Cell-based therapy in lung regenerative medicine

The Registry of the International Society for Heart and Lung Transplantation: Thirty-third Adult Lung and Heart–Lung Transplant Report—2016; Focus Theme: Primary Diagnostic Indications for Transplant

Chapter 9 - Preclinical and clinical applications of mesenchymal stem cell-based therapy in pulmonary diseases

For decades the pulmonary circulation was not considered as important as the systemic ("greater") circulation. However, pulmonary hypertension can arise because of many diseases of the heart and lung. Therefore, increasing efforts in research have been undertaken leading to a profound increase in understanding pulmonary vascular physiology and pathophysiology. This review discusses basic physiology, clinical concepts, and treatment options for pulmonary hypertension and the related right ventricular heart failure focusing on secondary pulmonary hypertension in patients during anesthetic procedures. Physiology of Pulmonary Circulation The pulmonary vascular bed is a high-flow, low-pressure circulation system. Pulmonary vessels have less resistance in comparison to the systemic circulation under normal conditions because of higher compliance of pulmonary precapillary arterioles with a thinner media and less smooth muscle cells (SMCs) compared with the corresponding systemic arterioles. In addition, the cross-sectional area of the pulmonary vascular bed is large and highly distensible with recruitable vessels available to accommodate increase in flow resulting in low pressure and low resistance. In contrast to the systemic arteries, pulmonary vessels constrict with hypoxia (Euler-Liljestrand reflex) and relax in the presence of hyperoxia. Furthermore, changes in cardiac output (CO), airway pressure, and gravity affect the pulmonary more than the systemic circulation. Increases in CO distend open vessels and recruit previously closed vessels. Therefore, the cross-sectional area of pulmonary circulation enlarges and results in a decrease in pulmonary vascular resistance (PVR). An increase in CO has very little effect on pulmonary arterial pressure (PAP) because of the recruitment and distension of pulmonary vessels. An increased PAP or left atrial pressure (LAP) may also distend and recruit pulmonary vessels. Clinically, this means that enhanced CO caused by the administration of inotropic drugs or enlarged blood volume will passively decrease PVR. The contribution of intra- and extraalveolar vessels accounts for the unique U-shaped relationship between lung volume and PVR, which is minimal at functional residual capacity and increased at large and small lung volumes (Fig. 1). Clinically, this may be observed when hyperinflation of the lungs greatly increases PVR.Figure 1: Relationship between lung volume and pulmonary vascular resistance (PVR) (123). RV = residual volume, FRC = functional residual capacity, TLC = total lung capacity.Gravity influences the distribution of blood flow in the pulmonary circulation. Blood flow as well as ventilation increases in the dependent areas of the lung. The relationship between alveolar and hydrostatic pressure implies important clinical consequences. Patients with unilateral lung disease should be positioned with the diseased side up as was already shown by Remolina et al. (1). Lying with the sick lung dependent resulted in the worst gas exchange and the lowest arterial oxygen pressure (1). Application of high levels of positive end-expiratory pressure (PEEP) will narrow the capillaries in the well-ventilated lung areas and divert flow to less well-ventilated or nonventilated areas. Therefore, a decrease in PaO2 is the consequence. Constantly changing hemodynamics, mechanical forces, and hormonal environment influence the vascular endothelium and the underlying SMCs. Under normal circumstances, the interaction between endothelium and SMCs results in a low vascular resistance in the pulmonary circulation. An increasing number of molecules seem to be involved in these biochemical transductions: L-arginine-nitric oxide (NO)-cyclic-3′-5′guanosine monophosphate (cGMP) pathway seems to have a predominant role (2). Physiological agonists such as bradykinin and acetylcholine as well as mechanical stress derived from pulmonary blood flow can activate endothelial cells. This results in oxidation of the guanidino-nitrogen atom of the amino acid L-arginine by the constitutive NO synthase to form and release NO. NO diffuses from endothelial cells to the SMCs, and produces relaxation by activating guanylate cyclase with increasing intracellular cGMP (2). Pulmonary endothelial cells (surface area of 200 m2 in the adult) have a very strategic location: they are exposed to the entire CO, link the pulmonary and systemic circulation, and also regulate SMC tone by signaling to the vascular wall. Endothelial cells both seem to sense local shear stress and to initiate a response (e.g., production of NO) in order to accommodate rapid changes of blood flow. NO and prostacyclin (PGI2) support oxygenation and lung inflation in dilating the pulmonary vasculature after birth, keeping a key regulatory role in the lung because of its prompt, local powerful action compared with a brief half-life time (3). Pathophysiology of Pulmonary Hypertension It is now understood that the balance between vasoconstrictors and vasodilators and mitogenic and antimitogenic factors derived from the endothelium is disturbed in situations with an increase in PAP (4, 5). Endothelial dysfunction is promoted by hypoxia, acidosis, free radicals (6), inflammatory mediators, shear stress caused by increased pulmonary blood flow from left-to-right intracardiac shunt (7), and fibrin from thromboembolism (8). There is no widely accepted range for normal PAP. However, mean PAP >25 mm Hg (normal 15 mm Hg) at rest or >30 mm Hg with exercise is generally accepted as indicative for pulmonary hypertension (9). The enhanced pressure in the pulmonary circulation is associated with an increase in PVR and results in a progressive inability of the right ventricle (RV) to sustain its output leading to RV hypertrophy and RV failure depending from acute or chronic changes (4). In a study by Vizza et al. (10), prevalence of RV dysfunction (defined as RV ejection fraction [RVEF] <45%) was significantly higher in patients with pulmonary hypertension compared with all other groups with end-stage pulmonary disease. Determination of PVR is difficult in clinical settings even if the respective variables such as LAP, mean PAP, pulmonary capillary wedge pressure (PCWP), and thermodilution CO are measured directly because of their intrinsic inaccuracies. Therefore, indirect estimations use the equation: Normal PVR is approximately 1.1–1. 