Fas antigen (CD95) in pure erythroid cell line AS-E2 is induced by interferon-gamma and tumor necrosis factor-alpha and potentiates apoptotic death

Cytotoxic activities of CD8⁺ T cells collaborate with macrophages to protect against blood-stage murine malaria

The protective immunity afforded by CD8⁺ T cells against blood-stage malaria remains controversial because no MHC class I molecules are displayed on parasite-infected human erythrocytes. We recently reported that rodent malaria parasites infect erythroblasts that express major histocompatibility complex (MHC) class I antigens, which are recognized by CD8⁺ T cells. In this study, we demonstrate that the cytotoxic activity of CD8⁺ T cells contributes to the protection of mice against blood-stage malaria in a Fas ligand (FasL)-dependent manner. Erythroblasts infected with malarial parasites express the death receptor Fas. CD8⁺ T cells induce the externalization of phosphatidylserine (PS) on the infected erythroblasts in a cell-to-cell contact-dependent manner. PS enhances the engulfment of the infected erythroid cells by phagocytes. As a PS receptor, T-cell immunoglobulin-domain and mucin-domain-containing molecule 4 (Tim-4) contributes to the phagocytosis of malaria-parasite-infected cells. Our findings provide insight into the molecular mechanisms underlying the protective immunity exerted by CD8⁺ T cells in collaboration with phagocytes.

DOI:http://dx.doi.org/10.7554/eLife.04232.001

The immune system consists of several different types of cell that work together to prevent infection and disease. For example, immune cells called cytotoxic CD8⁺ T cells kill tumor cells or other cells that are infected. To do so, the CD8⁺ T cells must recognize certain molecules on the surface of the tumor or infected cells and bind to them.

Malaria is an infectious disease caused by the Plasmodium parasite, which is transferred between individuals by mosquitoes. The parasite is able to evade the immune system—so much so that it is not well understood how the immune system tries to respond to stop the infection. This has made it difficult to develop a vaccine that protects against malaria.

During the latter stages of a malaria infection, the parasite infects the host's red blood cells. It was long believed that CD8⁺ T cells did not help to eliminate the red blood cells that had been infected by Plasmodium. However, recent work in mice suggested that CD8⁺ T cells do respond to infected erythroblasts—precursor cells that develop into red blood cells—and that CD8⁺ T cells help protect mice against blood-stage malaria.

Now, Imai et al. describe how the CD8⁺ T cells in mice help to kill erythroblasts infected with Plasmodium yoelli, a species of the parasite used to study malaria in mice. The infected cells display a protein called Fas on their surface. Imai et al. found that, during a malaria infection, the CD8⁺ T cells produce a protein that can interact with Fas. This interaction causes the infected cell to move a signaling molecule to its outside surface, which encourages another type of immune cell to engulf and destroy the infected cell. This knowledge of how CD8⁺ T cells fight Plasmodium parasites in the bloodstream could now help to develop new types of blood-stage vaccine for malaria.

DOI:http://dx.doi.org/10.7554/eLife.04232.002

Elevated NF-kappaB responses and FLIP levels in leukemic but not normal lymphocytes: reduction by salicylate allows TNF-induced apoptosis

As its name suggests, tumor necrosis factor (TNF) is known to induce cytotoxicity in a wide variety of tumor cells and cell lines. However, its use as a chemotherapeutic drug has been limited by its deleterious side effects of systemic shock and widespread inflammatory responses. Some nonsteroidal antiinflammatory drugs, such as sodium salicylate, have been shown to have a chemopreventive role in certain forms of cancer. Here, we reveal that sodium salicylate selectively enhances the apoptotic effects of TNF in human erythroleukemia cells but does not affect primary human lymphocytes or monocytes. Sodium salicylate did not affect the intracellular distribution of TNF receptors (TNFRs) but stimulated cell surface TNFR2 shedding. Erythroleukemia cells were shown to possess markedly greater basal NF-kappaB responses and elevated Fas-associated protein with death domain-like IL-1 converting enzyme (FLIP) levels. Sodium salicylate achieved its effects by reducing the elevated NF-kappaB responsiveness and FLIP levels and restoring the apoptotic response of TNF rather than the proliferative/proinflammatory effects of the cytokine in these cancer cells. Inhibition of NF-kappaB or FLIP levels in human erythroleukemia cells by pharmacological or molecular-biological means also resulted in switching the character of these cells from a TNF-responsive proliferative phenotype into an apoptotic one. These findings expose that the enhanced proliferative nature of human leukemia cells is caused by elevated NF-kappaB and FLIP responses and basal levels, reversible by sodium salicylate to allow greater apoptotic responsiveness of cytotoxic stimuli such as TNF. Such findings provide insight into the molecular mechanisms by which human leukemia cells can switch from a proliferative into an apoptotic phenotype.

