Activation-induced T cell death occurs at G1A phase of the cell cycle

Regulation of CD4+ T-cell contraction during pathogen challenge

Signals orchestrating productive CD4+ T-cell responses are well documented; however, the regulation of contraction of CD4+ T-cell effector populations following the resolution of primary immune responses is not well understood. While distinct mechanisms of T-cell death have been defined, the relative importance of discrete death pathways during the termination of immune responses in vivo remains unclear. Here, we review the current understanding of cell-intrinsic and -extrinsic variables that regulate contraction of CD4+ T-cell effector populations through multiple pathways that operate both initially during T-cell priming and later during the effector phase. We discuss the relative importance of antigen-dependent and -independent mechanisms of CD4+ T-cell contraction during in vivo responses, with a special emphasis on influenza virus infection. In this model, we highlight the roles of greater differentiation and presence in the lung of CD4+ effector T cells, as well as their polarization to particular T-helper subsets, in maximizing contraction. We also discuss the role of autocrine interleukin-2 in limiting the extent of contraction, and we point out that these same factors regulate contraction during secondary CD4+ T-cell responses.

The TCL1 oncoprotein inhibits activation-induced cell death by impairing PKCtheta and ERK pathways

The TCL1/MTCP1 oncogenes were identified on the basis of their involvement in T-cell prolymphocytic leukemia (T-PLL). TCL1 and MTCP1 proteins directly interact with AKT and modulate the AKT signal-transduction pathway, but the relevance of this mechanism in leukemogenesis remains unclear. We investigate the biologic functions of TCL1 in the T-cell lineage using various cell lines, and primary malignant and normal lymphocytes. In the Jurkat cell line, expression of TCL1 had no effect in unstimulated cells, whereas it abrogated activation-induced cell death (AICD). These cellular effects were concomitant with a major inhibition by TCL1 of PKCtheta and ERK pathways. Secondly, the TCL1-driven T-cell leukemia cell line SUP-T11 was shown to have impaired PKCtheta and ERK phosphorylation upon stimulation, which were restored by TCL1 inhibition using RNA interference. Finally, defects in these pathways were also observed in primary malignant (T-PLL) and transduced normal T lymphocytes expressing TCL1. Altogether, our data demonstrated that TCL1 inhibits AICD in T cells by blocking PKCtheta and ERK activation, upon cellular activation.

Cell cycle dependency of caspase activation in Fas-induced apoptosis in leukemia cells

The relationship between apoptosis and the cell cycle remains unclear. In the present study we have investigated the relationship between cell cycle progression and the activation of caspases (caspase-3 and caspase-8) in Fas (CD95)-mediated apoptosis in asynchronously growing leukemia cells. We found that cells expressing the active form of caspase-3 were cyclin A/B1 and Ki-67 negative but cyclin E positive, whereas expression of the active form of caspase-8 was detected in cyclin A/B1/E-negative and Ki-67-negative cells. In addition, both the activation of caspases and Fas-mediated apoptosis were completely abolished when leukemia cells were arrested in early G1 phase. Using post-sorting western blot analysis, we demonstrated that caspase-3 and caspase-8 were activated in p27-negative cells. These results suggest that caspase-3 would be activated in cells entering into late G1 or early S phase, and caspase-8 would be activated in middle or late G1 phase. The speed of cell cycle progression from G1 to S phase might be influential in the speed of caspase activation and induction of Fas-mediated apoptosis.

Differential role of ICAM ligands in determination of human memory T cell differentiation

Background

Leukocyte Function Antigen-1 (LFA-1) is a primary adhesion molecule that plays important roles in T cell activation, leukocyte recirculation, and trans-endothelial migration. By applying a multivariate intracellular phospho-proteomic analysis, we demonstrate that LFA-1 differentially activates signaling molecules.

Results

Signal intensity was dependent on both ICAM ligand and LFA-1 concentration. In the presence of CD3 and CD28 stimulation, ICAM-2 and ICAM-3 decreased TGFβ1 production more than ICAM-1. In long-term differentiation experiments, stimulation with ICAM-3, CD3, and CD28 generated IFNγ producing CD4+CD45RO+CD62L-CD11a^BrightCD27- cells that had increased expression of intracellular BCL2, displayed distinct chemokine receptor profiles, and exhibited distinct migratory characteristics. Only CD3/CD28 with ICAM-3 generated CD4+CD45RO+CD62L-CD11a^BrightCD27- cells that were functionally responsive to chemotaxis and exhibited higher frequencies of cells that signaled to JNK and ERK1/2 upon stimulation with MIP3α. Furthermore, these reports identify that the LFA-1 receptor, when presented with multiple ligands, can result in distinct T cell differentiation states and suggest that the combinatorial integration of ICAM ligand interactions with LFA-1 have functional consequences for T cell biology.

Conclusion

Thus, the ICAM ligands, differentially modulate LFA-1 signaling in T cells and potentiate the development of memory human T cells in vitro. These findings are of importance in a mechanistic understanding of memory cell differentiation and ex vivo generation of memory cell subsets for therapeutic applications.