4 Wood units or about 90–120 dynes · s · cm−5, and a PVR >300 dynes · s · cm−5 is indicative of pulmonary hypertension. Pulmonary blood flow and volume are not always equal to or correlated with CO, because of intracardiac or other shunts. Vascular Remodeling Chronic pulmonary hypertension leads to structural alterations of the pulmonary vasculature and to a progression of histological changes known as "vascular remodeling" (11). Under physiological conditions, pulmonary arterioles are thin-walled vessels with the media occupying only 7% of the vessel thickness. The major finding in remodeled vessels caused by a chronic stimulus like hypoxia is the increase in SMCs in already muscularized arteries and extension of SMCs into vessels that are normally thin and nonmuscular (12) and thickening of the adventitial layer (13). Thickening of the adventitial layer is the result of marked proliferation of the fibroblasts, which has been shown to be modulated by protein kinase C and mitogen-activated protein kinase (14). After the proliferative response, there is an increase in adventitial connective tissue including a switch in SMC phenotype to a more synthetic cell that is responsible for deposition of increased connective tissue (12). This deposition of most notably collagen is probably a protective mechanism strengthening the vascular wall against the increase in intravascular pressure (15). Additionally, damage to intimal endothelium as well as intimal hyperplasia and fibrosis can be detected (5). The central regulatory function of the pulmonary endothelium is underlined by the fact that a dysfunctional endothelium can influence the development of pulmonary hypertension at the levels of coagulation control, vasomotor tone, and pulmonary vascular remodeling (16). The major stimulus for remodeling is hypoxia (17). The increase in PVR is predominantly caused by the hypoxic pulmonary vasoconstriction (HPV), which resides primarily distal to lobar arteries and proximal to the capillaries and occurs in resistance arterioles 30–300 μm in diameter (18). Pulmonary arteriolar SMC oxygen-sensitive voltage-dependent potassium channels seem to have an important role for initiating HPV (19). Inhibition of these channels by decreased PO2 inhibits outward potassium current, causing membrane depolarization and calcium entry through voltage-dependent calcium channels (19). The main determinant of HPV is alveolar PO2, but mixed venous PO2 contributes to approximately 20% of the response (19). HPV is inhibited by substances such as substance P, atrial natriuretic peptides, by mediators such as PGI2 and NO, by increased LAP, by increased alveolar pressure, and by alkalosis. Enhanced HPV, however, could be observed with acidosis, by using epidural anesthesia and by inhibition of cyclooxygenase or NO synthase (19). Subsequent increases in PAP are predominantly caused by vascular remodeling as well as secondary polycythemia. Proliferation and differentiation of pericytes and intermediate cells to SMCs are a consequence of chronic exposure to hypoxia followed by elastin and collagen synthesis and deposition (20). In addition, vasomotor function may be altered in these remodeled vessels (20). Among the candidates of biochemical mediators of hypoxia-induced pulmonary hypertension are voltage-gated potassium channels, mitochondrial oxygen sensing, and an imbalance in vasoactive factors (see below) (21). Remodeling also happens in inflammation secondary to sepsis (22) and in patients with chronic lung disease (23) and adult respiratory distress syndrome (ARDS) (24). The inflammation process and increased blood flow lead to vascular remodeling by damaging endothelial cells and disturbs the sensitive balance of pulmonary tone. Under physiological conditions, these cells eliminate factors that initiate SMC proliferation such as angiotensin II, endothelin, thromboxane A2, prostaglandin H2 and O2−(25). Alternatively, damaged endothelial cells may fail to produce inhibitory factors, possible heparin-like substances, which normally decrease SMC proliferation. Removal of the inciting stimulus can lead to reversal of structural changes. Etiology of Pulmonary Hypertension Pulmonary hypertension is caused by a variety of acute and chronic pulmonary diseases with an increase in PAP (Table 1). In contrast to secondary pulmonary hypertension, primary pulmonary hypertension (PPH) is not related to a known underlying disease.Table 1: World Health Organization Diagnostic Classification of Pulmonary Hypertension (1998) (21)PPH PPH is rare (1–2 per million in the general population), affects more women than men (1.7:1), sometimes has a familial link (6% of all cases), and has a poor prognosis (median survival 2–3 yr after diagnosis). PAP usually is >60 mm Hg in these patients (9, 26). PPH is a diagnosis of exclusion, because it is idiopathic. Hypoxemia and increased PVR resulting in an increased PAP can lead to RV failure and death. Altered vasoreactivity alone is not responsible for PPH. Vascular remodeling contributes to the vasculopathic process. However, regression of extensive changes caused by hypertension can occur. Secondary Pulmonary Hypertension Secondary pulmonary hypertension is more common, the increases in PAP are generally less severe (mean PAP <40 mm Hg) (26), and it is also more frequently seen in the perioperative period. Therefore, in this review, we have focused on secondary forms of pulmonary hypertension. Most cases of pulmonary hypertension are secondary to cardiac or pulmonary disease (Table 1) and may be reversible in some cases. Clinical manifestations might be overshadowed by the symptoms of the underlying disease (Table 2). Blood vessel changes are not confined to PPH but are also found in many forms of secondary pulmonary hypertension. Congenital cardiac disease with an atrial or ventricular septal defect causes an increase in pulmonary blood flow. Over time, in response to the increased blood flow, the PVR is increased and eventually will exceed systemic vascular resistance (SVR). Under these