Contribution of gut microbiota to the development of Crohn's disease: Insights gained from fecal microbiota transplantation studies in mice

Abstract

We would like to present some new thoughts on the publication in the journal published in August 2024 in World Journal of Gastroenterology. We specifically focused on the alterations in the intestinal tract, mesenteric adipose tissue (MAT), and systemic inflammatory changes in mice following fecal flora transplantation into a mouse model of Crohn's disease (CD). Accumulating evidence suggests that the occurrence of CD is influenced by environmental factors, host immune status, genetic susceptibility, and flora imbalance. One microbiota-based intervention, fecal microbiota transplantation, has emerged as a potential treatment option for CD. The MAT is considered a "second barrier" around the inflamed intestine. The interaction between gut microbes and inflammatory changes in MAT has attracted considerable interest. In the study under discussion, the authors transplanted fetal fecal microorganisms from patients with CD and clinically healthy donors, respectively, into 2,4,6-trinitrobenzene sulfonic acid-induced CD mice. The research explored the complex interplay between MAT, creeping fat, inflammation, and intestinal flora in CD by evaluating intestinal and mesenteric lesions, along with the systemic inflammatory state in the mice. This article provides several important insights. First, the transplantation of intestinal flora holds significant potential as a therapeutic strategy for CD, offering hope for patients with CD. Second, it presents a novel approach to the diagnosis and treatment of CD: The inflammatory response in CD could potentially be assessed through pathological or imaging changes in the MAT, and CD could be treated by targeting the inflammation of the MAT.

Key Words: Fecal microbiota transplantation; Mesenteric adipose tissue; Creeping fat; Inflammation; Intestinal fibrosis; Crohn's disease

Core Tip: Mesenteric adipose tissue and creeping fat (CrF) may play a potential role in Crohn's disease (CD) progression, providing new insights and promising avenues for therapeutic intervention. Fecal microbiota transplantation (FMT) from healthy donors may partially mitigate intestinal inflammatory infiltration, improve intestinal permeability, enhance barrier function, and reduce systemic inflammation in patients with CD. Conversely, FMT from CD patients exacerbates inflammatory changes and contributes to intestinal wall fibrosis. In the present study, we found that the gut microbiota played a multifaceted role, mediating the properties of CrF and influencing the inflammatory and fibrotic phenotypes associated with CD.

TO THE EDITOR

Crohn's disease (CD) is a chronic, non-specific inflammatory condition affecting the digestive tract, commonly presenting with symptoms such as abdominal pain, diarrhea, and weight loss. It is characterized by recurrent episodes of chronic inflammation, often leading to complications such as strictures, abscesses, and fistulas[1]. The pathogenesis of CD remains incompletely understood, but accumulating evidence suggests that aberrant immune activation, driven by the interaction of genetic susceptibility and environmental factors affecting the gut microbiota, may play a role in the pathogenesis of CD[2].

The gut is home to more than 10 billion microorganisms, referred to as the "digestive flora". Nearly 90% of human diseases, such as obesity, diabetes, depression, and gastrointestinal diseases, are associated with the composition of the gut flora. Under normal conditions, a dynamic physiological balance is maintained between gut microbes and the host. However, a microecological imbalance can occur when large-scale or persistent disturbances exceed the microecosystem's capacity to regulate itself and maintain homeostasis. The assessment of intestinal microecological imbalance involves evaluating the relative proportions of beneficial and harmful bacterial species, their abundance, and the microbial diversity. In patients with CD, dysbiosis of the intestinal microbiota is characterized by reduced bacterial diversity, an expansion of invasive presumptive flora, and a reduction in protective flora[3,4]. In addition, a prospective study evaluating the fecal metagenomes of patients with inflammatory bowel disease undergoing biologic therapies found that patients with CD who responded to treatment showed a higher baseline abundance of butyrate producers, suggesting that the gut microbiome may be predictive of therapy response[5].

In 1932, Krohn et al first described the proliferation of mesenteric adipose tissue (MAT) in the vicinity of intestinal inflammation[6]. Histopathologically, it differs from healthy MAT in terms of an altered extracellular matrix, markedly smaller adipocyte size, and a specific microenvironment characterized by elevated levels of adipokines and immune cell infiltration[1]. Creeping fat (CrF) is defined as the hyperplastic hypertrophy of adipose tissue near the diseased bowel. Creeping inflammatory mesenteric fat hyperplasia (CrF) is a well-known feature of CD, especially in ileal stricture disease[7]. CrF differs from normal MAT in that it contains elevated levels of cytokines, including tumor necrosis factor-alpha (TNF-α), interleukin (IL)-1 beta (IL-1β), and IL-6, which are indicators of CD-related inflammation. Additionally, levels of adipokines, such as leptin, resistin, chemotaxin, and adiponectin, are elevated, whereas the beneficial adipokine lipocalin is downregulated. The intestinal microbiota may persist in CrF, mediating the migration and proliferation of adipose tissue to the lesion site. Gut microorganisms that breach the mesentery after damage to the intestinal barrier may be the underlying cause of creeping fat formation. Inflammation-related barrier dysfunction and dysbiosis of the intestinal microbiota have emerged as critical contributors to the pathogenesis of CD. Transplantation of intestinal flora from patients with CD into 2,4,6-trinitrobenzene sulfonic acid (TNBS)-induced CD mice was evaluated for intestinal permeability, markers of intestinal barrier function, levels of cytokines, adipokines, and inflammatory factors in the blood, along with histopathological changes. The results showed that CD-fecal microbiota transplantation (FMT) exacerbated the progression of CD. Taken together, these findings suggest that targeting MAT and CrF may offer promising therapeutic avenues for managing intestinal inflammation and fibrosis in patients with CD[8-10].

