Appendix and Ulcerative Colitis: a Key to Explaining the Pathogenesis and Directing Novel Therapies?

Abstract

The vermiform appendix is generally considered a redundant organ, but recent evidence suggests that the appendix could contribute to the pathogenesis of inflammatory bowel diseases, in particular ulcerative colitis (UC), and may even have a therapeutic role; however, mechanisms of the appendix involvement remain unclear. Here, we highlight current evidence on the link between the appendix and UC and consider plausible therapeutic implications. A literature search was conducted using PubMed and PubMed Central from inception to Nov 2021 using the terms “Appendix”, “UC”, “Appendix & UC,” “Appendectomy”, and “Peri-appendicular patch,” including only articles published in English. Reference lists from the selected studies were manually searched and reviewed to gather additional related reports. Inflammation around the appendix (“peri-appendicular patch”) has been frequently observed in UC patients without other cecal involvement, and this inflammation can even precede the onset of UC. Epidemiologic studies propose that appendectomy reduces the risk of developing UC or even the risk of flare after UC is diagnosed, although this remains controversial. We reviewed studies showing altered host-microbe interactions in the appendix in UC, which suggest that the appendix could act as a priming site for disease via alterations in the immune response and changes in microbiota carried distally to the colon. In summary, recent literature suggests a possible role for microbes and immune cells within the appendix; however, the role of the appendix in the pathogenesis of UC remains unclear. Further research could clarify the therapeutic potential related to this organ.

Key Messages.

What is already known?

• Appendectomy appears to be protective in ulcerative colitis (UC).

• Periappendicular involvement in UC remains unexplained.

What is new here?

• The appendix may have a key role in the pathogenesis of UC.

• We propose that microbes and immune cells originating in the appendix could fuel inflammation in the colon in patients with UC.

• Many gaps in knowledge remain, highlighting the need for further research on the role of the appendix in UC.

How can this study help patient care?

• Understanding how the appendix is involved in UC could assist in defining disease pathogenesis, identifying patients at risk, and possibly introduce novel therapies for UC.

Introduction

Inflammatory bowel diseases (IBD), including Crohn disease (CD) and ulcerative colitis (UC), are characterized by chronic, relapsing intestinal inflammation, with incidence rates rising globally.¹ The precise etiology remains poorly understood, yet it is thought to involve genetic susceptibility, alterations in intestinal microbiota, epithelial barrier defects, immune dysregulations, and environmental factors; this ultimately drives extreme and uncontrolled immune response against resident microbiota via the activation of innate immunity and mucosal lymphoid response.^2–4

Ulcerative colitis is generally limited to the colon and rectum and is characterized by superficial inflammation of the mucosa and submucosa.³ The pathogenesis of UC is complex and multifactorial. Increased mucosal permeability, or leaky gut, is postulated in UC, although it is mostly recognized as a hallmark for CD.⁵ Recently, Kuwada et al demonstrated that most UC patients (103 of 112) have anti-integrin αvβ6 auto-antibodies, with higher titers in more severe disease; because αvβ6 integrin plays an essential role in epithelial barrier functions, this study supports possible mechanisms underlying increased permeability in UC.⁶ Furthermore, the characteristic gut inflammation in UC is likely driven by excessive pro-inflammatory reactions within the mucus layer, epithelial cells, and lymphoid tissues.^7,8

Mucosal inflammation in UC typically begins in the rectum and extends proximally in a continuous manner to a transition zone (excluding cases of complete pancolitis), with the occasional exception of the peri-appendicular region. The appendix, which opens into the cecum, has been shown to display important immune functions and impacts on the colon. Recent studies have focused attention on a potential role for the appendix in the pathogenesis of UC, although clinical studies remain mostly correlative.⁹

The vermiform appendix has been considered a vestigial organ in the human intestine for decades, as evidenced by the common clinical decision to remove the appendix during surgery (appendectomy), even in cases without a clear pathology.^10,11 Interestingly, several studies support a role for this seemingly dispensable organ in the development and maturation of the immune system, maintenance of the intestinal microbiome, and development of gut-associated lymphoid tissue (GALT), which is especially abundant in the appendix.^12–14 The location of the appendix—which is protected from the fecal flow in the colon—and its narrow tube morphology have led to a thought that infection and antibiotics could have less of an impact on its microbiota. As a result, researchers have speculated that the appendix may in fact function as a “safe house” for the normal bacteria of the colon, thereby acting as a reservoir for the commensal/beneficial intestinal microbiota that could repopulate the intestinal tract following antibiotic use or infections. This is well demonstrated in the case of Clostridioides difficile, where prior appendectomy has been shown to increase the risk of recurrence for this infection.^15–17