conditions, blood flow is shunted from right to left, often referred to as Eisenmenger syndrome. In contrast, high-pressure shunts such as those associated with truncus arteriosus, ventricular septal defects, or patent ductus arteriosus (28) cause pulmonary hypertension much earlier and it is often more severe.Table 2: Common Symptoms and Signs of Pulmonary Hypertension (27)The relevance of pulmonary hypertension in the perioperative period was shown in a study by Reich et al. (29). In patients undergoing coronary artery bypass grafting with cardiopulmonary bypass (CPB), the development of pulmonary hypertension was a significant predictor of increased mortality and perioperative myocardial infarction (29). The severity of the observed vasoconstriction correlates with the extent of CPB-induced endothelial injury (30). Levels of thromboxane A2(31) and endothelin (32) were increased after CPB, whereas PGI2 and NO levels were reduced (33). For a long time, it was thought that CPB did not cause any pulmonary endothelial injury due to ischemia-reperfusion because of the protection via the vasa vasorum of the bronchial circulation. It has been shown, however, that total CPB may cause complete cessation and reestablishment of pulmonary artery flow, with inadequate pulmonary endothelial blood supply by the vasa vasorum, resulting in an ischemia-reperfusion injury (34). The intraoperative injury and postoperative endothelial dysfunction of the pulmonary endothelium is promoted by the following factors (2): Preoperative status of the pulmonary vascular bed, i.e., pulmonary hypertension, valvular pathology; chronic obstructive pulmonary disease, pulmonary thromboembolism, and shear stress caused by increases in pulmonary blood flow and pressure of left-right intracardiac shunt. Bando et al. (35) demonstrated in patients with congenital heart disease that preoperative pulmonary hypertension, absence of mixed venous saturation monitoring, and absence of prophylactic α-blockade significantly increased postoperative pulmonary hypertension. Intraoperative vasospastic stimuli, such as hypoxia, hypercarbia, acidosis, duration of total CPB, ischemia-reperfusion injury, free radical formation, inflammatory mediators, pulmonary leukosequestration, excess thromboxane or endothelin production, and microemboli. Postoperative factors such as adrenergic tone, atelectasis, and HPV. Other examples of pulmonary hypertension during anesthesia are shown in Table 3.Table 3: Occurrence of Pulmonary Hypertension During Anesthesia (36)Symptoms and Diagnosis of Pulmonary Hypertension The most common clinical signs of pulmonary hypertension are dyspnea and fatigue (9) (see also Table 2). These major symptoms might be explained through the associated decrease in CO; however, an exact etiology is still missing. On physical examination, a prominent P2 heart sound, a tricuspid regurgitation murmur, an atrial (S3), or ventricular (S4) heart sound might be heard. The chest radiograph can display an enlarged main pulmonary artery and enlarged hilar vessels, whereas the electrocardiogram shows a right axis deviation suggesting right ventricular hypertrophy. However, the validity of acute changes in the electrocardiogram attributed to right heart insufficiency in the intraoperative period is limited. Transesophageal echocardiography (TEE) as a more advanced technique diagnoses pulmonary hypertension indirectly attributed to RV enlargement, paradoxical interatrial and interventricular movement, partial systolic closure of pulmonary valve, tricuspid regurgitation, and an increased RV systolic pressure (Fig. 2). There are two approaches to obtaining hemodynamic measurements using TEE. By using spatial imaging methods, cardiac chamber volumes can be estimated to obtain both preload and stroke volume. In addition, Doppler-based methods can be used to estimate both right ventricular filling and CO (37). Because of the inherent difficulty in developing a simple geometric model of the RV, regression equations may inaccurately predict ventricular volumes. In addition, TEE measurements are further altered by changing loading conditions, which alter both the size and geometry of the RV. Although TEE evaluation of RV volume can be difficult to determine in adults, high-quality images are routinely obtained in pediatric patients. One of the main limitations on further use of TEE is the availability of equipment as well as costs of echocardiography machines and probes, and expertise (37). The definitive diagnosis is obtained by right heart catheterization with direct measurement of PAP, right atrial pressure (RAP), PCWP, and CO.Figure 2: Transesophageal echocardiography for diagnosis of pulmonary hypertension. Dilated right ventricle (RV) in the midesophageal four chamber view. The figure shows the classical septal (S) interventricular shift to the left side. RA = right atrium, LA = left atrium, LV = left ventricle.RV and Pulmonary Hypertension Like the pulmonary circulation, the RV was not considered as important as the left ventricle (LV) in maintaining normal hemodynamics and counted for a long time as a "quantite negliable" (38). Today it is recognized that RV and LV are interdependent and both have vitally important functions. The RV is a thin-walled, highly compliant, but poorly contractile chamber. Under normal loading conditions and when function is not compromised, RV ejects blood against 25% of the afterload of the LV, resulting in a smaller RV wall thickness (39). The RV is bound by the RV free wall and the interventricular septum. Failure of the septum (e.g., because of ischemia) to contract normally will decrease RV systolic function. Blood supply for the RV and the septum depends mostly on whether a right or left dominant, or a "balanced" coronary circulation is present. Usually the right and the left anterior descending coronary artery supply the septum and parts of the free wall of the RV. The continuous pressure gradient between the aorta and the RV (coronary perfusion pressure) is responsible for the coronary blood flow to the RV free wall throughout systole and diastole (40). Therefore, the RV blood/oxygen supply is proportional to the systemic pressure but inversely proportional to the RV pressure. Systemic hypotension or increased RV pressure results in a decreased RV coronary perfusion pressure (41). Vlahakes et al. (42) previously showed that right heart