Antigen-induced death of mature T lymphocytes: analysis by flow cytometry

CD25 blockade protects T cells from activation-induced cell death (AICD) via maintenance of TOSO expression

CD25 monoclonal antibody binding to the alpha-chain of the Interleukin-2 (IL-2) receptor, blocks high-affinity IL-2 binding, thereby preventing complete T-cell activation and being of ample importance in transplantation medicine and potentially the treatment of autoimmune disease. However, CD25 antibodies do not only block T-cell activation but also prevent activation-induced cell death (AICD) attributing a dual function to IL-2. In this study, the modulation of the genomic expression profile of human peripheral blood mononuclear cells (PBMC) with therapeutic concentrations of humanized anti-CD25 mAb was investigated. PBMC were stimulated with CD3 antibody OKT-3 together with recombinant IL-2 in the absence or presence of anti-CD25 mAb. RNA was extracted and subjected to microarray analysis on U133A microarrays (Affymetrix). Anti-CD25 treatment inhibited several genes typically expressed during T-cell activation including granzyme B, signalling lymphocyte activation molecule, family member 1 (SLAMF1), CD40-Ligand (CD40-L), IL-9 and interferon (IFN)-gamma. Interestingly, anti-CD25 mAb also blocked the expression of several genes important for susceptibility to apoptosis, such as death receptor 6 (DR6) or reversed IL-2-mediated repression of anti-apoptotic genes, such as Fas apoptotic inhibitory molecule 3 (FAIM3)/TOSO. Functional significance of DR6 and TOSO expression in IL-2-dependent T-cell activation was subsequently evaluated by RNA interference in AICD: While siRNA specifically directed against DR6 did not modulate FAS-L-mediated apoptosis induction in primary T cells, down-regulation of TOSO significantly increased susceptibility to apoptosis, emphasizing an important role for TOSO in IL-2-mediated AICD.

Development of autoimmune arthritis with aging via bystander T cell activation in the mouse model of Sjögren's syndrome

OBJECTIVE

A wide spectrum of extraglandular manifestations may occur in patients with Sjogren's syndrome (SS), but the mechanisms responsible for in vivo progression are still obscure. We undertook this study to evaluate the age-related changes during the development of extraglandular autoimmune lesions, including arthritis, in the murine model of primary SS, and to evaluate the possible relationship between age-related disturbance of activation-induced cell death and the in vivo kinetics against autoantigens.

METHODS

A total of 126 NFS/sld mice were investigated at ages 2, 4, 6, 10, 12, 18, 20, and 24 months. Cytokine production was tested using culture supernatants from anti-CD3 monoclonal antibody-stimulated T cells. Anti-single-stranded DNA (anti-ssDNA) antibodies, Ig isotypes (IgG1, IgG2a), rheumatoid factor (RF), and anti-type II collagen (anti-CII) antibodies were detected by enzyme-linked immunosorbent assay. Proliferative T cell responses against each of 3 recombinant alpha-fodrin proteins and against CII were analyzed.

RESULTS

Autoimmune arthritis developed in SS model mice until age 24 months. Significant elevations in serum levels of RF, anti-ssDNA antibodies, and anti-CII antibodies were found in aging SS model mice. A high titer of serum autoantibodies against alpha-fodrin fragments (containing different epitopes that were originally identified in primary SS model mice) was frequently detected in young and aged SS model mice. Moreover, we found that alpha-fodrin autoantigen induced Th1 immune responses and accelerated disturbance of Fas-mediated T cell apoptosis in aged SS model mice.

CONCLUSION

These results indicate that age-related disturbance of activation-induced cell death via bystander T cell activation may play a crucial role in the development of autoimmune arthritis in a murine model of SS.

Apoptosis and estrogen deficiency in primary Sjögren syndrome

PURPOSE OF REVIEW

Primary Sjögren syndrome is an autoimmune disorder characterized by lymphocytic infiltrates and destruction of the salivary and lacrimal glands, and systemic production of autoantibodies to the ribonucleoprotein particles SS-A/Ro and SS-B/La. The purpose of this review is to discuss recent advances in the pathogenesis of primary Sjögren syndrome.

