Ligation of either CD2 or CD28 rescues CD4+ T cells from HIV-gp120-induced apoptosis

Bivalent binding of staphylococcal superantigens to the TCR and CD28 triggers inflammatory signals independently of antigen presenting cells

Staphylococcus aureus superantigens (SAgs) such as staphylococcal enterotoxin A (SEA) and B (SEB) are potent toxins stimulating T cells to produce high levels of inflammatory cytokines, thus causing toxic shock and sepsis. Here we used a recently released artificial intelligence-based algorithm to better elucidate the interaction between staphylococcal SAgs and their ligands on T cells, the TCR and CD28. The obtained computational models together with functional data show that SEB and SEA are able to bind to the TCR and CD28 stimulating T cells to activate inflammatory signals independently of MHC class II- and B7-expressing antigen presenting cells. These data reveal a novel mode of action of staphylococcal SAgs. By binding to the TCR and CD28 in a bivalent way, staphylococcal SAgs trigger both the early and late signalling events, which lead to massive inflammatory cytokine secretion.

SARS-CoV-2 Spike Does Not Possess Intrinsic Superantigen-like Inflammatory Activity

Multisystem inflammatory syndrome in children (MIS-C) is a rare hyperinflammatory disease occurring several weeks after SARS-CoV-2 infection. The clinical similarities between MIS-C and the toxic shock syndrome, together with the preferential expansion of T cells with a T-cell receptor variable β chain (TCRVβ) skewing, suggested a superantigen theory of MIS-C. For instance, recent in silico modelling evidenced the presence of a highly conserved motif within SARS-CoV-2 spike protein similar in structure to the superantigenic fragment of staphylococcal enterotoxin B (SEB). However, experimental data on the superantigenic activity of the SARS-CoV-2 spike have not yet been provided. Here, we assessed the superantigenic activity of the SARS-CoV-2 spike by analysing inflammatory cytokine production in both Jurkat cells and the peripheral blood CD4⁺ T cells stimulated with the SARS-CoV-2 spike or SEB as a control. We found that, unlike SEB, the SARS-CoV-2 spike does not exhibit an intrinsic superantigen-like activity.

Binding of Staphylococcal Enterotoxin B (SEB) to B7 Receptors Triggers TCR- and CD28-Mediated Inflammatory Signals in the Absence of MHC Class II Molecules

The inflammatory activity of staphylococcal enterotoxin B (SEB) relies on its capacity to trigger polyclonal T-cell activation by binding both T-cell receptor (TCR) and costimulatory receptor CD28 on T cells and MHC class II and B7 molecules on antigen presenting cells (APC). Previous studies highlighted that SEB may bind TCR and CD28 molecules independently of MHC class II, yet the relative contribution of these interactions to the pro-inflammatory function of SEB remained unclear. Here, we show that binding to MHC class II is dispensable for the inflammatory activity of SEB, whereas binding to TCR, CD28 and B7 molecules is pivotal, in both human primary T cells and Jurkat T cell lines. In particular, our finding is that binding of SEB to B7 molecules suffices to trigger both TCR- and CD28-mediated inflammatory signalling. We also provide evidence that, by strengthening the interaction between CD28 and B7, SEB favours the recruitment of the TCR into the immunological synapse, thus inducing lethal inflammatory signalling.

A non-conserved amino acid variant regulates differential signalling between human and mouse CD28

CD28 superagonistic antibodies (CD28SAb) can preferentially activate and expand immunosuppressive regulatory T cells (Treg) in mice. However, pre-clinical trials assessing CD28SAbs for the therapy of autoimmune diseases reveal severe systemic inflammatory response syndrome in humans, thereby implying the existence of distinct signalling abilities between human and mouse CD28. Here, we show that a single amino acid variant within the C-terminal proline-rich motif of human and mouse CD28 (P²¹² in human vs. A²¹⁰ in mouse) regulates CD28-induced NF-κB activation and pro-inflammatory cytokine gene expression. Moreover, this Y²⁰⁹APP²¹² sequence in humans is crucial for the association of CD28 with the Nck adaptor protein for actin cytoskeleton reorganisation events necessary for CD28 autonomous signalling. This study thus unveils different outcomes between human and mouse CD28 signalling to underscore the importance of species difference when transferring results from preclinical models to the bedside.