performance is directly related to systemic pressure during pulmonary hypertension. RV function is very sensitive to increased RV preload or afterload, septic shock, coexistent LV dysfunction, or right coronary artery occlusive disease (43, 44). In a fundamental study, Urabe et al. (45) demonstrated that an increase in the afterload of the RV resulted in a rightward shift of the relationship between perfusion pressure and regional shortening. Also, end-diastolic segment lengths increased significantly after banding of the pulmonary artery. In contrast to LV performance, RV function is relatively sensitive to increases in afterload. Acute increase in mean PAP above approximately 40 mm Hg results in a significant decrease in RVEF even in the presence of a normal RV contractility. However, gradual increases in afterload are well tolerated, because the RV has time to assemble new sarcomeres in parallel to increase wall thickness (39). In the presence of decreased RV contractility, the RV is even more susceptible to acute increases in afterload. The decrease in RVEF results from a disproportionate increase in end-systolic volume compared with end-diastolic volume (41). The RV is less preload-responsive than the LV such that a given increase in preload results in a smaller increase in stroke work. Therefore, attempts at volume loading may be less effective in increasing RV output compared with volume loading of the In addition, volume loading may even the of RV oxygen (41). Vlahakes that two important in the of right heart RV afterload be reduced and systemic pressure be or Normal and are for maintaining normal RV function The of is as important for RV as it is for LV RV performance is by both intrinsic factors of the RV as well as factors afterload, of the pressure, right coronary artery perfusion pressure, LV performance, and the contractile of the interventricular of the LV function like coronary artery disease, heart valvular heart disease, or systemic hypertension influence RV function by ventricular Also, a RV and right can shift the interatrial and interventricular septum and the left and LV end-diastolic volume. venous pressure not on RV filling pressure and volume because it is dependent on the compliance of the RV in the tricuspid methods, and imaging are and more in the RV RV dysfunction caused by increased PAP was demonstrated in patients with chronic pulmonary disease and in patients ventilation can also RV the interventricular septum paradoxical resulting in right heart and decreased LV chamber size In postoperative a decrease in RVEF is observed at levels because of of alveolar vessels and increase in RV afterload significant was between RVEF and increasing of airway pressure by ventilation of the underlying disease has Because symptoms often arise in pulmonary hypertension, the after of signs of RV insufficiency is yr in for lung disease, as well as systemic for chronic lung are of of pulmonary hypertension. of which causes and and for can also definitive of congenital heart diseases can and mortality from postoperative pulmonary hypertension common clinical is the of the right heart because of chronic left heart In this of the LV has of the right heart by means of coronary or coronary artery bypass treatment and should PVR, which can be with the following and factors the pulmonary oxygenation with oxygen of respiratory acidosis, mm Hg) of a to of respiratory of the lung of release caused by stress and of options for RV dysfunction with increased PVR whereas positive inotropic drugs are the treatment of in RV dysfunction with normal PVR. of RV of RV preload should be considered if central venous pressure is mm Hg It is however, that preload of the RV should be estimated by a simple volume in an increase in mean arterial blood pressure and can increase RVEF with volume if RV is normal and PAP is increased only However, administration of volume also can have in patients with right heart treatment in for (defined as patients no increase in to the with central venous pressure mm of RV of the of the pulmonary circulation of CO and RV failure previously or vasodilators should be keeping in to a significant decrease in the decrease in the in RV stroke volume, systemic hypotension can be the consequence. is thought to produce by calcium channels has other such as the to NO synthase activate and release in both endothelial and SMCs. of SMCs is by both release of NO and direct of SMCs has a half-life it is relatively for the pulmonary circulation. It has been shown to be a and of other drugs to produce pulmonary in of patients with PPH a but there are no observed because of the small In contrast to treatment with that to reduced PAP and PVR in patients with secondary pulmonary hypertension treatment did not any This might be explained by the fact that on their to vascular remodeling The angiotensin reduced PAP and PVR in patients with secondary pulmonary hypertension 4 after have been shown to be effective in the treatment of pulmonary hypertension secondary to connective tissue vascular disease but not in patients with treatment was because of the on venous The of with calcium in secondary pulmonary hypertension seems to on the of PAP, i.e., the higher the of PAP, the less effective the of to the of on PAP in patients with pulmonary showed that a in PAP in of The in PAP in patients with the and the in PAP with clinical Although these drugs have shown their they have severe if used it is that calcium treatment should be to to Patients with pulmonary hypertension and severe clinical right heart failure (mean mm CO should be from treatment with calcium because of the inotropic of these by or more responsible for the of monophosphate leading to an increased of these with increased LV and pulmonary given have been used in patients with pulmonary hypertension after cardiac was shown for cardiac patients with increased PVR to pulmonary systemic It also seems to have an pulmonary effect to The PGI2 was in to PAP in PPH It is by the vascular endothelium and via prostaglandin to cyclase with following increase of PGI2 production is in patients with pulmonary hypertension of PGI2 may in to these are an inhibition of both and SMC proliferation from treatment of PGI2 or its also PAP in pulmonary hypertension of the pulmonary hypertension after heart in or pulmonary hypertension secondary to connective tissue diseases in a

