Can intestinal microbiota and circulating microbial products contribute to pulmonary arterial hypertension?

Abstract

Pulmonary arterial hypertension (PAH) is a fatal disease with a median survival of only 5–7 yr. PAH is characterized by remodeling of the pulmonary vasculature causing reduced pulmonary arterial compliance (PAC) and increased pulmonary vascular resistance (PVR), ultimately resulting in right ventricular failure and death. Better therapies for PAH will require a paradigm shift in our understanding of the early pathophysiology. PAC decreases before there is an increase in the PVR. Unfortunately, present treatment has little effect on PAC. The loss of compliance correlates with extracellular matrix remodeling and fibrosis in the pulmonary vessels, which have been linked to chronic perivascular inflammation and immune dysregulation. However, what initiates the perivascular inflammation and immune dysregulation in PAH is unclear. Alteration of the gut microbiota composition and function underlies the level of immunopathogenic involvement in several diseases, including atherosclerosis, obesity, diabetes mellitus, and depression, among others. In this review, we discuss evidence that raises the possibility of an etiologic role for changes in the gut and circulating microbiome in the initiation of perivascular inflammation in the early pathogenesis of PAH.

INTRODUCTION

Pulmonary arterial hypertension (PAH) continues to be a fatal and costly disease despite progress in understanding the pathogenesis and treatment over the last two decades. Median survival is only 5–7 yr (6, 36). In-hospital mortality continues to be high at 6% (3). Moreover, the economic burden for PAH-related health care costs in the United States is estimated to be approximately $29 billion per year (3). PAH is characterized by remodeling of the pulmonary vasculature causing reduced pulmonary arterial compliance (PAC) and increased pulmonary vascular resistance (PVR), ultimately resulting in right ventricular (RV) failure and death (34, 94). The Food and Drug Administration has approved 11 PAH-specific drug therapies for the treatment of PAH. These target three different pathways: the endothelin pathway, the nitric oxide pathway, and the prostacyclin pathway (28). These PAH-specific therapies are predominantly pulmonary vasodilators, which reduce PVR but have little effect on PAC (95). These drug therapies increase exercise capacity modestly and reduce hospital admission but are expensive and not curative (3, 28). Since 2005, no new therapeutic pathways have been identified for the treatment of PAH (34). Therefore, it is imperative that we reconsider the paradigms in our understanding of PAH and explore additional mechanisms that may be involved in the pathophysiology of this disease.

PAC is a measure of the stiffness of the pulmonary arteries. It represents the pulsatile afterload of the right ventricle, which accounts for approximately one-fourth of the RV afterload. In contrast, PVR is a measure of the steady afterload of the RV, which accounts for the remaining three-fourths of the RV afterload. In PAH, PAC decreases and PVR increases, leading to increased RV afterload. Whereas the decrease in PAC is attributed to the extracellular matrix remodeling of the pulmonary vasculature, the increase in PVR results from the intimal endothelial cell proliferation and medial smooth muscle hypertrophy/proliferation causing narrowing of the luminal cross-sectional area of the vasculature.

There is clear evidence in both preclinical and clinical studies that the decrease in PAC due to extracellular matrix remodeling precedes the increase in PVR due to intimal endothelial cell proliferation and medial smooth muscle hypertrophy. In fact, there is growing evidence that the decrease in PAC due to extracellular matrix remodeling causes intimal endothelial cell proliferation and medial smooth muscle hypertrophy. The extracellular matrix remodeling and fibrosis in the pulmonary vessels, which correlate with loss of compliance, have been linked to chronic perivascular inflammation and immune dysregulation (92). In fact, perivascular inflammation has been described in PAH from the earliest pathological studies (32). Many papers provide evidence of systemic inflammation in patients with PAH (35, 83). However, events and factors that initiate and/or exacerbate the immune dysregulation and perivascular inflammation in PAH remain unknown. Identifying the important factors that initiate and/or exacerbate maladaptive inflammatory responses in PAH will help us to understand the early pathophysiology, to diagnose patients earlier, and to develop novel therapies for PAH. In this review, we discuss evidence stimulating the hypothesis that there is an etiologic role for changes in gut and circulating microbiota in the initiation of perivascular inflammation in the early pathogenesis of PAH.

