Pan-neurotrophin receptor p75 contributes to neuronal hyperreactivity and airway inflammation in a murine model of experimental asthma

ProBDNF and its receptors in immune-mediated inflammatory diseases: novel insights into the regulation of metabolism and mitochondria

Immune-mediated inflammatory diseases (IMIDs) consist of a common and clinically diverse group of diseases. Despite remarkable progress in the past two decades, no remission is observed in a large number of patients, and no effective treatments have been developed to prevent organ and tissue damage. Brain-derived neurotrophic factor precursor (proBDNF) and receptors, such as p75 neurotrophin receptor (p75^NTR) and sortilin, have been proposed to mediate intracellular metabolism and mitochondrial function to regulate the progression of several IMIDs. Here, the regulatory role of proBDNF and its receptors in seven typical IMIDs, including multiple sclerosis, rheumatoid arthritis, systemic lupus erythematosus, allergic asthma, type I diabetes, vasculitis, and inflammatory bowel diseases, was investigated.

In vivo functions of p75NTR: challenges and opportunities for an emerging therapeutic target

The p75 neurotrophin receptor (p75^NTR) functions at the molecular nexus of cell death, survival, and differentiation. In addition to its contribution to neurodegenerative diseases and nervous system injuries, recent studies have revealed unanticipated roles of p75^NTR in liver repair, fibrinolysis, lung fibrosis, muscle regeneration, and metabolism. Linking these various p75^NTR functions more precisely to specific mechanisms marks p75^NTR as an emerging candidate for therapeutic intervention in a wide range of disorders. Indeed, small molecule inhibitors of p75^NTR binding to neurotrophins have shown efficacy in models of Alzheimer's disease (AD) and neurodegeneration. Here, we outline recent advances in understanding p75^NTR pleiotropic functions in vivo, and propose an integrated view of p75^NTR and its challenges and opportunities as a pharmacological target.

Neurotrophins in Asthma

PURPOSE OF REVIEW

Asthma is a chronic airway disease that affects more than 300 million people worldwide. Current treatment focuses on symptomatic relief by temporally dampening inflammation and relaxing the airway. Novel combative strategies against asthma and hopefully a cure are yet to be developed. The goal of this review is to summarize recent literature on neurotrophins (NTs) in experimental models and clinical settings of asthma research.

RECENT FINDINGS

We highlight studies of early phases of asthma that collectively reveal a profound impact of elevated NT levels following initial detrimental insults on long-term airway dysfunction. We hope this review will foster insights into the complex interaction between NTs, nerves, immune cells, and airway structural cells during a critical time window of development and disease susceptibility. Future studies are required to better understand the role of NTs in asthma pathophysiology and to evaluate whether NTs and their receptors may serve as new drug targets.

Neurotrophin Receptor p75NTR Regulates Immune Function of Plasmacytoid Dendritic Cells

Plasmacytoid dendritic cells (pDCs) regulate innate and adaptive immunity. Neurotrophins and their receptors control the function of neuronal tissue. In addition, they have been demonstrated to be part of the immune response but little is known about the effector immune cells involved. We report, for the first time, the expression and immune-regulatory function of the low affinity neurotrophin receptor p75 neurotrophin receptor (p75NTR) by the antigen-presenting pDCs, mediated by toll-like receptor (TLR) 9 activation and differential phosphorylation of interferon regulatory factor 3 and 7. The modulation of p75NTR on pDCs significantly influences disease progression of asthma in an ovalbumin-induced mouse model mediated by the TLR9 signaling pathway. p75NTR activation of pDCs from patients with asthma increased allergen-specific T cell proliferation and cytokine secretion in nerve growth factor concentration-dependent manner. Further, p75NTR activation of pDCs delayed the onset of autoimmune diabetes in RIP-CD80GP mice and aggravated graft-versus-host disease in a xenotransplantation model. Thus, p75NTR signaling on pDCs constitutes a new and critical mechanism connecting neurotrophin signaling and immune response regulation with great therapeutic potential for a variety of immune disorders.

The nervous system of airways and its remodeling in inflammatory lung diseases

Inflammatory lung diseases are associated with bronchospasm, cough, dyspnea and airway hyperreactivity. The majority of these symptoms cannot be primarily explained by immune cell infiltration. Evidence has been provided that vagal efferent and afferent neurons play a pivotal role in this regard. Their functions can be altered by inflammatory mediators that induce long-lasting changes in vagal nerve activity and gene expression in both peripheral and central neurons, providing new targets for treatment of pulmonary inflammatory diseases.

An NT4/TrkB-dependent increase in innervation links early-life allergen exposure to persistent airway hyperreactivity

Children who are exposed to environmental respiratory insults often develop asthma that persists into adulthood. In this study, we used a neonatal mouse model of ovalbumin (OVA)-induced allergic airway inflammation to understand the long-term effects of early childhood insults on airway structure and function. We showed that OVA sensitization and challenge in early life led to a 2-fold increase in airway smooth muscle (ASM) innervation (P<0.05) and persistent airway hyperreactivity (AHR). In contrast, OVA exposure in adult life elicited short-term AHR without affecting innervation levels. We found that postnatal ASM innervation required neurotrophin (NT)-4 signaling through the TrkB receptor and that early-life OVA exposure significantly elevated NT4 levels and TrkB signaling by 5- and 2-fold, respectively, to increase innervation. Notably, blockade of NT4/TrkB signaling in OVA-exposed pups prevented both acute and persistent AHR without affecting baseline airway function or inflammation. Furthermore, biophysical assays using lung slices and isolated cells demonstrated that NT4 was necessary for hyperreactivity of ASM induced by early-life OVA exposure. Together, our findings show that the NT4/TrkB-dependent increase in innervation plays a critical role in the alteration of the ASM phenotype during postnatal growth, thereby linking early-life allergen exposure to persistent airway dysfunction.

Soluble toll-like receptor 4 reversed attenuating effect of Chinese herbal Xiao-Qing-Long-Tang on allergen induced nerve growth factor and thymic stromal lymphopoietin

Xiao-Qing-Long-Tang (XQLT) is known to regulate allergic immune reactions. The aim of this study was to investigate the effects of XQLT on allergen-induced cytokines and associated signaling pathways. An acute allergic mouse model was used to investigate the effects of XQLT on nerve growth factor (NGF) during an allergic reaction, while human pulmonary alveolar epithelial cells (HPAEpiCs) were used to investigate the effects of XQLT on Dermatophagoides pteronyssinus group 2 (Der p 2)-induced NGF, p75 neurotrophin receptor (p75NTR) and thymic stromal lymphopoietin (TSLP) expression. XQLT was demonstrated to inhibit NGF- and p75NTR-related allergic reactions in the mouse model. XQLT also reduced the expression of Toll-like receptor 4 (TLR4) in the lungs of the model mice. XQLT inhibited Der p 2-induced NGF, TSLP and p75NTR expression in the HPAEpiC cell line. The use of recombinant soluble TLR4 (sTLR4) in a competitive assay partially attenuated the inhibitory effect of XQLT on NGF, TSLP and p75NTR expression in HPAEpiC cells. The inhibitory effect of XQLT on the Ser536 phosphorylation of p65 (nuclear factor-κB; NF-κB), a TLR4-induced factor, was also attenuated by sTLR4. In conclusion, XQLT inhibited Der p allergen-induced NGF, p75NTR and TSLP expression. This inhibition was attenuated by sTLR4. The mechanism of action of XQLT may be correlated with TLR4, a primary receptor in the innate immune system. The findings of this study may focus the search for pharmacological targets of XQLT onto TLR4, which is important in the allergen presentation pathway.

