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

Detection of Aberrant CD58 Expression in a Wide Spectrum of Lymphoma Subtypes: Implications for Treatment Resistance

CD58 or lymphocyte function-associated antigen-3, is a ligand for CD2 receptors on T and NK cells and is required for their activation and target cell killing. We recently showed a trend toward higher frequency of CD58 aberrations in patients with diffuse large B-cell lymphoma (DLBCL) who progressed on chimeric antigen receptor-T-cell treatment compared with those who responded. Given that CD58 status may be an important measure of T-cell-mediated therapy failure, we developed a CD58 immunohistochemical assay and evaluated CD58 status in 748 lymphomas. Our results show that CD58 protein expression is downregulated in a significant proportion of all subtypes of B-, T-, and NK-cell lymphomas. CD58 loss is significantly related to poor prognostic indicators in DLBCL and to ALK and DUSP22 rearrangements in anaplastic large-cell lymphoma. However, it is not associated with overall or progression-free survival in any of the lymphoma subtypes. As eligibility for chimeric antigen receptor-T-cell therapy is being extended to a broader spectrum of lymphomas, mechanisms of resistance, such as target downregulation and CD58 loss, may limit therapeutic success. CD58 status is therefore an important biomarker in lymphoma patients who may benefit from next-generation T-cell-mediated therapies or other novel approaches that mitigate immune escape.

Dual Mechanisms of Metabolism and Gene Expression of the CCRF-CEM Leukemia Cells under Glucocorticoid Treatment

BACKGROUND

Glucocorticoids play an essential part in anti-leukemic therapies, but resistance is a crucial event for the prognosis of the disease. Glucocorticoids influence the metabolic properties of leukemic cells. The inherent plasticity of clinically evolving cancer cells justifies the characterization of drug-induced early oncogenic pathways, which represent a likely source of detrimental secondary effects.

AIM

The present work aims to investigate the effect of glucocorticoids in metabolic pathways in the CCRF-CEM leukemic cells. Metabolic factors and gene expression profiles were examined in order to unravel the possible mechanisms of the CCRF-CEM leukemic cell growth dynamics.

METHODS

CCRF-CEM cells were used as a model. Cells were treated with prednisolone with concentrations 0-700 μM. Cell culture supernatants were used for glucose, lactic acid, LDH, Na⁺, K⁺ and Ca⁺⁺ measurements. Cytotoxicity was determined with flow cytometry. Microarray analysis was performed using two different chips of 1.2 k and 4.8 k genes. Gene Ontology enrichment analysis was applied to find metabolism- and GC-related genes.

RESULTS

Higher prednisolone concentrations inhibited glucose uptake, without exhibiting any cytotoxic effects. Glucose consumption did not correlate with the total cell population, or the viable population, indicating that growth is not directly proportional to glucose consumption. Neither of the subpopulations, i.e., viable, necrotic, or apoptotic cells, contributed to this.

CONCLUSIONS

Different types of leukemic cells seem to exhibit different patterns of glucose metabolism. Both resistant and sensitive CCRF-CEM cells followed the aerobic pathway of glycolysis. There is probably a rapid change in membrane permeability, causing a general shutdown towards everything that is outside the cell. This could in part also explain the observed resistance. Glucocorticoids do not enter the cell passively anymore and therefore no effects are observed. Based on our observations, ion concentrations are measurable factors both in vitro and in vivo, which makes them possible markers of glucocorticoid cytotoxic action.

Regulation of key molecules of immunological synapse by T11TS immunotherapy abrogates Cryptococcus neoformans infection in rats

Cryptococcus neoformans infects and disseminates in hosts with diminished T cell responses. The immunomodulator T11TS (T11 target structure) had profound potential in glioma as well as C. neoformans infected model for disease amelioration. It is been established by our group that T11TS potentiates Calcineurin-NFAT pathway in T cells of C. neoformans infected rats. We investigated the upstream Immunological Synapse (IS) molecules that are vital for the foundation of initial signals for downstream signaling, differentiation and proliferation in T cells. Improved RANTES level in the T11TS treated groups suggests potential recruitment of T cells. Down-regulation of TCRαβ, CD3ζ, CD2, CD45 and CD28 molecules by cryptococcus were boosted after T11TS therapy. Heightened expression of inhibitory molecule CTLA-4 in cryptococcosis was dampened by T11TS. The decline of MHC I, MHC II and CD80 expression on macrophages by C. neoformans were enhanced by T11TS. The dampening of positive regulators and upsurge of negative regulators of the IS during cryptococcosis was reversed with T11TS therapy resulting in enhanced clearance of fungus from the lungs as envisaged by our histological studies. This preclinical study with T11TS opens a new prospect for potential immunotherapeutic intervention against the devastating C. neoformans infection with positive aspect for the long-term solution and a safer immunotherapeutic regimen.

Loading of Primary Human T Lymphocytes with Citrate-Coated Superparamagnetic Iron Oxide Nanoparticles Does Not Impair Their Activation after Polyclonal Stimulation

For the conversion of immunologically cold tumors, characterized by a low T cell infiltration, into hot tumors, it is necessary to enrich T cells in the tumor area. One possibility is the use of magnetic fields to direct T cells into the tumor. For this purpose, primary T cells that were freshly isolated from human whole blood were loaded with citrate-coated superparamagnetic iron oxide nanoparticles (SPION^Citrate). Cell toxicity and particle uptake were investigated by flow cytometry and atomic emission spectroscopy. The optimum loading of the T cells without any major effect on their viability was achieved with a particle concentration of 75 µg Fe/mL and a loading period of 24 h. The cellular content of SPION^Citrate was sufficient to attract these T cells with a magnet which was monitored by live-cell imaging. The functionality of the T cells was only slightly influenced by SPION^Citrate, as demonstrated by in vitro stimulation assays. The proliferation rate as well as the expression of co-stimulatory and inhibitory surface molecules (programmed cell death 1 (PD-1), lymphocyte activation gene 3 (LAG-3), T cell immunoglobulin and mucin domain containing 3 (Tim-3), C-C motif chemokine receptor 7 (CCR7), CD25, CD45RO, CD69) was investigated and found to be unchanged. Our results presented here demonstrate the feasibility of loading primary human T lymphocytes with superparamagnetic iron oxide nanoparticles without influencing their viability and functionality while achieving sufficient magnetizability for magnetically controlled targeting. Thus, the results provide a strong fundament for the transfer to tumor models and ultimately for new immunotherapeutic approaches for cancer treatment.