CD28 transmits co-stimulatory signals for the activation of both mouse and human T cells, but in vivo hyperactivation of CD28 has opposite effects on system immunity. Here, the authors show that a single amino acid difference between mouse and human CD28 dictates this function distinction via differential recruitment of Nck.

Mechanisms of apoptosis induction by the HIV-1 envelope

The envelope glycoprotein complex (Env) of human immunodeficiency virus-1 (HIV-1) can induce apoptosis by a cornucopia of distinct mechanisms. A soluble Env derivative, gp120, can kill cells through signals that are transmitted by chemokine receptors such as CXCR4. Cell surface-bound Env (gp120/gp41), as present on the plasma membrane of HIV-1-infected cells, can kill uninfected bystander cells expressing CD4 and CXCR4 (or similar chemokine receptors, depending on the Env variant) by at least three different mechanisms. First, a transient interaction involving the exchange of lipids between the two interacting cells ('the kiss of death') may lead to the selective death of single CD4-expressing target cells. Second, fusion of the interacting cells may lead to the formation of syncytia which then succumb to apoptosis in a complex pathway involving the activation of several kinases (cyclin-dependent kinase-1, Cdk1; checkpoint kinase-2, Chk2; mammalian target of rapamycin, mTOR; p38 mitogen-activated protein kinase, p38 MAPK; inhibitor of NF-kappaB kinase, IKK), as well as the activation of several transcription factors (NF-kappaB, p53), finally resulting in the activation of the mitochondrial pathway of apoptosis. Third, if the Env-expressing cell is at an early stage of imminent apoptosis, its fusion with a CD4-expressing target cell can precipitate the death of both cells, through a process that may be considered as contagious apoptosis and which does not involve Cdk1, mTOR, p38 nor p53, yet does involve mitochondria. Activation of some of the above- mentioned lethal signal transducers have been detected in patients' tissues, suggesting that HIV-1 may indeed trigger apoptosis through molecules whose implication in Env-induced killing has initially been discovered in vitro.

Mitochondrial Apoptosis Induced by the HIV-1 Envelope

The envelope glycoprotein complex (Env), encoded by the human immunodeficiency virus (HIV-1), kills uninfected cells expressing CD4 and/or the chemokine receptor CXCR4 or CCR5, via at least three independent mechanisms. First, the soluble Env product gp120 can induce the apoptotic cell death of lymphocytes, neurons, and myocardiocytes, via interaction with surface receptors. Second, Env present on the surface of HIV-1 infected cells can transiently interact with cells expressing CD4 and CXCR4/CCR5, thereby provoking a hemifusion event that results in the death of the uninfected cell. Third, the interaction between Env on infected cells and its receptors on uninfected cells can result in syncytium formation. Such syncytia undergo apoptosis after a phase of latency. In several models of Env-induced apoptosis, early signs of mitochondrial membrane permeabilization (MMP) become manifest. Such signs include a loss of the mitochondrial transmembrane potential and the release of cytochrome c and AIF. The mechanisms of Env-triggered apoptotic MMP may involve an elevation of cytosolic Ca(2+), reactive oxygen species and/or the transcriptional activation of p53, with the consequent expression of pro-apoptotic proteins such as Bax, which permeabilizes mitochondrial membranes. The implications of these findings for the pathophysiology of HIV-1 infection is discussed.

The associated regulators and signal pathway in rIL-16/CD4 mediated growth regulation in Jurkat cells

IL-16 is a ligand and chemotactic factor for CD4+ T cells. IL-16 inhibits the CD3 mediated lymphocyte activation and proliferation. The effects of IL-16 on the target cells are dependent on the cell type, the presence of co-activators etc. To understand the regulation function and mechanism of IL-16 on target cells, we used a 130 a.a. recombinant IL-16 to study its effects on the growth of Jurkat T leukemia cells in vitro. We found that the rIL-16 stimulated the proliferation of Jurkat cells at low dose (10(-9)M), but inhibited the growth of the cells at higher concentration (10(-5)M). Results showed that 10(-5) M of rIL-16 treatment induced an enhanced apoptosis in Jurkat cells. The treatment blocked the expression of FasL, but up-regulated the c-myc and Bid expression in the cells. Pre-treatment of PKC inhibitor or MEK1 inhibitor markedly increased or decreased the rIL-16 induced growth-inhibiting effects on Jurkat cells, respectively. The results suggested that the rIL-16 might be a regulator for the growth or apoptosis of Jurkat cells at a dose-dependent manner. The growth-inhibiting effects of rIL-16 might be Fas/FasL independent, but, associated with the activation of PKC, up-regulated expression of c-Myc and Bid, and the participation of the ERK signal pathway in Jurkat cells.