Xiao-Qing-Long-Tang shows preventive effect of asthma in an allergic asthma mouse model through neurotrophin regulation

BACKGROUND

This study investigates the effect of Xiao-Qing-Long-Tang (XQLT) on neurotrophin in an established mouse model of Dermatophagoides pteronyssinus (Der p)-induced acute allergic asthma and in a LA4 cell line model of lung adenoma. The effects of XQLT on the regulation of nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF), airway hyper-responsiveness (AHR) and immunoglobulin E were measured.

METHODS

LA4 cells were stimulated with 100 μg/ml Der p 24 h and the supernatant was collected for ELISA analysis. Der p-stimulated LA4 cells with either XQLT pre-treatment or XQLT co-treatment were used to evaluate the XQLT effect on neurotrophin.Balb/c mice were sensitized on days 0 and 7 with a base-tail injection of 50 μg Dermatophagoides pteronyssinus (Der p) that was emulsified in 50 μl incomplete Freund's adjuvant (IFA). On day 14, mice received an intra-tracheal challenge of 50 μl Der p (2 mg/ml). XQLT (1g/Kg) was administered orally to mice either on days 2, 4, 6, 8, 10 and 12 as a preventive strategy or on day 15 as a therapeutic strategy.

RESULTS

XQLT inhibited expression of those NGF, BDNF and thymus-and activation-regulated cytokine (TARC) in LA4 cells that were subjected to a Der p allergen. Both preventive and therapeutic treatments with XQLT in mice reduced AHR. Preventive treatment with XQLT markedly decreased NGF in broncho-alveolar lavage fluids (BALF) and BDNF in serum, whereas therapeutic treatment reduced only serum BDNF level. The reduced NGF levels corresponded to a decrease in AHR by XQLT treatment. Reduced BALF NGF and TARC and serum BDNF levels may have been responsible for decreased eosinophil infiltration into lung tissue. Immunohistochemistry showed that p75NTR and TrkA levels were reduced in the lungs of mice under both XQLT treatment protocols, and this reduction may have been correlated with the prevention of the asthmatic reaction by XQLT.

CONCLUSION

XQLT alleviated allergic inflammation including AHR, IgE elevation and eosinophil infiltration in Der p stimulated mice by regulating neurotrophin and reducing TARC. These results revealed the potential pharmacological targets on which the XQLT decotion exerts preventive and therapeutic effects in an allergic asthma mouse model.

Nerve growth factor exacerbates allergic lung inflammation and airway remodeling in a rat model of chronic asthma

Nerve growth factor (NGF) is critical in the pathogenesis of allergic airway inflammation in vivo and induces proliferation of airway smooth muscle cells and matrix metalloproteinase-9 (MMP-9) expression in vitro. However, the effects of NGF on chronic pulmonary diseases of allergic origin remain unknown. To investigate the effects of NGF on lung inflammation and airway remodeling, 32 Wistar rats were randomly divided into four groups: control, NGF, ovalbumin (OVA) and anti-rat-β-NGF antibody (anti-NGF). Aerosolized OVA was administered to the rats in the NGF, OVA and anti-NGF groups to generate the asthmatic rat model, and NGF or anti-NGF was administered 3 h prior to OVA inhalation every two days. On day 70, bronchial responsiveness tests, bronchoalveolar lavage (BAL) and cell counting were conducted. The levels of serum OVA-specific immunoglobulin E (IgE) and of T-helper cell type-2 (Th2) cytokines [interleukin (IL)-4 and IL-13] in the BAL fluid were measured by enzyme-linked immunosorbent assay. The expression levels of NGF protein and MMP-9 mRNA, and the activity of MMP-9 in the lungs were detected by western blot analysis, quantitative polymerase chain reaction and gelatin zymography analysis, respectively. Our results showed that NGF significantly increased eosinophilic airway inflammation, persistent airway hyperresponsiveness (AHR), the serum levels of OVA-specific IgE and the levels of Th2 cytokines in the BAL fluid, and also increased the expression levels and activity of MMP-9. However, anti-NGF treatment significantly inhibited eosinophilic airway inflammation, persistent AHR and airway remodeling. The results showed that NGF may have exacerbated the development of airway inflammation, AHR and airway remodeling through a Th2 pathway and by increasing the level of MMP-9 expression. Therefore, anti-NGF is potentially beneficial for preventing and treating patients with asthma.

Expression of nerve growth factor and matrix metallopeptidase-9/tissue inhibitor of metalloproteinase-1 in asthmatic patients

OBJECTIVE

The aim of this study was to measure the level of nerve growth factor (NGF) in bronchial specimens from humans and to determine whether it correlated with not only clinical characteristics of asthma such as percent eosinophils, Th2 cytokine levels, and pulmonary function, but also metallopeptidase-9 (MMP-9) and tissue inhibitor of metalloproteinases-1 (TIMP-1).

METHODS

Fifty-three people participated; 42 had asthma. The participants underwent bronchoscopy and the specimens were analyzed. The participants' clinical data including pulmonary function tests were reviewed.

RESULTS

Bronchoalveolar lavage fluid (BALF) from patients with asthma had a significantly higher level of NGF compared with that from participants without asthma. NGF level showed a positive correlation with the percentage of eosinophils in both BALF and serum. The concentration of NGF did not correlate with that of Th2 cytokines interleukin (IL)-4, IL-5, and IL-13 in BALF or parameters of pulmonary function including degree of airway hyperresponsiveness (ARH). The levels of MMP-9 and TIMP-1 in BALF were higher in asthma patients than in participants without asthma. The levels of NGF correlated with TIMP-1 levels but not with MMP-9 in the whole participants.

CONCLUSIONS

This study shows that NGF correlates with levels of eosinophils, a major effector cell in asthma. The high expression of NGF and TIMP-1 in asthma patients and the moderate correlation between NGF and TIMP-1 in the entire group of asthma subjects suggest a possible association between NGF and TIMP-1, which may influence asthma pathogenesis.

Epigenetics and miRNA emerge as key regulators of smooth muscle cell phenotype and function

Regulation of phenotypic plasticity in smooth muscle requires an understanding of the mechanisms regulating phenotype-specific genes and the processes dysregulated during pathogenesis. Decades of study in airway smooth muscle has provided extensive knowledge of the gene expression patterns and signaling pathways necessary to maintain and alter smooth muscle cell phenotype. With this solid foundation, the importance and complexity of inheritable epigenetic modifications and mechanisms silencing gene expression have now emerged as fundamental components regulating aspects of inflammation, proliferation and remodeling.

Enhanced nerve growth factor expression by mast cells does not differ significantly between idiopathic and allergic rhinitis

BACKGROUND

The role of neurotrophins in allergic rhinitis (AR) has been well studied, but it has not been evaluated in idiopathic rhinitis (IR).

OBJECTIVE

We aimed to evaluate the nasal β-nerve growth factor (β-NGF) expressions of mast cells in patients with AR and IR.

METHODS

Seventeen patients with house dust mites-induced persistent moderate/severe allergic rhinitis (mean age: 29.7 ± 11.96), 14 patients with idiopathic rhinitis (mean age, 29.3 ± 10.62), and 16 healthy controls (29.9 ± 11.57) were included in the study. Nasal biopsy specimens were taken from the posterior part of the inferior turbinate from all of the study subjects. Nasal β-nerve growth factor and its receptors, pan-neurotrophin receptor p75, and tyrosine kinase A (trkA) were assessed with an immunofluorescence assay. Mast cells were determined by both an immunofluorescence assay and immunohistochemistry as tryptase-positive cells.

RESULTS

The β-NGF, trkA, and p75 receptor counts were significantly higher in AR and IR patients than in the control group (P < .001, for each), but they were not different between AR and IR patients. Similarly, the ratio of β-NGF+ mast cells/total mast cells and the ratio of β-NGF+ mast cells/total β-NGF+ cells in AR and IR patients was found to be elevated when compared with the control group (P < .001, P < .001, P < .001, and P = .046, respectively); furthermore, the 2 ratios were not statistically different between the 2 patient groups.

