virus negative hosts. Results also suggest that myxoma disrupts the ability of the host to clear T. retortaeformis as worm intensities were consistently high and remained high in old rabbits. However, the co-infection model has to include some immune-mediated nematode regulation to be consistent with field data, indicating that the T_H1–T_H2 dichotomy is not complete. We conclude that seasonal myxoma outbreaks enhance host susceptibility to the nematode and generate highly infected hosts that remain infectious for a longer time. Finally, the virus–nematode co-infection increases heterogeneities among individuals and potentially has a large effect on parasite transmission.

Allele-specific silencing of Alzheimer's disease genes: the amyloid precursor protein genes with Swedish or London mutations

Alzheimer's disease (AD) is the most common cause of dementia in humans. A pathological hallmark in the brain of an AD patient is extracellular amyloid plaques formed by accumulated beta-amyloid protein (Abeta), a metabolic product of amyloid precursor protein (APP). Studies have revealed a strong genetic linkage in the early-onset familial form (<60 years old) of AD. For example, some mutant APPs are transmitted dominantly and are segregated with inheritance of early onset AD. These mutants facilitate Abeta production. The "Swedish" mutations (APP(SW)) and the "London" mutation (APP(LON)) are examples of these mutants. Selective silencing of these mutant alleles holds therapeutic promise for AD. Here we show that the expression of the mutant APPs was selectively inhibited by RNA interference. The best selectivity was obtained when the mismatches were centrally placed in the antisense strand of small interfering RNAs. Introducing an additional mismatch in the antisense strand may improve the selectivity. The addition of a G at 5' end of the antisense strand may enhance the efficacy of gene silencing by RNA interference. Our results illustrate the guiding principles for selection of targeted sequences to achieve allele-specific silencing. The sequences that are effective to silence APP(SW) and APP(LON) as identified in this study may be useful in both in vivo and in vitro studies to investigate the pathophysiological role of APP(SW) and APP(LON) in AD development.

From teratocarcinomas to embryonic stem cells

The recent derivation of human embryonic stem (ES) cell lines, together with results suggesting an unexpected degree of plasticity in later, seemingly more restricted, stem cells (so-called adult stem cells), have combined to focus attention on new opportunities for regenerative medicine, as well as for understanding basic aspects of embryonic development and diseases such as cancer. Many of the ideas that are now discussed have a long history and much has been underpinned by the earlier studies of teratocarcinomas, and their embryonal carcinoma (EC) stem cells, which present a malignant surrogate for the normal stem cells of the early embryo. Nevertheless, although the potential of EC and ES cells to differentiate into a wide range of tissues is now well attested, little is understood of the key regulatory mechanisms that control their differentiation. Apart from the intrinsic biological interest in elucidating these mechanisms, a clear understanding of the molecular process involved will be essential if the clinical potential of these cells is to be realized. The recent observations of stem-cell plasticity suggest that perhaps our current concepts about the operation of cell regulatory pathways are inadequate, and that new approaches for analysing complex regulatory networks will be essential.

Lentiviral expression of HIV-1 Vpr induces apoptosis in human neurons

Our recent studies have demonstrated that extracellular, recombinant human immunodeficiency virus type I (HIV-1) Vpr protein is highly neurotoxic in the microenvironment of differentiated mature human neurons and undifferentiated neuronal precursors. Although most of the direct neurotoxic effects of HIV-1 have been attributed previously to the envelope gene product, gp120, and the Tat regulatory protein, it was demonstrated that Vpr protein caused apoptosis comparable to that induced by gp120 protein in a dose-dependent manner in the neuronal system. Having observed the neurocytopathic effects of extracellular Vpr protein previously, the effects of virally expressed Vpr on nondividing, terminally differentiated human neurons were investigated. An HIV-1-based three-plasmid expression vector system was utilized to study the effects of intracellularly expressed Vpr. These virion preparations were then used to transduce neurons generated from the human neuronal precursor NT2 cell-line. Intracellularly expressed Vpr induced apoptosis within terminally differentiated neurons, as demonstrated by TUNEL assays. Additionally, virions lacking Vpr expression did not significantly induce apoptosis within these neurons. These results suggest that HIV-1 Vpr may also be leading directly to selective neurotoxicity through intracellular expression. Furthermore, human apoptosis gene microarray comparisons exhibited an up-regulation of Bcl-2-related mRNA, as well as other apoptosis genes involved in the mitochondrial apoptotic pathway, for the Vpr-transduced neuronal cells, when compared to Vpr-negative controls. Thus, Vpr delivered intracellularly, as well as extracellularly, is involved in the induction of significant neuronal apoptosis and may be one of the molecular mechanisms in HIV-1-induced encephalopathy.

