gp160 of HIV or anti-CD4 monoclonal antibody ligation of CD4 induces inhibition of JNK and ERK-2 activities in human peripheral CD4+ T lymphocytes

Anti-viral opportunities during transcriptional activation of latent HIV in the host chromatin

Human immunodeficiency virus (HIV) when integrated into a host chromosome exists in a transcriptionally inactive but replication-competent state. Such latent infection represents a major challenge to HIV eradication efforts because a permanent virus reservoir resided in the infected cell is able to spike the viral load on immune suppression or during interruption of highly active anti-retroviral therapy. Understanding the molecular mechanisms that control HIV proviral latency and its reactivation could provide new perspectives on host factors as therapeutic targets for abolishing cellular reservoirs of dormant HIV. Although the control of HIV latency is multifactorial, chromatin structure and the chromatin-associated transcriptional machinery are known to be important factors. For instance, transcription initiation of the HIV provirus involves a complex molecular interplay between chromatin-associated proteins and the virus-encoded trans-activator, Tat. The first part of this review discusses our current understanding of the elements involved in HIV transcriptional activation and viral mRNA elongation, mainly post-translational modifications of HIV Tat and its interactions with host chromatin-modifying enzymes and chromatin-remodeling complexes. The second part highlights new experimental therapeutic approaches aimed at administrating activators of HIV gene expression to reduce or eliminate the pool of latently HIV-infected cells.

RNA viruses and the mitogenic Raf/MEK/ERK signal transduction cascade

The Raf/MEK/ERK signal transduction cascade belongs to the mitogen-activated protein kinase (MAPK) cascades. Raf/MEK/ERK signaling leads to stimulus-specific changes in gene expression, alterations in cell metabolism or induction of programmed cell death (apoptosis), and thus controls cell differentiation and proliferation. It is induced by extracellular agents, including pathogens such as RNA viruses. Many DNA viruses are known to induce cellular signaling via this pathway. As these pathogens partly use the DNA synthesis machinery for their replication, they aim to drive cells into a proliferative state. In contrast, the consequences of RNA virus-induced Raf/MEK/ERK signaling were less clear for a long time, but since the turn of the century the number of publications on this topic has rapidly increased. Research on this virus/host-interaction will broaden our understanding of its relevance in viral replication. This important control center of cellular responses is differently employed to support the replication of several important human pathogenic RNA viruses including influenza, Ebola, hepatitis C and SARS corona viruses.

Cutting edge: CD4 is the receptor for the tick saliva immunosuppressor, Salp15

Salp15 is an Ixodes scapularis salivary protein that inhibits CD4+ T cell activation through the repression of TCR ligation-triggered calcium fluxes and IL-2 production. We show in this study that Salp15 binds specifically to the CD4 coreceptor on mammalian host T cells. Salp15 specifically associates through its C-terminal residues with the outermost two extracellular domains of CD4. Upon binding to CD4, Salp15 inhibits the subsequent TCR ligation-induced T cell signaling at the earliest steps including tyrosine phosphorylation of the Src kinase Lck, downstream effector proteins, and lipid raft reorganization. These results provide a molecular basis to understanding the immunosuppressive activity of Salp15 and its specificity for CD4+ T cells.

Biological activities on T lymphocytes of a baculovirus-expressed chimeric recombinant IgG1 antibody with specificity for the CDR3-like loop on the D1 domain of the CD4 molecule

