TCR-independent and caspase-independent apoptosis of murine thymocytes by CD24 cross-linking

NFATc1 induction by an intronic enhancer restricts NKT γδ cell formation

In thymus, the ablation of T cell receptor (TCR)-activated transcription factor NFATc1 or its inducible isoforms during the double-negative (DN) stages of thymocyte development leads to a marked increase in γδ thymocytes whereas the development of αβ thymocytes remains mostly unaffected. These γδ thymocytes are characterized by the upregulation of the promyelocytic leukemia zinc-finger factor (PLZF), the "master regulator" of natural killer T (NKT) cell development, and the acquisition of an NKT γδ cell phenotype with higher cell survival rates. The suppressive function of NFATc1 in NKT γδ cell formation critically depends on the remote enhancer E2, which is essential for the inducible expression of NFATc1 directed by its distal promoter P1. Thus, the enhancer deciphers a strong γδ TCR signal into the expression of inducible NFATc1 isoforms resulting in high levels of NFATc1 protein that are essential to control the numbers of NKT γδ cells.

Dendritic cell expression of CD24 contributes to optimal priming of T lymphocytes in lymph nodes

CD24 is a GPI anchored cell surface glycoprotein whose function as a co-stimulatory molecule has been implicated. However, the function of CD24 on antigen presenting cells during T cell responses is not well understood. Here we show that in the CD24-deficient host, adoptively transferred CD4+ T cells undergo inefficient expansion and have accelerated cell death in lymph nodes, which results in insufficient priming of T cells. Insufficient expansion of T cells in the CD24-deficient host was not due to host anti-CD24 response by NK, T and B lymphocytes. Transgenic expression of CD24 on DC in CD24-/- mice restored T cell accumulation and survival in draining lymph nodes. Consistent with these findings, MHC II tetramer staining also revealed that an antigen-specific polyclonal T cell response was reduced in lymph nodes of CD24-/- mice. Taken together, we have revealed a novel role of CD24 on DC in optimal T cell priming in lymph nodes. These data suggest that CD24 blockade should lower unwanted T cell responses such as those in autoimmune diseases.

CD24 and APC Genetic Polymorphisms in Pancreatic Cancers as Potential Biomarkers for Clinical Outcome

Background

There are no validated biomarkers that correlate with the prognosis of pancreatic ductal adenocarcinoma (PDA). The CD24 and adenomatous polyposis coli (APC) genes are important in the malignant transformation of gastrointestinal cells. This study examined APC and CD24 genetic polymorphisms and their possible impact on survival of patients with PDA.

Methods

Clinical and pathological data as well as blood samples for extracting DNA were obtained for 73 patients with PDA. Real-time PCR assessed genetic variants of APC (I1307K and E1317Q), and four different single nucleotide polymorphisms (SNPs) in the CD24 gene: C170T (rs52812045), TG1527del (rs3838646), A1626G (rs1058881) and A1056G (rs1058818).

Results

The median age at diagnosis was 64 (41–90) years. Thirty-one patients (42.5%) were operable, 16 (22%) had locally advanced disease and 26 (35.5%) had disseminated metastatic cancer. The malignancy-related mortality rate was 84%. Median survival was 14 months (11.25–16.74). Survival was similar for wild-type (WT), heterozygous and homozygous variants of the APC or CD24 genes. The three most frequent CD24 SNP combinations were: heterozygote for A1626G and WT for the rest of the alleles (14% of patients), heterozygote for C170T, A1626G, A1056G and WT for the rest (14% of patients), and heterozygote for C170T, A1056G and WT for the rest (10% of patients). All patients were APC WT. The first two groups were significantly younger at diagnosis than the third group.

Conclusions

Specific polymorphisms in the APC and CD24 genes may play a role in pancreatic cancer development. Correlation with survival requires a larger cohort.

