Lymphocyte activation by HIV-1 envelope glycoprotein

Intercellular Transport of Viral Proteins

Viruses are vehicles to exchange genetic information and proteins between cells and organisms by infecting their target cells either cell-free, or depending on cell-cell contacts. Several viruses like certain retroviruses or herpesviruses transmit by both mechanisms. However, viruses have also evolved the properties to exchange proteins between cells independent of viral particle formation. This exchange of viral proteins can be directed to target cells prior to infection to interfere with restriction factors and intrinsic immunity, thus, making the target cell prone to infection. However, also bystander cells, e.g. immune cell populations, can be targeted by viral proteins to dampen antiviral responses. Mechanistically, viruses exploit several routes of cell-cell communication to exchange viral proteins like the formation of extracellular vesicles or the formation of long-distance connections like tunneling nanotubes. Although it is known that viral nucleic acids can be transferred between cells as well, this chapter concentrates on viral proteins of human pathogenic viruses covering all Baltimore classes and summarizes our current knowledge on intercellular transport of viral proteins between cells.

Impact of human immunodeficiency virus on pulmonary vascular disease

With the advent of anti-retroviral therapy, non-AIDS-related comorbidities have increased in people living with HIV. Among these comorbidities, pulmonary hypertension (PH) is one of the most common causes of morbidity and mortality. Although chronic HIV-1 infection is independently associated with the development of pulmonary arterial hypertension, PH in people living with HIV may also be the outcome of various co-morbidities commonly observed in these individuals including chronic obstructive pulmonary disease, left heart disease and co-infections. In addition, the association of these co-morbidities and other risk factors, such as illicit drug use, can exacerbate the development of pulmonary vascular disease. This review will focus on these complex interactions contributing to PH development and exacerbation in HIV patients. We also examine the interactions of HIV proteins, including Nef, Tat, and gp120 in the pulmonary vasculature and how these proteins alter the endothelial and smooth muscle function by transforming them into susceptible PH phenotype. The review also discusses the available infectious and non-infectious animal models to study HIV-associated PAH, highlighting the advantages and disadvantages of each model, along with their ability to mimic the clinical manifestations of HIV-PAH.

Extracellular Vesicles Carry HIV Env and Facilitate Hiv Infection of Human Lymphoid Tissue

Cells productively infected with HIV-1 release virions along with extracellular vesicles (EVs) whose biogenesis, size, and physical properties resemble those of retroviruses. Here, we found that a significant number of EVs (exosomes) released by HIV-1 infected cells carry gp120 (Env), a viral protein that mediates virus attachment and fusion to target cells, and also facilitates HIV infection in various indirect ways. Depletion of viral preparations of EVs, in particular of those that carry gp120, decreases viral infection of human lymphoid tissue ex vivo. Thus, EVs that carry Env identified in our work seem to facilitate HIV infection and therefore may constitute a new therapeutic target for antiviral strategy.

Characterization of gp120 Binding to the CD4 Antigen and Detection of Specific Inhibitors

The first step in the attack of human immunodeficiency virus (HIV-1) on human T lymphocytes is binding of the viral coat protein, gp120, to the CD4 antigen on the surface of the cells. The present study examines 125I-gp120 binding to CEM and U937 cells. The results of this study show that this process is more complex than previously thought, consisting of multiple binding components, some of which may reflect processes occurring after the actual binding. Despite its complexity, measurement of the inhibition of 125I-gp120 binding by monoclonal antibodies to the CD4 antigen or to gp120 generally parallels direct measurement of the inhibition of HIV-1 binding to cells expressing the CD4 antigen (CD4 positive). Furthermore, aurintricarboxylic acid and pentosan polysulphate, inhibitors of the association of HIV-1 with cells, also inhibit 125I-gp120 binding. This is the first case in which a small molecule has been shown to inhibit gp120 binding using a classical radioligand binding assay. Two other inhibitors of HIV-1 association with cells, dextran sulphate and chondroitin sulphate, were less effective as 125I-gp120 binding inhibitors, suggesting that prevention HIV-1/cell association can be produced by mechanisms other than direct inhibition of gp120/CD4 binding.

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.

Recent developments in human immunodeficiency virus-1 latency research

Almost 30 years after its initial discovery, infection with the human immunodeficiency virus-1 (HIV-1) remains incurable and the virus persists due to reservoirs of latently infected CD4(+) memory T-cells and sanctuary sites within the infected individual where drug penetration is poor. Reactivating latent viruses has been a key strategy to completely eliminate the virus from the host, but many difficulties and unanswered questions remain. In this review, the latest developments in HIV-persistence and latency research are presented.

Short communication: HIV antigen-specific reactivation of HIV infection from cellular reservoirs: implications in the settings of therapeutic vaccinations

Therapeutic vaccinations using human immunodeficiency virus (HIV) antigens in HIV-infected patients on antiretroviral therapy (ART) have so far been attempted with the purpose of inducing CTL response. However, they can also be useful as a strategy for activation of latent HIV reservoir, which is thought to be mainly comprised of latently infected HIV-specific memory CD4 cells, eventually leading to elimination of the virus. The present study was carried out to explore the ability of different HIV antigens to activate HIV replication as assessed by intracellular P24 detection as well as to induce T cell responses in terms of cytokine expression by flow cytometry after stimulation of PBMCs from HIV-infected patients. HIV antigens were found to be able to activate most of the CD4 T cells harboring proviral DNA. HIV-1 Pol and Env were responsible for induction of higher HIV replication in terms of both magnitude and frequency followed by Gag and Nef. As opposed to this, Pol and Env contributed to fewer numbers of polyfunctional CD8 cells desirable for elimination of HIV-infected cells in comparison to Gag and Nef. Thus, HIV antigens may provide a strategy for the activation of a latent reservoir. It was observed that HIV replication started as early as half an hour after in vitro activation indicating a stringent need for maintaining effective concentrations of antiretroviral drugs to prevent further spread of HIV during this process. HIV-infected cells were found to be responsible for higher IL-10 secretion after activation, which could also serve as one of the reasons for suppressed CD8 responses to Pol and Env as more HIV-infected CD4 cells would be secreting IL-10 in response to these antigens. Since IL-10 blockade helped to improve immune responses in terms of cytokine secretion, it should be considered in settings of therapeutic vaccination to improve CTL responses, which will ultimately limit the persistence of the viral reservoir.