Lung diseases are associated with high morbidity and mortality rates, thereby jeopardizing human health and imposing a great burden on society. Currently, lung diseases are mainly treated with medications, oxygen therapy and mechanical ventilation, but these approaches are unable to effectively reduce the mortality rate. Therefore, lung transplantation remains the ultimate treatment for various chronic lung diseases, but this treatment is also hindered by the limited availability of lung sources, immature technology and a low survival rate after transplantation. With constant changes in the environment, pathogens, type and amount of harmful substances and the prevalence of respiratory diseases, there is an urgent need to identify alternative treatment methods. Research on stem cell therapy has been very successful in recent years, and mesenchymal stem cells (MSCs), together with their secretory bodies, play a significant therapeutic role. Extracellular vesicles of MSCs (MSC-EVs) are also major components of the paracrine secretion of MSCs, including exosomes, microvesicles, and apoptotic bodies, among which exosomes are the most typical. MSC-EVs are believed to be present in various tissues of the human body where they can carry proteins, DNA, RNA and biologically active factors, just to name a few. They can also transmit various biological signals to participate in different biological activities, including the maintenance of homeostasis within the tissue. Several studies have further demonstrated that MSCs and their generated extracellular vesicles play an important role in the treatment of diseases. In this paper, the origin, properties and roles of MSCs and MSC-EVs are reviewed, the mechanisms of different lung diseases, the limitations of current therapeutic options and the roles of MSC-EVs in Chronic Obstructive Pulmonary Disease, asthma, infectious lung disease, lung cancer, pulmonary fibrosis, pulmonary arterial hypertension, and acute lung injury/ acute respiratory distress syndrome are also discussed (Figure 1). In addition, the current limitations and possible future research directions are also discussed in view of providing new ideas for the role of MSC-EVs in the treatment of lung diseases.

