Effects of antigen presentation on superantigen-induced apoptosis mediated by Fas/Fas ligand interactions in human T cells

Effective activation alleviates the replication block of CCR5-tropic HIV-1 in chimpanzee CD4+ lymphocytes

Human immunodeficiency virus type 1 (HIV-1) originated in chimpanzees; yet, several previous studies have shown that primary HIV-1 isolates replicate poorly in chimpanzee CD4+ T lymphocytes in vitro and in vivo. The reasons for this apparent restriction are not understood. Here, we describe a new activation protocol that led to a reproducible expansion and activation of chimpanzee CD4+ T lymphocytes in vitro. Using this protocol, we uncovered species-specific differences in the activation profiles of human and chimpanzee CD4+ T-cells, including HLA-DR and CD62L. Moreover, we found that improved activation facilitated the replication of both CXCR4 and CCR5-tropic HIV-1 in CD4+ T-cell cultures from over 30 different chimpanzees. Thus, the previously reported "replication block" of CCR5-tropic HIV-1 in chimpanzee lymphocytes appears to be due, at least in large part, to suboptimal T-cell activation.

Effect of Staphylococcus aureus and Streptococcus uberis on apoptosis of bovine mammary gland lymphocytes

The aim of this study was to determine whether lymphocyte apoptosis is modulated by infections caused by Staphylococcus aureus and Streptococcus uberis. Samples of cell populations were obtained by lavage of the mammary glands at 4 intervals (24, 48, 72 and 168 h) following infection. The percentage of apoptotic lymphocytes peaked at 168 h after challenge with S. aureus or S. uberis. Subsequent experiments focused on in vitro cultivation of mammary gland lymphocytes with S. aureus and S. uberis. These experiments showed a lower percentage of apoptotic lymphocytes following 3h of cultivating cells with bacteria than after cultivation without bacteria. The results demonstrate that during both experimental infection of bovine mammary glands with S. aureus or S. uberis and during in vitro cultivation of lymphocytes with S. aureus or S. uberis, apoptosis of lymphocytes is delayed.

Induction of endotoxin tolerance inhibits alloimmune responses

It was recently reported that the induction of endotoxin tolerance (ET), which is defined as a reduced response to a lipopolysaccharide (LPS) challenge following the first LPS encounter, inhibits major histocompatibility complex (MHC)-restricted antigen presentation. This raises the question whether alloimmune responses can be inhibited by inducing ET in transplant donors. C57BL/6 mice were treated with a low dose of LPS prior to a challenge with a high dose of LPS to induce ET. Hearts from endotoxin-tolerized C57BL/6 mice were transplanted to BALB/c mice. The survival of the endotoxin-tolerized heart allografts was significantly prolonged. By using irradiated splenocytes from C57BL/6 mice and allogeneic splenocytes from BALB/c mice, a mixed lymphocyte reaction (MLR) assay was performed. The MLR assay used CFSE, and revealed that the splenocytes from the endotoxin-tolerized mice failed to induce the proliferation of allogeneic CD4(+) and CD8(+) T cells. Cytokine analyses of the supernatant of the MLR culture using endotoxin-tolerized stimulators revealed a distinct shift in the Th 1/Th 2 balance toward the Th 2-type response. The induction of ET increased the proportion of myeloid-related dendritic cells (DCs) expressing molecules necessary for antigen presentation, which favor the development of a Th 2 response; however, it reduced the proportion of lymphoid-related DCs expressing those molecules, which favor the development of the Th 1 response. Although the relevance of these findings with regard to the prolonged survival of the endotoxin-tolerized heart allografts remains to be elucidated, this is the first study to demonstrate that the induction of ET in donor animals inhibits alloimmune responses.

Unique features of bovine lymphocytes exposed to a staphylococcal enterotoxin

We previously demonstrated that stimulation of bovine peripheral blood mononuclear cells (PBMCs) with staphylococcal enterotoxin C (SEC), led to an inversion of the CD4⁺:CD8⁺ T cell ratio and generation of an atypical CD8⁺ T cell subpopulation expressing CD26. In the present study, we examined T cell apoptosis and proliferation profiles of PBMC subpopulations in cultures stimulated with SEC. Unlike when stimulated with concanavalin A, nucleic acid synthesis in bovine PBMC cultures stimulated with SEC was low during the first four days but increased greatly on day 5. In contrast, nucleic acid synthesis in human PBMC cultures stimulated with SEC increased continuously. To investigate the mechanism of delayed bovine T cell proliferation, various cell phenotypes were monitored. The inversion of the bovine CD4⁺:CD8⁺ T cell ratio in PBMC cultures stimulated by SEC was associated with higher proliferation and lower apoptosis of CD8⁺ T cells compared to CD4⁺ T cells. The mRNA levels for interleukin (IL)-4 and IL-13 were sustained over 4 days but IL-12 mRNA levels dropped to background on day 2. These data suggest that SEC induces a prolonged Th-2-biased microenvironment, and together with the inversion of the bovine CD4⁺:CD8⁺ T cell ratios in bovine PBMC cultures with SEC, may in part explain the inability of the mammary immune system to establish an effective response to Staphylococcus aureus infections.