[A pathophysiological role of cytochrome p450 involved in production of reactive oxygen species]

Dysregulation of the production of reactive oxygen species (ROS) determines cellular function. Cytochrome P450s (CYPs) regulates ROS production and contributes to the process of cell death. This review summarizes our recent findings, focusing on the involvement of CYPs in pathophysiology induced by ROS. 1. Quinone toxicity in hepatocytes: CYPs require electrons supplied from NADPH-cytochrome P450 reductase (NPR) during the process of metabolism. NPR also provides electrons to quinone compounds, which compete with CYPs over electrons. Inhibition of CYPs shifts NPR's electron flow more to quinones, which accelerates the redox cycle to enhance ROS production and quinone toxicity. 2. Myocardial ischemia-reperfusion injury: Reperfusion of blood flow after coronary artery occlusion induces cell damage, as evident by the extension of myocardial infarct size and caspase-independent cell apoptosis. CYP2C6 appears to be a source for ROS production, since sulfaphenazole, a selective inhibitor of CYP2C6, reduces this damage. ROS produced by CYP2C6 during the reperfusion causes translational activation of Noxa and BimEL, as well as the suppression of caspase activation, resulting in caspase-independent apoptosis. 3. Primary hepatocyte apoptosis: Inhibition of catalase and glutathione peroxidase increases intracellular ROS and elicits caspase-independent hepatocyte apoptosis. SKF-525A, a pan-CYP inhibitor, suppresses these ROS increases and hepatocyte apoptosis. Increased ROS activates ERK and AP-1 by inhibition of tyrosine phosphatase, and inhibits BimEL degradation by proteasome. These results in the accumulation of mitochondrial BimEL, which then induces the release of cytochrome c and endonuclease G (EndoG). Increased ROS also keeps caspases inactivated. As a result, EndoG executes nucleosomal DNA fragmentation.

CD24-triggered caspase-dependent apoptosis via mitochondrial membrane depolarization and reactive oxygen species production of human neutrophils is impaired in sepsis

Apoptosis is the most common pathway of neutrophil death under both physiological and inflammatory conditions. In this study, we describe an apoptotic pathway in human neutrophils that is triggered via the surface molecule CD24. In normal neutrophils, CD24 ligation induces death through depolarization of the mitochondrial membrane in a manner dependent on caspase-3 and caspase-9 and reactive oxygen species. Proinflammatory cytokines such as TNF-α, IFN-γ, and GM-CSF upregulated the expression of CD24 in vitro, favoring the emergence of a new CD16(high)/CD24(high) subset of cultured neutrophils. We observed that CD24 expression (at both mRNA and protein levels) was significantly downregulated in neutrophils from sepsis patients but not from patients with systemic inflammatory response syndrome. This downregulation was reproduced by incubation of neutrophils from healthy controls with corticosteroids or with plasma collected from sepsis patients, but not with IL-10 or TGF-β. Decreased CD24 expression observed on sepsis neutrophils was associated with lack of functionality of the molecule, because cross-ligation of CD24 failed to trigger apoptosis in neutrophils from sepsis patients. Our results suggest a novel aspect of CD24-mediated immunoregulation and represent, to our knowledge, the first report showing the role of CD24 in the delayed/defective cell death in sepsis.

Apoptosis-regulated low-avidity cancer-specific CD8(+) T cells can be rescued to eliminate HER2/neu-expressing tumors by costimulatory agonists in tolerized mice

A major barrier to vaccines in cancer treatment is their failure to activate and maintain a complete cancer-specific CD8(+) effector T-cell repertoire. Low-avidity T cells are more likely to escape clonal deletion in the thymus when compared with high-avidity T cells, and therefore comprise the major population of effector T cells available for activation in patients with cancer. However, low-avidity T cells fail to traffic into the tumor microenvironment and function in eradicating tumor under optimal vaccination conditions as opposed to high-avidity T cells that escape clonal deletion and function in tumor killing. We used high- and low-avidity T-cell receptor transgenic CD8(+) T cells specific for the immunodominant epitope HER2/neu (RNEU420-429) to identify signaling pathways responsible for the inferior activity of the low-avidity T cells. Adoptive transfer of these cells into tumor-bearing vaccinated mice identified the members of apoptosis pathways that are upregulated in low-avidity T cells. The increased expression of proapoptotic proteins by low-avidity T cells promoted their own cell death and also that of other tumor-specific CD8(+) T cells within their local environment. Importantly, we show that this proapoptotic effect can be overcome by using a strong costimulatory signal that prevents the activation-induced cell death and enables the low-avidity T cells to traffic into the tumor and assist in tumor clearance. These findings identify new therapeutic opportunities for activating the most potent anticancer T-cell responses.

