The role and mechanism of gut-lung axis mediated bidirectional communication in the occurrence and development of chronic obstructive pulmonary disease

ABSTRACT

The current studies have shown that the occurrence and development of chronic obstructive pulmonary disease (COPD) are closely related to the changes in gut health and its microenvironment, and even some gut diseases have significant clinical correlation with COPD. The dysbiosis of gut microbiota observed in COPD patients also suggests a potential bidirectional interaction between the gut and lung. Communication between the gut and lung may occur through circulating inflammatory cells, gut microbial metabolites, and circulating inflammatory mediators, but the mechanism of bidirectional communication between the gut and lung in COPD is still under study. Therefore, more research is still needed in this area. In this review, we summarize recent clinical studies and animal models on the role of the gut-lung axis in the occurrence and development of COPD and its mechanisms, so as to provide ideas for further research in this field. In addition, we also summarized the negative effects of COPD medication on gut microbiota and the gut microbiota risk factors for COPD and proposed the potential prevention and treatment strategies.

Introduction

Chronic obstructive pulmonary disease (COPD) is a kind of chronic airway nonspecific inflammatory disease and is characterized by poorly reversible airflow obstruction and alveolar damage well as respiratory symptoms including upper respiratory tract inflammation, emphysema, cough, and increased airway mucus, which is the third leading cause of death globally.^1,2 The main treatment of COPD is to optimize lung function and inhibit disease progression through pharmacological and non-pharmacological interventions. Pharmacological interventions mainly include inhaled corticosteroids,³ antibiotics,⁴ and anticholinergic drugs.⁵ Non-pharmacological interventions include smoking cessation,⁶ dietary modification,⁷ and lung rehabilitation.⁸ These management measures are effective only in stabilizing disease progression but are suboptimal in preventing or delaying deterioration in lung function. Together with the rising incidence rates and the socioeconomic burden it imposes, has made it a social problem of increasing concern.⁹ Therefore, the comprehensive investigation of the pathogenic mechanisms underlying COPD is essential to early diagnosis, prevent or slow the progression of COPD and reduce mortality.

The remarkable development of the gut microbiota research in recent years has led to an unprecedented paradigm for disease management.¹⁰ The gut system is the largest organ in the human body. In host gut, a large number of bacteria, fungi, archaea and viruses form a vast ecosystem known as the gut microbiota. The human gut microbiota is composed of approximately 10¹⁴ microbes, equivalent to 4 × 10⁶ genes, which constitutes the unique gene pool in each human body.¹¹ The significant role of such a vast ecosystem in human health cannot be over looked. With the development of high-throughput sequencing and metagenomics, we can obtain information on the dynamic changes of human gut microbiota, thus revealing the mystery of the interaction between gut microbiota and diseases. The interaction between gut microbiota and host mainly includes immune regulation, microbial metabolites, microbial components and neuromodulation, among which microbial metabolites are a particularly important way.¹⁰ The gut microbiota and its derived metabolites have an important impact on host innate immunity, nutrient metabolism, and tissue and organ development.¹² The COPD patients are often accompanied by gut microbiota imbalance, which can lead to disorders of the mucosal microenvironment, such as the gut barrier and the immune system, which may have negative effects on the gut and lung.¹³ A recent study observed increased abundance of Firmicutes, decreased abundance of Bacteroidetes, and increased abundance of Streptococcus and Alistipes at the genus level in feces of COPD patients.¹⁴ Streptococcus is one of the common strains that cause lung infection in patients with COPD.¹⁵ Gastrointestinal diseases are more prevalent in COPD patients compared to healthy individuals. A retrospective study of 1,228 COPD patients showed that most COPD patients had at least one gastrointestinal symptom.¹⁶ Plasma zonulin, a marker of gut leakage, was observed to be elevated in patients with COPD, and higher levels of zonulin were found in plasma of patients with moderate and severe COPD compared with patients with mild COPD,¹⁷ which suggests that there is a certain degree of impairment of intestinal barrier function in COPD patients. COPD can not only lead to gut microbiota disorder and gastrointestinal diseases, but also vice versa. When gut microbiota is disordered, it can also affect lung function. For example, patients with gastrointestinal diseases such as inflammatory bowel disease (IBD) experience disruptions in their gut microbiota, leading to decreased lung function in these individuals.¹⁸ This suggests a bidirectional communication between the gut and the lung, which is known as the gut-lung axis. Diet, as an important factor affecting gut microbiota and gut health, has an important effect on the gut-lung axis. For example, in a cross-sectional study of 7,486 participants, those who consumed prebiotics and yogurt had a 25% lower incidence of COPD.¹⁹ Another study of middle-aged and elderly people found that dietary fiber intake greater than 15.10 g per day was effective in reducing the prevalence of COPD.²⁰ Similarly, a meta-analysis suggested that adding 10 g of total fiber, cereal fiber, or fruit fiber per day reduced the risk of COPD by 26%, 21%, and 37%, respectively.²¹ The protective effect of dietary fiber on COPD may be due to the gut microbiota metabolizing dietary fiber to form SCFAs, which can improve lung function by regulating immune homeostasis and gut barrier function.^22,23

At present, we have made a little progress in the field of the mechanism of the gut microbiota in the pathogenesis of COPD. However, revealing the precise role of actual changes in gut microbiota in the pathogenesis of COPD does not seem to be an easy task due to the limitations of clinical individual heterogeneity. Gut microbiota-derived metabolites seem to be another way to reveal the precise interaction between gut microbiota and disease. So far, some studies have found an association between metabolites of gut microbiota, such as short-chain fatty acids (SCFAs) and trimethylamine N-oxide (TMAO), and COPD.^22,24,25 For example, SCFAs can protect lung function by regulating immune homeostasis, enhancing gut barrier function and inhibiting pathogen translocation.^22,26–28 However, compared with other diseases, the progress in the relationship between gut microbiota metabolites and COPD is still insufficient. Especially for TMAO, we only stay on its correlation with COPD,²⁴ and there is no direct evidence to reveal the mechanism of action between the two. The rapid development of metagenomics, targeted or non-targeted metabolomics and other technologies has brought important means to reveal the complex relationship between gut microbiota, its metabolites and human health. Now, more and more attention has been paid to the relationship between gut microbiota mediated gut-lung axis and the pathogenesis of COPD. Therefore, this review provides a comprehensive overview of the gut microbiota and its associated metabolites in COPD, highlighting the interaction between gut microbiota and host in the pathogenesis of COPD. We believe that our work can provide guidance for future research in COPD and ultimately contribute to the development of management measures for COPD.

Gut-lung axis and COPD

Gut-lung axis

The distribution and function of the gastrointestinal tract and the respiratory tract are different in humans and animals, but they have the same embryonic origin, so they are similar in structure. The lung, trachea, respiratory epithelium, and gut all originate from the endoderm during embryonic development.²⁹ In addition, a previous study found that hyperactive Wnt signaling in lung progenitor cells expressing lung-specific genes can induce differentiation of lung progenitor cells into gut cell types.³⁰ Therefore, it is not difficult to understand the bidirectional crosstalk between the gut and the lung mediated by the gut-lung axis. The lung inflammation in COPD patients is characterized by changes in the number and function of immune cells, such as reduced antigen presentation capacity of macrophages in the lungs of COPD patients.^31,32 Changes in the gut environment are critical for the development and remodeling of lung immunity. Both the respiratory tract and the digestive tract belong to the mucosal immune system, and secreted immunoglobulin A (sIgA) produced by them is the common molecular basis of mucosal immunity in different parts of the body and also an important molecular mediator of the gut-lung axis.³³ sIgA is not only involved in the pathogenesis and progression of lung diseases, including COPD, asthma and idiopathic pulmonary fibrosis,³⁴ but also prevent the spread of pathogens in the body and regulate the composition and function of gut microbiota.³⁵ The poor outcome of germ-free mice exposed to acute infection and their susceptibility to allergic airway disease demonstrate the critical role of the gut microbiota in lung homeostasis and immunity.^36,37 In addition, the researchers detected the expression of lung function protein pulmonary surfactant protein A in the gut tissue of patients with gut inflammation, which demonstrate the similarity between the lung and gut again.³⁸ These similarities set the stage for bidirectional communication of the gut-lung axis.