INFLAMMATION IN PAH

There is ample clinical and experimental evidence indicating a role for inflammation in PAH (63). Abnormalities in both adaptive and innate immunity have been described in PAH. Clinically, PAH is associated with autoimmune diseases, including scleroderma and systemic lupus erythematosus, and also with infectious diseases, such as human immunodeficiency virus and schistosomiasis (79). There is perivascular accumulation of inflammatory cells including macrophages, T cells, and B cells in the pulmonary arteries in patients with PAH (85). PAH mortality is associated with increased levels of circulating cytokines (35, 83). Interestingly, patients with idiopathic PAH have increased serum autoantibody levels such as antinuclear antibodies (66). Lower circulating natural killer cells are associated with poor survival in patients with PAH, suggesting that they have a protective role (23, 55).

In addition to these observations in patients with PAH, which demonstrate an association between inflammation and PAH, several animal experimental studies prove that a “cause-and-effect” relationship between inflammation and PAH is possible. First, PAH can be induced experimentally in animals by exposure to various immune stimuli, including human immunodeficiency virus, schistosomiasis, and IL-6 overexpression (2, 30, 86). Second, depletion of inflammatory macrophages in chronic hypoxic calves and rats prevents remodeling of the pulmonary vascular extracellular matrix and pulmonary hypertension (PH; 26). Third, an imbalance of cluster of differentiation 4 (CD4) helper T (Th) cell subsets occurs in PAH lungs. While there is an increase in Th1, Th2, and Th17 CD4 cells that induces inflammatory responses, the number of regulatory T cells (Tregs) with anti-inflammatory effects is decreased. Athymic rats deficient in T cells and B cells develop more severe pulmonary vascular disease in response to Sugen and hypoxia. Immune reconstitution of athymic rats with Tregs reduces the severity of PH in response to Sugen and hypoxia, suggesting a role for Treg deficiency in the pathogenesis of PAH (89). Moreover, induction of endogenous Tregs attenuates development of pulmonary vascular inflammation and PH in mice following transverse aortic constriction (98). Finally, autoantibodies from monocrotaline-treated rats cause PAH in naïve rats, supporting the importance of abnormal B cells in the pathogenesis of PAH (20). Taken together, these data strongly suggest a major role for inflammation in the pathogenesis of PAH. However, an important unanswered question remains: What initiates and/or exacerbates the immune dysregulation and perivascular inflammation in PAH?

GUT MICROBIOTA AND IMMUNE DYSREGULATION

The gut contains trillions of microorganisms including various bacteria, fungi, viruses, and parasites, collectively called the “gut microbiota.” This microbiota is integral to the host physiology and contributes to many physiological functions, including maintenance of the mucosal barrier in the intestine, systemic immune regulation, energy homeostasis and metabolism, vitamin synthesis and degradation, and neurological development, among others (21). Drugs, diet, exercise, and genetic factors determine the individual gut microbiota composition and function (10, 21, 31, 47, 67, 80).

Alteration in the gut microbiota composition and function associated with a disease process is called “gut dysbiosis.” Changes in the gut microbiome (i.e., genomic composition of the gut microbiota) have been linked to the immunopathogenesis of numerous diseases, such as atherosclerotic vascular disease, obesity, diabetes mellitus (both type 1 and type 2), depression, alcoholic and nonalcoholic steatohepatitis, multiple sclerosis, and chronic lung allograft rejection, among others (9, 17, 21, 40, 41, 50, 51, 74, 102). The gut microbiota influences the host immune system through several different mechanisms. Gut dysbiosis can increase gut permeability (51), which may allow increased bacterial translocation and release of endotoxin into the splanchnic circulation. This in turn triggers activation of innate and adaptive immune elements, including macrophages and T cells, through engagement of microbe-associated molecular pattern receptors (16, 33, 59).