CD24 on thymic APCs regulates negative selection of myelin antigen-specific T lymphocytes

Negative selection plays a key role in the clonal deletion of autoreactive T cells in the thymus. However, negative selection is incomplete; as high numbers of autoreactive T cells can be detected in normal individuals, mechanisms that regulate negative selection must exist. In this regard, we previously reported that CD24, a GPI-anchored glycoprotein, is required for thymic generation of autoreactive T lymphocytes. The CD24-deficient 2D2 TCR transgenic mice (2D2(+) CD24(-/-) ), whose TCR recognizes myelin oligodendrocyte glycoprotein (MOG), fail to generate functional 2D2 T cells. However, it was unclear if CD24 regulated negative selection, and if so, what cellular mechanisms were involved. Here, we show that elimination of MOG or Aire gene expression in 2D2(+) CD24(-/-) mice - through the creation of 2D2(+) CD24(-/-) MOG(-/-) or 2D2(+) CD24(/) ∼Aire(-/-) mice - completely restores thymic cellularity and function of 2D2 T cells. Restoration of CD24 expression on DCs, but not on thymocytes also partially restores 2D2 T-cell generation in 2D2(+) CD24(-/-) mice. Taken together, we propose that CD24 expression on thymic antigen-presenting cells (mTECs, DCs) down-regulates autoantigen-mediated clonal deletion of autoreactive thymocytes.

Engineering a photoactivated caspase-7 for rapid induction of apoptosis

Apoptosis is a cell death program involved in the development of multicellular organisms, immunity, and pathologies ranging from cancer to HIV/AIDS. We present an engineered protein that causes rapid apoptosis of targeted cells in monolayer culture after stimulation with blue light. Cells transfected with the protein switch L57V, a tandem fusion of the light-sensing LOV2 domain and the apoptosis-executing domain from caspase-7, rapidly undergo apoptosis within 60 min after light stimulation. Constant illumination of under 5 min or oscillating with 1 min exposure had no effect, suggesting that cells have natural tolerance to a short duration of caspase-7 activity. Furthermore, the overexpression of Bcl-2 prevented L57V-mediated apoptosis, suggesting that although caspase-7 activation is sufficient to start apoptosis, it requires mitochondrial contribution to fully commit.

Interaction between the mouse homologue of CD99 and its ligand PILR as a mechanism of T cell receptor-independent thymocyte apoptosis

Here, we show that the interaction between two membrane proteins, the mouse homologue of CD99 (designated D4) and its ligand, paired immunoglobulin-like type 2 receptor (PILR), is one of the major mechanisms of thymocyte apoptosis. Using the polymeric fusion protein of PILR and IgG1 (PILR-Ig), we demonstrated that D4 ligation in the absence of T cell receptor (TCR) engagement leads to the induction of apoptosis, mainly at the double-positive stage of thymocytes. This was further confirmed by a blocking study in which blocking the interaction between D4 and PILR by soluble D4 protein led to reduced apoptosis in the fetal thymic organ culture with wild type and TCRalpha(-/-) mice. Furthermore, the dissection of intracellular signaling pathway demonstrated that D4 cross-linking led to caspase activation without any change in mitochondrial membrane potential. Based on these data, we propose a mechanism for thymocyte depletion in which the interaction between D4 and PILR delivers an active signal.

A new function for LAT and CD8 during CD8-mediated apoptosis that is independent of TCR signal transduction

The majority (>95%) of thymocytes undergo apoptosis during selection in the thymus. Several mechanisms have been proposed to explain how apoptosis of thymocytes that are not positively selected occurs; however, it is unknown whether thymocytes die purely by "neglect" or whether signaling through a cell-surface receptor initiates an apoptotic pathway. We have previously demonstrated that on double positive thymocytes the ligation of CD8 in the absence of TCR engagement results in apoptosis and have postulated this is a mechanism to remove thymocytes that have failed positive selection. On mature single positive T cells CD8 acts as a co-receptor to augment signaling through the TCR that is dependent on the phosphorylation of the adaptor protein, linker for activation of T cells (LAT). Here, we show that during CD8-mediated apoptosis of double positive thymocytes there is an increase in the association of CD8 with LAT and an increase in LAT tyrosine phosphorylation. Decreasing LAT expression and mutation of tyrosine residues of LAT reduced apoptosis upon crosslinking of CD8. Our results identify novel functions for both CD8 and LAT that are independent of TCR signal transduction and suggest a mechanism for signal transduction leading to apoptosis upon CD8 crosslinking.