CD4-Lck through TCR and in the absence of Vav exchange factor induces Bax increase and mitochondrial damage

In the present study, we aimed to demonstrate that CD4 may represent a critical turning point that governs the apoptotic and survival programs in T cells, without modifying the physical association with the TCR-CD3 complex. To address this issue, we have explored the possibility that the activation of CD4 may transduce apoptotic signals unless signaling effectors neutralize them. Our data show that in Jurkat T cells CD4 engagement by Leu3a mAb results in a rapid and strong increase of Lck kinase activity, subsequent alterations of mitochondrial membrane potential, and apoptosis. Critical parameters are coassociation of CD4/Lck with TCR/CD3 and up-regulation of the proapoptotic protein Bax. Indeed, Leu3a-mediated Lck activation failed to induce apoptotic features in Jurkat cells either defective for TCR/CD3 or overexpressing the antiapoptotic protein Bcl-2. Furthermore, we demonstrate that Leu3a treatment of Jurkat cells overexpressing Vav results in the inhibition of mitochondrial damage and apoptosis; this rescue effect is accompanied with a significant decrease of Bax expression observed in apoptotic cells. Our evidence that the activation of Lck activates in T cells apoptotic pathways which are counteracted by Vav, a signaling molecule that cooperates with CD28 to boost TCR signals, suggests a novel role for costimulation in protecting T cells from CD4-mediated cell death.

Human anergic CD4+ T cells can act as suppressor cells by affecting autologous dendritic cell conditioning and survival

T cell suppression exerted by regulatory T cells represents a well-established phenomenon, but the mechanisms involved are still a matter of debate. Recent data suggest that anergic T cells can suppress responder T cell activation by inhibiting Ag presentation by dendritic cells (DC). In this study, we focused our attention on the mechanisms that regulate the susceptibility of DC to suppressive signals and analyzed the fate of DC and responder T cells. To address this issue, we have cocultured human alloreactive or Ag-specific CD4+ T cell clones, rendered anergic by incubation with immobilized anti-CD3 Ab, with autologous DC and responder T cells. We show that anergic T cells affect either Ag-presenting functions or survival of DC, depending whether immature or mature DC are used as APC. Indeed, MHC and costimulatory molecule expression on immature DC activated by responder T cells is inhibited, while apoptotic programs are induced in mature DC and in turn in responder T cells. Ligation of CD95 by CD95L expressed on anergic T cells in the absence of CD40-CD40L (CD154) interaction are critical parameters in eliciting apoptosis in both DC and responder T cells. In conclusion, these findings indicate that the defective activation of CD40 on DC by CD95L+ CD154-defective anergic T cells could be the primary event in determining T cell suppression and support the role of CD40 signaling in regulating both conditioning and survival of DC.

Vav cooperates with CD28 to induce NF-kappaB activation via a pathway involving Rac-1 and mitogen-activated kinase kinase 1

CD28-delivered costimulatory signals are required to induce NF-kappaB activation in response to TCR stimulation. We have recently demonstrated that the mitogen-activated kinase kinase 1 (MEKK1), a kinase known to regulate the c-jun N-terminal kinase (JNK) pathway, is also involved in the CD28- and TCR-induced inhibitor of kappaB factor (IkappaB) kinases (IKK) and NF-kappaB activation. Searching for molecules that couple TCR and CD28 to MEKK1, we found that the guanine nucleotide exchange factor Vav synergized with CD28 stimulation in Jurkat cells to induce NF-kappaB transcriptional activity through the activation of IKKalpha and IKKbeta. Dominant negative mutants of Vav inhibited TCR- and CD28-NF-kappaB-dependent transcription by interfering with the activation of the IKK complex. Blocking Rac signaling downstream of Vav by dominant negative RacN17 exerts similar effects on IKK and NF-kappaB activation after TCR/CD28 stimulation. Finally, Vav-induced NF-kappaB activation in CD28 costimulated cells was inhibited by dominant negative MEKK(KM). These results identify Vav, Rac-1 and MEKK1 as components of a common pathway regulating both NF-kappaB and AP-1 that contributes to full activation of the CD28 response element (CD28RE).