The gut microbiota can impact the host immune system through release of immunomodulatory bacterial metabolites. Some of the most studied such products are short-chain fatty acids (SCFAs), such as butyrate, propionate, and acetate, which result from microbial fermentation of fiber in the diet. These SCFAs have anti-inflammatory activity. They activate or induce Tregs, either through activation of G protein-coupled receptors (GPCRs: GPCR43, GPCR41, and GPR109A) or through epigenetic modifications by inhibiting histone deacetylases (59, 90, 103). Microbial production of SCFAs promotes generation of Tregs in the colon (81). In addition, reduction of SCFA-producing bacteria levels in the gut decreases the circulating levels of SCFAs (71). Notably, butyrate also stimulates peroxisome proliferator-activated receptor-γ (PPARγ), which drives the colonic epithelium toward β-oxidation and maintenance of an anaerobic environment, which favors the obligate anaerobes in the colon (12). In contrast, the absence of butyrate allows increased expression of the inducible nitric oxide synthase and expansion of Proteobacteria, which are facultative anaerobes capable of nitrate respiration and are characteristically more abundant in dysbiotic states (12, 77).

Microbial products of tryptophan catabolism (indole, tryptamine, indolepropionic acid, indolelactic acid, indolealdehyde, indoleacrylic acid, etc.) constitute another group of metabolites that modulate various aspects of gastrointestinal physiology, including the gut barrier function and immunity, which ultimately may affect the pulmonary vasculature. Several tryptophan catabolites strengthen the mucosal barrier by enhancing expression of epithelial junctional complex molecules, increasing mucus production, and altering the balance of cytokines toward a less inflammatory state (4, 76). Some of these effects are mediated through the pregnane X receptor (PXR). PXR-deficient mice have dysregulated Toll-like receptor 4 (TLR4) expression, increased intestinal permeability, and heightened sensitivity toward different intestinal injury challenges, including nonsteroidal anti-inflammatory drugs, inflammation-induced edema, ischemia-reperfusion, and endotoxic shock (97). Catabolites of tryptophan also interact with the aryl hydrocarbon receptor (AhR), which is widely expressed at all barrier sites, including the gut, where it plays important roles in intestinal homeostasis (88). Specifically, AhR signaling is essential in maintenance of intraepithelial and innate lymphoid cell populations, promoting differentiation of Th17 cells and Tregs, and production of IL-22, a cytokine that participates in multiple barrier functions, including epithelial cell proliferation, mucus production, and production of antimicrobial peptides.

In addition, a causal link exists between increased risk of atherosclerotic vascular disease and circulating levels of trimethylamine N-oxide (TMAO; 91, 105), which has a vascular proinflammatory effect through oxidative stress (45). Plasma TMAO is produced in the liver by the oxidation of trimethylamine, which is generated by the gut bacteria from dietary components such as choline and carnitine. Thus, the potential for TMAO production is dependent on the individual composition of the intestinal microbiota and diet.

MICROBIAL TRANSLOCATION IN THE GUT AND THE CIRCULATING MICROBIOME

The pulmonary vasculature is vulnerable not only to endotoxin and bacterial metabolites but also to microbes that may translocate from the gut into the splanchnic venous circulation. Multiple indirect assays have been used historically to assess the gut barrier function, including measurements of oral probes of different sizes in serum or urine, as well as assessment of macromolecular flux across isolated segments of gastrointestinal tissue and morphometric investigations of the epithelial junction components (14). Altered intestinal epithelial integrity also correlates with plasma measurements of intestinal epithelial junctional components, such as zonulin and claudin-3, as well as markers of enterocyte damage, such as intestinal fatty acid-binding protein (58, 78, 87). These biomarkers are also typically correlated with circulating levels of endotoxin and various cytokines.