CONCLUSION

The increase in β-NGF-expressing mast cells does not differ between idiopathic and allergic rhinitis. Therefore, we propose that mast cells do play a role in the pathogenesis of IR as important as in that of AR.

A Shh/miR-206/BDNF cascade coordinates innervation and formation of airway smooth muscle

Dysfunctional neural control of airway smooth muscle (ASM) is involved in inflammatory diseases, such as asthma. However, neurogenesis in the lung is poorly understood. This study uses mouse models to investigate developmental mechanisms of ASM innervation, a process that is highly coordinated with ASM formation during lung branching morphogenesis. We show that brain-derived neurotrophic factor (BDNF) is an essential ASM-derived signal for innervation. Although BDNF mRNA expression is temporally dissociated with ASM formation and innervation, BDNF protein is coordinately produced through post-transcriptional suppression by miR-206. Using a combination of chemical and genetic approaches to modulate sonic hedgehog (Shh) signaling, a pathway essential for lung branching and ASM formation, we show that Shh signaling blocks miR-206 expression, which in turn increases BDNF protein expression. Together, our work uncovers a functional cascade that involves Shh, miR-206 and BDNF to coordinate ASM formation and innervation.

A Shh/miR-206/BDNF cascade coordinates innervation and formation of airway smooth muscle

Dysfunctional neural control of airway smooth muscle (ASM) is involved in inflammatory diseases, such as asthma. However, neurogenesis in the lung is poorly understood. This study uses mouse models to investigate developmental mechanisms of ASM innervation, a process that is highly coordinated with ASM formation during lung branching morphogenesis. We show that brain-derived neurotrophic factor (BDNF) is an essential ASM-derived signal for innervation. Although BDNF mRNA expression is temporally dissociated with ASM formation and innervation, BDNF protein is coordinately produced through post-transcriptional suppression by miR-206. Using a combination of chemical and genetic approaches to modulate sonic hedgehog (Shh) signaling, a pathway essential for lung branching and ASM formation, we show that Shh signaling blocks miR-206 expression, which in turn increases BDNF protein expression. Together, our work uncovers a functional cascade that involves Shh, miR-206 and BDNF to coordinate ASM formation and innervation.

Nerve growth factor derived from bronchial epithelium after chronic mite antigen exposure contributes to airway hyperresponsiveness by inducing hyperinnervation, and is inhibited by in vivo siRNA

BACKGROUND

Bronchial asthma is a chronic allergic airway inflammatory disease. Neurotrophins, including nerve growth factor (NGF), play an important role in the pathogenesis of asthma. However, the effects of NGF derived from epithelium on airway hyperresponsiveness (AHR) after antigen sensitization/exposure remain uncertain.

OBJECTIVE

In this study, we examined the role of NGF on AHR after chronic antigen exposure and the effect of inhibiting NGF by in vivo siRNA on AHR exacerbation.

METHODS

We generated chronic mouse models of bronchial asthma using house-dust mite antigen (Dermatophagoides pteronyssinus; Dp). NGF concentrations in bronchoalveolar lavage fluid (BALF), lung histopathology, hyperresponsiveness, and related neuronal peptides and cytokines in supernatants of lung homogenates were determined.

RESULTS

NGF in BALF was increased in a dose- and time-dependent manner, and was expressed primarily in bronchial epithelium. Nerve fibres and substance P-positive fibres were detected in subepithelium of Dp-sensitized and challenged mice over 4 weeks of mite antigen exposure. AHR was positively correlated with NGF concentration and nerve fibre innervation. AHR, modulation of innervation, and increased substance P were inhibited by in vivo administration of siRNA that targeted NGF, although the inhibition of NGF did not affect allergic inflammation and subepithelial fibrosis.

CONCLUSION AND CLINICAL RELEVANCE

These findings suggest that NGF derived from bronchial and alveolar epithelium plays an important role in AHR after chronic exposure to mite antigen. NGF inhibition could potentially manage bronchial asthma, including AHR.

Nerve growth factor induces type III collagen production in chronic allergic airway inflammation

BACKGROUND

Excessive extracellular matrix deposition occurs as a result of repetitive injury-repair cycles and plays a central role in the pathogenesis of chronic inflammatory diseases, such as allergic asthma. The molecular mechanism leading to aberrant collagen deposition is not fully understood.

OBJECTIVE

We sought to test the hypothesis that increased nerve growth factor (NGF) production contributes to collagen deposition in the airways during chronic allergic airway inflammation.

METHODS

Antibody-blocking experiments were performed in an in vivo model for chronic allergic airway inflammation (allergic asthma), which is accompanied by matrix deposition in the subepithelial compartment of the airways, to study the profibrotic effect of NGF. The signaling pathways were delineated with in vivo and in vitro studies in primary lung fibroblasts.

RESULTS

Functional blocking of NGF in chronically affected mice markedly prevented subepithelial fibrosis. Transgenic overexpression of NGF in murine airways resulted in altered airway wall morphology with increased peribronchial collagen deposition and impaired lung physiology in the absence of inflammation. NGF exerted a direct effect on collagen expression in murine lung fibroblasts, which was mainly mediated through the activation of the receptor tropomyosin-related kinase A. NGF-induced collagen expression was dependent on downstream activation of p38 mitogen-activated protein kinase independent of the TGF-β1/mothers against decapentaplegic homolog (SMAD) pathway.

CONCLUSION

The results of this study demonstrate that NGF exerts profibrotic activities in the airways by inducing type III collagen production in fibroblasts independently of TGF-β1.

A single DH gene segment is sufficient for the establishment of an asthma phenotype in a murine model of allergic airway inflammation

BACKGROUND

We have previously shown that the allergic sensitization to ovalbumin does not represent a superantigen-like immune response. In gene-targeted mice (ΔD-iD) with a single modified Diversity gene segment (D(H)) of the immunoglobulin heavy chain, enriched for charged amino acids, the asthma phenotype in a murine model was markedly alleviated compared to wild-type animals.

OBJECTIVE

We now sought to determine whether the confinement to a single D(H) gene segment alone leads to a reduced allergic phenotype.

METHODS

We examined another gene-targeted mouse strain (ΔD-DFL) with a single D(H) gene segment which encodes for neutral amino acids, thus reflecting the preferential repertoire in wild-type mice. Mice were sensitized intraperitoneally to ovalbumin.

RESULTS

Despite the constraint to a single D(H) gene segment, ΔD-DFL mice mounted high total and allergen-specific IgG(1) and IgE serum levels after sensitization to ovalbumin. The affinity constants of allergen-specific IgG(1) antibodies did not differ between ΔD-DFL and wild type. Following challenge with aerosolized allergen, a marked local T(H)2 cytokine response and an eosinophilic airway inflammation developed. Quantitative histology revealed increased mucus production and intense goblet cell metaplasia which were identical to those in wild type. Moreover, ΔD-DFL mice developed an airway hyperreactivity to methacholine and to the specific allergen, which both did not differ from those in wild-type animals.

CONCLUSION

A single D(H) gene segment is sufficient for the establishment of the asthma phenotype in a murine model of allergic airway inflammation. Thus, the allergic phenotype depends on the amino acid composition and not on the diversity of the classical antigen-binding site.

