Stable indicator cell lines exhibiting HIV-1 tat function

Influenza virus assays based on virus-inducible reporter cell lines

BACKGROUND

Virus-inducible reporter genes have been used as the basis of virus detection and quantitation assays for a number of viruses. A strategy for influenza A virus-induction of a reporter gene was recently described. In this report, we describe the extension of this strategy to influenza B virus, the generation of stable cell lines with influenza A and B virus-inducible reporter genes, and the use of these cells in various clinically relevant viral assays. Each of the cell lines described herein constitutively express an RNA transcript that contains a reporter gene coding region flanked by viral 5￠- and 3￠-untranslated regions (UTR) and therefore mimics an influenza virus genomic segment. Upon infection of the cells with influenza virus the virus-inducible reporter gene segment (VIRGS) is replicated and transcribed by the viral polymerase complex resulting in reporter gene expression.

FINDINGS

Reporter gene induction occurs after infection with a number of laboratory strains and clinical isolates of influenza virus including several H5N1 strains. The induction is dose-dependent and highly specific for influenza A or influenza B viruses.

CONCLUSIONS

These cell lines provide the basis of simple, rapid, and objective assays that involve virus quantitation such as determination of viral titer, assessment of antiviral susceptibility, and determination of antibody neutralization titer. These cell lines could be very useful for influenza virus researchers and vaccine manufacturers.

Gene expression analysis from human immunodeficiency virus type 1 subtype C promoter and construction of bicistronic reporter vectors

We report the cloning and sequence analysis of the long terminal repeat (LTR) of several primary HIV-1 subtype C strains of India. Phylogenetically, all the LTRs and the paired env sequences clustered with subtype C reference strains. The LTRs demonstrated extensive polymorphism in the transcription factor binding sites (TFBS) within the enhancer and the modulator regions. We generated reporter vectors under the control of a select subset of the subtype C LTRs. The reporter vectors are distinguished by the simultaneous expression of two independent reporter genes, secreted alkaline phosphatase (SEAP) and enhanced green fluorescence protein (EGFP), in response to Tat. Expression of EGFP was facilitated by engineering an internal ribosome entry site (IRES) into the expression cassette. Although subtype C strains cause a large majority of the global infections, and important differences in the transcription factor binding sites have been identified in the subtype C promoter, few reporter vectors containing subtype C-LTR have been described. We analyzed gene expression from the C-LTR reporter vectors in different cell lines under diverse experimental conditions and compared it to the B-LTR reporter vector. The reporter vectors were responsive to Tat derived from diverse viral subtypes. Furthermore, a positive correlation was observed between the expression of the reporter genes and the viral structural protein p24 when the cells were infected with viral molecular clones. The LTR reporters we developed could be of significant use in the study of viral transactivation, in the evaluation of biological properties of viral subtypes, and in the screening for antiviral inhibitors.

Synergy between human immunodeficiency virus type 1 and Epstein-Barr virus in T lymphoblastoid cell lines

CR2 (CD21), the EBV receptor, was detected on three of four CD4-positive cell lines by indirect fluorescent labeling, and its corresponding mRNA was found by use of the reverse transcription-based polymerase chain reaction. To determine whether CR2 on CD4-positive cells was functional, their ability to be infected by EBV was analyzed. EBV DNA, EBV nuclear antigen 2 (EBNA-2A), and EBV-encoded small RNA (EBER1) transcripts could be detected in CR2-expressing CD4-positive cells following infection by the B95.8 strain of EBV. Analysis of the terminal region showed the EBV genome remained linear following infection, and copy number decreased with time. Since CD4-positive cell lines are targets for HIV-1 infection, the effects of EBV infection on HIV-1 expression were analyzed. HIV-1 replication was upregulated when CD4-positive cells were coinfected with EBV strain B95.8 but not P3HR-1K. These results suggested that EBNA-2 is involved in upregulation of HIV-1 expression in T lymphoblastoid cell lines. To test this hypothesis an EBNA-2-expression vector was transfected into T lymphoblastoid cell lines and HIV-1 expression measured. First, trans-activation of HIV-1 long terminal repeat (LTR) by Tat was enhanced by EBNA-2 type 1 expression. trans-Activation of the HIV-1 LTR by Tat was also enhanced when CD4-positive cells were infected by EBV (strain B95.8) encoding an intact EBNA-2, but not by P3HR-1K with a deleted EBNA-2. In addition, CD4-positive cell clones stably expressing EBNA-2 supported enhanced HIV-1 replication as measured by accumulation of reverse transcriptase activity and syncytium induction. This provides direct evidence that EBV infection can enhance HIV-1 replication in T cells. Whether this in vitro phenomenon contributes to disease progression in vivo remains to be determined.

A human chromosome 12-associated 83-kilodalton cellular protein specifically binds to the loop region of human immunodeficiency virus type 1 trans-activation response element RNA

trans activation of human immunodeficiency virus type 1 (HIV-1) involves the viral trans-activator protein (Tat) and a cellular factor(s) encoded on human chromosome 12 (HuChr12) that targets the trans-activation response element (TAR) in the viral long terminal repeat. Because nascent TAR RNA is predicted to form a secondary structure that specifically binds cellular proteins, we investigated the composition of the TAR RNA-protein complex for HuChr12-specific proteins. UV cross-linking of TAR RNA-nuclear protein complexes formed in vitro identified an 83-kDa protein in human cells and in a human-hamster hybrid cell containing only HuChr12. The 83-kDa TAR RNA-binding protein was absent in the parental hamster cells. TAR RNA mutations that inhibited binding of the 83-kDa protein in vitro also inhibited HuChr12-dependent Tat trans activation. These TAR mutations changed the native sequence or secondary structure of the TAR loop. The TAR RNA binding activity of the 83-kDa protein also correlated with a HuChr12-dependent increase in steady-state HIV-1 RNA expression during Tat trans activation. Our results suggest that either a species-specific 83-kDa TAR RNA loop-binding protein is directly encoded on HuChr12 or a HuChr12 protein(s) induces the expression of an 83-kDa TAR-binding protein in nonprimate cells.

