Programmed death of autoreactive thymocytes

Microsomal prostaglandin E2 synthase-1 and its inhibitors: Molecular mechanisms and therapeutic significance

Inflammation is closely linked to the abnormal phospholipid metabolism chain of cyclooxygenase-2/microsomal prostaglandin E₂ synthase-1/prostaglandin E₂ (COX-2/mPGES-1/PGE₂). In clinical practice, non-steroidal anti-inflammatory drugs (NSAIDs) as upstream COX-2 enzyme activity inhibitors are widely used to block COX-2 cascade to relieve inflammatory response. However, NSAIDs could also cause cardiovascular and gastrointestinal side effects due to its inhibition on other prostaglandins generation. To avoid this, targeting downstream mPGES-1 instead of upstream COX is preferable to selectively block overexpressed PGE₂ in inflammatory diseases. Some mPGES-1 inhibitor candidates including synthetic compounds, natural products and existing anti-inflammatory drugs have been proved to be effective in in vitro experiments. After 20 years of in-depth research on mPGES-1 and its inhibitors, ISC 27864 have completed phase II clinical trial. In this review, we intend to summarize mPGES-1 inhibitors focused on their inhibitory specificity with perspectives for future drug development.

Elimination of self-reactive T cells in the thymus: a timeline for negative selection

Two-photon microscopy and flow cytometry reveal the timing of thymocyte death and the surprisingly close coupling between cell death and phagocytosis during negative selection in thymic slices.

The elimination of autoreactive T cells occurs via thymocyte apoptosis and removal by thymic phagocytes, but the sequence of events in vivo, and the relationship between thymocyte death and phagocytic clearance, are unknown. Here we address these questions by following a synchronized cohort of thymocytes undergoing negative selection within a three-dimensional thymic tissue environment, from the initial encounter with a negative selecting ligand to thymocyte death and clearance. Encounter with cognate peptide–MHC complexes results in rapid calcium flux and migratory arrest in auto-reactive thymocytes over a broad range of peptide concentrations, followed by a lag period in which gene expression changes occurred, but there was little sign of thymocyte death. Caspase 3 activation and thymocyte loss were first detectable at 2 and 3 hours, respectively, and entry of individual thymocytes into the death program occurred asynchronously over the next 10 hours. Two-photon time-lapse imaging revealed that thymocyte death and phagocytosis occurred simultaneously, often with thymocytes engulfed prior to changes in chromatin and membrane permeability. Our data provide a timeline for negative selection and reveal close coupling between cell death and clearance in the thymus.

As an important safeguard against autoimmunity, T cells bearing autoreactive T cell antigen receptors are eliminated during their development in the thymus, a process known as negative selection. Although much is known about the molecular events involved in negative selection, surprisingly little is known about the dynamic aspects of the process. Here we examine a synchronized population of developing T cells (thymocytes) undergoing negative selection within three-dimensional living thymic tissue. We show that the initial encounter with negative selecting ligands results in migratory arrest, but in spite of this synchronous early response, individual thymocytes then undergo delayed and asynchronous entry into the death program between 2 and 12 hours thereafter. Using time-lapse two-photon imaging, we reveal that thymocyte death and the clearance of the dead cells invariably occur together, with many thymocytes already engulfed by a macrophage before the cell death-related changes in chromatin and membrane permeability are evident. These data provide a timeline of the major events during negative selection, and suggest close coupling between thymocyte death and clearance by macrophages.

Treg cell resistance to apoptosis in DNA vaccination for experimental autoimmune encephalomyelitis treatment

Background

Regulatory T (Treg) cells can be induced with DNA vaccinations and protect mice from the development of experimental autoimmune encephalomyelitis (EAE), a mouse model of multiple sclerosis (MS). Tacrolimus (FK506) has been shown to have functions on inducing immunosuppression and augmenting apoptosis of pathologic T cells in autoimmune disease. Here we examined the therapeutic effect of DNA vaccine in conjunction with FK506 on EAE.

Methodology/Principal Findings

After EAE induction, C57BL/6 mice were treated with DNA vaccine in conjunction with FK506. Functional Treg cells were induced in treated EAE mice and suppressed Th1 and Th17 cell responses. Infiltrated CD4 T cells were reduced while Treg cells were induced in spinal cords of treated EAE mice. Remarkably, the activated CD4 T cells augmented apoptosis, but the induced Treg cells resisted apoptosis in treated EAE mice, resulting in alleviation of clinical EAE severity.

Conclusions/Significance

DNA vaccine in conjunction with FK506 treatment ameliorates EAE by enhancing apoptosis of CD4 T cells and resisting apoptosis of induced Treg cells. Our findings implicate the potential of tolerogenic DNA vaccines for treating MS.

Evidence of apoptosis in some cell types due to pentachlorophenol (PCP) in Heteropneustes fossilis

The study aimed to clarify the role of apoptosis in pentachlorophenol (PCP) induced testicular, ovarian and renal cell genotoxicity of Heteropneustes fossilis. It was further intended to find the target germ cell type and assess the cellular and nuclear damage. Treatment of PCP was used for multiduration on the germinal tissues and they were processed to detect structural changes by light and electron microscopic evaluation and kidney cells for subsequent detection of DNA fragmentation by agarose gel electrophoresis. Findings suggest functional and morphological changes in the tissues are due to apoptosis, as evidenced by some biochemical and cytological signs. Histological observation on germinal epithelium reveals cell suicidal symptoms such as vacuolization, liquefied regions in the cytoplasm of oocytes, margination of nuclei, clumping of chromatin, and compaction of cytoplasmic organelle. Biochemical manifestation concurrent to this, is; cleavage of kidney cell DNA into low molecular weight fragments confirming apoptosis. Subsequently, it is further cleaved into nucleosome size fragments or its multiples. Ultra-structural histopathology and DNA studies conclusively lead to the PCP induced apoptosis in the exposed cell types. Results further support the usefulness of this assay in the related studies and its feasibility in generating a base line data.

Apoptosis and inflammation

During the last few decades it has been recognized that cell death is not the consequence of accidental injury, but is the expression of a cell suicide programme. Kerr et al. (1972) introduced the term apoptosis. This form of cell death is under the influence of hormones, growth factors and cytokines, which depending upon the receptors present on the target cells, may activate a genetically controlled cell elimination process. During apoptosis the cell membrane remains intact and the cell breaks into apoptotic bodies, which are phagocytosed. Apoptosis, in contrast to necrosis, is not harmful to the host and does not induce any inflammatory reaction. The principal event that leads to inflammatory disease is cell damage, induced by chemical/physical injury, anoxia or starvation. Cell damage means leakage of cell contents into the adjacent tissues, resulting in the capillary transmigration of granulocytes to the injured tissue. The accumulation of neutrophils and release of enzymes and oxygen radicals enhances the inflammatory reaction. Until now there has been little research into the factors controlling the accumulation and the tissue load of granulocytes and their histotoxic products in inflammatory processes. Neutrophil apoptosis may represent an important event in the control of intlamtnation. It has been assumed that granulocytes disintegrate to apoptotic bodies before their fragments are removed by local macrophages. Removal of neutrophils from the inflammatory site without release of granule contents is of paramount importance for cessation of inflammation. In conclusion, apoptotic cell death plays an important role in inflammatory processes and in the resolution of inflammatory reactions. The facts known at present should stimulate further research into the role of neutrophil, eosinophil and macrophage apoptosis in inflammatory diseases.