Recently, the next-generation sequencing technologies, which have enabled culture-independent studies of complex microbial communities, have also been increasingly applied to studies of the microbial DNA in blood, leading to the conceptual development of both healthy and dysbiotic human blood microbiota (15). The study of any low-biomass microbiota is technically challenging given the potential for contamination from the environment, reagents, and even venipuncture. However, this problem can be addressed by using appropriate controls, and there is reasonable agreement among different laboratories with regard to the composition of the circulating microbiome in healthy individuals (22, 29, 56, 100). Interestingly, the major phyla in the circulating microbiome are Proteobacteria and Actinobacteria, followed by Firmicutes and Bacteroidetes, whereas Firmicutes and Bacteroidetes are by far the dominant bacterial phyla in stool. However, Proteobacteria and Actinobacteria are more abundant in the small intestine and potentially more capable of translocation (25, 106). In addition, it is possible that other parts of the body including the mouth, skin, or lungs may contribute to the circulating microbiome (57). Patients with liver disease have decreased capacity for hepatic filtering of portal blood; therefore, as could be expected, the peripheral blood carries a higher burden of microbial DNA (19, 42, 60, 96, 104). At this time, the studies remain too preliminary to define any specific dysbiotic signature based on a bacterial community structure of the circulating microbiome. However, differences have been found in liver disease and other diseases, including cardiovascular disease, sepsis, chronic kidney disease, acute pancreatitis, and type 2 diabetes (22, 29, 44, 61, 73). The majority of bacteria found in blood are in the white blood cell compartment (56), which may be explained by their internalization at the barrier sites before entry into the bloodstream. It is not known what fraction of these bacteria is viable; however, even nonviable microbial products can stimulate the immune system (38).

EVIDENCE LINKING GUT DYSBIOSIS AND PAH

The etiologic role of the gut microbiota and altered circulating microbiome in the pathogenesis of PAH is unknown. However, there are several compelling observations, in preclinical models of PAH and in patients with PAH, that suggest a possible mechanistic role for gut and circulating microbiome dysbiosis.

First, gut dysbiosis occurs in experimental PAH. Wistar rats treated with Sugen and chronic hypoxia to cause PH demonstrate gut dysbiosis (13). On taxonomy-based analysis, compared with control rats, Sugen-hypoxia PAH rats have a threefold increase in the Firmicutes-to-Bacteroidetes ratio, which is also commonly associated with obesity (13, 43). This altered Firmicutes-to-Bacteroidetes ratio is primarily driven by less abundant Bacteroidetes families in PAH rats, with no significant changes in the relative presence of Firmicutes families (13). In addition, Sugen-hypoxia PAH rats have decreased acetate-producing bacterial genera in the gut and corresponding decreased serum acetate levels, but not butyrate or lactate levels, compared with controls (13). Although these data strongly associate PAH with gut dysbiosis, it is uncertain whether the gut microbiome changes in PAH rats are secondary to right heart failure resulting from PAH or whether they play a causative role in the early pathogenesis of immune dysregulation and pulmonary vascular remodeling. In Sugen-hypoxia PAH rats, gut dysbiosis and serum metabolite changes are observed within 2 wk of exposure to Sugen and hypoxia (13). This indicates that the gut microbiota changes, in a temporal sense, might have a causative role in the early pathogenesis of PAH.