In contrast, researchers have hypothesized that the appendix in UC may function as a reservoir for “dysbiotic microbiota” capable of promoting and re-establishing dysbiosis.^18,19 One study utilized the T cell receptor α chain knockout mouse model to show that colonic inflammation in UC could be suppressed by appendectomy, supporting a detrimental role for the appendix in this model.²⁰ Other studies support that appendectomy can reduce UC severity by preventing appendiceal immune stimulation of the intestine (possibly via secretion of pro-inflammatory cytokines from the appendix) and recolonization of abnormal/pathobiont bacteria in the colon from the appendix reservoir.^21–25 In fact, appendectomy has even been suggested as a potential therapy for UC, although with limited mechanistic evidence to support this intervention; the clinical rationale remains controversial. In this review, we aim to summarize the current knowledge on the vermiform appendix and its intriguing connection with UC, identify gaps in research, and offer ways to fill these gaps moving forward; our literature search included all case-control, cohort, randomized, observational studies, and review articles available on PubMed with keywords “Appendix,” “UC,” “Appendix & UC,” “Appendectomy,” and “Peri-appendicular patch.” As most data were not quantifiable, this review does not focus on meta-analysis but rather on the rationale and narrative connecting the appendix to UC.

Unique Features and Unclear Pathogenesis of UC: Opportunities and Remaining Questions

Two important factors affecting the development of IBD are the gut microbiota and the immune system.^26,27 Hallmark changes in the gut microbiota of UC patients include decreased diversity with reduced commensal microorganisms, along with increased abundance of pathobionts and pathogens. In fact, reduced bacterial richness predicts failure of intravenous steroids in children with acute severe UC²⁸; however, it is important to note that most studies on microbial involvement in UC are during active disease, limiting the support for causality in this case, which is likely better established for CD.²⁹ This imbalanced gut microbiota results in impaired intestinal homeostasis, which is essential to prevent extreme growth of harmful microbes and, consequently, overactive host immune responses; although it remains unclear whether these associations are causative.^8,30

The appendix has been implicated in UC through numerous interesting, yet unexplained, clinical observations.^21,31–34 For example, inflammation involving the orifice of the appendix, termed peri-appendicular or cecal patch (PAP), has been reported in UC patient cohorts in studies with an extensive frequency range, from 12% to 87%.^35,36 Furthermore, appendectomy has been suggested to provide protective effects in UC, and its use as an intervention could lead to lower relapse rates and decreased risk of colectomy in UC patients (both are discussed in detail below).²⁵

As researchers continue to reveal the role of the appendix in UC, it is important to consider the key questions that remain unanswered surrounding UC pathogenesis: Why is colonic inflammation only superficial in UC? Why is inflammation observed only in the colon and in a contiguous fashion? Why is the only site of noncontiguous inflammation located within the cecum, near the appendiceal neck? Can microbiota housed in other areas of the gut, including the appendix, explain these phenomena? We believe that the appendix may offer important insight and answers to these questions.

The Appendix: an Under-recognized Immune and Microbial Organ

The appendix first appeared in the annals of medical literature in the 1500s and was later recognized for its potential to become afflicted by inflammation (appendicitis) in the 1800s. This was followed by Charles Darwin’s mention of the organ in his vestigial theory, in which he proposed that the appendix was used by more “primitive species” for digesting food.³⁷ It was not until 2007 that researchers provided the first evidence for the role of the appendix in both digestion and immunity, acting as a possible safe house for beneficial microbes enlisted to re-establish the gut microbiota following loss of beneficial microflora (due to infection or antibiotic exposure, for example). In contrast, a pivotal role of systemic and mucosal immunity of the appendix in other mammals, such as rabbits, has been long established. For these reasons, removal of the appendix has been traditionally performed without much consideration of the underlying pathology.^10,11,15

The appendix is a hollow tube that is closed at one end and is connected to the cecum at the other. Much like the colon, the appendiceal wall comprises of serosa, muscularis externa, submucosa, and mucosa; however, the presence, quantity, and function of cells vary between the appendix and the colon. Specifically, lymphoid follicles can be found in greater numbers within the submucosa and lamina propria layers of the appendix wall compared with the limited lymph nodes of the lamina propria of the colon.^38,39Figure 1 schematically illustrates the layers of a healthy appendix and colon.

