Expression and regulation of CCL15 by human airway smooth muscle cells

BACKGROUND

Structural cells are an important reservoir of chemokines that coordinate the influx of various immune cells to the lungs of asthmatics. Airway smooth muscle cells (ASMC) are an important source of these chemokines. CCL15 is a recently described chemo-attractant for neutrophils, eosinophils, monocytes and lymphocytes.

OBJECTIVE

To determine the production and the regulation of CCL15 by ASMC and to investigate its production in asthmatic airways.

METHODS

Human ASMC were obtained from main bronchial airway segments of patients with mild, moderate and severe asthma. To induce chemokine production, cells were incubated with IL-4, IL-13, TNF-α or IFN-γ in presence or absence of dexamethasone, mithramycin A (SP-1 inhibitor) or the IKK-2 inhibitor, AS602868. CCL15 mRNA expression was evaluated by real-time PCR. Immunoreactive CCL15 was detected by immuno-fluorescence and CCL15 protein concentration in the supernatant was measured using ELISA.

RESULTS

CCL15 is constitutively expressed in human ASMC and is strongly up-regulated by TNF-α. This up-regulation is inhibited by dexamethasone, mithramycin A and AS602868. TNF-α-induced CCL15 levels can be synergistically enhanced by the presence of IFN-γ, at both the transcriptional and translation level. This synergism is NF-κB-dependent. Asthmatic biopsies demonstrated higher expression of CCL15 compared with non-asthmatic controls.

CONCLUSION AND CLINICAL RELEVANCE

Our results show that ASMC are a potent source of CCL15 in the airways and may directly participate in the recruitment of inflammatory cells to asthmatic airways. Targeting the production of CCL15 by ASMC might reduce the inflammatory response within the airways of asthmatic patients.

Signal transducer and activator of transcription 6 down-regulates toll-like receptor-4 expression of a monocytic cell line

BACKGROUND

Toll-like receptor 4 (TLR4), part of the bacterial lipopolysaccharide (LPS) receptor, is an important bridge between innate and adaptive immunity. Our previous studies have indicated reduced expression of TLR4 and reduced responsiveness to LPS in nasal mucosa of atopic adults compared with non-atopic adults. IL-4 and signal transducer and activator of transcription 6 (STAT6), which are increased in atopic patients, may have a role in modulating TLR4.

OBJECTIVE

To examine direct effects of IL-4 and STAT6 on TLR4 expression of U-937 monocytic cells.

METHODS

LPS responsiveness, under different conditions of U-937 cells was measured by nuclear factor (NF)-kappaB activation of transcription. TLR4 mRNA was quantified by real-time PCR and TLR4 surface expression was measured by flow cytometry. The promoter and 4.3 kb of the upstream region of TLR4 were cloned into a plasmid vector and transiently transfected into U-937 cells. Transfected cells were incubated with IL-4 and transcriptional activity was assayed by the luciferase assay. STAT6 was transfected to evaluate overexpression of this transcription factor. Cells were also incubated with Tyrphostin AG490 to inhibit tyrosine kinases.

RESULTS

NF-kappaB activation by LPS was inhibited by IL-4 pre-incubation but not when IL-4 was added at the same time as LPS. TLR4 mRNA expression was inhibited by IL-4 as early as 6 h but the effect was lost by 24 h. Surface expression of TLR4 was inhibited by IL-4 at 12 and 24 h, but returned to baseline at 48 h. IL-4 inhibited activity of the TLR4 promoter as early as 6 h, but, like the mRNA, these effects were transient. STAT6 overexpression enhanced the inhibition of the TLR4 promoter and prolonged it. Inhibition of TLR4 by IL-4 was abolished by pre-incubation with the tyrosine kinase inhibitor Tyrphostin AG490.

CONCLUSION

Our findings demonstrate that IL-4, through STAT6, can modulate TLR4 expression and suggests that Th2 cytokines can impact on the LPS responsiveness of cells.

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.

[Eosinophil apoptosis in asthma]

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.