Quantitative proteomic analysis of HIV-1 infected CD4+ T cells reveals an early host response in important biological pathways: protein synthesis, cell proliferation, and T-cell activation

Human immunodeficiency virus (HIV-1) depends upon host-encoded proteins to facilitate its replication while at the same time inhibiting critical components of innate and/or intrinsic immune response pathways. To characterize the host cell response on protein levels in CD4+ lymphoblastoid SUP-T1 cells after infection with HIV-1 strain LAI, we used mass spectrometry (MS)-based global quantitation with iTRAQ (isobaric tag for relative and absolute quantification). We found 266, 60 and 22 proteins differentially expressed (DE) (P-value ≤ 0.05) at 4, 8, and 20 hours post-infection (hpi), respectively, compared to time-matched mock-infected samples. The majority of changes in protein abundance occurred at an early stage of infection well before the de novo production of viral proteins. Functional analyses of these DE proteins showed enrichment in several biological pathways including protein synthesis, cell proliferation, and T-cell activation. Importantly, these early changes before the time of robust viral production have not been described before.

Interleukin-16 as a marker of Sézary syndrome onset and stage

INTRODUCTION

Sézary syndrome is one of the most common forms of cutaneous T cell lymphoma (CTCL). It is characterized by skin infiltration of malignant T cells. We examined interleukin-16, a potent T cell chemoattractant and cell-cycle regulator, as a prospective marker of disease onset and stage.

METHODS

The correlation of total intracellular interleukin-16 and surface CD26 was studied by flow cytometry. Confocal microscopy was performed to determine localization of interleukin-16 at different stages of the disease. The levels of interleukin-16 in plasma and culture supernatants were examined by enzyme-linked immunoassay. Additionally, lymphocytes from stage IB patients were cultured in the presence of interleukin-16 alone and in combination with interleukin-15, and their ability to survive and proliferate was determined by cell counts and [3H]TdR incorporation.

RESULTS

The data indicate that loss of both nuclear and intracellular pro-interleukin-16 highly correspond to disease stage, with a concomitant increase in secreted mature interleukin-16 in both culture supernatants and patients' plasma that peaks at stage IB. Loss of intracellular interleukin-16 strongly corresponded to loss of surface CD26, which has been shown to occur with more advanced stage of CTCL. Nuclear translocation of pro-interleukin-16 was not observed in late stages of Sézary syndrome, indicating this loss is not reversible.

CONCLUSIONS

We propose that it is feasible to use plasma levels of IL-16 as a potential diagnostic marker of Sézary syndrome and to use loss of intracellular IL-16 as a prognostic indicator of disease severity and stage.

Human immunodeficiency virus type 1 gp120 reprogramming of CD4+ T-cell migration provides a mechanism for lymphadenopathy

Infection by human immunodeficiency virus type 1 (HIV-1) is associated with decreases in peripheral CD4(+) T cells and development of lymphadenopathy. The precise mechanisms by which HIV-1 induces these changes have not been elucidated. T-cell trafficking through lymphoid tissues is facilitated by CCL21-mediated entry and sphingosine-1-phosphate (S1P)-mediated egress. Having previously determined that HIV-1 envelop glycoprotein, gp120, directly alters T-cell migration, we investigated whether gp120 without HIV-1 infection could influence the responses of CD4(+) T cells to the signals involved in T-cell trafficking through lymph tissue. Incubation of normal human T cells with gp120 for 1 h resulted in reprogramming of CD4 T-cell migratory responses by increasing sensitivity to CCL20 and CCL21 and complete inhibition of migration to S1P. Incubation of human T cells with gp120 prior to injection into NOD.CB17-Prkdc(scid)/J mice resulted in increases in lymph node accumulation of CD4(+) T cells, with reciprocal decreases in blood and spleen compared to T cells not exposed to gp120. The effects of gp120 required CD4 signaling mediated through p56(lck). These findings suggest that gp120 alone can alter CD4(+) influx and efflux from lymph nodes in a fashion consistent with the development of lymphopenia and lymphadenopathy.

HIV-1 integrates into resting CD4+ T cells even at low inoculums as demonstrated with an improved assay for HIV-1 integration

Human Immunodeficiency Virus Type 1 (HIV-1) establishes a latent reservoir early in infection that is resistant to the host immune response and treatment with highly active antiretroviral therapy (HAART). The best understood of these reservoirs forms in resting CD4(+) T cells. While it remains unclear how reservoirs form, a popular model holds that the virus can only integrate in activated CD4(+) T cells. Contrary to this model, our previous results suggest that HIV-1 can integrate directly into the genomes of resting CD4(+) T cells. However, a limitation of our previous studies was that they were conducted at high viral inoculum and these conditions may lead to cellular activation or saturation of restriction factors. In the present study, we tested if our previous findings were an artifact of high inoculum. To do this, we enhanced the sensitivity of our integration assay by incorporating a repetitive sampling technique that allowed us to capture rare integration events that occur near an Alu repeat. The new technique represents a significant advance as it enabled us to measure integration accurately down to 1 provirus/well in 15,000 genomes--a 40-fold enhancement over our prior assay. Using this assay, we demonstrate that HIV can integrate into resting CD4(+) T cells in vitro even at low viral inoculum. These findings suggest there is no threshold number of virions required for HIV to integrate into resting CD4(+) T cells.