Genetic Analysis of Susceptibility to Radiation-induced Apoptosis of Thymocytes in Mice

Genetic control of thymocyte susceptibility to radiation-induced apoptosis was investigated in BALB/cHeA, STS/A and five other strains of mice by counting pyknotic cells in a selected area of thymic cortex on histological specimens after whole-body X-irradiation. Number of dead cells increased almost linearly with doses (range 0.25-0.75 Gy) in BALB/cHeA and STS/A mice. However, dead cell counts in BALB/cHeA mice were more than twice those in STS/A mice at each dose. Of five other strains of mice, C57BL/6N and B10.BR mice exhibited a sensitive phenotype similar to BALB/cHeA mice, while C3H/HeAMsNrs and NFS/N mice showed a resistant phenotype similar to STS/A mice. A/J mice seemed to be rather resistant. A sex difference was not recognized in BALB/cHeA and STS/A mice. Resistance was dominant over susceptibility in the progenies of reciprocal crosses between the two strains, indicating an autosomal inheritance and no maternal effect. Segregation ratio of susceptible phenotype to resistant one in the backcrosses of female (BALB/cHeA x STS/A)F1 mice with male BALB/cHeA mice was not significantly different from 1:1 and all backcrosses of female (BALB/cHeA x STS/A)F1 mice with male STS/A mice exhibited a resistant phenotype. Results suggested that thymocyte susceptibility to radiation-induced apotosis is controlled by one major autosomal allele.

Effect of ZVAD-fmk on hepatocyte apoptosis after bile duct ligation in rat

AIM: Retention and accumulation of toxic hydrophobic bile salts within hepatocyte may cause hepatocyte toxicity by inducing apoptosis. Apoptosis is a pathway of cell death orchestrated by a family of proteases called caspases. Z-Val-Ala-Asp (OMe)-fluoromethyl ketone (ZVAD-fmk) is a cell-permeable irreversible inhibitor of caspase. The purpose of this study was to evaluate the possible effect of ZVAD-fmk on hepatocyte apoptosis after bile duct ligation in the rat.

METHODS: Male Sprague-Dawley rats, weighing 250-300 g, were randomized to five groups of five rats each. Group 1 underwent common bile duct ligation and simultaneous treatment with ZVAD-fmk (dissolved in dimethylsulfoxide (DMSO)). Group 2 underwent common bile duct ligation and simultaneous treatment with Z-Phe-Ala-fluoromethyl ketone ( ZFA-fmk, dissolved in DMSO). Group 3 underwent sham operation and simultaneous treatment with the same amount of DMSO. Group 4 underwent sham operation and simultaneous treatment with the same amount of normal saline. Group 5 underwent common bile duct ligation without other manipulation. After three days, liver tissue was harv-ested for histopathologic analysis and measurements of apoptosis.

RESULTS: When compared with sham operation, common bile duct ligation significantly increased hepatocyte apoptosis (P = 0.008) and ductular proliferation (P = 0.007). ZVAD-fmk significantly diminished the increased hepatocyte apoptosis and ductular proliferation after common bile duct ligation (P = 0.008 and P = 0.007, respectively). ZFA did not show the same effects.

CONCLUSION: Hepatocyte apoptosis and ductular proliferation significantly increased after common bile duct ligation. ZVAD-fmk effectively diminished the increased hepatocyte apoptosis and ductular proliferation after common bile duct ligation, whereas ZFA-fmk did not.

Massive telomere loss and telomerase RNA expression in dexamethasone-induced apoptosis in mouse thymocytes

Apoptosis is a well-recognized process of cell death occurring under several physiological and pathological conditions and represents the principal mechanism involved in cell selection in the thymus. Glucocorticoids are well known to stimulate apoptosis in rat thymocytes. However, it is unclear whether the same changes occur after in vivo glucocorticoid treatment in mice. Chromosomal stability and cell viability require a proficient telomeric end-capping function. Cells with critical telomere shortening and telomerase dysfunction undergo increased apoptosis. In turn, the change in telomere function in cells undergoing apoptosis is not fully characterized. In order to investigate this, we studied the changes in thymocytes after dexamethasone administration in BALB/c mice. The loss of normal thymocytes coincided with the appearance of small dense cells with characteristic features of apoptosis including condensed chromatin, internucleosomal DNA cleavage, and a "hypodiploid" peak on flow cytometry, which suggested that dexamethasone-induced thymocyte apoptosis in BALB/c mice could be considered a well-defined experimental model for studying apoptotic processes. Dexamethasone-treated thymocytes exhibited rapid and dynamic loss of telomeric sequences and up-regulation of telomerase RNA as an early event in the apoptotic process. Telomerase activity was unchanged in this event. Thereafter, telomere gain associated with an increase in telomerase activity occurred in the regenerative process of the thymus. These results suggest a role of telomere loss and up-regulation of telomerase RNA as key apoptosis sensors.

The thymosins. Prothymosin alpha, parathymosin, and beta-thymosins: structure and function

The studies on thymosins were initiated in 1965, when the group of A. White searched for thymic factors responsible for the physiological functions of thymus. To restore thymic functions in thymic-deprived or immunodeprived animals, as well as in humans with primary immuno-deficiency diseases and in immunosuppressed patients, a standardized extract from bovine thymus gland called thymosin fraction 5 was prepared. Thymosin fraction 5 indeed improved immune response. It turned out that thymosin fraction 5 consists of a mixture of small polypeptides. Later on, several of these peptides (polypeptide beta 1, thymosin alpha 1, prothymosin alpha, parathymosin, and thymosin beta 4) were isolated and tested for their biological activity. The research of many groups has indicated that none of the isolated peptides is really a thymic hormone; nevertheless, they are biologically important peptides with diverse intracellular and extracellular functions. Studies on these functions are still in progress. The current status of knowledge of structure and functions of the thymosins is discussed in this review.