Anti-human immunodeficiency virus type 1 gene therapy in human central nervous system-based cells: an initial approach against a potential viral reservoir

Studies have demonstrated that human immunodeficiency virus type 1 (HIV-1) infection of central nervous system (CNS)-based cells in vivo results in a series of devastating clinical conditions collectively termed acquired immune deficiency syndrome (AIDS) dementia complex (ADC). Gene therapy for these neurovirological disorders necessitates utilization of a vector system that can mediate in vivo delivery and long-term expression of an antiretroviral transgene in nondividing/postmitotic CNS cellular elements. The present studies focus on the transfer of an anti-HIV-1 gene to primary isolated CNS microvascular endothelial cells (MVECs) and neuronal-based cells, for its effects in protecting these cells from HIV-1 infection. By using an HIV-1-based vector system, it was possible to efficiently transduce and maintain expression of a marker transgene, beta-galactosidase (beta-Gal), in human CNS MVECs, human fetal astrocytes, plus immature and mature (differentiated) NT2 cells. Significant transduction of the marker gene, beta-Gal, in CNS-based cells prompted the utilization of this system with an anti-HIV-1 gene therapeutic construct, RevM10, a trans-dominant negative mutant Rev protein. Initially, it was not possible to generate any HIV-1 vector particles with the RevM10 gene in the transducing construct, because of inhibitory effects on the HIV-1 vector by this gene product. However, the vector could be partially rescued by adding an additional construct that supplied wild-type rev, in trans, during a multiple construct transfection in the packaging 293T cells. Thus, it was possible to significantly improve the titer of RevM10-expressing viral particles generated from these cells. Moreover, this RevM10 vector transduced the neuronal precursor cell line NT2, retinoic acid-differentiated human neurons (hNT) from the precursor cells, and primary isolated human brain MVECs with high efficiency. RevM10 generated from the HIV-1-based vector system potently inhibited replication of diverse HIV-1 strains in human CNS MVECs and neuronal cells. The data generated from these studies represent an initial approach for future development of anti-HIV-1 gene therapy in the CNS.

Human immunodeficiency virus infection in vitro activates naturally integrated human papillomavirus type 18 and induces synthesis of the L1 capsid protein

Human papillomavirus (HPV) infections are prevalent in human immunodeficiency virus (HIV)-positive individuals. To highlight the effect of HIV on HPV expression, HPV-18-positive HIV-permissive HeLa-T4 cells were either infected with HIV-1 or treated with Tat or with the cytokines IL-1alpha, IL-1beta, IL-6 and TNF-alpha. The presence of HPV-18 E1 (early) and L1 (late) transcripts was then determined by dot-blot or Northern blot hybridization with E1 and L1 or with genomic HPV-18 DNA probes, respectively. Protein extracts from parallel cultures were challenged by Western blotting with an antiserum raised against an L1-beta-galactosidase hybrid protein. Results indicated that HeLa-T4 cells constitutively express E1 and L1 transcripts. When cells were infected with HIV, the amounts of E1 and L1 RNAs increased with time, followed by the de novo appearance of L1 protein. E1 and L1 transcripts were also increased, in a dose-dependent manner, by treatment of uninfected cultures with Tat or with IL-6, but were not affected by IL-1alpha, IL-1beta and TNF- alpha. Neither Tat nor IL-6 could induce L1 translation. These findings raise the hypothesis that the increase of HPV shedding and of HPV-associated diseases in HIV-infected individuals could be due in part to a direct or cytokine-mediated action of HIV, in addition to the HIV-induced immunodeficiency.

Neuron-enriched second trimester human cultures: growth factor response and in vivo graft survival

Grafts of first trimester fetal tissue show limited survival and integration in the adult CNS. Alternative grafting strategies have been sought for treatment of neurodegenerative disease. We have developed cultures of human second trimester fetal tissues to study neuronal differentiation. Grafted into the SCID mouse striatum, aggregates of these cultures formed neuron-rich xenografts for at least 8 months. We examined the influence of various neurotrophic factors, including basic fibroblast growth factor (bFGF), brain-derived neurotrophic factor (BDNF), transforming growth factor-beta 1 (TGF-beta1), and hepatocyte growth factor (HGF), on the growth and differentiation of neuronal and glial cell populations. BDNF promoted the survival and differentiation of second trimester neurons whereas bFGF exhibited a strong proliferative effect on precursors and the astroglial population. Our data suggest that second trimester human fetal cultures contain neuroprogenitor cells that can be directed to the neuronal lineage. This process may be amplified by treatment with BDNF, which we hypothesize could improve the long-term in vivo survival of neuron-enriched grafts.

