Mesothelial cell modulation of pleural repair: thrombin stimulated mesothelial cells release prostaglandin E2

Pleural mesothelial cells in pleural and lung diseases

During development, the mesoderm maintains a complex relationship with the developing endoderm giving rise to the mature lung. Pleural mesothelial cells (PMCs) derived from the mesoderm play a key role during the development of the lung. The pleural mesothelium differentiates to give rise to the endothelium and smooth muscle cells via epithelial-to-mesenchymal transition (EMT). An aberrant recapitulation of such developmental pathways can play an important role in the pathogenesis of disease processes such as idiopathic pulmonary fibrosis (IPF). The PMC is the central component of the immune responses of the pleura. When exposed to noxious stimuli, it demonstrates innate immune responses such as Toll-like receptor (TLR) recognition of pathogen associated molecular patterns as well as causes the release of several cytokines to activate adaptive immune responses. Development of pleural effusions occurs due to an imbalance in the dynamic interaction between junctional proteins, n-cadherin and β-catenin, and phosphorylation of adherens junctions between PMCs, which is caused in part by vascular endothelial growth factor (VEGF) released by PMCs. PMCs play an important role in defense mechanisms against bacterial and mycobacterial pleural infections, and in pathogenesis of malignant pleural effusion, asbestos related pleural disease and malignant pleural mesothelioma. PMCs also play a key role in the resolution of inflammation, which can occur with or without fibrosis. Fibrosis occurs as a result of disordered fibrin turnover and due to the effects of cytokines such as transforming growth factor-β, platelet-derived growth factor (PDGF), and basic fibroblast growth factor; which are released by PMCs. Recent studies have demonstrated a role for PMCs in the pathogenesis of IPF suggesting their potential as a cellular biomarker of disease activity and as a possible therapeutic target. Pleural-based therapies targeting PMCs for treatment of IPF and other lung diseases need further exploration.

Pathophysiology of the pleura

The pleural mesothelial cell is an essential cell in maintaining the normal homeostasis of the pleural space and it is also a central component of the pathophysiologic processes affecting the pleural space. In this review, we will review the defense mechanisms of the pleural mesothelium and changes in pleural physiology as a result of inflammatory, infectious, and malignant conditions with a focus on cytokine and chemokine networks. We will also review the processes involved in the pathogenesis of pleural fibrosis.

Respiratory and circulatory effects of parietal pleural afferent stimulation in rabbits

Respiratory symptoms accompanying pleural diseases combine dyspnea, tachypnea, rapid shallow breathing, and sometimes hypotension. There are no experimental data on the changes in respiratory and circulatory functions elicited by the activation of pleural afferents. After removal of all muscles covering the 5th to 10th intercostal spaces, we investigated in paralyzed, vagotomized rabbits the changes in phrenic discharge, transpulmonary pressure, and systemic arterial pressure in response to an outwardly directed force exerted on the parietal pleura or the local application of solutions containing lactic acid or inflammatory mediators. Mechanical stimulation of the pleura induced an immediate decrease in both integrated phrenic discharge and arterial blood pressure, the responses being positively correlated with the magnitude of force applied on the pleura. No accompanying changes in ventilatory timing, transpulmonary pressure, or heart rate were measured. Lactic acid solution also elicited an inhibition of phrenic activity and a fall in blood pressure. Section of the internal intercostal nerves supplying the stimulated intercostal spaces totally abolished the responses to mechanical stimulation or lactic acid. An inflammatory mixture elicited only modest respiratory and circulatory effects. We concluded that an acute mechanical distension of the parietal pleura as well as its chemical stimulation by lactic acid elicit a marked inhibition of phrenic motoneurons combined to a reduction of the sympathetic outflow to the circulatory system.

Identification and properties of parietal pleural afferents in rabbits

Although pain and dyspnoea are common symptoms in pleural diseases, there are few studies on the sensory innervation of the pleura. Using rabbits, after removal of all muscles in the intercostal space to be studied, we investigated the afferents of the internal intercostal nerve by applying to the internal thoracic wall pieces of gauze soaked in warmed (37 degrees C), buffered saline (mechanical stimulation) or solutions containing lactic acid, inflammatory mediators or capsaicin (chemical stimulation). The afferent conduction velocity ranged from 0.5 to 14 m s(-1). Most units (97%) were activated by mechanical stimulation of the pleura (local positive pressure range = 4.5-8.5 cmH2O) and we found a linear relationship between the discharge rate of afferents and the force applied to the thoracic wall. The majority of mechanosensitive units (70%) also responded to one or several chemical agents. Thus, the afferents were activated by lactic acid (49%) and/or a mixture of inflammatory mediators (50%). Local application of capsaicin elicited an initial increased or decreased background afferent activity in 57% of the afferents, a delayed decrease in firing rate being noted in some units initially activated by capsaicin. Capsaicin blocked the afferent response to a further application of inflammatory mediators but did not affect the mechanosensitive units. Thus, sensory endings connected with thin myelinated and unmyelinated fibres in the internal intercostal nerve detect the mechanical and chemical events of pleural diseases.