Second, gut dysbiosis leads to increased gut permeability allowing translocation of gut bacteria and/or bacterial products, such as endotoxin. This innate immune phenomenon has been implicated in the pathogenesis of PAH. Serum endotoxin levels are elevated in some experimental models of pulmonary vascular disease (65, 93). Common bile duct ligation (CBDL) in a rat recapitulates human pulmonary vascular disease related to cirrhosis (93). CBDL rats develop the classic hepatopulmonary syndrome with biliary cirrhosis, hypoxemia with an increased alveolar-arterial gradient, alveolar capillary dilatation, and increased lung microvascular density. In addition, our prior work described an increased medial thickness and loss of lumen of the resistance pulmonary arterioles in CBDL rats compared with sham animals (93). In this experimental model, circulating endotoxin levels are elevated, which in turn recruits and activates pulmonary intravascular macrophages. In the presence of cirrhosis, portosystemic shunting may occur, in which case the endotoxin released from the gut into the splanchnic venous circulation enters directly into the pulmonary circulation, where it can activate intravascular macrophages (27). Endotoxin interacts with TLR4 in the macrophages and activates the nuclear factor-κB (NF-κB) signaling pathway leading to increased secretion of cytokines and growth factors (93). The muscular and nonmuscular pulmonary arteries in CBDL rat lungs show accumulation of CD68⁺ macrophages with near occlusion of some of the resistance arterioles. These macrophages are in an activated inflammatory stage as demonstrated by positive NF-κB staining in the nucleus (93). NF-κB is a transcription factor that translocates into the nucleus during inflammatory activation. In contrast, sham animals have no pulmonary intravascular macrophage accumulation and have low levels of tissue macrophages, which are also not activated. Treatment of CBDL rats with the antibiotic norfloxacin decreases pulmonary intravascular macrophage accumulation and reduces pulmonary vascular remodeling (62). Furthermore, depletion of the pulmonary intravascular macrophages in the CBDL rats prevents, as well as reverses, the pulmonary vascular changes (93). These data together suggest that increased circulating microbial products originating in the gut, such as endotoxin, play a central role in the pathogenesis of pulmonary vascular changes due to cirrhosis through activation of macrophages. Monocrotaline-induced PAH rats also have increased gut permeability, increased circulating levels of endotoxin in the portal vein, and increased circulating levels of soluble CD14, a marker of macrophage activation, in the systemic venous blood (65).

Mutations in the Bmpr2 gene encoding bone morphogenic protein receptor type 2 underlie 80% of heritable PAH, but disease penetrance is only 10–20%, indicating a requirement for an additional trigger or triggers (49, 53). Similar to humans carrying a BMPR2 mutation, Bmpr2^+/− mice also have only mild or no elevation of pulmonary artery pressures at baseline (7). However, chronic administration of lipopolysaccharide (LPS; endotoxin is the lipid A portion of the LPS) causes PH in Bmpr2^+/− mice compared with littermate controls, suggesting that endotoxin-induced inflammation can be an additional trigger for disease penetrance (82). Compared with littermate controls, LPS-administered Bmpr2^+/− mice have increased lung IL-6 cytokine expression, coupled with an increase in the phosphorylation of signal transducer and activator of transcription 3 (STAT3), which is the major downstream signaling pathway of IL-6 (82). Likewise, in in vitro studies, acute administration of LPS increases IL-6 production and phospho-STAT3 activation in both mouse Bmpr2^+/− and human BMPR2 mutant pulmonary artery smooth muscle cells that are haplo-insufficient for BMPR2 (82). This is associated with increased expression of TLR4 and increased superoxide levels (82). These data suggest that Bmpr2 deficiency predisposes to inflammation. Thus, gut dysbiosis leading to increased gut permeability and raised circulating endotoxin levels may be a potential additional trigger for phenotypic manifestation of PAH in BMPR2 mutation carriers.

Translocation of gut bacteria, serum endotoxemia, and activation of macrophages occur in human PAH as well. Patients with idiopathic PAH and heritable PAH have increased serum levels of endotoxin and soluble CD14 compared with healthy controls (65). In patients with PAH, the increase in serum CD14 levels parallels the increase in serum endotoxin levels (65). Furthermore, patients with PAH have increased blood TLR4 expression compared with healthy controls (18). The Abernethy malformation is a congenital anomaly of the splanchnic vasculature in which portal venous blood is diverted into the inferior vena cava creating a congenital portosystemic shunt. Patients with the Abernethy malformation often develop pulmonary vascular disease, including portopulmonary hypertension and hepatopulmonary syndrome (46, 75, 84). In the Abernethy malformation there is no hemodynamic left-to-right shunt, such as an atrial septal defect or ventricular septal defect, to explain the PH. Commensal gut bacteria and endotoxin released from the gut bypass the liver through the portosystemic shunt, avoiding hepatic uptake and inactivation, pass through the right heart, and enter the pulmonary vasculature. In the lungs they activate macrophages, leading to pulmonary arteriovenous malformations, capillary dilatation, and proliferative arteriopathy of the distal pulmonary arteries (27, 93). Liver transplantation with the correction of the portosystemic shunt reverses the pulmonary vascular changes in patients with the Abernethy malformation (37). These observations strongly suggest an etiologic role even for the normal gut microbiota in the pathogenesis of PAH.

