Human immunodeficiency virus gp120 and derived peptides activate protein tyrosine kinase p56lck in human CD4 T lymphocytes

CD45RO regulates the HIV-1 gp120-mediated apoptosis of T cells by activating Lck

CD45 has been reported to regulate the HIV-1 gp120-induced apoptosis of Jurkat cells. Here, we demonstrate that the extracellular domain of CD45 plays an important role in this function. We observed that CD45RO-transfected cells, but not cells transfected with other CD45 isoforms, underwent significant apoptosis induced by gp120. However, a CD45RA-transfected cell line treated with an O-glycan inhibitor was able to undergo apoptosis. The role of the extracellular domain of CD45 was further confirmed using CD45 isoform-transfected cell lines by analyzing the phosphorylation of Lck, which is a direct substrate of CD45 tyrosine phosphatase, and by using an Lck inhibitor. These results suggest that CD45RO modulates HIV-1 gp120-induced apoptosis by regulating the activity of Lck.

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.

HIV: cell binding and entry

The first step of the human immunodeficiency virus (HIV) replication cycle-binding and entry into the host cell-plays a major role in determining viral tropism and the ability of HIV to degrade the human immune system. HIV uses a complex series of steps to deliver its genome into the host cell cytoplasm while simultaneously evading the host immune response. To infect cells, the HIV protein envelope (Env) binds to the primary cellular receptor CD4 and then to a cellular coreceptor. This sequential binding triggers fusion of the viral and host cell membranes, initiating infection. Revealing the mechanism of HIV entry has profound implications for viral tropism, transmission, pathogenesis, and therapeutic intervention. Here, we provide an overview into the mechanism of HIV entry, provide historical context to key discoveries, discuss recent advances, and speculate on future directions in the field.

The inositol 5-phosphatase SHIP-1 and adaptors Dok-1 and 2 play central roles in CD4-mediated inhibitory signaling

CD4 functions to enhance the sensitivity of T cells to antigenic peptide/MHC class II. However, if aggregated in isolation, e.g. in the absence of T cell receptor (TCR), CD4 can transduce yet undefined signals that lead to T cell unresponsiveness to antigen and apoptosis. In Human Immunodeficiency Virus-1 (HIV-1) disease, CD4(+) T cell loss can result from gp120-induced CD4 signaling in uninfected cells. We show here that CD4 aggregation leads to Lck-dependent phosphorylation of the RasGAP adaptors Downstream of kinase-1/2 (Dok-1/2) and the inositol 5-phosphatase-1 (SHIP-1) and association of the two molecules. Studies using SHIP-1 shRNA, knockout mice and decoy inhibitors further indicate that CD4-mediated inhibition of TCR-mediated T cell activation is SHIP-1 and Dok-1/2 dependent, and involves SHIP-1 hydrolysis of Phosphatidylinositol 3,4,5-trisphosophate (PI(3,4,5)P3) needed for TCR signaling. Our studies provide evidence for a novel mechanism by which ill-timed CD4-mediated signals activated by ligands such as HIV-1 gp120 lead to disarmament of the immune system.

CD4-mediated regulatory T-cell activation inhibits the development of disease in a humanized mouse model of allergic airway disease

BACKGROUND

Based on their potency to control allergic diseases, regulatory T (Treg) cells represent a promising target for novel strategies to interfere with allergic airway inflammation. We have previously demonstrated that stimulation of the CD4 molecule on human Treg cells activates their suppressive activity in vitro and in vivo.

OBJECTIVE

We sought to determine the effect of CD4-mediated Treg-cell activation on pulmonary inflammation in a humanized mouse model of allergic airway inflammation.

METHODS

PBMCs obtained from donors allergic to birch pollen or from healthy donors were injected into NOD-severe combined immunodeficiency γc(-/-) mice, followed by allergen airway challenges and analysis of airway responsiveness and inflammation. For Treg-cell activation, mice were treated with the CD4-binding, lck-activating recombinant HIV-1 surface protein gp120 after sensitization prior to allergen challenge. Control experiments with CD25-depleted PBMCs were performed to evaluate the role of Treg cells.

RESULTS

PBMCs from allergic donors but not from healthy donors induced airway inflammation and airway hyperresponsiveness. Treatment with gp120 prior to allergen challenge abrogated airway hyperresponsiveness and reduced the inflammatory immune response. In contrast, treatment had no effect on inflammation and airway hyperresponsiveness in mice that received CD25-depleted PBMCs, demonstrating Treg-cell dependency of disease prevention.

CONCLUSION

Allergic airway inflammation can be prevented by stimulation of human Treg cells by CD4. These results suggest a clinical potential of Treg-cell activation by high-affinity CD4 ligands in allergic diseases.

HIV-1 binding to CD4 on CD4+CD25+ regulatory T cells enhances their suppressive function and induces them to home to, and accumulate in, peripheral and mucosal lymphoid tissues: an additional mechanism of immunosuppression

The establishment and persistence of many chronic infections have been demonstrated to depend on restraint of the vigor of the anti-microbial immune responses by CD4+CD25+ regulatory T (Treg) cells. In HIV-infected individuals, Treg cells suppress both HIV-specific and general CD4+ and CD8+ T cell responses. Increases of CD4+CD25+ Treg cell function during viral infections might be mediated by host-derived pro-inflammatory molecules or directly by viral infection or binding. We examined the effect HIV has upon binding to CD4+CD25+ Treg cells by exposing human purified CD4+CD25+ T cells from healthy donors to HIV-1 in vitro and assessing their Treg-associated functional marker profile and suppressive activities. We found that HIV-1 binding increased their suppressor activities by 2- to 5-fold, which was accompanied by enhanced expression of Treg-associated functional markers sCTLA-4, glucocorticoid-induced tumor necrosis factor receptor and FoxP3. Moreover, HIV-1 binding extended the survival of CD4+CD25+ Treg cells and up-regulated the expression of homing receptors CD62L and integrin alpha4beta7, which in turn would result in Treg cells migrating more rapidly to the peripheral lymph nodes and mucosal lymphoid tissues where anti-HIV immune responses are occurring. Importantly, CD4+CD25+ Treg cells exposed to HIV were not susceptible to homing-induced apoptosis like are other resting CD4+ cells following HIV-1 binding. We show that CD4+CD25+ Treg cells respond directly to HIV-1 itself through HIV gp120 interactions with CD4 molecules. Collectively, our findings explain a mechanism that contributes to the abnormal accumulation of intensified Treg cells in lymphoid and mucosal tissues in HIV patients, resulting in impairment of immune responses which would greatly help HIV persistence.

