HIV-1 envelope glycoproteins induce activation of activated protein-1 in CD4+ T cells

The eMERGE genotype set of 83,717 subjects imputed to ~40 million variants genome wide and association with the herpes zoster medical record phenotype

The Electronic Medical Records and Genomics (eMERGE) network is a network of medical centers with electronic medical records linked to existing biorepository samples for genomic discovery and genomic medicine research. The network sought to unify the genetic results from 78 Illumina and Affymetrix genotype array batches from 12 contributing medical centers for joint association analysis of 83,717 human participants. In this report, we describe the imputation of eMERGE results and methods to create the unified imputed merged set of genome‐wide variant genotype data. We imputed the data using the Michigan Imputation Server, which provides a missing single‐nucleotide variant genotype imputation service using the minimac3 imputation algorithm with the Haplotype Reference Consortium genotype reference set. We describe the quality control and filtering steps used in the generation of this data set and suggest generalizable quality thresholds for imputation and phenotype association studies. To test the merged imputed genotype set, we replicated a previously reported chromosome 6 HLA‐B herpes zoster (shingles) association and discovered a novel zoster‐associated loci in an epigenetic binding site near the terminus of chromosome 3 (3p29).

Global phosphoproteomics of CCR5-tropic HIV-1 signaling reveals reprogramming of cellular protein production pathways and identifies p70-S6K1 and MK2 as HIV-responsive kinases required for optimal infection of CD4+ T cells

BACKGROUND

Viral reprogramming of host cells enhances replication and is initiated by viral interaction with the cell surface. Upon human immunodeficiency virus (HIV) binding to CD4+ T cells, a signal transduction cascade is initiated that reorganizes the actin cytoskeleton, activates transcription factors, and alters mRNA splicing pathways.

METHODS

We used a quantitative mass spectrometry-based phosphoproteomic approach to investigate signal transduction cascades initiated by CCR5-tropic HIV, which accounts for virtually all transmitted viruses and the vast majority of viruses worldwide.

RESULTS

CCR5-HIV signaling induced significant reprogramming of the actin cytoskeleton and mRNA splicing pathways, as previously described. In addition, CCR5-HIV signaling induced profound changes to the mRNA transcription, processing, translation, and post-translational modifications pathways, indicating that virtually every stage of protein production is affected. Furthermore, we identified two kinases regulated by CCR5-HIV signaling-p70-S6K1 (RPS6KB1) and MK2 (MAPKAPK2)-that were also required for optimal HIV infection of CD4+ T cells. These kinases regulate protein translation and cytoskeletal architecture, respectively, reinforcing the importance of these pathways in viral replication. Additionally, we found that blockade of CCR5 signaling by maraviroc had relatively modest effects on CCR5-HIV signaling, in agreement with reports that signaling by CCR5 is dispensable for HIV infection but in contrast to the critical effects of CXCR4 on cortical actin reorganization.

CONCLUSIONS

These results demonstrate that CCR5-tropic HIV induces significant reprogramming of host CD4+ T cell protein production pathways and identifies two novel kinases induced upon viral binding to the cell surface that are critical for HIV replication in host cells.

HIV glycoprotein gp120 enhances mesenchymal stem cell migration by upregulating CXCR4 expression

BACKGROUND

HIV infection and/or the direct pathogenic effects of circulating HIV proteins impairs the physiological function of mesenchymal stem cells (MSCs), and contribute to the pathogenesis of age-related clinical comorbidities in people living with HIV. The SDF-1/CXCR4 pathway is vital for modulating MSC proliferation, migration and differentiation. HIV glycoprotein gp120 inhibits SDF-1 induced chemotaxis by downregulating the expression and function of CXCR4 in monocytes, B and T cells. The influence of gp120 on CXCR4 expression and migration in MSCs is unknown.

METHODS

We investigated CXCR4 expression and SDF-1/CXCR4-mediated MSC migration in response to gp120, and its effect on downstream signaling pathways: focal adhesion kinase (FAK)/Paxillin and extracellular signal-regulated kinase (ERK).

RESULTS

Gp120 upregulated MSC CXCR4 expression. This potentiated the effects of SDF-1 in inducing chemotaxis; FAK/Paxillin and ERK pathways were over-activated, thereby facilitating actin stress fiber reorganization. CXCR4 blockage or depletion abrogated the observed effects.

CONCLUSION

Gp120 from both T- and M- tropic HIV strains upregulated CXCR4 expression in MSCs, resulting in enhanced MSC chemotaxis in response to SDF-1.

GENERAL SIGNIFICANCE

HIV infection and its proteins are known to disrupt physiological differentiation of MSC; increased gp120-driven migration amplifies the total MSC population destined for ineffective and inappropriate differentiation, thus contributing to the pathogenesis of HIV-related comorbidities. Additionally, given that MSCs are permissive to HIV infection, initial cellular priming by gp120 results in increased expression of CXCR4 and could lead to co-receptor switching and cell tropism changes in chronic HIV infection and may have implications against CCR5-knockout based HIV cure strategies.

The orientation of HIV-1 gp120 binding to the CD4 receptor differentially modulates CD4+ T cell activation

Progressive quantitative and qualitative decline of CD4(+) T cell responses is one hallmark of HIV-1 infection and likely depends on several factors, including a possible contribution by the HIV-1 envelope glycoprotein gp120, which binds with high affinity to the CD4 receptor. Besides virion-associated and cell-expressed gp120, considerable amounts of soluble gp120 are found in plasma or lymphoid tissue, predominantly in the form of gp120-anti-gp120 immune complexes (ICs). Because the functional consequences of gp120 binding to CD4(+) T cells are controversially discussed, we investigated how gp120 affects TCR-mediated activation of human CD4(+) T cells by agonistic anti-CD3 mAb or by HLA class II-presented peptide Ags. We show that the spatial orientation of gp120-CD4 receptor binding relative to the site of TCR engagement differentially affects TCR signaling efficiency and hence CD4(+) T cell activation. Whereas spatially and temporally linked CD4 and TCR triggering at a defined site promotes CD4(+) T cell activation by exceeding local thresholds for signaling propagation, CD4 receptor engagement by gp120-containing ICs all around the CD4(+) T cell undermine its capacity in supporting proximal TCR signaling. In vitro, gp120 ICs are efficiently captured by CD4(+) T cells and thereby render them hyporesponsive to TCR stimulation. Consistent with these in vitro results we show that CD4(+) T cells isolated from HIV(+) individuals are covered with ICs, which at least partially contain gp120, and suggest that IC binding to CD4 receptors might contribute to the progressive decline of CD4(+) T cell function during HIV-1 infection.