A baculovirus-expressed chimeric recombinant IgG1 (rIgG1) antibody, with Cgamma1 and Ckappa human constant domains, was derived from the murine monoclonal antibody (mAb) 13B8.2, which is specific for the CDR3-like loop of the CD4 molecule and which inhibits HIV-1 replication. Chimeric rIgG1 antibody 13B8.2 blocked, in a dose-dependent manner, antigen presentation through inhibition of subsequent IL-2 secretion by stimulated T cells. The one-way mixed lymphocyte reaction was abrogated by previous addition of baculovirus-produced rIgG1 13B8.2 in the T-cell culture. Anti-proliferative activity of rIgG1 was demonstrated on CD3-activated CD4+ T lymphocytes from healthy donors, such effect being associated with reduced IL-2 secretion of activated T cells. On the other hand, no proliferation inhibition was observed on CD4+ T lymphocytes activated with phorbol ester plus ionomycin, suggesting that rIgG1 13B8.2 preferentially acts on a proximal TCR-induced signaling pathway. Treatment of DBA1/J human CD4-transgenic mice with 100 microg of recombinant antibody for three consecutive days led to in vivo recovery of rIgG1 antibody 13B8.2 both coated on murine T lymphocytes and free in mouse serum, without CD4 depletion or down-modulation. These findings predict that the baculovirus-expressed chimeric rIgG1 anti-CD4 antibody 13B8.2 is a promising candidate for immunotherapy.

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.

A therapeutic CD4 monoclonal antibody inhibits TCR-zeta chain phosphorylation, zeta-associated protein of 70-kDa Tyr319 phosphorylation, and TCR internalization in primary human T cells

The molecular mechanisms mediating the inhibitory effects of a humanized CD4 mAb YHB.46 on primary human CD4(+) T cells were investigated. Preincubation of T cells with soluble YHB.46 caused a general inhibition of TCR-stimulated protein tyrosine phosphorylation events, including a reduction in phosphorylation of p95(vav), linker for activation of T cells, and Src homology 2 domain-containing leukocyte protein of 76-kDa signaling molecules. A marked reduction in activation of the Ras/mitogen-activated protein kinase pathway was also observed. Examination of the earliest initiation events of TCR signal transduction showed that YHB.46 inhibited TCR-zeta chain phosphorylation together with recruitment and tyrosine phosphorylation of the zeta-associated protein of 70-kDa tyrosine kinase, particularly at Tyr(319), as well as reduced recruitment of p56(lck) to the TCR-zeta and zeta-associated protein of 70-kDa complex. These inhibitory events were associated with inhibition of TCR endocytosis. Our results show that the YHB.46 mAb is a powerful inhibitor of the early initiating events of TCR signal transduction.

The use of monoclonal antibodies to restore self-tolerance in established autoimmunity

The author hopes to convince the reader that the data presented argue for a stage during the development of IDDM when beta-cell destruction can be counteracted and tolerance to beta cells restored, provided the immune aggression is arrested. This argument constitutes a solid rationale for immunointervention in established IDDM, especially by using potent agents such as CD3. The future for the application of monoclonal antibodies not only in autoimmunity but also in transplantation is exiting. With the development of humanized monoclonal antibodies, therapeutic uses for them are likely to expand. Enormous progress has been made in the last 15 years, and it is likely that before a similar time period has elapsed, monoclonal antibodies will have become standard tools that will dispense the need for long-term immunosuppression and its inherent dangers in various clinical arenas.

Modulation of human lymphocyte proliferative response with aging

Previously, we have demonstrated age-associated alterations in transmembrane signaling. One of the most reproducible alterations found in the immune response with aging is the decrease of lymphocyte proliferation on stimulation with various different mitogens. Here, we confirm that proliferative responses to stimulation with phytohaemagglutin (PHA), recombinant human IL-2, or anti-CD3 monoclonal antibody are all greater in the young (20-25 years) than old (60-87 years) population. We attempted to modulate the proliferative response using various agents acting at different levels of transmembrane signaling (pertussis toxin, cholera toxin, isoproterenol, PMA, Ca ionophore A23187), as well as at the level of the lymphocyte plasma membrane (methyl-beta-cyclodextrin, MBCD), or by using antioxidant vitamins (Vitamin E or C). None of these agents was able to restore effectively the proliferative response of lymphocytes from the aged to the level of young subjects. Even the combination of A23187 and PMA acting directly on calcium metabolism and protein kinase C activity was insufficient to restore the decreased mitogenic capacity of T cells from elderly subjects. Cyclodextrin, which decreases the cholesterol content of the membrane, increased the proliferative response of lymphocytes of elderly subjects, but not to the level of the young. Vitamin E had a very strong inhibitory effect on lymphocyte stimulation in both the age groups, except in combination with MBCD in T cells of the elderly, while Vitamin C had no significant modulatory effect. MAPK ERK and p38 activation was found to be decreased with aging in T cells after anti-CD3 mAb stimulation. Vitamin E but not Vitamin C strongly inhibited MAPK ERK or p38 activation. The direct activation of certain molecules or the modulation of the cholesterol content of the membrane seems to be effective immunomodulatory interventions with aging.