Gut microbiota-driven metabolites are important mediators mediating the gut-lung axis. Gut microbiota-driven metabolites circulate in the blood, promoting bidirectional communication between the lung and gut, and exerting different effects on individual organs. SCFAs, as important metabolites of gut microbiota, are essential for the establishment of bidirectional communication between the gut-lung axis.³⁹ SCFAs are mainly derived from the fermentation of dietary fiber by gut microbiota. The levels of circulating SCFAs in high-fiber fed mice are higher than those in low-fiber fed mice, and they show resistance to allergic inflammation in the lung.²⁶ Butyrate, as an important representative of SCFAs, is essential for the maintenance of gut barrier function and the maturation and differentiation of gut immune cells.^40,41 Gut-derived butyrate can cross the wall of gut and enter the peripheral blood system, not only regulate the differentiation and maturation of immune cells in the bone marrow cavity, but also reach the lung and directly regulate the activity of regulatory T cells (Treg) through G protein-coupled receptors (GPCRs).^42–44 In addition, SCFAs are energy sources of colonic epithelial cells, which maintain the anaerobic environment of the gut lumen, inhibit the proliferation of Proteobacteria, and ultimately maintain the integrity of the gut barrier function by promoting the transition of colonic epithelial cells to β-oxidation.⁴⁵ The integrity of gut barrier function also inhibits the translocation of pathogens in the gut lumen and indirectly protects lung function.⁴⁶

Lipopolysaccharide (LPS), as a microbial component, is an important component of the outer membrane of gram-negative bacteria. When the gut barrier function is damaged, LPS will cross the gut barrier and be transported throughout the body through the blood circulation system, promoting bidirectional communication between the gut and the lung. LPS is essential for human health, and its mechanism of immune activation is different from other immune agents. It is the only substance that activates lung phagocytes by binding to TLR4.⁴⁷ LPS can activate NF-κB and MAPK by binding to TLR4 receptor, leading to the production of pro-inflammatory cytokines (such as IL-1, IL-6 and TNF-α) and ultimately leading to acute lung injury.^48,49

In a word, the gut and lung share many similarities in structure, development and immunomodulatory mechanisms, which are the biological and anatomical basis for the gut-lung axis. Furthermore, immune cell interactions, gut microbiota metabolites and microbial components (such as LPS) mediate the bidirectional regulation between gut and lung (Figure 1).

Figure 1.

Bidirectional communication between gut and lung mediated by gut-lung axis. The bidirectional interaction between the gut and lungs is mainly achieved through immune responses, microbial composition, and metabolites of the gut microbiota. Dendritic cells receive antigens to activate T-lymphocytes and B-lymphocytes mediated immune responses. LPS, as an important component of gram-negative bacteria in gut microbiota, can regulate lung immune homeostasis through the circulatory system. Gut microbiota metabolites, such as SCFAs, promote bidirectional communication between the gut and lungs through blood circulation. DCs, dendritic cells; LPS, lipopolysaccharide; SCFAs, short-chain fatty acids; sIgA, secretory immunoglobulin A.

Clinical correlation between COPD and gut diseases

Clinical studies have shown an association between gastrointestinal diseases and COPD. Results of people from Taiwan, China-based cohort study show that patients with COPD have a higher risk of functional gastrointestinal disorders than healthy people.⁵⁰ Similarly, studies have shown that the incidence of Crohn’s disease (CD) and ulcerative colitis (UC) in patients with COPD is 55% and 30% higher than that in the general population, respectively.⁵¹ A cohort study showed that IBD was associated with an increased risk of all-cause mortality in COPD patients. In addition, IBD increases the risk of death due to respiratory diseases in COPD patients with asthma.⁵² First-degree relatives of COPD patients are at increased risk of developing CD. Shared environmental factors may contribute to this, but familial risk factors cannot be excluded.⁵³ Atrophic gastritis and gut nutrient absorption dysfunction exist in COPD patients.^54,55 Patients with IBD also have respiratory symptoms such as airway inflammation and sputum production.⁵⁶ Cigarette smoke exposure is an important risk factor for COPD. During smoking, nicotine accumulates in the gut and leads to the activation of AMPKα.⁵⁷ In addition, a retrospective cohort study showed that irritable bowel syndrome(IBS) patients had a significantly increased risk of developing COPD.⁵⁸ In a retrospective cohort study, the cumulative incidence of COPD increased significantly in patients with IBS over more than 10 years of follow-up, and those who were managed for IBS had significantly lower risk for developing COPD.⁵⁸ There is also a potential association between COPD and colorectal cancer. A Mendelian randomization analysis showed that COPD can increase the risk of colorectal cancer, and fatty acid metabolism is a potential mediator of the two diseases.⁵⁹ Other subtypes of gastrointestinal diseases such as gastroesophageal reflux disease showed a higher risk of COPD.^60,61 A data review of 141,057 COPD patients in the Korean national Health Insurance Database showed that gastroesophageal reflux disease is one of the most common comorbidities of COPD.⁶⁰ Cigarette smoke is a risk factor for COPD, and nicotine may lead to an increased probability of reflux events.⁶² Gastroesophageal reflux may increase the severity of acute COPD by irritating the airways and vagal.⁶³

Mechanism of the association between COPD and gut disease

The gut microbiota is an important mediator of the gut-lung bidirectional communication mediated by the gut-lung axis. According to Global Initiative for Chronic Obstructive Lung Disease (GOLD) guidelines, one study recruited 73 healthy controls, 67 patients with COPD severity stages I and II (COPD I-II) and 32 COPD severity stages IIII and IV (COPD III-IV) patients, and transplanted fecal microbiota from these three groups into mice, the results show that fecal microbiota from COPD III-IV patients resulted in significantly higher levels of IL-1β and TNF-α in the plasma of mice. And the percentage of leukocytes in bronchial lavage fluid increased. Compared with mice receiving fecal microbiota from healthy people, the total number of CD3+T lymphocytes, auxiliary T lymphocytes and cytotoxic T lymphocytes in peripheral blood of mice receiving fecal microbiota from COPD I-II and COPD III-IV significantly increased, and the total number of B lymphocytes decreased.⁶⁴ In addition, the α smooth-muscle actin and matrix metalloproteinase 2 levels were significantly elevated in mice receiving the fecal microbiota of COPD III-IV patients, which led to airway remodeling and increased lung mucus secretion in the mice.⁶⁴ A recent study has shown that Pediococcus pentosaceus SMM914 can regulate the transformation of gut microbiota in COPD mice into a community structure characterized by the production of SCFAs and antioxidant metabolites, and activate the tryptophane-melatonin pathway of mice to increase the content of 6-hydroxymelatonin and hypotaurine in lung tissue, which increase the secretion of anti-inflammatory cytokines, and ultimately alleviate the inflammatory response of COPD mice.⁶⁵ Nucleotide-binding oligomerization domain containing 2 (NOD2) is closely related to the occurrence and development of CD. Mutations in the NOD2 gene have been reported in 15% of CD patients.⁶⁶ The defect of NOD2 leads to increased levels of the serum cytokines IL-1β and IL-12, which can lead to impaired gut barrier function.^67,68 And in COPD patients and mouse models, IL-12 was associated with activation of CD8 cytotoxic T-cell and natural killer (NK) cells.^69,70 Plasma levels of C-reactive protein (CRP) and IL-6 are higher in COPD patients than in healthy individual, and circulating IL-6 and CRP were associated with the risk of IBD in a prospective case-control study.^71,72 Pro-inflammatory cytokines such as TNF-α and IL-1β, which are excessively elevated in some gastrointestinal diseases, are also important risk factors for exacerbating COPD progression.^73,74 Group 2 innate lymphoid cells (ILC2) are the innate counterparts of adaptive T lymphocytes, which strongly modulate COPD pathogenesis. The elevated levels of ILC2 and IL-5, a cytokine that favors ILC2 activation and is partly produced by macrophages, were observed in peripheral blood of patients with COPD.⁷⁵ Studies have shown that ILC2, as a tissue-resident cell, can also migrate between tissues, and IL-25-induced inflammatory ILC2 are circulating cells produced by resting ILC2 residing in the lamina propria of the gut. Inflammatory ILC2 can migrate to different tissues, including the lungs,⁷⁶ which may explain the interaction between gut disease and COPD. However, there is still a lack of direct evidence of bidirectional communication between COPD and gut diseases mediated by ILC2.

Changes of gut microbiota in COPD

The gastrointestinal tract contains hundreds of species of bacteria, which are closely involved in the physiological functions of the body through metabolites, immune regulation, microbial components, and neuro modulation. Gut microbiota plays an important role in metabolism, genetics and immune regulation. So far, studies have found that alterations and dysbiosis of gut microbiota is closely related to acute pancreatitis, gut diseases, cardiovascular diseases and cancer.^41,77–79 In recent years, many studies have found that gut microbiota is involved in the pathogenesis of COPD, and changes in gut microbiota have been observed in both animal models and COPD patients (Figure 2).^80–82 We summarize the overview of gut microbiota in COPD, which lays a foundation for further exploration of the role and mechanism of gut microbiota in COPD (Table 1).

Figure 2.

Changes of gut microbiota composition in COPD. Changes in gut microbiota were observed in both animal models and clinical studies. Clinically, the dominant bacteria in the gut microbiota of COPD patients at different stages of development were different. COPD, chronic obstructive pulmonary disease; COPD I, COPD severity stages I; COPD I-II, COPD severity stages I and II; COPD II-III, COPD severity stages II and III; COPD III-IV, COPD severity stages III and IV.

Table 1.

Changes of gut microbiota in COPD.

