Wu-Meng Jin, Si-Yue Song, Xiao-Fen Xu, Cheng-Ping Wen, Chang-Feng Hu

College of Basic Medical Sciences, Zhejiang Chinese Medical University, Hangzhou 310053, China

Primary biliary cholangitis (PBC, formerly known as primary biliary cirrhosis) is a chronic female-predominant inflammatory and cholestatic liver disease with a prolonged course and poor prognosis [1] . It is characterized by anti-mitochondrial autoantibody (AMA), immunoglobulin M and lymphocyte infiltration in the portal area of liver tissue. A recent systemic review has shown a steady rise both in incidence and prevalence of PBC, with 5.31/10 0 0 0 0 persons and 40.2/10 0 0 0 0 persons, respectively [2] .It can cause fatigue, pruritus and is associated with a high risk of cirrhosis and liver failure if left untreated [3] . The pathogenesis of PBC may be related to genetic, environmental, immune factors and individual susceptibility, but the specific etiology is not clear.Several previous studies [ 4 , 5 ] have found that gut microbiota plays an important role in a variety of liver diseases, including PBC. The bidirectional crosstalk between liver and microbiome appears to be crucial in maintaining health and could be therapeutically targeted,such as by fecal microbiota transplantation.

A series of basic and clinical studies have shown the association between intestinal microbiota and cholestatic liver diseases. In early-stage PBC patients, the gut was depleted of some potentially beneficial bacteria but enriched in some bacterial taxa containing opportunistic pathogens [6] . Furthermore, an epidemiological study has identified that the exposure to urinary tract infection in susceptible host, predominantly byEscherichiacoli, is one of the risk factors for PBC [7] . As known, the gut microbiota generally influences the host immunity, and the immune system also regulates the microbiota by protecting the integrity of the intestinal barrier and immunological rejection in turn [8] . Dysregulation of the gut microbiota might act as a critical factor in the progress of the diseases. A recent clinical research has disclosed a comprehensive signature of gut microbiota in PBC and reported that gut microbiome dysbiosis could be partially ameliorated after ursodeoxycholic acid(UDCA) treatment (which is the first-line treatment of PBC and effective in preventing disease progression in about two thirds of the patients) [9] . Another basic study [10] has taken advantage of a well-characterized murine model of PBC to investigate the influences of gut microbiota on autoimmune cholangitis. As noted,intestinal microbiome disturbances cause cholangitis in mice and the altered composition of gut microbiota by administration of antibiotics significantly alleviates T-cell-mediated infiltration and bile duct damage. Therefore, gut microbiota could break the immune homeostasis and thus directly involved in the pathogenesis of PBC.

However, it is not clear whether the altered gut microbiome is a trigger for PBC or simply a reflection of the disease status.Hence, there are several mechanisms regarding the immunological impact on gut microbiota in the diseases [11] . It is known that the dominant autoepitope of PBC is the E2 subunit of the pyruvate dehydrogenase complex (PDC-E2) of the mitochondria, which is recognized by the AMA [12] . Interestingly, recent data have shown that the gram-negative bacteria, such asEscherichiacoli,HelicobacterandSphingomonas, in the gut microbial community of susceptible hosts have the relevant protein closely resembling the PDC-E2 [ 13 , 14 ]. Subsequently, the enterohepatic circulation transports the potential microbes or metabolites of gut microbiota from the gut to liver through the portal venous system. In this process,dominant epitopes recognized by the immune cells induce extensive cross-reactivity between autoepitopes and microbial mimics through the mechanism of molecular mimicry, playing a role in the induction of AMA [15] . Indeed, several studies have revealed that AMA in the serum from PBC patients could react with bacterium sequences [16–18] . One study has discovered that 97.5% AMApositive patients with PBC have serum reactivity against the three definedSphingomonasproteins [17] . Another study revealed the interaction between AMA and protein components ofEscherichia coliisolated from stool specimens of PBC patients, suggesting that the bacteria may promote the disease progression by initiating immune damage patterns in susceptible hosts [18] . Furthermore, the dysbiosis of gut microbiota impairs the mucosal immune system.The aberrant activation of T cells, specific chemokine receptors and integrins expressed in mucosal lymphocytes are abnormally recruited from the gut into the liver, triggering autoimmune responses upon recognition of the antigens [19] . As aforementioned,compositional and functional alterations in the microbiota are crucial regulators in the potential involvement of gut lymphocyte homing.

