Surfactant nebulization does not alter airflow obstruction and bronchial responsiveness to histamine in asthmatic children

Revisiting the role of pulmonary surfactant in chronic inflammatory lung diseases and environmental exposure

Pulmonary surfactant is a crucial and dynamic lung structure whose primary functions are to reduce alveolar surface tension and facilitate breathing. Though disruptions in surfactant homeostasis are typically thought of in the context of respiratory distress and premature infants, many lung diseases have been noted to have significant surfactant abnormalities. Nevertheless, preclinical and clinical studies of pulmonary disease too often overlook the potential contribution of surfactant alterations - whether in quantity, quality or composition - to disease pathogenesis and symptoms. In inflammatory lung diseases, whether these changes are cause or consequence remains a subject of debate. This review will outline 1) the importance of pulmonary surfactant in the maintenance of respiratory health, 2) the diseases associated with primary surfactant dysregulation, 3) the surfactant abnormalities observed in inflammatory pulmonary diseases and, finally, 4) the available research on the interplay between surfactant homeostasis and smoking-associated lung disease. From these published studies, we posit that changes in surfactant integrity and composition contribute more considerably to chronic inflammatory pulmonary diseases and that more work is required to determine the mechanisms underlying these alterations and their potential treatability.

The Role of Surfactant in Lung Disease and Host Defense against Pulmonary Infections

Pulmonary surfactant is essential for life as it lines the alveoli to lower surface tension, thereby preventing atelectasis during breathing. Surfactant is enriched with a relatively unique phospholipid, termed dipalmitoylphosphatidylcholine, and four surfactant-associated proteins, SP-A, SP-B, SP-C, and SP-D. The hydrophobic proteins, SP-B and SP-C, together with dipalmitoylphosphatidylcholine, confer surface tension-lowering properties to the material. The more hydrophilic surfactant components, SP-A and SP-D, participate in pulmonary host defense and modify immune responses. Specifically, SP-A and SP-D bind and partake in the clearance of a variety of bacterial, fungal, and viral pathogens and can dampen antigen-induced immune function of effector cells. Emerging data also show immunosuppressive actions of some surfactant-associated lipids, such as phosphatidylglycerol. Conversely, microbial pathogens in preclinical models impair surfactant synthesis and secretion, and microbial proteinases degrade surfactant-associated proteins. Deficiencies of surfactant components are classically observed in the neonatal respiratory distress syndrome, where surfactant replacement therapies have been the mainstay of treatment. However, functional or compositional deficiencies of surfactant are also observed in a variety of acute and chronic lung disorders. Increased surfactant is seen in pulmonary alveolar proteinosis, a disorder characterized by a functional deficiency of the granulocyte-macrophage colony-stimulating factor receptor or development of granulocyte-macrophage colony-stimulating factor antibodies. Genetic polymorphisms of some surfactant proteins such as SP-C are linked to interstitial pulmonary fibrosis. Here, we briefly review the composition, antimicrobial properties, and relevance of pulmonary surfactant to lung disorders and present its therapeutic implications.

Pulmonary surfactant in the airway physiology: a direct relaxing effect on the smooth muscle

Beside alveoli, surface active material plays an important role in the airway physiology. In the upper airways it primarily serves in local defense. Lower airway surfactant stabilizes peripheral airways, provides the transport and defense, has barrier and anti-edematous functions, and possesses direct relaxant effect on the smooth muscle. We tested in vitro the effect of two surfactant preparations Curosurf® and Alveofact® on the precontracted smooth muscle of intra- and extra-pulmonary airways. Relaxation was more pronounced for lung tissue strip containing bronchial smooth muscle as the primary site of surfactant effect. The study does not confirm the participation of ATP-dependent potassium channels and cAMP-regulated epithelial chloride channels known as CFTR chloride channels, or nitric oxide involvement in contractile response of smooth muscle to surfactant.By controlling wall thickness and airway diameter, pulmonary surfactant is an important component of airway physiology. Thus, surfactant dysfunction may be included in pathophysiology of asthma, COPD, or other diseases with bronchial obstruction.

Therapeutic use of surfactant components in allergic asthma

Pulmonary surfactant is a complex mixture of lipids and proteins that reduces the surface tension at the air-liquid interface. In addition to its biophysical function, some surfactant components play an important role for the innate and adaptive immunity of the lung. A negative modulation of the surfactant function was observed in allergic asthma leading to the assumption that the therapeutic application of surfactant components might be beneficial in this disease. So far, there are a number of preclinical and already some clinical studies demonstrating various effects of different surfactant components that were administered with preventive or therapeutic aim in allergic asthma. This review summarizes the current knowledge on the possibilities to treat allergic asthma with surfactant components.