Sensitivity to gemcitabine and its metabolizing enzymes in neuroblastoma

PURPOSE

We examined the activity of gemcitabine against neuroblastoma in vitro and in vivo. In addition, we investigated the cellular mechanisms of high sensitivity to the agent in neuroblastoma cells.

EXPERIMENTAL DESIGN

We examined 11 neuroblastoma cell lines for sensitivity to gemcitabine and other chemotherapeutic agents used clinically for neuroblastoma. The in vivo sensitivity of neuroblastoma to gemcitabine was determined in xenograft models. Furthermore, the major metabolic enzymes of gemcitabine were assessed and compared in leukemia and carcinoma cells. Apoptosis and mitochondrial membrane potentials were also evaluated.

RESULTS

The IC50s for gemcitabine in 11 neuroblastoma lines ranged between 3 nmol/L and 4 micromol/L. The high activity of gemcitabine against neuroblastoma was confirmed in animal models. Interestingly, enzymes in neuroblastoma cells involved in the metabolism of deoxycytidine analogue have unique characteristics among solid tumors. The median of deoxycytidine kinase activity in neuroblastoma lines was similar to that in leukemia lines, which have low IC50s for cytarabine. Cytidine deaminase (CDA) activity in neuroblastoma was hardly detectable and significantly lower than that in carcinoma. The defect of CDA activity was associated with negative expression of mRNA. Furthermore, gemcitabine-induced apoptosis was observed irrespective of the caspase-8 status of neuroblastoma cells, which indicates that apoptosis depends on the mitochondrial pathway.

CONCLUSIONS

Neuroblastoma is highly sensitive to gemcitabine. Although the cellular mechanism involved in sensitivity to gemcitabine is multifactorial, low CDA activity may contribute high sensitivity in neuroblastoma cells. These results suggest that clinical application of gemcitabine to the treatment of neuroblastoma is warranted.

A dual role of IFN-alpha in the balance between proliferation and death of human CD4+ T lymphocytes during primary response

Type I IFNs (IFN-alphabeta) enhance immune responses, notably T cell-mediated responses, in part by promoting the functional activities of dendritic cells. In this study, we analyzed the direct impact of IFN-alpha on proliferative and apoptotic signals upon in vitro activation of human naive CD4+ T lymphocytes. We demonstrate that IFN-alpha protects T cells from the intrinsic mitochondrial-dependent apoptosis early upon TCR/CD28 activation. IFN-alpha acts by delaying entry of cells into the G1 phase of the cell cycle, as well as by increasing Bcl-2 and limiting Bax activation. Later, upon activation, T cells that were exposed to IFN-alpha showed increased levels of surface Fas associated with partially processed caspase-8, a key component of the extrinsic apoptotic pathway. Caspase-8 processing was augmented furthermore by Fas ligation. Overall, these findings support a model whereby IFN-alpha favors an enhanced clonal expansion, yet it sensitizes cells to the Ag-induced cell death occurring at the end of an immune response. These observations point to a complex role of type I IFN in regulating the magnitude of proliferation and survival of naive CD4+ T cells during primary response and underline how crucial could be the timing of exposure to this cytokine.

Discrete Event Modeling of CD4+ Memory T Cell Generation

Studies of memory T cell differentiation are hampered by a lack of quantitative models to test hypotheses in silico before in vivo experimentation. We created a stochastic computer model of CD4+ memory T cell generation that can simulate and track 10(1)-10(8) individual lymphocytes over time. Parameters for the model were derived from experimental data using naive human CD4+ T cells stimulated in vitro. Using discrete event computer simulation, we identified two key variables that heavily influence effector burst size and the persistent memory pool size: the cell cycle dependent probability of apoptosis, and the postactivation mitosis at which memory T cells emerge. Multiple simulations were performed and varying critical parameters permitted estimates of how sensitive the model was to changes in all of the model parameters. We then compared two hypotheses of CD4+ memory T cell generation: maturation from activated naive to effector to memory cells (model I) vs direct progression from activated naive to memory cells (model II). We find that direct progression of naive to memory T cells does not explain published measurements of the memory cell mass unless postactivation expansion of the memory cell cohort occurs. We conclude that current models suggesting direct progression of activated naive cells to the persistent memory phenotype (model II) do not account for the experimentally measured size of the postactivation CD4+, Ag-specific, memory T cell cohort.