Modulation of feline immunodeficiency virus infection by stromal cell-derived factor

The alpha-chemokine receptor CXCR4 has recently been shown to support syncytium formation mediated by strains of feline immunodeficiency virus (FIV) that have been selected for growth in the Crandell feline kidney cell line (CrFK-tropic virus). Given that both human and feline CXCR4 support syncytium formation mediated by FIV, we investigated whether human stromal cell-derived factor (SDF-1) would inhibit infection with FIV. Human SDF-1alpha and SDF-1beta bound with a high affinity (K(D)s of 12.0 and 10.4 nM, respectively) to human cells stably expressing feline CXCR4, and treatment of CrFK cells with human SDF-1alpha resulted in a dose-dependent inhibition of infection by FIV(PET). No inhibitory activity was detected when the interleukin-2 (IL-2)-dependent feline T-cell line Mya-1 was used in place of CrFK cells, suggesting the existence of a CXCR4-independent mechanism of infection. Furthermore, neither the human beta-chemokines RANTES, MIP-1alpha, MIP-1beta, and MCP-1 nor the alpha-chemokine IL-8 had an effect on infection of either CrFK or Mya-1 cells with CrFK-tropic virus. Envelope glycoprotein purified from CrFK-tropic virus competed specifically for binding of SDF-1alpha to feline CXCR4 and CXCR4 expression was reduced in FIV-infected cells, suggesting that the inhibitory activity of SDF-1alpha in CrFK cells may be the result of steric hindrance of the virus-receptor interaction following the interaction between SDF and CXCR4. Prolonged incubation of CrFK cells with SDF-1alpha led to an enhancement rather than an inhibition of infection. Flow cytometric analysis revealed that this effect may be due largely to up-regulation of CXCR4 expression by SDF-1alpha on CrFK cells, an effect mimicked by treatment of the cells with phorbol myristate acetate. The data suggest that infection of feline cells with FIV can be mediated by CXCR4 and that, depending on the assay conditions, infection can be either inhibited or enhanced by SDF-1alpha. Infection with FIV may therefore prove a valuable model in which to study the development of novel therapeutic interventions for the treatment of AIDS.

Teratocarcinomas and human embryology: pluripotent human EC cell lines. Review article

The histogenesis of germ cell tumours (GCT) reflects the normal processes of gametogenesis, fertilisation and subsequent embryonic cell differentiation. Understanding the mechanisms that control the differentiation of embryonal carcinoma (EC) cells into a variety of embryonic and extraembryonic cell types is pertinent to understanding the progression of GCT. EC cells also provide a tool for analysing the mechanisms that control differentiation during embryonic development, and particularly the mechanisms that control differentiation along alternative cell line, NTERA2, into neurones and other cell types in response to agents such as retinoic acid, HMBA and the bone morphogenetic proteins. We have also compared the pluripotent NTERA2 EC cells with other human EC cell lines that exhibit a much reduced capacity for cell differentiation. A variety of genes are activated during NTERA2 differentiation. In particular we have identified a novel human member of the wnt family. This wnt gene is activated following retinoic acid induction of differentiation but is later down-regulated as the cells mature into neurones and other cell types. We have also observed expression of genes belonging to the Frizzled family, which is likely to include genes encoding receptors for the wnt gene products. Thus in the NTERA2 system, genes pertinent to regulating cell differentiation during embryonic development are activated and appear to play a role in modulating how these pluripotent human EC cells differentiate.

Identification of a chemokine receptor encoded by human cytomegalovirus as a cofactor for HIV-1 entry

The human cytomegalovirus encodes a beta-chemokine receptor (US28) that is distantly related to the human chemokine receptors CCR5 and CXCR4, which also serve as cofactors for the entry into cells of human immunodeficiency virus-type 1 (HIV-1). Like CCR5, US28 allowed infection of CD4-positive human cell lines by primary isolates of HIV-1 and HIV-2, as well as fusion of these cell lines with cells expressing the viral envelope proteins. In addition, US28 mediated infection by cell line-adapted HIV-1 for which CXCR4 was an entry cofactor.