T cell exhaustion implications during transplantation

Exhaustion of lymphocyte function, particularly T cell exhaustion, due to prolonged exposure to a high load of foreign antigen is commonly seen during chronic viral infection as well as antitumor immune responses. This phenomenon has been associated with a determined molecular mechanism and phenotypic manifestations on the cell surface. In spite of investigation of exhaustion, mostly about CD8 responses toward viral infections, recent studies have reported that chronic exposure to antigen may develop exhaustion in CD4 + T cells, B cells, and NK cells. Little is known with respect to lymphocyte exhaustion during transplantation and its effect on aberrant anti-graft responses. Through a same mechanobiology observed during chronic exposure of foreign viral antigens, alloantigen persistence mediated by allograft could develop a favorable circumstance for exhaustion of T cells responding to allograft. However, to achieve better manipulation approaches of this event to reduce the complications during transplantation, we need to be armed with a bulk of knowledge with regard to quality and quantity of T cell exhaustion occurring in various allografts, the kinetics of exhaustion development, the impression of immunosuppressive agents on the exhaustion, and the influence of exhaustion on graft survival and immune tolerance.

T cell exhaustion and immune-mediated disease-the potential for therapeutic exhaustion

T cell exhaustion represents a continuous spectrum of cellular dysfunction induced during chronic viral infection, facilitating viral persistence and associating with poor clinical outcome. Modulation of T cell exhaustion can restore function in exhausted CD8 T cells, promoting viral clearance. Exhaustion has also been implicated as playing an important role in anti-tumour responses, whereby exhausted tumour-infiltrating lymphocytes fail to control tumour progression. More recently exhaustion has been linked to long-term clinical outcome in multiple autoimmune diseases but, in contrast to cancer or infection, it is associated with a favourable clinical outcome characterised by fewer relapses. An increasing understanding of key inhibitory signals promoting exhaustion has led to advances in therapy for chronic infection and cancer. An increasing understanding of this biology may facilitate novel treatment approaches for autoimmunity through the therapeutic induction of exhaustion.

TGFβ-dependent expression of PD-1 and PD-L1 controls CD8(+) T cell anergy in transplant tolerance

CD8⁺ T cell anergy is a critical mechanism of peripheral tolerance, poorly investigated in response to immunotherapy. Here, using a pancreatic islet allograft model and CD3 antibody therapy, we showed, by single cell gene profiling, that intragraft CD8⁺ lymphocytes coexpressing granzyme B and perforin were selectively depleted through the Fas/FasL pathway. This step led to long-standing anergy of the remaining CD8⁺ T cells marked by the absence of cytotoxic/inflammatory gene expression also confirmed by transcriptome analysis. This sustained unresponsiveness required the presence of the alloantigens. Furthermore, tissue-resident CD8⁺ lymphocytes produced TGFβ and expressed the inhibitory receptors PD-1 and PD-L1. Blockade of TGFβ downregulated PD-1 and PD-L1 expression and precipitated graft rejection. Neutralizing PD-1, PD-L1 or TGFβRII signaling in T cells also abrogated CD3 antibody-induced tolerance. These studies unravel novel mechanisms underlying CD8⁺ T cell anergy and reveal a cell intrinsic regulatory link between the TGFβ and the PD-1/PD-L1 pathways.

DOI:http://dx.doi.org/10.7554/eLife.08133.001

The immune system is always on guard for signs of infection or cells that have become diseased. When these signs are identified, a subset of white blood cells called CD8⁺ T cells leap into action, multiply in number and then act to eliminate the potential threat. While this response is essential to fighting off infections and other diseases like cancer, it can backfire in people with an organ transplant. Indeed, the CD8⁺ T cells can target and attack the cells of the transplanted organ causing the body to reject the organ.

One way to avoid transplant rejection would be to turn off CD8⁺ T cells that have learned to recognize cells from the transplant. In fact, studies in 2012 and 2013 showed that treating transplanted animals with an antibody that binds T cells protects a transplanted organ from attack. This treatment had to be given after the CD8⁺ T cells had recognized and began targeting the transplanted organ to be effective. But it was not clear exactly how this antibody treatment protected the transplant.

Now, Baas, Besançon et al. – including some of the same researchers involved in the earlier studies – show that the antibodies used in the treatment selectively target and eliminate the attacking CD8⁺ T cells. This leaves behind only inactive CD8⁺ T cells that don’t harm the transplant. To do this, Baas, Besançon et al. transplanted pancreatic cells from mice into other mice with a diabetes-like disorder. Next, the experiments compared gene expression in CD8⁺ T cells found within the transplanted tissue in mice that were treated with the antibody and those that were not treated. The expression of many genes for toxic molecules was stopped after treatment with the antibody leaving the CD8⁺ T cells in an inactive state.

In addition, the treated CD8⁺ T cells expressed more of a certain type of receptor (called PD-1 and PD-L1) that acts as inhibitory checkpoint for the immune system. So, Baas, Besançon et al. treated transplanted mice with both the T cell-eliminating antibody and antibodies that block these inhibitory receptors to see what would happen. The transplanted organs were quickly attacked and rejected. This shows that the inhibitory receptors play a crucial role in helping to shut down attacking CD8⁺ T cells in the initial antibody treatment and allowed long-term survival of the transplanted organs. Blocking another protein called TGFβ in antibody-treated mice also caused organ rejection. The findings help explain how these antibodies protect transplanted organs and may help scientists trying to develop new anti-transplant rejection drugs in the future.

DOI:http://dx.doi.org/10.7554/eLife.08133.002

A CD2 high-expressing stress-resistant human plasmacytoid dendritic-cell subset

Human plasmacytoid dendritic cells (pDCs) were considered to be a phenotypically and functionally homogeneous cell population; however, recent analyses indicate potential heterogeneity. This is of major interest, given their importance in the induction of anti-viral responses and their role in creating immunologically permissive environments for human malignancies. For this reason, we investigated the possible presence of human pDC subsets in blood and bone marrow, using unbiased cell phenotype clustering and functional studies. This defined two major functionally distinct human pDC subsets, distinguished by differential expression of CD2. The CD2(hi) and CD2(lo) pDCs represent discontinuous subsets, each with hallmark pDC functionality, including interferon-alpha production. The rarer CD2(hi) pDC subset demonstrated a significant survival advantage over CD2(lo) pDC during stress and upon exposure to glucocorticoids (GCs), which was associated with higher expression of the anti-apoptotic molecule BCL2. The differential sensitivity of these two human pDC subsets to GCs is demonstrated in vivo by a relative increase in CD2(hi) pDC in multiple myeloma patients treated with GCs. Hence, the selective apoptosis of CD2(lo) pDC during stress represents a novel mechanism for the control of innate responses.