STUDY OVERVIEW AND DISCUSSION

This prospective study included 12 patients diagnosed with CD at Xiangya Second Hospital. Colonic tissue and MAT specimens were collected from both diseased and non-diseased colonic regions of these patients. The study showed that inflammatory cell infiltration and fibrotic changes were significantly more pronounced in the MAT lesion group compared to the non-lesion group. Fecal donors were obtained from five patients with CD (one female and four male) and five healthy controls. TNBS-induced CD mice exhibited substantially lower body weights and shorter colon lengths than control mice. These reductions in body weight and colon length were partially reversed by FMT (NC-FMT) using the healthy donor samples. TNBS significantly increased intestinal permeability in the mice, which was partially reduced by NC-FMT. However, following CD-FMT, intestinal permeability remained higher in TNBS-treated mice compared to those receiving NC-FMT. Moreover, TNBS treatment significantly decreased the levels of occludin, a key marker of intestinal barrier function, in the colonic tissues. This decrease was partially reversed by NC-FMT. However, CD-FMT potentiated these changes. TNBS-treated colon samples showed significantly elevated levels of IL-6, IL-1β, TNF-α, leptin, and lipocalin compared to control mice, and NC-FMT partially abolished these effects. However, similar effects were still observed, and CD-FMT further enhanced these effects. Finally, NC-FMT partially attenuated fibrosis, while CD-FMT led to an increase in fibrotic areas.

This article examined the MAT hyperplasia and hypertrophy, as well as intestinal fibrosis, were addressed through FMT, offering new insights into the pathogenesis and treatment of CD. The study demonstrated significant histopathological changes in CrF and intestinal tissue in patients with CD, including structural disruptions, inflammatory infiltrates, and fibrotic changes. There was an active inflammatory response characterized by a significant increase in pro-inflammatory factors, such as IL-6, IL-1β, and TNF-α, as well as chemokines, including monocyte chemotactic protein-1, within the MAT of patients with CD. These factors contributed to the recruitment and activation of immune cells in the inflamed intestinal regions. Additionally, a microbial signature was identified in the MAT of patients with CD, suggesting that MAT-associated microbiota play a role in the progression of CD[11]. Using a TNBS-induced mouse model of CD, the study demonstrated that FMT with CD exacerbated tissue congestion and inflammatory cell infiltration. In contrast, FMT from clinically healthy donors partially ameliorated intestinal permeability, barrier function, and levels of serum cytokines and adipokines in CD mice, underscoring the key feature that NC-FMT ameliorates CD. In addition, collagen deposition and alpha-smooth muscle actin (α-SMA) expression were significantly higher in the intestinal tissues of patients with CD compared with healthy controls, revealing a role of intestinal fibrosis in the progression of CD. CrF and intestinal stenosis frequently co-occur in the same bowel segment of patients with CD[12]. Obstructed bowel segments are also often associated with significant MAT hypertrophy. CD-FMT further increased the expression of fibrotic markers, including α-SMA and vimentin, in the colon tissues of mice with TNBS-induced CD, underscoring the multifaceted role of gut microbiota in shaping the inflammatory and fibrotic landscape of CD.

STRENGTHS AND LIMITATIONS

This article examines the complex interactions between MAT, peristaltic fat, and intestinal flora in the mechanisms underlying CD development, an area that has seen limited investigation in previous literature. The study concludes that MAT and CrF may contribute to the progression of CD, offering new insights and promising opportunities for clinical therapeutic interventions in patients. However, there remain several aspects that warrant further exploration. In the mouse model of chronic colitis with fibrosis induced by TNBS, incorporating additional mouse fibrosis models could enhance the study. This would be particularly useful for observing the anti-inflammatory effects of FMT and its potential to reduce fibrosis severity. The study does not provide sufficient clinical data to validate the use of FMT in treating CD, highlighting the need for further research to evaluate the practical implications of these findings for clinical application. Additionally, we are eager to see the long-term data regarding FMT treatment for CD.

FUTURE RESEARCH DIRECTIONS

Future research should focus on several key areas. First, mounting evidence suggests that both the functional involvement and structural changes in MAT affect the progression of CD. Therefore, for the diagnosis and prognosis of patients with CD, evaluating inflammatory lesions in the MAT at the earliest stage possible, such as through research and development of the use of magnetic resonance elastography to study the viscoelasticity of MAT in CD, is crucial. Second, MAT is considered to act as a potential secondary barrier around the inflamed bowel, providing local anti-inflammatory functions and protecting against systemic inflammatory responses. In the future, studies should investigate MAT as a distinct target tissue, which could yield insights for the development of effective drugs for CD.

CONCLUSION

This prospective study investigated and discussed the inflammatory alterations in MAT and CrF, along with the systemic immune response in TNBS-induced CD mice during the progression of CD. It also examined intestinal permeability and intestinal barrier function in TNBS-induced CD mice that underwent either CD-FMT or NC-FMT in a controlled study. The study concluded that gut microbiota played a crucial role in the histopathology of CD by influencing the properties of CrF and modulating the inflammatory and fibrotic phenotypes associated with CD. Therefore, targeting MAT and CrF may offer therapeutic potential for patients with CD.