Henao-Mejia J, Elinav E, Jin C, Hao L, Mehal WZ, Strowig T, et al. Inflammasome-mediated dysbiosis regulates progression of NAFLD and obesity. Nature 2012;482:179-185. (Reprinted with permission.)
Non-alcoholic fatty liver disease (NAFLD) is the hepatic manifestation of metabolic syndrome and the leading cause of chronic liver disease in the Western world. Twenty per cent of NAFLD individuals develop chronic hepatic inflammation (non-alcoholic steatohepatitis, NASH) associated with cirrhosis, portal hypertension and hepatocellular carcinoma, yet the causes of progression from NAFLD to NASH remain obscure. Here, we show that the NLRP6 and NLRP3 inflammasomes and the effector protein IL-18 negatively regulate NAFLD/NASH progression, as well as multiple aspects of metabolic syndrome via modulation of the gut microbiota. Different mouse models reveal that inflammasome-deficiency-associated changes in the configuration of the gut microbiota are associated with exacerbated hepatic steatosis and inflammation through influx of TLR4 and TLR9 agonists into the portal circulation, leading to enhanced hepatic tumour- necrosis factor (TNF)-α expression that drives NASH progression. Furthermore, co-housing of inflammasome-deficient mice with wild-type mice results in exacerbation of hepatic steatosis and obesity. Thus, altered interactions between the gut microbiota and the host, produced by defective NLRP3 and NLRP6 inflammasome sensing, may govern the rate of progression of multiple metabolic syndrome-associated abnormalities, highlighting the central role of the microbiota in the pathogenesis of heretofore seemingly unrelated systemic auto-inflammatory and metabolic disorders.
Emerging literature suggests a critical role for inflammation in driving the progression of chronic liver diseases (CLDs), such as nonalcoholic fatty liver disease (NAFLD), as well as other manifestations of metabolic syndrome (MetS). Likewise, it is increasingly apparent that our commensal intestinal bacteria are crucial to the development and maintenance of a healthy immune system and may play a role in diverse inflammatory diseases. Given the close anatomical and functional connection between the gut and the liver, and their cooperative role in maintaining immunological tolerance, it stands to reason that imbalances in the gut could influence liver disease, although this has rarely been systematically investigated. The recent study by Henao-Mejia et al.1 demonstrated that perturbations in intestinal flora exacerbated disease in two mouse models of fatty liver disease (FLD). Previous studies in mice support the hypothesis that CLD of different etiologies is associated with perturbations in gut microflora,2 but this is the first study to demonstrate gut flora as a driver of liver disease. This study underlines the complex role the innate immune system plays in liver biology and the challenges for developing safe, effective immunotherapies to treat FLD.1
The human gut microbiome is a complex ecological system performing vital physiological functions, whose composition may be as influential as our own genetic makeup in health and disease.3 In addition to metabolic functions, intestinal microbes regulate the development of a healthy immune system. Conversely, genetic or acquired host factors affecting microbial composition and function can, in turn, modulate disease susceptibility. Many environmental factors also modulate the intestinal microbiome, including diet, antibiotics and other drugs, and environmental toxins. Together, host, environment, and microbes create a balanced, symbiotic state. Perturbations in this balance (“dysbiosis”) are increasingly linked to diverse pathologies, including extraintestinal diseases.3
NAFLD encompasses a range of conditions, from simple steatosis to the proinflammatory, profibrotic nonalcoholic steatohepatitis (NASH), which is associated with a risk of cirrhosis and hepatocellular cancer. The reasons some individuals with NAFLD have NASH whereas others follow a more benign course are not understood. Several correlates of NAFLD severity and NASH development have been identified, including liver inflammation. Multiple triggers of liver inflammation may be relevant in NAFLD, including products of damaged cells within the liver, as in other diseases characterized by “sterile” inflammation,4 and/or microbial stimuli of commensal origin. Such inflammatory stimuli are primarily detected by professional innate immune cells, as well as nonhematopoietic sentinel cells (e.g., intestinal and liver epithelial cells [ECs]). Several danger-sensing systems cooperate to protect the host from invasion and injury, including the Toll-like receptors (TLRs) that trigger proinflammatory gene expression and the nod-like receptor (NLR) family that has diverse functions in gene regulation and proteolytic pathways. Select members of the NLR family (NLRP1, NLRP3, NLRP6, and NLRC4) are danger sensors that, upon activation, form molecular machines (‘inflammasomes’) that regulate immune responses by processing cytokine precursors into their active forms, enabling their secretion (e.g., the interleukin [IL]-1β and IL-18). In addition to their role in defense against microbes, NLRs have emerged as critical sensors of endogenous danger signals4 and cooperate with other immune pathways to control commensal organisms, thereby maintaining intestinal homeostasis.5
Henao-Mejia et al. studied the composition of the microbiome and severity of FLD in mice genetically deficient in either the sensor or signaling components of two distinct inflammasomes (NLRP3 and NLRP6) whose pathways converge on common cytokine targets. Absence of innate immune control by the NLRP3 or NLRP6 inflammasomes perturbed the normal gut microbial balance, causing the appearance of Prevotellaceae and a proportional increase in Porphyromonadaceae within the gut microflora. The latter was further increased when mice were subject to a methionine-choline–deficient diet (MCDD) to induce FLD. Inflammasome-dependent dysbiosis led to increased delivery of proinflammatory microbial components (specifically, TLR4 and TLR9 agonists) to the liver and exacerbated steatosis and inflammation by increased production of the proinflammatory cytokine, tumor necrosis factor (TNF). A causal link between dysbiosis and disease exacerbation was established by antibiotic treatment, which ameliorated disease, and by transferring the perturbed microbiota and disease phenotype to wild-type mice by cohabitation with inflammasome-deficient mice. Interestingly, the exacerbation of FLD observed in the inflammasome-deficient MCDD model did not rely on hematopoietic or hepatocyte inflammasomes, implicating an alternate cell population in controlling intestinal homeostasis, most likely intestinal ECs, which possess inflammasomes. Overall, this study suggests that a competent innate immune system is necessary to maintain intestinal homeostasis, and that perturbations in gut flora can promote hepatic steatosis, inflammation and, potentially, NAFLD progression. These results are in keeping with the prevailing view that chronic inflammation is a key step in the development of profibrotic disease (NASH); however, the incidence or severity of fibrosis in inflammasome-deficient mice was not addressed in this study.