Idiopathic pulmonary fibrosis (IPF) and chronic obstructive pulmonary disease (COPD) are chronic, progressive and lethal lung diseases. The incidence of IPF and COPD increases with age, independent of exposure to common environmental risk factors. At present, there is limited understanding of the relationship between ageing and the development of chronic lung diseases. One hypothesis is that chronic injury drives to exhaustion the local and systemic repair responses in the lung. These changes are accentuated during ageing where there is a progressive accumulation of senescent cells. Recently, stem cells have emerged as a critical reparative mechanism for lung injury. In this review, we discuss the repair response of bone marrow-derived mesenchymal stem cells (B-MSC) after lung injury and how their function is affected by ageing. Our own work has demonstrated a protective role of B-MSC in several animal models of acute and chronic lung injury. We recently demonstrated the association, using animal models, between age and an increase in the susceptibility to develop severe injury and fibrosis. At the same time, we have identified functional differences between B-MSC isolated from young and old animals. Further studies are required to understand the functional impairment of ageing B-MSC, ultimately leading to a rapid stem cell depletion or fatigue, interfering with their ability to play a protective role in lung injury. The elucidation of these events will help in the development of rational and new therapeutic strategies for COPD and IPF.

Since the recognition of nitric oxide (NO) as a key endothelial-derived vasodilator molecule in 1987, the field of NO research has expanded to encompass many areas of biomedical research. It is now well established that NO is an important signaling molecule throughout the body. The therapeutic potential of inhaled NO as a selective pulmonary vasodilator was suggested in a lamb model of pulmonary hypertension and in patients with pulmonary hypertension in 1991.1,2 Because NO is scavenged by hemoglobin (Hb) on diffusing into the blood and is thereby rapidly inactivated, the vasodilatory effect of inhaled NO is limited largely to the lung. This is in contrast to intravenously infused vasodilators that can cause systemic vasodilation and severe systemic arterial hypotension. Recent data indicate that inhaled NO can be applied in various diseases. For example, studies suggest that inhaled NO is a safe and effective agent to determine the vasodilatory capacity of the pulmonary vascular bed. This article summarizes the pharmacology and physiology of inhaled NO and reviews the current uses of inhaled NO for the treatment, evaluation, and prevention of cardiovascular and respiratory diseases. ### Chemistry of NO Gas NO is a colorless, odorless gas that is only slightly soluble in water.3 NO and its oxidative byproducts (eg, NO2 and N2O4) are produced by the partial oxidation of atmospheric nitrogen in internal combustion engines, in the burning cinder cones of cigarettes, and in lightning storms. Medical-grade NO gas is produced under carefully controlled conditions, diluted with pure nitrogen, and stored in the absence of oxygen. The recent article by Williams4 provides a review of the chemistry of NO. ### Therapeutic Versus Endogenous NO Concentrations in the Airway Although early studies of inhaled NO in the treatment of pulmonary hypertension used concentrations of 5 to 80 ppm, it has since been realized that concentrations >20 ppm provide little additional …