p56lck, LFA-1 and PI3K but not SHP-2 interact with GM1- or GM3-enriched microdomains in a CD4-p56lck association-dependent manner

We previously showed that the association of CD4 and G(M3) ganglioside induced by CD4 ligand binding was required for the down-regulation of adhesion and that aggregation of ganglioside-enriched domains was accompanied by transient co-localization of LFA-1 (lymphocyte function-associated antigen-1), PI3K (phosphoinositide 3-kinase) and CD4. We also showed that these proteins co-localized with the G(M1) ganglioside that partially co-localized with G(M3) in these domains. In the present study, we show that CD4-p56(lck) association in CD4 signalling is required for the redistribution of p56(lck), PI3K and LFA-1 in ganglioside-enriched domains, since ganglioside aggregation and recruitment of these proteins were not observed in a T-cell line (A201) expressing the mutant form of CD4 that does not bind p56(lck). In addition, we show that although these proteins associated in different ways with G(M1) and G(M3), all of the associations were dependent on CD4-p56(lck) association. Gangliosides could associate with these proteins that differ in affinity binding and could be modified following CD4 signalling. Our results suggest that through these associations, gangliosides transiently sequestrate these proteins and consequently inhibit LFA-1-dependent adhesion. Furthermore, while structural diversity of gangliosides may allow association with distinct proteins, we show that the tyrosine phosphatase SHP-2 (Src homology 2 domain-containing protein tyrosine phosphatase 2), also required for the down-regulation of LFA-1-dependent adhesion, transiently and partially co-localized with PI3K and p56(lck) in detergent-insoluble membranes without association with G(M1) or G(M3). We propose that CD4 ligation and binding with p56(lck) and their interaction with G(M3) and/or G(M1) gangliosides induce recruitment of distinct proteins important for CD4 signalling to form a multimolecular signalling complex.

HIV-1 gp120 inhibits TLR9-mediated activation and IFN-{alpha} secretion in plasmacytoid dendritic cells

Plasmacytoid dendritic cells (pDCs) play a central role in innate and adaptive immune responses against viral infections. pDCs secrete type I IFNs and proinflammatory cytokines upon stimulation by either TLR7 or TLR9. Throughout the course of HIV infection, the production of type-I IFNs is profoundly impaired, and total pDC cell counts in peripheral blood correlates inversely with viral load and positively with CD4(+) T cell count. The origin of these defects is unclear. pDCs express CD4, CCR5, and CXCR4, the primary receptor and coreceptors, respectively, for the HIV envelope; yet little is known concerning the effects of the viral envelope on these cells. Here, we show that exposure of pDCs to gp120 results in the suppression of activation of these cells. This suppression is specific for TLR9-mediated responses, because TLR7-mediated responses are unaffected by gp120. gp120 also suppressed TLR9-mediated induction of proinflammatory cytokines and expression of CD83, a marker of DC activation. Finally, gp120 suppressed pDC-induced cytolytic activity of natural killer cells. Taken together, these data demonstrate that the direct interaction of HIV-1 gp120 with pDCs interferes with TLR9 activation resulting in a decreased ability of pDCs to secrete antiviral and inflammatory factors that play a central role in initiating host immune responses against invading pathogens.

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.

The role of glycosphingolipids in HIV signaling, entry and pathogenesis

Although HIV uses CD4 and coreceptors (CCR5 and CXCR4) for productive infection of T cells, glycosphingolipids (GSL) may play ancillary roles in lymphoid and non-lymphoid cells. Interactions of the HIV Envelope Glycoprotein (Env) with GSL may help HIV in various steps of its pathogenesis. Physical-chemical aspects of the interactions between HIV Env and GSL leading to CD4-dependent entry into lymphocytes, the role of GSL in HIV transcytosis, and CD4-independent entry into non-lymphoid cells are reviewed. An overview of signaling properties of HIV receptors is provided with some speculation on how GSL may play a role in these events by virtue of being in membrane rafts. Finally, we summarize how interactions between HIV and coreceptors leading to signaling and/or fusion can be analyzed by the use of various tyrosine kinase and cytoskeletal inhibitors.

HIV-1 gp120-induced TNF-{alpha} production by primary human macrophages is mediated by phosphatidylinositol-3 (PI-3) kinase and mitogen-activated protein (MAP) kinase pathways

Human immunodeficiency virus type 1 (HIV-1) infection is initiated by binding of the viral envelope glycoprotein gp120 to CD4 followed by a chemokine receptor, but these interactions may also take place independently from infection. gp120 stimulation of primary human macrophages is known to trigger production of cytokines implicated in pathogenesis, particularly tumor necrosis factor alpha (TNF-alpha), but the mechanisms have not been determined. We sought to define the pathways responsible for TNF-alpha secretion by monocyte-derived macrophages (MDM) following HIV-1 gp120 stimulation. MDM exposure to recombinant macrophage-tropic (R5) gp120 led to dose- and donor-dependent release of TNF-alpha, which was cyclohexamide-sensitive and associated with up-regulated message. Pretreatment with specific inhibitors of the mitogen-activated protein kinases (MAPK) extracellular signal-regulated kinase 1/2 (ERK-1/2; PD98059, U0126) and p38 (SB202190, PD169316) inhibited the secretion of TNF-alpha. gp120-elicited TNF-alpha production was also blocked by phosphatidylinositol-3 kinase (PI-3K) inhibitors (wortmannin, LY294002). Moreover, PI-3K inhibition ablated gp120-induced phosphorylation of p38 and ERK-1/2. The response was inhibited by a CC chemokine receptor 5 (CCR5)-specific antagonist, indicating that CCR5 was in large part responsible. These results indicate that gp120-elicited TNF-alpha production by macrophages involves chemokine receptor-mediated PI-3K and MAPK activation, that PI-3K is an upstream regulator of MAPK in this pathway, and that p38 and ERK-1/2 independently regulate TNF-alpha production. These gp120-triggered signaling pathways may be responsible for inappropriate production of proinflammatory cytokines by macrophages, which are believed to play a role in immunopathogenesis and in neurological sequelae of AIDS.

