Although certain studies have shown that the heritable risk of Crohn's disease (CD) is greater than that of UC, the risk of the disease in first-degree relatives of UC patients is still four times greater than that of the general population, suggesting that the risk of genetic factors remains a contributor to the occurrence of UC (13, 14). The first genome-wide association studies based on IBD identified IL23R, a gene encoding the pro-inflammatory cytokine interleukin (IL)-23, whose abnormalities and mutations are associated with the development of CD and UC (15). To date, more than 240 risk loci have been identified to be associated with IBD (16). It was interesting that only 4–7% of UC occurrence can be explained through known risk loci, but genetic factors appear to be more important for early IBD that develops in children (17, 18).

The disease-associated loci of involving genes with different functions, such as the innate and adaptive immune systems, cytokine signaling, lymphocyte activation, and response function to microbial molecules (17). For example, variants in CARD9, a bridging protein involved in antifungal innate immunity, enhance the host response to the fungus and increase the production of inflammatory cytokines, and is also considered as one of the genetic risk factors for CD and UC (19). Moreover, the ADCY7 gene is expressed in hematopoietic cells and encodes a protein, adenylate cyclase 7, which converts ATP to cAMP to participate in the regulation of host innate and adaptive immunity (20, 21). And it is reported that the missense variant of ADCY7 increases the risk of UC. For other IBD risk loci, a complete description was given in the research article published in 2017 (16).

So far, the genetic risk of CD has been well studied, and a large number of studies have shown a strong genetic predisposition for CD. For example, disorders of the NOD2 gene have been identified as an essential causative factor for CD occurrence (22). At the same time, researches on the genetic risk of UC and IBD is continuing, which not only facilitates the development of the pathogenesis of IBD and the differentiation of IBD subtypes but also contributes to the updating of therapeutic targets and drugs or programs for the disease.

Genetic, environmental and microbial factors have been identified as risk factors of UC (22). However, UC is ultimately linked with immune abnormalities, which indicated immunological factors may be the central link (1). The human immune system can be divided into innate immunity and adaptive immunity according to functions, and the available evidence points to the involvement of both innate and adaptive immunity in the pathogenesis of UC (23).

The innate immune response is the first line of defense against any attack, and the neutrophils, dendritic cells or macrophages that mediate innate immunity has been confirmed to be involved in the pathogenesis of UC. The earliest feature of intestinal inflammation is the infiltration of neutrophils into the mucosa and epithelium, and is present throughout the period of active inflammation of the intestine. The tissue and epithelial barrier disruption are caused by neutrophils with oxidative and proteolytic damage, as well as promote the release of pro-inflammatory cytokines to perpetuate inflammation (24). Dendritic cells are also involved in the expression of Toll-like receptors (TLRs) and the recognition of microbiota (25). Increased activation and sensitivity of mature dendritic cells and significantly higher the level of TLR2 and TLR4 expressed by mucosal dendritic cells are observed in UC patients, which lead to abnormal activation of signaling pathways such as the nuclear factor κB (NF-κB) to promote the inflammatory cascade (26). Macrophages can be polarized into classically activated pro-inflammatory or alternatively activated anti-inflammatory macrophages depending on the stimulus (27). The pro-inflammatory macrophages are induced by pathogen-associated molecular patterns and involved in the production of pro-inflammatory cytokines such as tumor necrosis factor-α (TNF-α), IL-1β and IL-6 to intensify inflammation (28). In addition, the polarization of pro-inflammatory macrophages has been found to promote the development of colitis. In conclusion, excessive abnormalities in innate immunity and the occurrence of inflammatory cascade aggravate the emergence and persistence of local inflammation in the colon, which is closely associated with the occurrence of UC.

After recognizing of antigens, the dendritic cells and macrophages will present antigens to T cells and B cells, leading to the activation of adaptive immunity (29). And the abnormalities of adaptive immunity are another risk factor of UC (30). UC is thought to be a disease mediated by T helper type (Th) 2 cells, which are involved in the secretion of IL-4, IL-5 and IL-13. Published studies have shown the increase in IL-5 and IL-13 secretion in UC patients, with IL-13 affecting the intestinal epithelium and disrupting tight junctions to cause an inflammatory state (31, 32). In addition, abnormalities in Th9 and Th17 cells provide evidence for the involvement of adaptive immunity in UC. Th9 cells are associated with the release of IL-9, which can inhibit the repair of intestinal epithelial cells and increase the concentration of TNF-α (33). Similarly, Th17 cells are also involved in the release of pro-inflammatory cytokines such as IL-17A, IL-17F, IL-21 and IL-22, and increased Th17 cells expression has been observed in UC patients (34, 35). In summary, current evidence suggests that both innate immunity and adaptive immunity are critical in the pathogenesis of UC (1).