Molecular mechanisms of sulfasalazine-induced T-cell apoptosis

Impaired apoptosis of T-lymphocytes is involved in the development of chronic inflammatory disorders. Previously we have shown that the anti-inflammatory drug sulfasalazine induces apoptosis in a murine T-lymphocyte cell line. The aims of the present study were to expand these observations to human systems and to analyse the molecular basis for sulfasalazine-induced apoptosis. Sulfasalazine induces apoptosis both in Jurkat cells, a human T-leukaemia cell line (ED50 value approximately 1.0 mM), and in primary human peripheral blood T-lymphocytes (ED50 value approximately 0.5 mM). In contrast SW620 colon carcinoma cells or primary human synoviocytes are not affected at these concentrations suggesting a cell type-specific sensitivity to sulfasalazine. Sulfasalazine triggers the mitochondrial accumulation of Bax and induces a collapse of the mitochondrial transmembrane potential (deltapsi(m)). Sulfasalazine causes cytochrome c release from mitochondria and subsequent activation of caspase-3 and downstream substrates. However, the pan-caspase inhibitor Z-VAD.fmk fails to inhibit sulfasalazine-induced apoptosis. Sulfasalazine stimulates mitochondrio-nuclear translocation of the novel apoptogenic factor apoptosis-inducing factor (AIF) and triggers large-scale DNA fragmentation, a characteristic feature of AIF-mediated apoptosis. Sulfasalazine-induced DeltaPsi(m) loss, AIF redistribution, and cell death are fully prevented by overexpression of Bcl-2. In conclusion, our data suggest that sulfasalazine-induced apoptosis of T-lymphocytes is mediated by mitochondrio-nuclear translocation of AIF and occurs in a caspase-independent fashion. Sulfasalazine-induced apoptosis by AIF and subsequent clearance of T-lymphocytes might thus provide the molecular basis for the beneficial therapeutic effects of sulfasalazine in the treatment of chronic inflammatory diseases.

Role of Ca2+ in the intracellular signaling pathway of anti-IgM-induced apoptosis in the human B-cell line, MBC-1, established from Burkitt lymphoma

The role of Ca2+ in the intracellular signal transduction process that causes antibody-induced apoptotic cell death in B-cells is not completely understood. We previously established a B-cell line (MBC-1) from a patient with Burkitt lymphoma at the leukemic stage that demonstrated the typical morphology and internucleosomal DNA fragmentation of apoptosis when treated with anti-immunoglobulin (Ig)M antibody. This antibody-induced cell death was partially inhibited by pretreatment with ethyleneglycol-bis-tetraacetic acid (EGTA) and actinomycin-D. FK506, an immunosupressive agent and calcineurin inhibitor, also partially rescued the anti-IgM antibody-induced death of MBC-1 cells. These results show that the calcium signaling pathway, which leads to a change in gene expression, plays an important role in anti-IgM-induced apoptosis in MBC-1 cells. Flow cytometric measurement of the cytosolic free Ca2+ concentration ([Ca2+]i) showed that nontoxic concentrations of 4-bromo-calcium ionophore A23187 (Ca2+ IP) increased [Ca2+]i more than did the anti-IgM antibody. A brief Ca2+ spike was observed on anti-IgM antibody treatment, but a gradual increase and decrease were observed when the cells were treated with Ca2+ IP at a nontoxic concentration of 1 microg/mL. These findings suggest that interpretations differ for the 2 patterns of calcium signaling and that the brief spiked elevation of Ca2+ produces distinct biological and cellular responses compared to the gradual increase and decrease of [Ca2+]i. Our results support the hypothesis that Ca2+ plays a significant role as a multifunctional second messenger providing specific information to the nucleus in anti-IgM antibody-induced apoptosis in MBC-1 cells.

Extensive cell death in thymocytes in colon 26-induced cachectic mice

Extensive atrophy has been reported to occur in the thymus in a cancer-burden state but the mechanisms of this atrophy have not been fully elucidated. We investigated changes in the thymus in tumour-bearing mice inoculated with two subclones of the murine colon 26 adenocarcinoma cell line: clone 5 (non-cachectic) and clone 20 (cachectic). In clone 20 mice, body weights and thymocyte numbers decreased significantly compared with controls. Flow cytometric analysis of the thymocytes demonstrated that the frequency of single positive cells (CD4+ CD8- and CD4- CD8+) was significantly increased and that of double positive cells (CD4+ CD8+) was significantly decreased in clone 20 mice and, to a lesser extent, in clone 5 mice compared with controls. Serum levels of interleukin 6 and granulocyte-macrophage colony-stimulating factor (GM-CSF) were significantly elevated. These results suggested that thymocyte apoptosis was accelerated in the cancer-cachectic state, and increased GM-CSF might be partly responsible for thymic atrophy.

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)

An overview of the immune system

We are continually exposed to organisms that are inhaled, swallowed, or inhabit our skin and mucous membranes. Whether these organisms penetrate and cause disease is a result of both the pathogenicity of the organism (the virulence factors at its disposal) and the integrity of host defence mechanisms. The immune system is an interactive network of lymphoid organs, cells, humoral factors, and cytokines. The essential function of the immune system in host defence is best illustrated when it goes wrong; underactivity resulting in the severe infections and tumours of immunodeficiency, overactivity in allergic and autoimmune disease. In this review we have covered the normal function of the immune system in recognising, repelling, and eradicating pathogens and other foreign molecules.

Resistance to glucocorticoid-induced apoptosis in PLP peptide-specific T cell clones from patients with progressive MS

Glucocorticoids (GC) are commonly used to treat inflammatory disorders such as multiple sclerosis (MS) and may exert their immunosuppressive activity by inducing apoptosis in activated lymphocytes. However, unlike relapsing-remitting MS patients, those with progressive disease respond poorly to GC treatment. The data in this communication indicate that PLP peptide-specific T cell clones from progressive, but not relapsing-remitting MS patients are resistant to GC-induced apoptosis in vitro, in a fashion associated with expression of B-7 co-stimulatory molecules. Thus, failure to respond to GC treatment may reflect defect in apoptosis that develop during the progressive stages of chronic inflammatory disease.

Clustering of distinct autoimmune diseases associated with functional abnormalities of T cell survival in children

To ascertain whether alterations of lymphocyte switching off may be associated with clustering of autoimmune diseases in children, Fas- and C2-ceramide-induced cell death was evaluated on T cell lines derived from three patients affected by clustering of autoimmune disorders. Three patterns were found: patient 3 was resistant to Fas- and C2-ceramide, patient 1 was resistant to Fas, but sensitive to C2-ceramide, patient 2 was resistant to C2-ceramide, but sensitive to Fas. By contrast, Fas- and C2-ceramide-induced cell death was normal in five children with systemic juvenile rheumatoid arthritis, five children with insulin-dependent diabetes and 10 age-matched healthy controls. Surface expression of Fas was low in patient 1, but normal in patients 2 and 3. Together with normal Fas transcripts, patients 2 and 3 displayed a transcript 152 bp longer than the normal one retaining intron 5. Our data indicate that polyreactive autoimmune syndromes may be associated with heterogeneous alteration of the immune response switching-off system.

Neutrophils die in the gingival crevice, periodontal pocket, and oral cavity by necrosis and not apoptosis

BACKGROUND

Neutrophils play important roles in the homeostasis of periodontal tissues. However, remarkably little is known about how these cells live and die in the gingival crevice and periodontal pocket. We have examined whether subgingival neutrophils die by necrosis or apoptosis and have begun to study the mechanisms controlling the functional lifespan of these cells.