References	Sample (sample size)	Differential taxa feature in COPD
Naijian Li et al.64	COPD I-II (67) COPD III-IV (32) Healthy controls (73)	COPD I-II vs. healthy controls ↑:Prevotellaceae COPD III-IV vs. healthy controls ↓:Bacteroidetes, Bacteroidaceae, Fusobacteriaceae.
Mei Wang et al.82	COPD patients (40) Healthy controls (40)	↑:Proteobacteria, Acidobacteria, Synergistetes, Tenericutes, Ruminococcus, Blautia, Eubacterium, Sphingomonas, Turicibater, Enterobacter, Enterococcus ↓:observed otus, chao1, shannon, simpson, Firmicutes, Lentisphaerae, Cyanobacteria, Subdoligranulum, Bilophila, Faecalibacterium, Phenylobacterium, Barnesiella, Brevundimonas, Lachnospira, Anaerostipes, Bifidobacterium
Kate L. Bowerman et al.2	COPD patients (28) Healthy controls (29)	↑:Streptococcus, Rothia, Romboutsia, Intestinibacter, Streptococcus spp., Romboutsia timonensis, Intestinibacter bartlettii. ↓:Bacteroides, Roseburia, Lachnospira, Coprobacter fastidiosus, Coprobacter secundus, Rikenellaceae genus RC9, Christensenellales family CAG-74
Yu-Chi Chiu et al.83	COPD I (20) COPD II (20) COPD III-IV (20)	COPD III-IV vs. COPD I ↑:Fusobacterium, Aerococcus, Tyzzerella 4, Dialister ↓:Bacteroidetes, Ruminococcaceae NK4A214 group, Lachnoclostridium COPD III-IV vs. COPD II ↑:Veillonella, Corynebacterium 1, Romboutsia, Aerococcus COPD I vs. COPD II ↓:Megasphaera
Yanhong Yang et al.84	CS-induced mice (10) Control mice (10)	↑:Proteobacteria, Cyanobacteria
K. Tomoda et al.85	CS-induced ale albino rats (5) Control ale albino rats (5)	↓:Bifidobacterium
Lai, HC et al.81	CS-induced mice (10) Control mice (10)	↑: Lachnospiriaceae ↓: Erysipelotrichaceae, Bacteroidales, Ruminococcaceae
X. Wang et al.86	Active smoking rats (9) Control rats (10)	↑:Cyanobacteria, Firmicutes, Tenericutes,TM7 ↓: Bacteroidetes, Clostridiales, Turicibacterales, Clostridiaceae, Turicibacteraceae, Clostridium, Turicibacter, Clostridium perfringens

Abbreviations: COPD, chronic obstructive pulmonary disease; COPD III-IV, COPD severity stages III and IV; COPD I-II, COPD severity stages I and II; COPD I, COPD severity stages I; COPD II, COPD severity stages II; CS, cigarette smoke.

COPD patients

In recent years, more and more studies have observed gut microbiota dysbiosis in COPD patients.⁸⁷ A cohort study showed that the relative abundance of Bacteroidetes, Bacteroidaceae, Fusobacteriaceae were lower but that of Proteobacteria, Acidobacteria, Synergistetes, Acidobacteria, Tenericutes, Ruminococcus, Blautia, Eubacterium, Sphingomonas, Turicibater, Enterobacter, Enterococcus were higher in COPD III-IV patients than in healthy population. The abundance of Prevotellaceae was higher in COPD I-II group compared to the healthy population.⁶⁴ Furthermore, transplantation of gut microbiota from COPD patients resulted in decreased lung function and emphysema in mice, and isolated commensal bacterium Parabacteroides goldsteinii could ameliorate COPD.^64,81 In another study, chao1, shannon, simpson and observed_otus were significantly lower in COPD patients than in healthy controls.⁸² Compared with healthy individuals, the abundance of Proteobacteria, Acidobacteria, Synergistetes, Acidobacteria, Tenericutes, Ruminococcus, Blautia, Eubacterium, Sphingomonas, Turicibater, Enterobacter, Enterococcus increased in COPD patients. The abundance of Firmicutes, Lentisphaerae, Cyanobacteria, Subdoligranulum, Bilophila, Faecalibacterium, Phenylobacterium, Barnesiella, Brevundimonas, Lachnospira, Anaerostipes, and Bifidobacterium decreased. Enterococcus, Ruminococcus, Blautia and Enterobacter were associated with the immune status of COPD patients.⁸² In a study of 28 COPD patients and 29 healthy controls, several bacteria, including Streptococcus and multiple members of the family Lachnospiraceae, were also associated with reduced lung function.² Streptococcus was positively correlated with bilirubin, allo-threonine, N-acetyl-cadaverine, and 2-acetamidobutanoate. And negatively correlated with N-carbamoyl glutamate, N-acetylglutamate, 2’-deoxyadenosine, harmane, suberate, sebacate, undecanedioate, dodecanedipate.² In a clinical study involving 60 patients with COPD, according to GOLD guidelines, COPD patients were divided into COPD I patients, COPD II patients, and COPD III-IV patients. The composition of gut microbiota in COPD I patients and COPD II patients was similar. Compared with COPD I patients, Fusobacterium, Aerococcus, Tyzzerella 4, and Dialister were enriched in the gut microbiota of patients with COPD III-IV. Compared with COPD II, Veillonella, Corynebacterium 1, Romboutsia, and Aerococcus were enriched in the gut microbiota of patients with COPD III-IV.⁸³

Animal models

The model of COPD in C57BL/6 mice induced by cigarette smoke exposure are common animal models of COPD. The abundance of Proteobacteria and Cyanobacteria in gut microbiota of COPD mice induced by cigarette smoke exposure is increased.⁸⁴ KEGG pathway enrichment analysis showed that cigarette smoke exposure induced changes in metabolic pathways in the liver of COPD mice, such as bile acid metabolism, amino acid metabolism, and energy metabolism.⁸⁴ In rats, cigarette smoke exposure resulted in a decrease in the abundance of Bifidobacterium.⁸⁵ In addition, the reduction of Bifidobacterium in the gut microbiota of COPD mice has also been observed in other studies and Bifidobacterium are also potential probiotic prevention and treatment strategies for COPD.^88,89 Studies have shown that Bifidobacterium longum subsp.longum can alleviate the expression of inflammatory factors and adhesion factors, and reduce lung inflammation. The mutant strain with impaired acetic acid production also alleviated inflammation in COPD mice, suggesting that the therapeutic effect of Bifidobacterium longum subsp. longum on COPD is independent of acetic acid.⁸⁸ The transfer of gut microbiota from cigarette smoke-induced COPD in Nlrp6^−/− mice to wild-type mice through co-housing also led to airway inflammation in wild-type mice,⁹⁰ which indicated that cigarette smoke-regulated gut microbiota may be the cause of COPD morbidity. Depletion of gut microbiota by antibiotic cocktail can significantly alleviate emphysema related characteristics such as alveolar wall destruction in COPD mice. Transplantation of gut microbiota from the control group could also alleviate emphysema features in mice, and Parabacteroides goldsteinii may play a dominant role.⁸¹ Most studies have only looked at the composition of the microbiota in the cecal contents or feces of COPD mice/rats. Another study investigated the changes of microbiota in multiple regions of the digestive tract in active smoking rat model. The abundance of Bacteroidetes was reduced in the oral cavity of active smoking rat model, whereas that of Cyanobacteria was increased. In the cecal region, the abundance of Firmicutes was reduced in active smoking rat model, whereas the relative abundance of Tenericutes and TM7 was increased in the colon.⁸¹

There is no doubt that COPD can lead to gut microbiota disorder and COPD-regulated gut microbiota can promote the development of COPD. However, most of the current research only stays in the changes of microbiota, and the pathogenesis of COPD promoted by such changes and the dominant bacteria in this process are still unclear. Therefore, it is necessary to conduct in-depth research on this.⁸⁶

Potential mechanisms of gut microbiota in the pathogenesis of COPD

Usually, Bacteroidetes, Firmicutes, Proteobacteria and Actinobacteria are dominant in the gut microbiota of healthy population.⁹¹ The disorder of gut microbiota may affect the respiratory system by causing gut mucosal damage.¹⁰ Mucins secreted by gut epithelial cells limit the migration of harmful bacteria in the gut to epithelial tissues. Once the gut mucosa is damaged, this restriction will be lifted, and the risk of bacterial translocation from the gut lumen to distant organs will increase, which may lead to severe systemic reactions.⁹² Gut microbiota translocation may play a role in the gut-lung axis in COPD patients (Figure 3). The presence of Enterobacteriaceae in the lung during acute lung injury demonstrated the presence of translocation of gut microbiota in the gut-lung axis.^93,94 Increased zonulin, a marker of gut leakage, was observed in the plasma of COPD patients,¹⁷ which indicates that the gut barrier function of COPD patients is impaired. A decrease in the abundance of Clostridium and Turicibacter has been observed in smoker-induced COPD rat model,⁸⁶ Clostridium and Turicibacter are important commensal bacteria in the gut with the ability to regulate gut barrier function and inflammatory and immune responses associated with gut diseases.^95,96 The impairment of gut barrier function caused by COPD can cause the translocation of bacteria and their products, in which bacterial toxins such as enterotoxins and LPS can promote the development of COPD, and the inflammation associated with gut microbiota may be systemic.^97–100 In addition, gut commensal bacteria, including Bacteroides and Bifidobacterium, can induce the production of antimicrobial peptides and sIgA,⁹³ Clostridium can also promote anti-inflammatory Treg cell responses.¹⁰¹ In the respiratory tract, Streptococcus activates host p38 mitogen-activated protein kinase (MAPK) in TLR-dependent manner to amplify the inflammatory response.¹⁰² Enrichment of Streptococcus in gut microbiota and depletion of Bacteroides and Bifidobacterium are frequently observed in COPD patients and animal models.^2,14,82,85 In addition, Proteobacteria dominated dysbiosis has been observed in COPD patients and animal models,^82,84 which may be related to the expression of inflammatory genes in lung leukocytes caused by Proteobacteria, while Bacteroidetes are associated with gene expression profiles characteristic of lung tissue remodeling.¹⁰³ Microbial components also play a role in regulating inflammation. Polysaccharide A (PSA) derived from Bacteroides fragilis can prevent allergic airway inflammation in mice by inducing IL-10 production by T cells.¹⁰⁴ SCFAs can directly or indirectly protect lung function by regulating lung immune homeostasis and gut barrier function. Reduced SCFAs content was observed in the feces of COPD patients,⁶⁴ which may be related to the depletion of SCFAs producing bacteria (such as Clostridium, Bifidobacterium and Ruminococcaceae) caused by COPD.^82,85,86

Figure 3.