CD95/phosphorylated ezrin association underlies HIV-1 GP120/IL-2-induced susceptibility to CD95(APO-1/Fas)-mediated apoptosis of human resting CD4+T lymphocytes

CD95(APO-1/Fas)-mediated apoptosis of bystander uninfected T cells exerts a major role in the HIV-1-mediated CD4+ T-cell depletion. HIV-1 gp120 has a key role in the induction of sensitivity of human lymphocytes to CD95-mediated apoptosis through its interaction with the CD4 receptor. Recently, we have shown the importance of CD95/ezrin/actin association in CD95-mediated apoptosis. In this study, we explored the hypothesis that the gp120-mediated CD4 engagement could be involved in the induction of susceptibility of primary human T lymphocytes to CD95-mediated apoptosis through ezrin phosphorylation and ezrin-to-CD95 association. Here, we show that gp120/IL-2 combined stimuli, as well as the direct CD4 triggering, on human primary CD4(+)T lymphocytes induced an early and stable ezrin activation through phosphorylation, consistent with the induction of ezrin/CD95 association and susceptibility to CD95-mediated apoptosis. Our results provide a new mechanism through which HIV-1-gp120 may predispose resting CD4(+)T cell to bystander CD95-mediated apoptosis and support the key role of ezrin/CD95 linkage in regulating susceptibility to CD95-mediated apoptosis.

The HIV Env-mediated fusion reaction

The current general model of HIV viral entry involves the binding of the trimeric viral envelope glycoprotein gp120/gp41 to cell surface receptor CD4 and chemokine co-receptor CXCR4 or CCR5, which triggers conformational changes in the envelope proteins. Gp120 then dissociates from gp41, allowing for the fusion peptide to be inserted into the target membrane and the pre-hairpin configuration of the ectodomain to form. The C-terminal heptad repeat region and the leucine/isoleucine zipper region then form the thermostable six-helix coiled-coil, which drives the membrane merger and eventual fusion. This model needs updating, as there has been a wealth of data produced in the last few years concerning HIV entry, including target cell dependencies, fusion kinetic data, and conformational intermediates. A more complete model must include the involvement of membrane microdomains, actin polymerization, glycosphingolipids, and possibly CD4 and chemokine signaling in entry. In addition, kinetic experiments involving the addition of fusion inhibitors have revealed some of the rate-limiting steps in this process, adding a temporal component to the model. A review of these data that may require an updated version of the original model is presented here.

HIV receptors on lymphocytes

Human immunodeficiency virus infection, despite tremendous efforts in research and prevention, is spreading at an increasing speed, especially in developing countries. Currently available therapeutic approaches significantly extend the lifespan of HIV-infected people, but their use is associated with a severe drug regimen, several undesirable side effects, and high cost. Therefore, the scientific community is steadfastly pursuing novel strategies for inhibiting viral replication, promoting a better immune response, and developing an effective vaccine. Recent research on HIV receptors has introduced new concepts in the field, showing that expression of receptors, although necessary for virus entry, is subordinate to quality of expression, so that efficient infection occurs when receptors are properly colocalized. In addition, intracellular signaling triggered by HIV receptors has been shown to play important roles in pathogenesis by inducing apoptosis of bystander cells. Induction of some pathways of intracellular signaling, however, can instead suppress HIV replication, so that modulation of these pathways constitutes an additional target to be exploited for therapy or vaccines. This article reviews the most exciting aspects of these novel findings and discusses their practical application in the fight against HIV infection.

HIV-1 infection is facilitated in T cells by decreasing p56lck protein tyrosine kinase activity

Several studies have suggested an important role for the protein tyrosine kinase p56lck (Lck) in HIV infection; however, the exact nature of this role remains unclear. Using a series of well characterized Jurkat-derived cell lines having a wide range of Lck kinase activity, our results showed that, while the entry of HIV-1 into these cell lines was similar, the kinetics of virus production by these cells were very different. Cells expressing a kinase-inactive Lck showed accelerated viral replication, whereas, cells expressing Lck with normal or elevated enzymatic activity showed a delay in virus replication that was proportional to the initial level of endogenous Lck activity. The cell line having the highest initial Lck kinase activity showed the slowest rate of productive HIV-1 infection. Analysis of 2-LTR circles revealed that this inhibitory effect of Lck was not due to inhibition of reverse transcription of HIV-1 genome or migration of the proviral DNA into the nuclei. This affect of Lck was confirmed in additional studies that used either the S1T cell line lacking completely Lck or where the Lck activity was altered in Jurkat cells prior to infection. S1T cells showed a 3- to 12-fold increase in the level of infection compared to Jurkat cells despite similar CD4 and chemokine coreceptor expression and cell doubling times. Pretreatment of Jurkat with an antisense lck oligodeoxynucleotide inhibited the synthesis of functional Lck and facilitated the viral replication by the cells as did expressing a dominant-negative mutant Lck which increased the productive infection>3-fold. Conversely, whereas IL-16 had no affect on productive infection in S1T cells that lack Lck, IL-16 pretreatment of Jurkat cells resulted in an immediate (within 5 min) and sustained and gradual (over 5 h) increase in Lck activity that resulted in a reduction of HIV-1 replication that paralleled the increasing Lck kinase activity. These results show that the enzymatic activity of Lck kinase can affect viral replication, that a lack of, or decreased Lck activity facilitates viral replication. Conversely, Lck can mediate a delay in HIV-1 infection that is proportional to the initial endogenous Lck enzyme activity.