Kidins220/ARMS contributes to airway inflammation and hyper-responsiveness in OVA-sensitized mice

BALB/c mice were sensitized and challenged with ovalbumin. We hypothesized that Kidins220/ARMS influences airway inflammation and hyper-responsiveness during allergic airway challenge, and assessed it by intranasal administration of anti-NGF antibody or anti-ARMS antibody to mice. Airway resistance was measured using a sealed whole-body plethysmograph. Total cell numbers and the percentage of different inflammatory cells in BALF were counted. Expression of IL-1β, IL-4 and TNF-α were determined by ELISA, and NF-κB activation determined by EMSA. Kidins220/ARMS expression was observed in ovalbumin-sensitized mice by immunofluorescence or western blotting. IL-1β, IL-4, and TNF-α were overexpressed and NF-κB activation increased after allergen challenge compared with controls. After treatment with anti-ARMS or anti-NGF, levels of IL-1β, IL-4 and TNF-α and NF-κB activation were reduced in comparison with those of ovalbumin-sensitized mice. These results suggest that NGF-mediated Kidins220/ARMS signaling participates in the pathogenesis of asthma, and contributes to airway inflammation and hyper-responsiveness in ovalbumin-sensitized mice.

The role of neurotrophins in the pathophysiology of allergic rhinitis

PURPOSE OF REVIEW

Allergic rhinitis is characterized by allergic airway inflammation and a hyperresponsiveness to nonspecific stimuli which is partly neuronally controlled. In this regard, neurotrophins are prime candidates as mediators of neuronal and immunological plasticity and they will be the focus of the current review.

RECENT FINDINGS

Neurotrophins including nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) are expressed in the nasal mucosa. The majority of NGF expression has been found in eosinophil granulocytes, the glandular apparatus and peripheral nerves. As shown recently, nasal allergen provocation upregulates BDNF expression in nasal mucosa and NGF expression on peripheral nerves and nasal lavage in patients with allergic rhinitis. In this regard, increased BDNF expression positively correlates with the maximum increase in total nasal symptom score. The neurotrophin receptors including pan-neurotrophin receptor p75, tyrosine kinase A (trkA) and trkB are expressed in nasal tissue. TrkA is expressed on endothelial, p75 on peripheral nerves and trkB on nasal mucosa mast cells that decreases after allergen provocation. The expression of these neurotrophin receptors is increased on peripheral blood eosinophils in allergic rhinitis compared with nonatopic controls. Further, BDNF and NGF exert immunomodulatory functions on eosinophils of patients with allergic rhinitis. Finally, eosinophils of patients with allergic rhinitis are capable of BDNF and NGF production.

SUMMARY

Neurotrophins represent prime candidates in upper airway pathophysiology in allergic rhinitis. Research on neurotrophins in allergic rhinitis is thus becoming a progressively more exciting field and may reveal new and promising therapeutic options for the future.

Combined effects of chronic nicotine and acute virus exposure on neurotrophin expression in rat lung

Strong epidemiologic evidence indicates that tobacco smoke influences frequency and severity of respiratory infections. Previously, we have shown that infection with respiratory syncytial virus upregulates expression of neurotrophic factors and receptors in the lungs, but the effect of tobacco exposure on neurotrophins is unknown. Therefore, we first sought to determine the expression of neurotrophic pathways in lungs of rats chronically exposed to nicotine, and then we studied the interactions between pollution and infection by inoculating virus after nicotine exposure. Expression of the neurotrophins nerve growth factor (NGF) and brain-derived neurotrophic factor, of their high-affinity tyrosine kinase receptors (trkA and trkB, respectively), and of the low-affinity receptor p75(NTR) was measured in the lungs of nicotine-exposed rats both at the mRNA level by reverse-transcription polymerase chain reaction and at the protein level by enzyme-linked immunoassay. Nicotine increased NGF expression both at the mRNA and protein level and also created a receptor imbalance deriving from increased expression of the pro-inflammatory p75(NTR) receptor without any concomitant change in the high-affinity trkA receptor. Viral infection after chronic nicotine exposure exerted an additive effect on NGF expression, and resulted in exaggerated neurogenic airway inflammation that was abolished by selective inhibition. In conclusion, nicotine levels comparable to those found in smokers are per se able to upregulate the expression of critical neurotrophic molecules in the respiratory tract, and combination of an acute infection following chronic nicotine exposure produces more severe neurotrophic dysregulation and neurogenic-mediated inflammation compared to either infection or nicotine alone.

Airway-specific recruitment of T cells is reduced in a CD26-deficient F344 rat substrain

Asthma is a chronic inflammatory disease affecting the airways. Increased levels of T cells are found in the lungs after the induction of an allergic-like inflammation in rats, and flow cytometry studies have shown that these levels are reduced in CD26-deficient rats. However, the precise anatomical sites where these newly recruited T cells appear primarily are unknown. Therefore, we quantified the distribution of T cells in lung parenchyma as well as in large, medium and small airways using immunohistochemical stainings combined with morphometric analyses. The number of T cells increased after the induction of an allergic-like inflammation. However, the differences between CD26-deficient and wild-type rats were not attributable to different cell numbers in the lung parenchyma, but the medium- and large-sized bronchi revealed significantly fewer T cells in CD26-deficient rats. These sites of T cell recruitment were screened further using immunohistochemistry and quantitative real-time polymerase chain reaction with regard to two hypotheses: (i) involvement of the nervous system or (ii) expression of chemokines with properties of a T cell attractor. No topographical association was found between nerves and T cells, but a differential transcription of chemokines was revealed in bronchi and parenchyma. Thus, the site-specific recruitment of T cells appears to be a process mediated by chemokines rather than nerve-T cell interactions. In conclusion, this is the first report showing a differential site-specific recruitment of T cells to the bronchi in a CD26-deficient rat substrain during an asthma-like inflammation.

Atropine-enhanced, antigen challenge-induced airway hyperreactivity in guinea pigs is mediated by eosinophils and nerve growth factor

Although anticholinergic therapy inhibits bronchoconstriction in asthmatic patients and antigen-challenged animals, administration of atropine 1 h before antigen challenge significantly potentiates airway hyperreactivity and eosinophil activation measured 24 h later. This potentiation in airway hyperreactivity is related to increased eosinophil activation and is mediated at the level of the airway nerves. Since eosinophils produce nerve growth factor (NGF), which is known to play a role in antigen-induced airway hyperreactivity, we tested whether NGF mediates atropine-enhanced, antigen challenge-induced hyperreactivity. Antibody to NGF (Ab NGF) was administered to sensitized guinea pigs with and without atropine pretreatment (1 mg/kg iv) 1 h before challenge. At 24 h after challenge, animals were anesthetized, vagotomized, paralyzed, and ventilated. Electrical stimulation of both vagus nerves caused bronchoconstriction that was increased in challenged animals. Atropine pretreatment potentiated antigen challenge-induced hyperreactivity. Ab NGF did not affect eosinophils or inflammatory cells in any group, nor did it prevent hyperreactivity in challenged animals that were not pretreated with atropine. However, Ab NGF did prevent atropine-enhanced, antigen challenge-induced hyperreactivity and eosinophil activation (assessed by immunohistochemistry). This effect was specific to NGF, since animals given control IgG remained hyperreactive. These data suggest that anticholinergic therapy amplifies eosinophil interactions with airway nerves via NGF. Therefore, therapeutic strategies that target both eosinophil activation and NGF-mediated inflammatory processes in allergic asthma are likely to be beneficial.