Potential mechanisms of gut microbiota in the pathogenesis of COPD. the disorder of gut microbiota in COPD patients can lead to increased gut permeability, increase the risk of gut bacterial translocation and LPS transfer, and thus promote lung inflammation. The decrease in the abundance of Bacteroides and bifidobacterium can affect the host immune homeostasis by inhibiting the production of sIgA and antimicrobial peptides. Bacteroidetes derived PSA can also prevent airway inflammation by regulating the activity of T lymphocytes to produce IL-10. LPS, Lipopolysaccharide; sIgA, secreted immunoglobulin A; PSA, polysaccharide A.

Although we have observed the changes of gut microbiota in patients with COPD, the results from different studies are quite different and even contradictory.^82,86 This is mainly due to the influence of confounding factors such as individual differences, sampling methods, environment, genetics and living habits. Although animal models avoid the above confounding factors to a certain extent, due to the differences in lung anatomy and physiology between mice and humans, it is difficult to capture the changes of chronic bronchitis and emphysema simultaneously in a single model. An ideal animal model should recapitulate the main features of the human disease, namely chronic bronchitis, mucus hypersecretion, small airway remodeling, and emphysema. However, all described animal models exhibit only a subset of these features.¹³ Therefore, in the future, large-scale clinical cohort studies are warranted to investigate the characteristics of gut microbiota in COPD and the role of landmark bacteria in the pathogenesis of COPD.

The relationship between gut microbiota metabolites and the pathogenesis of COPD

Metabolites derived from the gut microbiota are a major mode of crosstalk between gut microbiota and host. At present, studies have revealed the role and mechanism of metabolites driven by gut microbiota in a variety of diseases. Here, we summarize the metabolites driven by gut microbiota in COPD (Figure 4), which provides innovative ideas for the subsequent exploration of the pathogenesis of COPD and the development of clinical diagnosis and treatment strategies.

Figure 4.

Effects and mechanisms of gut microbiota metabolites on lung health. the content of SCFAs in feces of COPD patients is decreased. ① SCFAs can inhibit the phosphorylation of STAT1 by inhibiting the binding of STAT1 and HDAC3, and promote the transformation of pulmonary macrophages into the anti-inflammatory M2 phenotype; ② SCFAs can alleviate lung inflammation by inhibiting the recruitment and activation of mast cells and eosinophils mediated by Th9 cells; ③ SCFAs can inhibit the transition of ILC2 to inflammatory ILC2, thereby alleviating lung inflammation. The increased plasma TMAO levels in COPD patients are associated with all-cause mortality. TMAO stimulates bone-marrow-derived macrophages to secrete chemokines and inflammatory factors, leading to excessive proliferation and migration of pulmonary vascular smooth muscle cells, which is a potential mechanism of pulmonary vascular remodeling. STAT1, signal transducer and activator of transcription 1; HDAC3, histone deacetylase 3; ILC2, group 2 innate lymphoid cells; TMAO, trimethylamine N-oxide.

SCFAs

SCFAs are the most intensively studied bacterial metabolites in the gastrointestinal tract, which are mainly derived from the microbial fermentation of indigestible foods in the gastrointestinal tract. The effects of microbiota-derived SCFAs are not only limited to the gastrointestinal tract, but also disseminate into the bloodstream and then circulate to various organs throughout the body, including the maintenance of healthy homeostasis in the lung.¹⁰⁵ SCFAs are essential for host health homeostasis and immune regulation. The regulation of host immunity by SCFAs is mainly through binding to GPCRs, which in addition couple to different downstream effector molecules, leading to different outcomes of cell functions and responses in different cell types.^106,107 The pleiotropic functions of SCFAs in the gut have been well studied,^108–111 but the question of how commensal-derived SCFAs link the gut to the lung is only beginning to be revealed.¹¹² There is no doubt that SCFAs have a good protective effect on lung function. A positive correlation between higher fecal acetate levels and forced expiratory volume in first second was observed in patients with emphysema, and exogenous acetate supplementation reduced alveolar destruction and proinflammatory cytokine production in mice model of emphysema.¹¹³ One study found that compared to healthy controls, COPD patients showed a Prevotella-dominated gut type and lower SCFAs in feces, including acetic acid, isobutyric acid and isovaleric acid, and that the severity of COPD patients was associated with reduced SCFAs concentrations in feces.⁶⁴ This could be an adverse subsequent events of gut microbiota disturbance, which is again demonstrated by antibiotic-induced gut microbiota imbalance leading to SCFAs reduction and aggravating the development of emphysema in mice.¹¹⁴ Another study found that gavage of 3 × 10⁸ CFU/mL acetate-producing Bifidobacterium longum subsp. longum was found to alleviate lung inflammation and butyrate depletion in the cecum of mice in a COPD model induced by 8 weeks of cigarette smoke exposure.⁸⁸ Gut microbiota-derived SCFAs could directly or indirectly regulate the immune homeostasis of the lung, thereby alleviating the development of COPD.^115–117 One study showed that bacterial pneumonia caused by methicillin-resistant Staphylococcus aureus (MRSA) infection in mice resulted in a significant decrease in the abundance of butyrate producer unclassified_f__Lachnospiraceae in the gut, as well as a significant decrease in the levels of butyrate in gut and serum, while exogenous butyrate supplementation decreased the binding of histone deacetylase 3 (HDAC3) to signal transducer and activator of transcription 1 (STAT1), inhibited the phosphorylation of STAT1, and thus altered the polarization of lung macrophages toward the anti-inflammatory M2 phenotype. Ultimately, the colonization of the lung by MRSA is reduced and the lung tissue morphology is improved.¹¹⁸ Th9 cells can promote allergic lung inflammation by recruiting and activating eosinophils and mast cells. One study observed that supplementing mice with butyrate alleviated lung inflammation and mucus production in ovalbumin (OVA)-induced type 2 airway inflammation and hypersensitivity mice, while adoptive transfer of Th9 cells restored lung inflammation, suggesting that butyrate may inhibit lung inflammation by suppressing Th9-mediated immune responses.¹¹⁹ In a model of acute exacerbation COPD (AECOPD) induced by cigarette smoke and LPS, the amount of inflammatory ILC2 cells in the lungs and colon of mice was significantly increased, and the contents of SCFAs in the gut was significantly decreased. Butyrate treatment reduced the proportion of inflammatory ILC2 cells in the colon and lung tissue of AECOPD mice and inhibited inflammatory ILC2 cells phenotypes and the secretion of inflammatory factors from the colon and lung of normal mice.¹²⁰ Propionate and butyrate can also alleviate the inflammatory response in COPD patients by inhibiting the activity of histone deacetylase (HDAC) and regulating the generation of Treg.¹¹⁵ In addition, SCFAs, as an important energy source of colonic epithelial cells, can enhance the gut barrier function, thereby effectively inhibiting the spread of pathogens and endotoxin in the gut lumen.^26–28 There is no doubt that SCFAs are closely related to the occurrence and development of COPD, but further studies are needed to clarify the mechanism.

TMAO

TMAO is a related metabolite of the gut microbiota. Nearly a decade ago, TMAO was thought to predict the risk of cardiovascular disease.¹²¹ In recent years, its relationship with lung disease has gradually been revealed. Some bacteria in the gut microbiota can convert choline from food into trimethylamine (TMA), which can be circulated through the portal vein to the liver and produced by flavin-containing monooxygenases (FMOs) to TMAO.¹²² TMAO is produced in the liver and transported through the bloodstream to various organs in the body, including the heart and lungs.¹²³ Results of a prospective follow-up study showed that elevated circulating TMAO was associated with all-cause mortality in patients with COPD. And the patients who died had a significantly higher median admission TMAO compared to the survivors.²⁴ TMAO can lead to inflammation, resulting in structural damage to lung blood vessels. Studies have shown that the increase of TMAO concentration in the blood can lead to damage of vascular endothelial function, induce oxidative stress, and eventually lead to vascular inflammation.^124,125 One study found that TMAO significantly increased the expression levels of Kng1, Cxcl1, Cxcl2, Cxcl6, and IL-6 in bone-marrow-derived macrophages. In addition, the macrophage conditioned medium after TMAO treatment increased the excessive proliferation and migration of pulmonary artery smooth muscle cells, but the treatment of TMAO itself did not have this effect, suggesting that TMAO may lead to pulmonary vascular remodeling by stimulating the production of chemokines and cytokines by macrophages.¹²⁶ Compared with SCFAs, our knowledge of TMAO in the pathogenesis of lung diseases such as COPD is still insufficient, which suggests that we should pay more attention to TMAO in order to comprehensively and deeply explain the adverse effects and molecular mechanism of TMAO on COPD.