Abnormalities of the immune system are certainly important for the pathogenesis of UC, which is also a major reason for the appearance and persistence of local inflammation in UC patients. However, except for the immune system, the mucus and epithelial barriers are the first physical and chemical lines of defense protecting the intestinal epithelium from pathogens and antigens. Mucus barrier disorders in the colon are also a possible cause of UC. Moreover, a recent study has shown that the disorders of mucus barrier were not related to local inflammation and immune response, but the response of goblet cells to microbial alterations (74). In addition, defects in the mucus barrier occur early in the onset of UC, further leads to increased intestinal permeability and exposure to antigens, thus activating abnormal immune responses (22, 23). This suggests that it was necessary to consider other risk factors. Notably, there is growing evidence that dysbiosis of intestinal microbiota is associated with disruption of the intestinal barrier, increased permeability, and increased antigen exposure.

Bacteria occupy the absolute advantage of intestinal microbiota, but fungi, viruses, and archaea are also important parts of the intestinal microbiota. Microbiota maintains a dynamic balance through complex interactions and connections, and the homeostasis is a key to maintaining host healthy. The composition of intestinal microbiota and the interactions between microorganisms are often altered by the risk factors such as genes, eating habits, unhealthy lifestyles, and drug abuse. For example, a long-term high-fat diet thins intestinal mucus, increases intestinal permeability, and impairs the tight junctions of intestinal epithelial cells, leading in intestinal barrier dysfunction (75). Additionally, the intestinal barrier function tends to deteriorate as the body matures (76). Increased intestinal permeability and breakdown of the intestinal barrier allow bacteria to readily pass past the epithelium and colonize the mucosa, hence amplifying their effect on the intestine (77). Although intestinal commensal microbiota contributes to the regulation of intestinal epithelial and immune function in the physiological state, risk factors such as diet, environment, and age can also result in dysbiosis of the intestinal microbiota, as revealed by decreased diversity and changes in specific microbial species and abundance (3). Inequality within each microbial community and changed interactions contribute to overall microbial homeostasis imbalance (78). Reduced probiotic bacteria and a rise in pathogenic and opportunistic pathogenic bacteria are significant aspects of microbial homeostasis imbalance in UC patients (79–81). The intestinal microbiota can directly interact with the immune system of the body (82). Certain Clostridium bacteria demonstrate pro-inflammatory properties by penetrating and mucosalizing the intestinal mucosa. Similarly, increasing bifidobacteria abundance has been shown to stimulate Th1 cell-mediated immune responses (83). Additionally, TLRs identification of bacterial and fungal surface components results in excessive activation of TLRs signaling pathways, triggering innate and adaptive immunity and chronic inflammation when the gut barrier is disrupted and pathogenic bacteria colonize. The metabolites generated by intestinal microbiota play a role in modulating host immune responses, such as SCFAs and bile acids, which are critical for sustaining anti-inflammatory effects and safeguarding intestinal barrier function (84, 85). Likewise, a decrease in the absolute abundance of SCFAs-producing bacteria is another unique symptom of microbial dysregulation in UC patients. Thus, the combination of reduced intestinal barrier function, microbial homeostasis imbalance, and lack of beneficial microbial products results in disruption of the intestinal epithelium and an abnormal immune response, which is a significant contributor to the development of UC.

The intestinal microbiota is a very complex and extensive system. Although the mechanisms of bacteria and fungi in the occurrence of UC are increasingly obvious, members such as viruses and archaea are also related to UC. Therefore, more in-depth studies are needed to explore the relationship between the intestinal microbiota and UC. At the same time, drugs and treatment methods focusing on restoring intestinal microbiota homeostasis may be the key to the treatment of UC.

DSS is normally used to induce experimental colitis by dissolving it in water and provided ad libitum for several days to animals. It is generally believed that DSS can induce acute UC in SD rats and Winstar rats at a concentration of 2–5% added to drinking water for 5–9 days. At the same time, adding 3–5% of DSS in drinking water for 5–8 days can induce acute UC in C57BL/6 mice and BALB/C mice (88). Multiple repeated drinking cycles are required if model is necessary to induce chronic UC (89). DSS can cause complete loss of intestinal epithelial cells to destroy the epithelial barrier. The lamina propria and submucosa are subsequently exposed to antigens and microbiota in the intestinal lumen, which eventually induced inflammation (90). During the administration of DSS, animals showed weight loss, thickening of stool, bloody stool and diarrhea etc. (91). Histopathological changes such as epithelial denuding, loss of mucin and goblet cells, submucosal edema, hyperemia and erosions, infiltration of inflammatory cells in the lamina propria and loss of crypt structures could be observed (92, 93).

