The lung and the gut: common origins, close links

Pathophysiology of acute lung injury in patients with acute brain injury: the triple-hit hypothesis

It has been convincingly demonstrated in recent years that isolated acute brain injury (ABI) may cause severe dysfunction of peripheral extracranial organs and systems. Of all potential target organs and systems, the lung appears to be the most vulnerable to damage after ABI. The pathophysiology of the bidirectional brain-lung interactions is multifactorial and involves inflammatory cascades, immune suppression, and dysfunction of the autonomic system. Indeed, the systemic effects of inflammatory mediators in patients with ABI create a systemic inflammatory environment ("first hit") that makes extracranial organs vulnerable to secondary procedures that enhance inflammation, such as mechanical ventilation (MV), surgery, and infections ("second hit"). Moreover, accumulating evidence supports the knowledge that gut microbiota constitutes a critical superorganism and an organ on its own, potentially modifying various physiological functions of the host. Furthermore, experimental and clinical data suggest the existence of a communication network among the brain, gastrointestinal tract, and its microbiome, which appears to regulate immune responses, gastrointestinal function, brain function, behavior, and stress responses, also named the "gut-microbiome-brain axis." Additionally, recent research evidence has highlighted a crucial interplay between the intestinal microbiota and the lungs, referred to as the "gut-lung axis," in which alterations during critical illness could result in bacterial translocation, sustained inflammation, lung injury, and pulmonary fibrosis. In the present work, we aimed to further elucidate the pathophysiology of acute lung injury (ALI) in patients with ABI by attempting to develop the "double-hit" theory, proposing the "triple-hit" hypothesis, focused on the influence of the gut-lung axis on the lung. Particularly, we propose, in addition to sympathetic hyperactivity, blast theory, and double-hit theory, that dysbiosis and intestinal dysfunction in the context of ABI alter the gut-lung axis, resulting in the development or further aggravation of existing ALI, which constitutes the "third hit."

Development of severe colitis is associated with lung inflammation and pathology

Inflammatory bowel diseases (IBD) such as Crohn’s disease and ulcerative colitis are chronic relapsing diseases that affect the gastrointestinal tract, most commonly the colon. A link between the gut and the lung is suggested since patients with IBD have an increased susceptibility for chronic inflammatory lung disease. Furthermore, in the absence of overt lung disease, IBD patients have worsened lung function and more leukocytes in sputum than healthy individuals, highlighting a conduit between the gut and lung in disease. To study the gut-lung axis in the context of IBD, we used TCRδ-/- mice, which are highly susceptible to dextran sulfate sodium (DSS) due to the importance of γδ T cells in maintenance of barrier integrity. After induction of experimental colitis using DSS, the lungs of TCRδ-/- mice exhibited signs of inflammation and mild emphysema, which was not observed in DSS-treated C57BL/6 mice. Damage to the lung tissue was accompanied by a large expansion of neutrophils in the lung parenchyma and an increase in alveolar macrophages in the lung wash. Gene expression analyses showed a significant increase in Csf3, Cxcl2, Tnfa, and Il17a in lung tissue in keeping with neutrophil infiltration. Expression of genes encoding reactive oxygen species enzymes and elastolytic enzymes were enhanced in the lungs of both C57BL/6 and TCRδ-/- mice with colitis. Similarly, surfactant gene expression was also enhanced, which may represent a protective mechanism. These data demonstrate that severe colitis in a susceptible genetic background is sufficient to induce lung inflammation and tissue damage, providing the research community with an important tool for the development of novel therapeutics aimed at reducing co-morbidities in IBD patients.

Epidemiology and prevalence of pulmonary sequestration in Chinese population, 2010-2019

BACKGROUND

Pulmonary sequestration (PS) is the second common congenital lung malformation and has been known for over 150 years. However, there is a scarcity of epidemiological studies on it. This study aimed to characterize the epidemiology of pulmonary sequestration in Chinese population in the recent decade by using a nationwide database.

METHODS

Using data from the Chinese Birth Defects Monitoring Network during 2010-2019, the prevalence rates for PS were calculated by birth year, maternal age, residence area, geographical region, and infant sex. Variations in prevalence and changes over time were further examined. Other variables of interest for analysis included the pregnancy outcomes of affected infants, the prenatal diagnosis, and the co-occurring anomalies of PS cases.