Other metabolites

In addition to SCFAs and TMAO, other gut microbiota-dependent metabolites have been less explored in COPD. A study found that Pediococcus pentosaceus SMM914 synthesizes L-tryptophanamide, 5-hydroxy-L-tryptophane, and 3-sulfino-L-alanine to activate the tryptophane-melatonin pathway, increasing the amount of 6-hydroxymelatonin and hypotaurine in the lungs, which reduced the polarization of macrophages and enhanced the anti-inflammatory and antioxidant capacity of COPD mice.⁶⁵ Hypotaurine is the precursor of taurine, and it can also show excellent antioxidant properties under physiological conditions.¹²⁷ It has been found that Bacteroides uniformis in gut microbiota can promote the production of taurine, thereby inhibiting the activation of NF-κB and IL-17 signaling pathway in neutrophils, and ultimately controlling the inflammatory response of pancreatitis.¹²⁸ However, there is a lack of research about the effect and mechanism of taurine on lung related diseases. Currently, little is known about the impact of gut microbiota metabolites on lung diseases such as COPD, so it is necessary to further reveal the relationship between gut microbiota metabolites and the pathogenesis of COPD.

Gut virome and COPD

The virus content in human feces reaches 10⁹-10¹⁰ virus-like particles (VLPs)/g, which is an important component of gut microbiota with high abundance, temporal persistence and remarkable diversity.¹²⁹ The human gut virome is less characterized than the gut bacteriome and is often referred to as the “dark matter” in the human gut microbiota.¹³⁰ Recent studies suggest that gut virome may play an important role in the ecological processes of the gut microbiota and further influence human health and disease. At present, specific changes of gut virome have been found in IBD, diabetes, hypertension and other diseases.¹³¹ Bacteriophages have shown promising therapeutic potential in a variety of diseases by precisely regulating the composition of gut microbiota.^132–134 Thus, improving our understanding of the gut virome is critical to reveal the complex interplay between the gut microbiota and human health.

The cross-kingdom interactions between bacteriophages and bacteria and between viruses/bacteriophages and the host immune system are the main ways in which the human gut virome regulates the host.¹³¹ Bacteriophages, as natural parasites of bacteria, can change the composition of bacteria in the gut and regulate the metabolism of bacteria in the gut.¹³⁵ The ability of bacteriophages to shape gut bacterial communities depends largely on their life cycle (including lytic, temperate/lysogenic, pseudolysogenic, and bacterial budding life cycles). Lytic bacteriophages inject their genomes into bacterial cells to produce a large number of viral particles, resulting in bacterial death, which can effectively regulate the composition of the gut bacteriome.¹³⁶ In addition, the large number of viruses/bacteriophages present in the mucosal layer of the gastrointestinal tract is essential for the establishment and regulation of host innate and adaptive immunity. Variable Ig-like proteins of a set of bacteriophages capsids have been reported to bind to mucin glycoproteins on the host gut mucous layer through the interaction between Ig-like proteins and mucin-displayed glycans.¹³⁷ Bacteriophages that adhere to the mucous layer of the gut can help build an innate immune barrier by providing an antimicrobial upfront defense against pathogens in the gut.¹³⁵ In addition, bacteriophages can interact with the human immune system to maintain host immune homeostasis and influence disease progression. For example, there was increased infiltration of interferon gamma (IFN-γ)-producing CD4+ and CD8+ T cells in the gut mucosa of germ-free mice fed with E. coli bacteriophage or T4 bacteriophage. In a mice model of UC induced by dextran sodium sulfate, E. coli bacteriophages can aggravate UC by activating TLR9 and IFN-γ-dependent pathways,¹³⁸ which suggests that bacteriophages play an important role in the pathogenesis of UC and may well be potential therapeutic targets.

In addition to gut diseases, the relationship between extra-intestinal diseases and gut virome has also been gradually revealed. A study analyzed the gut virome of 674 children aged 1 year from the Copenhagen Prospective Studies on Asthma in Childhood₂₀₁₀ (COPSAC2010) mother – child cohort and conducted a longitudinal assessment of asthma diagnoses. The results showed significant differences in the relative abundance of caudoviruses and microviruses between children with and without asthma before age 5. In addition, the relative abundance of bacteriophage life cycle was shown to be associated with asthma.¹³⁹ Another study that conducted deep metagenomics sequencing of fecal virus-like particles in 92 subjects from China found that the abundance and diversity of gut virome were lower in COPD patients compared to healthy individuals. Among them, the abundance of Clostridium phage, Myoviridae sp., and Synechococcus phage decreased significantly in COPD patients, and they were significantly positively correlated with pulmonary ventilation function. In addition, virus-bacterial interactions were weakened in COPD patients compared to healthy individuals.¹⁴⁰ These results suggest that the gut virome plays an important role in the development of COPD and other lung diseases. However, the mechanism of action of the gut virome in COPD is still unclear, which suggests that we should still pay more attention to the point in the future.

Gut mycobiome and COPD

Fungi, as eukaryotes, have a long history of life origin. For a long time, because of the public impression of fungal infections, human fungi play a negative role in the vast majority of cases. In addition, because gut fungi account for less than 0.1% of the total microbial cells in the gut microecosystem, they have been neglected as a research area.¹⁴¹ Thanks to the continuous progress and development of deep sequencing technology, more and more fungal species in the human gut have been paid attention to by researchers, including key fungal species that are critical to human health or have an important impact on the development of diseases.¹⁴² For example, candida spp can obtain available nutrients, secondary metabolites, and influence the production of antimicrobial peptides through cell contact, competition, or cooperation, with complex effects on the composition and function of gut bacteria.^143–146

Over the past few decades, researchers have studied the gut mycobiome genome by targeting fungal 18S rRNA gene, or ribosomal DNA (rDNA) internal transcribed spacer (ITS) regions, which has greatly advanced our understanding of the human gut mycobiome genome.¹⁴⁷ For example, 18S rRNA and 18S rDNA sequencing techniques observed an increase in gut fungal diversity in CD patients. ITS2 sequencing results showed that the ratio of Basidiomycota/Ascomycota increased, the proportion of Candida albicans increased, and the level of Saccharomyces cerevisiae decreased in IBD patients.¹⁴⁸ The gut mycobiome were characterized by ITS2 sequencing of CD patients and their non-CD first-degree relatives from 9 familial clusters living in northern France-Belgium, as well as healthy individuals from 4 families living in the same region. The results showed an increase in the total fungal load and a higher prevalence of Cystofilobasidiaceae family and C. glabrata species during CD. In addition, the abundance of Candida tropicalis increased in patients with CD compared to non-CD relatives.¹⁴⁹ Further animal studies have shown that long-term oral antifungals aggravate the severity of UC leading to gut mycobiome dysregulation, characterized by Candida spp. decreased and Aspergillus, Wallemia and Epicoccum spp. increase.¹⁵⁰ In addition to gastrointestinal diseases, the gut mycobiome is also closely related to the occurrence of diseases of extra-intestinal organs. One study found that patients with alcoholic hepatitis had a low diversity of gut mycobiome and an overgrowth of Candida spp.. In addition, the increase of serum anti-Saccharomyces cerevisiae antibodies was associated with increased mortality in patients with alcoholic hepatitis.¹⁵¹ In mice, chronic alcohol management was found to increase the number of fungi in the gut as well as the transport of fungal β-glucan into the systemic circulation, β-glucan can induce liver inflammation via the C-type lectin-like receptor CLEC7A on Kupffer cells.¹⁵² The gut mycobiome is also closely related to the occurrence and development of lung-related diseases. Antibiotic-induced disruption of the gut microbiota in mice leads to overgrowth of Candida albicans in the gut and T-cell-dependent airway inflammation, but the mechanism was not elucidated in this study.¹⁵³ Another study delves deeper into this mechanism, the results show that Candida albicans overproliferates in antibiotic-treated mice, and that Candida albicans can promote M2 macrophage polarization by synthesizing prostaglandin E-2 (PGE₂), thereby exacerbating allergic airway inflammation in mice.¹⁵⁴ Oral fluconazole (0.5 mg/mL) was used to clear the Candida albicans of mice in gut, followed by oral Candida albicans to establish a mouse model of Candida albicans overgrowth. Compared with the control group (oral fluconazole only and no Candida albicans treatment), oral administration of Candida albicans can accelerate the progression of bleomycin-induced pulmonary fibrosis. IL-17A neutralizing antibody treatment slowed the severity of Candida albicans accelerated pulmonary fibrosis, but this was not observed in the control group. This suggests that IL-17A may be involved in pulmonary fibrosis aggravated by Candida albicans.¹⁵⁵ However, while reducing the abundance of Candida albicans, fluconazole could also increase the abundance of Aspergillus amstelodami, Epicoccum nigrum, and Wallemia mellicola. Single intragastric administration of live conidia from these three fungi or conidia from Wallemia mellicola alone to mice with allergic airway disease was found to increase the severity of the disease.^150,156 In addition, Candida albicans can increase airway inflammation in mice and humans by activating the Th17 response.^157,158 The expansion of Th17 cells leads to an increased chronic immune response against Aspergillus fumigatus in the airways of patients with inflammatory respiratory diseases, including asthma and COPD.¹⁵⁷ However, the role of gut mycobiome in the occurrence and development of COPD remains unclear.