METHODS

Neutrophils collected from clinically healthy gingival crevices, periodontal pockets, and the oral cavity (saliva) were examined with Hoechst dye 33342, propidium iodide/eithidium bromide, and FITC-annexin V to identify live, dead, and apoptotic cells. Blood neutrophils were cultured for 17 to 20 hours in the presence or absence of gingival crevice washings (GC-w) to study the effect of GC-w on neutrophil apoptosis. In addition, endotoxin was removed from GC-w by affinity resin to investigate the contribution of LPS to the inhibitory effect of GC-w on blood neutrophils.

RESULTS

The percentage of dead neutrophils in all subgingival sites and in all oral samples far exceeded the percentage of apoptotic neutrophils. In all 3 locations, approximately 30% of neutrophils were dead, whereas less than 1% of neutrophils were apoptotic. We conclude that the majority of neutrophils which lose their viability within gingival crevices, periodontal pockets and the oral cavity die by necrosis and not by apoptosis. Washings obtained from clinically healthy gingival crevices (GC-w) variably, but significantly, delayed apoptosis of peripheral blood neutrophils (mean suppression 45.7% +/- SD 22.3). Removal of endotoxin from GC-w significantly reduced this inhibitory effect.

CONCLUSIONS

Our findings provide insights into the mechanisms of neutrophil death and the control of the functional lifespan of neutrophils in gingival crevices and periodontal pockets and therefore into the pathogenesis of periodontal diseases.

Apoptosis of murine neonatal T cells

Recent evidence supports the idea that T cells in neonatal animals are developmentally mature in their capacity to mount protective helper and cytotoxic responses. Nonetheless, neonates fall prey to infections which have little effect on adults and they often fail to mount mature responses to environmental, experimental, or vaccine antigens. To reconcile these observations, it may be important to consider the potential role of apoptosis in neonatal immune responses. In adults, apoptosis is well established as a centrally important process in the homeostasis of cellular immune responses. Activated T cells deprived of IL-2 undergo cytokine withdrawal-induced apoptosis. Previously activated T cells can also be triggered by secondary stimulation to undergo activation induced apoptosis. This review summarizes our current state of knowledge of apoptosis of murine neonatal T cells and discusses the possible impact(s) of this apoptosis on neonatal immune responses in vivo.

DNA fragmentation pattern induced in thymocytes by sulphur mustard

Sulphur mustard (HD) is a blister agent for which no specific therapy exists. The mechanism of cell injury caused by HD is not well understood. This study examined DNA damage in thymocytes exposed to a range of HD concentrations over a time course of 1-24 h. Thymocytes incubated with HD showed an increase in the production of DNA fragments of the type frequently associated with apoptosis, namely, initial formation of large fragments of 30-50, 200-300 and > 700 kilobase pairs (kbp), followed by further degradation to produce an internucleosomal 'ladder' of oligomers of approximately 180 base pairs (bp). Pulsed field electrophoresis analysis of thymocytes incubated with HD detected breakdown of the chromatin up to 3 h before a corresponding increase in the low molecular weight (MW) oligonucleosomal fragments could be seen on conventional agarose gels. These results suggest that cells damaged by HD poisoning may be irretrievably committed to cell death sooner after exposure than previous studies suggested. The nature of the DNA fragments produced suggested that apoptosis may represent a component of the pathway of cell death induced by HD. These aspects may have implications for the search for specific therapeutic reagents effective in the prevention or treatment of HD poisoning.

Disruption of T cell tolerance to self-immunoglobulin causes polyclonal B cell stimulation followed by inactivation of responding autoreactive T cells

Scavenger receptor (SR)-specific delivery by maleylation of a ubiquitous self-protein, Ig, to SR-bearing APCs results in self-limiting induction of autoimmune effects in vivo. Immunization with maleyl-Ig breaks T cell tolerance to self-Ig and causes hypergammaglobulinemia, with increases in spleen weight and cellularity. The majority of splenic B cells show an activated phenotype upon maleyl-Ig immunization, leading to large-scale conversion to a CD138+ phenotype and to significant increases in CD138-expressing splenic plasma cells. The polyclonal B cell activation, hypergammaglobulinemia, and autoreactive Ig-specific T cell responses decline over a 2-mo period postimmunization. Following adoptive transfer, T cells from maleyl-Ig-immune mice taken at 2 wk postimmunization can induce hypergammaglobulinemia in the recipients, but those taken at 10 wk postimmunization cannot. Hypergammaglobulinemia in the adoptive transfer recipients is also transient and is followed by an inability to respond to fresh maleyl-Ig immunization, suggesting that the autoreactive Ig-specific T cells are inactivated peripherally following disruption of tolerance. Thus, although autoreactive T cell responses to a ubiquitous self-Ag, Ig, are induced by SR-mediated delivery to professional APCs in vivo resulting in autoimmune pathophysiological effects, they are effectively and rapidly turned off by inactivation of these activated Ig-specific T cells in vivo.

Inhibition of nuclear factor kappa B and induction of apoptosis in T-lymphocytes by sulfasalazine

1. Chronic inflammatory diseases have been shown to be associated with NF-kappaB activation and impaired apoptosis of immune cells. The aim of the present study was to investigate if sulfasalazine and its colonic metabolites 5-aminosalicylic acid (5ASA) and sulfapyridine affect NF-kappaB/Rel activation and viability of T-lymphocytes. 2. Sulfasalazine inhibits NF-kappaB/Rel activation in the murine T-lymphocyte cell line RBL5 using electrophoretic mobility shift assays. In transfection assays sulfasalazine treatment for 4 h inhibits kappaB-dependent transcription with an IC50 value of approximately 0.625 mM. 3. Higher doses or prolonged treatment result in cell death of T-lymphocytes in a dose- and time-dependent manner. Cell death is caused by apoptosis as judged by DNA fragmentation, annexin V and Apo 2.7 staining. Induction of apoptosis is a fast event with 50% apoptotic cells after a 4 h incubation with 2.5 mM sulfasalazine. The ED50 value for apoptosis induction after 24 h treatment was approximately 0.625 mM. 4. In contrast, 5ASA and sulfapyridine neither inhibit NF-kappaB/Rel activation nor induce apoptosis in T-lymphocytes at doses up to 5.0 mM. 5. These results demonstrate that sulfasalazine, but not 5ASA or sulfapyridine, strongly inhibits NF-kappaB activation and potently induces apoptosis in T-lymphocytes. Inhibition of NF-kappaB/Rel activation and subsequent clearance of activated T-lymphocytes by apoptosis might thus explain the beneficial effects of sulfasalazine in the treatment of chronic inflammatory disorders.

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.

A specific inhibitor of apoptosis decreases tissue injury after intestinal ischemia-reperfusion in mice

PURPOSE

Apoptosis is a stereotypical pathway of cell death that is orchestrated by a family of cysteine endoproteases called caspases. This study examined the effect of apoptosis inhibition with a specific caspase inhibitor on murine intestinal viability after ischemia-reperfusion (IR).