Innate immune dysfunction in HIV infection: effect of HIV envelope-NK cell interactions

We have previously described a number of NK cell dysfunctions in HIV-viremic individuals. In the present study, we performed DNA microarray analysis followed by phenotypic and functional characterization in an effort to investigate which HIV envelope glycoproteins (gp120) affect the physiologic functions of NK cells. Upon treatment of NK cells with HIV gp120, DNA microarray analyses indicated up-regulation of several categories of genes that are associated with apoptosis, suppression of both cellular proliferation and survival, as well as down-regulation of genes that play a vital role in cell proliferation, innate immune defense mechanism, and cell survival. Both subtypes of gp120 suppressed NK cell cytotoxicity, proliferation, and the ability to secrete IFN-gamma. NK cells exposed to X4-subtype HIV gp120 showed a significant decrease in the levels of CC chemokines, while exposure to R5-subtype HIV gp120 had minimal effect. Extended exposure to HIV gp120 resulted in apoptosis of NK cells, further validating the microarray data. Our data demonstrate that exposure of NK cells to HIV envelope proteins results in profound cellular abnormalities at the level of gene expression as well as generic cell functions. These findings are likely to be a consequence of a direct HIV gp120-mediated effect on NK cells. Identification of specific surface receptors on NK cells that interact with HIV envelope proteins might explain how HIV is capable of circumventing innate immune defense mechanisms and establishing infection in susceptible individuals.

HIV-1 gp120 induces NFAT nuclear translocation in resting CD4+ T-cells

The replication of human immunodeficiency virus (HIV) in CD4+ T-cells is strongly dependent upon the state of activation of infected cells. Infection of sub-optimally activated cells is believed to play a critical role in both the transmission of virus and the persistence of CD4+ T-cell reservoirs. There is accumulating evidence that HIV can modulate signal-transduction pathways in a manner that may facilitate replication in such cells. We previously demonstrated that HIV gp120 induces virus replication in resting CD4+ T cells isolated from HIV-infected individuals. Here, we show that in resting CD4+ T-cells, gp120 activates NFATs and induces their translocation into the nucleus. The HIV LTR encodes NFAT recognition sites, and NFATs may play a critical role in promoting viral replication in sub-optimally activated cells. These observations provide insight into a potential mechanism by which HIV is able to establish infection in resting cells, which may have implications for both transmission of HIV and the persistence of viral reservoirs.

Preferential feline immunodeficiency virus (FIV) infection of CD4+ CD25+ T-regulatory cells correlates both with surface expression of CXCR4 and activation of FIV long terminal repeat binding cellular transcriptional factors

Previously, we have characterized feline CD4+ CD25+ T-regulatory (Treg) cells with regard to their immune regulatory properties and ability to support feline immunodeficiency virus (FIV) replication in vitro and in vivo. Our studies showed that while CD4+ CD25+ cells were capable of replicating FIV in the presence of interleukin-2 (IL-2) alone, CD4+ CD25- cells harbored a latent infection that required a strong mitogenic stimulus to activate virus replication. In the present study, we investigated the mechanisms governing the preferential replication of FIV in highly purified CD4+ CD25+ Treg cells compared to their CD4+ CD25+ counterparts. Studies aimed at elucidating mechanisms regulating infection of these cells revealed that CD4+ CD25- cells were less susceptible to FIV binding and entry than CD4+ CD25+ cells, which correlated with increased surface expression of FIV coreceptor CXCR4. In addition, the number of CD4+ CD25+ cells that expressed the primary receptor CD134 was greater than for CD4+ CD25- cells. Although increased permissiveness to FIV infection of CD4+ CD25- cells following mitogenic stimulation correlated strongly with upregulation of surface CXCR4, it did not correlate with CD134 expression. Further, study of intracellular factors regulating FIV replication revealed that CD4+ CD25+ but not CD4+ CD25- T cells showed constitutive and IL-2-responsive transactivation of activating transcription factor, CAAT enhancer binding protein, and activating protein 1 transcription factors that are important for FIV replication. These factors were upregulated in CD4+ CD25- T cells following ConA stimulation, which correlated with FIV replication. This is the first report elucidating the mechanisms that allow for productive lentiviral infection of CD4+ CD25+ Treg cells.

Cutting edge: IL-16/CD4 preferentially induces Th1 cell migration: requirement of CCR5

IL-16 binds to CD4 and induces a migratory response in CD4(+) T cells. Although it has been assumed that CD4 is the sole receptor and that IL-16 induces a comparable migratory response in all CD4(+) T cells, this has not been investigated. In this study, we determined that IL-16 preferentially induces a migratory response in Th1 cells. Because chemokine receptor CCR5 is expressed predominantly in Th1 cells and is physically associated with CD4, we investigated whether IL-16/CD4 stimulation was enhanced in the presence of CCR5. Using T cells from CCR5(null) mice, we determined that IL-16-induced migration was significantly greater in the presence of CCR5. The presence of CCR5 significantly increased IL-16 binding vs CD4 alone; however, IL-16 could not bind to CCR5 alone. Because CD4(+)CCR5(+) cells are prevalent at sites of inflammation, this intimate functional relationship likely plays a pivotal role for the recruitment and activation of Th1 cells.

Human immunodeficiency virus type 1 (HIV-1) induces activation of multiple STATs in CD4+ cells of lymphocyte or monocyte/macrophage lineages

Human immunodeficiency virus type 1 (HIV-1) impacts the activation state of multiple lineages of hematopoietic cells. Chronic HIV-1 infection among individuals with progressive disease can be associated with increased levels of activated signal transducers and activators of transcription (STATs) in peripheral blood mononuclear cells. To investigate interactions between HIV-1 and CD4(+) cells, activated, phosphorylated STAT proteins in nuclear extracts from lymphocytic and promonocytic cell lines as well as primary monocyte-derived macrophages were measured. Levels of activated STATs increased six- to tenfold in HUT78 and U937 cells within 2 h following exposure to virions. The response to virus was dose-dependent, but kinetics of activation was delayed relative to interleukin-2 or interferon-gamma. Activation of STAT1, STAT3, and STAT5 occurred with diverse viral envelope proteins, independent of coreceptor use or viral replication. Envelope-deficient virions had no effect on STAT activation. Monoclonal antibody engagement of CD4 identified a novel role for CD4 as a mediator in the activation of multiple STATs. Results provide a model for HIV-1 pathogenesis in infected and noninfected hematopoietic cells.