Quantitative phosphoproteomics reveals extensive cellular reprogramming during HIV-1 entry

Receptor engagement by HIV-1 during host cell entry activates signaling pathways that can reprogram the cell for optimal viral replication. To obtain a global view of the signaling events induced during HIV-1 entry, we conducted a quantitative phosphoproteomics screen of primary human CD4(+) T cells after infection with an HIV-1 strain that engages the receptors CD4 and CXCR4. We quantified 1,757 phosphorylation sites with high stringency. The abundance of 239 phosphorylation sites from 175 genes, including several proteins in pathways known to be impacted by HIV-receptor binding, changed significantly within a minute after HIV-1 exposure. Several previously uncharacterized HIV-1 host factors were also identified and confirmed through RNAi depletion studies. Surprisingly, five serine/arginine-rich (SR) proteins involved in messenger RNA splicing, including the splicing factor SRm300 (SRRM2), were differentially phosophorylated. Mechanistic studies with SRRM2 suggest that HIV-1 modulates host cell alternative splicing machinery during entry in order to facilitate virus replication and release.

Pertussis toxin B-oligomer suppresses IL-6 induced HIV-1 and chemokine expression in chronically infected U1 cells via inhibition of activator protein 1

Pertussis toxin B-oligomer (PTX-B) inhibits HIV replication in T lymphocytes and monocyte-derived macrophages by interfering with multiple steps of the HIV life cycle. PTX-B prevents CCR5-dependent (R5) virus entry in a noncompetitive manner, and it also exerts suppressive effects on both R5- and CXCR4-dependent HIV expression at a less-characterized postentry level. We demonstrate in this study that PTX-B profoundly inhibits HIV expression in chronically infected promonocytic U1 cells stimulated with several cytokines and, particularly, the IL-6-mediated effect, a cytokine that triggers viral production in these cells independently of NF-kappaB activation. From U1 cells we have subcloned a cell line, named U1-CR1, with increased responsiveness to IL-6. In these cells, PTX-B neither down-regulated the IL-6R nor prevented IL-6 induced signaling in terms of STAT3 phosphorylation and DNA binding. In contrast, PTX-B inhibited AP-1 binding to target DNA and modified its composition with a proportional increases in FosB, Fra2, and ATF2. PTX-B inhibited IL-6-induced HIV-1 long-terminal repeat-driven transcription from A, C, E, and F viral subtypes, which contain functional AP-1 binding sites, but failed to inhibit transcription from subtypes B and D LTR devoid of these sites. In addition, PTX-B inhibited the secretion of IL-6-induced, AP-1-dependent genes, including urokinase-type plasminogen activator, CXCL8/IL-8, and CCL2/monocyte chemotactic protein-1. Thus, PTX-B suppression of IL-6 induced expression of HIV and cellular genes in chronically infected promonocytic cells is strongly correlated to inhibition of AP-1.

Cellular gene expression upon human immunodeficiency virus type 1 infection of CD4(+)-T-cell lines

The expression levels of approximately 4,600 cellular RNA transcripts were assessed in CD4(+)-T-cell lines at different times after infection with human immunodeficiency virus type 1 strain BRU (HIV-1(BRU)) using DNA microarrays. We found that several classes of genes were inhibited by HIV-1(BRU) infection, consistent with the G(2) arrest of HIV-1-infected cells induced by Vpr. These included genes involved in cell division and transcription, a family of DEAD-box proteins (RNA helicases), and all genes involved in translation and splicing. However, the overall level of cell activation and signaling was increased in infected cells, consistent with strong virus production. These included a subgroup of transcription factors, including EGR1 and JUN, suggesting they play a specific role in the HIV-1 life cycle. Some regulatory changes were cell line specific; however, the majority, including enzymes involved in cholesterol biosynthesis, of changes were regulated in most infected cell lines. Compendium analysis comparing gene expression profiles of our HIV-1 infection experiments to those of cells exposed to heat shock, interferon, or influenza A virus indicated that HIV-1 infection largely induced specific changes rather than simply activating stress response or cytokine response pathways. Thus, microarray analysis confirmed several known HIV-1 host cell interactions and permitted identification of specific cellular pathways not previously implicated in HIV-1 infection. Continuing analyses are expected to suggest strategies for impacting HIV-1 replication in vivo by targeting these pathways.

Inhibition of HIV-1-mediated syncytium formation and virus replication by the lipophosphoglycan from Leishmania donovani is due to an effect on early events in the virus life cycle

Previous findings have indicated that the major surface molecule of Leishmania, lipophosphoglycan (LPG), could abrogate HIV-1-induced syncytium formation and virus replication. In the present work, we were interested in characterizing this inhibitory process. Data from a new luciferase-based semiquantitative assay for syncytium formation, relying on the coincubation of a T-cell line containing an HIV-1 LTR-driven luciferase construct with a cell line chronically infected with HIV-1, confirmed that LPG was indeed a strong inhibitor of HIV-1-dependent syncytium formation and that this inhibition was dose-dependent. As determined by flow cytometric analyses, this inhibition was not apparently due to downregulation of CD4, CXCR4 or LFA-1, three distinct surface glycoproteins known to be important in HIV-1 mediated syncytium formation. Furthermore, LPG did not seem to affect signal transduction pathways in T cells as judged by measurement of HIV-1 LTR-driven reporter gene activity upon treatment with different stimuli. However, pretreatment of either of the cell lines used in the assay with LPG led to a significant decrease of virus-mediated syncytium formation, which was further accentuated when both cell lines were pretreated. LPG inhibition of HIV-1 replication was next assessed. When measuring either infection with luciferase-encoding recombinant HIV-1 particles or multinucleated giant cell formation following an acute virus infection, we again observed that LPG was efficient at blocking HIV-1 replication. Specific assays probing different steps of viral entry demonstrated that attachment was not hindered by LPG but that viral entry was modulated, suggesting that LPG targets a postbinding step. Hence, incorporation of LPG into a target cell membrane could influence its fluidity and diminish both the virus-cell and cell-to-cell fusion processes initiated by HIV-1.