Disease relevance of T11TS-induced T-cell signal transduction through the CD2-mediated calcineurin-NFAT pathway: Perspectives in glioma immunotherapy

Malignant glioma is the most lethal of a wide array of CNS neoplasms. Its onset and progression are markedly associated with profound immunosupression and paralysis of T-cell survival and proliferation. Myriad immunotherapeutic strategies are presently used to target such T-cell anomalies in glioma. Our recent work has highlighted use of the novel glycopeptide, the CD2 ligand, T11 target structure (T11TS) as an immunotherapeutic agent against experimentally induced glioma in rats. We have shown that T11TS causes multi-target modulation of key components of the T-cell - antigen presenting cell (APC) immunological synapse. This consequently triggers T-cell activation so as to reverse glioma-induced changes to physiological levels. T11TS administration also causes CD2 upregulation. Earlier we also found T11TS to cause enhanced proliferation of both CD4+ and CD8+ T-cells in glioma conditions. These findings led us to believe that downstream CD2-stimulated "alternative pathway" of calcineurin-NFAT could be a possible target for modulation by T11TS. In the present paper we thus show that immunotherapy with T11TS induces a multi-targeted approach towards activation of this "alternative pathway" of T-cell signaling providing an immunotherapeutic advantage against glioma. We show here that T11TS immunotherapy causes positive modulations of the CD2 pathway-associated proteins, viz., p59fyn, protein kinase C-θ (PKC-θ), calcineurin and nuclear factor for activation of T-cells (NFAT) and hint that this may accord greater survival and proliferation advantage to T-cells of the glioma-bearing animals for augmented defence against glioma. These findings help open a molecular immunotherapeutic door - one which is directed towards clinical studies for glioma-immunotherapy using T11TS.

CD8+ T cells: GITR matters

As many members of the tumor necrosis factor receptor superfamily, glucocorticoid-induced TNFR-related gene (GITR) plays multiple roles mostly in the cells of immune system. CD8⁺ T cells are key players in the immunity against viruses and tumors, and GITR has been demonstrated to be an essential molecule for these cells to mount an immune response. The aim of this paper is to focus on GITR function in CD8⁺ cells, paying particular attention to numerous and recent studies that suggest its crucial role in mouse disease models.

Induction of granzyme B and T cell cytotoxic capacity by IL-2 or IL-15 without antigens: multiclonal responses that are extremely lytic if triggered and short-lived after cytokine withdrawal

The purpose of these studies was to determine the minimal requirements to induce granzyme B, cytotoxic granules and perforin-dependent lytic capacity. To our surprise, both IL-2 and IL-15 induced not only proliferation, but also profound granzyme B and lytic capacity from CD8+ T cells in the absence of antigen or TCR-stimulation. Mouse splenocytes were incubated with mouse r-IL-2 or r-IL-15 for three days, tested by anti-CD3 redirected lysis and examined for intracellular granzyme B and for T cell activation markers. With 10(-8) M IL-2 or IL-15, there was excellent lytic activity at 1:1 effector to target ratios mediated by T cells from wild-type but not from perforin-gene-ablated mice, consistent with multiclonal activation. Lower interleukin concentrations induced less lytic activity. Granzyme B was undetectable on day 0, and greatly elevated on day 3 in CD44hi CD8+ T cells as detected by flow cytometry. Cytokines alone elevated the granzyme B as much as concanavalin A combined with the cytokines. Some ex vivo CD8+ T cells were CD122+, as were the cultured granzyme B+ cells, thus both populations had low-affinity receptors for the interleukins. Only some of the activated cells were proliferating as detected by CFSE labeling. When the cytokines were withdrawn, the cells lost lytic activity within 24 h and then within the next 24 h, died. Our results suggest that high concentrations of either IL-2 or IL-15 will activate the lytic capacity and granzyme B expression of many T cells and that antigen recognition is not required.

T helper lymphocytes rescue CTL from activation-induced cell death

T cell activation is characterized by a vast expansion of Ag-specific T cells followed by an equally extensive reduction in T cell numbers. This decline is due, in part, to activation-induced apoptosis of the responding T cells during repeated encounter with Ag. In the current study, we used solid-phase MHC class I/peptide monomers to cause activation-induced cell death (AICD) of previously activated CD8 T cells in an Ag-specific manner. AICD occurred rapidly and was mediated primarily by Fas-FasL interactions. Most interestingly, we observed that Th cells could provide survival signals to CTL significantly reducing the level of AICD. Both Th1 and Th2 subsets were capable of protecting CTL from AICD, and a major role for soluble factors in this protection was ruled out, as cell-to-cell contact was an essential component of this Th-mediated protection. Upon encounter with Ag-expressing tumor cells, CTL underwent significant apoptosis. However, in the presence of Th cells, the CTL not only were protected against death, but also had significantly greater lytic ability. In vivo tumor protection studies using peptide immunization showed that the activation of Ag-specific Th cells was crucial for optimal protection, but did not affect the magnitude of the CTL response in the lymphoid tissues. In this study, we examine the type of help that CD4 T cells may provide and propose a model of Th cell-CTL interaction that reduces CTL death. Our results show a novel role for Th cells in the maintenance of CTL responses.

GITR, a member of the TNF receptor superfamily, is costimulatory to mouse T lymphocyte subpopulations

GITR (glucocorticoid-induced TNFR family related gene) is a member of the TNFR superfamily (TNFRSF) that is expressed in different cell types, including T lymphocytes. Because of a high homology in its cytoplasmic region with other known costimulatory members of the TNFRSF, we investigated whether GITR played a costimulatory role in T lymphocyte subpopulations. Our results show that the proliferation response of CD8+ and CD4+ peripheral T cell subpopulations was potentiated when a GITR costimulus was added to an anti-CD3 stimulus. Furthermore, expression of the main activation-induced receptor (IL-2Ralpha) and production of IL-2 and IFN-gamma were increased more with a GITR costimulus than with anti-CD3 alone. GITR stimulation also enhanced anti-CD3-induced ERK phosphorylation, suggesting that GITR is involved in MAPK-pathway activation. Interestingly, CD4+CD25+ regulatory T cell (Treg cell) proliferation was triggered by the GITR costimulus; Treg cell proliferation was paralleled by the loss of the anergic phenotype and suppressor activity. Nevertheless, unstimulated GITR(-/-) CD4+CD25+ and GITR(+/+) CD4+CD25+ cells were equally able to exert suppressor activity on CD4+CD25- responder cells. These results indicate a novel function for GITR as costimulatory molecule of T cell subsets.