The membrane-microfilament linker ezrin is involved in the formation of the immunological synapse and in T cell activation

Dynamic interactions between membrane and cytoskeleton components are crucial for T cell antigen recognition and subsequent cellular activation. We report here that the membrane-microfilament linker ezrin plays an important role in these processes. First, ezrin relocalizes to the contact area between T cells and stimulatory antigen-presenting cells (APCs), accumulating in F-actin-rich membrane protrusions at the periphery of the immunological synapse. Second, T cell receptor (TCR)-mediated intracellular signals are sufficient to induce ezrin relocalization, indicating that this protein is an effector of TCR signaling. Third, overexpression of the membrane binding domain of ezrin perturbs T cell receptor clustering in the T cell-APC contact area and inhibits the activation of nuclear factor for activated T cells (NF-AT).

Cytokines and HIV-1: interactions and clinical implications

Cytokines play an important role in controlling the homoeostasis of the immune system. Infection with HIV results in dysregulation of the cytokine profile in vivo and in vitro. During the course of HIV-1 infection secretion of T-helper type 1 (Th1) cytokines, such as interleukin (IL)-2, and antiviral interferon (IFN)-gamma, is generally decreased, whereas production of T helper type 2 (Th2) cytokines, IL-4, IL-10, proinflammatory cytokines (IL-1, IL-6, IL-8) and tumour necrosis factor (TNF)-alpha, is increased. Such abnormal cytokine production contributes to the pathogenesis of the disease by impairing cell-mediated immunity. A number of cytokines have been shown to modulate in vitro HIV-1 infection and replication in both CD4 T lymphocytes and cells of macrophage lineage. HIV-inductive cytokines include: TNF-alpha, TNF-beta, IL-1 and IL-6, which stimulate HIV-1 replication in T cells and monocyte-derived macrophages (MDM), IL-2, IL-7 and IL-15, which upregulate HIV-1 in T cells, and macrophage-colony stimulating factor, which stimulates HIV-1 in MDM. HIV-suppressive cytokines include: IFN-alpha, IFN-beta and IL-16, which inhibit HIV-1 replication in T cells and MDM, and IL-10 and IL-13, which inhibit HIV-1 in MDM. Bifunctional cytokines such as IFN-gamma, IL-4 and granulocyte-macrophage colony-stimulating factor have been shown to have both inhibitory and stimulatory effects on HIV-1. The beta-chemokines, macrophage-inflammatory protein (MIP)-1alpha, MIP-1beta and RANTES are important inhibitors of macrophage-tropic strains of HIV-1, whereas the alpha-chemokine stromal-derived factor-1 suppresses infection of T-tropic strains of HIV-1. This review outlines the interactions between cytokines and HIV-1, and presents clinical applications of cytokine therapy combined with highly active antiretroviral therapy or vaccines.

Changes in host cell molecules acquired by circulating HIV-1 in patients treated with highly active antiretroviral therapy and interleukin-2

OBJECTIVE

To analyse cell membrane proteins (CMP) acquired by HIV-1 present in the plasma of asymptomatic patients, and their modifications after a cycle of highly active antiretroviral therapy (HAART) and interleukin (IL)-2.

DESIGN AND METHODS

Plasma samples from eight drug-naive asymptomatic subjects underwent immobilized antibody capture (IAC) to detect CMP on the surface of circulating HIV-1. The CMP considered were lymphocyte subset markers (CD45RA, CD45RO), activation markers (HLA-DR), adhesion molecules (LFA-3), costimulatory proteins (B7-2), lymph-node homing receptors (CD62L) and pro-apoptosis molecules (FasL). This analysis was repeated after one cycle of HAART + IL-2, after virus rebound.

RESULTS

LFA-3, followed by CD45RO and HLA-DR, are the most represented CMP on the surface of circulating virions in naive asymptomatic patients; CD45RA, CD62L, B7-2 and FasL are detected only occasionally. After rebound, a significant reduction of CD45RO and HLA-DR, but not of LFA-3, is observed on virions, whereas CD45RA and CD62L, as well as other molecules, are not affected, remaining almost undetectable.