Attachment of human immunodeficiency virus-1 (HIV-1) particles bearing host-encoded B7-2 proteins leads to nuclear factor-kappa B- and nuclear factor of activated T cells-dependent activation of HIV-1 long terminal repeat transcription

Previous studies have shown that human immunodeficiency virus type-1 (HIV-1) can incorporate several surface proteins of host origin. Recent findings indicate that host-encoded cell surface constituents retain their functionality when found embedded into the viral envelope. The primary objective of the current study was to define whether interaction between some specific virion-bound host proteins with their natural cognate ligands present on target cells could mediate intracellular signaling cascade(s). For this purpose, we have generated a whole series of isogenic virus stocks (NL4-3 backbone) bearing or not bearing on their surface foreign CD28, CD54 (ICAM-1), CD80 (B7-1) or CD86 (B7-2) proteins. Our results indicate that incubation of human T lymphoid cells with virions bearing host-derived B7-2 proteins and anti-CD3 antibody can potently activate HIV-1 long terminal repeat-driven gene expression. This up-regulating effect necessitates the involvement of nuclear factor-kappa B (NF-kappa B) and nuclear factor of activated T cells (NFAT) as revealed by the use of vectors coding for dominant negative versions of both transcription factors (i.e. I kappa B alpha S32A/36A and dnNFAT) and band shift assays. The increase of NF-kappa B activity was abolished when infection with B7-2-bearing HIV-1 particles was performed in the presence of the fusion protein CTLA-4 Ig suggesting that the interaction between virally embedded B7-2 and CD28 on the target cell is responsible for the observed NF-kappa B induction. The findings presented here provide the first demonstration that host-encoded proteins acquired by HIV-1 can mediate signal transduction events.

Molecular pathways in virus-induced cytokine production

Virus infections induce a proinflammatory response including expression of cytokines and chemokines. The subsequent leukocyte recruitment and antiviral effector functions contribute to the first line of defense against viruses. The molecular virus-cell interactions initiating these events have been studied intensively, and it appears that viral surface glycoproteins, double-stranded RNA, and intracellular viral proteins all have the capacity to activate signal transduction pathways leading to the expression of cytokines and chemokines. The signaling pathways activated by viral infections include the major proinflammatory pathways, with the transcription factor NF-kappaB having received special attention. These transcription factors in turn promote the expression of specific inducible host proteins and participate in the expression of some viral genes. Here we review the current knowledge of virus-induced signal transduction by seven human pathogenic viruses and the most widely used experimental models for viral infections. The molecular mechanisms of virus-induced expression of cytokines and chemokines is also analyzed.

Role of 14-3-3epsilon, c-Myc/Max, and Akt phosphorylation in HIV-1 gp 120-induced mesangial cell proliferation

Focal glomerulosclerosis (FGS) is the predominant glomerular lesion in patients with human immunodeficiency virus (HIV)-associated nephropathy. Initial mesangial cell hyperplasia and subsequent hypoplasia are common features of FGS. In the present study we evaluated the effect of HIV-1 glycoprotein (gp) 120 on human mesangial cell (HMC) growth. HIV-1 gp 120 stimulated HMC proliferation at lower concentrations, whereas it suppressed cell proliferation at higher concentrations. In parallel to the modulation of cell growth, gp 120 at low concentrations resulted in an increase in the expression of c-Myc, Max, and 14-3-3epsilon proteins and phosphorylation of ATP-dependent tyrosine kinases (Akt) at Ser(473). However, the expression of these proteins decreased with increasing concentrations of gp 120. Furthermore, gp 120 also exhibited a dose-dependent inhibition of Akt phosphorylation at Ser-473 without any significant alteration of Akt expression. Little or no effects of gp 120 were observed on the expression of extracellular signal-regulated kinase (ERK), phospho-ERK, Bcl-2, and Bax proteins. At a higher concentration, gp 120 not only promoted HMC apoptosis but also enhanced expression of Fas and FasL. These results suggest that HIV-1 gp 120 induces alterations in conflicting survival signaling pathways that contribute to the potential dual effects of gp 120 in promoting or inhibiting HMC proliferation.

Markers of Lymphocyte Homing Distinguish CD4 T Cell Subsets That Turn Over in Response to HIV-1 Infection in Humans

In HIV-1 infection, the abrupt rise in CD4 T cells after effective antiretroviral therapy has been viewed as a measure of HIV-1-related CD4 T cell turnover in the steady state. The early (2–4 wk) response is reportedly dominated by CD4 T cells with a memory (CD45RO) phenotype. It is controversial whether the measurement of steady-state kinetics identifies cells that otherwise would have been recruited into a short-lived, virus-producing pool or reflects lymphoid redistribution/sequestration. We performed detailed phenotypic and kinetic analysis of CD4 T cell subsets in 14 patients. Turnover occurs in memory (CD45RO) as well as naive (CD45RA) cells, if the latter are present at baseline. Most of the turnover occurs in those memory (CD45RO) and naive (CD45RA) cells that are programmed for recirculation through lymphoid organs (CD62L+ and CD44low), whereas very little turnover occurs in memory cells (CD45RO) destined for recirculation from blood to tissue (CD62L− and CD44high). Turnover occurs in both activated (CD25+ and HLA-DR+) and nonactivated populations, although it is restricted to CD38-positive cells, indicating that turnover does not measure cells that are already infected. More likely, turnover occurs in cells that replace infected cells or are on their way to becoming infected. Taken together, markers of lymphocyte trafficking better describe cell turnover related to virus replication than do naive and memory markers per se, and lymph organs, not tissue-destined cells or peripheral blood cells, appear to be the important site of virus replication and CD4 T cell turnover, destruction, and redistribution.

Obligatory cross-talk with the tyrosine kinases assembled with the TCR/CD3 complex in CD4 signal transduction

Dissection of the CD4 signal transduction pathway has revealed striking similarities with the TCR/CD3 pathway. Furthermore, downstream signaling by CD4 is impaired in cells lacking surface TCR, suggesting a role for the TCR/CD3 complex in CD4 signal transduction. We have investigated the molecular basis for the dependence of CD4 signaling on TCR/CD3 expression. Using the phosphotyrosine binding domains of the Shc adaptor and the Fyn kinase, which both participate in CD4 signaling, as baits, we show that CD4 induces tyrosine phosphorylation of a subset of the proteins phosphorylated in response to TCR/CD3 engagement. The phosphoprotein patterns were dramatically altered in cells defective for TCR/CD3 expression, and were recoverable by reconstitution of correctly assembled TCR, suggesting that CD4 uses TCR/CD3-associated tyrosine kinases to signal. Among the tyrosine kinases associated with the resting TCR/CD3 complex, only Fyn is activated following CD4 engagement. The failure of Fyn to become phosphorylated in cells defective for TCR expression underlines the unique role of TCR/CD3 associated Fyn in CD4 signal transduction. While no calcium mobilization was measurable in cells defective for TCR/CD3 expression in response to CD4 engagement, the Ras/MAP kinase pathway could be partially activated. Thus, CD4 activates at least two signaling pathways, and tyrosine kinases associated with the TCR/CD3 complex are key components of one of these pathways.