The lectin jacalin induces phosphorylation of ERK and JNK in CD4+ T cells

The CD4 molecule plays an essential role in mediating the transduction of intracellular signals by functioning as a coreceptor for the complex T cell receptor/CD3 and also acts as the primary receptor for human immunodeficiency virus (HIV). Several authors have shown evidence that jacalin, a plant lectin, binds to CD4 and inhibits in vitro HIV infection. We analyzed jacalin-induced intracellular signaling events in CD4(+) T cells and have shown that cell activation resulted in tyrosine phosphorylation of intracellular substrates p56(lck), p59(fyn), ZAP-70, p95 (vav), phospholipase C-gamma1, and ras activation, as assessed by conversion of ras guanosine 5'-diphosphate to ras guanosine 5'-triphosphate. We further examined extracellular regulated kinase (ERK) and c-jun NH(2)-terminal kinase (JNK) phosphorylation following stimulation with jacalin. The data indicate that the kinetics of JNK phosphorylation is delayed. Optimum phosphorylation of ERK2 was observed by 10 min, and that of JNK was observed by 30 min. Pretreatment with gp120 followed by stimulation with jacalin resulted in marked inhibition of all of the aforementioned intracellular events. The data presented here provide insight into the intracellular signaling events associated with the CD4 molecule-jacalin-gp120 interactions and HIV-induced CD4(+) T cell anergy. Jacalin may be used as a possible tool for the study of CD4-mediated signal transduction and HIV-impaired CD4(+) T cell activation.

gp120-mediated induction of the MAPK cascade is dependent on the activation state of CD4(+) lymphocytes

The capacity of the HIV-1 envelope glycoprotein gp120 to induce intracellular signals is thought to contribute to HIV-1 pathogenesis. Here, we report that gp120 binding resulted in activation of the mitogen-activated protein kinase (MAPK) in CD4(+) lymphocytes prestimulated through their T-cell receptor (TCR). However, gp120 did not activate this pathway in either freshly isolated quiescent T cells or nonproliferating CD4(+) lymphocytes prestimulated with the interleukin-7 (IL-7) cytokine. This response was not solely dependent on proliferation per se because proliferating IL-7-prestimulated umbilical cord (UC)-derived T lymphocytes did not exhibit significant MAPK activation upon gp120 binding. Nevertheless, like peripheral blood lymphocytes, MAPK recruitment was induced by gp120 in UC T cells following TCR prestimulation. The lack of a gp120-mediated signaling response was not due to decreased gp120 receptor levels; CD4 expression was modified neither by IL-7 nor by TCR engagement, and high levels of functional CXCR4 were present on IL-7-treated lymphocytes. In addition to CD4 and CXCR4, recent evidence suggests that glycosphingolipids in raft microdomains serve as cofactors for HIV-1 fusion. The ganglioside GM1, a marker of rafts, was augmented in TCR-stimulated but not IL-7-stimulated T lymphocytes, and disruption of rafts inhibited gp120-induced signaling. Thus, stimulation of a mitogenic pathway by gp120 appears to require receptor binding in the context of membrane microdomains. These studies reveal a mechanism via which gp120 may differentially modulate the fate of activated and quiescent T cells in vivo.

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.

Inhibition of the calcium release-activated calcium (CRAC) current in Jurkat T cells by the HIV-1 envelope protein gp160

The HIV-1 envelope glycoprotein gp120/160 has pleiotropic effects on T cell function. We investigated whether Ca(2+) signaling, a crucial step for T cell activation, was altered by prolonged exposure of Jurkat T cells to gp160. Microfluorometric measurements showed that Jurkat cells incubated with gp160 had smaller (approximately 40%) increases in [Ca(2+)](i) in response to phytohemagglutinin and had a reduced Ca(2+) influx (approximately 25%). gp160 had similar effects on Jurkat cells challenged with thapsigargin. We used the patch clamp technique to record the Ca(2+) current, which is responsible for Ca(2+) influx and has properties of the calcium release-activated Ca(2+) current (I(CRAC)). gp160 reduced I(CRAC) by approximately 40%. The inhibitory effects of gp160 were antagonized by staurosporine (0.1 microm), an inhibitor of protein-tyrosine kinases and protein kinase Cs (PKCs), and by Gö 6976 (5 microm), an inhibitor acting especially on PKC alpha and PKC beta I. 12-O-Tetradecanoyl phorbol 13-acetate (16 nm), a PKC activator, reproduced the effects of gp160 in untreated cells. A Western blotting analysis of PKC isoforms alpha, beta I, delta, and zeta showed that only the cellular distribution of PKC alpha and -beta I were significantly modified by gp160. In addition, gp160 was able to modify the subcellular distribution of PKC alpha and PKC beta I caused by phytohemagglutinin. Therefore the reduction in I(CRAC) caused by prolonged incubation with gp160 is probably mediated by PKC alpha or -beta I.

Lessons from similarities between SLE and HIV infection

OBJECTIVE

We attempted to obtain deeper understanding of systemic lupus erythematosus (SLE) and human immunodeficiency virus (HIV) infection through comparative studies between both diseases.

METHOD

For this purpose, we reviewed and discussed lessons from similarities in both diseases based on our own and reported findings in literatures.

RESULT

Such comparative studies may contribute to the progress in understanding the clinical or pathogenetic features of these diseases.

CONCLUSION

Further studies into the relationship between SLE and HIV infection may bring to light important clues to assist in the development of better treatments for each disease.

The role of the CD4 receptor versus HIV coreceptors in envelope-mediated apoptosis in peripheral blood mononuclear cells

We examined the role of CD4, CXCR4, and CCR5 in HIV envelope-mediated apoptosis by measuring the response of activated PBMCs to recombinant envelope proteins derived from CXCR4- and CCR5-utilizing viruses. Apoptosis of T cells was assessed by annexin-V staining and TdT-mediated dUTP-biotin nick-end labeling. Treatment of CCR5Delta32 homozygote PBMCs with a CCR5-specific envelope induced apoptosis in T cells, demonstrating that envelope--CD4 interactions are sufficient to induce apoptosis. However, a CXCR4-specific envelope induced higher levels of apoptosis than a CCR5-specific envelope, suggesting that envelope-mediated apoptosis can be enhanced by envelope--CXCR4 interactions. We conclude that envelope can induce apoptosis in T cells independently of the coreceptor specificity of a given envelope, or the expression profile of CXCR4 or CCR5 on a target cell. However, envelope--coreceptor interactions, and in particular, envelope--CXCR4 interactions, can contribute to this process.

