Glucocorticoid-induced apoptosis in human eosinophils: mechanisms of action.

Prominent blood and tissue eosinophilia is clinically manifested in a number of inflammatory states, particularly in allergic diseases. Corticosteroids are the most effective anti-inflammatory drugs used in the treatment of eosinophilic disorders, including bronchial asthma. Their beneficial effects result, among others, from (i) the suppression of the synthesis and the effects of eosinophil survival factors, (ii) the direct induction of eosinophil apoptosis and (iii) the stimulation of their engulfment by professional phagocytic cells. Failure of steroids to propagate apoptotic signals and to promote eosinophil clearance may explain the corticoresistance observed in a proportion of asthmatic patients. Accordingly, studies on the intracellular mechanisms involved in eosinophil corticosensitivity and resistance may provide a valuable tool for identifying new and selective molecular targets to therapeutically resolve otherwise persistent eosinophilic inflammation. In this review, the intracellular cascade of events involved in corticosteroid-mediated eosinophil apoptotic death is discussed and compared to the signalling pathway governing this process in the established model of dexamethasone-induced thymocyte apoptosis.

Involvement of caspases and of mitochondria in Fas ligation-induced eosinophil apoptosis: modulation by interleukin-5 and interferon-gamma.

In this study, we examined the relative importance of caspases and mitochondria in Fas-mediated eosinophil apoptosis. Stimulation of human peripheral blood eosinophils with an agonistic anti-human Fas monoclonal antibody, but not with control IgM, induced a time-dependent increase in their apoptosis, which was associated with a loss in mitochondrial transmembrane potential (DeltaPsi(m)) and with caspase-8 and caspase-3 activation. Interleukin (IL)-5 and interferon (IFN)-gamma, two cytokines known to prolong eosinophil survival, inhibited Fas-mediated apoptosis and caspase activation but poorly affected the decrease in DeltaPsi(m). Eosinophil incubation with bongkrekic acid, an inhibitor of the mitochondrial permeability transition pore (MPTP) opening, failed to modify Fas-mediated loss in DeltaPsi(m), caspase activation, and apoptosis. In contrast, caspase inhibitors markedly reduced eosinophil apoptosis without significantly affecting DeltaPsi(m) dissipation. We conclude that caspase-8 and caspase-3 activation, but not MPTP opening, mediate Fas-induced eosinophil apoptosis and are the main targets for the protective effect of IL-5 and IFN-gamma.

Regulation of peroxisome proliferator-activated receptor gamma expression in human asthmatic airways: relationship with proliferation, apoptosis, and airway remodeling.

Airway inflammation and alterations in cellular turnover are histopathologic features of asthma. We show that the expression of peroxisome proliferator-activated receptor gamma (PPAR gamma), a nuclear hormone receptor involved in cell activation, differentiation, proliferation, and/or apoptosis, is augmented in the bronchial submucosa, the airway epithelium, and the smooth muscle of steroid-untreated asthmatics, as compared with control subjects. This is associated with enhanced proliferation and apoptosis of airway epithelial and submucosal cells, as assessed by the immunodetection of the nuclear antigen Ki67, and of the cleaved form of caspase-3, respectively, and with signs of airway remodeling, including thickness of the subepithelial membrane (SBM) and collagen deposition. PPAR gamma expression in the epithelium correlates positively with SBM thickening and collagen deposition, whereas PPAR gamma expressing cells in the submucosa relate both to SBM thickening and to the number of proliferating cells. The intensity of PPAR gamma expression in the bronchial submucosa, the airway epithelium, and the smooth muscle is negatively related to FEV(1) values. Inhaled steroids alone, or associated with oral steroids, downregulate PPAR gamma expression in all the compartments, cell proliferation, SBM thickness, and collagen deposition, whereas they increase apoptotic cell numbers in the epithelium and the submucosa. Our findings have demonstrated that PPAR gamma (1) is a new indicator of airway inflammation and remodeling in asthma; (2) may be involved in extracellular matrix remodeling and submucosal cell proliferation; (3) is a target for steroid therapy.

Regulation of IL-1beta generation by Pseudo-ICE and ICEBERG, two dominant negative caspase recruitment domain proteins.