HIV-1 reactivation in resting peripheral blood mononuclear cells of infected adults upon in vitro CD4 cross-linking by ligands of the CDR2-loop in extracellular domain 1

HIV-1 infects resting peripheral blood mononuclear cells (PBMCs) but remains inactive state until subsequent cell activation. We have demonstrated that the cross-linking of cell surface CD4 by gp120-anti-gp120 immune complexes or heat-inactivated HIV-1 (iHIV-1) is sufficient to trigger activation signals leading to virus reactivation (9). In this study, we demonstrate that NF-kappaB nuclear translocation and stimulation of virus production by iHIV-1 were strictly linked to the concentrations of viral proteins used as exogenous stimuli. Moreover, we further investigated the physiologic relevance of these observations. When submitted to an in vitro CD4 cross-linking by iHIV-1, PBMCs from HIV-1-infected patients were found to produce virus. This viral reactivation was associated with increased NF-kappaB nuclear translocation in patients' PBMCs. Additionally, virus reactivation in resting PBMCs infected in vitro with HIV-1 was found to be specifically induced by ligands of the CDR2-loop in domain 1 (D1) of CD4 (virus envelope and anti-CD4 monoclonal antibodies). In contrast, virus reactivation was not observed following CD4 oligomerization by antibodies that bind other epitopes in D1, including the D1/CDR3-loop. Finally, soluble CD4 (sCD4) prevented virus reactivation by D1/CDR2-loop ligands. Our results indicate that the signaling events initiated in PBMCs by oligomerization of CD4 at the D1/CDR2-loop can trigger HIV-1 upregulation in infected individuals.

IL-16 as an Anti-Inflammatory Cytokine in Rheumatoid Synovitis

T lymphocytes are a major component of the inflammatory infiltrate in rheumatoid synovitis, but their exact role in the disease process is not understood. Functional activities of synovial T cells were examined by adoptive transfer experiments in human synovium-SCID mouse chimeras. Adoptive transfer of tissue-derived autologous CD8+ T cells induced a marked reduction in the activity of lesional T cells and macrophages. Injection of CD8+, but not CD4+, T cells decreased the production of tissue IFN-γ, IL-1β, and TNF-α by >90%. The down-regulatory effect of adoptively transferred CD8+ T cells was not associated with depletion of synovial CD3+ T cells or synovial CD68+ macrophages, and it could be blocked by Abs against IL-16, a CD8+ T cell-derived cytokine. In the synovial tissue, CD8+ T cells were the major source of IL-16, a natural ligand of the CD4 molecule that can anergize CD4-expressing cells. The anti-inflammatory activity of IL-16 in rheumatoid synovitis was confirmed by treating synovium-SCID mouse chimeras with IL-16. Therapy for 14 days with recombinant human IL-16 significantly inhibited the production of IFN-γ, IL-1β, and TNF-α in the synovium. We propose that tissue-infiltrating CD8+ T cells in rheumatoid synovitis have anti-inflammatory activity that is at least partially mediated by the release of IL-16. Spontaneous production of IL-16 in synovial lesions impairs the functional activity of CD4+ T cells but is insufficient to completely abrogate their stimulation. Supplemental therapy with IL-16 may be a novel and effective treatment for rheumatoid arthritis.

Inhibitory effect of interleukin-16 on interleukin-2 production by CD4+ T cells

Signalling through CD4 by human immunodeficiency virus (HIV)-1 envelope glycoprotein (gpl20) and/or anti-CD4 antibodies can promote T-cell activation and anergy. Interleukin (IL)-16 is a competence growth factor for CD4+ T cells that can induce a G0 to G1 cell cycle transition but cannot induce cell division. The receptor of this cytokine is thought to be the CD4 molecule, although the binding epitope of IL-16 differs from that of HIV. We have demonstrated that both HIV-1/gp120 and IL-16 induced CD4+ T-cell dysfunction, as indicated by suppression of mitogen-induced IL-2 production. Two anti-CD4 antibodies with different binding sites on CD4 also showed an inhibitory effect on IL-2 production. These results indicate that promotion of CD4+ T-cell anergy via the CD4 molecule does not depend on the binding sites of the CD4 ligands.

An anti-CD4 (CDR3-loop) monoclonal antibody inhibits human immunodeficiency virus type 1 envelope glycoprotein-induced apoptosis

Inhibition of human immunodeficiency virus type 1 (HIV-1)-inducing programmed cell death (PCD) by anti-CD4 monoclonal antibodies (mAbs) was investigated using DNA intercalant YOPRO-1 assay. We found that 13B8.2, an mAb that binds the CDR3-like loop in domain 1 (D1) of CD4, protected infected CEM cell cultures against HIV-1-induced PCD. Protection was not observed using another anti-CD4 mAb (BL4) that binds D1-D2, suggesting that the mechanism involved in cell protection against HIV-1-induced PCD requires engagement of precise CD4 epitopes. Because 13B8.2 is known to inhibit syncytia formation and virus transcription, this mAb could inhibit HIV-1-induced PCD by (1) inhibiting virus gene expression, (2) preventing viral envelope-CD4 interaction, and/or (3) interfering with apoptotic signals. Our data indicated that the absence of enhanced PCD in infected cell cultures treated with 13B8.2 mAb probably was the result of inhibition of HIV-1 replication and virus spread. Moreover, 13B8.2 mAb was found to inhibit PCD mediated by membrane-expressed HIV-1 envelope glycoproteins. Finally, we found that 13B8.2 mAb displayed no protective interference with apoptotic signal induced by Fas, dexamethasone, and serum withdrawal.