METHODS

C57Bl6 X SV129 mice underwent segmental small bowel ischemia by vascular isolation of 10 cm of terminal ileum. In separate experiments, the ischemic time was varied from 30 to 130 minutes with a reperfusion interval of 6 hours. The degree of small bowel injury was quantified from 1 to 5 (increasing severity) by standardized, blinded histologic grading. The degree of apoptosis was assessed with a specific assay (terminal deoxyamcleotydil transferase-mediated deoxyuridine triphosphate nick end labeling [TUNEL]) and quantified by calculating the apoptotic index (apoptotic cells/10 high-power fields). To evaluate for activation of interleukin-1beta converting enzyme we measured tissue mature interleukin-1beta levels using a specific enzyme-linked immunosorbent assay. To evaluate the effect of apoptosis inhibition on intestinal viability after IR, mice received 3.0 mg of the caspase inhibitor ZVAD (N-benzyloxycarbonyl Val-Ala-Asp-Ome-fluoromethylketone) subcutaneously before and after IR in five divided doses (n = 11), the same dose of ZFA (N-benzyloxycarbonyl Phe-Ala fluoromethylketone), a structurally similar molecule with no anticaspase activity (n = 9), or sham operation (n = 6).

RESULTS

A linear relationship existed between ischemic interval and histologic grade (r = 0.69, P <.006). Increasing the ischemic interval from 0 to 50 minutes was associated with a fivefold increase in apoptotic index (P =.05). Ischemic bowel was measured to have an average of 57.3 +/- 7.8 pg/mL whereas normal bowel had an average of 1.8 +/- 0.5 pg/mL of mature interleukin-1beta present. Mice tolerated multiple injections of ZVAD and ZFA without signs of toxicity. Animals treated with ZVAD (apoptosis inhibitor) had little injury after 50 minutes of ischemia and 6 hours of reperfusion (injury grade 1.8) compared with sham controls (injury grade 1.2, P =.7) and had significantly less injury than mice treated with ZFA (placebo) (injury grade 3.0, P <.006).

CONCLUSIONS

Increasing ischemic interval in a segmental small bowel murine IR model is associated with increased histologic injury and augmented apoptosis as evidenced by increased TUNEL staining and interleukin-1beta converting enzyme activation. Inhibition of apoptosis with a specific caspase inhibitor significantly diminishes the degree of small bowel injury.

Mutant p53: epigenetic mutator of the T-cell receptor via induction of methylation

The mechanism and effects of epigenetic alterations in human carcinogenesis are not well understood, except that cancers often have alterations in the methylation status of their genomes. Additionally, human cancers, including aggressive T-cell leukemias and lymphomas, have a high frequency of p53 mutations, particularly missense mutations, which raises the possibility of gain-of-new-function proteins, but the new proteins' oncogenic functions are mechanistically ill-defined. To investigate the mechanisms behind the high prevalence of p53 tumor suppressor gene mutations in aggressive or relapsed T-cell leukemias, we transfected Jurkat cells null for p53 protein with a temperature-sensitive p53 mutant. We showed that this mutant p53 abrogated expression of the T-cell antigen receptor (TCR) by affecting the methylation of an at least 20-kb region of DNA, 5'to the TCR beta-chain gene enhancer region, which includes TCRbetaC1 and betaC2. Expression of the TCR is restored when the temperature is reduced to 32 degrees C, at which temperature the mutant p53 regains wild-type function. The TCR, a common site of dysfunction in T-cell malignancies, is the principal signal transduction moiety controlling both T-cell activation and activation-induced apoptosis. These results suggest a new role for mutant p53-as an epigenetic mutator, bridging p53, methylation, and transcriptional silencing-and suggest novel mechanisms in immunosuppression and cancer progression.

The Src-Homology Domain 2-Bearing Protein Tyrosine Phosphatase-1 Inhibits Antigen Receptor-Induced Apoptosis of Activated Peripheral T Cells

Restimulation of Ag receptors on peripheral T lymphocytes induces tyrosine phosphorylation-based signaling cascades that evoke Fas ligand expression and induction of Fas-mediated programmed cell death. In view of the role for the Src homology domain 2-bearing protein tyrosine phosphatase-1 (SHP-1) in modulating TCR signaling, we investigated the influence of SHP-1 on TCR-mediated apoptosis by assaying the sensitivity of peripheral T cells from SHP-1-deficient viable motheaten (mev) mice to cell death following TCR restimulation. The results of these studies revealed mev peripheral T cells to be markedly more sensitive than wild-type cells to induction of cell death following TCR stimulation. By contrast, PMA/ionophore and anti-Fas Ab-induced apoptotic responses were no different in mev compared with wild-type activated cells. Enhanced apoptosis of TCR-restimulated mev lymphocytes was associated with marked increases in Fas ligand expression as compared with wild-type cells, but was almost abrogated in both mev and wild-type cells by Fas-Fc treatment. Thus, the increased sensitivity of mev T cells to apoptosis following TCR restimulation appears to reflect a TCR-driven phenomenon mediated through up-regulation of Fas-Fas ligand interaction and induction of the Fas signaling cascade. These findings, together with the hyperproliferative responses of mev peripheral T cells to initial TCR stimulation, indicate that SHP-1 modulation of TCR signaling translates to the inhibition of both T cell proliferation and activation and, as such, is likely to play a pivotal role in regulating the expansion of Ag-stimulated T cells during an immune response.

Induction of apoptosis in mouse thymocytes by microcolin A and its synthetic analog

Microcolin A (Mic-1), a marine-derived compound, has been shown to be a novel antiproliferative and immunosuppressive agent. We investigated the ability of Mic-1 and its chemosynthetic analog, microcolin A3 (Mic-3), to induce apoptosis in murine thymocytes. Following incubation of the cells with Mic-1 (10-100 nM) or Mic-3 (10-100 nM), internucleosomal DNA fragmentation in apoptotic cells was detected by agarose gel electrophoresis and the diphenylamine (DPA) assay; the presence of hypodiploid nuclei assessed by propidium iodide (PI) staining; and the percentages of apoptotic and necrotic cells quantified by morphological observation and fluorescein labeled annexin-V binding. Our results show that both Mic-1 and Mic-3 are potent inducers of apoptosis in thymocytes depending on drug concentration and time of exposure, with Mic-3 being more potent than Mic-1 in the induction of apoptosis. Furthermore, flow cytometric analysis using monoclonal antibodies specific to thymocyte subpopulations showed that the proportion of the early immature CD4+ CD8+ T-cell subpopulation in thymocytes was selectively decreased by both agents with a corresponding increase of other subpopulations, indicating that CD4+ CD8+ T cells are the most likely targets of Mic-1 and Mic-3. These in vitro results suggest that the antiproliferative and immunosuppressive properties of both compounds are possibly associated with apoptosis-inducing events and imply that they may have additional potential value as antineoplastic agents.