Relative resistance in the development of T cell anergy in CD4+ T cells from simian immunodeficiency virus disease-resistant sooty mangabeys

Despite high viral loads, T cells from sooty mangabey (SM) monkeys that are naturally infected with SIV but remain clinically asymptomatic, proliferate and demonstrate normal Ag-specific memory recall CD4(+) T cell responses. In contrast, CD4(+) T cells from rhesus macaques (RM) experimentally infected with SIV lose Ag-specific memory recall responses and develop immunological anergy. To elucidate the mechanisms for these distinct outcomes of lentiviral infection, highly enriched alloreactive CD4(+) T cells from humans, RM, and SM were anergized by TCR-only stimulation (signal 1 alone) and subsequently challenged with anti-CD3/anti-CD28 Abs (signals 1 + 2). Whereas alloreactive CD4(+)T cells from humans and RM became anergized, surprisingly, CD4(+) T cells from SM showed marked proliferation and IL-2 synthesis after restimulation. This resistance to undergo anergy was not secondary to a global deficiency in anergy induction of CD4(+) T cells from SM since incubation of CD4(+) T cells with anti-CD3 alone in the presence of rapamycin readily induced anergy in these cells. The resistance to undergo anergy was reasoned to be due to the ability of CD4(+) T cells from SM to synthesize IL-2 when incubated with anti-CD3 alone. Analysis of phosphorylated kinases involved in T cell activation showed that the activation of CD4(+) T cells by signal 1 in SM elicited a pattern of response that required both signals 1 + 2 in humans and RM. This function of CD4(+) T cells from SM may contribute to the resistance of this species to SIV-induced disease.

Immune-based approaches for control of HIV infection and viral-induced immunopathogenesis

Due to the limited efficacy of the current antiretroviral drug regimens in completely eradicating HIV and reconstituting the immune system, AIDS research is turning toward immune-based therapy to complement highly active antiretroviral therapy. Here we review potential mechanisms of protective cellular immunity and current HIV-specific immune-based strategies and discuss the rationale for novel hypothetical immunologic approaches for modulation of host antiviral immunity. One of the mechanisms by which the immune system exerts antiviral effects is via leukocyte generation of anti-HIV factors. Recent observations in this area of research suggest that non-HIV antigens can stimulate the in vitro production of anti-HIV activity by leukocytes from healthy uninfected individuals and HIV-infected patients. These findings may provide insights for the design of novel therapeutic or prophylactic approaches, which might contribute to modulating immune system control of HIV infection.

Effects of human immunodeficiency virus type 1 on CD4 lymphocyte subset activation

The pathogenesis of the decline of CD4 lymphocyte counts accompanying the typical course of HIV-1 infection is not completely defined and might be related to a differential susceptibility of naive and memory cells to HIV-1 exposure. Here, we examined the effects induced by heat-inactivated HIV-1 virions on these lymphocyte populations. Exposure of CD45RA naive T cells to inactivated viral particles induced a marked decrease of both mitogenic responses and activation-induced apoptosis. Conversely, the growth of CD45RO cells was less severely restrained. Analysis of intracellular levels of cell cycle regulatory proteins revealed an arrest at the G1/S restriction point of the naive but not memory subset. This effect was associated with alterations in phosphotyrosine profile and with a marked decrease of ERK and NJK kinase activation. Finally, up-regulation of the cAMP-dependent protein kinase A (PKA) activity induced by mitogens was not affected by virus. Altogether, these findings show that interaction of HIV-1 with the T cell surface is sufficient to inhibit the proliferative response of the CD4CD45RA subset by disturbing proximal TCR signaling. This mechanism would affect renewal of naive lymphocytes, contributing in such a way to the impairment of T cell turnover during the course of HIV-1 infection.