RECENT FINDINGS

Although several candidate autoantigens including alpha-fodrin have been reported in Sjögren syndrome, the pathogenic roles of the autoantigens in initiation and progression of SS are still unclear. It is possible that individual T cells activated by an appropriate self antigen can proliferate and form a restricted clone. Recent evidence suggests that the apoptotic pathway plays a central role in tolerizing T cells to tissue-specific self antigen, and may drive the autoimmune phenomenon. Cleavage of certain autoantigens during apoptosis may reveal immunocryptic epitopes that could potentially induce autoimmune response. The studies reviewed imply that Fas-mediated cytotoxicity and caspase-mediated alpha-fodrin proteolysis are involved in the progression of tissue destruction in Sjögren syndrome. Fas ligand (FasL), and its receptor Fas are essential in the homeostasis of the peripheral immune system. It can be considered that a defect in activation-induced cell death of effector T cells may result in the development of autoimmune exocrinopathy in Sjögren syndrome.

SUMMARY

Although the mechanisms by which estrogen deficiency influences autoimmune lesions remain unclear, it is possible that antiestrogenic actions might be a potent factor in the formation of pathogenic autoantigens.

Differential recognition and activation thresholds in human autoreactive GAD-specific T-cells

The activation requirements of autoreactive CD4(+) T-cells were investigated in GAD65-specific HLA-DR0401-restricted clones derived from a diabetic patient using major histocompatibility complex (MHC) class II tetramers (TMrs) as stimulating agents. Despite the fact that TMrs loaded with an immunodominant-altered GAD peptide (TMr-GAD) bound a limited number of T-cell receptors, they were capable of efficiently delivering activation signals. These signals ranged from the early steps of phospholipase C (PLC)-gamma(1) phosphorylation and Ca(2+) mobilization to more complex events, such as CD69 upregulation, cytokine mRNA transcription and secretion, and proliferation. All the effects triggered by TMr-GAD were dose dependent. On the contrary, [(3)H]-thymidine incorporation decreased at high TMr-GAD concentrations because of activation-induced cell death (AICD) after initial proliferation. Lower-avidity clones (as defined by TMr-GAD binding) were less sensitive to activation as well as less susceptible to AICD compared with higher-avidity clones. Induction of apoptosis is a potential immunomodulatory target for therapeutic applications of MHC class II multimers, but the relative resistance of low-avidity T-cells may limit its benefits.

Role of the passive apoptotic pathway in graft-versus-host disease

Donor T cells have been shown to undergo apoptosis during graft-vs-host disease (GVHD). Although active apoptosis mediated through Fas/Fas ligand interactions has been implicated in GVHD, little is known about the role of the passive apoptotic pathway. To examine this question, we compared the ability of normal donor T cells and T cells overexpressing the antiapoptotic protein, Bcl-x(L), to mediate alloreactive responses in vitro and lethal GVHD in vivo. In standard MLCs, T cells that overexpressed Bcl-x(L) had significantly higher proliferative responses but no difference in cytokine phenotype. Overexpression of Bcl-x(L) prolonged survival of both resting and alloactivated CD4(+) and CD8(+) T cells as assessed by quantitative flow cytometry, accounting for the higher proliferative responses. Analysis of engraftment in murine transplantation experiments demonstrated an increase in donor T cell chimerism in animals transplanted with Bcl-x(L) T cells, suggesting that overexpression of Bcl-x(L) prolonged T cell survival in vivo as well. Notably, transplantation of Bcl-x(L) T cells into nonirradiated F(1) recipients also significantly exacerbated GVHD as assessed by mortality and pathological damage in the gastrointestinal tract. However, when mice were irradiated no difference in GVHD mortality was observed between animals transplanted with wild-type and Bcl-x(L) T cells. These data demonstrate that the passive apoptotic pathway plays a role in the homeostatic survival of transplanted donor T cells. Moreover, the susceptibility of donor T cells to undergo passive apoptosis is a significant factor in determining GVHD severity under noninflammatory but not inflammatory conditions.