We report here the identification and functional characterization of two new human caspase recruitment domain (CARD) molecules, termed Pseudo-interleukin-1beta converting enzyme (ICE) and ICEBERG. Both proteins share a high degree of homology, reaching 92% and 53% identity, respectively, to the prodomain of caspase-1/ICE. Interestingly, both Pseudo-ICE and ICEBERG are mapped to chromosome 11q22 that bears caspases-1, -4- and -5 genes, all involved in cytokine production rather than in apoptosis. We demonstrate that Pseudo-ICE and ICEBERG interact physically with caspase-1 and block, in a monocytic cell line, the interferon-gamma and lipopolysaccharide-induced secretion of interleukin-1beta which is a well-known consequence of caspase-1 activation. Moreover, Pseudo-ICE, but not ICEBERG, interacts with the CARD-containing kinase RICK/RIP2/CARDIAK and activates NF-kappaB. Our data suggest that Pseudo-ICE and ICEBERG are intracellular regulators of caspase-1 activation and could play a role in the regulation of IL-1beta secretion and NF-kappaB activation during the pro-inflammatory cytokine response.

[Eosinophil apoptosis in asthma]. / L'apoptose des éosinophiles dans l'asthme.

Eosinophils play a major role in the onset and maintenance of bronchial inflammation and tissue injury in asthma. Like other leukocytes, eosinophils present in excessive numbers in inflamed tissues are removed by apoptosis. This phenomenon, also called 'programmed cell death', allows elimination of dangerous or redundant cells, thereby ensuring maintenance of tissue homeostasis. It has been suggested that a defect in eosinophil apoptosis would participate in the development and persistence of allergic airways inflammation in asthma. Eosinophil apoptosis, as well as the expression and function of various molecules determining this process, are closely regulated by various stimuli, including cytokines, lipid mediators and growth factors released by various cell types and by the eosinophil itself, as well as exogenous molecules, such as glucocorticoids. These stimuli have been shown to alter the expression and function of different molecules involved in the cascade of events characterising the apoptotic process, particularly Bcl-2 family proteins and the pro-apoptotic membrane glycoprotein, Fas. These observations, together with a better understanding of the mechanisms underlying eosinophil apoptosis, will help to more clearly define the molecular events involved in accumulation of these cells in blood and tissues and to identify potential new targets for the treatment of allergic diseases.

Apoptosis, proliferation, and expression of Bcl-2, Fas, and Fas ligand in bronchial biopsies from asthmatics.

The in situ apoptosis and the expression of molecules involved in this process, such as Bcl-2, Fas, and its ligand, Fas ligand (FasL), were examined in bronchial biopsies from healthy control subjects and from steroid-untreated or -treated asthmatics, using terminal transferase-mediated deoxyuridyltriphosphate nick-end labeling and immunohistochemical techniques, respectively. Bronchial submucosa from steroid- untreated asthmatics showed an increase in the number of eosinophils and a decrease in that of apoptotic cells compared with that of control subjects, but no significant changes in the number of T lymphocytes or in that of cells expressing Bcl-2, Fas, or FasL. Treatment with steroids reduced airway eosinophilia and augmented the proportion of apoptotic eosinophils. Compared with control subjects or untreated patients, steroid-treated asthmatics exhibited increased expression of Bcl-2, Fas, FasL, and of proliferating cell nuclear antigen (PCNA) in their bronchial epithelium, without changes in the number of apoptotic cells. Moreover, the intensity of the expression of Bcl-2, Fas, and FasL correlates well with that of PCNA. We conclude that steroids may reduce the inflammatory cell infiltrate in the bronchial submucosa in part by promoting eosinophil apoptosis and by inducing the expression of FasL on bronchial epithelial cells. Treatment with steroids may also augment survival and proliferation of epithelial cells, possibly via the expression of Bcl-2 and PCNA.

Role of adenylate cyclase-hemolysin in alveolar macrophage apoptosis during Bordetella pertussis infection in vivo.

Bordetella pertussis induces in vitro apoptosis of murine alveolar macrophages by a mechanism that is dependent on expression of bacterial adenylate cyclase-hemolysin. Using a murine respiratory model, we found in this study that intranasal infection with a parental B. pertussis strain, but not with an isogenic variant deficient in the expression of all toxins and adhesins, induced a marked neutrophil accumulation in the bronchoalveolar lavage fluid and an early decrease in macrophage numbers. These phenomena paralleled a time-dependent rise in the proportion of apoptotic nuclei, as detected by flow cytometry, and of macrophages which had engulfed apoptotic bodies. Apoptotic death of bronchopulmonary cells was observed exclusively following intranasal infection with bacteria reisolated from lungs of infected animals and not with B. pertussis collected after in vitro subculture. Using the terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling technique coupled to fluorescence microscopy and morphological analysis, we established that the apoptotic cells in bronchoalveolar lavage fluids were neutrophils and macrophages. Histological analysis of the lung tissues from B. pertussis-infected mice showed increased numbers of apoptotic cells in the alveolar compartments. Cellular accumulation in bronchoalveolar lavage fluids and apoptosis of alveolar macrophages were significantly attenuated in mice infected with a mutant deficient in the expression of adenylate cyclase-hemolysin, indicating a role of this enzyme in these processes.