Synthesis of glucocorticoid-induced leucine zipper (GILZ) by macrophages: an anti-inflammatory and immunosuppressive mechanism shared by glucocorticoids and IL-10

Glucocorticoids and interleukin 10 (IL-10) prevent macrophage activation. In murine lymphocytes, glucocorticoids induce expression of glucocorticoid-induced leucine zipper (GILZ), which prevents the nuclear factor kappaB (NF-kappaB)-mediated activation of transcription. We investigated whether GILZ could account for the deactivation of macrophages by glucocorticoids and IL-10. We found that GILZ was constitutively produced by macrophages in nonlymphoid tissues of humans and mice. Glucocorticoids and IL-10 stimulated the production of GILZ by macrophages both in vitro and in vivo. Transfection of the macrophagelike cell line THP-1 with the GILZ gene inhibited the expression of CD80 and CD86 and the production of the proinflammatory chemokines regulated on activation normal T-cell expressed and secreted (CCL5) and macrophage inflammatory protein 1alpha (CCL3). It also prevented toll-like receptor 2 production induced by lipopolysaccharide, interferongamma, or an anti-CD40 mAb, as well as NF-kappaB function. In THP-1 cells treated with glucocorticoids or IL-10, GILZ was associated with the p65 subunit of NF-kappaB. Activated macrophages in the granulomas of patients with Crohn disease or tuberculosis do not produce GILZ. In contrast, GILZ production persists in tumor-infiltrating macrophages in Burkitt lymphomas. Therefore, GILZ appears to play a key role in the anti-inflammatory and immunosuppressive effects of glucocorticoids and IL-10. Glucocorticoid treatment stimulates GILZ production, reproducing an effect of IL-10, a natural anti-inflammatory agent. The development of delayed-type hypersensitivity reactions is associated with the down-regulation of GILZ gene expression within lesions. In contrast, the persistence of GILZ gene expression in macrophages infiltrating Burkitt lymphomas may contribute to the failure of the immune system to reject the tumor.

Molecular mechanisms of pre-T cell receptor-induced survival

En route to maturing as T cell receptor (TCR) alphabeta-expressing cells, the development of thymocytes is contingent on expression of a pre-TCR complex comprising a TCRbeta chain paired with a surrogate TCRalpha chain, pre-Talpha (pTalpha). The pre-TCR has been proposed to promote cell survival, proliferation, differentiation, and lineage commitment. However, the precise molecular mechanisms governing this variety of effects remain elusive. Here, we present a cellular system designed to biochemically dissect signals elicited upon pre-TCR expression. Using the T cell line 4G4 stably transfected with one of the two known pTalpha isoforms or selective pTalpha deletion mutants and TCRbeta, we were able to observe that expression of a functional pre-TCR complex is sufficient to control the levels of surface Fas protein, the stimulation of mitogen-activated and stress-regulated kinases, and the activation status of the p53 antioncogene. We demonstrate that this regulation has a major impact on the expression of important regulators of apoptosis, such as Bcl-2 family members, and the cell cycle, such as p21(WAF). Furthermore, we show here that cells expressing a functional pre-TCR are more resistant to different types of DNA damage-induced apoptosis and that these effects are contingent on an intact cytoplasmic tail of pTalpha. We finally propose that the presence of a functional pre-TCR complex triggers many intracellular pathways capable of driving and ensuring thymocyte survival in the presence of DNA damage.

Critical role of caspases in the regulation of apoptosis and proliferation of mucosal T cells

BACKGROUND & AIMS

Caspases are critical mediators of apoptosis and proliferation of peripheral blood T cells (PBT), but their role in lamina propria T cells (LPT), a cell population highly susceptible to apoptosis, has not been explored.

METHODS

RA(+), RO(+) PBT, and LPT were activated with CD3, CD2, and CD28 antibodies, and caspase activity, apoptosis, and proliferation were measured by a fluorometric assay, DNA content, and thymidine incorporation, respectively. Levels of FLIP, an endogenous inhibitor of caspase 8, were measured by immunoblotting.

RESULTS

In RA(+) and RO(+) PBT, activation leads to significant increase of caspase activity but not cell death, whereas in LPT a lower elevation of caspase activity was followed by a marked degree of apoptosis. Based on the results of its inhibition, caspase 8 seemed to be essential for LPT apoptosis but, in contrast to RA(+) PBT, had no effect on proliferation. In addition, compatible with their differential susceptibility to apoptosis, levels of FLIP were lower in LPT than PBT.

CONCLUSIONS

The high susceptibility of LPT to apoptosis is associated with a distinct regulation of caspase 8 activity, which seems to reflect their mucosal origin rather than simply their memory status. This unique behavior may allow proper control of mucosal T-cell proliferation while still permitting elimination by apoptosis in the face of excessive antigenic pressure.

Molecular mechanisms of immunomodulatory activity of glucocorticoids

The development and function of cells in the immune system are regulated by many intrinsic and extrinsic factors. One class of molecule that affects immune cells belongs to the neuroendocrine system and the best-studied mediators in this category are glucocorticoids. These are small lypophilic molecules that participate in a wide number of normal and pathologic processes. This paper concentrates on their physiologic and pharmacologic effects on the immune response.