Nerve growth factor and neurotrophin-3 mediate survival of pulmonary plasma cells during the allergic airway inflammation

Allergen-specific Abs play a pivotal role in the induction and maintenance of allergic airway inflammation. During secondary immune responses, plasma cell survival and Ab production is mediated by extrinsic factors provided by the local environment (survival niches). It is unknown whether neurotrophins, a characteristic marker of allergic airway inflammation, influence plasma cell survival in the lung. Using a mouse model of allergic asthma, we found that plasma cells from the lung and spleen are distinct subpopulations exhibiting differential expression patterns of neurotrophins and their receptors (Trks). In vitro, the nerve growth factor (NGF) and neurotrophin-3 (NT3) led to a dose-dependent increase in viability of isolated pulmonary plasma cells due to up-regulation of the antiapoptotic Bcl2 pathway. In parallel, the expression of transcription factors that stimulate the production of immunoglobulins (X-box binding protein 1 and NF-kappaB subunit RelA) was enhanced in plasma cells treated with NGF and NT3. These findings were supported in vivo. When the NGF pathway was blocked by intranasal application of a selective TrkA inhibitor, sensitized mice showed reduced numbers of pulmonary plasma cells and developed lower levels of allergen-specific and total serum IgE in response to OVA inhalation. This suggests that in the allergic airway inflammation, NGF/TrkA-mediated pulmonary IgE production contributes significantly to serum-IgE levels. We conclude that the neurotrophins NGF and NT3 act as survival factors for pulmonary plasma cells and thus are important regulators of the local Ab production in the allergic airway disease.

The role of neuro-immune cross-talk in the regulation of inflammation and remodelling in asthma

Despite recent advances in the development of anti-asthmatic medication, asthma continues to be a major health problem worldwide. The symptoms of asthmatic patients include wheezing, chest tightness, cough and shortness of breath, which, together with airway hyperresponiveness, previously have been attributed to a dysfunction of airway nerves. However, research in the last two decades identified Th2-sensitization and the subsequent allergic reaction to innocuous environmental antigens as a basic immunological mechanism leading to chronic airway inflammation. Recent evidence suggests that the development of allergic asthma is influenced by events and circumstances in early childhood and even in utero. Allergen, ozone or stress exposure, as well as RSV infection in early life could be able to induce irreversible changes in the developing epithelial-mesenchymal trophic unit of the airways. The co-existence of chronic inflammation and neural dysfunction have recently drawn attention to the involvement of interaction pathways between the nervous and the immune system in the airways. Intensive basic research has accumulated morphological as well as functional evidence for the interaction between nerves and immune cells. Neuropeptides and neurotrophins have come into focus of attention as the key mediators of neuro-immune interactions, which lead to the development of several pharmacological compounds specifically targeting these molecules. This review will integrate our current knowledge on the involvement of neuro-immune pathways in asthma on the cellular and molecular level. It will summarize the results of pharmacological studies addressing the potential of neuropeptides and neurotrophins as novel therapeutic targets in asthma.

Composition of the immunoglobulin classic antigen-binding site regulates allergic airway inflammation in a murine model of experimental asthma

BACKGROUND

When bound to mast cell FcepsilonRI, IgE serves as antigen receptor for allergic reactions, permitting specific identification of the allergen. Although the core of the classic antigen-binding site is heavy chain complementarity determining region 3 (CDR-H3), recent studies suggest that allergens might also bind IgE in a superantigen-like fashion outside the classic antigen-binding site.

OBJECTIVE

We sought to evaluate the contribution of the classic CDR-H3-centric antigen-binding site to the development of an allergic phenotype.

METHODS

Using a murine model of experimental asthma, we characterized a gene-targeted mouse strain expressing an altered range of CDR-H3s (DeltaD-iD mice) in response to the hydrophobic allergen ovalbumin (OVA). Mutant and wild-type (wt) mice were sensitized intraperitoneally with OVA; non-sensitized mice served as controls.

RESULTS

We found the composition of the classic CDR-H3-centric antigen-binding site to be critical for the development of characteristic aspects of allergic asthma. (i) Compared with wt animals, DeltaD-iD mice showed a significantly less pronounced OVA-induced rise in allergen-specific IgE levels and hence in total serum IgE levels. (ii) In addition, DeltaD-iD mice demonstrated a significant reduction in eosinophilic airway inflammation, as well as in interleukin-4 (IL-4), IL-5 and IL-13 levels in BAL fluids.

CONCLUSION

Allergic sensitization and airway inflammation depend on the composition of the predominant CDR-H3 repertoire, suggesting that the classic CDR-H3-centric antigen-binding site plays a crucial role in creating the immunological interface between allergen and IgE. Our results further emphasize a central role of IgE, not only in mediating but also in regulating the allergic immune response.

Role of p75NTR in female rat urinary bladder with cyclophosphamide-induced cystitis

Previous studies demonstrated changes in urinary bladder neurotrophin content and upregulation of neurotrophin receptors, TrkA and the p75 neurotrophin receptor (p75(NTR)), in micturition reflex pathways after cyclophosphamide (CYP)-induced cystitis. p75(NTR) can bind nerve growth factor (NGF) and modulate NGF-TrkA binding and signaling. We examined p75(NTR) expression and the role of p75(NTR) in the micturition reflex in control and CYP-treated rats. p75(NTR) Immunoreactivity was present throughout the urinary bladder. CYP-induced cystitis (4 h, 48 h, chronic) increased (P < or = 0.05) p75(NTR) expression in whole urinary bladder as shown by Western blotting. The role of p75(NTR) in bladder function in control and CYP-treated rats was determined using conscious cystometry and immunoneutralization or PD90780, a compound known to specifically block NGF binding to p75(NTR). An anti-p75(NTR) monoclonal antibody or PD90780 was infused intravesically and cystometric parameters were evaluated. Both methods of p75(NTR) blockade significantly (P < or = 0.05) decreased the intercontraction interval and void volume in control and CYP-treated rats. Intravesical infusion of PD90780 also significantly (P < or = 0.001) increased intravesical pressure and increased the number of nonvoiding contractions during the filling phase. Control intravesical infusions of isotype-matched IgG and vehicle were without effect. Intravesical instillation of PD90780 significantly (P < or = 0.01) reduced the volume threshold to elicit a micturition contraction in control rats (no inflammation) and CYP-treated in a closed urinary bladder system. These studies demonstrate 1) ubiquitous p75(NTR) expression in urinary bladder and increased expression with CYP-induced cystitis and 2) p75(NTR) blockade at the level of the urinary bladder produces bladder hyperreflexia in control and CYP-treated rats. The overall activity of the urinary bladder reflects the balance of NGF-p75(NTR) and NGF-TrkA signaling.

Modulation of neurotrophin and neurotrophin receptor expression in nasal mucosa after nasal allergen provocation in allergic rhinitis

BACKGROUND

Patients with allergic rhinitis (AR) feature both allergic airway inflammation and a hyperresponsiveness to nonspecific stimuli which is partly neuronally controlled. Still, it is unclear whether or not neurotrophins are involved in airway pathophysiology of AR and in nasobronchial interaction.

METHODS

Nine AR patients with mono-allergy to grass pollen and nine healthy controls underwent nasal allergen provocation (NP). Serum samples, nasal and bronchial biopsies were taken before (T(0)) and 24 h after (T(24)) NP. Pan-neurotrophin receptor p75(NTR), tyrosine kinase A (trkA), trkB, nerve growth factor (NGF), and brain-derived neurotrophic factor (BDNF) were assessed with immunohistochemistry, and NGF and BDNF levels with ELISA.

RESULTS

At T(24), BDNF and NGF were upregulated in nasal mucosa (P < 0.05) and increased in the peripheral blood of AR compared with T(0). The increase in nasal BDNF expression correlated positively with the maximum increase in total nasal symptom score in AR (P = 0.02). p75(NTR) was expressed on peripheral nerves and epithelial layer, trkA on endothelial cells, and trkB on mast cells. trkB + mast cells significantly decreased after NP in AR (P < 0.01). NP did not modulate p75(NTR) and trkA expression in nasal mucosa and had no effect on the expression of neurotrophins and receptors in bronchial mucosa.

CONCLUSION

This study shows that neurotrophins and their receptors are expressed in human airways. Allergic rhinitis was characterized by a modulation of BDNF, NGF, and trkB in nasal mucosa after NP and a correlation of nasal BDNF with the maximal increase of total nasal symptom score. Therefore, our data suggest that neurotrophins participate in upper-airway pathophysiology in AR, whereas their role in nasobronchial interaction remains unclear.