Possible role of organ-specific autoantigen for Fas ligand-mediated activation-induced cell death in murine Sjögren's syndrome

Activation-induced cell death (AICD) is a well-known mechanism of peripheral T cell tolerance that depends upon an interaction between Fas and Fas ligand (FasL). In this study, we demonstrate that the administration of a soluble form of anti-FasL Ab, FLIM58, results in severe destructive autoimmune exocrinopathy in the murine model of human Sjögren's syndrome (SS), and we found that an organ-specific autoantigen may play an important role on down-modulation of AICD. A high titer of serum autoantibodies against 120-kDa alpha-fodrin autoantigen was detected in the FLIM58-treated mice, and splenic T cell culture supernatants contained high levels of IFN-gamma. In vitro T cell apoptosis assay indicated that FasL-mediated AICD is down-regulated by autoantigen stimulation in spleen cells from the murine SS model, but not from Fas-deficient MRL/lpr mice and FasL-deficient MRL/gld mice. FasL undergo metalloproteinase-mediated proteolytic processing in their extracellular domains, resulting in the release of soluble trimeric ligands (soluble FasL). We showed that the processing of soluble FasL occurs in autoantigen-specific CD4(+) T cells, and that a significant increase in expressions of metalloproteinase-9 mRNA was observed in spleen cells from SS model mice. These findings indicate that the increased generation of soluble FasL inhibits the normal AICD process, leading to the proliferation of effector CD4(+) T cells in the murine SS model.

Products of fourth-stage larvae of Oesophagostomum dentatum induce proliferation in naïve porcine mononuclear cells

Infection of pigs with Oesophagostomum dentatum is a major cause of economic losses in pig productions. Whether infection with this nematode results in a protective immunity is still in debate and information about immune-modulating properties of O. dentatum are lacking. The present study investigated the question whether products of O. dentatum larvae modulate the proliferative response of porcine blood mononuclear cells (poMNC) in vitro. The poMNC of naïve and O. dentatum-infected pigs were cultured for 72 h in the presence of products (total homogenates and culture supernates) derived from third- (L3) and fourth-stage larvae (L4) of O. dentatum. Numbers of vital cells and blast-transformed cells were determined flow cytometrically. No larvae product induced an accelerated death of poMNC in vitro. In contrast, products of L4 (but not L3) significantly increased the numbers of vital poMNC in vitro (up to 187%). In addition, L4 products (homogenates and supernates, 0.1-10 microg/ml) but not those of L3 induced significant blastogenesis of poMNC. This was seen with poMNC from naïve and from O. dentatum-infected animals. In spite of these effects, the larvae products were not able to modulate the mitogen-induced (Concanavalin A) poMNC proliferation of naïve and infected animals. In summary, larvae of O. dentatum contain and secrete products with potential immunomodulatory capacity for porcine peripheral blood mononuclear cells. The differential effects of L3 and indicate that the parasite alters its set immunomodulatory substances during its development. This has to be considered in further studies and may help to identify the mediators involved.

In vivo lymphocyte activation and apoptosis by lectins of the Diocleinae subtribe

This paper reports the overall effects of three lectins, extracted from Canavalia brasiliensis, Dioclea violacea, and D. grandiflora, on BALB/c mice popliteal draining lymph nodes. These lectins have presented high stimulatory capacity on lymph node T cells. Additionally, they were able to induce apoptosis and inflammation (frequently associated with high endothelial venule necrosis). The data presented here suggest that the Diocleinae lectins studied can stimulate in vivo T cell activation and apoptosis, as well as present important side effects.

Soluble HLA class I/CD8 ligation triggers apoptosis in EBV-specific CD8+ cytotoxic T lymphocytes by Fas/Fas-ligand interaction

In the present study, we report that allogeneic soluble HLA class I (sHLA-I) molecules isolated from serum induce apoptosis on EBV-specific CD8(+) Fas(+) cytotoxic T lymphocytes (CTL). CTL apoptosis is induced by the binding of sHLA-I molecules to CD8 and its extent depends on the time of incubation with sHLA-I molecules. Apoptosis is triggered by the interaction of Fas(+) CTL with soluble Fas-ligand, which is released following the binding of sHLA-I antigens to CD8 molecules. These results suggest that sHLA-I molecules may regulate immune responses by inducing apoptosis in virus-specific CTL.