Although advances in deep sequencing-based technologies have led to our understanding of the role of gut mycobiome in the gut and extra-intestinal organs, this understanding is still very limited compared to our understanding of the gut bacterial community. A major reason for the lack of understanding of the role of gut mycobiome in disease pathogenesis is the limited availability of reference genomes. Although the Unified Human Gastrointestinal Genome (UHGG) has collected more than 230,000 gut microbial genomes from more than 5,400 species,¹⁵⁹ these resources lack information on gut fungi. Although some measures have been taken to culture gut fungi and construct fungal genomes from fecal metagenomes,^160,161 these measures only cover a small fraction of gut fungi. The limited availability of reference genomes leads to inaccurate quantification of gut fungal species in high-throughput sequencing concentrations, which greatly limit our understanding and knowledge of this important part of the gut mycobiome. A recent study established a cultivated gut fungi (CGF) catalog, which included 760 fungal genomes from 206 species obtained from stool samples of healthy individuals, including 69 species previously unidentified. In addition, the 7,660,447 protein-coding genes encoded by the CGF genome were organized into 643,717 non-redundant protein clusters, 77% of which were not present in respiratory/digestive tract fungi in the human-associated fungi (PHF) catalog, indicating a 247% increase in human intestinal fungal protein sequences.¹⁶² The establishment of the CGF genome greatly expands the genomic library of gut mycobiome, provides new insights into the biological significance of the human gut fungal group, and contributes a valuable resource for exploring the composition and functional diversity of the human gut microbiota and the role that gut fungal groups play in human health and disease.

Effects of COPD medication on gut microbiota

The interaction between gut microbiota and drugs is complex and bidirectional. The use of drugs, especially antibiotics, has a strong effect on the composition and function of the gut microbiota. Vice versa, the gut microbiota can also influence an individual’s response to a drug by enzymatically altering its structure, bioavailability, bioactivity, or toxicity. For patients with COPD, antibiotics, corticosteroids and anticholinergic are the main medication, and there is an interaction between these drugs and gut microbiota.

Several important antibiotics, including penicillins, cephalosporins and tetracycline, are clinically used in the treatment of COPD. A randomized controlled trial comparing azithromycin, clarithromycin, erythromycin, roxithromycin and moxifloxacin in the treatment of COPD, the results showed that azithromycin and erythromycin were the most effective in COPD patients.¹⁶³ Some reports also reflects a negative effect of antibiotic treatment. For example, a course of antibiotics lasting 7 days or longer was associated with more overall adverse events than a shorter course of antibiotics (shorter than 6 days).¹⁶⁴ In a study of smoking-induced emphysema models in mice, antibiotic treatment was found to worsen emphysema by causing disruption to the gut microbiota, showing higher IL-6 and IFN-γ levels and an increase in inflammatory cells in bronchial lavage fluid.¹¹⁴ At present, the main concern about antibiotic is the spread of antibiotic-resistant bacteria and disruption of the gut microbiota. In one study, COPD patients treated with long-term antibiotics (median duration of antibiotic treatment was 40 days) had significantly higher abundance of antibiotic resistance genes (ARGs) compared to healthy individuals. The main categories are aminoglycoside, beta-lactam, and macrolide-lincosamide-streptogramin resistance genes and the abundances of Enterococcus and Escherichia were positively correlated with ARGs and antibiotic treatment days.¹⁶⁵ After 30 weeks of antibiotic treatment in nude mice, Klebsiella was found to grow and become one of the most common microbes.¹⁶⁶ These microorganisms can be potentially harmful to the health of the host. We cannot deny the great role of antibiotics in the treatment of COPD, but we cannot ignore the potential risks of antibiotics. The effect of antibiotic use may be related to the treatment dose, route and duration, so we should use antibiotics more cautiously in the treatment of COPD patients.

Anticholinergic and steroids are considered to be the main therapeutic agents for COPD, and can be used individually or in combination in clinical practice.¹⁶⁷ But these drugs affect the composition and function of the gut microbiota. A case-control study observed that anticholinergic was associated with a decrease in peripheral blood butyric acid concentration.¹⁶⁸ One study conducted an association analysis of 51 drugs in the TwinsUK cohort and their relationship to gut microbiota. The results showed that inhaled anticholinergic were negatively correlated with the abundance and α diversity of Ruminococcaceae and Peptococcaceae, which suggest that non-oral medications may indirectly affect the gut microbiota.¹⁶⁹ A retrospective study found that patients who used inhaled corticosteroids prior to an acute exacerbation of COPD had a significantly elevated risk of developing bacterial lung infections, and patients who used systemic corticosteroids had a higher risk of developing bacterial lung infections than those who used inhaled corticosteroids.¹⁷⁰ Prenatal corticosteroid therapy is an effective treatment to reduce the incidence of neonatal respiratory distress syndrome by promoting fetal lung maturation.¹⁷¹ However, a recent study found that prenatal dexamethasone exposure (PDE) can lead to increased susceptibility to various diseases in adulthood, possibly due to the effect of PDE on the composition of the gut microbiota. PDE led to a decrease in the abundance of 13 bacteria genera including Akkermansia, Bacteroides and Parabacteroides, and an increase in the relative abundance of 11 bacteria genera including Klebsiella in the gut of girls at 6 months of age. The abundance of 8 species including Parabacteroides, Lachnoclostridium and Bacteroides decreased in male infants, while the abundance of 9 species including Atopobium increased. PDE was also found in the offspring of rats to lead to a decreased abundance of Lactobacillus reuteri and an increased abundance of Klebsiella pneumonia 6 weeks after birth, while cholestatic liver injury was observed. Fecal microbiota transplantation (FMT) results showed that rats receiving fecal bacteria from the offspring of PDE also exhibited cholestatic liver injury, suggesting that PDE-induced changes in the offspring gut microbiota may increase their susceptibility to cholestatic liver injury.¹⁷² Notably, increased Klebsiella abundance in the gut was observed in both rats and humans in this study, and Klebsiella infection plays an important role in COPD and COPD exacerbation.^173,174 In addition, gut microbiota also has an impact on drug metabolism. The hepatic cytochrome P450 3A (CYP3A) is a major corticosteroid metabolizing enzyme in the liver, which is closely related to the gut microbiota.^175,176 In particular, the human gut microbiota has a significant influence on the activity of CYP3A through the regulation of Cyp3a11 gene expression.¹⁷⁷ Therefore, differences in gut microbiota between different individuals may affect the metabolism of drugs such as corticosteroids by affecting host CYP3A, and ultimately lead to differences in drug efficacy between different individuals.

These findings suggest that the interaction between the gut microbiota and drugs is complex and bidirectional: the use of drugs can affect the composition and function of the gut microbiota, and thus the health of the body. Conversely, the gut microbiota can also influence drug metabolism, resulting in different individuals showing different responses to specific drugs. All in all, uncovering the complex interactions between the gut microbiota and drugs is not an easy task, and each drug seems to interact with the gut microbiota in a unique way, making it difficult to come up with a common mechanism. Therefore, understanding how the gut microbiota metabolizes drugs and influences the effectiveness of medications will open up the possibility of regulating the gut microbiota to improve medications.

Gut microbiota risk factors for COPD

Smoking

Smoking is a detrimental habit that is prevalent all over the world. Epidemiological studies have shown that smoking is a major risk factor for COPD and gastrointestinal diseases.¹⁷⁸ Smoking cessation can effectively halt the advancement of COPD. A study using the Korean National Health Insurance Service (NHIS) database showed that patients who quit smoking within two years of their COPD diagnosis had a 17% lower risk of all-cause mortality and a 44% lower risk of cardiovascular death than COPD patients who continued to smoke.¹⁷⁹ Another multicentre cohort study using the Korean COPD Subgroup Study (KOCOSS) database, which recruited patients from 54 medical centers in Korea, compared the clinical characteristics of smoking COPD patients with nonsmoking COPD and showed that smoking COPD patients had a higher incidence of emphysema according to the radiological findings, and over 5 years of follow-up data, smoking patients with COPD have a more rapid decline in lung function.¹⁸⁰ In addition, the effects of smoking behavior on COPD differ by sex, with female smokers having a higher risk of COPD than male smokers.¹⁸¹

However, recent research suggests that the imbalance in gut microbiota caused by smoking plays a key role in this process. Various components in smoke, such as nicotine, interact with the gut microbiota in the gut, resulting in adverse effects on the health of the host.^85,182 Numerous studies have shown that smoking can remodels the gut microbiota. For example, compared with nonsmokers, the abundance of Prevotella, Veillonella, Bacteroides, Acidaminococcus and Oscillospira increased in the fecal of smokers and the abundance of Firmicutes and Proteobacteria was reduced.^183,184 Another study in healthy males showed that smoking led to a high abundance of Actinobacteria and a low abundance of Bacteroides, and an increased proportion of Gram-negative bacteria in the gut.¹⁸⁵ LPS is an important component of the outer membrane of most Gram-negative bacteria, as well as an important virulence factor, which can interact with the host pattern recognition receptors (such as toll-like receptors, TLRs) to produce immune inflammatory effects in the human body. When LPS activates TLR4, it causes a series of intracellular signal transmission that releases inflammatory factors such as IL-1β and TNF-α, ultimately leading to COPD,^186,187 and LPS can also activate neutrophil-derived extracellular vesicles to induce COPD-like disease.¹⁸⁸ In addition, smoking causes a decrease in the amount of SCFAs in the gut, which may be related to a decrease in the abundance of SCFAs producing bacteria.¹⁸⁴ It has been found that SCFAs content and population of Bifidobacterium in cecum of rats exposed to cigarette smoke for 4 weeks are significantly reduced, and the pH of cecum contents is increased.⁸⁵ In mice, the intragastric administration of cigarette smoke condensate inhibited the production of antimicrobial peptides and reduced bactericidal capacity, resulting in a dysregulation of gut microbiota, an increase in the abundance of Erysipellaceae, and a decrease in the abundance of Rikenellaceae and Eisenbergiella.¹⁸⁹ These results suggest that smoking may alter the host gut environment, thereby affecting the composition of the gut microbiota.