Apart from direct interaction with the immune system in gutliver axis, bile acids, the end products of hepatic cholesterol metabolism, are also one of the important mediators of the gutliver crosstalk. As pleiotropic signaling molecules, bile acids interact with gut microbiota through the signaling pathway mediated by bile acid receptors, such as transmembrane G-protein-coupled receptor 5 (TGR5) and farnesoid X-activated receptor (FXR) expressed in the liver including the biliary tree as well as in the intestinal epithelial cells [20] . Recent data highlight the role of bile acids-activated TGR5 as a major inhibitor of cytokine expression and secretion, thereby further reducing biliary and intestinal inflammation [21] . Additional data suggest that the activation of the nuclear receptor FXR transcription in intestine protects the liver from cirrhosis in mice by regulating the hepatic bile acids synthesis [22] . Notably, bile acids are synthesized in the liver via both classical and alternative pathways involving more than 17 different enzymes, including the rate-limiting enzyme cholesterol 7α-hydroxylase (CYP7A1), which is down-regulated by gut microbiota and in turn, attenuates the activity of classical pathway.On the one hand, perturbations of microbiota and their metabolites exert a profound influence on bile acid pool and modulate the activity of bile acids-activated receptors. On the other hand, bile acids have direct antibacterial effects by inhibiting the growth of certain bile sensitive bacteria and affect microbiota indirectly via nuclear receptors [23] . It is noteworthy that bile acids have effects on both bacterial membrane and intestinal mucosa. Furthermore, there is evidence that bile acids may alter critical proteins in microbiota through unfolding, aggregation and disulfide stress, thereby altering their biological functions.As the disease progresses, bile secretion and transport are impaired, with the weakened function of bile acids, leading to further intestinal microbiota disorders. In general, bacteria tend to overgrow and translocate in the small intestine causing a series of subsequent detrimental effects due to biliary obstruction. Collectively, the interaction between gut microbiota and bile acids is bidirectional, associated with the initiation and progression of PBC.

By altering the bile acid pool in PBC patients, it is possible to correct a part of gut microbiota dysregulation and reduce bile duct damage caused by cholestasis. As already mentioned, UDCA is well recognized as the first-line treatment for PBC that can effectively improve patients’ survival and significantly decrease the risk of severe complications. However, some patients have poor improvement after taking the drugs. Currently, regulation of gut microbiota is expected to be a potential therapeutic target for PBC. Fecal microbiota transplantation, as an innovative clinical therapy, can improve the composition and function of gut microbiota in recipients via introducing the specially treated fecal suspension from healthy donors into the intestines of patients. A study [24] performed microbiota transplantation in 10 patients with cholestatic liver disease to restore the disrupted microbiota and ameliorate bacterial imbalances. Overall, the bacterial diversity improved in all patients after fecal microbiota transplantation in early week and alkaline phosphatase levels decreased in 30% patients. Another phase I clinical safety trial also has found that the patients with decompensated cirrhosis could restore metabolic capability after fecal microbiota transplantation by inducing the secretion of short-chain fatty acids and modulating bile acid profiles, especially in the shortterm after treatment [25] . Therefore, regulation of gut microbiota by prebiotics, probiotics or small molecules with therapeutic benefits harvested from the microbiota may be the new tools for PBC.

In summary, gut microbiota plays a considerable role in the progression of PBC by regulating enterohepatic circulation and immune responses. They are able to alter bile acid pool, and in turn, bile acids can shape the gut microbiota community. There is still much remained to be discovered about the specific mechanisms between microbiota, bile acids and PBC. In order to carry out fecal microbiota transplantation precisely, accurate identification and quantitative analysis of gut microbiota should be the primary step to develop a new therapeutic approach. Therefore, future safety and efficacy trials are required to better refine the underlying mechanisms of prebiotics, probiotics and fecal microbiota transplantation in order to explore the role of microbiota as diagnostic and prognostic tools.

Acknowledgments

None.

CRediT authorship contribution statement

Wu-Meng Jin: Data curation, Investigation, Writing - original draft. Si-Yue Song: Investigation, Writing - review & editing. Xiao-Fen Xu: Investigation, Writing - review & editing. Cheng-Ping Wen: Conceptualization, Writing - review & editing. Chang-Feng Hu: Conceptualization, Funding acquisition, Writing - review &editing.

Funding

The study was partially supported by a grant from the Natural Science Foundation of Zhejiang Province ( LY20B050 0 06 ).

Ethical approval

Not needed.

Competing interest

No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article.