Binding of human immunodeficiency virus type 1 to CD4 and CXCR4 receptors differentially regulates expression of inflammatory genes and activates the MEK/ERK signaling pathway

We have previously shown that binding of human immunodeficiency virus type 1 (HIV-1) virions to CD4 receptors stimulates association of Lck with Raf-1 and results in the activation of Raf-1 kinase in a Ras-independent manner. In the present study, we demonstrate that HIV-1 envelope glycoproteins of both T-cell-tropic and macrophagetropic strains rapidly activate the ERK/mitogen-activated protein (MAP) kinase pathway and the binding of nuclear transcription factors (AP-1, NF-kappaB, and C/EBP) and stimulate expression of cytokine and chemokine genes. The activation of this signaling pathway requires functional CD4 receptors and is independent of binding to CXCR4. Binding of the natural ligand stromal cell-derived factor 1 (SDF-1) to CXCR4, which inhibits entry of T-cell-tropic HIV-1, activates also the ERK/MAP kinase pathway. However, SDF-1 did not affect the CD4-mediated expression of cytokine and chemokine genes. These results provide firm molecular evidence that binding of HIV-1 envelope glycoproteins to CD4 receptor initiates a signaling pathway(s) independent of the binding to the chemokine receptor that leads to the aberrant expression of inflammatory genes and may contribute significantly to HIV-1 replication as well as to deregulation of the immune system.

The protein tyrosine kinase p56lck is required for triggering NF-kappaB activation upon interaction of human immunodeficiency virus type 1 envelope glycoprotein gp120 with cell surface CD4

We have previously shown that NF-kappaB nuclear translocation can be observed upon human immunodeficiency virus type 1 (HIV-1) binding to cells expressing the wild-type CD4 molecule, but not in cells expressing a truncated form of CD4 that lacks the cytoplasmic domain (M. Benkirane, K.-T. Jeang, and C. Devaux, EMBO J. 13:5559-5569, 1994). This result indicated that the signaling cascade which controls HIV-1-induced NF-kappaB activation requires the integrity of the CD4 cytoplasmic tail and suggested the involvement of a second protein that binds to this portion of the molecule. Here we investigate the putative role of p56(lck) as a possible cellular intermediate in this signal transduction pathway. Using human cervical carcinoma HeLa cells stably expressing CD4, p56(lck), or both molecules, we provide direct evidence that expression of CD4 and p56(lck) is required for HIV-1-induced NF-kappaB translocation. Moreover, the fact that HIV-1 stimulation did not induce nuclear translocation of NF-kappaB in cells expressing a mutant form of CD4 at position 420 (C420A) and the wild-type p56(lck) indicates the requirement for a functional CD4-p56(lck) complex.

Signaling and functional properties of interleukin-16

Interleukin-16 is secreted from a variety of immune cells as a peptide of 17 kDa which aggregates into tetrameric form essential for IL-16s direct interaction with and cross linking of its receptor, the CD4 antigen. IL-16 stimulation of CD4+ cells results in the induction of cell motility, and in addition can function as a competence growth factor for CD4+ lymphocytes. These activities suggest that IL-16 could play a role in the accumulation and activation of CD4+ cells recruited to sites of inflammation. Along those lines, IL-16 has been identified at sites of inflammation associated with several different disease states. Its function as a competence growth factor specifically for CD4+ T cells may be useful for immune reconstitution in immunodeficiency diseases such as AIDS.

The Ick protein tyrosine kinase is not involved in antibody-mediated CD4 (CDR3-loop) signal transduction that inhibits HIV-1 transcription

Monoclonal antibodies (mAb) that bind to the immunoglobulin CDR3-like region in the D1 domain of the CD4 molecule can inhibit the HIV-1 life cycle in CD4-positive T cells and lymphoblastoid cell lines at the stage of transcription. This antiviral effect requires the integrity of the cytoplasmic tail of CD4 which is known to act as a signal transduction region through its association with the protein tyrosine kinase (PTK) p56lck. In this study, we investigated the putative role of this PTK in transducing inhibitory signals that act on HIV-1 replication after triggering by anti-CDR3-like region antibody treatment of infected T cell lines. CEM (CD4+/p56lck + inducible), MT2 (CD4+/p56lck - repressed), HSB-2 (CD4-/p56lck + constitutively), HSB-2 WTCD4 (CD4+/p56lck + constitutively), HSB-2 CD4.402 (CD4+ truncated form which lacks the cytoplasmic domain/p56lck + constitutively), and HSB-2 CD4mut (CD4+ unable to bind lck/p56lck + constitutively) were exposed to HIV-1 and cultured in medium supplemented with an anti-CDR3-like region-specific antibody or a control anti-CD4 mAb which does not inhibit HIV-1 transcription. We found that CDR3-loop-mediated inhibitory signals are efficiently transduced in CD4-positive cells which demonstrate a constitutive activation of p56lck or in CD4-positive cells lacking p56lck expression. Moreover, inhibitory signals were transduced in HSB-2 CD4mut cells expressing a cell surface CD4 with a double cysteine mutation in its cytoplasmic tail that renders the molecule unable to bind p56lck, but not HSB-2 CD4.402 cells expressing a truncated form of CD4 which lacks the cytoplasmic domain. These results indicate that the p56lck plays no direct role in this process and suggests the existence of another signaling partner for CD4.

Inhibition of HIV-1 replication by a monoclonal antibody directed toward the complementarity determining region 3-like domain of CD4 in CD45 expressing and CD45-deficient cells

Monoclonal antibodies directed toward the complementarity determining region (CDR)3-like loop of the aminoterminal domain of CD4 have been shown to inhibit the replication of human immunodeficiency virus (HIV) in CD4 positive T cells. The mechanism of action of these antibodies is not yet elucidated, although several observations suggest that they inhibit viral transcription by signal transduction through the CD4 molecule, potentially implicating the activation of a protein tyrosine kinase (PTK) cascade. Since CD45 is the major protein tyrosine phosphatase associated to the plasma membrane in T cells, and has been shown to regulate the activity of several PTK, we postulated that CD45 may be necessary for the inhibitory action of the CDR3-like specific anti-CD4 antibodies. Therefore we tested the effect of one such anti-CD4 monoclonal antibody, 13B8.2, in repressing HIV replication in CD45 positive cell lines and CD45 deficient variants. Our data show that cells respond to 13B8.2 postinfection treatment regardless of CD45 expression, indicating that neither CD45 nor PTK regulated by CD45 are implicated in the mechanism of action of this antibody.