In vitro and clinical immunomodulatory effects of a novel Pentaherbs concoction for atopic dermatitis

BACKGROUND

Our group recently reported a randomized and placebo-controlled clinical trial on the efficacy of a twice-daily concoction of five herbal ingredients (Pentaherbs formulation, PHF) in treating children with atopic dermatitis (AD).

OBJECTIVES

To investigate the immunomodulatory effects that may be induced by PHF treatment.

METHODS

We investigated the effects of PHF on cytotoxicity and proliferation of phytohaemagglutinin (PHA)- and staphylococcal enterotoxin B (SEB)-stimulated peripheral blood mononuclear cells (PBMC) isolated from buffy coat of blood donors. PHF-induced immunomodulation for five inflammatory mediators in cultured PBMC was measured by reverse transcription-polymerase chain reaction and enzyme-linked immunosorbent assay. The effects of a 3-month, open-label study of PHF on circulating inflammatory mediators in children with AD were also assessed.

RESULTS

PHF at up to 1 mg mL(-1) dose-dependently suppressed PBMC proliferation. The addition of PHF to cultured PBMC reduced supernatant concentrations of brain-derived neurotrophic factor (BDNF), interferon (IFN)-gamma and tumour necrosis factor (TNF)-alpha in response to PHA, and BDNF and thymus and activation-regulated chemokine (TARC) following SEB stimulation. PHF increased epithelial cell-derived neutrophil activating peptide-78 levels in culture supernatants. At the RNA level, PHF suppressed the transcription of BDNF, TARC, IFN-gamma and TNF-alpha. Twenty-eight children with AD were treated with PHF for 3 months, and their mean plasma concentrations of BDNF and TARC decreased significantly from 1798 pg mL(-1) and 824 pg mL(-1) at baseline to 1378 pg mL(-1) and 492 pg mL(-1) (P = 0.002 and 0.013, respectively) upon study completion.

CONCLUSIONS

PHF possesses in vitro and in vivo immunomodulatory properties that may mediate the clinical efficacy observed in AD treatment.

Neuroimmune crosstalk in asthma: dual role of the neurotrophin receptor p75NTR

BACKGROUND

Neurotrophins have been implicated in the pathogenesis of asthma because of their ability to induce airway inflammation and to promote hyperreactivity of sensory neurons, which reflects an important mechanism in the pathogenesis of airway hyperreactivity. Neurotrophins use a dual-receptor system consisting of Trk-receptor tyrosine kinases and the structurally unrelated p75NTR. Previous studies revealed an important role of p75NTR in the pathogenesis of allergic asthma.

OBJECTIVES

The aim of the study was to investigate the precise mechanisms of neurotrophins in neuroimmune interaction, which can lead to both airway inflammation and sensory nerve hyperreactivity in vivo.

METHODS

Mice selectively expressing p75NTR in immune cells or nerves, respectively, were generated. After sensitization and allergen provocation, hyperreactivity of sensory nerves was tested in response to capsaicin. Airway inflammation was analyzed on the basis of differential cell counts and cytokine levels in bronchoalveolar lavage fluids.

RESULTS

Allergic mice selectively expressing p75NTR in immune cells showed normal inflammation but no sensory nerve hyperreactivity, whereas mice selectively expressing p75NTR in nerve cells had a diminished inflammation and a distinct sensory nerve hyperreactivity.

CONCLUSION

Our data indicate that p75NTR plays a dual role by promoting hyperreactivity of sensory nerves and airway inflammation. Additionally, our study provides experimental evidence that development of sensory nerve hyperreactivity depends on an established airway inflammation in asthma. In contrast, development of airway inflammation seems to be independent from sensory nerve hyperreactivity.

CLINICAL IMPLICATIONS

Because of its dual function, antagonization of p75NTR-mediated signals might be a novel approach in asthma therapy.

Cell type-specific regulation of brain-derived neurotrophic factor in states of allergic inflammation

BACKGROUND

Brain-derived neurotrophic factor (BDNF) is a molecule influencing neuronal proliferation and differentiation. In states of allergy, it may orchestrate inflammatory changes by linking the immune system with the nervous system. Because the precise regulation of gene transcription in mast cells MCs is not clear, the present studies assessed the gene regulation of BDNF in this inflammatory cell type.

METHODS

Transcriptional expression of BDNF in human skin was studied in isolated cells using RT-PCR. In situ lesional MC BDNF protein expression was analysed by immunohistochemistry and related to the differential staining of MCs and functional effects of BDNF on HaCaT keratinocytes.

RESULTS

BDNF mRNA expression was found in isolated human skin MCs, keratinocytes, and fibroblasts. Also, low levels were found in endothelial cells and melanocytes. BDNF protein expression was found in situ in lesional and non-lesional MCs. A significantly decreased expression of BDNF protein was found in atopic dermatitis lesional MCs when compared with control MC expression. Functional in vitro experiments demonstrated that a decrease in BDNF stimulation led to increased secretion rates for stem cell factor and IL-8 in HaCaT keratinocytes.

CONCLUSION

The demonstration of a decreased level of BDNF gene transcription in lesional MCs points to a differential regulation of MC-released neutrotrophins in cutaneous allergic inflammation. Topically administered neurotrophin receptor-modulating compounds should be receptor target specific and not universally acting in diseases such as atopic dermatitis or allergic asthma.

Perinatal maternal application of Lactobacillus rhamnosus GG suppresses allergic airway inflammation in mouse offspring

BACKGROUND

Clinical studies indicate that maternal exposure to probiotic bacteria may protect from the development of allergic disease later in life.

OBJECTIVE

The purpose of this study was to analyse the effects of a perinatal Lactobacillus rhamnosus GG (LGG) supplementation on the development of allergic disorders in offspring.

METHODS

Female BALB/c mice received intragastric LGG every other day before conception, during pregnancy and lactation (perinatal supplementation group) or before conception and during pregnancy only (prenatal supplementation group). Cytokine expression of placental tissues was examined. Offspring of LGG-supplemented and sham-exposed mothers were sensitized to Ovalbumin (OVA), followed by aerosol allergen challenges. Development of experimental asthma was assessed by bronchoalveolar lavage analysis, lung histology and lung function measurement. Cytokine production of splenic mononuclear cells was analysed following in vitro stimulation.

RESULTS

Intestinal colonization with LGG was observed in mother mice only, but not in the offspring. However, a reduced expression of TNF-alpha, IFN-gamma, IL-5 as well as IL-10 was observed in mice derived from perinatally LGG-supplemented mothers, whereas IL-13 and IL-4 expression remained unchanged. Moreover, in offspring of prenatally or perinatally LGG-supplemented mothers allergic airway and peribronchial inflammation as well as goblet cell hyperplasia were significantly reduced as compared with mice derived from non-supplemented mothers. In contrast, airway hyperresponsiveness to methacholine was not affected. Exposure to LGG during pregnancy only shifted the placental cytokine expression pattern with a markedly increased TNF-alpha level.

CONCLUSION

Our data suggest that LGG may exert beneficial effects on the development of experimental allergic asthma, when applied in a very early phase of life. Immunological effects are, at least in parts, mediated via the placenta, probably by induction of pro-inflammatory cell signals.