Among the inflammasome sensors investigated, NLRP3 or NLRP6, but not NLRC4, were required to maintain intestinal homeostasis and reduce disease burden. NAFLD exacerbation was the result of impaired IL-18 production, and independent of the archetypal inflammasome-activated IL-1 pathway, in accord with previous studies implicating IL-18 in maintaining intestinal epithelial integrity.5, 7Intestinal permeability was not directly addressed in the present study, but increased permeability in NLRP3- or NLRP6-inflammasome-deficient mice might explain the enhanced delivery of TLR4/9 agonists to the liver. Polymorphisms that compromise mucosal immunity (including by NLRP3) are also associated with dysbiosis and inflammatory bowel disease in humans.8 On the other hand, there is also evidence to suggest a detrimental role for inflammasome activation in intestinal inflammation, suggesting multiple, potentially conflicting roles for inflammasomes and their substrate cytokines. The outcome of inflammasome activation is likely to be dependent on additional factors, such as the location of activation (e.g., epithelium and lamina propria), and cross-talk with other inflammatory pathways. Nevertheless, intestinal dysbiosis was sufficient to exacerbate hepatic steatosis and inflammation, even in the absence of hematopoietic and hepatocyte inflammasomes that may mediate proinflammatory effects. Importantly, inflammasome-deficient mice still possessed a repertoire of microbial sensors, including TLRs, which were required for NAFLD exacerbation.
Although MCDD mice are commonly used to model inflammation and fibrosis, the model lacks many of the metabolic phenotypes associated with NAFLD. The investigators extended their findings by studying an obese mouse model, the leptin-receptor deficient db/db mice that develop NAFLD, impaired intestinal barrier function, and glucose intolerance when fed a high fat diet (HFD). Inflammasome-deficient db/db mice, or db/db mice cohoused with inflammasome-deficient mice, showed increased hepatocyte injury, steatosis, and inflammation, compared to singly housed db/db mice. The inflammasome-dependent gut microbiota also exacerbated metabolic abnormalities in HFD db/db mice, including obesity, glucose intolerance, and insulin resistance (IR). These abnormalities were microflora dependent because antibiotic treatment reversed the metabolic phenotype. There was little overlap between the microbial perturbations observed in this or other studies,2 supporting the view that specific microorganisms are not required to drive FLD and highlighting the potential involvement of multiple redundant pathways.
The negative effect of inflammasome deficiency on liver inflammation and metabolism observed in the db/db model apparently contradicts earlier studies on the role of inflammasomes in mouse models of MetS. In obese mouse models, overnutrition leads to adipose tissue inflammation by lipotoxic NLRP3 activators, such as ceramide.9 Thus, NLRP3-deficient mice were resistant to the development of HFD-induced obesity, IR, and hepatic steatosis in an IL-1-dependent manner.9, 10IL-1 is also implicated in the pathogenesis of human metabolic disease and is a promising therapeutic target.11 By contrast, IL-18 deficiency induced obesity and IR,12 in line with the present study. Together, these studies suggest a dominant proinflammatory role for IL-1β in metabolic disease, as opposed to the homeostatic control IL-18 exerts on the composition of the intestinal microbiome. In addition to mechanistic explanations for the inconsistencies in mouse disease models, microbial diversity may also contribute because genetically identical mice housed in different environments have distinct gut flora and disease susceptibility. Adding a further layer of complexity, intestinal microbes have been shown to regulate fat storage and adiposity,13 in addition to their proinflammatory potential. Thus, the complex interplay between the microbiome and hierarchy of pro- and anti-inflammatory mechanisms likely leads to a distinct intestinal balance, and therefore disease susceptibility, in each individual mouse strain/model or human subject.
The present study supports the hypothesis that dysbiosis may cause proinflammatory changes in the liver that exacerbate NAFLD and, potentially, influence the development of NASH. Here, dysbiosis resulted from impaired immunity, but it could potentially arise as the result of a range of host or environmental factors. Although alterations in intestinal permeability and microbial composition have previously been associated with advanced liver disease in humans,14 the role of the gut microflora in driving disease progression at earlier stages has not been investigated. In one NAFLD study, increased intestinal permeability correlated with hepatic steatosis, but not the presence of NASH.15 As in mice, genetic or acquired immune deficiencies could plausibly modulate the microbiome in such a way as to predispose to, or exacerbate, NAFLD. Polymorphisms in innate immune loci (TLR4, CD14, and TNF) are associated with NASH in humans, but their effect on the gut microbiome has not been investigated.16 Ultimately, the balance between pathological and protective immune responses along the gut-liver axis will be determined by the complex interplay between diverse immune stimuli and host immune status, which, in turn, are further modulated by tissue injury and environmental factors. Research into the gut liver axis is already progressing rapidly, with the recent publication of another landmark article showing that intestinal microbes promoted the development of hepatocellular carcinoma, a long-term consequence of chronic liver injury.17 This research has exciting implications for human studies, which are likely to be even more complex given our enormous genetic, environmental, and microfloral diversity.