Chemokine expression in human erythroid leukemia cell line AS-E2: macrophage inflammatory protein-3alpha/CCL20 is induced by inflammatory cytokines

OBJECTIVE

Normal and malignant hematopoietic cells are shown to express and secrete various cytokines and chemokines, some of which are believed to play an important role in normal and abnormal hematopoiesis in an autocrine/paracrine manner. To explore the possibility of a cytokine/chemokine network participating in the pathophysiology of anemic disorders, we evaluated the ability of inflammatory cytokines to induce chemokine expression using erythroid progenitor cells.

METHODS

Erythropoietin-dependent human leukemia cell line AS-E2 was used as a model of erythroid colony-forming unit (CFU-E) cells. The expression of mRNA of 8 chemokines was examined using RT-PCR, before and after TNF-alpha, IFN-gamma, and IL-1beta stimulation. For MIP-3alpha, the promoter activity was analyzed by luciferase assay and secretion was confirmed by ELISA. The expression of CCR6, the specific receptor for MIP-3alpha, was analyzed by RT-PCR and flow cytometry.

RESULTS

Unstimulated AS-E2 cells constitutively expressed transcripts for MCP-4, IP-10, PF-4, IL-8, and MIP-3alpha. Stimulation with TNF-alpha, IFN-gamma, and IL-1beta upregulated MIP-3alpha mRNA expression and induced its protein secretion. Luciferase assay revealed that these cytokines could upregulate promoter activity of the MIP-3alpha gene, possibly through the NF-kappaB pathway. CCR6 mRNA was detected and its intracellular expression was confirmed.

CONCLUSION

These data suggest that inflammatory cytokine-stimulated erythroid progenitors secrete MIP-3alpha, which may function in an autocrine/paracrine manner. Furthermore, the existence of intracellular CCR6 suggests the involvement in cytokine signaling of a MIP-3alpha-dependent internal autocrine mechanism. These mechanisms may play a role in pathophysiology of anemic disorders, such as secondary anemia and bone marrow failure syndromes.

The impact of anaemia on outcome in cancer

Anaemia is not an inconsequential side effect of cancer and its treatment should not be ignored. Current practice for anaemia management varies and its role in influencing outcome in cancer patients is under recognized. As a common complication of cancer, anaemia is prevalent in virtually all tumour types to varying degrees. Predictive factors for anaemia include baseline haemoglobin concentration, decrease in haemoglobin concentration within the first month of treatment, tumour type, duration of treatment and prior blood transfusions. Interest in the prognostic significance of anaemia in cancer patients has generated extensive clinical research. Data is now published in a wide range of tumour types confirming that anaemia is a negative prognostic indicator of outcome (e.g. survival, disease-free recurrence and local relapse), with the strongest association in patients receiving radiotherapy. The association has also been documented in patients undergoing chemotherapy and chemoradiation. A retrospective meta-analysis has shown an overall 65% increased risk of death associated with anaemia in cancer patients. The impact of anaemia as an independent prognostic factor for outcome may be mediated by several factors, however the emerging consensus is on the central role of tumour hypoxia. It has been nearly 50 years since R. Thomlinson and L. Gray (British Journal of Cancer 1955, 9: 539) first documented the existence of hypoxia in tumours and it is now well accepted that tumour hypoxia protects tumour cells from therapeutic damage directly by reducing the availability of oxygen-free radicals which are necessary for optimal impact of radiotherapy, certain chemotherapeutic agents and photodynamic therapy. The indirect effects include the impact of hypoxia on gene expression, which affects genetic stability, proliferation kinetics and cellular metabolism. There has been an emergence of preclinical and circumstantial data over recent years that are suggestive of the ability to correct the negative effect of anaemia on outcome by the use of repeated blood transfusions or recombinant human erythropoietin. This has led to some attempts to measure the impact on survival in cancer patients of treating anaemia, but early attempts have served to underline the complexity of the relationship and have produced unexpected results.