COPD 2020: new directions needed.

Pulmonary hypertension (PH) is an important complication of chronic parenchymal lung diseases. The current definition of PH requires an abnormal resting mean pulmonary arterial pressure >20 mmHg during right heart catheterization (RHC). The most common chronic lung conditions that cause PH are chronic obstructive pulmonary disease (COPD) and idiopathic pulmonary fibrosis (IPF), but PH can occur in the context of other parenchymal lung diseases including combined pulmonary fibrosis and emphysema (CPFE), sarcoidosis, lymphangioleiomyomatosis (LAM), and pulmonary Langerhans cell histiocytosis (PLCH). PH that occurs due to chronic lung diseases or chronic hypoxia (high altitude or severe sleep-disordered breathing) comprises Group 3 PH, whereas conditions such as sarcoidosis, LAM, and PLCH have multiple or multifactorial mechanisms and are classified into Group 5 PH.

Pathophysiology of Pulmonary Hypertension in Chronic Parenchymal Lung Disease

Chronic lung diseases are strongly associated with pulmonary hypertension (PH), and even mildly elevated pulmonary arterial pressures are associated with increased mortality. Chronic obstructive pulmonary disease (COPD) is the most common chronic lung disease, but few of these patients develop severe PH. Not all these pulmonary pressure elevations are due to COPD, although patients with severe PH due to COPD may represent the largest subgroup within patients with COPD and severe PH. There are also patients with left heart disease (group 2), chronic thromboembolic disease (group 4, CTEPH) and pulmonary arterial hypertension (group 1, PAH) who suffer from COPD or another chronic lung disease as co-morbidity. Because therapeutic consequences very much depend on the cause of pulmonary hypertension, it is important to complete the diagnostic procedures and to decide on the main cause of PH before any decision on PAH drugs is made. The World Symposia on Pulmonary Hypertension (WSPH) have provided guidance for these important decisions. Group 2 PH or complex developmental diseases with elevated postcapillary pressures are relatively easy to identify by means of elevated pulmonary arterial wedge pressures. Group 4 PH can be identified or excluded by perfusion lung scans in combination with chest CT. Group 1 PAH and Group 3 PH, although having quite different disease profiles, may be difficult to discern sometimes. The sixth WSPH suggests that severe pulmonary hypertension in combination with mild impairment in the pulmonary function test (FEV1 > 60 and FVC > 60%), mild parenchymal abnormalities in the high-resolution CT of the chest, and circulatory limitation in the cardiopulmonary exercise test speak in favor of Group 1 PAH. These patients are candidates for PAH therapy. If the patient suffers from group 3 PH, the only possible indication for PAH therapy is severe pulmonary hypertension (mPAP ≥ 35 mmHg or mPAP between 25 and 35 mmHg together with very low cardiac index (CI) < 2.0 L/min/m2), which can only be derived invasively. Right heart catheter investigation has been established nearly 100 years ago, but there are many important details to consider when reading pulmonary pressures in spontaneously breathing patients with severe lung disease. It is important that such diagnostic procedures and the therapeutic decisions are made in expert centers for both pulmonary hypertension and chronic lung disease.

Extracellular vehicles (EVs) are nanoscale lipid bilayer vesicles that carry biologically active biomolecule cargos like proteins, lipids, and nucleic acids (DNA, RNA) outside of the cell. Blood (serum/plasma), urine, and bronchoalveolar lavage fluid are all examples of biofluids from which they may be collected. EVs play a vital role in intracellular communication. The molecular signature of EVs largely depends on the parental cell's status. EVs are classified into two groups, (1) exosomes (originated by endogenous route) and (2) microvesicles (originated from the plasma membrane, also known as ectosomes). The quantity and types of EV cargo vary during normal conditions compared to pathological conditions (chronic inflammatory lung diseases or lung cancer). Consequently, EVs contain novel biomarkers that differ based on the cell type of origin and during lung diseases. Small RNAs (e.g., microRNAs) are transported by EVs, which is one of the most rapidly evolving research areas in the field of EVs biology. EV-mediated cargos transport small RNAs that can result in reprograming the target/recipient cells. Multiple chronic inflammatory lung illnesses, such as chronic obstructive pulmonary disease, asthma, pulmonary hypertension, pulmonary fibrosis, cystic fibrosis, acute lung injury, and lung cancer, have been demonstrated to be regulated by EV. In this review, we will consolidate the current knowledge and literature on the novel role of EVs and their small RNAs concerning chronic lung diseases (CLDs). Additionally, we will also provide better insight into the clinical and translational impact of mesenchymal stem cells-derived EVs as novel therapeutic agents in treating CLDs.