Differential Effects of Interleukin-13 on Cytomegalovirus and Human Immunodeficiency Virus Infection in Human Alveolar Macrophages

Alveolar macrophages, which form a principal line of defense against a variety of pulmonary pathogens, may themselves be infected by viruses like human immunodeficiency virus-1 (HIV-1), which impair their defensive functions. Interleukin-13 (IL-13), a multifunctional cytokine, has been considered for therapeutic use based on its potent inhibition of HIV-1 in these cells. We have further examined the effects of IL-13 on alveolar macrophages under conditions that reflect those seen in acquired immune deficiency syndrome, where this cell type is often infected by the opportunistic pathogen human cytomegalovirus (HCMV). Alveolar macrophages exposed to both HCMV and HIV-1 consistently exhibited higher levels of HIV-1 replication than cells exposed to HIV-1 alone. HIV-1 production was strongly suppressed in alveolar macrophages treated with IL-13 regardless of whether or not the cultures were coinfected with HCMV. However, IL-13 treatment markedly enhanced the expression of HCMV in otherwise latently infected macrophages in a dose dependent manner. These unexpected differential effects of IL-13 on host-virus interactions are important considerations in guiding its potential therapeutic applications.

CD4-independent association between HIV-1 gp120 and CXCR4: functional chemokine receptors are expressed in human neurons

BACKGROUND

Chemokines are a family of proteins that chemoattract and activate immune cells by interacting with specific receptors on the surface of their targets. We have shown previously that chemokine receptors including the interleukin-8 receptor B (CXCR2) and the Duffy blood group antigen are expressed on subsets of neurons in various regions of the adult nervous system.

RESULTS

Using a combination of immunohistochemical staining and receptor binding studies, we show that hNT cells, which are differentiated human neurons derived from the cell line NTera2, express functional chemokine receptors of the C-X-X and C-C types. These chemokine receptors include CXCR2, CXCR4, CCR1 and CCR5. We demonstrate high-affinity binding of both types of chemokines to hNT neurons and dose-dependent chemotactic responses to these chemokines in differentiated, but no t undifferentiated, NTera 2 cells. In addition, we show that the envelop glycoprotein from the T-cell-tropic human immunodeficiency virus 1 (HIV-1) strain IIIB is a CD4-independent, dose-dependent inhibitor of the binding of stromal cell-derived factor 1 to its receptor, CXCR4.

CONCLUSIONS

These data support recent findings that members of the chemokine family, including CCR5 and LESTR/Fusin (CXCR4), function as coreceptors in combination with CD4 for HIV-1 invasion. This is the first report of functional expression of chemokine receptors on human neurons. Furthermore, our studies provide for direct CD4-independent association of the viral envelope protein of the HIV-1 strain III with the chemokine receptor CXCR4.

Congenital cytomegalovirus infection in infants infected with human immunodeficiency virus type 1

OBJECTIVES

To determine the rate of in utero transmission of cytomegalovirus (CMV) in perinatally HIV-exposed infants and to determine whether coinfection with CMV in early life affects outcome.

METHODS

Infants born to HIV-infected women between March, 1988, and March, 1995, were evaluated (n = 206). Congenital or in utero CMV infection was defined as a positive CMV culture or shell vial assay on urine obtained in the first 3 weeks of life. HIV-infected infants either had positive serology beyond 18 months of age or, for an infant younger than 18 months, had a positive HIV PCR or HIV culture on at least two separate occasions.

RESULTS

There were 30 HIV-infected and 171 uninfected infants (144 who seroreverted and 27 infants with at least 2 negative HIV PCR or culture results and normal immunologic studies during the first 6 months of age). Urine culture for CMV was obtained during the first 3 weeks of life on 154 infants: 24 of 30 (80%) HIV-infected infants; and 130 of 171 (76%) HIV-uninfected infants. Overall 10 of 154 (6.5%) infants were infected with CMV: 5 of 24 (21%) HIV-infected and 5 of 130 (3.8%) HIV-uninfected infants. The rate of in utero CMV infection was significantly higher in HIV-infected infants (P = 0.008). Dually infected infants were more immunosuppressed than their CMV-negative counterparts. At 3 months of age the percentage of CD4+ T lymphocytes (P = 0.0021) and CD4:CD8 ratios (P = 0.0018) were significantly lower in the CMV-infected infants than in the CMV-uninfected infants. At 6 months of age the absolute CD4+ T lymphocyte counts (P = 0.0038), percentage of CD4+ T lymphocytes (P = 0.044) and CD4:CD8 ratios (P = 0.037) were significantly lower in the CMV-infected infants. The mean survival of HIV-infected infants who were coinfected with CMV in early life (5 in utero and 1 perinatally infected infant identified at 7 weeks) was 24.77 months. Kaplan-Meier survival analysis indicated a trend toward decreased survival in the infants who were coinfected with CMV in early life (P = 0.088).