Dexamethasone-Induced Thymocyte Apoptosis: Apoptotic Signal Involves the Sequential Activation of Phosphoinositide-Specific Phospholipase C, Acidic Sphingomyelinase, and Caspases

Glucocorticoid hormones (GCH) have been implicated as regulators of T-lymphocyte growth and differentiation. In particular, it has been reported that GCH can induce thymocyte apoptosis. However, the molecular mechanisms responsible for this GCH-induced death have not been clarified. In this work, the biochemical events associated with apoptosis induced by Dexamethasone (Dex), a synthetic GCH, in normal mouse thymocytes, have been analyzed. Results indicate that Dex-induced thymocyte apoptosis is attributable to an early ceramide generation caused by the activation of an acidic sphingomyelinase (aSMase). Caspase activity plays a crucial role in Dex-induced apoptosis and is downstream the aSMase activation in that inhibition of the early ceramide generation inhibits caspase activation and thymocyte death. Moreover, Dex treatment rapidly induces diacylglycerol (DAG) generation, through a protein kinase C (PKC) and G-protein–dependent phosphatidylinositol-specific phospholipase C (PI-PLC), an event which precedes and is required for aSMase activation. Indeed, PI-PLC inhibition by U73122 totally prevents Dex-induced aSMase activity, ceramide generation, and consequently, caspase activation and apoptosis. All these effects require Dex interaction with GCH receptor (GR), are countered by the GR antagonist RU486, and precede the GCH/GR-activated transcription and protein synthesis. These observations indicate that GCH activates thymocyte death through a complex signaling pathway that requires the sequential activation of different biochemical events.

Dexamethasone-Induced Thymocyte Apoptosis: Apoptotic Signal Involves the Sequential Activation of Phosphoinositide-Specific Phospholipase C, Acidic Sphingomyelinase, and Caspases

Glucocorticoid hormones (GCH) have been implicated as regulators of T-lymphocyte growth and differentiation. In particular, it has been reported that GCH can induce thymocyte apoptosis. However, the molecular mechanisms responsible for this GCH-induced death have not been clarified. In this work, the biochemical events associated with apoptosis induced by Dexamethasone (Dex), a synthetic GCH, in normal mouse thymocytes, have been analyzed. Results indicate that Dex-induced thymocyte apoptosis is attributable to an early ceramide generation caused by the activation of an acidic sphingomyelinase (aSMase). Caspase activity plays a crucial role in Dex-induced apoptosis and is downstream the aSMase activation in that inhibition of the early ceramide generation inhibits caspase activation and thymocyte death. Moreover, Dex treatment rapidly induces diacylglycerol (DAG) generation, through a protein kinase C (PKC) and G-protein–dependent phosphatidylinositol-specific phospholipase C (PI-PLC), an event which precedes and is required for aSMase activation. Indeed, PI-PLC inhibition by U73122 totally prevents Dex-induced aSMase activity, ceramide generation, and consequently, caspase activation and apoptosis. All these effects require Dex interaction with GCH receptor (GR), are countered by the GR antagonist RU486, and precede the GCH/GR-activated transcription and protein synthesis. These observations indicate that GCH activates thymocyte death through a complex signaling pathway that requires the sequential activation of different biochemical events.

Fas and Fas ligand messenger ribonucleic acid and protein expression in the rat corpus luteum during apoptosis-mediated luteolysis

Apoptosis has been found to occur during regression of the corpus luteum (CL) in many species. The Fas (APO-1/CD95) receptor, a transmembrane protein that induces apoptosis in the cell when bound to Fas ligand (FasL), may be involved. This study established and quantitated the presence and regulation of Fas receptor and FasL in the rat CL during pregnancy and postpartum. Using immunohistochemistry, FasL was localized in CL during pregnancy and postpartum. Fas was localized at Day 1 of pregnancy and at the time of luteolysis. Both Fas and FasL mRNA were found to be expressed throughout pregnancy and postpartum using reverse transcription-polymerase chain reaction (RT-PCR). Relative quantitative RT-PCR established that expression of FasL mRNA increased significantly at Day 22 of pregnancy and decreased by Day 3 postpartum. Spontaneous apoptosis of rat CL placed in an in vitro culture model with serum-free medium was examined by analysis of extracted DNA using 3' end-labeling. Treatment with an anti-rat Fas monoclonal antibody demonstrated a reduction in the occurrence of spontaneous apoptosis. These data support a role for Fas receptor and FasL in rat CL apoptosis during luteolysis.

G10.3 monoclonal antibody identifies novel functional cell surface structures expressed by normal B lymphocytes and various malignant cell lines

G10.3, a unique monoclonal antibody (mAb), was produced to better characterize lymphocyte subsets. In the present study, we show that this mAb identifies 118, 83 and 51 kDa cell surface sialylated glycoproteins on the immunizing cell line YTindi. The reactivity of G10.3 mAb is restricted in normal cells to B lymphocytes, whereas within tumoral cell lines various lymphoid and non-lymphoid cells were found positive. Interestingly, functional studies revealed that triggering G10.3 mAb reactive molecules with soluble antibody led to an inhibition of growth and to an induction of programmed cell death in tumor cell lines expressing high levels of reactive molecules.

Ligation of CD5 on resting B cells, but not on resting T cells, results in apoptosis

The CD5 molecule is expressed by a B cell subset. We have demonstrated that resting B cells do not proliferate in response to CD5 ligation, whereas cells preactivated with anti-IgM and IL-2 do so. Here, we specifically studied the effects of anti-CD5 and anti-IgM on apoptosis of CD5+ B cells. Both ligation of CD5 or of surface IgM (sIgM) resulted in apoptosis. This started earlier following ligation of CD5 than with sIgM, and both responses were time dependent. CD5-induced apoptosis was independent of the epitope recognized or the way the antibody was presented to the B cells. CD5+ B cells were more sensitive to IgM-induced apoptosis than CD5 B cells. Engagement of CD5 or CD3 expressed by T cells failed to induce apoptosis. Our data indicate differences in the function of CD5 molecules on tonsillar B cells, compared with blood T cells and suggest that cross-linking CD5 on B cell activates specific pathways responsible for apoptosis.

Stimulation of CD40 in human bladder carcinoma cells inhibits anti-Fas/APO-1 (CD95)-induced apoptosis

CD40 and the CD95 (Fas/APO-1 antigen) are both members of the tumor necrosis factor receptor family. Whereas CD40 mediates a strong growth stimulatory signal in B cells, engagement of the CD95 receptor leads to growth inhibition and induction of apoptosis. As it has been reported that CD40 activation may rescue B cells from undergoing apoptosis, we were interested to see whether it had a similar effect in other cells expressing the CD40 receptor. We used epithelial tumor cells from the urinary bladder, a cell type that frequently expresses CD40 but for which no clear function of the molecule has been assigned. We found that the ligation of CD95 with the antibody anti-APO-1 induced apoptosis in most of the cell lines tested. Stimulation of CD40 with antibodies or a soluble construct of the CD40 ligand was shown to protect cells from apoptosis, as demonstrated by their ability to suppress the growth inhibition exerted by the anti-APO-1 antibody. Our results show that CD40 stimulation make cells less vulnerable to apoptosis induced via CD95 and suggest that CD40 expression on epithelial tumors may be associated with cell survival.