Pulmonary hypertension (PH) is a common complication of chronic respiratory diseases and particularly of chronic obstructive pulmonary disease (COPD) and interstitial lung diseases (ILD). Owing to its frequency COPD is by far the most common cause of PH. It is generally a mild to moderate PH, pulmonary artery mean pressure (PAP) usually ranging between 20 and 25 mm Hg, but PH may worsen during exercise, sleep, and particularly during exacerbations of the disease. These acute increases in PAP may lead to the development of right heart failure. A small proportion of COPD patients may present "disproportionate" PH defined by a resting PAP >35 to 40 mm Hg. The prognosis is particularly poor in these patients. PH is relatively frequent in advanced ILD and particularly in idiopathic pulmonary fibrosis. As in COPD the diagnosis is suggested by Doppler echocardiography, but the confirmation still requires right heart catheterization. As in COPD, functional (alveolar hypoxia) and morphological factors (vascular remodeling, destruction of the pulmonary parenchyma) explain the elevation of pulmonary vascular resistance that leads to PH. Also as in COPD PH is most often mild to moderate. In ILD the presence of PH predicts a poor prognosis. The treatment of PH relies on long-term oxygen therapy. "New" vasodilator drugs have rarely been used in COPD and ILD patients exhibiting severe PH. In advanced ILD the presence of PH is a supplemental argument for considering lung transplantation.

C-proSP-B: A Possible Biomarker for Pulmonary Diseases?

Acute lung injury (ALI) is a pulmonary illness with a high burden of morbidity and mortality around the world. Chronic lung diseases also represent life-threatening situations. Metastasis-associated lung adenocarcinoma transcript 1 (MALAT1) is a type of long non-coding RNA (lncRNA) and is highly abundant in lung tissues. MALAT1 can function as a competitive endogenous RNA (ceRNA) to impair the microRNA (miRNA) inhibition on targeted messenger RNAs (mRNAs). In this review, we summarized that MALAT1 mainly participates in pulmonary cell biology and lung inflammation. Therefore, MALAT1 can positively or negatively regulate ALI and chronic lung diseases (e.g., chronic obstructive pulmonary disease (COPD), bronchopulmonary dysplasia (BPD), pulmonary fibrosis, asthma, and pulmonary hypertension (PH)). Besides, we also found a MALAT1-miRNA-mRNA ceRNA regulatory network in acute and chronic lung diseases. Through this review, we hope to cast light on the regulatory mechanisms of MALAT1 in ALI and chronic lung disease and provide a promising approach for lung disease treatment.

Given the failure of pharmacological interventions in acute respiratory distress syndrome (ARDS), researchers have been actively pursuing novel strategies to treat this devastating, life-threatening condition commonly seen in the intensive care unit. There has been considerable research on harnessing the reparative properties of stem and progenitor cells to develop more effective therapeutic approaches for respiratory diseases with limited treatment options, such as ARDS. This review discusses the preclinical literature on the use of stem and progenitor cell therapy and cell-based gene therapy for the treatment of preclinical animal models of acute lung injury (ALI). A variety of cell types that have been used in preclinical models of ALI, such as mesenchymal stem cells, endothelial progenitor cells, and induced pluripotent stem cells, were evaluated. At present, two phase I trials have been completed and one phase I/II clinical trial is well underway in order to translate the therapeutic benefit gleaned from preclinical studies in complex animal models of ALI to patients with ARDS, paving the way for what could potentially develop into transformative therapy for critically ill patients. As we await the results of these early cell therapy trials, future success of stem cell therapy for ARDS will depend on selection of the most appropriate cell type, route and timing of cell delivery, enhancing effectiveness of cells (i.e., potency), and potentially combining beneficial cells and genes (cell-based gene therapy) to maximize therapeutic efficacy. The experimental models and scientific methods exploited to date have provided researchers with invaluable knowledge that will be leveraged to engineer cells with enhanced therapeutic capabilities for use in the next generation of clinical trials.