Specific and effective targeting cancer immunotherapy with a combination of three bispecific antibodies

For the purpose of establishing a new adoptive immunotherapy for bile duct carcinoma (BDC), we previously constructed two kinds of bispecific antibodies (bsAbs), anti-MUC1 x anti-CD3 (M x 3) and anti-MUC1 x anti-CD28 (M x 28), which activate T cells and form bridges between them and MUC1-expressing tumor cells. In our previous studies [Cancer Res. 56 (1996) 4205] specific targeting therapy (STT) consisting of i.v. administration of lymphokine activated killer cells with a T cell phenotype (T-LAK) sensitized with two kinds of bsAbs to human BDC-grafted severe combined immunodeficient (SCID) mice demonstrated remarkable inhibition of tumor growth. However, complete cures could not be obtained. In order to improve antitumor efficacy, we have paid attention to anti-CD2 monoclonal antibodies (mAbs), thought to play an important roles in signal transduction in T cell activation or control of T cell receptor (TCR)-driven activation. Therefore, we developed another bsAb, anti-MUC1 x anti-CD2 (M x 2), in order to examine if this would show synergism with the two previously described bsAbs. The combination of the three bsAbs (M x 3, M x 28 and M x 2 bsAbs) showed highest cytotoxicity against MUC1-expressing BDC cells when given simultaneously with peripheral blood mononuclear cells (PBMCs) or T-LAK cells in vitro. When 2 x 10(7) T-LAK cells sensitized with different combinations of bsAbs were administered four times i.v. to BDC-grafted SCID mice, the best therapeutic result was obtained with a combination of all three bsAbs. These results indicate usefulness of combination of three bsAbs for targeting cancer immunotherapy.

CD44 co-stimulates apoptosis in thymic lymphomas and T cell hybridomas

Thymic lymphomas and hybridomas vary in their sensitivity to dexamethasone (DEX). Identical variance has been demonstrated in our laboratory for apoptosis of such cells by primary thymic epithelial cells or a cell line (TEC). We have also shown that apoptosis induced by TEC was partially mediated by TEC-derived glucocorticoids (GC). We studied the responses of various thymic lymphomas and hybridomas to TEC and DEX. Of these cells, PD1.6 and 2B4 were sensitive whereas B10 were relatively resistant to either inducer. In the present study we found that TEC and DEX synergize in inducing B10 cell apoptosis. B10 cells could also undergo apoptosis by TEC, conditional upon the presence of a TEC-sensitive cell (PD1.6 or 2B4). Contact between TEC and B10 was essential for apoptosis to occur. Thus, TEC may provide two signals, one mediated by GC and the other requiring cell to cell contact. We then analyzed the involvement of co-stimulatory or adhesion molecules in the TEC-induced apoptosis of thymic lymphoma cells. Soluble anti-CD44 antibodies but not anti-CD18, CD2 or CD28, inhibited TEC-induced apoptosis of PD1.6. Dimerization of CD44 by immobilized antibodies augmented DEX-induced apoptosis of all the lymphomas tested. CD44 cross-linkage up-regulated expression of the pro-apoptotic protein Bax, and down-regulated the anti-apoptotic protein, Bclx(L), in the presence of DEX. Taken together, the data suggest that CD44 enhances the apoptotic response of T lymphoma cells to DEX, and that CD44 modulates TEC-induced apoptosis of thymic lymphomas.

Modulation of T-cell activation by the glucocorticoid-induced leucine zipper factor via inhibition of nuclear factor kappaB

Previously a novel gene was identified that encodes a glucocorticoid-induced leucine zipper (GILZ) whose expression is up-regulated by dexamethasone. This study analyzed the role of GILZ in the control of T-cell activation and its possible interaction with nuclear factor kappaB (NF-kappaB). Results indicate that GILZ inhibits both T-cell receptor (TCR)-induced interleukin-2/interleukin-2 receptor expression and NF-kappaB activity. In particular, GILZ inhibits NF-kappaB nuclear translocation and DNA binding due to a direct protein-to-protein interaction of GILZ with the NF-kappaB subunits. Moreover, GILZ-mediated modulation of TCR-induced responses is part of a circuit because TCR triggering down-regulates GILZ expression. These results identify a new molecular mechanism involved in the dexamethasone-induced regulation of NF-kappaB activity and T-cell activation. (Blood. 2001;98:743-753)

Western and Chinese antirheumatic drug-induced T cell apoptotic DNA damage uses different caspase cascades and is independent of Fas/Fas ligand interaction

Spontaneous or therapeutic induction of T cell apoptosis plays a critical role in establishing transplantation tolerance and maintaining remission of autoimmune diseases. We investigated the mechanisms of apoptosis induced by Chinese and Western antirheumatic drugs (ARDs) in human T cells. We found that hydroxychloroquine, Tripterygium wilfordii hook F, and tetrandrine (Tet), but not methotrexate, at therapeutic concentrations can cause T cell death. In addition, Tet selectively killed T cells, especially activated T cells. Although ARD-induced cytotoxicity was mediated through apoptotic mechanisms, Fas/Fas ligand interaction was not required. We further demonstrated that the processes of phosphatidylserine externalization and DNA damage along the ARD-induced T cell apoptotic pathway could operate independently, and that selective inhibition of DNA damage by caspase inhibitors did not prevent T cells from undergoing cell death. Moreover, we found that Tet- and Tripterygium wilfordii hook F-induced T cell DNA damage required caspase-3 activity, and hydroxychloroquine-induced T cell DNA damage was mediated through a caspase-3- and caspase-8-independent, but Z-Asp-Glu-Val-Asp-fluomethyl ketone-sensitive, signaling pathway. Finally, the observation that ARD-induced activation of caspase-3 in both Fas-sensitive and Fas-resistant Jurkat T cells indicates that Fas/Fas ligand interaction plays no role in ARD-induced T cell apoptosis. Our observations provide new information about the complex apoptotic mechanisms of ARDs, and have implications for combining Western and Chinese ARDs that have different immunomodulatory mechanisms in the therapy of autoimmune diseases and transplantation rejection.

Identification of Fas-L-expressing apoptotic T lymphocytes in normal human peripheral blood : in vivo suicide

Fas-L molecules expressed by in vitro stimulated T cells may be critically involved in suicidal activation-induced cell death (AICD) of such cells through engagement of their Fas receptors. A similar suicide of T cells was postulated to occur even in vivo, to eliminate dangerous activated lymphocytes; however, the demonstration of suicidal AICD of T cells in healthy humans in vivo is still lacking. We therefore investigated the possible occurrence of Fas-L-linked suicidal apoptosis of T cells in normal human peripheral blood. For this purpose, we took advantage of immunoelectron microscopy, which allows simultaneous visualization of the morphological apoptotic cellular changes together with surface expression of Fas-L molecules. Very few T lymphocytes were observed showing the ultrastructural features of apoptotic lymphocytes; these occasional apoptotic T cells, together with the majority of the normal T cell population, expressed the Fas molecule on the plasma membrane, as expected. Interestingly, the apoptotic cells were also Fas-L-positive, whereas normal T cells were Fas-L-negative. Such Fas-L-associated T cell suicide operating in vivo in healthy individuals is presumably able to suppress immune responses and prevent autoreactivity, thus maintaining the homeostasis of human blood.