The nerve growth factor and its receptors in airway inflammatory diseases

The nerve growth factor (NGF) belongs to the neurotrophin family and induces its effects through activation of 2 distinct receptor types: the tropomyosin-related kinase A (TrkA) receptor, carrying an intrinsic tyrosine kinase activity in its intracellular domain, and the receptor p75 for neurotrophins (p75NTR), belonging to the death receptor family. Through activation of its TrkA receptor, NGF activates signalling pathways, including phospholipase Cgamma (PLCgamma), phosphatidyl-inositol 3-kinase (PI3K), the small G protein Ras, and mitogen-activated protein kinases (MAPK). Through its p75NTR receptor, NGF activates proapoptotic signalling pathways including the MAPK c-Jun N-terminal kinase (JNK), ceramides, and the small G protein Rac, but also activates pathways promoting cell survival through the transcription factor nuclear factor-kappaB (NF-kappaB). NGF was first described by Rita Levi-Montalcini and collaborators as an important factor involved in nerve differentiation and survival. Another role for NGF has since been established in inflammation, in particular of the airways, with increased NGF levels in chronic inflammatory diseases. In this review, we will first describe NGF structure and synthesis and NGF receptors and their signalling pathways. We will then provide information about NGF in the airways, describing its expression and regulation, as well as pointing out its potential role in inflammation, hyperresponsiveness, and remodelling process observed in airway inflammatory diseases, in particular in asthma.

The contribution of neurotrophins to the pathogenesis of allergic asthma

The neurotrophins nerve growth factor, brain-derived neurotrophic factor, NT-3 (neurotrophin 3) and NT-4 are known for regulating neuron development, function and survival. Beyond this, neurotrophins were found to exert multiple effects on non-neuronal cells such as immune cells, smooth muscle and epithelial cells. In allergic asthma, airway inflammation, airway obstruction, AHR (airway hyperresponsiveness) and airway remodelling are characteristic features, indicating an intensive interaction between neuronal, structural and immune cells in the lung. In allergic asthma patients, elevated neurotrophin levels in the blood and locally in the lung are commonly observed. Additionally, structural cells of the lung and immune cells, present in the lung during airway inflammation, were shown to be capable of neurotrophin production. A functional relationship between neurotrophins and the main features of asthma was revealed, as airway obstruction, airway inflammation, AHR and airway remodelling were all shown to be stimulated by neurotrophins. The aim of the present review is to provide an overview of neurotrophin sources and target cells in the lung, concerning their possible role as mediators between structural cells, immune cells and neurons, connecting the different features of allergic asthma.

Neuro-immune interaction in allergic asthma: role of neurotrophins

The nature of persistent airway hyperreactivity and chronic inflammation in asthma remains unclear. It has been suggested that bi-directional neuro-immune interaction plays an important role in the pathogenesis of this disease, leading to enhanced airway narrowing after contact with unspecific stimuli, as well as infiltration, activation and degranulation of several immune cell subtypes. Important mediators in neuro-immune cross-talk are neurotrophins, which are produced by cells at the site of inflammation. In addition to modulating the function of several leucocyte subsets, they play an important role in the synthesis of neuropeptides by sensory nerve cells. Neuropeptides have been shown to cause smooth-muscle contraction and, in addition, modulate the production of pro-inflammatory molecules by leucocytes. The aim of the present review is to provide an overview of the molecular mechanisms by which neurotrophins and neuropeptides are involved in neuro-immune cross-talk in allergic asthma.

Allergic airway inflammation induces tachykinin peptides expression in vagal sensory neurons innervating mouse airways

BACKGROUND

Allergic airway inflammation has been shown to induce pro-inflammatory neuropeptides such as tachykinin peptides substance P (SP) and neurokinin A (NKA) together with related peptide like calcitonin gene-related peptide (CGRP) in nodose sensory neurons innervating guinea-pig airways.

OBJECTIVE

The present study was designed to examine the effects of allergen sensitization and challenge on the SP/NKA expression in the jugular-nodose ganglion neurons innervating the murine airways.

METHODS

Using retrograde neuronal tracing technique in combination with double-labelling immunohistochemistry, the expression of SP/NKA was investigated in a murine model of allergic airway inflammation.

RESULTS

Allergic airway inflammation was found to induce the expression of SP/NKA (13.2+/-1.43% vs. 5.8+/-0.37%, P<0.01) in large-diameter (>20 microm) vagal sensory neurons retrograde labelled with Fast blue dye from the main stem bronchi.

CONCLUSION

Based on the induction of tachykinins in airway-specific large-sized jugular-nodose ganglia neurons by allergic airway inflammation, the present study suggests that allergen sensitization and challenge may lead to de novo induction of tachykinins in neurons. This may partly contribute to the pathogenesis of airways diseases such as allergic airway inflammation.

Inhibition of pan neurotrophin receptor p75 attenuates diesel particulate-induced enhancement of allergic airway responses in C57/B16J mice

Recent investigations have linked neurotrophins, including nerve growth factor (NGF), neurotrophin-3 (NT-3), and brain-derived neurotrophic factor (BDNF), to allergic airways diseases. Antibody blockade of NGF attenuates airway resistance in allergic mice. Diesel exhaust particle (DEP) exposure has been linked to asthma exacerbation in many cities with vehicular traffic congestion. We tested the hypothesis that DEP-induced enhancement of the hallmark features of allergic airway disease in a murine model is dependent on the function of the pan neurotrophin receptor p75. Ovalbumin (OVA)-sensitized C57B1/6J mice were intranasally instilled with an antibody against the p75 receptor or saline alone 1 h before OVA challenge. The mice were then exposed nose-only to the PM2.5 fraction of SRM2975 DEP or air alone for 5 h beginning 1 h after OVA challenge. Two days later, air-exposed OVA-allergic mice developed a small but insignificant increase in methacholine-induced airflow obstruction relative to air-exposed, vehicle-sensitized mice. DEP-exposed OVA-allergic mice had a significantly greater degree of airway obstruction than all other groups. Instillation of anti-p75 significantly attenuated the DEP-induced increase in airway obstruction in OVA-allergic mice to levels similar to non-sensitized mice. The DEP-induced exacerbation of allergic airway responses may, in part, be mediated by neurotrophins.

Novel strategies of aerosolic pharmacotherapy

The pulmonary administration of drugs plays a crucial role in the management of various respiratory and systemic diseases. While the cellular properties of airway epithelial cells offer a great potential to deliver drugs into the lungs or the circulation, only little is known about the exact transport pathways. Recently, the high-affinity proton-coupled drug and peptide transporter PEPT2 was identified in the human respiratory tract. The expression of transporter mRNA and protein was localized to the airway epithelium and alveolar type II pneumocytes. In addition, transport studies revealed transporter-mediated uptake of substrates into epithelial cells indicating that the transporter is the molecular basis for the transport of peptides and peptidomimetic drugs in pulmonary epithelial cells. Since genotype analysis revealed no significant differences amongst different transporter genotypes concerning expression and function, the transporter displays an interesting novel target for pulmonary delivery of drugs.

Neurotrophins in allergic diseases: From neuronal growth factors to intercellular signaling molecules

Understanding the complex pathophysiology of allergic diseases has been a main challenge of clinical and experimental research for many years. It is well known that the allergic inflammation triggers neuronal dysfunction and structural changes in the diseased tissues such as the airways or the skin. Recent evidence has emerged that the inflammatory response is also controlled by resident tissue cells such as neurons and structural cells. Therefore, signaling molecules that mediate inflammatory interactions among immune, neuronal, and structural cells are becoming a focus of allergy research. Neurotrophins, a family of homologous growth factors initially discovered in the nervous system, display such bidirectional signaling. The expression of neurotrophins, such as nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF), is highly upregulated during allergic inflammation. Neurons, structural cells, and invading immune cells were now identified not only as sources but also as targets of neurotrophins within the inflamed tissue. In this review, we provide an actual overview of the role of neurotrophins in the pathobiology of allergic diseases. We discuss recent findings in human and animal studies such as the regulation of neurotrophin expression during allergic inflammation and the effect of neurotrophins on the development and magnitude of allergic reactions.