HIV envelope induces virus expression from resting CD4+ T cells isolated from HIV-infected individuals in the absence of markers of cellular activation or apoptosis

Resting CD4(+) T cells containing integrated HIV provirus constitute one of the long-lived cellular reservoirs of HIV in vivo. This cellular reservoir of HIV had been thought to be quiescent with regard to virus replication based on the premise that HIV production in T cells is inexorably linked to cellular activation as determined by classical activation markers. The transition of T cells within this HIV reservoir from a resting state to an activated HIV-producing state is believed to be associated with a shorten life span due to susceptibility to activation-associated apoptosis. Evidence is mounting, however, that HIV production may occur in T cells that have not undergone classic T cell activation. HIV encodes several proteins, including envelope and Nef, which trigger a variety of signaling pathways associated with cellular activation, thereby facilitating HIV replication in nondividing cells. The present study demonstrates that production of infectious virus from resting CD4(+) T cells isolated from HIV-infected individuals can be induced following exposure of these cells to HIV-1 recombinant (oligomeric gp140) envelope protein. Envelope-mediated induction of HIV expression occurs in the presence of reverse transcriptase inhibitors and is not associated with markers of classic T cell activation, proliferation, or apoptosis. The ability of HIV envelope to induce virus replication in HIV-infected resting CD4(+) T cells without triggering apoptosis provides a mechanism for the virus itself to directly participate in the maintenance of HIV production from this cellular reservoir.

HIV envelope induces a cascade of cell signals in non-proliferating target cells that favor virus replication

Certain HIV-encoded proteins modify host-cell gene expression in a manner that facilitates viral replication. These activities may contribute to low-level viral replication in nonproliferating cells. Through the use of oligonucleotide microarrays and high-throughput Western blotting we demonstrate that one of these proteins, gp120, induces the expression of cytokines, chemokines, kinases, and transcription factors associated with antigen-specific T cell activation in the absence of cellular proliferation. Examination of transcriptional changes induced by gp120 in freshly isolated peripheral blood mononuclear cells and monocyte-derived-macrophages reveals a broad and complex transcriptional program conducive to productive infection with HIV. Observations include the induction of nuclear factor of activated T cells, components of the RNA polymerase II complex including TFII D, proteins localized to the plasma membrane, including several syntaxins, and members of the Rho protein family, including Cdc 42. These observations provide evidence that envelope-mediated signaling contributes to the productive infection of HIV in suboptimally activated T cells.

Ligation of human CD4 interferes with antigen-induced activation of primary T cells

The CD4 molecule functions to enhance T cell activation when it is co-aggregated with the T cell receptor for antigen (TCR) by MHC class II antigenic peptide complexes. However, independent ligation of CD4 has been shown to negatively effect signaling through the TCR in vitro. The interaction between the HIV-1 envelope glycoprotein gp120 and CD4 is a central event in the pathogenesis of AIDS and may contribute to immune deficiency via both direct and indirect mechanisms, including lytic infection of T cells and induction of CD4 signaling events resulting in apoptosis and anergy. Analysis of the consequences of interactions between CD4 and gp120 have yielded contradictory results presumably because most of these studies have focused on T cell clones of questionable relevance to the in vivo target of the virus. Here, we analyzed the effects of CD4 ligation on freshly isolated cells of human CD4 transgenic mice, and show that huCD4 preligation, in the absence of human CXCR4, has an inhibitory effect on both early and late T cell activation events. CD4 signaling negatively regulates the response to antigen, as well as to anti-TCR mAb. In addition, we show here that this negative signal requires the cytoplasmic tail of CD4. These results suggest that in HIV infected patients the interaction of gp120 with CD4 induces unresponsiveness of CD4+ T cells to subsequent activation by antigen.

Activation of CD8 T cells induces expression of CD4, which functions as a chemotactic receptor

It was previously shown that costimulation of CD8(+) lymphocytes results in de novo expression of CD4. This study expanded on this observation to investigate the function of CD4 on CD8 cells. The ability of costimulated CD8 cells to respond to interleukin 16 (IL-16), a ligand that binds CD4 and induces cellular chemotaxis, was examined. IL-16-mediated ligation of CD4 expressed on CD8 T cells was found to induce an intracellular signal that directs migration of these cells in vitro. Thus, expression of CD4 on a CD8 lymphocyte has functional importance and may serve to control distribution of newly activated CD8 T cells in vivo.

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.

V3 induces in human normal cell populations an accelerated macrophage-mediated proliferation--apoptosis phenomenon of effector T cells when they respond to their cognate antigen

The semi-conserved domain of V3 of HIV-1 was synthesised in a lipopeptide form to be presented on the surface of liposome particles. Composite liposomes were constructed with entrapped tetanus toxoid as a recall antigen (lipo-V3/TT liposomes) to study the influence of V3 on effector T cells of human normal peripheral lymphocyte populations. We demonstrated that lipo-V3/TT liposomes induce a V3-specific response characterised by an early, enhanced proliferation of effector CD4+ T cells, followed by a sharp apoptosis. The phenomenon required the presence of monocyte-derived macrophages and CD4+ T cells, but it was qualitatively and quantitatively distinct from the normal soluble antigen-mediated antigen presenting cell: T cell interaction. Presence of the beta-chemokine RANTES in the culture medium inhibited the phenomenon, suggesting that V3 plays a costimulatory role that involves the chemokine receptor CCR5 pathway during the process of antigen presentation to T cells. This observation may be very important if it occurs also in HIV-1 infection, as it may explain the selective and progressive depletion of non-infected effector CD4+ T cells.