CONCLUSIONS

Assuming that CMP on HIV-1 reflect the cellular origin of virions, activated T cells expressing CD45RO, HLA-DR, and LFA-3 may be the main source of HIV-1 in asymptomatic patients. After a cycle of HAART + IL-2, followed by therapy interruption, CD45RA and CD62L are detected on virions rarely, indicating that even during virus rebound, expanded naive T cells do not become a major target of virus replication. Furthermore, the presence of HLA-DR on rebound HIV-1 is decreased, consistent with decreased activation of the HIV-producing cells. More extensive investigation may clarify the significance of these findings with respect to pathogenesis.

CD28 utilizes Vav-1 to enhance TCR-proximal signaling and NF-AT activation

The mechanism through which CD28 costimulation potentiates TCR-driven gene expression is still not clearly defined. Vav-1, an exchange factor for Rho GTPases thought to regulate, mainly through Rac-1, various signaling components leading to cytokine gene expression, is tyrosine phosphorylated upon CD28 engagement. Here, we provide evidence for a key role of Vav-1 in CD28-mediated signaling. Overexpression of Vav-1 in Jurkat cells in combination with CD28 ligation strongly reduced the concentration of staphylococcus enterotoxin E/MHC required for TCR-induced NF-AT activation. Surprisingly, upon Vav-1 overexpression CD28 ligation sufficed to activate NF-AT in the absence of TCR engagement. This effect was not mediated by overexpression of ZAP-70 nor of SLP-76 but necessitated the intracellular tail of CD28, the intactness of the TCR-proximal signaling cascade, the Src-homology domain 2 (SH2) domain of Vav-1, and SLP-76 phosphorylation, an event which was favored by Vav-1 itself. Cells overexpressing Vav-1 formed lamellipodia and microspikes reminiscent of Rac-1 and Cdc42 activation, respectively, for which the SH2 domain of Vav-1 was dispensable. Together, these data suggest that CD28 engagement activates Vav-1 to boost TCR signals through a synergistic cooperation between Vav-1 and SLP-76 and probably via cortical actin changes to facilitate the organization of a signaling zone.

Engagement of CD4 before TCR triggering regulates both Bax- and Fas (CD95)-mediated apoptosis

In the present study, we have aimed at clarifying the CD4-dependent molecular mechanisms that regulate human memory T cell susceptibility to both Fas (CD95)-dependent and Bcl-2-dependent apoptotic pathways following antigenic challenge. To address this issue, we used an experimental system of viral and alloantigen-specific T cell lines and clones and two ligands of CD4 molecules, Leu-3a mAb and HIV gp120. We demonstrate that CD4 engagement before TCR triggering suppresses the TCR-mediated neosynthesis of the Flice-like inhibitory protein and transforms memory T cells from a CD95-resistant to a CD95-susceptible phenotype. Moreover, evidence that the apoptotic programs were executed while Fas ligand mRNA expression was inhibited led us to analyze Bcl-2-dependent pathways. The data show that the engagement of CD4 separately from TCR influences the expression of the proapoptotic protein Bax independently of the anti-apoptotic protein Bcl-2, whereas Ag activation coordinately modulates both Bax and Bcl-2. The increased expression of Bax and the consequent dissipation of the mitochondrial transmembrane potential (DeltaPsim) suggest a novel immunoregulatory function of CD4 and demonstrate that both passive cell death and activation-induced cell death are operative in CD4+ memory T cells. Furthermore, analysis of the mechanisms by which IL-2 and IL-4 cytokines exert their protective function on CD4+ T cells in the presence of soluble CD4 ligands shows that they were able to revert susceptibility to Bax-mediated but not to CD95-dependent apoptotic pathways.

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.