Engagement of the α2β1 integrin inhibits Fas ligand expression and activation-induced cell death in T cells in a focal adhesion kinase-dependent manner

T-cell receptor (TCR)-mediated apoptosis, also known as activation-induced cell death (AICD), plays an important role in the control of immune response and in the development of T-cell repertoire. Mechanistically, AICD has been largely attributed to the interaction of Fas ligand (Fas-L) with its cell surface receptor Fas in activated T cells. Signal transduction mediated by the integrin family of cell adhesion receptors has been previously shown to modulate apoptosis in a number of different cell types; in T cells, integrin signaling is known to be important in cellular response to antigenic challenge by providing a co-stimulatory signal for TCR. In this study we demonstrate that signaling via the collagen receptor 2β1 integrin specifically inhibits AICD by inhibiting Fas-L expression in activated Jurkat T cells. Engagement of the 2β1 integrin with monoclonal antibodies or with type I collagen, a cognate ligand for 2β1, reduced anti-CD3 and PMA/ionomycin-induced cell death by 30% and 40%, respectively, and the expression of Fas-L mRNA by 50%. Further studies indicated that the 2β1-mediated inhibition of AICD and Fas-L expression required the focal adhesion kinase FAK, a known component in the integrin signaling pathways. These results suggest a role for the 2β1 integrin in the control of homeostasis of immune response and T-cell development.

Cloning and Expression of a Short Fas Ligand: A New Alternatively Spliced Product of the Mouse Fas Ligand Gene

The Fas/FasL system mediates apoptosis in several different cell types, including T lymphocytes. Fas ligand (FasL), a 40-kD type II membrane protein also expressed in activated T cells, belongs to the tumor necrosis factor ligand family. We describe a new alternative splicing of mouse FasL, named FasL short (FasLs), cloned by reverse transcriptase-polymerase chain reaction. FasLs is encoded by part of exon 1 and part of exon 4 of FasL gene. The protein encoded by FasLs mRNA has a putative initiation code at position 756 and preserves the same reading frame as FasL, resulting in a short molecule lacking the intracellular, the transmembrane, and part of the extracellular domains. RNase protection and immunoprecipitation analysis showed that FasLs is expressed in nonactivated normal spleen cells and in hybridoma T cells and that it is upregulated upon activation by anti-CD3 monoclonal antibody (MoAb). Moreover, FasLs-transfected cells expressed soluble FasLs in the supernatant and became resistant to apoptosis induced by agonist anti-Fas MoAb. Thus, FasLs, a new alternative splicing of FasL, is involved in the regulation of Fas/FasL-mediated cell death.

The diversity of T-cell co-stimulation in the induction of antitumor immunity

T-cell activation has now been shown to require at least two signals. The first signal is antigen-specific, is delivered through the T-cell receptor (TCR) via the peptide/major histocompatibility complex (MHC), and causes the T cell to enter the cell cycle. The second, or co-stimulatory, signal is required for cytokine production and proliferation, and is mediated through ligand interaction on the surface of the T cell. This chapter deals with: 1) comparative studies on the use of a dual-gene construct of a recombinant vaccinia (rV) vector containing a tumor-associated antigen (TAA) gene and a co-stimulatory molecule gene vs the use of admixtures of rV-TAA and rV containing the co-stimulatory molecule to induce anti-tumor immunity; 2) the use of an admixture of vaccinia viruses containing a TAA gene and the B7-1 co-stimulatory molecule gene to induce a therapeutic response in a lung metastasis tumor model; 3) the antitumor efficacy of whole-tumor-cell vaccines in which the B7-1 co-stimulatory molecule is expressed in a tumor-cell vaccine via a vaccinia vs a retroviral vector; 4) the use of recombinant poxviruses containing the genes for the co-stimulatory molecules ICAM-1 or LFA-3 to induce antitumor immunity; and 5) the use of poxvirus vectors containing a triad of co-stimulatory molecules (B7-1, ICAM-1 and LFA-3) that synergize to enhance both CD4+ and CD8+ T-cell responses to a new threshold.

Induction of anti-tumor immunity elicited by tumor cells expressing a murine LFA-3 analog via a recombinant vaccinia virus

T cell activation requires binding of the T cell receptor to the major histocompatibility molecule-peptide complex in the presence of adhesion and/or costimulatory molecules such as B7-1 (CD80), B7-2 (CD86), ICAM-1 (CD54), and LFA-3 [corrected]. The major ligand of CD2 is CD48, the murine analog of human leukocyte function-associated antigen 3 (LFA-3). To determine the effect of LFA-3 expression on the immunogenicity of tumor cells, we constructed a recombinant vaccinia virus containing the murine LFA-3 gene (designated rV-LFA-3). rV-LFA-3 was shown to be functional in vitro in terms of expression of LFA-3, T cell proliferation, adhesion, and cytotoxicity. Subcutaneous inoculation of rV-LFA-3-infected murine colon adenocarcinoma tumor cells (MC38) into immunocompetent syngeneic C57BL/6 mice resulted in complete lack of tumor growth. Inoculation of MC38 cells infected with equal doses of control wild-type vaccinia virus resulted in tumor growth in all animals. In addition, partial immunological protection was demonstrated against subsequent challenge with uninfected parental tumor cells up to 56 days after vaccination with rV-LFA-3-infected cells. Anti-tumor memory was also demonstrated by using gamma-irradiated MC38 cells and cells from another carcinoma model (CT26). These studies demonstrate that expression of LFA-3 via a poxvirus vector can be used to induce anti-tumor immunity.

Mechanisms and assessment of lectin-mediated mitogenesis

The discovery of lectin-mediated mitogenesis by Nowell in 1960 stimulated interest in the properties of lectins while advancing knowledge of immunology. Although some lectins are polyclonal activators both in vitro and in vivo, others may display a broad range of activities toward human lymphocytes. Indeed, the same lectin (e.g., wheat germ agglutinin or Datura lectin) may be mitogenic, comitogenic, or antimitogenic, depending on the experimental conditions. An individual lectin may bind to several glycoproteins on the lymphocyte surface, resulting in interactions that may or may not be functionally relevant, and that may have opposing effects. Studies with lectins and with monoclonal antibodies (MAbs) have established that a surprisingly large variety of cell-surface molecules can influence the initiation and regulation of lymphocyte activation and proliferation. Interactions between lymphocytes and accessory cells are crucial; some signals are cell-mediated, but others depend on soluble cytokines. Mitogenic lectins presumably bind to the T-cell receptor complex and also promote a positive costimulatory signal leading to the synthesis of interleukin 2 and interleukin 2 receptors (IL-2R). Nonmitogenic, comitogenic, and antimitogenic lectin activities also probably act via accessory molecules involved in costimulation. Plant lectin-animal lymphocyte interactions presumably have no physiological significance, but it is suggested that the former mimics microbial superantigens, which may function in the colonization of host cells. Mitogenic stimulation of lymphocytes can be assessed in several ways. The standard technique measures [3H]-thymidine incorporation into DNA, but nonradioactive procedures are also available.