The lymphopenia mutation of the BB rat causes inappropriate apoptosis of mature thymocytes

BB rats develop autoimmune diabetes mellitus at a high frequency. A key factor in the development of the disease is an autosomal recessive mutation determining peripheral T cell lymphocytopenia. Previous studies have suggested that the lymphopenia could be caused by increased cell death. Here we demonstrate that the lyp mutation dramatically reduces the in vitro lifespan of the TCRhi single-positive thymocytes and peripheral T cells, without abolishing their capacity to proliferate. The reduced lifespan is due to an increased rate of apoptosis, and is detected in single-positive thymocytes displaying characteristics of cells which have undergone positive selection. The cell death defect does not affect the in vitro lifespan of peripheral B cells. Interestingly, stimulation can rescue peripheral lyp/lyp T cells from immediate cell death. We propose that the lymphopenia mutation prevents the accumulation of a normal T cell pool, including regulatory subsets, without preventing the activation and proliferation of reactive T cells, thereby creating conditions appropriate for the development of uncontrolled autoimmune responses.

Monocyte-Dependent Death of Freshly Isolated T Lymphocytes: Induction by Phorbolester and Mitogens and Differential Effects of Catalase

Resting T cells are resistant to anti-Fas (CD95) mAb-mediated apoptosis but undergo apoptosis when triggered by anti-CD3 mAb or phorbolester PMA in the presence of PMA-activated monocytes. In this study, PMA, as well as the mitogens PHA and Con A, was found to induce death of resting T cells in the presence of autologous or allogeneic monocytes, while PWM was ineffective. Although several established monocytic and myelocytic cell lines were potent accessory cells for the mitogen-induced expansion of T lymphocytes, they all failed to replace plastic-adherent monocytes in the induction of monocyte-dependent cell death (MDCD) by PMA or PHA. CD45RA-positive cord blood T cells were as susceptible as peripheral blood T cells from adult donors to PMA-stimulated induction of MDCD. Using optimal concentrations of phorbolester, MDCD was inhibited neither by Fas-Fc fusion protein or neutralizing anti-Fas mAb, nor by inhibitors of IL-1β-converting enzyme (ICE)-like proteases. In striking contrast, the H2O2 scavenger catalase completely prevented the PMA-stimulated T cell death, thereby revealing a potent mitogenic activity of PMA for human T cells in the presence of monocytes. Taken together, our results demonstrate that the accessory cell activity of monocytes/macrophages can be separated into “T cell death” and “T cell expansion” costimulatory functions, of which only the latter is mediated by established cell lines. Moreover, our results point to a pivotal role of reactive oxygen intermediates in the execution of MDCD triggered by PMA.

Apoptosis, graft rejection, and transplantation tolerance

Transplantation tolerance can be achieved through several mechanisms, including the action of suppressor cells, the induction of anergy, or the deletion of graft-reactive donor T cells. In this article, the possible involvement of programmed cell death (apoptosis) in allograft rejection and transplantation tolerance is discussed. The recent characterization of members of the tumor necrosis factor-alpha gene family has identified ligands (Fas ligand and TRAIL) and corresponding death receptors (DR). In rejected organ transplants, apoptotic cells are frequently encountered. Conversely, high-level expression of Fas ligand on the allograft correlates with graft acceptance in some models of organ transplantation. Furthermore, some of the immunosuppressive drugs currently in clinical use might exert their activity at least in part through effects on apoptotic pathways. From the available data, it can be inferred that apoptosis contributes to the outcome after organ transplantation, being involved both in graft rejection and in transplantation tolerance.