CONCLUSIONS

Congenital CMV infection is more common in HIV-infected infants than in HIV-uninfected infants. Infection with CMV in early life is associated with greater immunosuppression and may be associated with a more rapid progression of HIV infection in infants.

AIDS Dementia and HIV-1-Induced Neurotoxicity: Possible Pathogenic Associations and Mechanisms

AIDS Dementia Complex (ADC) is a syndrome of cognitive, behavioral, and motor deficits resulting from HIV-1 infection within the brain. ADC is characterized by variable degrees of neuronal cell death and gliosis that likely result, at least, in part from release of metabolic products, cytokines, and viral proteins from infected macrophages, although a unifying explanation for the neurological dysfunction has yet to be established. Major unanswered questions include: (i) do neurologic symptoms result from neuronal cell death and/or dysfunction in surviving neurons?; (ii) are viral genomic sequences determinants of neurotoxicity?; (iii) is HIV infection of neurons and astrocytes relevant to pathogenesis?, and (iv) what circulating factors within the brain affect neuronal cell survival and function? This review addresses the association between HIV-1 replication within the brain, production of potential neurotoxins and possible mechanisms of induction of neurotoxicity and neuronal dysfunction contributing to the pathogenesis of ADC. Copyright 1996 S. Karger AG, Basel

Interactions between human immunodeficiency virus type 1 and human cytomegalovirus in human term syncytiotrophoblast cells coinfected with both viruses

Human cytomegalovirus (HCMV) and human immunodeficiency virus type 1 (HIV-1) may interact in the pathogenesis of AIDS. The placental syncytiotrophoblast layer serves as the first line of defense of the fetus against viruses. We analyzed the patterns of replication of HIV-1 and HCMV in singly an dually infected human term syncytiotrophoblast cells cultured in vitro. Syncytiotrophoblast cells exhibited restricted permissiveness for HIV-1, while HCMV replication was restricted at the level of immediate-early and early gene products in the singly infected cells. We found that the syncytiotrophoblasts as an overlapping cell population could be coinfected with HIV-1 and HCMV. HIV-1 replication was markedly upregulated by previous or simultaneous infection of the cells with HCMV, whereas prior HIV-1 infection of the cells converted HCMV infection from a nonpermissive to a permissive one. No simultaneous enhancement of HCMV and HIV-1 expression was observed in the dually infected cell cultures. Major immediate-early proteins of HCMV were necessary for enhancement of HIV-1 replication, and interleukin-6 production induced by HCMV and further increased by replicating HIV-1 synergized with these proteins to produce this effect. Permissive replication cycle of HCMV was induced by the HIV-1 tat gene product. We were unable to detect HIV-1 (HCMV) or HCMV (HIV-1) pseudotypes in supernatant fluids from dually infected cell cultures. Our results suggest that interactions between HIV-1 and HCMV in coinfected syncytiotrophoblast cells may contribute to the transplacental transmission of both viruses.

The effects of cytomegalovirus on human immunodeficiency virus replication in brain-derived cells correlate with permissiveness of the cells for each virus