Human eosinophils express bcl-2 family proteins: modulation of Mcl-1 expression by IFN-gamma.

The expression of the Bcl-2 family proteins Bax, Mcl-1, Bcl-2, and Bcl-xL, was examined in human peripheral blood eosinophils or in umbilical-cord-blood-derived eosinophils. Immunoblot analysis disclosed high amounts of the proapoptotic factor Bax in freshly purified eosinophils of both types. Although cord-blood-derived eosinophils expressed easily detectable levels of Mcl-1, Bcl-2, and Bcl-xL, only traces or no expression of these three antiapoptotic proteins were found in peripheral blood eosinophils. Incubation of both eosinophil types for 1 to 3 days in a cytokine-deprived medium led to apoptosis, without changes in the expression of Bax, Mcl-1, Bcl-2, or Bcl-xL. Although addition of interleukin-5 or interferon-gamma (IFN-gamma) to the culture medium increased the survival of both eosinophil types, a rise in the levels of Mcl-1 was observed only in IFN-gamma-treated cord-blood eosinophils. Together, these results indicate that human eosinophils have a specific profile of Bcl-2-family protein expression that depends on their maturation status and may be modulated by stimuli that influence their survival.

[Eosinophilia and interleukin-5: role in pulmonary atopic processes]. / L'eosinophile et l'interleukine-5: role dans les processus atopiques pulmonaires.

Infiltration of some tissues including the lung by activated eosinophil leukocytes is a characteristic feature of allergic disorders. In asthma patients, eosinophils have been shown to promote and prolong inflammation of bronchial mucosa. This effect is due to the release by eosinophils of numerous inflammation-stimulating mediators and cytokines as well as cationic proteins known to damage lung epithelium. Differentiation of eosinophils from their hematopoietic precursors as well as subsequent processes including maturation, chemotaxis, and activation are controlled mainly by interleukin-5 (IL-5). This cytokine is produced not only by a subpopulation of T-lymphocyte helpers, named Th2, but also by mastocytes and eosinophils themselves. A massive increase in IL-5 concentrations as well as in mRNA for IL-5 synthesis has been detected in serum, broncho-alveolar lavage fluid, and bronchial mucosa of asthmatic patients. In animal models mimicking clinical manifestations of lung allergy, neutralization of IL-5 using specific antibodies inhibits eosinophilic infiltration into the airways and improves respiratory function. These findings suggest that release of IL-5 during atopic disorders is a cardinal step in the development of inflammation and lesions in asthmatic patients. Treatment with drugs that selectively inhibit the synthesis and/or effects of IL-5 could represent a new therapeutic strategy not only for allergic disorders but also for other diseases associated with blood and tissue eosinophilia.

Fas-mediated apoptosis in cultured human eosinophils.

Previous studies have shown that cytokine-dependent eosinophils undergo apoptosis, yet the mechanisms governing this phenomenon remain obscure. Fas antigen is a transmembrane glycoprotein belonging to the tumor necrosis factor receptor family. Cross-linking of Fas antigen in numerous cell types leads to apoptosis. In the present study, we examined the potential role of Fas antigen in the apoptosis of purified blood eosinophils from healthy donors. Cytokine-deprived eosinophils exhibited a time-dependent loss in viability, accompanied by an increase in the number of apoptotic nuclei and in the expression of Fas antigen and its mRNA, as shown by flow cytometry and reverse transcriptase-polymerase chain reaction, respectively. Cross-linking of Fas antigen with an agonistic anti-Fas monoclonal antibody (MoAb) induced a dose- and time-dependent increase in the number of apoptotic nuclei. Furthermore, using an in vitro coculture system, we showed engulfment of anti-Fas MoAb-treated eosinophils by monocyte-derived macrophages. Finally, incubation of eosinophils with the corticosteroid, dexamethasone, induced apoptosis and augmented that triggered by anti-Fas MoAb. Together, these observations suggest that Fas antigen expression and activation is involved in the apoptosis of human eosinophils and may contribute to the resolution of inflammatory allergic reactions in which eosinophil accumulation is a prominent feature.