Due to the difficulty of sample collection, most current studies analyze the composition of gut microbiota by detecting stool samples of subjects, which does not reflect the authentic gut microbiota. The gut is not a uniform mix of species and environments, and different regions and microenvironments in the gut have unique physical and biochemical conditions, which leads to differences in the composition of the gut microbiota at different spatial scales.¹⁹⁰ The results of duodenal mucosal microbiome analysis of humans undergoing upper gastrointestinal endoscopy showed that Streptococcus and Veillonella were more abundant in smokers, while Neisseria and Prevotella were less abundant.¹⁹¹ Another study found that compared with nonsmokers, smokers had a relatively high abundance of Enterobacteriaceae, mainly Escherichia-Shigella and Klebsiella, and the abundance of Escherichia-Shigella was 20.51-fold higher than that of nonsmokers.¹⁹² Streptococcus, as an opportunistic pathogen, is associated with decreased lung function and can lead to the deterioration of COPD.¹⁹³ In addition, in a prospective cohort study, Klebsiella was detected in sputum culture in both stable COPD and AECOPD patients, and the isolation rate of Klebsiella was higher in acute exacerbation than in stable COPD patients, Klebsiella is a common clinical pathogen of pulmonary inflammation,¹⁷⁴ which suggested that Klebsiella plays a pathogenic role in acute exacerbation of COPD.¹⁷³ In conclusion, smoking may affect the composition of gut microbiota (such as excessive proliferation of pathogenic bacteria) by changing the gut microenvironment, thereby regulating the host immune inflammatory response and the production of SCFAs, and ultimately leading to the deterioration of COPD.

Diet

In the past few years, studies have found that dietary patterns have an important impact on the development of COPD.¹⁹⁴ Insufficient intake of fiber and other nutrients is related to the occurrence and development of COPD.¹⁹⁵ A meta-analysis showed that the risk of COPD was significantly increased in Western dietary patterns (high intake of red or processed meat, refined grains, saturated fatty acids, and sweets).¹⁹⁶ On the other hand, the Mediterranean diet pattern (characterized by a high intake of vegetables, fruits, and whole grains) helps maintain lung function.^197,198 Epidemiological studies from cross-sectional and longitudinal studies consistently show that high intake of fruits, vegetables, whole grains, and dietary fiber is negatively associated with the risk of COPD.^199–207 In addition, a meta-analysis showed that every 50 g/week increase in processed red meat intake was associated with an 8% increase in COPD risk.²⁰⁸ The Western diet pattern is rich in choline, which is converted by the gut microbiota into TMA, which is converted in the liver to form TMAO.^121,209 TMAO is associated with all-cause mortality in patients with COPD. After 6 years of follow-up in 189 patients with COPD exacerbations, all-cause mortality was 55.6%, and patients who died showed higher circulating TMAO levels compared to those who survived.²⁴ In animal studies, it has been found that long-term Western diet can cause the mitochondrial activity of mouse intestinal epithelial cells to decline, which further destroys the hypoxia of the gut environment. The increase of oxygen creates favorable conditions for the proliferation of E. coli, and ultimately strengthens the respiration-dependent choline catabolism of E. coli, which also increases circulating TMAO levels.²¹⁰ This may be related to the increase in E. coli abundance caused by the Western diet disrupting the hypoxic state of the gut. A healthy gut is rich in SCFAs producing bacteria such as Prevotella, Bacteroides, Ruminococcaceae and Lachnospiraceae. These SCFAs help maintain an anaerobic environment in the gut.²¹¹ However, long-term Western diet feeding can inhibit the proliferation of SCFAs producing bacteria such as Bifidobacterium, Akkermansia and Bacteroides, resulting in the decrease of SCFAs content,^212,213 which may be related to the disruption of intestinal hypoxia caused by long-term Western diet. COPD III-IV patients had lower levels of SCFAs in stools compared to COPD I-II patients and healthy volunteers.⁶⁴ In addition, patients with digestive disorders have lower levels of SCFAs in plasma and stool and are more likely to develop lung disease.¹⁰⁵ In general, the long-term Western diet pattern will lead to the destruction of gut hypoxia, resulting in the proliferation of aerobic bacteria and facultative anaerobic bacteria, thus destroying the composition of gut microbiota, resulting in the inhibition of SCFAs production. In addition, choline, which is rich in the Western diet, is metabolized into TMA by the gut microbiota and enters the liver, thereby increasing the level of circulating TMAO and increasing the risk of COPD.

Most patients with severe COPD also present with thin body mass and often in a state of malnutrition or undernutrition, termed “pulmonary cachexia syndrome” (PCS), which is mainly characterized by muscle wasting. Muscle wasting in COPD patients not only leads to decreased exercise capacity, but also is a major mortality factor independent of airflow obstruction.^214,215 Nutritional supplementation therapy can effectively maintain and improve muscle and exercise capacity in patients with COPD. It has been shown that a low-carbohydrate diet can significantly improve lung function in patients with COPD compared to a conventional high-carbohydrate diet.²¹⁶ Fruits and vegetables are rich in antioxidants, minerals, vitamins, flavonoids, and fiber, which are beneficial for acute and chronic respiratory diseases.²¹⁷ Vitamin D deficiency is associated with the early development of COPD, which may be related to the immunomodulatory effect of vitamin D, while vitamin D also alleviates the myopathy/muscle weakness.^218,219 One study found that colonization of the gut by a protective Escherichia coli strain during pneumonia can alter signaling of the insulin-like growth factor 1/phosphatidylinositol 3-kinase/AKT pathway without affecting host metabolism and energy intake, which is required for the prevention of muscle wasting.²²⁰ This finding suggests that gut microbiota may play a role in muscle wasting caused by cachexia. Nutrient intake is an important factor affecting the composition of gut microbiota. For example, low serum vitamin D level can lead to a decrease in the diversity of gut microbiota and the abundance of Bacteroides, Prevotella and Clostridiales.²²¹ However, we still lack a systematic understanding of how nutrient intake affects cachexia induced muscle wasting by influencing gut microbiota composition.

Aging

COPD is a disease associated with aging, and older individuals are at higher risk of developing it than younger individuals. As the world populations ages, about 20% of the world’s population is expected to be over the age of 65 years old by 2050, and 80% of these older people will live in low- and middle-income countries.²²² The susceptibility of older people to multiple diseases, including COPD, will pose a significant public health challenge. A cross-sectional study comparing disease severity in older and younger COPD patients found that older COPD patients, especially those 75 years and older, had significantly increased rates of dyspnea, exercise tolerance, and severe exacerbations compared to younger people. The prevalence of comorbidities, including cardiovascular disease and cancer, was significantly higher in older COPD patients.²²³ Many lung diseases in the elderly are associated with changes in the gut microbiota.^224,225 People over 65 years old showed greater individual differences in their gut microbiota compared to younger people,²²⁶ but numerous studies consistently show that a decline in the abundance of probiotics like Bifidobacterium and Lactobacillus in the gut of the elderly, while the abundance of certain pathogenic bacteria, like Clostridium difficile and Enterobacteriaceae, has increased.^227–230 Colonization of the lungs by Enterobacteriaceae was observed during acute lung injury, possibly as a result of increased permeability of the gut and lung epithelium.²³¹ Numerous studies consistently show that the increased epithelial permeability in both the lung and gut in elderly or senescent animal individuals.^232–234 The “leakage” of epithelial barrier function makes it easier for microbes and microbial metabolites to be transported between the two organs, providing more opportunities for pathogens and endotoxins to enter the lungs through lymph and blood. COPD is closely related to immune imbalance mediated by Th17/Treg ratio.²³⁵ Several mucosal associated bacteria in the gut microbiota (such as E. coli) and LPS can stimulate Tregs and drive the pro-inflammatory Th17 response to exacerbates COPD.^236–238 This enhanced bidirectional communication results in a loss of compartmentalization between the gut and lungs, which may be responsible for the increased COPD susceptibility. As a potential therapeutic target susceptible to intervention, exploring the role of gut microbiota in COPD and its mechanisms in older adults is of great significance to alleviate the global public health burden.