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.

Isopentenyl pyrophosphate, a mycobacterial non-peptidic antigen, triggers delayed and highly sustained signaling in human gamma delta T lymphocytes without inducing eown-modulation of T cell antigen receptor

The Vgamma9Vdelta2 T cell subset, which represents up to 90% of the circulating gammadelta T cells in humans, was shown to be activated, via the T cell receptor (TcR), by non-peptidic phosphorylated small organic molecules. These phosphoantigens, which are not presented by professional antigen-presenting cells, induce production of high amounts of interferon-gamma and tumor necrosis factor (TNF-alpha). To date, the specific signals triggered by these antigens have not been characterized. Here we analyze proximal and later intracellular signals triggered by isopentenyl pyrophosphate (IPP), a mycobacterial antigen that specifically stimulates Vgamma9Vdelta2 T cells, and compare these to signals induced by the non-physiological model using an anti-CD3 antibody. During antigenic stimulation we noticed that, except for the proximal p56(lck) signal, which is triggered early, the signals appear to be delayed and highly sustained. This delay, which likely accounts for the delay observed in TNF-alpha production, is discussed in terms of the ability of the antigen to cross-link and recruit transducing molecules mostly anchored to lipid rafts. Moreover, we demonstrate that, in contrast to anti-CD3 antibody, IPP does not induce down-modulation of the TcR.CD3 complex, which likely results in the highly sustained signaling and release of high levels of TNF-alpha.

Use of human CD4 transgenic mice for studying immunogenicity of HIV-1 envelope protein gp120

HIV-1 envelope protein, gp120, is a major immunogenic protein of the AIDS virus. A specific feature of this protein is its interaction with the receptor protein, human CD4, an important component of the immune system. This interaction might affect the immunogenic properties of the gp 120 and modulate the immune response towards HIV. To test this hypothesis we used human CD4-transgenic mice for immunization with gp120. The dynamics of the immune response towards gp120, CD4 and other proteins was followed. The results show that the primary immune response to gp120 (two weeks) developed somewhat faster in CD4-transgenic mice versus non-transgenic mice. Both animals, however, ultimately mounted the same level of response over time. The primary immune response to gp120 when complexed with soluble CD4 before the immunization, developed similarly in both groups. The secondary immune response was earlier and markedly stronger in non-transgenic mice compared with the transgenic mice where a less efficient memory response to gp120 was observed. The ability of gp120 to directly interact with CD4+ helper lymphocytes appears to affect the humoral response towards this antigen. Moreover, these effects illustrate how viral modulation of these cells may in turn lead to potentially different states of immunological equilibrium.

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.

TNF-alpha-induced secretion of C-C chemokines modulates C-C chemokine receptor 5 expression on peripheral blood lymphocytes

Peripheral blood lymphocytes express CCR5, a chemokine receptor for immune cell migration and calcium signaling that serves as an important coreceptor for the HIV. After in vitro stimulation, CCR5 expression is dramatically increased on mature T lymphocytes, especially on the CD45RO+ memory subset. In this study, we report that TNF-alpha delays the surface expression of CCR5 on PBLs after activation and diminishes CCR5 irrespective of its initial level. Functional loss of CCR5 is reflected in a decreased capability of the treated cells to migrate and signal calcium after MIP-1beta stimulation. The effect is mediated via the p80 type II TNF receptor (TNFR2), which induces NF-kappaB among other factors, leading to an enhanced secretion of the chemokines macrophage-inflammatory protein-1alpha, macrophage-inflammatory protein-1beta, and RANTES. Expression of these chemokines directly down-regulates CCR5. These findings reveal a new regulatory mechanism utilized by activated peripheral T cells to modulate their chemotaxis and potentially other functions mediated by CCR5, including the infection of T lymphocytes by macrophage-tropic HIV strains.

Role of the CD1a molecule in the superantigen-induced activation of MHC class II negative human thymocytes

Bacterial superantigens (Sag) are potent activators of T cells. This T-cell activation has been described as an MHC class II dependent phenomenon. We have observed that human thymocytes depleted of MHC class II positive cells are still able to proliferate in response to the staphylococcal enterotoxin A (SEA). This proliferation was clearly inhibited by the addition of monoclonal antibodies directed against the CD1a molecule. In contrast, monoclonal antibodies directed against the CD1b and CD1c molecules have no effect on the Sag-induced activation of the CD2 (+) MHC class II (-) thymocytes. We next examined the ability of the CD1a molecule to transmit transmembrane signals. Results obtained indicate that CD1a ligation on these thymocytes induced tyrosine phosphorylation of the p56(lck) tyrosine kinase. Signal transduction via CD1a is further confirmed by the observation of a significant intracellular calcium flux (Ca(i)(++)) in thymocytes following CD1a engagement. These data demonstrate that CD1a ligation induces a signal transduction pathway which has a potential role in the bacterial superantigen-induced activation of human CD2 (+) MHC class II (-) thymocytes.

HIV type 1-induced inhibition of CD45 tyrosine phosphatase activity correlates with disease progression and apoptosis, but not with anti-CD3-induced T cell proliferation

The tyrosine phosphatase CD45 is a key positive element in multiple lymphocyte signaling pathways. To understand the contribution of CD45 to HIV-1-induced T cell hyporesponsiveness and apoptosis we evaluated the CD45-associated tyrosine phosphatase activity of lymphocytes from patients with different stages of HIV-1 disease and compared it with CD45 expression, spontaneous and Fas-induced apoptosis, anti-CD3-induced T cell proliferation, distribution of CCR5 delta32/wt, and cytokine production. The proliferative response to anti-CD3 as well as the CD45-associated phosphatase activity were significantly reduced in progressors. In long-term nonprogressors (LTNPs) the proliferative response to anti-CD3 was also diminished, although to a lesser extent, while the tyrosine phosphatase activity was not significantly impaired. One-third of LTNPs were found positive for the 32-bp deletion of the CCR5 gene. This mutation had no effects on anti-CD3 proliferative response or CD45 phosphatase activity. A significant reduction in IL-2 and IFN-gamma was observed in both LTNPs and in normal progressors, whereas IL-4 production was significantly decreased only in progressors. Last, we observed a significant correlation between CD45 phosphatase activity and apoptosis. We therefore conclude that the impairment of CD45 tyrosine phosphatase activity correlates with disease progression and the level of T cell apoptosis, but not with anti-CD3-induced T cell proliferation. Moreover, we suggest that evaluation of CD45 tyrosine phosphatase activity may represent an additional tool with which to assess disease progression.