Desensitization of CXC chemokine receptor 4, mediated by IL-16/CD4, is independent of p56lck enzymatic activity

CCR5 and CXC chemokine receptor 4 (CXCR4) are coreceptors for CD4 as defined by HIV-1 glycoprotein (gp) 120 binding. Pretreatment of T cells with gp120 results in modulation of both CCR5 and CXCR4 responsiveness, which is dependent upon p56(lck) enzymatic activity. The recent findings that pretreatment of T cells with a natural CD4 ligand, IL-16, could alter cellular responsiveness to macrophage-inflammatory protein-1ss (MIP-1ss) stimulation, prompted us to investigate whether IL-16 could also alter CXCR4 signaling. These studies demonstrate that IL-16/CD4 signaling in T lymphocytes also results in loss of stromal derived factor-1alpha (SDF-1alpha)/CXCR4-induced chemotaxis; however, unlike MIP-1ss/CCR5, the effects were not reciprocal. There was no effect on eotaxin/CCR3-induced chemotaxis. Desensitization of CXCR4 by IL-16 required at least 10-15 min pretreatment; no modulation of CXCR4 expression was observed, nor was SDF-1alpha binding altered. Using murine T cell hybridomas transfected to express native or mutated forms of CD4, it was determined that IL-16/CD4 induces a p56(lck)-dependent inhibitory signal for CXCR4, which is independent of its tyrosine catalytic activity. By contrast, IL-16/CD4 desensitization of MIP-1ss/CCR5 responses requires p56(lck) enzymatic activity. IL-16/CD4 inhibition of SDF-1alpha/CXCR4 signals requires the presence of the Src homology 3 domain of p56(lck) and most likely involves activation of phosphatidylinositol-3 kinase. These studies indicate the mechanism of CXCR4 receptor desensitization induced by a natural ligand for CD4, IL-16, is distinct from the inhibitory effects induced by either gp120 or IL-16 on CCR5.

Interleukin 16: implications for CD4 functions and HIV-1 progression

In this article, David Center and colleagues clarify the controversies that have emerged over the unique structure of interleukin 16 and its anti-HIV-1 activity. Interleukin 16 is a ligand for CD4, and this implies CD4 acts as a sentinel receptor that can switch CD4+ T cells between immune and inflammatory functions.

HIV-induced T-cell syncytia release a two component T-helper cell chemoattractant composed of Nef and Tat

Using a newly developed gradient chamber to provide independent measurements of chemokinesis (stimulated motility) and chemotaxis (stimulated motility up a concentration gradient) of individual T-helper cells, it was recently demonstrated that HIV-induced T-cell syncytia release two distinct chemotactic activities that are separable by their rates of diffusion. The molecular masses of the two chemoattractant activities were estimated to be 30 and 120 kDa. The higher molecular mass activity was demonstrated to be the viral glycoprotein gp120. In an attempt to identify the lower molecular mass activity, chemotaxis and chemokinesis of T-helper cells were analyzed in individual concentration gradients of the virally encoded proteins Rev, p24, Tat and Nef. None functioned alone as a chemoattractant, but both Tat and Nef alone functioned as chemokinetic stimulants. When Tat and Nef were used together to generate parallel gradients, they stimulated chemotaxis. Antibody to either Tat or Nef neutralized the lower molecular mass chemotactic activity released by syncytia. The addition of antibody to the CD4 receptor or the addition of soluble CD4 inhibited high molecular mass chemotactic activity but not the low molecular mass chemotactic activity in HIV-induced syncytium-conditioned medium, demonstrating that the former but not the latter activity is mediated through the CD4 receptor. These results identify the combination of Nef and Tat as the lower molecular mass T cell chemoattractant released by HIV-induced syncytia, and provide the first evidence suggesting that parallel concentration gradients of two proteins are necessary for chemotaxis.

HIV-1 gp120 envelope protein modulates proliferation of human glomerular epithelial cells

Glomerular epithelial cells (GEC) have been demonstrated to undergo morphological alterations in human immunodeficiency virus (HIV)-associated focal glomerulosclerosis. In the present study, we evaluated the effect of HIV-1 gp120 envelope protein on the growth of cultured human (H) GEC. gp120 protein enhanced (P < 0.001) the proliferation of HGEC at lower concentrations. The mitogenic effect of gp120 protein on HGEC was further confirmed by enhanced accumulation of proliferating nuclear cell antigen (PCNA) by gp120 protein-treated cells, as compared with control cells. On the contrary, gp120 protein at higher concentrations suppressed (P < 0. 001) the growth of HGEC. To evaluate the mechanism of gp120 protein-induced HGEC growth suppression, we examined the effect of gp120 protein on HGEC apoptosis. gp120 protein at higher concentrations promoted the apoptosis of HGEC. At higher concentrations, gp120 protein also enhanced DNA fragmentation of HGEC. Anti-gp120 antibody attenuated the proliferative as well as the apoptotic effects of gp120 protein on HGEC. Because protein kinase C as well as tyrosine kinase inhibitors partially inhibited gp120-induced proliferation, gp120 appears to be activating both the protein kinase C and tyrosine kinase pathways. In addition, gp120 protein at lower concentrations enhanced mRNA expression of c-fos and at higher concentrations promoted mRNA expression of c-jun. We conclude that gp120 has a bimodal effect on proliferation of HGEC. This effect may be mediated through the activation of early growth genes.

Evidence of HIV type 1 glycoprotein 120 binding to recombinant N-methyl-D-aspartate receptor subunits expressed in a baculovirus system

Activation of the N-methyl-D-aspartate (NMDA) receptor by HIV-1 envelope glycoprotein 120 (gp120) is thought to represent at least one of the pathways causing neuronal damage in AIDS patients. In the present study, recombinant gp120 binding to NMDA receptor subunits expressed in a baculovirus system was examined by immunocytochemistry and a binding assay, using horseradish peroxidase (HRP)-conjugated and 125I-labeled recombinant gp120, respectively. We found that recombinant gp120 binds to Sf21 cells expressing epsilon1/zeta1 or epsilon2/zeta1 combined NMDA receptor subunits, but not to Sf21 cells infected with mock virus or Sf21 cells expressing a single epsilon1, epsilon2, or zeta1 NMDA receptor subunit. The binding was strongly blocked by unlabeled recombinant gp120, monoclonal anti-HIV-1 gp160 antibody, and a mixture of anti-epsilon1/epsilon2 and anti-zeta1 antibodies. The same results were obtained by flow cytometric analysis. These data suggest that HIV-1 gp120 may directly bind to the NMDA receptor. This evidence enhances our understanding of the mechanism of HIV-1-induced neuronal damage in AIDS patients.