Distinct Regulation of T-Cell Death by CD28 Depending on Both Its Aggregation and T-Cell Receptor Triggering: A Role for Fas-FasL

CD28 is a major coreceptor that regulates cell proliferation, anergy, and viability of T cells. The negative selection by T-cell receptor (TCR)-induced cell death of immature thymocytes as well as of activated human antigen-specific T-cell clone, requires a costimulatory signal that can be provided by CD28. Conversely, CD28-mediated signals increase expression of Bcl-XL, a survival gene, and promote survival of naive T cells cultured in the absence of antigen or costimulation. Because CD28 appears to both protect from, or induce T-cell death, one important question is to define the activation and cellular parameters that dictate the differential role of CD28 in T-cell apoptosis. Here, we compared different CD28 ligands for their ability to regulate TCR-induced cell death of a murine T-cell hybridoma. In these cells, TCR triggering induced expression of Fas and FasL, and cell death was prevented by anti-Fas blocking monoclonal antibody (MoAb). When provided as a costimulus, both CD28 MoAb and the B7.1 and B7.2 counter receptors downregulated, yet did not completely abolish T-cell receptor–induced apoptosis. This CD28 cosignal resulted in both upregulation of Bcl-XL and prevention of FasL expression. In marked contrast, when given as a single signal, CD28 MoAb or B7.1 and B7.2 induced FasL expression and resulted in T-cell death by apoptosis, which was dependent on the level of CD28 ligation. Furthermore, triggering of CD28 upregulated FasL and induced a marked T-cell death of previously activated normal peripheral T cells. Our results identify Fas and FasL as crucial targets of CD28 in T-cell death regulation and show that within the same cell population, depending on its engagement as a single signal or as a costimulus together with the TCR, CD28 can either induce a dose-dependent death signal or protect from cell death, respectively. These data provide important insights into the role of CD28 in T-cell homeostasis and its possible implication in neoplastic disorders.

© 1998 by The American Society of Hematology.

Distinct Regulation of T-Cell Death by CD28 Depending on Both Its Aggregation and T-Cell Receptor Triggering: A Role for Fas-FasL

CD28 is a major coreceptor that regulates cell proliferation, anergy, and viability of T cells. The negative selection by T-cell receptor (TCR)-induced cell death of immature thymocytes as well as of activated human antigen-specific T-cell clone, requires a costimulatory signal that can be provided by CD28. Conversely, CD28-mediated signals increase expression of Bcl-XL, a survival gene, and promote survival of naive T cells cultured in the absence of antigen or costimulation. Because CD28 appears to both protect from, or induce T-cell death, one important question is to define the activation and cellular parameters that dictate the differential role of CD28 in T-cell apoptosis. Here, we compared different CD28 ligands for their ability to regulate TCR-induced cell death of a murine T-cell hybridoma. In these cells, TCR triggering induced expression of Fas and FasL, and cell death was prevented by anti-Fas blocking monoclonal antibody (MoAb). When provided as a costimulus, both CD28 MoAb and the B7.1 and B7.2 counter receptors downregulated, yet did not completely abolish T-cell receptor–induced apoptosis. This CD28 cosignal resulted in both upregulation of Bcl-XL and prevention of FasL expression. In marked contrast, when given as a single signal, CD28 MoAb or B7.1 and B7.2 induced FasL expression and resulted in T-cell death by apoptosis, which was dependent on the level of CD28 ligation. Furthermore, triggering of CD28 upregulated FasL and induced a marked T-cell death of previously activated normal peripheral T cells. Our results identify Fas and FasL as crucial targets of CD28 in T-cell death regulation and show that within the same cell population, depending on its engagement as a single signal or as a costimulus together with the TCR, CD28 can either induce a dose-dependent death signal or protect from cell death, respectively. These data provide important insights into the role of CD28 in T-cell homeostasis and its possible implication in neoplastic disorders.

© 1998 by The American Society of Hematology.

Programmed cellular response to ionizing radiation damage

Three forms of radiation response were investigated to evaluate the hypothesis that cellular radiation response is the result of active molecular signaling and not simply a passive physicochemical process. The decision whether or not a cell should respond to radiation-induced damage either by induction of rescue systems, e.g. mobilization of repair proteins, or induction of suicide mechanisms, e.g. programmed cell death, appears to be the expression of intricate cellular biochemistry. A cell must recognize damage in its genetic material and then activate the appropriate responses. Cell type is important; the response of a fibroblast to radiation damage is both quantitatively and qualitatively different from that of a lymphocyte. The programmed component of radiation response is significant in radiation oncology and predicted to create unique opportunities for enhanced treatment success.

Cloning, characterization, and mapping of human homolog of mouse T-cell death-associated gene

To establish immunologic autotolerance, self-reactive immature thymocytes are eliminated by negative selection during T-cell development in the thymus. Self-reactive clones undergo apoptosis after stimulation via the T-cell receptor (TCR). The process of cell selection is determined by the dedication of the TCR for tolerogenic antigen/major histocompatibility complex. We have cloned a novel human gene that is highly homologous in the transmembrane and G protein-coupling domains to mouse T-cell death-associated gene 8 (TDAG8). The gene, human TDAG8 (hTDAG8), which belongs to the G protein-couple receptor superfamily, encodes a protein of 337 amino acids. An expressed sequence tag (EST) corresponding to hTDAG8 was identified from a human thyroid cDNA library and subsequently used to isolate a full-length genomic clone. Northern blot analysis revealed that the hTDAG8 gene is expressed predominantly in lymphoid tissues, including peripheral blood leukocytes, spleen, lymph nodes, and thymus. Stably transfected mammalian CHO cells were generated, and heterologous expression of hTDAG8 was confirmed by Northern blot analysis. Fluorescent in situ hybridization (FISH) revealed that hTDAG8 maps to human chromosome 14q31-32.1, a region in which abnormalities associated with human T-cell lymphoma or leukemia are found. Taken together, these data implicate the hTDAG8 gene in T-cell-associated diseases in humans, but its actual physiological and pathological role in the human immune system needs further investigation.

Apoptosis as a mechanism of peripheral blood mononuclear cell death after measles and varicella-zoster virus infections in children

Viral infections may induce an acquired form of immunodeficiency, generally lasting a few weeks. In the more severe form, such as HIV infection, the immunodeficiency is permanent. Programmed death of T cells represents one of the mechanisms by which HIV determines the T cell functional impairment, finally resulting in the destruction of T cells. In this study, we evaluated whether an altered regulation of apoptosis was also implicated in the anergy associated with the common measles or varicella-zoster virus (VZV) infections in infancy. A spontaneous apoptosis of peripheral blood mononuclear cells was observed in children who had suffered from these infections as long as 6 mo after the acute disease. Apoptosis was demonstrated through analysis of cellular DNA content, morphologic evidence of cell nuclei shrinkage, and by analysis of DNA degradation. Stimulation of T cells through anti-CD4 MAb increased the number of apoptotic cells with a maximal effect 72 h after the stimulation. Our results suggest that apoptosis may account for the anergy that follows acute viral infections in infancy.