MEK1 activation rescues Jurkat T cells from Fas-induced apoptosis

Although the protease cascade initiated by Fas (CD95, Apo-1) is well characterized, there remains little known about how kinase pathways may impact on Fas-mediated apoptosis. We recently observed that in T lymphocytes Fas strongly induced activation of JNK (c-Jun N-terminal kinase) but not of second messengers leading to activation of ERK (extracellular regulated kinase). Additionally, Fas-mediated apoptosis was significantly inhibited with PMA, a potent activator of the ERK signaling pathway. This suggested a model whereby activation of the ERK pathway might attenuate Fas-mediated apoptosis. This was confirmed in the current study by showing that activation of MEK1, the upstream regulator of ERK, reduces Fas-mediated apoptosis, whereas inhibition of MEK1 augments apoptosis by Fas. Furthermore, Fas-mediated apoptosis of Jurkat T cells is not affected by constitutively active or dominant negative variants that modulate the JNK pathway. These results demonstrate that Fas-induced JNK activation is not required for apoptosis by Jurkat T cells, but rather is more likely secondary to cell stress during the early phases of apoptosis. This is supported by the ability of the caspase blocker zVAD to inhibit both apoptosis and JNK activation by Fas.

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.

Ligands of CD4 inhibit the association of phospholipase Cgamma1 with phosphoinositide 3 kinase in T cells: regulation of this association by the phosphoinositide 3 kinase activity

We have previously shown that CD4 ligands inhibit interleukin-2 (IL-2) production and T cell proliferation in human peripheral CD4+ T lymphocytes, in an MHC-independent way. Two major pathways implicated in T cell activation are inhibited by binding of CD4 ligands to the CD4 molecule, i.e. Ca2+ signaling by phospholipase Cgamma1 (PLCgamma1), and ERK-2 activation, suggesting a p21ras inhibition. We have correlated these inhibitions with the disruption of multifunctional complexes containing PLCgamma1, p120GAP and Sam68, induced by T cell activation. We report here that T cell activation through the TCR/CD3 induces an association of the phosphoinositide 3 kinase (PI3 kinase) with PLCgamma1, both in peripheral CD4+ T lymphocytes and the HUT-78 CD4+ T cell line. PI3 kinase is present in the multifunctional complexes that we have described previously. Preincubation of human peripheral CD4+ T cells and HUT-78 CD4+ T cells with gp160 or a peptide analogue of the HLA class II DR molecule precludes the association of PLCgamma1 with PI3 kinase. We also demonstrate, using two specific inhibitors of PI3 kinase activity (LY294002 and wortmannin), that this activity plays a key role in the association of PLCgamma1 with PI3 kinase. Moreover, we demonstrate the implication of the PI3 kinase activity in the negative signal mediated by HIV gp160 binding to CD4 molecules. We propose that the products of the PI3 kinase are important mediators of the negative signaling induced by the binding of CD4 ligands to the CD4 molecule implicated in the regulation of the formation of multifunctional complexes.

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.