Granulocyte macrophage-colony stimulating factor and interleukin-3 increase expression of type II tumour necrosis factor receptor, increasing susceptibility to tumour necrosis factor-induced apoptosis. Control of leukaemia cell life/death switching

Tumour necrosis factor (TNF) induces apoptosis in a range of cell types via its two receptors, TNFR1 and TNFR2. Here, we demonstrate that proliferation and TNFR2 expression was increased in human leukaemic TF-1 cells by granulocyte macrophage-colony stimulating factor (GM-CSF) and interleukin-3 (IL-3), with TNFR1 expression unaffected. Consequently, they switch from a proliferative to a TNF-induced apoptotic phenotype. Raised TNFR2 expression and susceptibility to TNF-induced apoptosis was not a general effect of proliferation as IL-1beta and IFN-gamma both proliferated TF-1 cells with no effect on TNFR expression or apoptosis. Although raised TNFR2 expression correlated with the apoptotic phenotype, stimulation of apoptosis in GM-CSF-pretreated cells was mediated by TNFR1, with stimulation of TNFR2 alone insufficient to initiate cell death. However, TNFR2 did play a role in apoptotic and proliferative responses as they were blocked by the presence of an antagonistic TNFR2 antibody. Additionally, coincubation with cycloheximide blocked the mitotic effects of GM-CSF or IL-3, allowing only the apoptotic responses of TNF to persist. TNF life/death was also observed in K562, but not MOLT-4 and HL-60 human leukaemic cell types. These findings show a cooperative role of TNFR2 in the TNF life/death switching phenomenon.

Differential response of human lung epithelial cells to fas-induced apoptosis

The Fas (CD95)/Fas ligand (CD178) system plays an important role in epithelial damage during the acute respiratory distress syndrome. The goal of this study was to determine whether proximal and distal human lung epithelial cells differ in their sensitivity to Fas ligand (rh-sFasL), and whether the response of lung epithelium to Fas ligation is modulated by proinflammatory cytokines. Although the expression of both Fas message and protein was similar in proximal and distal lung epithelial cells, only distal cells became apoptotic when exposed to serial dilutions of rh-sFasL. Stimulation with tumor necrosis factor-alpha, interleukin-1beta, or interferon-gamma significantly increased the sensitivity of proximal cells to rh-sFasL, and exposure to either tumor necrosis factor-alpha or interferon-gamma enhanced the sensitivity of distal cells to Fas ligation. These findings suggest that in normal human lungs, the responses of the epithelium to Fas ligation become more pronounced from proximal to distal locations. Furthermore, proinflammatory cytokines sensitize lung epithelium to Fas-induced death. These findings are relevant for understanding the role of the Fas/FasL system in acute lung injury, in which epithelial damage occurs primarily in distal airway and alveolar epithelium, whereas sFasL is present throughout the airspaces.

The signaling pathways of erythropoietin and interferon-gamma differ in preventing the apoptosis of mature erythroid progenitor cells

Interferon (IFN)-gamma is a survival factor for mature erythroid progenitor cells. To elucidate related survival mechanisms, we compared the role of phosphatidylinositol 3-kinase (PI3-kinase) in the survival signals of IFN-gamma and erythropoietin (EPO). Human erythroid colony-forming cells (ECFCs) purified from peripheral blood were used, and Ly294002 was used as a PI3-kinase inhibitor. Treating ECFCs with a high concentration of Ly294002 (50 micromol/L) in the presence of EPO and/or IFN-gamma reduced cell viability by inducing apoptosis. However, treating cells with a lower concentration of Ly294002 (10 micromol/L) did not affect the antiapoptotic function of IFN-gamma and abolished the antiapoptotic effect of EPO. Adding IFN-gamma or EPO induced Bcl-x expression in ECFCs, as determined by Western blotting, and expression was suppressed in the presence of Ly294002. We also examined the phosphorylation of the protein kinase Akt, the downstream target of PI3-kinase. EPO stimulation significantly increased the level of Akt phosphorylation, but IFN-gamma did not. These results suggest that IFN-gamma plays a role in preventing the apoptosis of erythroid progenitor cells by affecting Bcl-x expression, thereby reducing the disruption of the mitochondrial transmembrane potential via PI3-kinase pathways that are related to but distinct from the EPO pathway.