HIV-1 envelope induces activation of caspase-3 and cleavage of focal adhesion kinase in primary human CD4(+) T cells

Binding of HIV type 1 (HIV-1) envelope glycoproteins to the surface of a CD4(+) T cell transduces intracellular signals through the primary envelope receptor, CD4, and a coreceptor, either CCR5 or CXCR4. Furthermore, envelope-CD4(+) cell interactions increase rates of apoptosis in peripheral blood mononuclear cells (PBMCs). We demonstrate that in primary T lymphocytes, recombinant HIV-1 envelope proteins induce the activation of caspase-3 and caspase-6, which belong to a family of cysteine proteases that, upon activation, promote programmed cell death. Envelope-mediated activation of caspase-3 and caspase-6 depended on envelope-CD4 receptor interactions; CCR5-utilizing as well as CXCR4-utilizing envelopes elicited this response. Focal adhesion kinase (FAK) is a substrate of both caspase-3 and caspase-6, and inactivation of FAK by these caspases promotes apoptosis. En-velope treatment of lymphocytes led to the cleavage of FAK in a manner consistent with caspase-mediated cleavage.

Induction of Phosphorylation and Intracellular Association of CC Chemokine Receptor 5 and Focal Adhesion Kinase in Primary Human CD4+ T Cells by Macrophage-Tropic HIV Envelope

Binding of HIV-1 envelope glycoproteins to the surface of a CD4+ cell transduces intracellular signals through the primary envelope receptor, CD4, and/or the envelope coreceptor, a seven-transmembrane chemokine receptor. Macrophage-tropic strains of HIV-1 preferentially use CCR5 as an entry coreceptor, whereas T cell-tropic strains use CXC chemokine receptor-4 for entry. Intracellular signals transduced by HIV-1 envelope may have immunopathogenic consequences, including anergy, syncytium formation, apoptosis, and inappropriate cell trafficking. We demonstrate here that a recombinant envelope protein derived from an M-tropic isolate of HIV-1 can transduce CD4-dependent as well as CCR5-dependent intracellular signals in primary human CD4+ T cells. Novel HIV-induced intracellular signals that were identified include tyrosine phosphorylation of focal adhesion kinase (FAK) and CCR5, which are involved in cell adhesion and chemotaxis, respectively. HIV envelope-induced cellular association of FAK and CCR5 was also demonstrated, suggesting that ligation of CD4 and CCR5 leads to the formation of an activation complex composed of FAK and CCR5. Activation of this signaling pathway by HIV-1 envelope may be an important pathogenic mechanism of dysregulated cellular activation and trafficking during HIV infection.

Human immunodeficiency virus-1 infection requires pertussis toxin sensitive G-protein-coupled signalling and mediates cAMP downregulation

The human immunodeficiency virus-1 (HIV-1) utilises CD4 and certain beta-chemokine receptors, mainly CCR-5 and CXCR4, for attachment and virus entry into T-lymphocytes and monocytes/macrophages. CD4 and beta-chemokine receptors participate in intracellular signalling via protein tyrosine kinases and G-protein-coupled signalling. The factors which influence HIV-1 replication and the intracellular signalling mechanisms elicited by the virus are not well understood. In this study, it was demonstrated that exposure of peripheral blood lymphocytes (PBLs) to a T-cell tropic strain of HIV-1 evokes signal(s) which results in downregulation of intracellular cAMP. In addition, pre-incubation of PBLs with the Gi-protein inhibitor Pertussis toxin mediated a significant inhibition of HIV-1 replication. These data strongly suggest that HIV-1 employs CD4 receptors and Gi-coupled proteins for entry into target cells and that productive HIV-1 infection is dependent on an active signalling event.

Tyrosine phosphorylation induced by C4 peptide constructs from HIV Gp120

Treatment of HUT78 cells with CD4-binding peptide constructs derived from the C4 domain of HIV-1 gp120 results in autophosphorylation of a src-related kinase, p56lck. This leads to p56lck activation and the subsequent phosphorylation of tyrosine residues in several intracellular proteins. The phosphorylation is specific to the C4 peptides as no new phosphorylation occurs when the cells are treated with control peptides or polymers. The induction of tyrosine phosphorylation by the C4 peptide constructs depends on the capability of the peptide to assume a helical conformation because similar peptide constructs that were not able to form helices did not induce cellular tyrosine phosphorylation.

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.

The protein tyrosine kinase p60c-Src is not implicated in the pathogenesis of the human autosomal recessive form of osteopetrosis: a study of 13 children

Osteopetrosis has been described in mice generated by homozygous gene disruption of c-src gene encoding for the p60c-Src protein tyrosine kinase (Src-/- mice). The similarities of bone histologic findings in this murine model to those observed in some patients first seen with autosomal recessive osteopetrosis, "malignant" osteopetrosis, led us to investigate the potential role of p60c-Src in the pathogenesis of malignant osteopetrosis in 13 children. In 4 patients a c-src mutation was ruled out by an intragenic microsatellite segregation study. In the other 9 we analyzed p60c-Src expression and function, as well as c-src sequence. The expression was normal in all of the patients tested. In addition, the tyrosine phosphorylation and kinase activity of p60c-Src were also normal in all of the patients. Moreover, in these patients, sequences of the coding region of c-src were identical to the published sequence of the human c-src complementary DNA. These results exclude a role for c-src in the pathogenesis of human malignant osteopetrosis in the 13 patients analyzed.

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.