B7 cosignal potentiates apoptosis of uninfected CD4+ T lymphocytic cell lines primed by HIV envelope proteins

In lymphoid organs, follicular dendritic cells (FDCs), monocytes, and macrophages are targets for HIV infection and reservoirs for infectious virus. Strikingly, the apoptotic cells in these sites are essentially uninfected CD4+ T lymphocytes, but lie in close proximity to infected cells or FDCs carrying trapped HIV virions. To decipher this apoptotic pathway, we have established a two-step experimental system that reproduces in vitro the HIV envelope protein-mediated apoptosis restricted to uninfected CD4+ T lymphocytic cell lines. In this assay, uninfected CD4+ T cell targets undergo apoptosis following an initial priming step on HeLa cells expressing functional HIV envelope proteins at their plasma membrane and a second and necessary stimulation step via the CD3-TCR complex. The CD4+ T lymphocytic cells susceptible to apoptosis are, in contrast, resistant to cell fusion mediated by HIV envelope protein and express SDF-1. FDCs and macrophages are known to be high B7 expressors. Thus in lymph nodes, the cells that have trapped HIV particles in immune complexes at the plasma membrane present both HIV envelope proteins and B7.1 at their surface. We mimicked this situation in vitro by priming CD4+ T lymphocytes on cells expressing the costimulatory molecule B7 in addition to HIV envelope proteins, and show that it resulted in an acceleration and a twofold increase in apoptosis. Finally, we characterized two enzymes, PI3Kinase and PI-PLC, which are both downstream effectors of the CD4 (HIV envelope protein receptor) and CD28 (B7 receptor) activation pathways, and that participated in the early steps of priming for apoptosis.

TCR Engagement Regulates Differential Responsiveness of Human Memory T Cells to Fas (CD95)-Mediated Apoptosis

In this work, we have tried to establish whether human memory T cells may be protected from Fas (CD95)-induced apoptosis when correctly activated by Ag, and not protected when nonspecifically or incorrectly activated. In particular, we wanted to investigate the molecular mechanisms that regulate the fate of memory T cells following an antigenic challenge. To address this issue, we chose an experimental system that closely mimics physiological T cell activation such as human T cell lines and clones specific for viral peptides or alloantigens. We demonstrate that memory T cells acquire an activation-induced cell death (AICD)-resistant phenotype when TCRs are properly engaged by specific Ag bound to MHC molecules. Ag concentration and costimulation are critical parameters in regulating the protective effect. The analysis of the mechanisms involved in the block of CD95 signal transduction pathways revealed that the crucial events are the inhibition of CD95-associated IL-1β-converting enzyme (ICE)-like protease (FLICE) activation and poly(ADP)-ribose polymerase cleavage, and the mRNA expression of FLICE-like inhibitory protein. Furthermore, we have observed that TCR-mediated neosynthesis of FLICE-like inhibitory protein mRNA is suppressed either by protein tyrosine kinase inhibitors or cyclosporin A. In conclusion, the present analysis of the effects of TCR triggering on the regulation of AICD suggests that AICD could be inhibited in human memory T cells activated in vivo by a foreign Ag, but may become operative when the Ag has been cleared.

Dendritic cells prevent CD95 mediated T lymphocyte death through costimulatory signals

T cell apoptosis is a mechanism regulating T cell homeostasis. Prolonged stimulation renders T cells susceptible to activation induced cell death (AICD), a process mediated through CD95 (Apo-1/Fas). While under some circumstances AICD can be prevented, little is known about molecules involved. Here, we wanted to assess whether dendritic cells (DC) have the capacity to prevent CD95-dependent AICD. T cells activated with PHA/PMA or anti-CD3 monoclonal antibody (mAb) were cocultured with increasing amounts of DC. While spontaneous T cell apoptosis amounted to 25%, the presence of an agonistic anti-CD95 antibody increased cell death to 64%. Addition of scalar amounts of DC prevented T cell apoptosis in a dose dependent fashion, where coculture of 10(5) DC/ml with 10(6) T cells/ml reduced apoptosis almost to baseline level (33%). Further addition of an anti-CD58 antibody partially abolished this protective effect. This was even more pronounced if anti-CD80 and anti-CD86 antibodies were added. Our findings suggest that dendritic cells are able to rescue T cells from AICD, with CD58 ligation playing a key role.