Peptide T blocks GP120/CCR5 chemokine receptor-mediated chemotaxis

We previously reported that certain short gp120 V2 region peptides homologous to vasaoactive intestinal peptide (VIP), such as "peptide T," were potent inhibitors of gp120 binding, infectivity, and neurotoxicity. The present study shows that synthetic V2-region-derived peptides have potent intrinsic chemotaxis agonist activity for human monocytes and also act as antagonists of high-affinity (0.1 pM) gp120-mediated monocyte chemotaxis. Selectivity is shown in that peptide T is more potent at suppressing M-tropic than T-tropic gp120 chemotaxis. Peptide T was also able to suppress monocyte chemotaxis to MIP-1beta, a chemokine with selectivity for CCR5 chemokine receptors, while chemotaxis of the more promiscuous ligand RANTES was not inhibited, nor was chemotaxis mediated by SDF-1alpha. In order to determine if peptide T mediated its gp120 antagonistic effects via modulation of CCR5 receptors, RANTES chemotaxis was studied using a CCR5 receptor-transfected HOS cell line. In this case, RANTES chemotaxis was potently inhibited by V2-region-derived short peptides. Peptide T also partially suppressed (125)I-MIP1-beta binding to human monocytes, suggesting action at a subset of MIP1-beta receptors. The V2 region of gp120 thus contains a potent receptor binding domain and synthetic peptides derived from this region modulate CCR5 chemokine receptor chemotactic signaling caused by either gp120 or chemokine ligands. The results have therapeutic implications and may explain recent clinical improvements, in that HIV/gp120 actions at CCR5 receptors, such as occur in the brain or early infection, would be susceptible to peptide T inhibition.

Reciprocal Desensitization of CCR5 and CD4 Is Mediated by IL-16 and Macrophage-Inflammatory Protein-1β, Respectively

The ability of HIV-1 gp120 to inhibit chemokine signaling prompted us to determine whether signaling through CD4 by a natural ligand, IL-16, could alter cellular responsiveness to chemokine stimulation. These studies demonstrate that IL-16/CD4 signaling in T lymphocytes results in a selective loss of macrophage-inflammatory protein (MIP)-1β/CCR5-induced chemotaxis. There was no effect on monocyte chemoattractant protein-2/CCR1, -2, or -3-induced chemotaxis. Desensitization of CCR5 by IL-16 required at least 10 min of pretreatment; no modulation of CCR5 expression was observed, nor was MIP-1β binding to CCR5 altered. Using murine T cell hybridomas transfected to express native or mutated forms of CD4, it was determined that IL-16/CD4 induces a p56lck-dependent signal that results in desensitization of CCR5. The desensitization process is reciprocal and again selective, as prior CCR5 stimulation, but not CCR1, -2, or -3 stimulation, completely inhibits IL-16/CD4-induced T cell migration. Of interest, while p56lck enzymatic activity is not required for IL-16-induced migration, it was required for desensitization of CCR5. These studies indicate the existence of reciprocal receptor cross-desensitization between CD4 and CCR5 induced by two proinflammatory cytokines and suggest a selective relationship between the two receptors.

Identification of Domains in IL-16 Critical for Biological Activity

IL-16 is a proinflammatory cytokine implicated in the pathogenesis of asthma and other conditions characterized by recruitment of CD4+ T cells to sites of disease. It is postulated that CD4 is an IL-16 receptor, although other receptors or coreceptors may exist. Among several known functions, IL-16 is a chemoattractant factor for CD4+ T cells and it inhibits MLR. We previously reported that an oligopeptide corresponding to the 16 C-terminal residues of human IL-16 inhibits chemoattractant activity. To identify functional domains with greater precision, shorter oligonucleotides containing native or mutated C-terminal IL-16 sequences were tested for IL-16 inhibition. Within the 16 C-terminal residues, the minimal peptide RRKS (corresponding to Arg106 to Ser109) was shown to mediate inhibition of IL-16 chemoattractant activity. Inhibition was lost when either arginine was substituted with alanine. Point mutations in IL-16 revealed that Arg107 is critical for chemoattractant activity, but MLR inhibition was unaffected by mutation of Arg107 or even deletion of the C-terminal tail through Arg106. Deletion of 12 or 22 N-terminal residues of IL-16 had no impact on chemoattractant activity, but MLR inhibition was reduced. Deletion of 16 C-terminal plus 12 N-terminal residues abolished both chemoattractant and MLR-inhibitory activity of IL-16. These data indicate that receptor interactions with IL-16 that activate T cell migration are not identical with those required for MLR inhibition, and suggest that both N-terminal and C-terminal domains in IL-16 participate in receptor binding or activation.

Identification of a CD4 domain required for interleukin-16 binding and lymphocyte activation

Interleukin-16 (IL-16) activates CD4(+) cells, possibly by direct interaction with CD4. IL-16 structure and function are highly conserved across species, suggesting similar conservation of a putative IL-16 binding site on CD4. Comparison of the human CD4 amino acid sequence with that of several different species revealed that immunoglobulin-like domain 4 is the most conserved extracellular region. Potential interaction of this domain with IL-16 was studied by testing murine D4 sequence-based oligopeptides for inhibition of IL-16 chemoattractant activity and inhibition of IL-16 binding to CD4 in vitro. Three contiguous 12-residue D4 region peptides (designated A, B, and C) blocked IL-16 chemoattractant activity, with peptide B the most potent. Peptides A and B were synergistic for inhibition, but peptide C was not. Peptides A and B also blocked IL-16 binding to CD4 in vitro, whereas peptide C did not. CD4, in addition to its known function as a receptor for major histocompatibility complex class II, contains a binding site for IL-16 in the D4 domain. The D4 residues required for IL-16 binding overlap those previously shown to participate in CD4-CD4 dimerization following class II major histocompatibility complex binding, providing a mechanistic explanation for the known function of IL-16 to inhibit the mixed lymphocyte reaction.