Novel concepts of neuropeptide-based drug therapy: vasoactive intestinal polypeptide and its receptors

Chronic inflammatory airway diseases such as bronchial asthma or chronic obstructive pulmonary disease (COPD) are major contributors to the global burden of disease. Although inflammatory cells play the central role in the pathogenesis of the diseases, recent observations indicate that also resident respiratory cells represent important targets for pulmonary drug development. Especially targeting airway neuromediators offers a possible mechanism by which respiratory diseases may be treated in the future. Among numerous peptide mediators such as tachykinins, calcitonin gene-related peptide, neurotrophins or opioids, vasoactive intestinal polypeptide (VIP) is one of the most abundant molecules found in the respiratory tract. In human airways, it influences many respiratory functions via the receptors VPAC1, VPAC2 and PAC1. VIP-expressing nerve fibers are present in the tracheobronchial smooth muscle layer, submucosal glands and in the walls of pulmonary and bronchial arteries and veins. Next to its strong bronchodilator effects, VIP potently relaxes pulmonary vessels, and plays a pivotal role in the mediation of immune mechanisms. A therapy utilizing the respiratory effects of VIP would offer potential benefits in the treatment of obstructive and inflammatory diseases and long acting VIP-based synthetic non-peptide compounds may represent a novel target for drug development.

Nerve growth factor: the central hub in the development of allergic asthma?

Neurotrophins like nerve growth factor (NGF), originally described as nerve growth factors in neuronal development, have been implicated in many physiological processes in the last years. They are now regarded as important factors involved in the resolution of pathological conditions. NGF has profound effects on inflammation, repair and remodeling of tissues. However, in the lung these beneficial effects can transact into disease promoting actions, e.g., in allergic inflammation or respiratory syncytial virus (RSV) infection. Overproduction of NGF then enhances inflammation, and promotes (neuronal) airway hyperreactivity and neurogenic inflammation. We hypothesize that NGF overexpression in certain vulnerable time windows during infancy could be a major risk factor for the development of asthma symptoms.

Chronic cough due to occupational factors

Within the large variety of subtypes of chronic cough, either defined by their clinical or pathogenetic causes, occupational chronic cough may be regarded as one of the most preventable forms of the disease. Next to obstructive airway diseases such as asthma or chronic obstructive pulmonary disease, which are sometimes concomitant with chronic cough, this chronic airway disease gains importance in the field of occupational medicine since classic fiber-related occupational airway diseases will decrease in the future.

Apart from acute accidents and incidental exposures which may lead to an acute form of cough, there are numerous sources for the development of chronic cough within the workplace. Over the last years, a large number of studies has focused on occupational causes of respiratory diseases and it has emerged that chronic cough is one of the most prevalent work-related airway diseases. Best-known examples of occupations related to the development of cough are coal miners, hard-rock miners, tunnel workers, or concrete manufacturing workers.

As chronic cough is often based on a variety of non-occupational factors such as tobacco smoke, a distinct separation into either occupational or personally -evoked can be difficult. However, revealing the occupational contribution to chronic cough and to the symptom cough in general, which is the commonest cause for the consultation of a physician, can significantly lead to a reduction of the socioeconomic burden of the disease.

The vanilloid receptor as a putative target of diverse chemicals in multiple chemical sensitivity

The vanilloid receptor (TRPV1 or VR1), widely distributed in the central and peripheral nervous system, is activated by a broad range of chemicals similar to those implicated in Multiple Chemical Sensitivity (MCS) Syndrome. The vanilloid receptor is reportedly hyperresponsive in MCS and can increase nitric oxide levels and stimulate N-methyl-D-aspartate (NMDA) receptor activity, both of which are important features in the previously proposed central role of nitric oxide and NMDA receptors in MCS. Vanilloid receptor activity is markedly altered by multiple mechanisms, possibly providing an explanation for the increased activity in MCS and symptom masking by previous chemical exposure. Activation of this receptor by certain mycotoxins may account for some cases of sick building syndrome, a frequent precursor of MCS. Twelve types of evidence implicate the vanilloid receptor as the major target of chemicals, including volatile organic solvents (but not pesticides) in MCS.

What targets have knockouts revealed in asthma?

There have been numerous studies of mice rendered genetically deficient of various genes in the context of allergic inflammatory airway disease. These studies have provided invaluable information about basic immune processes, but have also been considered to be useful in predicting novel pharmacological targets. In this review, the effect of a wide range of individual knockouts (KO) on the development of asthma-like pathologies in mice is compiled and considered. How the results of these studies compare with effects of agents that interfere with the function of each gene product, where known, is also described. Finally, a personal view of the utility of these studies in drug development is presented.

Neurotrophin effects on eosinophils in allergic inflammation

Elevated neurotrophin concentrations have been shown in nasal and bronchoalveolar lavage fluids as well as in the sera of patients with allergic rhinitis and asthma. Concentration of nerve growth factor correlated with disease severity, bronchial hyperreactivity, and levels of mediators released from eosinophils. Due to the release of cationic proteins, oxygen species, and cytokines after degranulation, eosinophils contribute to tissue damage and can influence airway hyperresponsiveness in asthma. It has been hypothesized that neurotrophins may be involved in the development of eosinophilia and in activation of these cells. The aim of this review is to elucidate the direct and indirect mechanisms of neurotrophins contributing to eosinophilia in allergic diseases.

Prenatal lipopolysaccharide-exposure prevents allergic sensitization and airway inflammation, but not airway responsiveness in a murine model of experimental asthma

BACKGROUND

Epidemiological evidence underlines the impact of prenatal environmental factors on the development of postnatal allergies. In this regard an inverse correlation between lipopolysaccharide (LPS) exposure and development of childhood allergy has been found.

OBJECTIVE

To assess the impact of prenatal LPS exposure on the development of postnatal respiratory allergies in a mouse model of experimental asthma.

METHODS

Female BALB/c mice were exposed to LPS before conception and during pregnancy. Several weeks after birth offspring were sensitized to ovalbumin (OVA) followed by aerosol allergen challenges.

RESULTS

Prenatal, maternal LPS-exposure enhanced neonatal IFN-gamma, but not IL-4 and IL-2 production. OVA sensitization of prenatally LPS-exposed mice was accompanied by a marked suppression in anti-OVA IgG1 and IgE as well as unchanged IgG2a antibody responses, paralleled by a significant reduction in IL-5 and IL-13 levels following mitogenic stimulation of splenic leucocytes. Assessment of bronchoalveolar lavage fluids following allergen challenges revealed a marked reduction in eosinophils and macrophages in these mice. Surprisingly, development of airway hyper-responsiveness, a hallmark of bronchial asthma, was not affected.

CONCLUSION

This study provides first experimental evidence that LPS may already operate in prenatal life in order to modulate the development of allergies in the offspring.

Nerve growth factor and its receptors in asthma and inflammation

Nerve growth factor (NGF) is a high molecular weight peptide that belongs to the neurotrophin family. It is synthesized by various structural and inflammatory cells and activates two types of receptors, the TrkA (tropomyosin-receptor kinase A) receptor and the p75(NTR) receptor, in the death receptor family. NGF was first studied for its essential role in neuronal growth and survival. Recent reports indicate that it may also help mediate inflammation, especially in the airways. Several studies in animals have reported that NGF may induce bronchial hyperresponsiveness, an important feature of asthma, by increasing sensory innervation. It may also induce migration and activation of inflammatory cells, which infiltrate the bronchial mucosa, and of structural cells, including epithelial, smooth muscle cells and pulmonary fibroblasts. Increased NGF expression and release is observed in asthma patients after bronchial provocation with allergen. Taken together, the data from the literature suggest that NGF may play a role in inflammation, bronchial hyperresponsiveness and airway remodelling in asthma and may help us to understand the neuro-immune cross-talk involved in chronic inflammatory airway diseases.