Interleukin-6 (IL-6) Prevents Activation-Induced Cell Death: IL-2–Independent Inhibition of Fas/fasL Expression and Cell Death

Triggering of the TCR/CD3 complex with specific antigen or anti-CD3 monoclonal antibody initiates activation-induced cell death (AICD) in mature T cells, an effect also mediated by the Fas/FasL system. We have previously shown that CD2 stimulation rescues T cells from TCR/CD3-induced apoptosis by decreasing the expression of Fas and FasL. In the present study, we examined whether the endogenous production of IL-2 plays a role in the effects mediated by CD2 triggering. The results indicated that transcription of Fas/FasL is controlled by interleukin-2 (IL-2) production and that CD2 triggering rescues a T-cell hybridoma from AICD via decreased production of IL-2. To ascertain whether modulation of IL-2 may be a general mechanism of AICD control, we examined other stimuli, capable of modulating the expression of the Fas/FasL system and the ensuing AICD, for ability to affect production of IL-2. We found that IL-6 reduced the level of TCR/CD3-induced apoptosis and the expression of Fas/FasL, yet failed to inhibit IL-2 production. Because IL-2 is involved in both apoptosis and activation events, these results indicate that, in contrast to CD2, which inhibits apoptosis and T cell activation, IL-6 inhibits apoptosis but not IL-2–induced activation. These observations may provide the basis for differential control of T-cell activation and apoptosis.

A Critical Role for CD48 Antigen in Regulating Alloengraftment and Lymphohematopoietic Recovery After Bone Marrow Transplantation

The binding of CD2, present on T cells, to its counterreceptor CD48 facilitates adhesion, signaling, alloantigen-induced cytokine production, and cytotoxic T-lymphocyte responses. Because these T-cell functions have been implicated in graft-versus-host disease (GVHD) pathogenesis, we have analyzed the effects of the CD2:CD48 pathway on GVHD mediated by CD4+ and CD8+ T cells infused into sublethally irradiated recipients. CD4+ T-cell–mediated, and to a lesser extent, CD8+ T-cell–mediated GVHD was inhibited by CD2 + 48 monoclonal antibody (MoAb) infusion. To assess the effects of combined MoAb infusion on alloengraftment, two different alloengraftment bone marrow transplantation (BMT) models were used. In both, MoAb infusion markedly inhibited alloengraftment and hematopoietic recovery post-BMT. To determine if the adverse effects on lymphohematopoiesis in the allogeneic BMT recipients were caused by an immune or nonimmune mechanism, studies were performed in congenic BMT recipients to preclude an immune mechanism as the cause for delayed recovery post-BMT. MoAb infusion resulted in impaired lymphohematopoietic recovery in congenic BMT recipients and markedly reduced day 12 colony-forming unit–spleen formation in syngeneic BMT recipients, consistent with a nonimmune mediated mechanism. Because the spleen is a site of early hematopoietic recovery post-BMT, studies were performed using adult splenectomized syngeneic BMT recipients. MoAb infusion delayed recovery in both nonsplenectomized and splenectomized recipients post-BMT, indicating that the delayed hematopoietic recovery was not the consequence of an abnormal homing pattern of hematopoietic progenitors to the spleen early post-BMT. CD48 MoAb was necessary and sufficient for the inhibition of GVHD lethality and delayed lymphohematopoietic effects of the combined MoAb regimen. CD48 MoAb was found to induce a profound modulation of CD48 antigen expression on BM cells, suggesting that the CD48 antigen may have an important function in hematopoiesis in the BM compartment. Taken together, these data provide evidence that the CD48 antigen plays a critical role in regulating hematopoiesis in post-BMT.

Interleukin-6 (IL-6) Prevents Activation-Induced Cell Death: IL-2–Independent Inhibition of Fas/fasL Expression and Cell Death

Triggering of the TCR/CD3 complex with specific antigen or anti-CD3 monoclonal antibody initiates activation-induced cell death (AICD) in mature T cells, an effect also mediated by the Fas/FasL system. We have previously shown that CD2 stimulation rescues T cells from TCR/CD3-induced apoptosis by decreasing the expression of Fas and FasL. In the present study, we examined whether the endogenous production of IL-2 plays a role in the effects mediated by CD2 triggering. The results indicated that transcription of Fas/FasL is controlled by interleukin-2 (IL-2) production and that CD2 triggering rescues a T-cell hybridoma from AICD via decreased production of IL-2. To ascertain whether modulation of IL-2 may be a general mechanism of AICD control, we examined other stimuli, capable of modulating the expression of the Fas/FasL system and the ensuing AICD, for ability to affect production of IL-2. We found that IL-6 reduced the level of TCR/CD3-induced apoptosis and the expression of Fas/FasL, yet failed to inhibit IL-2 production. Because IL-2 is involved in both apoptosis and activation events, these results indicate that, in contrast to CD2, which inhibits apoptosis and T cell activation, IL-6 inhibits apoptosis but not IL-2–induced activation. These observations may provide the basis for differential control of T-cell activation and apoptosis.