Figure 1.

A schematic illustration of the appendix and colon layers with their lymphoid tissues and commensal microbes. The appendix and colon layers consist of serosa, muscularis externa, submucosa, lamina propria, and mucosa. In contrast to the colon, the appendix has more abundant and pronounced lymphoid follicles and likely a different microbiome. This figure is created with BioRender.com.

Appendicitis, an acute appendiceal inflammatory process, develops under unclear specific circumstances, and its cause remains obscure. The lifetime risk of appendicitis is 8.7% in men and 6.7% in women, which makes appendicitis the most common clinical abdominal emergency requiring proper surgical intervention, with the peak age of its occurrence in the second and third decades of life (much like IBD). The surgical removal of this organ has been the primary intervention for decades; however, need for surgery has recently been questioned by some pioneering studies.^37,40 Common histologic and microbial characteristics of acute appendicitis include mucosal ulcerations, an altered microbial profile, transmural penetration of neutrophils, and finally perforation and serositis.^41,42

Recent evidence suggests that specific alterations in appendiceal microbiota could play a crucial role in the pathogenesis of acute appendicitis.⁴² A study by Hattori et al indicated that the increased diversity of some anaerobic bacteria, such as Bacillus species, Fusobacterium nucleatum, and Bilophila wadsworthia, is linked to the development of acute appendicitis.⁴³ There are a number of microbes that have been cultured from the appendix in acute appendicitis cases, including Clostridioides difficile, Bacteroides fragilis, Escherichia coli, and Fusobacterium spp; some of these bacteria are also associated with IBD.^44–48 Arlt et al showed that there is a noticeable reduction in the abundance and diversity of microbial communities in inflamed appendix tissues in comparison with noninflamed appendices.⁴⁹ However, these studies do not prove causality but rather an association, as changes in microbes may just reflect the inflammatory process.

Beyond appendicitis, research has begun to support a role for the appendix in gut homeostasis and disease pathogenesis. The findings that support this idea are primarily related to the immune system, as host interactions with the gut microbiota play an important regulatory role in immune functions through stimulation of pro- and anti-inflammatory responses. Indeed, opportunistic pathogens cause inflammation during infection while some symbiotic microbiota trigger anti-inflammatory responses, resulting in immune system education. Studies in germ-free animal models have proven the importance of microbes in the maturation of host immunity. Germ-free animals display considerable defects in GALT development and antibody production, as well as rather smaller size and number of Peyer patches and mesenteric lymph nodes (MLNs) in comparison with conventionally housed animals.⁵⁰ A study by Rhee et al demonstrated that specific commensal bacteria can promote GALT development in germ-free appendix of rabbit models, signifying the dynamic relationship between microbiota and host immune system.⁵¹ Therefore, a potential role for the appendix in homeostasis and disease pathogenesis is likely mediated by microbes and immunity.

Microbial and Immune Links Between the Appendix and UC

As microbes and the immune system are very intimately connected in the gut, it is difficult to completely separate these 2 factors when discussing IBD pathogenesis.

Inflammatory bowel disease animal models have shown that intestinal inflammation is significantly reduced in a germ-free environment, demonstrating a critical role for intestinal microbiota in the development of IBD.⁵² Some pathogens linked to IBD include Mycobacterium avium, C. difficile, E. coli, Listeria monocytogenes, and Campylobacter sp.⁵³ Interestingly, mucin-producing goblet cells are decreased in UC patients, with a reduction of MUC2, resulting in a thinner colonic mucous layer and disrupted mucosa with changes in the microbial composition, even in the uninvolved terminal ileum.⁵⁴ As a result, bacteria can penetrate to reach the epithelial cells and stimulate host-cells immunity in genetically susceptible individuals.^4,26,27