Particulate matter

Traditionally, smoking is widely recognized as the most important risk factor for COPD, but it is now recognized that about one quarter to one third of cases cannot be explained by smoking alone.²³⁹ In a report by the American Thoracic Society, the population attribution ratio of smoking to COPD in most clinical studies was < 80%, indicating that other risk factors besides smoking also play an important role in the development and development of COPD.²⁴⁰ Globally, particulate matter (PM) air pollution is a major threat to environmental governance and public health, and working and living in an environment with high concentrations of particulate matter for long periods of time can have serious health consequences. PM2.5, as an important component of PM, is closely related to the incidence of COPD. One meta-analysis found that every 10 ug/m³ increase in PM2.5 was associated with a 1.4–2.5% increase in COPD hospitalizations.²⁴¹ Studies have found that long-term exposure to PM2.5 can lead to reduced movement of respiratory cilia, increased mucus, and ultimately reduced cilia’s ability to clear the airway. It has been found that PM2.5 can inhibit the expression level of 8-oxyguanine DNA glycosylase 1 (OGG1) in type II alveolar epithelial cells, thereby inhibiting its proliferation and self-renewal.^242,243 In addition, microbiome imbalances caused by PM2.5 are also associated with the development of COPD.²⁴⁴ One study found that for every 10 ug/m³ increase in PM2.5 concentration in the air, α-diversity in COPD patients decreased by 2.16%. Moreover, Clostridium sp. CAG127, Enterococcus faecium, and Ruthenibacterium lactatiformans showed a correlation with PM2.5.²⁴⁵ An animal study showed that exposure to particulate matter for 24 weeks resulted in an inflammatory response in the lungs of rats and pathological changes characteristic of COPD. At the same time, the gut microbiota diversity of rats decreased, the serum endotoxin content increased, and the SCFAs content decreased.²⁴⁶ Another study showed that low levels of air pollution could increase the level of Fusobacteria and Verrucomicrobia in the lungs of rats, increase the abundance of Bacteroidetes and Proteobacteria and reduce Firmicutes in the gut microbiota.²⁴⁷ There is a symbiotic relationship between the hosts of Fusobacteria, which may lead to opportunistic infection in the host.²⁴⁸ At present, more and more studies have shown that Proteobacteria can play a role in gut diseases (such as IBD) by regulating the production of pro-inflammatory interleukin and immunoglobulin.²⁴⁹ However, in recent years, studies have found that Proteobacteria also plays a role in lung diseases, but the mechanism of action is still unclear.²⁵⁰

Prevention and treatment strategies

Dietary regulation

Prevention and treatment strategies based on nutritional regulation can not only change the innate immunity of the body and reduce systemic inflammatory response, but also be a potential non-pharmacological method without side effects to prevent and treat COPD, respiratory diseases and smoking-related cancers.¹⁸ A prospective study showed that a higher Alternate Healthy Eating Index 2010 (AHEI-2010) (reflecting high intakes of whole grains, polyunsaturated fatty acids, nuts, and long chain omega-3 fats and low intakes of red/processed meats, refined grains, and sugar sweetened drinks) is associated with a lower risk of COPD and is critical in COPD multi-intervention programs.²⁵¹ In a population-based prospective study of men, a strong negative association was found between total fiber intake and grain fiber intake and COPD among smokers and ex-smokers. Vegetable and fruit fiber intake only reduced COPD risk in current smokers.²⁰⁵ Another prospective study of Swiss women found that long-term high dietary fiber intake was associated with a 30% lower risk of COPD. High intakes of grain and fruit fiber were associated with a lower risk of COPD, but not with plant fiber intake.²⁰⁷ A high-fiber diet mainly results in an increase in circulating SCFAs by reducing the Firmicutes/Bacteroides ratio and fiber metabolism in the gut microbiota,²⁵² which can enhance gut barrier function, regulate body immunity, improve lung function and inhibit inflammatory response by the gut-lung axis.²⁵³ Increasing dietary fiber intake, as a nutritional regulation strategy that is easy to intervene and has no side effects, is of great significance for the prevention and alleviation of COPD and the enhancement of lung function.

Probiotics

Probiotics can positively regulate host immunity and pathogen microbiota. The intake of probiotics alone or supplementation with functional foods containing probiotics can reduce oxidative damage, free radical clearance rate, and local or systemic inflammatory response in humans.^254–256 A study have found that oral administration of Bifidobacterium longum can improve the immune response and survival rate of mice infected with Klebsiella pneumoniae.²⁵⁷ The combination of multi strain probiotic significantly reduced plasma C-reactive protein and 8-isoprostane levels, and also alleviated gut barrier function in patients with COPD.²⁵⁸ Lactobacillus rhamnosus can alleviate the balance of pro-inflammatory and anti-inflammatory factors in human bronchial epithelial cells induced by cigarette smoke (CS) exposure, and can alleviate the inflammatory response in the airway and lung parenchyma of CS-induced COPD mice, reduce inflammatory cell infiltration and the production of pro-inflammatory cytokines.²⁵⁹ Supplementation with acetate-producing Bifidobacterium longum subsp. longum alleviated lung inflammation, the expression of inflammatory cytokines and adhesion factors, and attenuated cigarette smoke-induced depletion of cecal butyrate.⁸⁸ A prospective, multi-center, randomized trial showed that long-term oral administration of Lactobacillus rhamnosus GG significantly slowed the progression from moderate to severe COPD.²⁶⁰ The above studies suggest that the development of probiotic strains and their functional components has the potential to become alternative drugs for the prevention or treatment of COPD. However, these mechanisms must be confirmed in large-scale randomized controlled trials before clinical application.

FMT

FMT has systemic effects on the human body and has been shown to have significant therapeutic effects on a variety of diseases.^261–263 One study found that the gut microbiota of mice with COPD induced by cigarette smoke exposure was sufficient to increase lung inflammation in antibiotic-treated mice while suppressing colonic immune responses.² This suggests that gut microbiota is involved in the pathogenesis of COPD. FMT increased the abundance of Bacteroidetes and Lachnospiraceae, which can catabolize fibers into SCFAs, thereby suppressing local and systemic inflammation and attenuating the development of emphysema.²⁵² The disease characteristics of COPD were correlated with the relative abundance of members of the Muribaculaceae, Desulfovibrionaceae and Lachnospiraceae family members, the microbiota from CS-induced COPD was associated with down-regulated glucose and starch metabolism. The disease outcomes can be improved with complex carbohydrate supplementation.² In conclusion, gut microbiota may be a potential therapeutic target for COPD, but there are no clinical studies of FMT in the treatment of COPD. In the future, more animal experiments and clinical trials are needed to explore the effect and mechanism of FMT in the treatment of COPD before large-scale clinical application.

Future perspectives

With the in-depth understanding of the “gut – lung axis”, the bidirectional crosstalk between the gut and the lung has been gradually revealed, indicating that the gut microenvironment and the homeostasis of gut microbiota play an important role in maintaining lung immune homeostasis. Therefore, the development of COPD prevention and treatment strategies targeting gut microbiota can be considered in future studies. The disruption of the gut microenvironment (including impairment of gut barrier function, gut inflammation, and oxidative stress) and disruption of gut microbiota may influence the degree of lung inflammation and immune homeostasis through the “gut-lung axis”. As described earlier, the presence of gut-associated bacteria in the lungs was observed during acute lung injury, suggesting that there is also a migration mechanism between the gut and lungs. However, this mechanism still needs to be further understood to better elucidate the interaction mechanism between gut microbiota and lung microbiota and to provide possible targets for prevention and treatment of COPD to improve the clinical outcome of patients with COPD. The use of fluorescently labeled bacteria could provide more solid evidence for the hypothesis of gut origin of lung bacteria.²⁶⁴ Up to now, the role and mechanism of gut microbiota in regulating local and systemic immune responses in a variety of diseases have been extensively and deeply studied, but the research in COPD is still limited to correlation studies at the phylum and genus levels. In future studies, it is necessary to use multi-omics combined analysis to further reveal the bacteria that play a key regulatory role in the occurrence of COPD and the target of gut microbiota on the host immune response and metabolic pathway, so as to provide a solid theoretical basis for the clinical prognosis of COPD patients. Metagenomics could be used to more accurately locate the key species and functional genes in the gut microbiota, so as to provide accurate diagnosis and treatment strategy for COPD patients.

In addition, the medication has brought light to COPD patients, but many studies have also observed its negative effects on gut microbiota, which leads to increased susceptibility of COPD patients to other adverse events, such as lung infection and emphysema. Due to the diversity of the interaction mechanisms between drugs and gut microbiota, it is difficult to conclude the common mechanism of the action of different drugs and gut microbiota. Therefore, we should pay more attention to carry out large-scale clinical cohort studies to systematically study the interaction between common drugs and gut microbiota in the future research, so as to provide a reliable theoretical basis for establishing a more scientific, standardized and rigorous clinical medication strategy for COPD patients.

Abbreviations

COPD

Chronic obstructive pulmonary disease

IBD

inflammatory bowel disease

SCFAs

short-chain fatty acids

TMAO

trimethylamine N-oxide

TMA

trimethylamine

sIgA

secreted immunoglobulin A

Treg

regulatory T cells

LPS

Lipopolysaccharide

CD

Crohn’s disease

UC

ulcerative colitis

IBS

irritable bowel syndrome

NOD2

Nucleotide-binding oligomerization domain containing 2

NK

natural killer

CRP

C-reactive protein

ILC2

Group 2 innate lymphoid cells

HDAC3

histone deacetylase 3

STAT1

signal transducer and activator of transcription 1

AECOPD

acute exacerbation COPD

ARGs

antibiotic resistance genes

FMT

Fecal microbiota transplantation

CYP3A

cytochrome P450 3A

PM

particulate matter

CS

cigarette smoke

Funding Statement

The work was supported by the National Natural Science Foundation of China [32072746]; Innovation Foundation for Doctor Dissertation of Northwestern Polytechnical University [CX2023021].

Disclosure statement

No potential conflict of interest was reported by the author(s).