Human cytomegalovirus (HCMV) is commonly found in the brains of patients with AIDS and in some cases can be detected in the same cells as can human immunodeficiency virus type 1 (HIV-1). In this study, we analyzed the patterns of replication of HIV-1 and HCMV in singly infected cells and the effects of dual infection in human brain-derived cell lines of three different origins: neuroblastoma cell lines SK-N-MC and SY5Y; astrocytoma/glioblastoma cell lines U373-MG and Hs 683; and undifferentiated glioblastoma cell lines A172 and T98G. To bypass the restriction at the adsorption/penetration step in these CD4-negative cells, we used HIV-1 (amphotropic retrovirus) pseudotypes. These HIV-1 pseudotypes infected the majority of the cells in the cultures and expressed high levels of HIV-1 gene products in all except the SY5Y cells. The cell lines differed in the ability to support HCMV infection, but coinfection with HIV-1 had no effect on HCMV replication. The A172 cells were completely nonpermissive for HCMV gene expression, while HCMV replication in the singly infected T98G and SK-N-MC cell lines was restricted at the level of some early gene products. This resulted in complete and partial inhibition, respectively, of viral DNA synthesis. Dual infection of the A172, T98G, and SK-N-MC cells had no effect on HIV-1 replication. The other three cell lines, U373-MG, Hs 683, and SY5Y, were fully permissive for HCMV replication. In the U373-MG and Hs 683 cells, HCMV markedly inhibited the synthesis of HIV-1 gene products. In contrast, a transient stimulation of HIV-1 production followed by a repression was observed in the dually infected SY5Y cells. We conclude from these results that under conditions in which both HIV-1 and HCMV can undergo fully permissive infection, HCMV can repress HIV-1 gene expression. In cells in which HCMV replication is limited but HIV-1 replicates well, there is no effect on HIV-1 gene expression. However, activation of HIV-1, at least transiently, may occur in cells in which HIV-1 gene expression is limited. These studies suggest that a threshold level of some HIV-1 gene product(s) may obscure activation or promote repression of HIV replication by HCMV.

Possible supplemental mechanisms in the pathogenesis of AIDS

Multiple and diverse mechanisms have been proposed as supplements to the HIV-1 virus in the destruction of CD4+ cells and the pathogenesis of AIDS. But it is now realized that 100 times more CD4+ cells are infected with HIV-1 than was originally thought to be the case, and many antigen-presenting cells are infected as well. In addition to the direct cytopathic effect of the virus, one or a few supplemental mechanisms may well suffice to explain the progressive loss of CD4+ cells, e.g., the considerable variation in the virus and/or the destruction of uninfected CD4+ cells by one immunological mechanism or another. However, it is not yet possible to state confidently which additional mechanism(s) is important. Identification of the nature of this supplemental process has become essential for successful, nonharmful intervention.

Differentiation-dependent human immunodeficiency virus long terminal repeat regulatory elements active in human teratocarcinoma cells

We have examined the transcriptional utilization of the human immunodeficiency virus type 1 (HIV-1) long terminal repeat (LTR) under differentiating conditions by using the embryonal carcinoma cell line NTERA-2. NTERA-2 cells undergo two distinct pathways of terminal differentiation, to a neuronal phenotype in response to retinoic acid and to a nonneuronal phenotype in response to hexamethylene bisacetamide. To identify LTR regulatory elements active in each cell type we used a set of HIV LTR linker substitution mutants, which contain mutations that progressively replace adjacent 18-bp segments across the U3 region and into the R region (between nucleotides -453 and +15 relative to the transcription start site). Although each differentiating cell type showed utilization of expected key elements (e.g., NF-kappa B, SP1, TATA) in the 3' portion of the LTR (+1 to -112), the data indicated differentiation-dependent differences in the utilization of these elements. In addition, regions showing dramatic differentiation-dependent effects were detected in the 5' portion of the LTR (-112 to -453), in positions where transcription control elements have not been described previously. The marked differences in the sets of LTR regulatory elements required by each cell type indicate that the LTR can function under a variety of differentiation conditions. Together with previous findings, the data suggest that the complexity of the HIV LTR for transcriptional control is much greater than was previously thought and that the LTR maintains elements which facilitate transcription in many cell types.

Isolation and characterization of an infectious molecular clone of the MN strain of HIV-1

Infectious molecular clones of the human immunodeficiency virus (HIV) have been very important tools for the analysis of regulatory gene functions and the study of differential cell tropism. We have cloned and characterized a proviral sequence of HIVmn from mn strain infected H9 cells. This clone, called KP1, was found to be infectious for different cell lines and human peripheral blood lymphocytes (PBL). KP1 proviral DNA was detected in HUT-78 cells and human PBL by polymerase chain reaction (PCR) analysis after infection of these cells with cell-free supernatants from KP1 transfected human rhabdomyosarcoma (RD) cells. To the best of our knowledge, this is the first report of an infectious molecular clone of HIVmn which is a representative of one of the most prevalent strains of HIV-1 in North America and Europe. Biologically active clones of a broadly antigenic strain such as HIVmn will be extremely useful in therapeutic approaches for AIDS.