Differential Role of CTLA-4 in Regulation of Resting Memory Versus Naive CD4 T Cell Activation

Regulation of peripheral T cell responses is critical for preserving self tolerance. Memory T cells have a lower threshold for activation through the TCR and are thought to be less dependent on costimulation than naive T cells, suggesting a requirement for more stringent regulation of memory T cells. We have recently shown that CD4 engagement apart from the TCR results in the inactivation of memory, but not naive, CD4 T cells. We show here that this inhibition requires ligation of CTLA-4, in that blocking CTLA-4-B7 interactions restores memory CD4 T cell responsiveness. Early signaling through CTLA-4 is possible because resting memory, but not naive, CD4 T cells contain intracellular stores of CTLA-4 that are continuously recycled between the cytoplasm and the cell surface. This mechanism ensures that low intensity TCR engagements, which are thought to be important for peripheral T cell longevity, do not cause memory T cell activation but instead raise their threshold for costimulatory signals. This may give memory T cells an extended lifespan with a reduced risk of inappropriate activation.

Fyn and ZAP-70 are required for Vav phosphorylation in T cells stimulated by antigen-presenting cells

In T cells, triggering of the T cell antigen receptor or of the co-stimulatory receptor CD28 can direct tyrosine phosphorylation of the signaling protein Vav. We investigated the role played by the protein tyrosine kinases Fyn, Lck, and ZAP-70 in these processes in a T cell hybridoma after physiological stimulation of the T cell receptor (TCR) and CD28. A dominant-negative mutant approach based on overexpression of catalytically inactive alleles of these kinases showed that CD28-induced Vav phosphorylation preferentially requires Fyn, whereas ZAP-70 had no role. Consistently, Vav was strongly phosphorylated in Lck-deficient JCAM-1 cells after CD28 ligation. In contrast, ZAP-70 appeared to control TCR-directed Vav phosphorylation. However, overexpression of ZAP-70 carrying a mutated Tyr315, contained within a motif previously suggested to be a Vav Src homology 2 domain binding site, had little or no effect. Immunoprecipitation assays showed that phosphorylated Vav associated with Fyn after CD28 triggering and that this interaction, likely to involve binding of Fyn Src homology 2 domain to Vav, was more strongly detectable after concomitant CD28 and TCR stimulation. These data suggest that Fyn plays a major role in controlling Vav phosphorylation upon T cell activation and that the mechanism implicating ZAP-70 in this process may be more complex than previously anticipated.

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

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

CD28 expression in pediatric human immunodeficiency virus infection

Increased apoptosis of lymphocytes represents a key event of immune destruction in HIV infection. In this study it was investigated at which stage of the disease and in which T lymphocyte subpopulation (CD4+ or CD8+) protection against apoptosis may be lost as measured by decreased CD28 expression. In 26 HIV-infected and 20 healthy children, as well as 10 infants exposed to HIV, expression of CD28 and the apoptosis-related marker CD95 was studied by fluorescence-activated cell sorting analysis. According to established Centers for Disease Control and Prevention definitions, children were divided into three immunologic categories. In the CD8 population, patients in category 1 already showed a markedly decreased mean CD28 (36.2%+/-16.1 SD) and increased CD95 expression (48.8+/-24.1%), compared with the age-matched control group (67.7+/-14.4%, 15.8+/-8.9%). In the CD4 population, mean CD28 and CD95 expression was not altered in category 1 patients. Of the exposed children, the child with the lowest CD28 expression on CD8 cells was determined later to be infected with HIV. Significant immunophenotypical alterations are observed in early stage pediatric HIV infection, which may indicate an early loss of protection against apoptosis in the CD8+ T cell population.

CD28 affects the earliest signaling events generated by TCR engagement

The efficiency and magnitude of T cell responses are influenced by ligation of the co-stimulatory receptor CD28 by B7 molecules expressed on antigen-presenting cells (APC). In contrast to most previous studies in which agonistic anti-TCR/CD3 and anti-CD28 antibodies were employed, here we have investigated the contribution of CD28 to T cell activation under physiological conditions of antigen presentation. Jurkat T cells and primary T cells from TCR-transgenic mice stimulated with superantigen and antigen, respectively, presented by B7-expressing APC were utilized. In both systems we show that inhibiting CD28/B7 interaction resulted in impaired TCR-induced tyrosine phosphorylation of the signal-transducing zeta chain and ZAP-70. Consistent with a blockade of TCR-proximal signaling events, Jurkat cells stimulated in the absence of CD28 ligation were found to have strongly diminished tyrosine phosphorylation of cellular substrates and downstream signaling pathways such as Ca2+/calcineurin, ERK/MAPK and JNK. Our results provide evidence for a role of CD28 in enhancing TCR signaling capacity during the earliest stages of T cell:APC interaction.