Targeting CD24 for treatment of colorectal and pancreatic cancer by monoclonal antibodies or small interfering RNA

CD24 is a potential oncogene reported to be overexpressed in a large variety of human malignancies. We have shown that CD24 is overexpressed in 90% of colorectal tumors at a fairly early stage in the multistep process of carcinogenesis. Anti-CD24 monoclonal antibodies (mAb) induce a significant growth inhibition in colorectal and pancreatic cancer cell lines that express the protein. This study is designed to investigate further the effects of CD24 down-regulation using mAb or small interfering RNA in vitro and in vivo. Western blot analysis showed that anti-CD24 mAb induced CD24 protein down-regulation through lysosomal degradation. mAb augmented growth inhibition in combination with five classic chemotherapies. Xenograft models in vivo showed that tumor growth was significantly reduced in mAb-treated mice. Similarly, stable growth inhibition of cancer cell lines was achieved by down-regulation of CD24 expression using short hairpin RNA (shRNA). The produced clones proliferated more slowly, reached lower saturation densities, and showed impaired motility. Most importantly, down-regulation of CD24 retarded tumorigenicity of human cancer cell lines in nude mice. Microarray analysis revealed a similar pattern of gene expression alterations when cells were subjected to anti-CD24 mAb or shRNA. Genes in the Ras pathway, mitogen-activated protein kinase, or BCL-2 family and others of oncogenic association were frequently down-regulated. As a putative new oncogene that is overexpressed in gastrointestinal malignancies early in the carcinogenesis process, CD24 is a potential target for early intervention in the prevention and treatment of cancer.

CD24 cross-linking induces apoptosis in, and inhibits migration of, MCF-7 breast cancer cells

Background

The biological effects of CD24 (FL-80) cross-linking on breast cancer cells have not yet been established. We examined the impact of CD24 cross-linking on human breast cancer cell line MCF-7.

Methods

MCF-7 and MDA-MB-231 cells were treated with anti-rabbit polyclonal IgG or anti-human CD24 rabbit polyclonal antibodies to induce cross-linking, and then growth was studied. Changes in cell characteristics such as cell cycle modulation, cell death, survival in three-dimensional cultures, adhesion, and migration ability were assayed after CD24 cross-linking in MCF-7.

Results

Expression of CD24 was analyzed by flow cytometry in MDA-MB-231 and MCF-7 cells where 2% and 66% expression frequencies were observed, respectively. CD24 cross-linking resulted in time-dependent proliferation reduction in MCF-7 cells, but no reduction in MDA-MB-231 cells. MCF-7 cell survival was reduced by 15% in three-dimensional culture after CD24 cross-linking. Increased MCF-7 cell apoptosis was observed after CD24 cross-linking, but no cell cycle arrest was observed in that condition. The migration capacity of MCF-7 cells was diminished by 30% after CD24 cross-linking.

Conclusion

Our results showed that CD24 cross-linking induced apoptosis and inhibited migration in MCF-7 breast cancer cells. We conclude that CD24 may be considered as a novel therapeutic target for breast cancer.

The novel oncogene CD24 and its arising role in the carcinogenesis of the GI tract: from research to therapy

CD24 was first described in the early 1980s and only attributed to scattered publications, referred to as a cell surface molecule in hematopoiesis. Recently, studies are accumulating to show that CD24 conveys a function in cell-to-cell interaction and regulation of proliferation and adhesion. CD24 appears to be highly expressed in a large variety of human cancers and to contribute to the acceleration of tumor growth and metastases shedding by binding to platelet (P)-selectin, L1 and by evoking--to date unknown--intracellular signal pathways. Anti-CD24 monoclonal antibodies thus act as a promising cancer treatment as was shown in the setting of gastrointestinal cancers. Recent articles also correlate CD24 expression with the identification of 'tumor stem cells'.