Cytokines and chemokines in HIV infection: implications for therapy

HIV infection is associated with both a hyperactivity of the immune system and decreased immune responses against specific antigens. A similar pattern is observed when considering cytokine production in HIV-infected patients. Several cytokines are spontaneously produced at an increased level, whereas other cytokines playing an important role during cell-mediated immune responses are produced at a low level following stimulation. This deregulation of cytokine production may participate to the immune deficiency, both by impairing immune responses and by accelerating CD4+ T lymphocyte destruction. Chemokine receptors have recently been shown to function as coreceptors for the virus, and to govern its cellular tropism. Heterogeneous expression of chemokine receptor may contribute to differences in infectability as well as in rate of progression of the disease between individuals. Better understanding of the role of cytokines and chemokines in HIV infection suggests new therapeutic approaches where administration of cytokines or cytokine antagonists may allow the immune system to function in better conditions, to stimulate antiviral and antiinfectious immune defenses, and to limit viral spread.

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.

Down-modulation of the ZAP-70 protein tyrosine kinase in macaque T lymphocytes infected with SIVsmmPBj14

The simian immunodeficiency virus SIV-PBj14 is the most virulent primate lentivirus identified to date. Other SIV strains, including the parental SIVsmm9, require mitogen-activated peripheral blood mononuclear cells (PBMC) for replication in vitro; however, SIV-PBj14 replicates in quiescent pig-tailed macaque PBMC and induces cellular proliferation, consistent with its in vivo pathogenesis. To identify mechanisms involved in SIV-PBj14-induced T-cell proliferation, kinases important in early T-cell receptor-mediated activation pathways were studied. Immunoblot analyses showed that ZAP-70 protein, a tyrosine kinase, was downregulated, primarily in CD8+ T cells, as early as 30 minutes after in vitro infection of quiescent macaque PBMC with SIV-PBj 14. Furthermore, this downregulation required the presence of either CD4+ T cells or adherent cells or both cell populations. In agreement with the in vitro results, ZAP-70 expression was downregulated in macaque PBMC, spleen, and rectal lymph node cells as early as 2 days after rectal inoculation of pig-tailed macaques with SIV-PBj14. This phenomenon, however, was not observed in cells obtained from distal lymph nodes to which the virus had not disseminated, implying that the presence of SIV-PBj14 is necessary to induce downregulation of ZAP-70.

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.

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.

The Raf-1/mitogen-activated protein kinase kinase-1/extracellular signal-regulated-2 signaling pathway as prerequisite for interleukin-2 gene transcription in lectin-stimulated human primary T lymphocytes

It has been shown that stimulation of lymphoid cells causes the activation of the extracellular signal-regulated-2 (ERK-2) which activates nuclear factor of activated T cells (NF-AT), a transcription factor involved in the regulation of interleukin-2 (1L2) gene transcription. ERK-2 is activated via a kinase cascade initiated by activation of the G protein p21Ras followed by phosphorylation and activation of Raf-1 and mitogen-activated protein kinase kinase-1 (MEK-1). Activation of this pathway has been described primarily in human T cell lines; however, using primary T lymphocytes from transgenic mice, a recent study has shown that a blockade of this cascade did not perturb lymphocyte stimulation and proliferation. In the present paper, we studied in human primary T cells the possible involvement of the Raf-1/MEK-1/ERK-2 pathway upon stimulation by jacalin, a mitogenic lectin which specifically stimulates CD4+ lymphocytes. We show here that the mitogen-activated protein (MAP) kinase pathway was stimulated in human purified lymphocytes upon activation with jacalin. Moreover, activation of this pathway appeared to be essential, since its blockade by a specific inhibitor of the MEK-1 kinase abolished IL2 gene transcription; in contrast, in T cells stimulated with phytohemagglutinin M(PHA), another potent T cell mitogenic lectin, blockade of MEK-1 reduced but did not totally inhibit either ERK-2 phosphorylation or IL2 mRNA expression. This shows, as already suggested, that another pathway in addition to the Raf-1/MEK-1/ERK-2 kinase cascade could be triggered in T cell activation. Jacalin stimulation therefore appeared to be a good model for the specific activation of the MAP kinase pathway in human primary T lymphocytes, which would allow the characterisation of drugs specifically targeted to this particular pathway.

Regulation of the serine-base exchange enzyme system by CD4: effects of monoclonal antibodies, jacalin, interleukin 16 and the HIV membrane protein gp120

Phosphatidylserine (PtdSer) is synthesized by an exchange of the polar head group of phospholipids for a serine residue. The enzyme responsible for this reaction, the serine-base exchange enzyme system (serine-BEES) is inhibited during lymphocyte activation. We show here that triggering the CD4 cell surface molecule in several CD4+ T-cell lines regulates the serine-BEES activity, thus resulting in marked changes in PtdSer synthesis. CD4 ligands able to generate an activating signal in T-cells such as the lectin jacalin, down-regulate the synthesis of PtdSer. In contrast, monoclonal antibodies (mAbs) directed against the CD4 molecule, such as IOT4 and IOT4a, which have previously been described as generating an inhibitory signal to T-cells, induced an up-regulation of the serine-BEES and impaired CD3-induced inhibition of PtdSer synthesis. Similarly, the HIV-gp120 envelope glycoprotein, in both soluble and cross-linked forms, induces an increase in PtdSer synthesis. The protein tyrosine kinase p56lck participates in the regulation of serine-BEES activity because the effect of CD4 mAbs was additive to that of amino-hydroxyflavone, an inhibitor of p56lck. Also, CD4 mAbs were inactive in J Cam 1.6 cells or when the CD3 signals were bypassed by using thapsigargin. These results demonstrate that the CD4 surface molecule can transmit both activating and inhibiting intracellular signals depending on the CD4 ligand used. We suggest that PtdSer synthesis would be one of the intracellular signals that could explain the opposite effects of different CD4 ligands on T-cells.