In the appendix, the microbiome may affect the host through interactions between appendiceal biofilms (commensal microbial communities, typically contained within the mucus layer) and the colon; this interaction is hypothesized to result in reintroduction of commensal microbes into the colon after they have been eliminated from the gut during antibiotic use, infection, or chronic diarrheal diseases.^13,16,55 Although empiric research is limited, biofilms appear to be more abundant in the appendix in comparison with the colon and may allow commensal microbiota survival and permit their re-inoculation of the gut while excluding pathogens, improving clearance of pathobionts and pathogens and keeping them away from epithelial cells to avoid overstimulation of immune system.^55,56 Presence of considerable lymphoid tissue in the appendix further supports observations regarding the formation of biofilms and provides more backing to the idea that the appendix can play a role as a reservoir for commensal microbiota within biofilms.^57,58

Furthermore, Motta et al showed that gut biofilms are disrupted in UC (possibly related to the thinner mucous layer observed in the colon, but this has not been reported in the appendix); this could lead to dispersion of pathobionts, which can then invade intestinal epithelial cells and trigger colonic inflammation. We therefore speculate that the appendix could repopulate the colon with pathobionts (instead of commensals), fueling inflammation; its removal can prevent the pathobiont scatter.⁵⁹ Despite this research, little is known about the appendix microbial composition in healthy individuals, let alone in IBD.

Different types of immune cells, including antigen-presenting cells (dendritic cells and macrophages), T helper (Th), regulatory T (Treg), and natural killer T (NKT) cells have an essential role in the pathogenesis of UC through inducing, regulating, and suppressing inflammation. Pro-inflammatory cytokines such as tumor necrosis factor (TNF)-α, interleukin (IL)-6, IL-9, IL-13, and IL-33 are involved in this inflammatory process. Activated CD4⁺ T cells, which produce inflammatory mediators and cytokines, play a crucial role in the pathogenesis of UC. Indeed, Th2 cells and their cytokines, IL-4 in particular, have been considered to impact the development of UC.^60–62

The appendix, as a biological structure with important immunologic activities, could have a critical role in the immunopathogenesis of UC, as the production of inflammatory cytokines in this organ could trigger cascade responses. The appendix is a secondary lymphoid organ, containing multifollicular GALT with similar properties to Peyer patches. This positions the appendix as an inductive site for adaptive immune responses. Its submucosal layer is mainly composed of B cells and CD4⁺ Th cells, with an overlying follicle associated epithelium and activated antigen-presenting cells.^18,31 Luminal microbes and gut antigens are sampled by microfold (M) cells, specialized epithelial cells for sampling of antigen above intestinal Peyer patches and isolated lymphoid follicles, and then presented to adaptive immune cells within the GALT.^63,64 Enriched with GALT, the appendix may play an essential role in the detection of microbes and removal of pathogens. The lymphocytic content of the appendix differs in comparison with the rest of the gastrointestinal tract, displaying noticeable abundance of NKT cells, activated T cells, and immunoglobulin (Ig)A- and IgG-producing B cells, which are specific to the appendix and play a significant role as effectors in the immune responses to microbes.^65,66 The chemokine CCL21 is present on the luminal surface of high endothelial venules and lymphatic endothelial cells in parafollicular regions and might be an influential factor for these lymphocytes’ abundance. This chemokine boosts the recruitment of B and T lymphocytes to the lymphoid tissue of the appendix, as well as the movement of activated dendritic cells (DCs) back to lymph nodes through binding to CCR7.^41,66

The appendix lumen contains secretory IgA (sIgA), produced by appendiceal B cells. Secretory immunoglobulin A stimulates bacterial agglutination in the appendix, and mucin traps these agglutinated bacteria in the mucus layer, forming biofilms.⁵⁶ The IgA-producing lymphocytic B cells of the appendix also migrate to the colon, providing increased protection against pathobiont and pathogen species.⁶⁷ Immunoglobulin A is the most plentiful class of antibodies in the lumen of humans’ intestine, acting as a prime line of defense in protecting the gut epithelium from toxins and external pathogens with high affinity to mediate their elimination. In contrast, IgA attaches to commensal bacteria with low affinity to control their abundance and composition.^68,69 Intestinal IgA and bacteria have a mutualistic relationship, in which a diverse selection of IgA allows for maintenance of a balanced and diverse microbiota. This can boost extension of Foxp3⁺regulatory T cells, which can sustain homeostatic IgA responses in a regulatory situation. In a model of appendectomized mice, there was a postponed and reduced accumulation of IgA⁺ cells observed in the colon, leading to alterations in the colonic fecal microbiota composition. Likely as a result of this alteration in mucosal immunity, 4 weeks after an appendectomy there was a relative increased abundance of many disease-associated bacterial groups, such as Bacilli, Erysipelotrichi, and Gammaproteobacteria in appendectomized mice. However, other (likely mutualistic) microbial groups, including Mollicutes, Deferribacteres, Betaproteobacteria, and Deltaproteobacteria, were less abundant.⁶⁷