The immunoregulatory roles of lung surfactant collectins SP-A, and SP-D, in allergen-induced airway inflammation

It has become increasingly evident that pulmonary surfactant proteins, SP-A and SP-D, present in the alveolar and bronchial epithelial fluid linings, not only play significant functions in the innate defense mechanism against pathogens, but also are involved in immunomodulatory roles, which result in the protection against, and resolution of, allergen-induced airway inflammation. Studies on allergen-sensitized murine models, and asthmatic patients, show that SP-A and SP-D can: specifically bind to aero-allergens; inhibit mast cell degranulation and histamine release; and modulate the activation of alveolar macrophages and dendritic cells during the acute hypersensitive phase of allergic response. They also can alleviate chronic allergic inflammation by inhibiting T-lymphocyte proliferation as well as increasing phagocytosis of DNA fragments and clearance of apoptotic cell debris. Furthermore, it has emerged, from the studies on SP-D-deficient mice, that, when these mice are challenged with allergen, they develop increased eosinophil infiltration, and abnormal activation of lymphocytes, leading to the production of Th2 cytokines. Intranasal administration of SP-D significantly attenuated the asthmatic-like symptoms seen in allergen-sensitized wild-type, and SP-D-deficient, mice. These important findings provide a new insight of the role that surfactant proteins play in handling environmental stimuli and in their immunoregulation of airway inflammatory disease.

Therapeutic effect of surfactant protein D in allergic inflammation of mite-sensitized mice

BACKGROUND

Surfactant protein D (SP-D) is involved in the innate immunity within the lung and may have important roles in modulating the inflammatory process of asthma.

OBJECTIVE

To examine the potential immunomodulating role of SP-D on the allergic response in mice, and its interaction with the alveolar macrophages (AMs) during allergic inflammation.

METHODS

A recombinant 60 kDa fragment of human SP-D (rfh SP-D), Survanta, and budesonide were administrated, respectively, to Der p-sensitive BALB/c mice before or after allergen challenge (AC). Total and differential cell counts, levels of cytokines in bronchoalveolar lavage fluids(BALFs), and levels of Der p-specific IgE and IgG1 antibodies in sera, were assayed. The production of nitric oxide (NO), and inducible NO synthase (iNOS) expression, in AMs, were determined by ELISA and RT-PCR, respectively.

RESULTS

Instillation of rfh SP-D to sensitized mice 6 h after AC (therapeutic), but not 24 h before AC (preventive), markedly reduced infiltration of eosinophils, and also reduced levels of IL-4, IL-5, eotaxin, and TNF-alpha but elevated levels of IFN-gamma in the BALF. These effects were comparable with those obtained with budesonide treatment, whereas Survanta did not have a suppressive effect, either before or after AC. There was significant inhibition of NO production in the rfh SP-D pre-treated AMs of allergen-sensitized mice, but not in naive mice.

CONCLUSIONS

These results indicate that rfh SP-D has a therapeutic effect on allergen-induced bronchial inflammation, and that this might be because of its inhibitory effect on NO and TNF-alpha production by AMs, and it thus prevents the development of T-helper type 2 cytokine response.

Impact of a soluble phospholipase A2 inhibitor on inhaled allergen challenge in subjects with asthma

The possible roles of secretory phospholipases A2 (sPLA2) in asthma include the release of arachidonic acid from cellular membranes, generation of lysophospholipids, sPLA2-mediated activation of cPLA2 with increased leukotriene production, and surfactant degradation. LY333013 is a potent inhibitor of sPLA2. This study examined the impact of two doses of LY333013 vs. placebo on allergen-induced bronchoconstriction following inhaled allergen challenge in atopic asthmatics. Fifty subjects were randomly assigned to treatment, and 40 subjects completed the study. A double-blind, placebo-controlled, random order, crossover study design was used. LY333013 had no impact on the primary outcome variables of the areas under the FEV1 response curve early (0-3 hours) (AUC(early)) and late (3-8 hours) (AUC(Iate)) following inhaled allergen challenge. No significant drug-related adverse effects were observed. The response to inhaled allergen challenge was reproducible and confirms the utility of this technique as a model in which to screen compounds for further testing in asthmatic patients.

Natural Porcine Surfactant Augments Airway Inflammation after Allergen Challenge in Patients with Asthma

There is increasing evidence for a role of pulmonary surfactant in asthma and allergic inflammation. In murine asthma models, recent studies have demonstrated that surfactant components downregulate the allergic inflammation. Therefore, we tested the hypothesis that in individuals with mild asthma, a natural porcine surfactant preparation (Curosurf) given before segmental allergen challenge can reduce the allergic airway inflammation. Ten patients with asthma and five healthy control subjects were treated in two segments with either Curosurf or vehicle followed by local allergen challenge. Six additional patients with asthma received Curosurf before allergen challenge in one segment as above, but the second segment was instilled with Curosurf without allergen challenge. Unexpectedly, surfactant treatment augmented the eosinophilic inflammation 24 hours after allergen challenge. A direct chemotactic effect of Curosurf was excluded. However, levels of eotaxin and interleukin-5 were increased in bronchoalveolar lavage after Curosurf treatment, whereas IFN-gamma-levels and numbers of IFN-gamma(+) T cells were decreased. Curosurf had no influence on spreading and retention of allergen determined by allergen uptake in mice. These findings demonstrate that treatment with a natural porcine surfactant results in an augmentation of the eosinophilic inflammation after allergen challenge that is more likely due to immunomodulatory effects than to biophysical properties of the surfactant.