RESULTS

During the study period, we identified an average prevalence rate of 0.31, 0.11, and 0.42 per 10,000 live and still births for the isolated, non-isolated, and overall PS, respectively. An upward trend was observed for each category of PS. The prevalence rates varied significantly by maternal age (< 20 years, 0.34/10,000; 20-24 years, 0.33/10,000; 25-29 years, 0.45/10,000; 30-34 years, 0.46/10,000; ≥ 35 years, 0.36/10,000), residence area (urban vs. rural, 0.51/10,000 vs. 0.30/10,000), geographical region (western, 0.33/10,000; eastern, 0.49/10,000; central, 0.43/10,000), and by infant sex (male vs. female, 0.45/10,000 vs. 0.38/10,000). Non-isolated PS cases were more likely born prematurely than isolated cases (15.29% vs. 7.83%). 40.28% and 33.80% of non-isolated cases were accompanied by additional respiratory, and circulatory system malformations, respectively.

CONCLUSIONS

The study presents for the first time the prevalence of pulmonary sequestration in Chinese population. The rising prevalence and relatively poor perinatal outcome of affected fetuses or newborns indicate the necessity to improve perinatal management of PS.

C-Fiber Degeneration Enhances Alveolar Macrophage-Mediated IFN-α/β Response to Respiratory Syncytial Virus

Stimulation of unmyelinated C fibers, the nociceptive sensory nerves, by noxious stimuli is able to initiate host responses. Host defensive responses against respiratory syncytial virus (RSV) infection rely on the induction of a robust alpha/beta interferon (IFN-α/β) response, which acts to restrict viral production and promote antiviral immune responses. Alveolar macrophages (AMs) are the major source of IFN-α/β upon RSV infection. Here, we found that C fibers are involved in host defense against RSV infection. Compared to the control mice post-RSV infection, degeneration and inhibition of C fibers by blockade of transient receptor potential vanilloid 1 (TRPV1) lowered viral replication and alleviated lung inflammation. Importantly, AMs were markedly elevated in C-fiber-degenerated (KCF) mice post-RSV infection, which was associated with higher IFN-α/β secretion as measured in bronchoalveolar lavage fluid (BALF) samples. Degeneration of C fibers contributed to the production of vasoactive intestinal peptide (VIP), which modulated AM and IFN-α/β levels to protect against RSV infection. Collectively, these findings revealed the key role of C fibers in regulating AM and IFN-α/β responses against RSV infection via VIP, opening the possibility for new therapeutic strategies against RSV. IMPORTANCE Despite continuous advances in medicine, safe and effective drugs against RSV infection remain elusive. As such, host-RSV interactions and host-directed therapies require further research. Unmyelinated C fibers, the nociceptive sensory nerves, play an important role in regulating the host response to virus. In the present study, from the perspective of neuroimmune interactions, we clarified that C-fiber degeneration enhanced the AM-mediated IFN-α/β response against RSV via VIP, providing potential therapeutic targets for the treatment of RSV infection.

Gut-lung Microbiota Interactions in Chronic Obstructive Pulmonary Disease (COPD): Potential Mechanisms Driving Progression to COPD and Epidemiological Data

This paper focuses on the gut-lung axis in the context of Inflammatory Bowel Disease (IBD) and Chronic Obstructive Pulmonary Disease (COPD), highlighting the key role played by microbial dysbiosis and the impact of environmental and genetic factors on the innate and acquired immune system and on chronic inflammation in the intestinal and pulmonary tracts. Recent evidence indicates that Antigen-Presenting Cells (APCs) perform regulatory activity influencing the composition of the microbiota. APCs (macrophages, dendritic cells, B cells) possess membrane receptors known as Pattern Recognition Receptors (PRRs), a category of toll-like receptors (TLRs). PRRs recognise distinct microbial structures and microbial metabolites called Signals, which modulate the saprophytic microbial equilibrium of the healthy microbiota by recognising molecular profiles associated with commensal microbes (Microbe-Associated Molecular Patterns, MAMPs). During dysbiosis, pathogenic bacteria can prompt an inflammatory response, producing PAMPs (Pathogen-Associated Molecular Patterns) thereby activating the proliferation of inflammatory response cells, both local and systemic. This series of regulatory and immune-response events is responsible (together with chronic infection, incorrect diet, obesity, etc.) for the systemic chronic inflammation (SCI) known as "low-grade inflammation" typical of COPD and IBD. This review looks at immunological research and explores the role of the microbiota, looking at two recent clinical studies, SPIROMICS and AERIS. There is a need for further clinical studies to characterize the pulmonary microbiota and to obtain new information about the pathogenesis of lung disease to improve our knowledge and treatment strategies and identify new therapeutic targets.