Additionally, follicular dendritic cells (FDCs) are present in appendiceal lymphoid follicles and naturally spread throughout the mucosa of the appendix during acute appendicitis, which can provide balance between tolerance and active immunity to commensal microorganisms that is vital to the inflammatory status.⁷⁰ Moreover, there is a distinct abundance of NK 1.1⁺ CD3⁺ T cells in the appendix that produce cytokines and chemokines swiftly following activation, possibly via intestinal bacteria, which results in inflammation.⁷¹

Natural killer T cells, a subset of CD1d-restricted T cells, can recognize endogenous and bacterial lipid antigens. Type 1 invariant NKT (iNKT) cells express a semi-invariant T cell receptor (TCR), Vα24-Jα18, in humans and Vα14-Jα18 in mice, revealing both adaptive and innate immune functions. Type 2 NKT cells express various TCRs, which have essential roles in regulating immunity to pathogens. These cells are abundant in the appendix compared with the small intestine and the colon, and their numbers tend to decline with age. Through producing IL-13, IL-17, TNFα, and IFN-γ, iNKTs can form an inappropriate Th1/Th2/Th17 type response and fuel inflammation in UC. Exposure to commensal microbes early in life might protect against the accumulation of iNKT cells and, consequently, the risk of UC. Figure 2 proposes a model for immune changes that could explain the link between the appendix and UC.^18,72–74

Figure 2.

The immune link between the appendix and ulcerative colitis. Interleukin (IL)-13, IL-17, INFγ, and TNFα, produced by NKT cells, lead to inflammation. Dendritic (DC) cells activate naïve T cells to T helper 2 cells, which could suppress or regulate the immune system. Follicular dendritic cells (FDCs) in interaction with B cells promote humoral immunity. Secretory IgA in the appendix is produced by B cells, resulting in the formation of biofilms. B cells can migrate to the colon, as well. This figure is created with BioRender.com.

The inflammation of the appendix in UC has a distinctive pattern compared with that observed during acute appendicitis, resembling the more chronic and autoimmune colonic inflammation seen during a UC flare, opposing the transmural histological variations observed during appendicitis.⁷⁵ This suggests that PAP inflammation in UC could in fact be a “skip lesion,” similar to that seen in CD, as well as a source for microbe- or immune cell-driven inflammation. Appendix inflammation is generally limited to the mucosa, resulting in lymphocytic alterations, including increased CD4/CD8 ratio in T lymphocytes, activation of CD25 in both T CD4⁺ and CD8⁺ lymphocytes (that are also increased in UC), as and more CD4⁺ CD69⁺ (early activation antigen) T cells. In keeping with its role as a secondary lymphoid organ, these variations indicate that CD4⁺ T cells may be activated in the appendix before migrating to the colon prior to the advent of UC. This could suggest that the appendix acts as a priming (and possibly triggering) site for UC.^31,76,77 Additionally, a remarkable enhancement in the differentiation of Paneth cell, goblet cell depletion, and also crypt abscesses incidence is seen in the appendix in UC with no noticeable neutrophil penetration, which is similar to the colonic inflammation seen in UC, rather than changes in acute appendicitis.^41,76

Together, we hypothesize that the appendix contributes to the host-microbe microenvironment, which enables the development of UC in the colon. Pathobionts present in biofilms in the appendix could seed the colon, and altered appendix immune cells could further drive inflammation, both contributing to the pathogenesis of UC. Figure 3 schematically illustrates the microbial and immune links between the appendix and UC. A list of cells found in the human appendix during ulcerative colitis with their presumed activities is provided in Table 1.

Figure 3.

Microbial and immunological links between the appendix and UC. The appendix and its biofilms are considered a safe house for resident bacteria, possibly seeding the colon with pathobionts in UC. Additionally, the appendix is rich in immune cells and can act as a priming site for UC, triggering inflammation. This figure is created with BioRender.com.

Table 1.

Immune cells in the human appendix and their potential link to UC.