Exercise training, by itself, improves exercise tolerance and functional capacity in patients with PAH (11). In fact, the improvement in 6-min walk distance seen with exercise training is significantly higher compared with the improvement in 6-min walk distance observed with PAH-specific vasodilator therapies (72 vs. ~40 m, respectively; 11, 48). Interestingly, there is emerging evidence that exercise-induced gut microbiota changes enhance exercise capacity (72). Exercise increases the gut Veillonella genus, which in turn improves exercise performance through its metabolic conversion of exercise-induced lactate into propionate (72). Thus, it is possible that the improvement in exercise capacity with exercise training in PAH might be mediated at least in part by gut microbiota changes.

Finally, gut dysbiosis can cause several immune dysregulations that have been reported in the pathogenesis of PAH. Increased TLR4 activation has been documented to occur in PAH (5). Gut dysbiosis has been associated with activation of Th1 and Th17 CD4 cells, increased secretion of IL-17, and downregulation of Tregs (8, 17, 50). Interestingly, Th1 and Th17 CD4 cell activation as well as Treg dysfunction have been implicated in the pathogenesis of PAH (63, 99). On the basis of these collective observations, it is certainly possible that gut dysbiosis plays a role in initiating immune dysregulation and early perivascular inflammation in PAH.

MECHANISMS BY WHICH GUT AND CIRCULATING MICROBIOME DYSBIOSIS MAY CAUSE PAH

There are two possible, non-mutually exclusive, mechanistic ways in which altered microbial composition in the gut and circulating microbiome might initiate perivascular inflammation in the pulmonary vasculature (Fig. 1): 1) an increase in gut permeability allowing microbial translocation in the gut and increased levels of circulating microbes or microbial products and 2) an altered microbial community structure that generates a proinflammatory metabolome (either an increase in proinflammatory metabolites or a decrease in anti-inflammatory metabolites). The loss of hepatic filtration and the detoxification of circulating microbial products, such as endotoxin, secondary to either cirrhosis or portosystemic shunts, may also favor the development of PAH. Likewise, increased genetic susceptibility, e.g., BMPR2 mutation, may synergize with the above mechanisms, through increased sensitivity to proinflammatory signals, to trigger PAH pathogenesis.

Fig. 1.

Proposed mechanisms through which gut and circulating microbiome dysbiosis causes lung perivascular inflammation in pulmonary arterial hypertension. AhR, aryl hydrocarbon receptor; BMPR2, bone morphogenic protein receptor type 2 gene; GPCR, G protein-coupled receptor; HDCA, histone deacetylase; PAH, pulmonary arterial hypertension; PXR, pregnane X receptor; SCFA, short-chain fatty acid; Th, helper T; TLR, Toll-like receptor; TMAO, trimethylamine N-oxide; Treg, regulatory T cell.

CHRONIC RIGHT HEART FAILURE AND SERUM ENDOTOXIN LEVELS

Chronic right heart failure, by itself, can also lead to increased circulating endotoxin levels in PAH. Chronic right heart failure causes intestinal congestion and reduced bowel perfusion leading to intestinal ischemia. This increases intestinal permeability, allowing translocation of gut bacteria and/or endotoxin (52, 69). In support of this, patients with untreated PAH with right heart failure have higher levels of circulating endotoxin and soluble CD14 compared with patients with treated PAH who are not in chronic right heart failure (65). Similarly, diuretic treatment has been shown to normalize serum endotoxin levels in patients with congestive heart failure. Hence, it is possible that the translocation of gut bacteria and/or endotoxin and activation of macrophages in patients with PAH may occur because of primary gut dysbiosis, right heart failure, or a combination of both (69, 70).