Lung surfactant in subacute pulmonary disease

Pulmonary surfactant is a surface active material composed of both lipids and proteins that is produced by alveolar type II pneumocytes. Abnormalities of surfactant in the immature lung or in the acutely inflamed mature lung are well described. However, in a variety of subacute diseases of the mature lung, abnormalities of lung surfactant may also be of importance. These diseases include chronic obstructive pulmonary disease, asthma, cystic fibrosis, interstitial lung disease, pneumonia, and alveolar proteinosis. Understanding of the mechanisms that disturb the lung surfactant system may lead to novel rational therapies for these diseases.

The role of surfactant in asthma

Pulmonary surfactant is a unique mixture of lipids and surfactant-specific proteins that covers the entire alveolar surface of the lungs. Surfactant is not restricted to the alveolar compartment; it also reaches terminal conducting airways and is present in upper airway secretions. While the role of surfactant in the alveolar compartment has been intensively elucidated both in health and disease states, the possible role of surfactant in the airways requires further research. This review summarizes the current knowledge on surfactant functions regarding the airway compartment and highlights the impact of various surfactant components on allergic inflammation in asthma.

Suppression of neural activity of bronchial irritant receptors by surface-active phospholipid in comparison with topical drugs commonly prescribed for asthma

BACKGROUND

Much indirect evidence has been put forward previously in support of the concept that surface-active phospholipid (SAPL) normally masks irritant receptors in the lungs and upper respiratory tract; but this physical barrier is deficient in asthmatics, imparting hyperresponsiveness of the bronchoconstrictor reflex.

OBJECTIVE

To determine whether exogenous SAPL applied to bronchial mucosa reduces the sensitivity of irritant receptors to a standard challenge used clinically to diagnose asthma and to compare the effects with those of corticosteroids and beta-stimulation.

METHODS

Nerve fibres in the vagi were monitored to record action potentials from irritant receptors identified in the upper airways of rat lungs in response to a methacholine challenge. SAPL in the form of dipalmitoyl phosphatidylcholine (PC) and phosphatidylglycerol (PG) - 7 : 3 PC:PG - was applied as a fine dry powder to enhance surface activity and, hence, chemisorption to epithelium. Comparison was also made with clinical doses of i.v. hydrocortisone and instilled salbutamol together with liquid or solid controls, as appropriate.

RESULTS

Neural activity of irritant receptors was found to be significantly (P = 0.0018) decreased by topical SAPL by 35.8% in response to a methacholine challenge in contrast to an increase of 11.2% in response to a solid (lactose) control. Instilled salbutamol and i.v. hydrocortisone also decreased responses to the same challenge by 43.4% and 14.7%, respectively, in contrast to a liquid (saline) control which increased by 24.5%.

CONCLUSIONS

Surface-active phospholipid has an appreciable effect upon irritant receptors in rat airways, reducing neural response to a methacholine challenge by an amount comparable to that of Salbutamol. These results support the concept of SAPL masking bronchial irritant receptors and warrant placebo-controlled clinical trials of this dry powder as a means of controlling asthma without the side-effects of current medication. Other possible roles discussed for the SAPL epithelial barrier include the exclusion of viruses and allergens.

Exogenous surfactant therapy and mucus rheology in chronic obstructive airway diseases

Exogenous surfactant is a specialized biomaterial used for substitution of the lipoprotein mixture normally present in the lungs-pulmonary surfactant. Respiratory Distress Syndrome is a disease of preterm infants mainly caused by pulmonary immaturity as evidenced by a deficiency of mature lung surfactant. Pulmonary surfactant is known to stabilize small alveoli and prevent them from collapsing during expiration. However, apart from alveoli, surfactant also lines the narrow conducting airways of the tracheobronchial tree. This paper reviews the role of this surfactant in the airways and its effect on mucus rheology and mucociliary clearance. Its potential role as a therapeutic biomaterial in chronic obstructive airway diseases, namely asthma, chronic bronchitis, and respiratory manifestations of cystic fibrosis, are discussed. This paper also attempts to elucidate the exact steps in the pathogenic pathway of these diseases which could be reversed by supplementation of exogenous surfactant formulations. It is shown that there is great potential for the use of present day surfactants (which are actually formulated for use in Respiratory Disease Syndrome) as therapy in the aforementioned diseases of altered mucus viscoelasticity and mucociliary clearance. However, for improved effectiveness, specific surfactant formulations satisfying certain specific criteria should be tailor-made for the clinical condition for which they are intended. The properties required to be fulfilled by the optimal exogenous surfactant in each of the above clinical conditions are enumerated in this paper.