Cells seen	Function in the appendix	Changes seen in UC	Proposed link
FoxP3+ CD25+ T cells	Regulate or suppress pro-inflammatory effects and metabolic inflammation. These cells are increased in the inflamed appendix.41	Increased level of expression in colonic lamina propria. Data for the appendix during UC are not available.78	These cells regulate immune response in UC although cell numbers seem insufficient to eradicate the inflammation.
Invariant Natural killer T (iNKT) cells	iNKT cells are abundant in the appendix and can produce cytokines and chemokines quickly after activation, which results in inflammation.79	Increased levels of expression.80	As iNKTs are numerous and can produce inflammatory cytokines, they could result in inflammation in the intestine of UC patients.
CD5+ B lymphocytes (B1 cells)	Abundant in a healthy appendix compared with the colon. They produce IgM Antibodies against pathogens and self-antigens.	Decreased level of expression inrectal mucosal samples of UC patients.41	Reduced number of these cells in UC may result in persistence of pathogens and consequently activation of immune system and inflammatory pathways.
CD19+ B cells	Common in healthy appendix in comparison to the colon and induce activation and function of B cells.81	Increased level of expression in rectal mucosal samples of UC patients.82	In UC, these cells could produce more inflammation and immune response.
CD4+ CD69+ Tcell	Activating marker for Treg cells, early activation cell surface maker on leukocytes, and activating function for triggering pro-inflammatory responses.83	Increased level of expression in colonoscopic biopsy specimens of the appendix.31	As these cells can trigger the inflammatory responses and are increased in UC, they could have a role in triggering or increasing the inflammation in the colon.

The Presumed Role of the Appendix in the Pathogenesis of UC

An important link between UC and the appendix is demonstrated in several cohort and case-control studies, showing that appendectomy is associated with reduced risk of developing UC, contrary to its increased risk for developing Crohn disease; in fact, prospective studies indicate that appendectomy may even be effective in the prevention or reducing the intensity of UC flares, leading to a milder disease course. Appendectomy appears to be most protective when it is performed before the age of 20 years, suggesting an age-dependent role. A study by Myrelid et al indicated that performing appendectomy prior to the development of UC, early in life, was associated with a less severe course of UC and a lower risk of colectomy.^84–86

Cheluvappa et al showed that appendectomy after appendicitis (AA) provided notable protection against subsequent experimental colitis; however, this group also showed in previous experiments that appendicitis or appendectomy alone were not protective, comparable to no intervention.⁸⁷ Similarly, Harnoy et al demonstrated that appendectomy without induced appendicitis increases the risk of colorectal neoplasia in mouse models of colitis and in UC patients, while AA ameliorated colitis.⁸⁸ Another study by Stellingwerf et al demonstrated that approximately half of therapy-refractory UC patients who had undergone appendectomy instead of colectomy had clinical response and entered endoscopic remission at 12 months and long-term follow-up, concluding that appendectomy could be beneficial in some patients.⁸⁹

Although most of these clinical studies provide only correlative evidence of the link between appendectomy and UC clinical outcomes, another study showed that appendectomy can lead to suppression of the Th17 pathway, a key pathway associated with inflammation in IBD, through downregulation of the pro-inflammatory chemokine CCL20. Appendectomy may also result in suppression of autophagy genes such as XBP1 and ULK1 (both IBD-related), and perhaps autoimmune damage in IBD.^22,90 Nevertheless, despite these observations, some authors argue that the decline in disease activity might be attributed to the natural course of UC, not to the appendectomy, highlighting the challenges with this area of research.⁹⁰ Furthermore, appendectomy reduces the host’s ability to sample luminal antigens, prevents production of appendiceal inflammatory cytokines, and inhibits re-establishment of commensal bacteria in the colon. Studies that link appendicitis and appendectomy to UC are listed in Table 2.

Table 2.

Studies linking appendicitis and/or appendectomy to UC.