HIV-1 Nef increases T cell activation in a stimulus-dependent manner

Lentiviral Nef increases viral replication in vivo, plays a direct role in pathogenesis, and increases viral particle infectivity. We now find that HIV Nef also increases the activation of T cells, a cellular state required for optimal viral replication. This enhancement is stimulant-dependent. As defined by IL-2 generation, activation of T cells stimulated with classical mitogens [phorbol 12-myristate 13-acetate (PMA) + anti-CD3, PMA + phytohemagglutinin, and PMA + ionomycin] is unaffected by the expression of Nef. However, Nef increases IL-2 secretion when cells are stimulated through the T cell receptor and the costimulus receptor (CD28). This increase in activation, which depends on Nef myristylation, is caused by an increase in the number of cells reaching full activation and not by an increase in the amount of IL-2 secreted per cell. These findings demonstrate that Nef lowers the threshold of the dual-receptor T cell activation pathway. The capacity of Nef to increase T cell activity may be very important in vivo when Nef is the predominant or the only viral gene product expressed.

Inhibition of Tyrosine Kinase Activation Blocks the Down-Regulation of CXC Chemokine Receptor 4 by HIV-1 gp120 in CD4+ T Cells

Because the binding of HIV-1 envelope to CD4 initiates a configurational change in glycoprotein 120 (gp120), enabling it to interact with fusion coreceptors, we investigated how this process interferes with the expression and function of CXC chemokine receptor 4 (CXCR4) in CD4+ T lymphocytes. A recombinant gp120 (MN), after preincubation with CD4+ T lymphocytes, significantly inhibited the binding and chemotaxis of the cells in response to the CXCR4 ligand stromal cell-derived factor-1α (SDF-1α), accompanied by a markedly reduced surface expression of CXCR4. gp120, but not SDF-1α, induced rapid tyrosine phosphorylation of src-like kinase p56lck in CD4+ T cells, whereas both gp120 and SDF-1α caused phosphorylation of the CXCR4. The tyrosine kinase inhibitor herbimycin A abolished the phosphorylation of p56lck and CXCR4 induced by gp120 in association with maintenance of normal expression of cell surface CXCR4 and a migratory response to SDF-1α. Thus, a CD4-associated signaling molecule(s) including p56lck is activated by gp120 and is required for the down-regulation of CXCR4.

T20/DP178, an Ectodomain Peptide of Human Immunodeficiency Virus Type 1 gp41, Is an Activator of Human Phagocyte N-Formyl Peptide Receptor

Human immunodeficiency virus type 1 (HIV-1) envelope protein gp41 mediates viral fusion with human host cells. The peptide segment T20/DP178, located in the C-terminus of the ectodomain of gp41, interacts with the N-terminal leucine zipper-like domain on gp41 to establish the fusogenic conformation of the virus. Synthetic T20/DP178 peptide is highly efficacious in inhibiting HIV-1 infection in vitro by disrupting the transformation of fusogenic status of viral gp41; thus, it has been proposed for clinical trial. We report that synthetic T20/DP178 is a chemoattractant and activator of human peripheral blood phagocytes but not of T lymphocytes. We further demonstrate that T20/DP178 specifically activates a seven-transmembrane, G-protein–coupled phagocyte receptor for N-formylated chemotactic peptides, formyl peptide receptor (FPR). Moreover, synthetic T20/DP178 analogs lacking N-terminal amino acids acted as FPR antagonists. Our results suggest that gp41 peptides regulate phagocyte function via FPR and identify a novel mechanism by which HIV-1 may modulate innate immunity.

T20/DP178, an Ectodomain Peptide of Human Immunodeficiency Virus Type 1 gp41, Is an Activator of Human Phagocyte N-Formyl Peptide Receptor

Human immunodeficiency virus type 1 (HIV-1) envelope protein gp41 mediates viral fusion with human host cells. The peptide segment T20/DP178, located in the C-terminus of the ectodomain of gp41, interacts with the N-terminal leucine zipper-like domain on gp41 to establish the fusogenic conformation of the virus. Synthetic T20/DP178 peptide is highly efficacious in inhibiting HIV-1 infection in vitro by disrupting the transformation of fusogenic status of viral gp41; thus, it has been proposed for clinical trial. We report that synthetic T20/DP178 is a chemoattractant and activator of human peripheral blood phagocytes but not of T lymphocytes. We further demonstrate that T20/DP178 specifically activates a seven-transmembrane, G-protein–coupled phagocyte receptor for N-formylated chemotactic peptides, formyl peptide receptor (FPR). Moreover, synthetic T20/DP178 analogs lacking N-terminal amino acids acted as FPR antagonists. Our results suggest that gp41 peptides regulate phagocyte function via FPR and identify a novel mechanism by which HIV-1 may modulate innate immunity.

A CD4-Independent Interaction of Human Immunodeficiency Virus-1 gp120 With CXCR4 Induces Their Cointernalization, Cell Signaling, and T-Cell Chemotaxis

The gp120 envelope glycoprotein of human immunodeficiency virus-1 (HIV-1) interacts with the CXCR4 chemokine receptor, but it is not known whether gp120 activates CXCR4-mediated signaling cascades in the same manner as its natural ligand, SDF1. We assessed the effects of wild-type gp120 and a mutant gp120 that interacts with CXCR4 but not CD4 on CD4−/CXCR4+ cells and CD4+/CXCR4+ cells, respectively. Under both experimental conditions, the interaction of CXCR4 and gp120 resulted in their CD4-independent cointernalization. Both molecules were translocated into early endosomes, whereas neither protein could be detected in late endosomes. Binding of gp120 to CXCR4 resulted in a CD4-independent phosphorylation of Pyk2 and an induction of chemotactic activity, demonstrating that this interaction has functional consequences. Interestingly, however, whereas SDF1 activated the ERK/MAP kinase pathway, this cascade was not induced by gp120. Together, these results suggest that the pathology of HIV-1 infection may be modulated by the distinct signal transduction pathway mediated by gp120 upon its interaction with CXCR4.