Preclinical evaluation of superantigen (staphylococcal enterotoxin B) in the intravesical immunotherapy of superficial bladder cancer

Superantigens are potent activators of T lymphocytes; therefore, their characteristics can be exploited in diseases where immunomodulation is known to be effective. In this study, we evaluated a new approach for the intravesical therapy of superficial bladder cancer. We investigated in coculture experiments if staphylococcal enterotoxin B (SEB)-activated PBMCs are able to induce apoptosis in human transitional cell carcinoma (TCC) cells. Additionally, we tested the toxicity and efficacy of SEB dissolved in NaCl 0.9% administered intravesically once weekly for 6 weeks in a rat bladder cancer model. To validate the coculture in vitro findings, we evaluated tumor stage, grade, apoptotic cells in the urothelium and stroma of the bladder and infiltration of the bladder wall by lymphocytes, macrophages and mononuclear cells. Coculture experiments revealed that SEB-activated PBMCs are able to kill TCC cells by inducing apoptosis. The intravesical toxicity study with a maximum dose of 100 microg/ml SEB demonstrated no side effects. In the intravesically SEB-treated animals (10 microg/ml), only 3 tumors remained vs. 15 persisting tumors in the control group. The remaining tumors of the therapy group showed a significant amount of apoptosis and granulocytes, mainly in the urothelium, whereas no relevant apoptosis or infiltration of the bladder with lymphocytes or macrophages was found in the control group. These preclinical findings suggest that SEB might be an interesting candidate for further clinical evaluation.

Exocytosis of CTLA-4 is dependent on phospholipase D and ADP ribosylation factor-1 and stimulated during activation of regulatory T cells

CTLA-4 is an essential protein in the regulation of T cell responses that interacts with two ligands found on the surface of APCs (CD80 and CD86). CTLA-4 is itself poorly expressed on the T cell surface and is predominantly localized to intracellular compartments. We have studied the mechanisms involved in the delivery of CTLA-4 to the cell surface using a model Chinese hamster ovary cell system and compared this with activated and regulatory human T cells. We have shown that expression of CTLA-4 at the plasma membrane (PM) is controlled by exocytosis of CTLA-4-containing vesicles and followed by rapid endocytosis. Using selective inhibitors and dominant negative mutants, we have shown that exocytosis of CTLA-4 is dependent on the activity of the GTPase ADP ribosylation factor-1 and on phospholipase D activity. CTLA-4 was identified in a perinuclear compartment overlapping with the cis-Golgi marker GM-130 but did not colocalize strongly with lysosomal markers such as CD63 and lysosome-associated membrane protein. In regulatory T cells, activation of phospholipase D was sufficient to trigger release of CTLA-4 to the PM but did not inhibit endocytosis. Taken together, these data suggest that CTLA-4 may be stored in a specialized compartment in regulatory T cells that can be triggered rapidly for deployment to the PM in a phospholipase D- and ADP ribosylation factor-1-dependent manner.

Activation-induced cell death in human T cells is a suicidal process regulated by cell density but superantigen induces T cell fratricide

Repeated ligation of the TCR results in apoptosis (activation-induced cell death; AICD). Superantigens such as Staphylococcal enterotoxin B (SEB) are particularly efficient at inducing AICD in T cells. We investigated whether apoptosis in human T cell subsets was due to fratricide (killing of neighboring cells) or suicide (cell autonomous death). AICD of Th1, Th2, Tc1, and Tc2 effector cells was dramatically enhanced at low cell densities and could be observed in single cell microcultures. AICD was unaffected by adhesion molecules or neighboring cells undergoing AICD, confirming the predominance of a suicidal mechanism. However, SEB was able to induce fratricidal apoptosis of type 1, but not type 2 cells. Fratricide was also observed when unstimulated T cells were exposed to activated Tc1 effector cells. Thus, AICD is tightly regulated to allow clonal T cell expansion and memory cell generation, but superantigens may subvert this process by allowing T cell fratricide.

Regulation of Fas-mediated apoptosis in CD2-fas transgenic mice

Fas-mediated apoptosis is an essential mechanism for maintenance of immune homeostasis. The expression of Fas is regulated at transcriptional and protein levels. Furthermore, several death domain molecules and caspases are crucial downstream mediators and executioners of Fas-mediated apoptosis. A tightly regulated interaction of these molecules ensures normal immune functions, including the execution of activation-induced cell death, T-cell mediated cytotoxicity, and surveillance of immune privileged tissues. In contrast, abnormally increased or decreased Fas-mediated apoptosis is a major pathogenic mechanism of several diseases, including systemic or tissue-specific autoimmune diseases and immune deficiency. Two CD2-fas transgenic mouse lines are described here to demonstrate the importance of controlling Fas-mediated apoptosis. Correction of Fas in Fas-mutant mice restored apoptosis function and ameliorated autoimmune symptoms, whereas a long-term enhancement of Fas expression in Fas-normal mice resulted in an increased acute-phase response and renal amyloidosis in aged transgenic mice.