Probiotics, prebiotics, and synbiotics to prevent or combat air pollution consequences: The gut-lung axis

Air pollution exposure is a public health emergency, which attributes globally to an estimated seven million deaths on a yearly basis We are all exposed to air pollutants, varying from ambient air pollution hanging over cities to dust inside the home. It is a mixture of airborne particulate matter and gases that can be subdivided into three categories based on particle diameter. The smallest category called PM_0.1 is the most abundant. A fraction of the particles included in this category might enter the blood stream spreading to other parts of the body. As air pollutants can enter the body via the lungs and gut, growing evidence links its exposure to gastrointestinal and respiratory impairments and diseases, like asthma, rhinitis, respiratory tract infections, Crohn's disease, ulcerative colitis, and abdominal pain. It has become evident that there exists a crosstalk between the respiratory and gastrointestinal tracts, commonly referred to as the gut-lung axis. Via microbial secretions, metabolites, immune mediators and lipid profiles, these two separate organ systems can influence each other. Well-known immunomodulators and gut health stimulators are probiotics, prebiotics, together called synbiotics. They might combat air pollution-induced systemic inflammation and oxidative stress by optimizing the microbiota composition and microbial metabolites, thereby stimulating anti-inflammatory pathways and strengthening mucosal and epithelial barriers. Although clinical studies investigating the role of probiotics, prebiotics, and synbiotics in an air pollution setting are lacking, these interventions show promising health promoting effects by affecting the gastrointestinal- and respiratory tract. This review summarizes the current data on how air pollution can affect the gut-lung axis and might impact gut and lung health. It will further elaborate on the potential role of probiotics, prebiotics and synbiotics on the gut-lung axis, and gut and lung health.

COPD and Gut–Lung Axis: How Microbiota and Host Inflammasome Influence COPD and Related Therapeutics

The exact pathogenesis of chronic obstructive pulmonary disease (COPD) remains largely unknown. While current management strategies are effective at stabilizing the disease or relief the symptoms, new approaches are required to target underlying disease process and reverse lung function deterioration. Recent research showed that pneumonia bacteria is critical in disease progression and gut microbiome is likely perturbed in COPD, which is usually accompanied by a decreased intestinal microbial diversity and a disturbance in immune system, contributing to a chronic inflammation. The cross-talk between gut microbes and lungs, termed as the “gut-lung axis,” is known to impact immune response and homeostasis in the airway. Although the gut and respiratory microbiota exhibit compositional differences, the gut and lung showed similarities in the origin of epithelia of both gastrointestinal and respiratory tracts, the anatomical structure, and early-life microbial colonization. Evidence showed that respiratory infection might be prevented, or at least dampened by regulating gut microbial ecosystem; thus, a promising yet understudied area of COPD management is nutrition-based preventive strategies. COPD patient is often deficient in nutrient such as antioxidant, vitamins, and fiber intake. However, further larger-scale randomized clinical trials (RCTs) are required to establish the role of these nutrition-based diet in COPD management. In this review, we highlight the important and complex interaction of microbiota and immune response on gut-lung axis. Further research into the modification and improvement of the gut microbiota and new interventions through diet, probiotics, vitamins, and fecal microbiota transplantation is extreme critical to provide new preventive therapies for COPD.

Links Between Inflammatory Bowel Disease and Chronic Obstructive Pulmonary Disease

Inflammatory bowel disease (IBD) and chronic obstructive pulmonary disease (COPD) are chronic inflammatory diseases of the gastrointestinal and respiratory tracts, respectively. These mucosal tissues bear commonalities in embryology, structure and physiology. Inherent similarities in immune responses at the two sites, as well as overlapping environmental risk factors, help to explain the increase in prevalence of IBD amongst COPD patients. Over the past decade, a tremendous amount of research has been conducted to define the microbiological makeup of the intestine, known as the intestinal microbiota, and determine its contribution to health and disease. Intestinal microbial dysbiosis is now known to be associated with IBD where it impacts upon intestinal epithelial barrier integrity and leads to augmented immune responses and the perpetuation of chronic inflammation. While much less is known about the lung microbiota, like the intestine, it has its own distinct, diverse microflora, with dysbiosis being reported in respiratory disease settings such as COPD. Recent research has begun to delineate the interaction or crosstalk between the lung and the intestine and how this may influence, or be influenced by, the microbiota. It is now known that microbial products and metabolites can be transferred from the intestine to the lung via the bloodstream, providing a mechanism for communication. While recent studies indicate that intestinal microbiota can influence respiratory health, intestinal dysbiosis in COPD has not yet been described although it is anticipated since factors that lead to dysbiosis are similarly associated with COPD. This review will focus on the gut-lung axis in the context of IBD and COPD, highlighting the role of environmental and genetic factors and the impact of microbial dysbiosis on chronic inflammation in the intestinal tract and lung.