The DSS-induced UC model has the characteristics of simple administration, easy control of drug dose and duration. Moreover, researchers can design acute or chronic experimental colitis by controlling the dose and duration of administration. Meanwhile, due to the induction of severe intestinal ulcer in animals, this model is also been interpreted as the animal model closest to human UC (94). Similarly, DSS-induced model also showed changes in the intestinal microbiota. The decrease in microbial diversity and abundance of beneficial symbiotic bacteria (Lactobacillus and Alistipes), and the increase of the abundance of pathogenic bacteria (Oscillibacter, Streptococcus, and Escherichia–Shigella) are similar to the changes of intestinal microbiota in UC patients (87, 95), which makes it also has advantages in studying the effects of drugs on the intestine microbiota. However, DSS-induced UC model also has obvious limitations. It is reported that DSS-induced inflammatory response began to ease naturally after 7 days, and the changes in the intestinal microbiota returned to normal levels after 21 days (87, 95). Besides, the sensitivity of certain types of mice to DSS is also different. After 5 days of acute administration, C57BL/6 mice induced colon inflammation, while BALB/C mice recovered simultaneously (94). Since UC itself is a chronic disease, DSS-induced model still lacks sufficient representation for UC.

Rats were first used in TNBS-induced experimental colitis modeling. 5-30 mg TNBS into 0.25 ml of 50% ethanol, and the colon ulcer and inflammation in rats were successfully induced by intraluminal drip administration. The BALB/C and C57BL/6 mice are also used for colitis models by 0.3–5.0 mg of TNBS mixed with 50% ethanol (96). In TNBS-induced animal model, the use of ethanol is not only a solvent or carrier but also can destroy epithelial barrier of animals. TNBS is a hapten reagent that induces the immune response of T cells to tentacle proteins and luminal antigens through acute oxidative stress, resulting in intestinal wall necrosis (97). After administration of TNBS, animals showed bloody diarrhea and weight loss. Histopathological changes can be observed in intestinal wall thickening, edema, bleeding, and ulcer (98). Chronic TNBS colitis is manifested with diffuse necrosis of the intestinal wall, involving the mucosa, submucosa and muscle layer. At the same time, there are significant edema and immune cell infiltration in submucosa (99).

The TNBS-induced colitis model has the advantages of rapid disease development, localized colon injury and low cost. Compared with DSS-induced UC model, animal inflammation does not begin to relieve until the 15th day, which seems to be more conducive to researchers to ensure the accuracy of their research results (97). However, TNBS-induced animal model is associated with the immune response mediated by Th1 cells, which is considered closer to the immunological changes of CD. Moreover, the intestinal microbial changes in TNBS-induced mice showed a decrease in α-diversity, possibly beneficial bacteria decreased and harmful bacteria increased, similar to those in patients with Crohn's disease (100). Interestingly, some experts believe that it causes intestinal ulcer in animals to be similar to UC (101). Hence, it is still doubtful whether the TNBS-induced colitis model can represent human UC.

OXA is also a hapten reagent that needs to be administered with ethanol to induce colitis in animals (102). Before administration, 3% OXA (100–150 μl) was dissolved in 40–50% ethanol to sensitize the skin of mice and induce experimental colitis (103). The sensitivity of different strains of mice to OXA is also different. SJL/J mice and C57BL/10 mice are highly susceptible to OXA, but C57BL/10 mice are more resistant than SJL/J mice. BALB/C mice are more suitable for inducing chronic colitis (104).

The OXA-induced mice have symptoms of diarrhea, bloody stool, and weight loss. Histopathological changes are mainly characterized by epithelial cell loss, inflammatory cell infiltration, edema, and occasionally crypt abscess (105). As a kind of hapten reagent, OXA-induced colitis in mice is closely related to Th2 cell-related immune response, which is similar to human UC (91, 106). Compared with other animal models induced by chemical drugs, The OXA-induced model is more suitable for studying the pathology of UC. However, due to the high mortality rate of OXA, it is rarely applied in practical research (94, 105). In experimental animals, whether OXA can cause microbial disorders similar to human UC still needs more research.