Delayed human immunodeficiency virus type 1-induced apoptosis in cells expressing truncated forms of CD4

It has been reported previously that cells expressing a truncated form of CD4 which lacks the cytoplasmic tail of the molecule (truncation at position 402) were not sensitive to human immunodeficiency virus type 1 (HIV-1)-induced apoptosis in an acute-phase model of infection (J. Corbeil, M. Tremblay, and D. D. Richman, J. Exp. Med. 183:39-48, 1996). The role played by the cytoplasmic domain of CD4 in HIV-1-induced apoptosis was reexamined here with clones of A2.01 cells expressing different forms of CD4 and the DNA intercalant YOPRO-1 assay. Six days after virus exposure, we found evidence of apoptosis in A2.01 cells expressing the wild-type CD4 (A2.01/CD4), whereas enhanced apoptosis remained absent in cultures of A2.01/CD4.401 and A2.01/CD4.403 cells (A2.01 cells which express CD4.401 and CD4.403 molecules with truncations at positions 401 and 403, respectively). However, cell death by apoptosis measured with YOPRO-1 was found in cultures of A2.01/CD4.401 and A2.01/CD4.403 cells 15 days after virus exposure. This result was confirmed with a terminal dUTP nick end-labeling assay and propidium iodide staining. The long lag time postinfection required for apoptosis to be observed in cultures of infected cells expressing truncated forms of CD4 was due to the delayed viral replication in these cells, as shown by monitoring of the viral reverse transcriptase activity and HIV-1 p24gag antigen expression. These results emphasize the relationship between virus replication and cell death by apoptosis.

Involvement of Extracellular Signal-Regulated Kinase Module in HIV-Mediated CD4 Signals Controlling Activation of Nuclear Factor-κB and AP-1 Transcription Factors

Although the molecular mechanisms by which the HIV-1 triggers either T cell activation, anergy, or apoptosis remain poorly understood, it is well established that the interaction of HIV-1 envelope glycoproteins with cell surface CD4 delivers signals to the target cell, resulting in activation of transcription factors such as NF-κB and AP-1. In this study, we report the first evidence indicating that kinases MEK-1 (MAP kinase/Erk kinase) and ERK-1 (extracellular signal-regulated kinase) act as intermediates in the cascade of events that regulate NF-κB and AP-1 activation upon HIV-1 binding to cell surface CD4. We found that CEM cells transfected with dominant negative forms of MEK-1 or ERK-1 do not display NF-κB activation after HIV-1 binding to CD4. In contrast, NF-κB activation was observed in these cells after PMA stimulation. Although the different cell lines studied expressed similar amounts of CD4 and p56lck, HIV-1 replication and HIV-1-induced apoptosis were slightly delayed in cells expressing dominant negative forms of MEK-1 or ERK-1 compared with parental CEM cells and cells expressing a constitutively active mutant form of MEK-1 or wild-type ERK-1. In light of recently published data, we propose that a positive signal initiated following oligomerization of CD4 by the virus is likely to involve a recruitment of active forms of p56lck, Raf-1, MEK-1, and ERK-1, before AP-1 and NF-κB activation.

Signals transduced through the CD4 molecule interfere with TCR/CD3-mediated ras activation leading to T cell anergy/apoptosis

It has been previously demonstrated that the occupancy of CD4 molecules by the HIV-1 envelope glycoprotein gp120 results in marked inhibition of T cell receptor-CD3 complex (TCR/CD3) activation-induced IL-2 secretion. To elucidate the mechanism of inhibitory effects of gp160 on T cell signaling, we have investigated the intracellular biochemical events and biological output in response to anti-CD3 mAb activation of purified peripheral blood CD4+ T cells from healthy donors with and without prior exposure to HIV-1 gp160. Pretreatment with gp160 resulted in marked inhibition of tyrosine phosphorylation of p59(fyn), PLC-gamma1, ras activation, and TNF-alpha secretion in anti-CD3 mAb activated CD4+ T cells, and a subset of CD4+ cells underwent activation-induced cell death. The data presented here provide insight into the mechanism by which the interaction of HIV-1 envelope glycoproteins with CD4 molecules may alter TCR/CD3-activation-induced signal transduction resulting in anergy and apoptosis with consequent functional deficiency of CD4+ T cells.

CD4 Cross-Linking (CD4XL) Induces RAS Activation and Tumor Necrosis Factor-α Secretion in CD4+ T Cells

CD4 molecules are the primary receptors for human immunodeficiency virus (HIV) and bind the envelope glycoprotein gp120 of HIV with high-affinity. We have previously shown that cross-linking of CD4 molecules (CD4XL) in normal peripheral blood mononuclear cells (PBMC) results in secretion of cytokines tumor necrosis factor-α (TNF-α) and interferon-γ (IFN-γ), but not of interleukin-2 (IL-2) or IL-4. To investigate the intracellular signaling events associated with CD4-gp120 interaction, we incubated CD4+ T cells from peripheral blood of HIV-negative healthy donors with HIV envelope protein gp160 alone or performed CD4XL with gp160 and anti-gp160 antibody. This procedure resulted in tyrosine phosphorylation of intracellular substrates p59fyn, zap 70, and p95vav and also led to ras activation, as assessed by conversion of rasGDP to rasGTP. The role of ras in CD4 signaling was further investigated using CD4+ Jurkat cells transfected with a dominant negative ras mutant. CD4+ T cells expressing dn-ras secreted significantly reduced levels of TNF-α in response to CD4XL. These studies indicate that interaction of HIV gp160 with CD4 molecules activates the ras pathway in T cells, which may result in the cells becoming unresponsive to subsequent stimulation.

CD4 Cross-Linking (CD4XL) Induces RAS Activation and Tumor Necrosis Factor-α Secretion in CD4+ T Cells

CD4 molecules are the primary receptors for human immunodeficiency virus (HIV) and bind the envelope glycoprotein gp120 of HIV with high-affinity. We have previously shown that cross-linking of CD4 molecules (CD4XL) in normal peripheral blood mononuclear cells (PBMC) results in secretion of cytokines tumor necrosis factor-α (TNF-α) and interferon-γ (IFN-γ), but not of interleukin-2 (IL-2) or IL-4. To investigate the intracellular signaling events associated with CD4-gp120 interaction, we incubated CD4+ T cells from peripheral blood of HIV-negative healthy donors with HIV envelope protein gp160 alone or performed CD4XL with gp160 and anti-gp160 antibody. This procedure resulted in tyrosine phosphorylation of intracellular substrates p59fyn, zap 70, and p95vav and also led to ras activation, as assessed by conversion of rasGDP to rasGTP. The role of ras in CD4 signaling was further investigated using CD4+ Jurkat cells transfected with a dominant negative ras mutant. CD4+ T cells expressing dn-ras secreted significantly reduced levels of TNF-α in response to CD4XL. These studies indicate that interaction of HIV gp160 with CD4 molecules activates the ras pathway in T cells, which may result in the cells becoming unresponsive to subsequent stimulation.