Mouse CD24 is required for homeostatic cell renewal

Under physiological conditions, some adult tissues retain a capacity for self-renewal. This property is attributable to the proliferation and differentiation of stem, transit-amplifying, and differentiating cells, which are regulated by cell-cell or cell-matrix interactions or by secreted factors. By gain and loss of function experiments, we demonstrate the involvement of mouse CD24 (mouse cluster of differentiation 24), which is a glycosyl phosphatidylinositol (GPI)-anchored cell-surface glycoprotein, in the regulation of homeostatic cell renewal. BrdU incorporation observations, at optical and electron-microscopic levels, have revealed increased cell proliferation in the developing brain and in the epithelia of mCD24-deleted mice. We have observed ectopic proliferative cells in the suprabasal layers of the mutant skin leading to a general disruption of basal and suprabasal layers. By contrast, ectopic mCD24 expression mediated by retroviral infection of the embryonic brain leads to a decreased number of clusters of cells generated in the progeny. Together, these results and our previous published data indicate that mCD24 contributes to the regulation of the production of differentiated cells by controlling the proliferation/differentiation balance between transit-amplifying and committed differentiated cells.

CD24 is a new oncogene, early at the multistep process of colorectal cancer carcinogenesis

BACKGROUND & AIMS

The aim of this study was to identify genes that play a role in colorectal cancer (CRC) carcinogenesis by analysis of differential gene expression of normal and transformed CRC cell lines.

METHODS

Gene expression array analysis ([RG-U34] GeneChip) was performed in normal and transformed rat intestinal epithelial cells before and after exposures to celecoxib. In particular, we were looking for (1) altered gene expression in the transformed cells that reverts to normal following exposure to a selective cyclooxygenase-2 inhibitor, (2) novel genes, and (3) genes encoding membrane receptors or ligands. As a validation of the results and for human patients, immunohistochemistry was performed on 398 biological samples from the gastrointestinal tract (normal, polyps, and adenocarcinomas). Human cancer cell lines were tested for their response to anti-CD24 monoclonal antibodies.

RESULTS

A total of 1081 genes were differently expressed following malignant transformation; 71 genes showed altered expression that reverted to normal following treatment with celecoxib, including the CD24 gene. Immunohistochemistry confirmed that increased expression of CD24 is an early event in CRC carcinogenesis. It was expressed in 90.7% of adenomas and 86.3% of CRCs. Very low expression was seen in normal epithelium (16.6%). Human cancer cell lines showed growth inhibition in response to the antibodies, according to their expression levels of CD24 and in dose- and time-dependent manners. These results were repetitive for 3 different antibodies.

CONCLUSIONS

CD24 is overexpressed in the colonic mucosa, already at an early stage of carcinogenesis. It may be a useful target for early detection and in CRC therapy.

CD24 plays an important role in the carcinogenesis process of the pancreas

Patients with pancreatic adenocarcinoma have a doom prognosis these tumors were previously proved to express high level of CD24. The current study was aimed to demonstrate that the treatment with monoclonal antibodies to CD24 is effective, in vitro, in pancreatic cancer cells, similar to what we had previously shown in the setting of colorectal cancer. Three human pancreatic cancer cell lines, Colo357, Panc1 and MIA-PaCa, were analyzed for their expression levels of CD24 by Western blot analysis. The correlation for the protein available on the cytoplasmic membrane was assessed by ELISA assay to plates coated with fixed cells using anti-CD24 Ab as the first binder. Human cancer cell lines were tested for their response to two different anti-CD24 monoclonal antibodies and a control antibody (mouse anti-GFP). Human pancreatic adenocarcinomas cell lines that express CD24 (Colo357 and Panc1 cells) showed growth inhibition in dose and time dependent manners. These results were repetitive for the two different antibodies. Growth rate was not affected in MIA-PaCa cells that do not express CD24, or when cells were treated with a control antibody. CD24 may play an important role in the carcinogenesis process of pancreatic cancer. It may serve as a useful target in the therapy of pancreatic cancer.