As is well known, the application of drugs is the key to successful modeling of chemically induced animal models. However, intestinal microbiota has also been found to play a role in different animal models. A study found that during the induction of colonic inflammation in mice with DSS, an increase in the proportion of cells expressing CD11c and TLR4 in the mesenteric lymph nodes of mice was consistent with an increase in the number of Enterobacteriaceae and Akkermansia on the colonic mucosa, implying that the response of the immune system to microbial recognition and the induction of inflammatory responses (107). Similarly, in TNBS and OXA-induced inflammation models, the abundance of Sulfate-reducing bacteria, which produce hydrogen sulfide that damages the colon, causes epithelial damage and inflammation, and induces a Th17 cell-associated immune response, was increased (108, 109).

All of these studies provide that microbiota is also vital in UC animal models. However, there are existed those different conclusions were from other experts. Gancarcikova et al. found the absence of microbiota did not affect the inflammatory effects of DSS when exposed antibiotic-treated germ-free mice to DSS (110). Another study found that in the absence of intestinal microbiota, the function of the intestinal barrier was also impaired although intestinal inflammation was significantly reduced in animals exposed to DSS (111). Obviously, the intestinal microbiota is a very complex organization, and different microorganisms play different roles. Therefore, further studies on the specific effects of microbiota in animal models are still necessary to resolve the current controversies, but this does not affect the use of animal models as a tool for UC research in subsequent studies.

The establishment of animal models is necessary to study the pathogenesis and pathological changes of UC. So far, the DSS-induced mice model has been most widely used. The intestinal microbiota of mice showed similar diversity to humans at the levels of Firmicutes, Bacteroidetes, and Proteobacteria. And the DSS-induced model is considered to be similar to the changes in the intestinal microbiota in UC patients. Therefore, DSS may be the current advantageous animal model for studying the relationship between intestinal microbiota and UC. However, the DSS-induced model may still be a limited result of obvious inflammation and rapid recovery of intestinal microbiota disorder, which still needs to be improved and developed.

Notably, many researchers have begun to use germ-free mice as an important tool to mimic the close relationship between intestinal microbiota and host (112). Studies ranged from single microorganisms to whole microbiota can be carried out in germ-free mice (23). For example, the intestinal bacteria highly coated with immunoglobulin A in IBD patients were isolated and cultured, and then transplanted into germ-free mice, which were eventually found to exhibit a high susceptibility to DSS-induced colitis (113). Consequently, germ-free mice may be a promising UC animal model, and the studies based on germ-free mice are also more conducive to revealing the specific link between intestinal microbiota and UC pathogenesis.

The use of 5-ASA dates back to 1941 when sulfasalazine (SASP) was used for the treatment of UC. SASP is composed of 5-ASA linked to sulfapyridine via a diazo bond, which can be cleaved by bacteria in the colon. After colonic bacterial azoreductase enzyme cleaves the diazo bond, 5-ASA achieves a high intraluminal concentration in the colon, makes it the active moiety of SASP (119, 120). To date, mesalazine and SASP are the main 5-ASA drugs used in the treatment of UC.

5-ASA acts on the colonic epithelium and exerts local mucosal anti-inflammatory effects by inhibiting cyclooxygenase and lipoxygenase, which subsequently leads to a decrease in prostaglandin and leukotriene production (121, 122). It has also been reported that 5-ASA inhibits the activation of the NF-κB signaling pathway, which promotes the transcription of pro-inflammatory cytokines and is an important mechanism in the pathogenesis of UC (123). In addition, 5-ASA also inhibits the function of active lymphocytes, macrophages, and natural killer cells in the inflammatory process, which can scavenge the reactive oxygen metabolites (122). In conclusion, the local mucosal anti-inflammatory effect of 5-ASA is the main mechanism of action for treating UC.

Both American Gastroenterology Association and European Crohn's and Colitis Organization recommend 5-ASA as the first-line therapy for mild to moderate UC (116, 117). And the 5-ASA compound is a pillar for patients with mild to moderate ulcerative colitis, both as induction and maintenance therapy. Studies have shown that one-third of mild to moderate UC patients achieved clinical remission and half of the patients had mucosal healing after 8 weeks of oral 5-ASA treatment (124). Moreover, up to three-fifths of patients showed significant clinical remission and endoscopic improvement with 5-ASA maintenance therapy, which strongly supports the effectiveness of 5-ASA (125). In terms of specific drug use, there was no significant difference in efficacy between mesalazine and SASP, but patients with SASP often experienced adverse events (124, 125). However, 5-ASA still has a high safety profile compared to corticosteroids and thiopurines, and long-term 5-ASA therapy may have a preventive effect on colorectal cancer (126, 127). Notably, several studies have demonstrated that 5-ASA (including SASP and mesalazine) can restore the microbial diversity and the abundance of beneficial bacteria and fungi, reduce the abundance of pathogenic bacteria, and increase the production of SCFAs in experimental colitis and UC patients, which implying that 5-ASA also has the potential to modulate gut microbial homeostasis (80, 128, 129). Unfortunately, 5-ASA is not available for all UC patients. First, there are still some patients who do not respond to or are intolerant of 5-ASA therapy, and such patients usually dependent on corticosteroids. Second, for patients with moderate to severe UC, guidelines suggest that 5-ASA should not be used for induction or maintenance therapy (116, 117). Thus, corticosteroids, thiopurines, and even advanced therapies are in development after 5-ASA.