HIV-1 envelope protein, gp120, has no effects on inositol phosphate production and metabolism in the Jurkat T-cell line either in the presence or absence of receptor stimulation

We have used HPLC techniques to investigate the effects of gp120 upon inositol phosphate turnover in Jurkat E6-1 CD4+ T-cells, to pursue previous reports that this viral coat protein: (a) inhibits receptor-activated inositol phosphate release; (b) stimulates basal inositol phosphate release; (c) inhibits inositol polyphosphate 5-phosphatase. Treatment of cells with up to 10 microg/ml gp120 from between 10 min and 24 h was without effect upon inositol phosphate turnover in both basal cells, and in C305 and OKT3 stimulated cells. This is the first report that biologically competent gp120 does not affect any aspect of inositol phosphate turnover in either basal or receptor-activated lymphocytes.

Transduction of activation signal that follows HIV-1 binding to CD4 and CD4 dimerization involves the immunoglobulin CDR3-like region in domain 1 of CD4

The role of CD4 during the human immunodeficiency virus type 1 (HIV-1) life cycle in T cells is not restricted to binding functions. HIV-1 binding to CD4 also triggers signals that lead to nuclear translocation of NF-kappaB and are important to the productive infection process. In addition to its cytoplasmic tail, in the ectodomain, the immunoglobulin (Ig) CDR3-like region of CD4 domain 1 seemed to play a role in this cascade of signals. We demonstrate in this work that the structural integrity of the CDR3-like loop is required for signal transduction. Substitutions of negatively charged residues by positively charged residues within the CDR3-like loop either inhibited NF-kappaB translocation after HIV-1 and gp120-anti-gp120 immune complexes binding to E91K,E92K mutants or induced its constitutive activation for E87K,D88K mutants. Moreover, A2.01-3B cells expressing the E91K,E92K mutant exhibited a lower HIV-1Lai replication. These cells, however, expressed p56(lck), demonstrated NF-kappaB translocation upon PMA stimulation, bound HIV-1Lai envelope glycoprotein with high affinity, and contained HIV-1 DNA 24 h after exposure to virus. E91K, E92K, and E87K,D88K mutant CD4 molecules were unable to bind a CD4 synthetic aromatically modified exocyclic, CDR3.AME-(82-89), that mimics the CDR3-like loop structure and binds to native cell surface CD4. This result together with molecular modeling studies indicates that the CDR3.AME-(82-89) analog binds to the CDR3-like loop of CD4 and strongly suggests that this region represents a site for CD4 dimerization. The negative charges on the CDR3-like loop thus appear critical for CD4-mediated signal transduction most likely related to CD4 dimer formation.

The effects of measles virus persistent infection on AP-1 transcription factor binding in neuroblastoma cells

Measles virus (MV) persistence in brain cells has broad effects on different cellular functions. We have previously shown that NS20Y clone, originally derived from C1300 neuroblastoma cells, persistently infected with MV (NS20Y/MS), displays constitutively elevated levels of c-fos and PKC mRNAs, implying MV-mediated effects on transcriptional regulation. Nonetheless, the mode by which virus affects the transcriptional machinery still remains obscure. In order to define this phenomenon, we studied the binding properties of major transcription factors (AP-1 and NFkappaB) in NS20Y/MS cells. Using electrophoretic mobility shift approach (EMSA) with the appropriate oligonucleotide probes, we have found that the persistent MV infection does not affect NFkappaB binding, while the AP-1 binding was significantly decreased. Similar inhibition was not observed in NS20Y cells acutely infected with MV. Anti-measles antibody-mediated restriction of viral gene expression restored AP-1 binding, thus suggesting that measles virus proteins may affect the components of the host transcriptional machinery.

Gene activating and proapoptotic potential are independent properties of different CD4 epitopes

CD4 engagement triggers an early signaling cascade which initiates late events such as transcription factor activation. The outcome of CD4 engagement is T-cell commitment to alternative, dramatically different fates, such as activation and apoptosis. We have tested a panel of anti-CD4 mAbs specific for different CD4 epitopes, as well as HIV-1 gp120, for the capacity to activate crucial early events such as enhancement of p56(lck) kinase activity and Shc phosphorylation. The same CD4 epitopes were characterized for their capacity both to deliver a gene activating signal and to program T-cells to activation dependent death. No correlation could be found between capacity of specific CD4 epitopes to deliver a gene activating signal and capacity to prime T-cells to apoptosis, suggesting that gene activating and proapoptotic potential are independent functions of CD4 epitopes. Furthermore, while triggering of the calcium pathway appears critical in NF-AT activation, optimal p56(lck) activation and Shc phosphorylation might be required for initiation of the apoptotic pathway.

Binding of HIV-1 to its receptor induces tyrosine phosphorylation of several CD4-associated proteins, including the phosphatidylinositol 3-kinase

Cell surface CD4 molecules are known to be important in several physiological responses of T lymphocytes. The use of human immunodeficiency virus (HIV) particles or purified gp120 molecules as CD4 cross-linking agents has been shown to result in a cascade of intracellular biochemical events. In addition, we and other have provided evidence suggesting that virus-mediated CD4 multimerization can lead to modulation of HIV-1 long terminal repeat-dependent activity and virus production. We were thus interested in measuring the effect of HIV-1 particles on intracellular tyrosine-phosphorylation levels, mostly of CD4-associated proteins. Using the T cell line CEM-T4, we observed that HIV-1 induces an increase in tyrosine phosphorylation of four major proteins physically complexed to the CD4 molecule. Immunoblot analysis permitted the identification of two of these proteins, p56lck and phosphatidylinositol 3-kinase (PI 3-kinase) p85 alpha. No concomitant variation in the level of these two CD4-associated proteins was observed after HIV-1-induced CD4 cross-linking. To our knowledge, this is the first report linking HIV-1-mediated CD4 multimerization to an increase in tyrosine phosphorylation of the PI 3-kinase complex. The four CD4-associated molecules described in this report are most likely implicated in virus-induced CD4-linked signaling events.