Abnormal Sonic hedgehog signaling in the lung of rats with esophageal atresia induced by adriamycin

BACKGROUND

Abnormal lung development was recently described in the rat model of esophageal atresia and tracheoesophageal fistula (EA-TEF). Since in this condition the ventral-to-dorsal switch of Shh expression in the foregut is disturbed, the present study tested the hypothesis that this abnormal expression at the emergence of the tracheobronchial bud might be translated into the developing lung.

METHODS

Pregnant rats received either 1.75 mg/kg i.p. adriamycin or vehicle from E7 to E9. Three groups were studied: control and adriamycin-exposed with and without EA-TEF. Embryos were recovered and the lungs were harvested and processed for reverse transcription polymerase chain reaction and immunofluorescence analysis of the Shh signaling cascade.

RESULTS

Shh signaling was downregulated at the late embryonic stage of lung development (E13) in embryos with EA-TEF. Throughout the subsequent stages of development, the expression of both Shh and its downstream components increased significantly and remained upregulated throughout gestation. Immunofluorescent localization was consistent with these findings.

CONCLUSION

Defective Shh signaling environment in the foregut is present beyond the emergence of lung buds and probably impairs lung development. Later in gestation, lungs exhibited a remarkable ability to upregulate the Shh cascade, suggesting a compensatory response. These findings may be relevant to understand pulmonary disease suffered by children with EA-TEF.

Transient in utero disruption of cystic fibrosis transmembrane conductance regulator causes phenotypic changes in alveolar type II cells in adult rats

Background

Mechanicosensory mechanisms regulate cell differentiation during lung organogenesis. We have previously demonstrated that cystic fibrosis transmembrane conductance regulator (CFTR) was integral to stretch-induced growth and development and that transient expression of antisense-CFTR (ASCFTR) had negative effects on lung structure and function. In this study, we examined adult alveolar type II (ATII) cell phenotype after transient knock down of CFTR by adenovirus-directed in utero expression of ASCFTR in the fetal lung.

Results

In comparison to (reporter gene-treated) Controls, ASCFTR-treated adult rat lungs showed elevated phosphatidylcholine (PC) levels in the large but not in the small aggregates of alveolar surfactant. The lung mRNA levels for SP-A and SP-B were lower in the ASCFTR rats. The basal PC secretion in ATII cells was similar in the two groups. However, compared to Control ATII cells, the cells in ASCFTR group showed higher PC secretion with ATP or phorbol myristate acetate. The cell PC pool was also larger in the ASCFTR group. Thus, the increased surfactant secretion in ATII cells could cause higher PC levels in large aggregates of surfactant. In freshly isolated ATII cells, the expression of surfactant proteins was unchanged, suggesting that the lungs of ASCFTR rats contained fewer ATII cells. Gene array analysis of RNA of freshly isolated ATII cells from these lungs showed altered expression of several genes including elevated expression of two calcium-related genes, Ca²⁺-ATPase and calcium-calmodulin kinase kinase1 (CaMkk1), which was confirmed by real-time PCR. Western blot analysis showed increased expression of calmodulin kinase I, which is activated following phosphorylation by CaMkk1. Although increased expression of calcium regulating genes would argue in favor of Ca²⁺-dependent mechanisms increasing surfactant secretion, we cannot exclude contribution of alternate mechanisms because of other phenotypic changes in ATII cells of the ASCFTR group.

Conclusion

Developmental changes due to transient disruption of CFTR in fetal lung reflect in altered ATII cell phenotype in the adult life.

Congenital pulmonary malformations associated with esophageal duplication and teratoma: prenatal to postnatal management

A unique case is reported of bronchopulmonary foregut malformation with associated benign teratoma. Antenatal Doppler ultrasound scan findings and postnatal management are described. We discuss the appropriate radiologic investigations and the necessity for surgical intervention. The possible embryogenesis is hypothesized.