Corticosteroids are used in patients with mild to moderate UC who are unresponsive or intolerant to 5-ASA therapy. In addition, corticosteroids are the treatment of choice for patients with moderate to severe UC. The first use of corticosteroids in treating UC was reported in 1955, with the surgical resection rate of the colon was significantly lower in UC patients treated with cortisone than in the placebo group (130). After that, the first generation of corticosteroids such as prednisone and hydrocortisone began to be widely used in UC. Corticosteroids act in the cell nucleus and ultimately play a role in regulating the immune response by inhibiting gene expression during transcription, down-regulating the production of transcription factor NF-κB and the expression of pro-inflammatory cytokines, and causing a decrease in the expression of adhesion molecules to limit the transfer of immune-inflammatory cells to inflammatory areas (131–133).

Since the immunosuppressive effects of first-generation corticosteroids are non-specific, inevitably, other body parts besides the intestinal inflammatory sites are also affected. As a result, up to 90% of UC patients experienced adverse effects after corticosteroid treatment (134), which led to the creation and development of second-generation corticosteroids such as beclomethasone dipropionate and budesonide. Compared to first-generation corticosteroids, second-generation corticosteroids can target the site of inflammation to exert local anti-inflammatory effects, to potentially reduce systemic corticosteroid concentrations (135). Similarly, the use of second-generation corticosteroids has an overall better safety profile and a reduced incidence of adverse events (136). However, all corticosteroids are absorbed by the body to some extent, and this leads to a continued occurrence of adverse events. In a study evaluating the safety of budesonide MMX, a novel Multimatrix formulation of budesonide, adverse events were reported in 31.8% of patients (137). In conclusion, as hormonal drugs, the therapeutic risk of corticosteroids rises with increasing dose and duration of exposure (138). In addition, steroid dependence is an issue that has to be considered (139). Therefore, corticosteroids are not recommended as the first choice for maintenance therapy, and corticosteroid use should be tapered or discontinued after clinical remission.

The first application of thiopurine for UC was in 1962 (140). To date, thiopurines are the most commonly used drug for maintenance treatment of UC after 5-ASA (141). Thiopurines are recommended for maintenance therapy without corticosteroids. In addition, the use of thiopurines is necessary for patients with moderate to severe UC who have developed steroid-refractory or steroid-dependent. Thiopurine analogs include azathioprine, mercaptopurine, and thioguanine, of which thioguanine is considered an atypical thiopurine drug, and is used only in those that have failed to respond to mercaptopurine and azathioprine in a few countries and regions (121). As a type of immunosuppressants, the mechanism of action of thiopurines may be the incorporation of their pharmacologically active metabolite, 6-thioguanine nucleotides, into DNA or RNA as false purine analogs, which lead to DNA damage, cell cycle arrest and apoptosis, and inhibition of nucleotide and protein synthesis (142). Ultimately, the expression of inflammatory genes can be inhibited (142).

Although thiopurines are thought to play a corticosteroid-sparing role in the treatment of steroid-dependent UC patients, it remains controversial in using thiopurines. Firstly, thiopurines have significant toxic effects, including bone marrow suppression, impaired red blood cell regeneration, and death in rare cases (143). Secondly, the use of thiopurines may increase the incidence of lymphoma and non-melanoma skin cancer in patients (144). Therefore, the treatment of thiopurines needs to be effective in a way that ensures a reduced risk of side effects, which often requires a rigorous evaluation by the physician.

Undoubtedly, 5-ASA, corticosteroids, and thiopurines have controlled the course of UC, and even saved the lives of a large number of UC patients. But the limitations of these drugs are also obvious and led to the search for more effective and safer drugs, which is an important reason for the rapid development of more advanced treatment strategies such as biologics and small molecules products. In addition, the microbial dysbiosis often occurs at the early stage in the occurrence of UC, suggesting that the therapeutic strategy based on restoring the microbial balance may be beneficial (145). Therefore, fecal microbiota transplantation (FMT), probiotics, prebiotics and synbiotics have become potential therapeutic approaches for UC and researched in fronter. It is also mentioned that TCM has potential advantages in the treatment of UC and should be paid more attention to (146).