A CD4-Independent Interaction of Human Immunodeficiency Virus-1 gp120 With CXCR4 Induces Their Cointernalization, Cell Signaling, and T-Cell Chemotaxis

The gp120 envelope glycoprotein of human immunodeficiency virus-1 (HIV-1) interacts with the CXCR4 chemokine receptor, but it is not known whether gp120 activates CXCR4-mediated signaling cascades in the same manner as its natural ligand, SDF1. We assessed the effects of wild-type gp120 and a mutant gp120 that interacts with CXCR4 but not CD4 on CD4−/CXCR4+ cells and CD4+/CXCR4+ cells, respectively. Under both experimental conditions, the interaction of CXCR4 and gp120 resulted in their CD4-independent cointernalization. Both molecules were translocated into early endosomes, whereas neither protein could be detected in late endosomes. Binding of gp120 to CXCR4 resulted in a CD4-independent phosphorylation of Pyk2 and an induction of chemotactic activity, demonstrating that this interaction has functional consequences. Interestingly, however, whereas SDF1 activated the ERK/MAP kinase pathway, this cascade was not induced by gp120. Together, these results suggest that the pathology of HIV-1 infection may be modulated by the distinct signal transduction pathway mediated by gp120 upon its interaction with CXCR4.

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 &gt;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.

TCR Activation Inhibits Chemotaxis Toward Stromal Cell-Derived Factor-1: Evidence for Reciprocal Regulation Between CXCR4 and the TCR

Stromal cell-derived factor-1 (SDF-1), a C-X-C family chemokine, is a potent T lymphocyte chemoattractant. We investigated the effects of T cell activation on the chemotactic response to SDF-1. Anti-CD3 Ab stimulation of either Jurkat T cells or murine peripheral CD4+ T lymphocytes produced a dramatic inhibition of SDF-1-induced chemotaxis. In contrast, the SDF-1 responses of Jurkat clones with deficiencies in key TCR signaling components (Lck, CD45, and TCR-β), were only marginally reduced by anti-CD3 stimulation. Similar to PMA treatment, which abolished both CXCR4 receptor expression and the chemotactic response of Jurkat cells to SDF-1, anti-CD3 Ab treatment reduced cell surface expression of CXCR4 to 65% of the control value, an effect that was blocked by protein kinase C inhibitors. Our data suggest that initial T cell activation events inhibit the response of Jurkat T cells to CXCR4 stimulation. In contrast, SDF-1 treatment resulted in a reduction of tyrosine phosphorylation of the TCR downstream effectors, ZAP-70, SLP-76, and LAT (linker for activation of T cells), suggesting that this chemokine potentially regulates the threshold for T cell activation.

Impaired CD45-associated tyrosine phosphatase activity during HIV-1 infection: implications for CD3 and CD4 receptor signalling

Proper function of the protein tyrosine phosphatase CD45 is required for the positive regulation of the activity of src tyrosine kinases p56lck and p59fyn which participate in T-cell receptor and CD4 receptor signalling. In this study, the effect of HIV-1 infection on the function of CD45-associated tyrosine phosphatase activity in the H9 T-cell line has been investigated with respect to CD3 and CD4 ligation. A significant reduction in CD45-associated phosphatase activity was observed following CD3 + CD4 ligation in virally infected cells, whereas CD45 activity was not compromised following CD3 receptor ligation. Dysfunctional CD45 activity in infected cells was not attributable to reduced receptor surface expression induced by HIV-1, since CD4, CD3 and CD45 expression levels were found to be intact. Defective CD45 activity correlated with inhibted downstream signalling events as evidenced by reduced CD4-associated tyrosine kinase activity and inhibition of PLC-gamma1. Impaired CD45 function is likely to play a critical role in the inhibition of CD3/CD4 signalling thereby contributing to HIV-1 pathogenesis.

HIV-1 Envelope gp120 Inhibits the Monocyte Response to Chemokines Through CD4 Signal-Dependent Chemokine Receptor Down-Regulation

Since HIV-1 infection results in severe immunosuppression, and the envelope protein gp120 has been reported to interact with some of the chemokine receptors on human T lymphocytes, we postulated that gp120 may also affect monocyte activation by a variety of chemokines. This study shows that human peripheral blood monocytes when preincubated with gp120 either purified from laboratory-adapted strains or as recombinant proteins exhibited markedly reduced binding, calcium mobilization, and chemotactic response to chemokines. The gp-120-pretreated monocytes also showed a decreased response to FMLP. This broad inhibition of monocyte activation by chemoattractants required interaction of gp120 with CD4, since the effect of gp120 was only observed in CD4+ monocytes and in HEK 293 cells only if cotransfected with both chemokine receptors and an intact CD4, but not a CD4 lacking its cytoplasmic domain. Anti-CD4 mAbs mimicked the effect of gp120, and both anti-CD4 Ab and gp120 caused internalization of CXCR4 in HEK 293 cells provided they also expressed CD4. Staurosporine blocked the inhibitory effect of gp120 on monocytes, suggesting that cellular signaling was required for gp120 to inhibit the response of CD4+ cells to chemoattractants. Our study demonstrates a broad suppressive effect of gp120 on monocyte activation by chemoattractants through the down-regulation of cell surface receptors. Thus, gp120 may be used by HIV-1 to disarm the monocyte response to inflammatory stimulation.

lck-Independent Inhibition of T Cell Antigen Response by the HIV gp120

Binding of the HIV envelope glycoprotein gp120 to CD4 inhibits T cell activation. We have used a murine T cell clone transfected with either wild-type human CD4 or mutated forms of CD4 to characterize the pathways involved in this inhibitory effect of gp120. Ag-induced proliferation of T cell clones transfected with human CD4 was completely inhibited in the presence of gp120, even though stimulation of this clone is independent of a CD4/MHC class II interaction. In addition, our results demonstrate that the inhibition by gp120 is not due to the sequestration of lck from TCR and does not require activation of lck by gp120. This suggests that CD4 can regulate the initiation of T cell activation independently of its interaction with lck. Moreover, we demonstrate that the nonresponsiveness induced by gp120 can be reversed by soluble CD4 when added early after onset of stimulation and that gp120 exerts its inhibitory effect when cells are in the G0 ≥ 1 phase of the cell cycle.

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.