Pathogenesis of pleural fibrosis

Pleural fibrosis resembles fibrosis in other tissues and can be defined as an excessive deposition of matrix components that results in the destruction of normal pleural tissue architecture and compromised function. Pleural fibrosis may be the consequence of an organised haemorrhagic effusion, tuberculous effusion, empyema or asbestos-related pleurisy and can manifest itself as discrete localised lesions (pleural plaques) or diffuse pleural thickening and fibrosis. Although the pathogenesis is unknown, it is likely that the complex interactions between resident and inflammatory cells, profibrotic mediators and coagulation, and fibrinolytic pathways are integral to pleural remodelling and fibrosis. It is generally considered that the primary target cell for pleural fibrosis is the subpleural fibroblast. However, increasing evidence suggests that mesothelial cells may also play a significant role in the pathogenesis of this condition, both by initiating inflammatory responses and producing matrix components. A greater understanding of the interactions between pleural and inflammatory cells, cytokines and growth factors, and blood derived proteins is required before adequate therapies can be developed to prevent pleural fibrosis from occurring.

Mesothelial cells: their structure, function and role in serosal repair

The mesothelium is composed of an extensive monolayer of specialized cells (mesothelial cells) that line the body's serous cavities and internal organs. Traditionally, this layer was thought to be a simple tissue with the sole function of providing a slippery, non-adhesive and protective surface to facilitate intracoelomic movement. However, with the gradual accumulation of information about serosal tissues over the years, the mesothelium is now recognized as a dynamic cellular membrane with many important functions. These include transport and movement of fluid and particulate matter across the serosal cavities, leucocyte migration in response to inflammatory mediators, synthesis of pro-inflammatory cytokines, growth factors and extracellular matrix proteins to aid in serosal repair, release of factors to promote both the deposition and clearance of fibrin, and antigen presentation. Furthermore, the secretion of molecules, such as glycosaminoglycans and lubricants, not only protects tissues from abrasion, but also from infection and possibly tumour dissemination. Mesothelium is also unlike other epithelial-like surfaces because healing appears diffusely across the denuded surface, whereas in true epithelia, healing occurs solely at the wound edges as sheets of cells. Although controversial, recent studies have begun to shed light on the mechanisms involved in mesothelial regeneration. In the present review, the current understanding of the structure and function of the mesothelium and the biology of mesothelial cells is discussed, together with recent insights into the mechanisms regulating its repair.

The many faces of transforming growth factor-beta in pleural diseases

Transforming growth factor (TGF)-beta is responsible for critical regulatory functions in many physiologic and pathologic processes. Emerging evidence suggests that these roles also apply to a multitude of pleural diseases. Both mesothelial cells and infiltrating cells in the pleural space can produce TGFbeta, and elevated TGFbeta concentrations have been found in pleural effusions and in pleural tissues during disease processes. Recent animal studies have suggested that TGFbeta can induce significant pleurodesis and probably plays a central role in the pathogenesis of pleural fibrosis. Paradoxically, TGFbeta may also stimulate increased pleural fluid formation, in part by inducing the production of vascular endothelial growth factor. TGFbeta also participates in the regulation of pleural inflammation and cell proliferation. Further research into the roles of TGFbeta in the pathogenesis of various pleural diseases is needed and may lead to the development of novel treatment strategies.

Induction of cyclooxygenase-2 in mesothelial cells in peritonitis caused by perforated ulcers--an immunohistochemical study in humans

BACKGROUND

Increasing evidence suggests that mesothelial cells contribute to the control of inflammation in the peritoneal cavity by secreting prostaglandins. A study has shown that cyclooxygenase (COX)-2 knockout mice die partly as a result of peritonitis.

AIM

To investigate the expression and location of COX in peritonitis associated with peptic ulcer perforation.

METHODS

Gastric and duodenal tissues were collected intraoperatively from nine and four patients, respectively, and immunohistochemical staining for COX-1 and COX-2 was performed.

RESULTS

Histologically, all patients had severe peritonitis around the perforation sites, into which many inflammatory cells and fibroblasts had infiltrated, and reactive mesothelial cells exhibited hyperplastic change. The COX-1 protein was not detected, whereas COX-2 was abundant in reactive mesothelial cells near the perforation site and disappeared away from the site. Macrophages and fibroblasts around the perforation site also revealed immunostaining for COX-2.

CONCLUSIONS

Our results showed that COX-2 protein is induced in mesothelial cells, as well as in macrophages and fibroblasts, in inflamed peritoneal tissues associated with peptic ulcer perforation, suggesting involvement of COX-2 in tissue repair.

Infection of the pleural space

Parapneumonic effusions are common accompaniments of pneumonia that require proper management to prevent progression to empyema. Management decisions require thoughtful individualization of care because of the multiple factors that affect outcome; no one algorithmic approach exists for all patients. Basic principles of care, however, apply to all patients and center on the early detection of infected pleural fluid and the rapid completion of effective pleural drainage and lung re-expansion, when indicated to decrease morbidity and mortality.