Btk is required for an efficient response to erythropoietin and for SCF-controlled protection against TRAIL in erythroid progenitors

Regulation of survival, expansion, and differentiation of erythroid progenitors requires the well-controlled activity of signaling pathways induced by erythropoietin (Epo) and stem cell factor (SCF). In addition to qualitative regulation of signaling pathways, quantitative control may be essential to control appropriate cell numbers in peripheral blood. We demonstrate that Bruton's tyrosine kinase (Btk) is able to associate with the Epo receptor (EpoR) and Jak2, and is a substrate of Jak2. Deficiency of Btk results in reduced and delayed phosphorylation of the EpoR, Jak2, and downstream signaling molecules such as Stat5 and PLCγ1 as well as in decreased responsiveness to Epo. As a result, expansion of erythroid progenitors lacking Btk is impaired at limiting concentrations of Epo and SCF. In addition, we show that SCF induces Btk to interact with TNF-related apoptosis-inducing ligand (TRAIL)–receptor 1 and that lack of Btk results in increased sensitivity to TRAIL-induced apoptosis. Together, our results indicate that Btk is a novel, quantitative regulator of Epo/SCF-dependent expansion and survival in erythropoiesis.

Therapeutic vaccination with tumor cells that engage CD137

Therapeutic cancer vaccination is based on the finding that tumors in both humans and experimental animals, such as mice, express potential immunological targets, some of which have high selectivity for cancer cells. In contrast to the successful vaccination against some infectious diseases, where most vaccines induce neutralizing antibodies that act prophylactically, the aim of therapeutic cancer vaccines is to treat established tumors (primarily micrometastases). Since most tumor-destructive immune responses are cell-mediated, therapeutic cancer vaccination needs to induce and expand such responses and also to overcome "escape" mechanisms that allow tumors to evade immunological destruction. Tumor antigens (as with other antigens) are presented by "professional" antigen-presenting cells, most notably dendritic cells (DC). Therefore DC that have been transfected or "pulsed" to present antigen provide a logical source of tumor vaccines, and some encouraging results have been obtained clinically as well as in preclinical models. An alternative and more physiological approach is to develop vaccines that deliver tumor antigen for in vivo uptake and presentation by the DC. Vaccines of the latter type include tumor cells that have been modified to produce certain lymphokines or express costimulatory molecules, as well as cDNAs, recombinant viruses, proteins, peptides and glycolipids which are often given together with an adjuvant. Several studies over the past 5 years have demonstrated dramatic therapeutic responses against established mouse tumors as a result of repeated injections of agonistic monoclonal antibodies (MAbs) to the costimulatory molecule CD137 (4-1BB). However, the clinical use of such MAbs may be problematic since they depress antibody formation, for example, to infectious agents. The alternative approach to transfect tumor cells to express the CD137 ligand (CD137L) increases their immunogenicity, but vaccination with tumor cells expressing CD137L is ineffective in several systems where injection of anti-CD137 MAb produces tumor regression. Recent findings indicate that a more effective way to engage CD137 towards tumor destruction is to transfect tumor cells to express a cell-bound form of anti-CD137 single-chain Fv fragments (scFv). Notably, tumors from melanoma K1735, growing either subcutaneously or in the lung, could be eradicated following vaccination with K1735 cells that expressed anti-CD137 scFv. This was in spite of the fact that K1735, as with many human neoplasms, expresses very low levels of MHC class I and has low immunogenicity. Similar results were subsequently obtained with other tumors of low immunogenicity, including sarcoma Ag104. We hypothesize that the concomitant expression of tumor antigen and anti-CD137 scFv effectively engages NK cells, monocytes and dendritic cells, as well as activated CD4(+) and CD8(+) T cells (all of which express CD137) so as to induce and expand a tumor-destructive Th1 response. While vaccines in the form of transfected tumor cells can be effective, at least in mouse models, the logical next step is to construct vaccines that combine genes that encode molecularly defined tumor antigens with a gene that encodes anti-CD137 scFv. Before planning any clinical trials, vaccines that engage CD137 via scFv need to be compared in demanding mouse models for efficacy and side effects with vaccines that are already being tested clinically, including transfected DC and tumor cells producing granulocyte-macrophage colony-stimulating factor.