A role for STAT5A/B in protection of peripheral T-lymphocytes from postactivation apoptosis: Insights from gene expression profiling

Activation of the transcription factors STAT5A and STAT5B by JAK1 and JAK3 tyrosine kinases is a key event in downstream signaling of gammac (common gamma chain)-family cytokine receptors in lymphoid cells. STAT5A/B-deficiency in mice causes, among other consequences, a reduced size and altered composition of the peripheral T-cell pool with predominance of an activated or memory-like population (CD4(+)/CD44(high)/CD62L(low)) and a proliferative deficiency following antigenic stimulation and subsequent IL-2 treatment. To further elucidate the critical function of STAT5A/B in homeostasis and activation of murine naïve peripheral T-lymphocytes, we have analyzed global gene expression of STAT5A/B-deficient versus wild-type splenic T-cells by profiling with high-density oligonucleotide arrays (Affymetrix). Comparison of (1) basal gene expression of untreated peripheral STAT5A/B-deficient and control T-cells and (2) immediate early gene induction upon in vitro stimulation of either population with IL-2 has revealed differential expression of a broad range of genes potentially contributing to the defects of STAT5A/B deficient T-cells. In the context of enhanced apoptotic rates of STAT5A/B(-/-)-T-cells in vivo and upon TCR-stimulation in culture our data suggest a role for STAT5 in post-activation survival beyond regulation of antiapoptotic Bcl-2 proteins and hence provide new insights into the nature of the late proliferative block in the T-cell compartment caused by STAT5-deficiency.

Sensitization of stefin B-deficient thymocytes towards staurosporin-induced apoptosis is independent of cysteine cathepsins

Stefin B (cystatin B) is an inhibitor of lysosomal cysteine cathepsins and does not inhibit cathepsin D, E (aspartic) or cathepsin G (serine) proteinases. In this study, we have investigated apoptosis triggered by camptothecin, staurosporin (STS), and anti-CD95 monoclonal antibody in the thymocytes from the stefin B-deficient mice and wild-type mice. We have observed increased sensibility to STS-induced apoptosis in the thymocytes of stefin B-deficient mice. Pretreatment of cells with pan-caspase inhibitor z-Val-Ala-Asp(OMe)-fluoromethylketone completely inhibited phosphatidylserine externalization and caspase activation, while treatment with inhibitor of calpains- and papain-like cathepsins (2S,3S)-trans-epoxysuccinyl-leucylamido-3-methyl-butane ethyl ester did not prevent caspase activation nor phosphatidylserine exposure. We conclude that sensitization to apoptosis induced by STS in thymocytes of stefin B-deficient and wild-type mice is not dependent on cathepsin inhibition by stefin B.

The Contraction Phase of Virus-Specific CD8+T Cells Is Unaffected by a Pan-Caspase Inhibitor

The effectiveness of protection conferred by CD8(+) memory T cells is determined by both their quality and their quantity, which suggests that vaccine efficacy might be improved if it were possible to increase the size of the memory pool. Approximately 90% of virus-specific CD8(+) T cells die during the contraction phase and, herein, we have attempted to increase the memory pool by reducing CD8(+) T cell death. CD8(+) T cell contraction has been attributed to apoptosis, or programmed cell death (PCD), which, classically, is dependent on caspases. Caspase-dependent PCD can be prevented by the pan-caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp (OMe)-fluoromethylketone (zVAD), and here we evaluate the effect of this compound on virus-specific T cell responses in mice. zVAD prevented caspase-dependent PCD of freshly isolated virus-specific T cells in tissue culture, and a fluorescent analog, FITC-VAD, entered CD8(+) T cells following in vivo injection. However, despite using 11 different regimens of zVAD administration in vivo, no significant effects on CD8(+) or CD4(+) memory T cell numbers were observed. Furthermore, the CD8(+) memory T cell responses to secondary virus infection were indistinguishable, both qualitatively and quantitatively, in zVAD-treated and normal mice. The absence of effect cannot be attributed to a technical flaw, because identical doses of zVAD were able to rescue mice from hepatocyte apoptosis and lethal intrahepatic hemorrhage, induced by inoculation of anti-Fas Ab. We conclude that the contraction phase of the virus-specific T cell response is unlikely to require caspase-dependent PCD. We propose that contraction can be mediated by an alternative, caspase-independent pathway(s).