Binding of human immunodeficiency virus type 1 to CD4 induces association of Lck and Raf-1 and activates Raf-1 by a Ras-independent pathway

We have analyzed CD4-mediated signaling during the early stages of human immunodeficiency virus type 1 (HIV-1) infection. Binding of purified HIV-1 virions or recombinant HIV-1 glycoprotein gp120 to CD4 receptors resulted in association and tyrosine phosphorylation and activation of tyrosine kinase Lck and serine/threonine kinase Raf-1. The association between Lck and Raf-1 was mediated by stimulation of the CD4 receptors, since it was abolished by preincubation of the virus with soluble CD4 and was not detected in CD4-negative A201 T cells. However, the Lck-Raf-1 association was restored in A201 cells permanently transfected with human CD4 cDNA and stimulated with anti-CD4 antibodies. In addition, a catalytically active Lck was required for the association of Lck and Raf-1. Surprisingly, the CD4-mediated signaling, induced by the HIV-1 binding, did not result in stimulation of the Ras GTP-binding activity or its association with Raf-1, indicating that the signaling pathway generated by the HIV-1 binding is not identical to the classical Ras/Raf-1 pathway. Furthermore, overexpression of activated Raf-1 in Jurkat T cells stimulated the HIV long terminal repeat promoter activity and significantly enhanced HIV-1 replication. This suggests that the Lck-Raf-1 pathway, rapidly stimulated by the binding of HIV-1 or gp120 to CD4 receptors, may play an essential role in the transcriptional activation of the integrated HIV-1 provirus as well as in its pathogenicity.

Impairment of phagosome-lysosome fusion in HIV-1-infected macrophages

Phagosome-lysosome fusion is critical for intracellular killing of most organisms and is inhibited by some viruses, notably influenza. We explored the effects of infection in vitro with HIV-1 (IIIB or Ada-M) on phagosome-lysosome fusion in blood monocyte-derived macrophages. After 8 days of infection, fusion was assessed from the fluorescence change occurring up to 2 h after labeling the lysosome compartment with acridine orange and loading of phagosomes with opsonized yeast. Compared with mock-infected control macrophages, the proportion of cells showing fusion after infection was reduced from a mean of 70% to a mean of 47% (p = 0.0001). Inhibition was seen with heat-killed HIV-1 IIIB but not virus-free filtrate. It was mimicked by recombinant gp 120 and blocked by soluble CD4 or antibody to CD4 but not by a neutralizing antibody to the V3 loop of gp 120. The inhibitory effect was seen 8 days after the original, transient exposure to gp 120. These results suggest that a lasting abnormality of phagosome-lysosome fusion results from interaction between gp 120 and CD4, contributing, perhaps, to the increased susceptibility to opportunistic infections of people infected with HIV.

Cyclosporin A sensitivity of the HIV-1 long terminal repeat identifies distinct p56lck-dependent pathways activated by CD4 triggering

The CD4 co-receptor interacts with nonpolymorphic regions of major histocompatibility complex class II molecules on antigen-presenting cells. This interaction results in the mobilization of a number of signaling mediators shared by the T cell receptor (TcR) signaling pathway and thus amplifies TcR-generated signals. We have investigated the outcome of CD4 engagement on the activation of both cellular transcription factors and the HIV-1 long terminal repeat (LTR). We show that CD4 triggering activates different pathways of HIV LTR activation which can be identified by their sensitivity to the immunosuppressant cyclosporin A. The response of the inducible cellular transcription factors involved in HIVLTR activation shows that both nuclear factor (NF)-kappa B and NF-AT mediate a cyclosporin A-sensitive response to CD4, while AP-1 is at least in part responsible for the cyclosporin A-insensitive response. Both pathways can, however, be blocked by a kinase-defective dominant negative p56lck mutant, supporting an essential role for p56lck kinase activity in CD4-dependent signal transduction. A functional analysis of different CD4 epitopes using either anti-CD4 mAb or HIV-1 gp120 reveals a common epitope-specific activation of both the LTR and of the transcription factors NF-kappa B and NF-AT.

Human immunodeficiency virus type 1 gp120 induces anergy in human peripheral blood lymphocytes by inducing interleukin-10 production

The effects of recombinant gp120 on the proliferative responses and cytokine production by normal peripheral blood mononuclear cells (PBMC) were investigated. gp120 inhibited in a dose-dependent fashion the anti-CD3 monoclonal antibody (MAb)- and concanavalin A-induced proliferative responses. The production of interleukin-2 (IL-2) and IL-4 was diminished by gp120 in the anti-CD3- and concanavalin A-stimulated cultures. In unstimulated PBMC, gp120 induced the production of considerable amounts of IL-10, gamma interferon, and tumor necrosis factor alpha. The gp120-induced reduction in the proliferative responses of PBMC was at least partially reversed by the addition of IL-2, anti-CD28 MAb, or transfectants expressing CD80, CD86, or CD40 but not with exogenous IL-4. Also, a neutralizing anti-IL-10 MAb reversed the inhibitory effect of gp120 on the proliferative responses whereas exogenous IL-10 further enhanced this inhibitory effect. These findings indicate that IL-10 plays an important role in the inhibitory effect of gp120 on PBMC proliferation. The ratio of CD3+CD4+ to CD3+CD8+ T cells was the same in gp120-treated and untreated cell cultures. No apoptosis in these two T-cell populations was observed. However, the number of activated CD3+CD4+ T cells and CD3+CD8+ T cells, as judged by CD25, CD69, and HLA-DR expression, was consistently reduced. gp120 induced the expression of IL-10 in the monocyte/macrophage population, and therefore gp120 also reduced the proliferative responses of CD4+ T-cell-depleted PBMC. Taken together, our observations point to the importance of the cytokine pattern changes and, in particular, the role of IL-10 (produced by the monocytes) in the inhibitory effect of gp120. This mechanism of gp120-induced immunosuppression, if operative in vivo, could contribute to the depressed immune responses associated with human immunodeficiency virus infection and thus have important implications for immunotherapeutic strategies to slow down disease progression in AIDS.