In vitro selection of HIV-1 resistant to an anti-CD4 monoclonal antibody that inhibits virus transcription

Phase I studies using monoclonal antibodies (mAbs) that bind to the Ig-CDR3-like loop in domain 1 of CD4 (e.g., 13B8-2 mAb) have already been documented for HIV-1-infected patients. In vitro, such mAbs do not inhibit virus to cell fusion but are able to inhibit virus envelope-mediated syncytia formation. Moreover, these mAbs inhibit Tat-induced activation of HIV-1 promoter and HIV-1 transcription in infected CD4+ cells. Here, we report the selection of escaped mutant virus or viruses derived from HIV-1Lai capable of replicating in vitro in the presence of concentrations of 13B8-2 mAb, that usually inhibit HIV-1Lai particle production. The escaped mutant virus or viruses, termed HIV-1Lai13EM, kept the major enzymatic restriction sites found in HIV-1Lai and remained sensitive to anti-CD4 mAb-, soluble CD4-, and recombinant gp120-mediated inhibition of syncytia formation. Possible genetic changes affecting the tat gene or the 5' long terminal repeat (LTR) were investigated. Partial sequence analysis of HIV-1Lai13EM and a control HIV-1Lai grown for 85 days in CEM cells, demonstrated that the first tat exon of these two viruses encoded identical proteins. Although a point mutation G>A was frequently encountered (6 of 13 sequences) in the LTRs of HIV-1Lai13EM at position -188 within the negative regulatory element (NRE), this mutation did not confer the escape mutant phenotype. Our study indicates that the mutant phenotype probably requires genetic changes in a region or regions outside the LTRs.

A major human immunodeficiency virus type 1-initiated killing pathway distinct from apoptosis

We have investigated the relative contribution of apoptosis or programmed cell death (PCD) to cell killing during acute infection with T-cell-tropic, cytopathic human immunodeficiency virus type 1 (HIV-1), by employing diverse strategies to inhibit PCD or to detect its common end-stage sequelae. When Bcl-2-transfected cell lines were infected with HIV-1, their viability was only slightly higher than that of control infections. Although the adenovirus E1B 19-kDa protein has been reported to be a stronger competitor of apoptosis than Bcl-2, it did not inhibit HIV-mediated cell death better than Bcl-2 protein. Competition for Fas ligand or inactivation of the Fas pathway secondary to intracellular mutation (MOLT-4 T cells) also had modest effects on overall cell death during acute HIV infection. In contrast to these observations with HIV infection or with HIV envelope-initiated cell death, Tat-expressing cell lines were much more susceptible (200% enhancement) to Fas-induced apoptosis than controls and Bcl-2 overexpression strongly (75%) inhibited this apoptotic T-cell death. PCD associated with FasR ligation resulted in the cleavage of common interleukin-1beta-converting enzyme (ICE)-protease targets, poly(ADP-ribose) polymerase (PARP) and pro-ICE, whereas cleaved products were not readily detected during HIV infection of peripheral blood mononuclear cells or T-cell lines even during periods of extensive cell death. These results indicate that one important form of HIV-mediated cell killing proceeds by a pathway that lacks the characteristics of T-cell apoptosis. Our observations support the conclusion that at least two HIV genes (env and tat) can kill T cells by distinct pathways and that an envelope-initiated process of T-cell death can be discriminated from apoptosis by many of the properties most closely associated with apoptotic cell death.

Increased Enzymatic Activity of the T-Cell Antigen Receptor-Associated Fyn Protein Tyrosine Kinase in Asymptomatic Patients Infected With the Human Immunodeficiency Virus

The immune system of patients infected with human immunodeficiency virus (HIV) is in a state of chronic activation; however, the nature of HIV-related immune activation is unknown. As normal T-cell activation involves early tyrosine phosphorylation induced by the T-cell antigen receptor-associated src-family protein tyrosine kinase p59fyn(T) (Fyn), we examined a potential role for this kinase in HIV-related immune dysfunction. We determined the relative specific kinase activity of Fyn in lysates of peripheral blood mononuclear cells from 47 normal control individuals tested negative for HIV-1 and -2, human T-cell lymphotropic virus Type I, hepatitis B virus (HBV), hepatitis C virus (HCV), and syphilis; 14 asymptomatic HIV-infected patients having near-normal CD4+ T-cell counts (350 to 980 CD4+ cells/μL); 4 patients with symptomatic acquired immunodeficiency syndrome (AIDS) (<30 CD4+ cells/μL); 13 patients having chronic infection with HBV (6 patients) or HCV (7 patients); and 6 patients with systemic lupus erythematosis (SLE). All patients with asymptomatic HIV disease were shown to have a profound increase (mean increase of 19-fold; range threefold to 56-fold increase; p = 1.33 × 10−9) in the relative specific kinase activity of Fyn compared to uninfected controls or patients with hepatitis or SLE. In contrast, patients with AIDS had an Fyn-specific kinase activity that was much less affected (mean increase of threefold; range onefold to sevenfold increase; p = 1.30 × 10−5). It was further shown that HIV infection affects the Fyn-specific kinase activity in CD8+-enriched cells, suggesting abnormal Fyn activity in both CD8+ as well as CD4+ T lymphocytes. Initial results implicate a role for the CSK protein tyrosine kinase as responsible for the abnormal Fyn kinase activity observed in HIV-infected patients. These data indicate early and chronic activation of Fyn as a unique HIV-related effect that has the potential to be diagnostic for early HIV infection and/or may serve as a prognostic indicator for advancement to full-blown AIDS. More importantly, sustained activation of the protein tyrosine kinase associated with T-cell antigen receptor function may result in, or contribute to, the immunopathogenic effects associated with HIV infection.

The lectin jacalin specifically triggers cell signaling in CD4+ T lymphocytes

The lectin jacalin was shown to specifically stimulate CD4(+) lymphocytes. This lectin, which presents a peptide highly similar to a sequence of the HIV external glycoprotein, interacts with CD4 and is able to inhibit in vitro HIV infection. Since jacalin binds also CD8, its mitogenic specificity cannot exclusively be attributed to its interaction with CD4. We therefore hypothesized that the lectin could trigger signals specifically associated with CD4. Here we show that jacalin triggers IL2 gene transcription only in CD4(+) lymphocytes. In parallel, we show that numerous proteins are tyrosine phosphorylated in this cell subset while only a restricted number of them are phosphorylated in CD8(+) cells. Moreover, we show that the tyrosine kinase p56lck, which is associated with both CD4 and CD8, is activated only in CD4(+) lymphocytes, making this lectin a good model for the study of cell signaling triggered in this restricted subpopulation.