T cell receptor (TCR)-induced death of immature CD4+CD8+ thymocytes by two distinct mechanisms differing in their requirement for CD28 costimulation: implications for negative selection in the thymus

Negative selection is the process by which the developing lymphocyte receptor repertoire rids itself of autoreactive specificities. One mechanism of negative selection in developing T cells is the induction of apoptosis in immature CD4+CD8+ (DP) thymocytes, referred to as clonal deletion. Clonal deletion is necessarily T cell receptor (TCR) specific, but TCR signals alone are not lethal to purified DP thymocytes. Here, we identify two distinct mechanisms by which TCR-specific death of DP thymocytes can be induced. One mechanism requires simultaneous TCR and costimulatory signals initiated by CD28. The other mechanism is initiated by TCR signals in the absence of simultaneous costimulatory signals and is mediated by subsequent interaction with antigen-presenting cells. We propose that these mechanisms represent two distinct clonal deletion strategies that are differentially implemented during development depending on whether immature thymocytes encounter antigen in the thymic cortex or thymic medulla.

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.

Differential effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin, bis(tri-n-butyltin) oxide and cyclosporine on thymus histophysiology

Recent advances in the histophysiology of the normal thymus have revealed its complex architecture, showing distinct microenvironments at the light and electron microscopic level. The epithelium comprising the major component of the thymic stroma is not only involved in the positive selection of thymocytes, but also in their negative selection. Dendritic cells, however, are more efficient than epithelial cells in mediating negative selection. Thymocytes are dependent on the epithelium for normal development. Conversely, epithelial cells need the presence of thymocytes to maintain their integrity. The thymus rapidly responds to immunotoxic injury. Both the thymocytes and the nonlymphoid compartment of the organ can be targets of exposure. Disturbance of positive and negative thymocyte selection may have a major impact on the immunological function of the thymus. Suppression of peripheral T-cell-dependent immunity as a consequence of thymus toxicity is primarily seen after perinatal exposure when the thymus is most active. Autoimmunity may be another manifestation of chemically mediated thymus toxicity. Although the regenerative capacity of thymus structure is remarkable, it remains to be clarified whether this also applies to thymus function. In-depth mechanistic studies on chemical-induced dysfunction of the thymus have been conducted with the environmental contaminants 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and bis(tri-n-butyltin)oxide (TBTO) as well as the pharmaceutical immunosuppressant cyclosporine (CsA). Each of these compounds exerts a differential effect on the morphology of the thymus, depending on the cellular targets for toxicity. TCDD and TBTO exposure results in cortical lymphodepletion, albeit by different mechanisms. An important feature of TCDD-mediated thymus toxicity is the disruption of epithelial cells in the cortex. TBTO primarily induces cortical thymocyte cell death. In contrast CsA administration results in major alterations in the medulla, the cortex remaining largely intact. Medullary epithelial cells and dendritic cells are particularly sensitive to CsA. The differential effects of these three immunotoxicants suggest unique susceptibilities of the various cell types and regions that make up the thymus.

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

Anti-CD3 monoclonal antibodies (MoAbs) and glucocorticoid hormones induce apoptosis in immature thymocytes and peripheral T lymphocytes. This process is inhibited by a number of growth factors, including interleukin-2 (IL-2), IL-3, and IL-4, as well as by triggering of the adhesion molecule CD44, which would indicate that signals generated by membrane receptors can modulate the survival of lymphoid cells. To investigate whether triggering of CD2 may also affect apoptosis in lymphoid cells, we analyzed the effect of stimu-lation with anti-CD2 MoAbs on T-cell apoptosis induced by two stimuli, anti-CD3 MoAbs and dexamethasone (DEX), using a hybridoma T-cell line and a T-helper cell clone. The results show that CD2 engagement decreased anti-CD3 MoAb-induced apoptosis, but did not influence DEX-induced cell death. Furthermore, the decrease appeared to be related to the expression of Fas/APO-1 (CD95) and Fas-ligand (Fas-L). In fact, we show that CD2 stimulation inhibits apoptosis by preventing the CD3-induced upregulation of Fas and Fas-L in a Fas-dependent experimental system. These data suggest that a costimulatory molecule may control a deletion pathway and may therefore contribute to the regulation of peripheral tolerance.

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

Anti-CD3 monoclonal antibodies (MoAbs) and glucocorticoid hormones induce apoptosis in immature thymocytes and peripheral T lymphocytes. This process is inhibited by a number of growth factors, including interleukin-2 (IL-2), IL-3, and IL-4, as well as by triggering of the adhesion molecule CD44, which would indicate that signals generated by membrane receptors can modulate the survival of lymphoid cells. To investigate whether triggering of CD2 may also affect apoptosis in lymphoid cells, we analyzed the effect of stimu-lation with anti-CD2 MoAbs on T-cell apoptosis induced by two stimuli, anti-CD3 MoAbs and dexamethasone (DEX), using a hybridoma T-cell line and a T-helper cell clone. The results show that CD2 engagement decreased anti-CD3 MoAb-induced apoptosis, but did not influence DEX-induced cell death. Furthermore, the decrease appeared to be related to the expression of Fas/APO-1 (CD95) and Fas-ligand (Fas-L). In fact, we show that CD2 stimulation inhibits apoptosis by preventing the CD3-induced upregulation of Fas and Fas-L in a Fas-dependent experimental system. These data suggest that a costimulatory molecule may control a deletion pathway and may therefore contribute to the regulation of peripheral tolerance.

Apoptosis down-regulates inflammation under the advancing epithelial wound edge: delayed patterns in diabetes and improvement with topical growth factors

BACKGROUND

Wound healing is involved in many aspects of care, ranging from anastomoses and skin incisions to foot ulcers and decubitus. Clinical healing failures are a major challenge to the physician and cause significant morbidity and mortality in select patient populations. As we are starting to understand more fully the mechanisms of impaired wound healing, the diabetic mouse (C57BL/KsJ-db/db) has been a good model for research. The diabetic wound exhibits significant delays in healing, previously identified as impaired cellular infiltration and granulation tissue formation. Apoptosis, or programmed cell death, is intimately involved in the regulation of inflammation and ultimately should play a role in the inflammatory phase of wound healing.

METHODS

To examine its role in wound healing, patterns of apoptosis in large, full-thickness cutaneous wounds were compared between groups of diabetic and nondiabetic mice.

RESULTS

Initially apoptosis was mainly limited to the wound edge and followed the advancing epithelial edge toward the center of the wound as healing progressed. Significant delays in the appearance of the apoptotic pattern were noted in the diabetic mice. Wounds in diabetic mice were then treated with topical application of growth factors. The delay in apoptotic pattern was reversed after treatment with the combination of insulin-like growth factor-II and platelet-derived growth factor, approaching levels in nondiabetic animals.

CONCLUSIONS

Apoptosis appears concurrently with reepithelialization of the wound and may signal the end of the inflammatory phase of healing at that site in the wound. One can speculate that a signal for apoptosis and down-regulation of inflammation in the wound is derived from the epithelium.