Reference	Study design	UC n=	Conclusion	Interpretation/Comment
Naganuma et al (2001)21	Retrospective Case-control	325	Reduced disease recurrence rate after appendectomy compared with those without appendectomy.	Appendectomy, in this Japanese population, could have a preventive effect on the development of UC and decrease recurrence.
Cosnes et al (2002)25	Prospective Observational	638	Appendectomized patients had a lower active disease rate than nonappendectomized.	Previous appendectomy can be beneficial for patients genetically at risk of developing UC, with a less severe disease course.
Lee et al (2015)32	Retrospective Cohort/Case-control	2648	No correlation between appendectomy before UC diagnosis and disease extent. No alteration in the disease course following appendectomy after UC diagnosis.	In this Korean study, appendectomy before or after UC diagnosis did not affect clinical course.
Harnoy et al (2016)88	Retrospective Cohort/ Animal research	232	Protective role of appendectomy in a murine model of colitis following appendicitis.However, increased risk of colorectal neoplasia was observed following appendicectomy without appendicitis in the murine model and UC patients.	Appendectomy following appendicitis is protective against UC.
Myrelid et al (2017)84	Retrospective Cohort	63711	Reduced risk of colectomy and UC-related hospital admissions for appendectomy in young age, prior to developing UC.	Appendectomy early in life can moderate UC intensity.
Chen et al (2018)91	Retrospective Case-control	402	No effect for appendectomy on the severity of UC.	In this Chinese population, there was no significant association between appendectomy and UC occurrence.
Stellingwerf et al (2019)89	Prospective Cohort	28	Appendectomy, in almost half of patients with refractory UC, resulted in clinical response while a noticeable portion were in endoscopic remission.	Appendectomy can be considered as an option in patients with therapy-refractory UC before colectomy.
Sahami et al (2019)92	Prospective Observational	30	Appendectomy was effective in ~30% of therapy-refractory UC patients, and after 1 year a significant portion of patients showed complete endoscopic remission.	Appendectomy may be beneficial for therapy-refractory UC patients.

Another important link between the appendix and UC is the common occurrence of inflammation within the PAP in UC patients, which can even precede the development of UC. This suggests that inflammation associated with the appendix could play a causative or triggering role or provide an early endoscopic marker for disease development. Rubin et al showed that 8% of patients with distal UC (29 of 367 patients) displayed PAP inflammation on endoscopy; and out of 43 colonoscopies performed on these 29 patients, 20 showed that histologic activity in the PAP paralleled the activity in the distal colon. This suggests that these 2 areas may be linked in their disease activity,^93,94 in contrast to the well-characterized and accepted continuous disease distribution in UC. In another study, 25% of 20 patients with PAP without accompanying typical UC developed UC after an average follow-up of 18.4 months; the authors concluded that PAP can occur before the development of UC.⁹⁵ Details on studies that link PAP lesions to UC are listed in Table 3.

Table 3.

Studies linking appendix involvement or PAP lesions to UC.

Reference	Study design	UC n=	Conclusion	Interpretation/Comment
Jahadi et al (1976)96	Retrospective Cohort	65	Appendiceal involvement was seen in 47% of UC patients. In 78% of these patients, similarity between the disease stages in the colon and in the appendix observed.	Despite the small number of the patients, this study suggests a positive link between the disease in the colon and in the appendix.
Goldblum & Appelman (1992)97	Retrospective Cohort	87	62% of UC patients had colitic changes in the appendix and involvement of cecum.	Appendiceal involvement happens with adjacent involved cecum in resected ulcerative pancolitis.
Kroft et al (1994)33	Retrospective Cohort	39	Detection of skip lesions to the appendix region in 15% of UC specimens.	Skip lesions could be seen in some UC cases, but the number of patients included in this study is small.
Okawa et al (1998)98	Prospective Observational	56	Observed active colitis at the opening of the appendix in 18% of UC patients.	All of these patients had mild disease.
Scott et al (1998)99	Retrospective Case-control	50	Observation of appendiceal inflammation in 48% (24/50) and 52% (14/27) of UC and Crohn’s diseases, respectively.	Appendiceal inflammation often happens as a skip lesion and histologically is similar to the colonic disease rather than acute appendicitis.
Yang et al (1999)100	Prospective Noninterventional	94	Detection of PAP in 26% of UC patients.	PAP observed as a skip lesion, mostly in patients with moderate disease.
Matsumoto et al (2002)36	Prospective Case-control	40	Detection of PAP in23 of 40UC patients.	PAP may express histologically active disease in patients with distal ulcerative colitis; the number of patients is low.
Byeon et al (2005)101	Prospective Observational	94	51% of UC patients had PAP.Risk of relapse and proximal disease extension after 5 years of remission was 69.2% and 44.5%, respectively, in patients with PAP.	PAP in regard to remission, recurrence, or the expansion in the proximal sites, does not have any prognostic role.
Rubin & Rothe (2010)94	Retrospective Cohort	622	Detection of PAP in 7.9% of UC patient.	There could be an association of PAP in a subset of UC patients.
Park et al (2011)95	Prospective Observational	20	25 % of patients with ulcerative colitis-like inflammatory lesions at the appendiceal orifice but without IBD, developed UC in an average time of 18.4 months.	PAP can precede development of UC, at least in some cases. limited number of patients.
Bakman et al (2011)34	Retrospective Case-control	123	PAP found in 4.8% of patients, and it was not associated with progressionto pancolitis or need for colectomy.	Isolated PAP is not a risk factor for future aggressive UC.
Anzai et al (2016)102	Retrospective Case-control	189	PAP observed in 13.7% (26 patients) of patients, including 19.1% (9 patients) with proctitis. Proximal extension observed in all patients with proctitis and PAP.	In patients with proctitis, there are links between PAP and subsequent proximal extension of mucosal inflammation.
Zhan et al (2016)103	Retrospective Cohort	427	Detection of PAP in 26.2% of UC patients.	Considering PAP features and proctitis can be a relatively specific feature for UC diagnosis.