A Critical Role for CD48 Antigen in Regulating Alloengraftment and Lymphohematopoietic Recovery After Bone Marrow Transplantation

The binding of CD2, present on T cells, to its counterreceptor CD48 facilitates adhesion, signaling, alloantigen-induced cytokine production, and cytotoxic T-lymphocyte responses. Because these T-cell functions have been implicated in graft-versus-host disease (GVHD) pathogenesis, we have analyzed the effects of the CD2:CD48 pathway on GVHD mediated by CD4+ and CD8+ T cells infused into sublethally irradiated recipients. CD4+ T-cell–mediated, and to a lesser extent, CD8+ T-cell–mediated GVHD was inhibited by CD2 + 48 monoclonal antibody (MoAb) infusion. To assess the effects of combined MoAb infusion on alloengraftment, two different alloengraftment bone marrow transplantation (BMT) models were used. In both, MoAb infusion markedly inhibited alloengraftment and hematopoietic recovery post-BMT. To determine if the adverse effects on lymphohematopoiesis in the allogeneic BMT recipients were caused by an immune or nonimmune mechanism, studies were performed in congenic BMT recipients to preclude an immune mechanism as the cause for delayed recovery post-BMT. MoAb infusion resulted in impaired lymphohematopoietic recovery in congenic BMT recipients and markedly reduced day 12 colony-forming unit–spleen formation in syngeneic BMT recipients, consistent with a nonimmune mediated mechanism. Because the spleen is a site of early hematopoietic recovery post-BMT, studies were performed using adult splenectomized syngeneic BMT recipients. MoAb infusion delayed recovery in both nonsplenectomized and splenectomized recipients post-BMT, indicating that the delayed hematopoietic recovery was not the consequence of an abnormal homing pattern of hematopoietic progenitors to the spleen early post-BMT. CD48 MoAb was necessary and sufficient for the inhibition of GVHD lethality and delayed lymphohematopoietic effects of the combined MoAb regimen. CD48 MoAb was found to induce a profound modulation of CD48 antigen expression on BM cells, suggesting that the CD48 antigen may have an important function in hematopoiesis in the BM compartment. Taken together, these data provide evidence that the CD48 antigen plays a critical role in regulating hematopoiesis in post-BMT.

A Phase II Study of BTI-322, a Monoclonal Anti-CD2 Antibody, for Treatment of Steroid-Resistant Acute Graft-Versus-Host Disease

BTI-322, a rat monoclonal IgG2b directed against the CD2 antigen on T cells and natural killer (NK) cells, blocks primary and memory alloantigen proliferative responses in vitro. We have evaluated the pharmacokinetics and safety of BTI-322 during treatment of 20 transplant recipients with steroid-refractory acute graft-versus-host disease (GVHD). Treatment consisted of BTI-322 by intravenous (IV) bolus or 30-minute infusion at approximately 0.1 mg/kg/d for 10 days in addition to continuing high-dose steroids and tacrolimus or cyclosporine. Pharmacokinetic sampling was performed in 10 patients; the t1/2 ± SE was 9.1 ± 1.3 hours, the Cmaxwas 2,549 ± 291 ng/mL, the Vd was 3.97 ± 0.95 L, and the Vd/kg was 0.05 ± 0.01 L/kg. Ten patients experienced transient dyspnea sometimes accompanied by nausea, vomiting, diarrhea, and tachycardia shortly after the initial bolus dose of drug, but serious drug-related adverse events were not seen during the remainder of the infusions. At the end of treatment (day 11), there were six patients with complete responses and five with a reduction in grade of GVHD for a total response rate of 55% (95% confidence interval [CI], 32% to 77%). Antibodies targeting CD2 may be active in the treatment of acute GVHD, and evaluation of a humanized form of BTI-322 is warranted.

A Phase II Study of BTI-322, a Monoclonal Anti-CD2 Antibody, for Treatment of Steroid-Resistant Acute Graft-Versus-Host Disease

BTI-322, a rat monoclonal IgG2b directed against the CD2 antigen on T cells and natural killer (NK) cells, blocks primary and memory alloantigen proliferative responses in vitro. We have evaluated the pharmacokinetics and safety of BTI-322 during treatment of 20 transplant recipients with steroid-refractory acute graft-versus-host disease (GVHD). Treatment consisted of BTI-322 by intravenous (IV) bolus or 30-minute infusion at approximately 0.1 mg/kg/d for 10 days in addition to continuing high-dose steroids and tacrolimus or cyclosporine. Pharmacokinetic sampling was performed in 10 patients; the t1/2 ± SE was 9.1 ± 1.3 hours, the Cmaxwas 2,549 ± 291 ng/mL, the Vd was 3.97 ± 0.95 L, and the Vd/kg was 0.05 ± 0.01 L/kg. Ten patients experienced transient dyspnea sometimes accompanied by nausea, vomiting, diarrhea, and tachycardia shortly after the initial bolus dose of drug, but serious drug-related adverse events were not seen during the remainder of the infusions. At the end of treatment (day 11), there were six patients with complete responses and five with a reduction in grade of GVHD for a total response rate of 55% (95% confidence interval [CI], 32% to 77%). Antibodies targeting CD2 may be active in the treatment of acute GVHD, and evaluation of a humanized form of BTI-322 is warranted.

A new dexamethasone-induced gene of the leucine zipper family protects T lymphocytes from TCR/CD3-activated cell death

By comparing mRNA species expressed in dexamethasone (DEX)-treated and untreated murine thymocytes, we have identified a gene, glucocorticoid-induced leucine zipper (GILZ), encoding a new member of the leucine zipper family. GILZ was found expressed in normal lymphocytes from thymus, spleen, and lymph nodes, whereas low or no expression was detected in other nonlymphoid tissues, including brain, kidney, and liver. In thymocytes and peripheral T cells, GILZ gene expression is induced by DEX. Furthermore, GILZ expression selectively protects T cells from apoptosis induced by treatment with anti-CD3 monoclonal antibody but not by treatment with other apoptotic stimuli. This antiapoptotic effect correlates with inhibition of Fas and Fas ligand expression. Thus, GILZ is a candidate transcription factor involved in the regulation of apoptosis of T cells.

CD28 can promote T cell survival through a phosphatidylinositol 3-kinase-independent mechanism

Phosphatidylinositol 3(PI3)-kinase is implicated in various biological responses, including protection from apoptosis, although its role in antigen-induced T cell death and the molecular effectors it triggers remains ill-defined. Here, we investigated the role of PI3-kinase activity in the prevention of T cell receptor/CD3-induced cell death by CD28. PI3-kinase inhibitors blocked the up-regulation of Bcl-X(L) by CD28, without impairing the prevention of T cell receptor/CD3-triggered apoptosis by CD28, hence showing the existence of a cell-survival pathway independent of PI3-kinase activity and up-regulation of Bcl-X(L). Instead, we show that up-regulation of FasL which is instrumental in CD3-induced apoptosis was prevented upon CD28 co-stimulation. These results indicate that PI3-kinase couples CD28 to Bcl-X(L) up-regulation and provide a molecular basis for the role of CD28 in cell survival through a PI3-kinase-independent mechanism including FasL down-regulation.