Cytokine regulation of human intestinal primary epithelial cell susceptibility to Fas-mediated apoptosis

The regulatory mechanisms of nontransformed intestinal epithelial cell apoptosis have not been thoroughly investigated. We determined the susceptibility and mechanism of Fas-mediated apoptosis in nontransformed human intestinal epithelial cells (HIPEC) in the presence and absence of inflammatory cytokines. Despite ample expression of Fas, HIPEC were relatively insensitive to Fas-mediated apoptosis in that agonist anti-Fas antibody (CH11) induced a <25% increase in HIPEC apoptosis. Pretreatment of HIPEC with interferon (IFN)-gamma, but not tumor necrosis factor-alpha or granulocyte-macrophage colony-stimulating factor, significantly increased CH11-induced apoptosis of these cells without increasing Fas expression. Increased apoptosis correlated with increased caspase 3 activation but not expression of procaspase 3. Also, there was a significant delay in the onset of Fas-mediated apoptosis in HIPEC, which correlated with the generation of an activated caspase 3 p22/20 subunit. HIPEC required both initiator caspases 8 and 9 activity but expressed significantly less of the zymogen form of these caspases than did control cells. IFN-gamma-mediated sensitization of HIPEC occurred upstream of caspase 9 activation and correlated with a small increase in procaspase 8 expression (<1-fold increase) and a significant increase in expression of an intermediate form (p35) of caspase 4 (3.3-fold increase).

Effects of overexpression of the SH2-containing inositol phosphatase SHIP on proliferation and apoptosis of erythroid AS-E2 cells

Previous studies have demonstrated that SH2-containing inositol phosphatase (SHIP) is involved in the control of B cell, myeloid cell and macrophage activation and proliferation. The goal of the present study was to examine the role of SHIP during proliferation and apoptosis in cells of the erythroid lineage. Wild-type and catalytically inactive SHIP proteins were overexpressed in the erythropoietin (EPO)-dependent cell line AS-E2. Stable overexpression of catalytically inactive SHIP decreased proliferation and resulted in prolonged activation of the extracellular signal-regulated protein kinases ERK1/2 and protein kinase B (PKB), while wild-type SHIP did not affect EPO-mediated proliferation or phosphorylation of ERK and PKB. When AS-E2 cells were EPO deprived a significant increase in apoptosis was observed in clones overexpressing wild type. Mutational analysis showed that this increase in apoptosis was independent of the enzymatic activity of SHIP. The enhanced apoptosis due to overexpression of SHIP was associated with an increase in caspase-3 and -9 activity, without a distinct effect on caspase-8 activity or mitochondrial depolarization. Moreover, in cells overexpressing SHIP apoptosis could be reduced by a caspase-3 inhibitor. These data demonstrate that in the erythroid cell line AS-E2 overexpression of catalytically inactive SHIP reduced proliferation, while overexpression of wild-type SHIP had no effect. Furthermore, overexpression of SHIP enhanced apoptosis during growth factor deprivation by inducing specific caspase cascades, which are regulated independently of the 5-phosphatase activity of SHIP.

Host defense, viruses and apoptosis

To thwart viral infection, the host has developed a formidable and integrated defense network that comprises our innate and adaptive immune response. In recent years, it has become clear that in an attempt to prevent viral replication, viral dissemination or persistent viral infection of the cell, many of these protective measures actually involve the induction of programmed cell death, or apoptosis. An initial response to viral infection primarily involves the innate arm of immunity and the killing of infected cells with cytotoxic lymphocytes such as natural killer (NK) cells through mechanisms that include the employment of perforin and granzymes. Once the virus has invaded the cell, however, a second host defense-mediated response is also triggered which involves the induction of a family of cytokines known as the interferons (IFNs). The IFNs, which are essential for initiating and coordinating a successful antiviral response, function by stimulating the adaptive arm of immunity involving cytotoxic T cells (CTLs), and by inducing a number of intracellular genes that directly prevent virus replication/cytolysis or that facilitate apoptosis. The IFN-induced gene family is now known to comprise the death ligand TRAIL, the dsRNA-dependent protein kinase (PKR), interferon regulatory factors (IRFs) and the promyelocytic leukemia gene (PML), all of which have been reported to be mediators of cell death. That DNA array analyses indicate that numerous cellular genes, many as yet uncharacterized, may similarly be induced by IFN, further emphasizes the likely importance that these cytokines have in the modulation of apoptosis. This likelihood is additionally underlined by the elaborate strategies developed by viruses to inhibit IFN-antiviral function and the mechanisms of cell death.