There is no regulatory role for induced nitric oxide in the regulation of the in vitro proliferative response of bovine mononuclear cells to mitogens, alloantigens or superantigens

Nitric oxide (NO) is a potent cellular mediator which has been shown to modulate several immune mechanisms. Between species, however, there are considerable differences regarding the signals required for induction of NO as well as the kind of cells capable of producing NO. The object of this study was to determine the kinetics of NO production of bovine blood mononuclear cells (boMNC) stimulated in vitro and to investigate whether it modulates their proliferative response following allogeneic (mixed leukocyte cultures, aMLC), mitogenic (PWM, Con A) or superantigenic (SEA, SEB) stimulation. NO production was indirectly determined with the Griess reagent measuring nitrite (NO2-). Significant but low amounts of NO could be detected as early as day 3 after in vitro stimulation and did noly slightly increase during the 6-8 day culture period. Superantigens (SEA, SEB) and aMLCs (4.3-5.2 microM NO2-) induced a significantly higher nitrite accumulation compared to Con A (2.6 microM NO2-). Generation of nitrite, most likely produced by monocytes/macrophages, could be inhibited by 1 mM N-monomethyl-L-arginine (NMLA). Flow cytometric characterization of various cellular responses revealed no differences between cultures with or without NMLA. This included the determination of blastogenesis, absolute numbers of viable cells, expression density of activation markers (MHC class II, IL-2R alpha) and cellular subpopulations (CD4+, CD8+, sIg+) among blasts. In addition, exogenously provided NO via SNOG in non-toxic concentrations (10(-5)-10(-4) M) did not alter the proliferative reaction of boMNC in vitro. The results suggest that NO is induced after in vitro stimulation of boMNC, however, at a low level, and without having any positive or suppressive effects on the so far tested cellular parameters of activation and proliferation.

Apoptosis: mechanisms and relation to AIDS

Infection with the human immunodeficiency virus (HIV) is considered to lead to the acquired immunodeficiency syndrome (AIDS) via the progressive loss of immune competence in the infected host. Recent research has highlighted that HIV may indirectly trigger an active cell suicide process, referred to as programmed cell death or apoptosis, that contributes to the decline in lymphocyte counts throughout the course of HIV infection. We review here the main host- and HIV-related factors actively involved in inducing lymphocyte apoptosis. Among them, the relationships linking HIV, the oxidant/antioxidant balance in the cellular redox system, tumor necrosis factor (TNF) and lymphocyte-associated ceramide generated through the activation of sphingomyelin pathway are receiving growing consideration. Recognizing the importance of apoptosis in AIDS pathogenesis may have a great impact on the design of new strategies for the treatment of the disease. Available data indicate that antioxidant compounds exert antiapoptotic activity. These compounds, in our opinion, should be used in combination regimens with antiretroviral drugs in the treatment of HIV-infected subjects.

The graft-versus-leukemia effect of post-transplant donor leukocyte infusion

Tumor relapse remains a major obstacle to the success of allogeneic bone marrow transplantation (BMT) as a treatment for leukemia. Due to limited treatment options, the outlook for most patients that relapse following allogeneic BMT has been poor. The infusion of normal immunocompetent leukocytes from the original marrow donor has become a promising new option for treating/preventing leukemia relapse in allogeneic BMT recipients. This form of treatment has often been referred to as donor leukocyte infusion (DLI) therapy. Our laboratory is using murine models of allogeneic BMT to address important unresolved issues regarding DLI therapy in an effort to make the treatment more effective. These include identification of the antileukemic effector cells, augmentation of the antileukemic effect, and understanding why graft-versus-host-disease (GVHD) is less severe than anticipated. This article reviews our work in murine models of DLI and introduces our current working hypotheses concerning DLI therapy.

Apoptosis of mature T lymphocytes: putative role in the regulation of cellular immune responses and in the pathogenesis of HIV infection

In this chapter, some aspects of programmed cell death, or apoptosis, of T lymphocytes are discussed. It has been recognized that transformed T cells and immature T lymphocytes can be triggered to undergo apoptosis. As in other cell systems, apoptosis is characterized by cell shrinkage, nuclear condensation, and DNA fragmentation that displays the characteristic "ladder" pattern of approximately 180-200 bp fragments. More recently, however, it has become clear that apoptosis is not restricted to immature thymocytes or transformed T lymphocytes, but can also occur in mature peripheral T cells. This raises the question of whether apoptosis plays a role as a mechanism in regulating cellular immune responses, which will be discussed in the following sections. We will also address the issue of the potential role of T cell apoptosis in pathophysiology. Here, we will concentrate on the infection with human immunodeficiency virus (HIV), where apoptosis is thought to contribute to the continuous decline in CD4+ T cells.