GUT MICROBIOTA AS A POTENTIAL THERAPEUTIC TARGET TO TREAT PAH

On the basis of the collective evidence suggesting a possible link between gut dysbiosis and the pathogenesis of PAH, one can conclude that modulation of the gut microbiota may be a potential therapeutic option to treat PAH. The composition of the gut microbiota can be modulated by diet, antibiotics, prebiotics, probiotics, and/or intestinal microbiota transplantation (IMT), also known as fecal microbiota transplantation (39). IMT is arguably the most potent way to alter a patient’s intestinal microbiota, and it is currently used clinically to treat antibiotic-refractory Clostridiodes difficile infection. However, multiple unknowns need to be considered in designing microbiota-targeting interventions to treat PAH. For example, in the case of an IMT-based strategy it may be important to optimize patient and donor selection, as well as explore the effects of such a treatment on the small bowel, where microbial translocation is more likely to occur. It is also necessary to consider more conventional pharmacologic treatments that recapitulate the beneficial effects of microbial metabolites. Some of these may already be available and are widely used for other conditions. For example, 5-aminosalicylic acid, which is commonly used to treat ulcerative colitis, activates PPARγ, mimicking similar actions induced by butyrate (68). Also, some of the beneficial effects of metformin, currently one of the first-line drugs to treat type 2 diabetes, are likely mediated via the intestinal microbiota through expansion of butyrate-producing bacteria (24, 101). Intriguingly, metformin has been shown to attenuate experimental PH (1, 54).

FUTURE DIRECTIONS

Although there are persuasive findings linking gut dysbiosis to the early pathogenesis of PAH, there are several pivotal unanswered questions. These need to be addressed before modulation of the gut microbiota is introduced as a therapeutic option to treat PAH. First, although gut dysbiosis has been described in experimental PAH, it has not been studied in human patients with PAH. Thus, the gut and circulating microbiota should be characterized in patients with PAH compared with healthy controls. Second, rigorous experimental studies should be conducted to ascertain the causative role of gut dysbiosis and altered circulating microbiome in the pathogenesis of PAH. Antibiotic therapy to modulate gut microbiota composition, alteration of diet to increase production of SCFAs such as acetate or butyrate, and transplantation of germ-free mice with the gut microbiota from experimental models of PAH or human patients with PAH are various approaches that can be undertaken to prove a cause-and-effect relationship between gut dysbiosis and PAH. Third, future studies should determine whether the increase in the circulating microbiome and/or endotoxin in PAH is secondary to gut dysbiosis, to right heart failure, or to a combination of both. Addressing these questions can aid in developing potential interventional strategies to target the microbiota therapeutically in patients with PAH.

CONCLUSION

Ample evidence confirms that inflammation plays an important role in the early pathogenesis of PAH. However, what initiates the early perivascular inflammation in PAH is unclear. In a recent in-depth review of the early pathogenesis of PAH, the authors state that “although the complexity of inflammatory response in the pathogenesis of PAH is recognized, the initial trigger of inflammation that is involved in PAH development is not identified” (64). We propose that the gut dysbiosis and circulating microbiome changes could be an initiating event causing perivascular inflammation in PAH. Changes in the gut and circulating microbiome have been implicated in the immunopathogenesis of many chronic disorders. There is strong circumstantial evidence to suggest an etiologic link between changes in the gut and circulating microbiota and the pathogenesis of PAH. Changes in the gut microbiota may also be important in exacerbating other forms of PH (World Health Organization groups 2–5).

Well-designed studies are needed in the future to prove a causative role for gut dysbiosis in the pathobiology of PAH. Further exploration of the gut and circulating microbiome changes in PAH has a huge potential to yield significant breakthroughs in the development of novel therapeutic tools for the treatment of PAH.

GRANTS

This work was supported by National Heart, Lung, and Blood Institute Grant 1-R01-HL-139797 (to Y. Chen).

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

T.T., A.K., and E.K.W. prepared figures; T.T., A.K., and E.K.W. drafted manuscript; T.T., A.K., Y.C., and E.K.W. edited and revised manuscript; T.T., A.K., Y.C., and E.K.W. approved final version of manuscript.