A Noncomitogenic CD2R Monoclonal Antibody Induces Apoptosis of Activated T Cells by a CD95/CD95-L-Dependent Pathway

Clonal expansion of activated T and B cells is controlled by homeostatic mechanisms resulting in apoptosis of a large proportion of activated cells, mostly through interaction between CD95 (Fas or Apo-1) receptor and its ligand CD95-L. CD2, which is considered as a CD3/TCR alternative pathway of T cell activation, may trigger activation-induced cell death, but the role of CD95/CD95-L interaction in CD2-mediated apoptosis remains controversial. We show here that the CD2R mAb YTH 655.5, which does not induce comitogenic signals when associated with another CD2 mAb, triggers CD95-L expression by preactivated but not resting T cells, resulting in CD95/CD95-L-mediated apoptosis. The critical role of CD95/CD95-L interaction was supported by complete inhibition in the presence of the antagonist CD95 mAb ZB4 and by blocking CD95-L synthesis and surface expression by cycloheximide, cyclosporin A, EGTA, or cytochalasin B. YTH 655.5 was shown to stimulate p56lck phosphorylation and enzymatic activity. However, p56lck activation is not sufficient to trigger apoptosis, because other CD2R and CD4 mAbs that activate p56lck do not induce apoptosis. In conclusion, CD2 can mediate nonmitogenic signals, resulting in CD95-L expression and apoptosis of CD95+ cells.

CD2 Rescues T Cells From T-Cell Receptor/CD3 Apoptosis: A Role for the Fas/Fas-L System

Anti-CD3 monoclonal antibodies (MoAbs) and glucocorticoid hormones induce apoptosis in immature thymocytes and peripheral T lymphocytes. This process is inhibited by a number of growth factors, including interleukin-2 (IL-2), IL-3, and IL-4, as well as by triggering of the adhesion molecule CD44, which would indicate that signals generated by membrane receptors can modulate the survival of lymphoid cells. To investigate whether triggering of CD2 may also affect apoptosis in lymphoid cells, we analyzed the effect of stimu-lation with anti-CD2 MoAbs on T-cell apoptosis induced by two stimuli, anti-CD3 MoAbs and dexamethasone (DEX), using a hybridoma T-cell line and a T-helper cell clone. The results show that CD2 engagement decreased anti-CD3 MoAb-induced apoptosis, but did not influence DEX-induced cell death. Furthermore, the decrease appeared to be related to the expression of Fas/APO-1 (CD95) and Fas-ligand (Fas-L). In fact, we show that CD2 stimulation inhibits apoptosis by preventing the CD3-induced upregulation of Fas and Fas-L in a Fas-dependent experimental system. These data suggest that a costimulatory molecule may control a deletion pathway and may therefore contribute to the regulation of peripheral tolerance.

CD2 Rescues T Cells From T-Cell Receptor/CD3 Apoptosis: A Role for the Fas/Fas-L System

Anti-CD3 monoclonal antibodies (MoAbs) and glucocorticoid hormones induce apoptosis in immature thymocytes and peripheral T lymphocytes. This process is inhibited by a number of growth factors, including interleukin-2 (IL-2), IL-3, and IL-4, as well as by triggering of the adhesion molecule CD44, which would indicate that signals generated by membrane receptors can modulate the survival of lymphoid cells. To investigate whether triggering of CD2 may also affect apoptosis in lymphoid cells, we analyzed the effect of stimu-lation with anti-CD2 MoAbs on T-cell apoptosis induced by two stimuli, anti-CD3 MoAbs and dexamethasone (DEX), using a hybridoma T-cell line and a T-helper cell clone. The results show that CD2 engagement decreased anti-CD3 MoAb-induced apoptosis, but did not influence DEX-induced cell death. Furthermore, the decrease appeared to be related to the expression of Fas/APO-1 (CD95) and Fas-ligand (Fas-L). In fact, we show that CD2 stimulation inhibits apoptosis by preventing the CD3-induced upregulation of Fas and Fas-L in a Fas-dependent experimental system. These data suggest that a costimulatory molecule may control a deletion pathway and may therefore contribute to the regulation of peripheral tolerance.