Sam68 Association with p120GAP in CD4+ T Cells Is Dependent on CD4 Molecule Expression

p120 GTPase-activating protein (p120GAP) is a major negative regulator of p21ras activity in several cell types including T cells. Catalytic activity of this enzyme is regulated in part by its interaction with several associated tyrosine-phosphorylated proteins. Sam68 was initially described as associated with p120GAP. It has been further established that Sam68 is a substrate of src kinases in mitosis and that it is not associated with p120GAP in transformed fibroblasts. We describe herein that Sam68 associates with p120GAP and PLCγ1 in human mature T cells and in a T cell line expressing the CD4 molecule HUT78 CD4+. This association is present in nonactivated cells and increases after anti-CD3 activation. It is dependent on CD4 expression and, in part, on the association of CD4 with p56lck, as shown by the strongly decreased association of Sam68 with p120GAP in the CD4− mutants, HUT78 CD4−, and by the reduced association of Sam68 with both p120GAP and p56lck in the HUT78 T cell line expressing a CD4 mutant unable to interact with p56lck, HUT78 C420/22. We propose that recruitment of Sam68, via CD4/p56lck, to the inner face of the plasma membrane may permit, via its docking properties, the correct association of key signaling molecules including PLCγ1 and p120GAP. This formation of transduction modules will enable the activation of different signaling cascades including the p21ras pathway and an array of downstream events, ultimately leading to T cell activation.

A novel complex of proteins binds the HIV-1 promoter upon virus interaction with CD4

HIV-1Lai13EM is a mutant isolate which is less sensitive than the parental HIV-1Lai strain to an in vitro treatment with 13B8-2 anti-CD4 monoclonal antibody (mAb) that generally inhibits transcription of HIV-1 and HIV-2. In contrast to other clade B viruses, this isolate carries a point mutation G > A at position -188 of the viral promoter. The fact that HIV-1NDK, a clade D virus insensitive to 13B8-2 mAb, also carries an A nucleotide at this position has brought our attention to the sequence surrounding position -188. Here we analyzed whether a DNA-binding molecule interacts with this region. Electrophoretic mobility shift assays performed with the -201/-175 HIV-1Lai wild-type sequence or the sequence containing a point mutation G > A at position -188 demonstrated their ability to bind a heterotrimeric complex induced in CEM cells by stimulation with heat-inactivated HIV-1.

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.

Cutting Edge: HIV-1 Infection Induces a Selective Reduction in STAT5 Protein Expression

HIV-1 infection is accompanied by qualitative and quantitative defects in CD4+ T lymphocytes. Loss of immune function in HIV patients is usually associated with a profound dysregulation of cytokine production. To investigate whether cytokine signaling defects occur during HIV infection, PHA blasts from healthy human donors were infected with two strains of HIV-1 and screened for the expression of STAT proteins used in cytokine signaling. A selective decrease in STAT5B was seen 8 days after infection with the BZ167 dual-tropic HIV isolate, but not with the Ba-L, M-tropic strain. Based on these findings, purified T cells from HIV-infected patients in different stages of disease were also tested for STAT expression; decreases in STAT5A, STAT5B, and STAT1α were observed in all patients. The reduction in STATs seen in vivo and in vitro after HIV infection may contribute to the loss of T cell function in HIV disease.

Activation and signal transduction via mitogen-activated protein (MAP) kinases in T lymphocytes

The various mitogen-activated protein (MAP) kinases have central roles in the signalling pathways of T lymphocytes. Their activation is uniquely dependent on dual phosphorylation of a serine/threonine and a tyrosine residue and is regulated by several levels of kinases in parallel cascades. In addition, both the MAP kinases and their upstream, activating kinases are regulated by several phosphatases. Although each of the MAP kinases have many cytoplasmic substrates, their ability to translocate to the nucleus means that they can transmit signals from the cytoplasm directly to transcription factors, which are sometimes nuclear bound. The MAP kinase cascades are activated in T lymphocytes by a variety of different external stimuli. They play an important role in transducing both the signal from T cell receptor and costimulatory molecules, on the T cell surface, and are able to regulate several of the transcription factors controlling the expression of critical genes, including that for IL-2. This review examines how the activation of several MAP kinases is regulated, their role in signal transduction initiated by a variety of stimuli, and how this may lead to different cellular responses.