A quantitative assay for trans-activation by HIV-1 Tat, using liposome-mediated DNA uptake and a parallel ELISA system

A cellular assay is described in which transient high-level expression of a heterologous reporter gene (chloramphenicol acetyltransferase, CAT) driven by the HIV LTR is used to determine trans-activation in a cell line constitutively expressing Tat. The use of a parallel ELISA system to determine effects on expression of CAT and of the neomycin phosphotransferase (NPT) marker gene effectively eliminated sample variability caused by cumulative processing errors or cell culture conditions. In addition the use of cationic liposome-mediated transfection minimized delay between DNA treatment that initiates trans-activation and addition of inhibitors, thereby eliminating background expression levels in treated samples. The assay has the potential to discriminate between inhibition of trans-activation and nonspecific effects such as inhibition of transfection and cytotoxicity. It has been adapted to a 96-well format suitable for high-throughput screening of natural products and synthetic chemicals.

The HIV-1 tat protein enhances E2-dependent human papillomavirus 16 transcription

Epidemiologic studies show an association between infection with human immunodeficiency virus type 1 (HIV-1) and human papillomavirus (HPV) associated anogenital disease. To investigate possible molecular mechanisms of HIV-1 modulation of HPV expression, we studied the effect of an HIV-1 regulatory protein, tat1, on gene expression directed by the upstream regulatory region (URR) of HPV type 16 (HPV 16). HPV 16 URR-directed chloramphenicol acetyltransferase (CAT) expression driven by the native HPV 16 promoter (P97) was increased in the presence of tat1 alone. Tat1 also reversed E2-mediated repression of P97-directed CAT expression. E2 mediated CAT expression with URR constructs containing the SV40 promoter was enhanced when tat1 and E2 were cotransfected. Using a cervical carcinoma cell line (SiHa), E2 enhancement of URR-directed gene expression was elevated in the presence of extracellular tat1 or during cocultivation with HIV-1-infected cells. These results show HIV can modulate HPV gene expression in cell culture and that the increased rate of HPV-associated cervical disease in asymptomatic HIV-seropositive women may result from HPV-HIV molecular interactions.

Human chromosome-dependent and -independent pathways for HIV-2 trans-activation

Human immunodeficiency virus (HIV types 1 and 2) replication is controlled by the interaction of viral-encoded regulatory proteins and host cellular proteins with the viral long terminal repeat (LTR). The presence of HIV-1 and HIV-2 trans-activator proteins, tat1 and tat2, respectively, greatly increases viral gene expression from their homologous LTRs. It is unclear if the cellular factors that support tat1-directed trans-activation of the HIV-1 LTR are the same for tat2 trans-activation of the HIV-2 LTR. Human-Chinese hamster ovary hybrid cell clones were used to probe for human chromosomes involved in regulating HIV-1 and HIV-2 tat-directed transactivation. DNA transfection experiments showed that the presence of human chromosome 12 in human-hamster hybrid clones was necessary for high-level tat-directed trans-activation of the HIV-1 and -2 LTR. Cross-trans-activation of the HIV-2 LTR by tat1 was found to be chromosome 12 independent. In addition, chromosome 12 did not support trans-activation of another human retrovirus (human T-cell leukemia virus type I). Our results suggest that HIV-1 and -2 have evolved to employ a cellular pathway(s) encoded on human chromosome 12 for supporting homologous tat-directed trans-activation. Trans-activation of the HIV-2 LTR by tat1 in chromosome 12-minus cells suggests that multiple cellular pathways can be recruited to trans-activate the HIV-2 LTR and that these pathways may have been important in an HIV-like progenitor virus.

Development of RD-tat cell lines: use in HIV recombination studies

The transactivator (tat) gene of human immunodeficiency virus (HIV) plays an essential role in the replication cycle of HIV. Previous studies have evaluated the extent and mechanistic aspects of tat-mediated transactivation using lymphoid and adherent non-lymphoid cells. We have exploited the transactivation property of the tat gene to achieve high levels of hybrid HIV resulting from recombination between HIV DNAs. For this purpose, we have generated stably transformed human rhabdomyosarcoma (RD) cell lines expressing tat gene product of HIV-1. Functional analysis of the cell lines for the presence of tat protein by transfecting HIV-long terminal repeat (LTR) linked to chloramphenicol acetyl transferase (CAT) revealed low, moderate, and high tat producer cell lines. RD-tat cell lines also showed enhanced virus production upon transfection of HIV-1 proviral DNA. Further, tat producer cell lines showed a high amount of hybrid virus in comparison to the control RD cells upon transfection of truncated viral DNAs. Thus, RD-tat cell lines would be valuable target cells for generating homogeneous viruses upon transfection of viral DNA.

Human chromosome 12 is required for elevated HIV-1 expression in human-hamster hybrid cells

Host cell factors act together with regulatory genes of the human immunodeficiency virus (HIV) to control virus production. Human-Chinese hamster ovary hybrid cell clones were used to probe for human chromosomes involved in regulating HIV gene expression. DNA transfection experiments showed that 4 of 18 clones had high levels of HIV gene expression measured by both extracellular virus production and transactivation of the HIV long terminal repeat in the presence of the trans-activator (tat) gene. Karyotype analyses revealed a 94% concordance (17/18) between human chromosome 12 and HIV gene expression. Other chromosomes had an 11 to 72% concordance with virus production.