Identification of a novel cytokine response element in the human IFN regulatory factor-1 gene promoter

The present study investigates the regulatory mechanisms involved in the cooperation between IFN-gamma and TNF-alpha to promote transcription from IFN regulatory factor-1 (IRF-1). A transient transfection analysis revealed that the region between -218 and -144, where +1 is the transcription start site, as well as previously reported downstream elements, ppkappaB and IFN-gamma activation site/kappaB, were required for the optimal response to the two cytokines. A subsequent DNase I footprint analysis showed that the region between -171 and -144 was inducibly protected with stimulation by TNF-alpha, and this protection was significantly enhanced with the combination of IFN-gamma and TNF-alpha. In an EMSA with the protected region as a probe, a TNF-alpha-inducible complex (C1) and an IFN-gamma-inducible complex (C2), but no synergy-specific DNA-protein complexes, were recognized. The C1 complex consisted of a pre-existing factor (p65/p50), whereas the C2 complex consisted of a newly synthesized IRF-1-related factor. A methylation interference assay revealed the critical G residues (from -167 to -151) for the DNA-protein complex formation specific to the cytokine response, and within this region the novel kappaB sequence, the promoter distal kappaB (pdkappaB) element (5'-GGGGAAG TAC-3'), was identified. Because the base substitutions over the pdkappaB region (from -171 to -144) affected not only the TNF-alpha-response but also that of IFN-gamma, this region might contribute to the cooperative action of the NF-kappaB subunits with the IRF-1-related factor. Finally, we demonstrated that none of the cis-acting elements, ppkappaB, pdkappaB, or IFN-gamma activation site/kappaB, is dispensable for the optimal synergism in response to IFN-gamma and TNF-alpha.

An essential contribution by IFN-gamma to CD8+ T cell-mediated rejection of pancreatic islet allografts

CD8+ T cells have long been considered to be the prototypical cytotoxic lymphocyte subpopulation. However, whether alloreactive CD8+ T cells require traditional cytolytic pathways such as perforin and Fas ligand (FasL) to mediate graft rejection has been a controversial issue. In the present studies, we examined the role of varied effector pathways in CD8+ T cell-mediated rejection of pancreatic islet allografts. Our goal was to systematically determine the relative requirements, if any, of perforin and FasL as well as the proinflammatory cytokine IFN-gamma in triggering graft destruction. To study CD8+ T cell effector pathways independently of other lymphocyte populations, purified alloreactive CD8+ T cells were adoptively transferred into severe combined immune-deficient (SCID) recipients bearing established islet allografts. Results indicate that to reject established islet allografts, primed CD8+ T cells do not require the individual action of the conventional cytotoxic effectors perforin and Fas ligand. In contrast, the ability to produce IFN-gamma is critical for efficient CD8+ T cell-mediated rejection of established islet allografts. Furthermore, alloreactive CD8+ TCR transgenic T cells (2C) also show IFN-gamma dependence for mediating islet allograft rejection in vivo. We speculate from these results that the production of IFN-gamma by alloreactive CD8+ T cells is a rate-limiting step in the process of islet allograft rejection.

The interferons and cell death: guardians of the cell or accomplices of apoptosis?

The interferons (IFNs) play an integral role in cellular host defense against virus infection and conceivably tumorigenesis. Despite over 50 years of research, however, the molecular mechanisms underlining IFN action remain to be fully elucidated, in part because of the large number of genes, with an uncharacterized function that appears to be induced by these cytokines. Although the majority of in vitro studies indicate that IFNs antiviral properties involve inhibiting viral replication while maintaining the integrity of the cell, numerous reports have now implicated that a number of IFN-induced genes, IFN transcriptional regulatory factors and IFN signaling molecules can also mediate apoptosis. Here, we review some of what is known about IFN's ability to invoke programmed cell death as part of an intricate arsenal intended to prevent viral infection and malignant disease.