Regulation of activation-induced Fas (CD95/Apo-1) ligand expression in T cells by the cyclin B1/Cdk1 complex

Fas (CD95/Apo-1) ligand-mediated apoptosis has been recognized as an important mechanism of cell-mediated cytotoxicity and maintenance of immune homeostasis. Chronically activated T cells undergo activation-induced cell death (AICD), which depends on simultaneous Fas and Fas ligand expression. Previous reports have suggested that AICD might be linked to cell cycle progression of T cells and therefore to the expression of cell cycle-related molecules. In particular, cyclin B1 has been implicated in the induction of AICD in T cells. In this study, we have investigated the role of cyclin B1 in AICD and the expression of effector molecules involved in this form of cell death. Our results show that inhibition of cyclin B1 blocks AICD in T cells through specific inhibition of Fas ligand expression but not Fas-induced apoptosis. This effect of cyclin B1 appears to be mediated through the cyclin B1/cyclin-dependent kinase 1 (Cdk1/Cdc2) complex because overexpression of cyclin B1 enhances FasL promoter activity, whereas a dominant-negative version of Cdk1 blocks Fas ligand promoter induction. We provide further evidence that cyclin B1/Cdk1 regulates FasL transcription through the regulation of NFkappaB activation because dominant-negative Cdk1 inhibits activation-induced NFkappaB reporter and Rel A-induced FasL promoter activity. In conclusion, our data support a link between cell cycle progression, activation-induced Fas ligand expression, and apoptosis in T cells.

Proteolytic regulation of Forkhead transcription factor FOXO3a by caspase-3-like proteases

Forkhead family transcription factors are critical regulators of cell cycle progression and apoptosis in hematopoietic cells. Here, we show that FOXO3a (also known as FKHRL1) is a new substrate of caspase-3-like proteases during apoptosis in T lymphocytes. FOXO3a was cleaved in vivo by caspases in leukemic Jurkat cells following engagement of Fas (CD95) receptor, staurosporine, and etoposide treatment, but not following engagement of CD99, a caspase-independent cell death inducer. Caspase-mediated cleavage of FOXO3a was also observed in CD4+ peripheral T cells subjected to activation-induced cell death. The expression of the death adapter FADD and caspase-8 was required for Fas-induced FOXO3a cleavage, but activation of survival pathways by overexpression of FLICE-inhibitory protein or phorbol myristate acetate treatment prevented it. FOXO3a was cleaved in vitro by caspase-3-like proteases at the consensus sequence DELD304A, releasing the N-terminal DNA-binding domain of FOXO3a from its C-terminal transactivating domain. Whereas full-length FOXO3a enhanced Forkhead response element-dependent transcription and apoptosis in Jurkat cells, both fragments were inactive to promote gene activation and cell death. In contrast, a caspase-resistant FOXO3a mutant exhibited enhanced transcriptional and proapoptotic activities. Together, these results indicate that the proteolytic cleavage of FOXO3a by caspases may represent a novel regulatory mechanism of FOXO3a activity during death receptors signaling.

NFATC2 transcription factor regulates cell cycle progression during lymphocyte activation: evidence of its involvement in the control of cyclin gene expression

Upon antigen stimulation, lymphocytes enter in cell cycle and proliferate, and most of the activated T cells die by apoptosis. Many of the proteins that regulate lymphocyte activation are Under the control of transcription factors belonging to the NFAT family. As previously demonstrated, NFATC2-/- mice consistently showed a marked increase in lymphocyte proliferation. Here, we evaluate the role of NFATC2 in regulating lymphocyte proliferation and its involvement in the control of cell cycle progression during lymphocyte activation. NFATC2-/- lymphocytes, including CD4+ T cells and B cells, hyperproliferated upon stimulation when compared with NFATC2+/+ cells. Analysis of cell death demonstrated that NFATC2-/- lymphocytes displayed an increased rate of apoptosis after antigen stimulation in addition to the hyperproliferation. Cell cycle analysis after antigen stimulation showed that NFATC2-/- cultures contained more cycling cells when compared with NFATC2+/+ cultures, which is related to a shortening in time of cell division upon activation. Furthermore, hyperproliferation of NFATC2-/- lymphocytes is correlated to an overexpression of cyclins A2, B1, E, and F. Taken together, our results suggest that the NFATC2 transcription factor plays an important role in the control of cell cycle during lymphocyte activation and may act as an inhibitor of cell proliferation in normal cells.

High levels of the onco-protein Gfi-1 accelerate T-cell proliferation and inhibit activation induced T-cell death in Jurkat T-cells

Gfi-1 is a nuclear zinc finger protein with the activity of a transcriptional repressor and the ability to predispose for the development of T-cell lymphoma when expressed constitutively at high levels. Whereas thymic T-cell precursors express endogenous Gfi-1, mature peripheral T-cells lack Gfi-1 but upregulate its expression transiently after antigenic stimulation and activation of Erk1/2 demonstrating a role of Gfi-1 in T-cell activation. Here we show that constitutive expression of Gfi-1 accelerates S phase entry of primary, resting T-cells upon antigenic stimulation. In addition, high level Gfi-1 expression inhibits phorbol ester induced G1 arrest and activation induced cell death in Jurkat T-cells. We demonstrate that these effects of Gfi-1 concur with lower absolute levels and hyperphosphorylation of the pocket protein pRb. Moreover, phorbol ester induced expression of the negative cell cycle regulator p21(WAF1) is blocked in the presence of Gfi-1. These findings suggest that Gfi-1 contributes to T-cell lymphomagenesis by overriding a late G1 cell cycle checkpoint which controls activation induced death and S phase entry of T-cells.