Remaining Research Questions and Opportunities

Very little is known about the microbiota of a normal appendix. In order to better understand the appendix’s role in the pathogenesis of UC, research must better define the appendiceal microbiota to determine the specific microbial changes in states of health and disease. In other words, the first and most important question is whether the appendix microbiome might be “dysbiotic” and may seed the colon with pathobionts in UC. As appendiceal microbes are aggregated within biofilms, they could be identified in healthy and unhealthy individuals to clarify whether they can trigger immunity and/or have impact on the gut barrier integrity. Additionally, studies should include larger number of patients to be able to account for patient diversity and increased the reliability of results, reducing false discoveries. One of the challenges in studying the appendix is the lack of cell lines or animal models that accurately mimic the human appendix. The appendix is also not easy to access in humans, outside of cases requiring its removal.

Similar to the colon, appendiceal microbiota could be affected by many factors, including diet, antibiotic, and immunosuppression therapies during both early and adult life. Therefore, microbe-altering therapies such as diet or targeted antibiotic therapy could be beneficial to impose a healthy microenvironment in the appendix and increase diversity and abundance of the bacteria in the appendix.

We acknowledge that some critical gaps remain to be addressed—yet we still believe that our hypothesis holds, suggesting that microbes and immune cells in the appendix fuel inflammation in the colon in UC. For example, it is unclear why these phenomena do not lead to inflammation in healthy individuals; we would suggest that they have normal microbes and immune cells, but this has not been assessed. Furthermore, why inflammation in UC is typically more prominent in the distal colon, with the exception of the peri-appendicular patch, remains to be determined (we speculate that other local factors define a threshold for overt inflammation). Considering that many questions about the appendiceal immunological function and its role in the pathogenesis of UC have remained disputed, more research on inflammatory cells and their functions is needed to illustrate the inflammation pathway regarding the appendix and this disease.

Conclusion

The vermiform appendix may play a critical role in the development of the intestinal immune system and the gut microbiome, giving it a key role in the pathogenesis of UC. Clinical, pathologic, and scientific observations have provided plausible links between the appendix and UC, suggesting it has a critical part in the pathogenesis of UC. Focusing future research on the unanswered questions could have substantial therapeutic implications for the development of novel therapeutics in UC.

Glossary

Abbreviations

AA

appendectomy after appendicitis

CD

Crohn disease

DC cells

dendritic cells

FDCs

follicular dendritic cells

FoxP3

forkhead box protein 3

GALT

gut-associated lymphoid tissue

IgA

immunoglobulin-A

IBD

inflammatory bowel diseases

IL

interleukin

iNKT

invariant natural killer cells

IHC

ischemic heart disease

MLNs

mesenteric lymph nodes

NKT

natural killer T cells

PAP

peri-appendicular patch

Tregs

regulatory T cells

sIgA

secretory IgA

TCR

T cell receptor

Th

T helper

Th17

T helper type 17

Th2

T helper type 2

TNF

tumor necrosis factor

UC

ulcerative colitis

Funding

This work was supported by a student award to NAF from the University of Alberta; The Wine lab is funded by operating grants including the Canadian Institutes of Health Research (CIHR) held by E.W., and Weston Family Foundation held by E.W. and H.A.

Conflict of Interest

The authors declare that there is no conflict of interests.