Down-regulation of CD28 via Fas (CD95): influence of CD28 on T-cell apoptosis

Following antigen engagement of the T-cell receptor (TCR), T-cell survival is largely dictated by the provision of additional signals, such as those from costimulatory receptors and cytokine receptors. Whilst CD28-mediated signalling is increasingly associated with survival, ligation of alternative T-cell antigens, such as Fas (CD95), can trigger apoptosis. The T-cell response following antigen engagement may therefore be influenced by the relative expression levels of these coreceptors as well as by the availability of their ligands (CD80/86 and Fas-L). In this study we demonstrate functional interplay between the death receptor Fas and the costimulatory receptor CD28 in human T cells. In Jurkat T cells, we show that Fas signalling leads to rapid and selective CD28 down-regulation, and that this is associated with a specific decrease in mRNA for CD28, indicating that mechanisms exist which target CD28 at a transcriptional level. Moreover, cells that down-regulate CD28 also undergo apoptosis. Studies on activated human peripheral blood T cells demonstrate that cells expressing high levels of CD28 are resistant to Fas-mediated apoptosis whereas cells expressing low levels are more susceptible, implicating CD28 in the provision of anti-apoptotic signals. Consistent with this hypothesis, direct ligation of CD28 using B7 transfectants concomitant with anti-Fas challenge protects from apoptosis. Since antigen-presenting cells may express Fas-L under certain circumstances, the maintenance of T-cell CD28 expression may be crucial for the prevention of Fas-mediated apoptosis during the course of antigen engagement.

Activation of Human T Cells with Superantigen (Staphylococcal Enterotoxin B) and CD28 Confers Resistance to Apoptosis via CD95

Ag recognition is an essential component for an effective T cell response. However, T cell activation is also subject to additional regulation by accessory molecules. CD28 provides essential costimulatory signals that allow T cells to proliferate, whereas molecules such as CTLA-4 and CD95 (Fas) appear to be negative regulators. Currently, which outcome predominates under conditions of antigenic challenge is poorly understood. In particular it has been suggested that one consequence of antigenic activation of T cells is the up-regulation of both CD95 and CD95 ligand, thereby exposing activated T cells to apoptotic death. We have investigated this possibility in normal human peripheral blood T cells triggered by the superantigen SEB either in the presence of endogenous APCs or transfectants expressing DR4 and CD80. In either case, we find that such activation does not expose the majority of T cells to anti-CD95-induced apoptosis as detected by annexin V externalization and DNA fragmentation. Furthermore, by phenotypically identifying, by flow cytometry, those cells that received both antigenic and costimulatory signals from those cells that did not, we observed that CD95-induced apoptosis was not seen in activated T cells receiving Ag and costimulatory signals via CD28. However, while not all T cells were stimulated by superantigen, CD95 expression was found to be homogeneously up-regulated, suggesting a mechanism whereby bystander cells might be made susceptible to CD95-induced death. We conclude that antigenic activation of T cells via the TCR and CD28 engagement provides protection from CD95-induced apoptosis.

The Potential of Iron Chelators of the Pyridoxal Isonicotinoyl Hydrazone Class as Effective Antiproliferative Agents II: The Mechanism of Action of Ligands Derived From Salicylaldehyde Benzoyl Hydrazone and 2-Hydroxy-1-Naphthylaldehyde Benzoyl Hydrazone

We have recently screened 36 analogues of the lipophilic iron (Fe) chelator, pyridoxal isonicotinoyl hydrazone (PIH), for their antiproliferative effect (Richardson et al, Blood 86:4295, 1995). Of these compounds, 1 chelator derived from salicylaldehyde benzoyl hydrazone (206) and 4 ligands derived from 2-hydroxy-1-naphthylaldehyde benzoyl hydrazone (308, 309, 311, and 315) showed pronounced antiproliferative activity, being far more effective than desferrioxamine (DFO). The present study was designed to investigate in detail the mechanism of action of these PIH analogues in a variety of neoplastic cell lines. This investigation showed that the analogues were far more active than DFO at inhibiting cellular proliferation and 3H-thymidine, 3H-leucine, and 3H-uridine incorporation. Additional experiments showed that, in contrast to DFO, the 5 analogues were potent at preventing 59Fe uptake from transferrin (Tf ) and increasing 59Fe release from cells at concentrations as low as 10 μmol/L. Examination of the distribution of 59Fe in neoplastic cells using native polyacrylamide gel electrophoresis (PAGE)/59Fe-autoradiography showed that most of the 59Fe taken up from Tf was incorporated into ferritin, although 3 other previously unrecognized components (bands A, B, and C) were also identified. Band C comigrated with 59Fe-citrate and was chelated on incubation of neuroblastoma cells with DFO, PIH, or the PIH analogues, with this compartment being the main intracellular target of these ligands. Further work showed that the effects of the chelators at inducing characteristics consistent with apoptosis or necrosis were cell line-specific, and while DFO increased the percentage of cells in the Go/G1 phases in all cell types, the effect of analogue 311 on the cell cycle was variable depending on the cell line. This study provides further evidence for the potential use of these Fe chelators as anticancer agents.