The immune system has an important role in recognizing and eliminating tumors. Transformed tumor cells express tumor-associated antigens (TAAs) that are not seen on normal cells[18]. These TAAs are recognized by the immune system, and T cells can be stimulated in response to cellular presentation of TAAs[19]. TAAs are presented along with the major histocompatibility complex (MHC) I or II by specialized antigen-presenting cells (APCs) that bind with T-cell receptors (TCRs). Activation of T cells requires a co-stimulatory signal which includes the interaction of TCR with MHC along with the interaction of CD-28 (stimulatory checkpoint expressed on T cells) with B7 (CD-80) present on APCs. This leads to T-cell proliferation, cytokine secretion, changes in gene expression and metabolism[20].

Tumors may use immune-checkpoint pathways as a mechanism of immune resistance, principally against T cells that are specific for TAAs[21]. Two well-studied immune-checkpoint receptors are CTLA-4 (CD152) and programmed cell death protein 1 (PD-1 or CD279). CLTA-4 is a negative regulator of T-cell-mediated anti-tumor responses. Expression of CTLA-4 is up-regulated upon TCR stimulation. This molecule competes with CD28 for binding to B7 on APCs, avoiding the costimulatory signal and blunting T-cell activation and proliferation[22]. PD-1 is also expressed on the surface of activated T cells. The interaction between PD-1 and programmed death ligand (PD-L1 and PD-L2), expressed on APCs, leads to T-cell inactivation. Additionally, PD-1 plays an important role to limit the activity of T cells in peripheral tissues through inflammatory response to infection and to limit autoimmunity[23].

Checkpoint inhibitors are monoclonal antibodies that block these pathways. Ipilimumab was the first checkpoint inhibitor immunotherapy approved by the FDA, in March 2011, for the treatment of melanoma. Ipilimumab is a fully humanized monoclonal antibody that competitively binds to CTLA-4 more efficiently when compared to B7 while preserving CD28 signaling[24]. Blockade of CTLA-4 signaling extends T-cell activation and reestablishes T-cell proliferation[25]. As CTLA-4 plays a crucial role in regulating tolerance to self-antigens, CTLA-4 blockade may lead to autoimmune damage to various organ systems, resulting in irAEs[26]. CTLA-4 blockade can initiate dysregulation of GI mucosal immunity which results in irAEs that comprise the esophagus, duodenum, stomach, ileum and colon[27]. The various types of ICIs are described in Table 1.

Mechanisms of IMC and other irAEs are not fully understood; however, CTLA-4 blockade removes CTLA4-mediated protection from autoimmunity and is responsible for a large spectrum of autoimmune-side effects[28]. Immune-related toxicities are mostly associated with the inflammatory reaction produced by immune system responses against specific organs and tissues[29]. Immune-related T-cell activation leads to the secretion of high levels of CD4 T-helper cell cytokines and cytolytic CD8 T-cell tissue infiltration[21]. Another potential mechanism for generating colitis following anti-CTLA4 antibody involves CD25+CD4+ regulatory T cells (Treg). These immunosuppressive regulatory cells constitutively express high levels of CTLA-4 and data show increased autoimmune diseases in mice lacking Treg cells[30,31]. Consequently, it has been hypothesized that an antibody to CTLA4 might diminish Treg cells and induce autoimmunity[32].

The enterocolitis related to ipilimumab has features similar to graft-versus-host disease. It has been proposed that a contributing factor to enterocolitis in this setting may be intestinal microflora and bacterial antigens, representing an area of future research for prophylaxis of enterocolitis in patients treated with ipilimumab[33,34].

Diarrhea and enterocolitis lie along a clinical spectrum where diarrhea is defined as increased stool frequency, and enterocolitis is defined as abdominal pain, rectal bleeding or the presence of mucus in stools with either clinical or radiologic objective evidence of entero-colonic inflammation, as defined by the American Society of Clinical Oncology (ASCO)[35]. The presence of enterocolitis increases the risk of other complications, including ileus, colonic distension, and toxic megacolon, intestinal perforation, or even death. The clinical severity of both diarrhea and colitis is graded according to the National Cancer Institute’s Common Terminology Criteria for Adverse Events (Table 2). Mild diarrhea (grade 1) is defined as less than 4 stools per day above baseline. Grade 2 diarrhea is defined as 4 to 6 stools per day above baseline, while grade 2 colitis is characterized by abdominal pain or blood or mucus in the stool. Severe diarrhea (grade 3) is defined as ≥ 7 stools per day above baseline, and grade 3 colitis is defined by the presence of peritoneal signs with ileus and fever consistent with bowel perforation. A grade 4 designation is distinct from grade 3, reflecting increased severity and the life-threatening nature of symptoms.

While irAEs can affect any portion of the GI tract, the lower GI tract is most commonly involved. Less commonly, the upper GI tract can be affected, manifesting as aphthous ulcers, esophagitis, and gastritis. IMC and diarrhea typically occur 5 wk-10 wk after the 2^nd or 3^rd doses of treatment, but they have also been documented to occur as late as 4 mo after the last dose and may even recur one to two years after discontinuation of treatment[13,36-38].

Combination therapies have so far only been approved for metastatic melanoma. Use of combined anti-CTLA4 and anti PD-1 agents results in increased frequency and severity of diarrhea and colitis than with the use of either agent alone[46-48]. They can also cause rarer forms of toxicities like pancreatitis and small bowel enteritis which warrants discontinuation of ICI treatment and initiation of immunosuppressive therapy.

Gut microbiome: Baseline microbiota composition may predict ipilimumab-induced colitis. In one prospective study of 34 patients whose pre-treatment fecal composition was analyzed, an increased baseline presence of Bacteroidetes species was found in patients who remained free of colitis after ipilimumab treatment[49]. Another study of 26 patients with metastatic melanoma treated with ipilimumab again showed that no-colitis related phylotypes were assigned to Bacteroidetes; most of the baseline colitis-associated phylotypes were related to Firmicutes[50]. Compared with those whose baseline microbiota was driven by Bacteroides, patients with a baseline microbiota enriched with Faecalibacterium and Firmicutes had longer progression-free survival and overall survival.

Autoimmune disorders: Patients with a history of autoimmune disease, are at risk for worsening of their autoimmune disease while on immune checkpoint blockade, but immune-mediated toxicities are often mild and manageable without discontinuation of treatment[51]. Patients that have experienced irAEs with prior checkpoint inhibitor therapy are at risk of developing irAEs following treatment with a different class of ICI; irAE risk also increases with dual therapy[51,52].

Other risk factors: A recent series demonstrated that non-steroidal anti-inflammatory drug use was associated with an increased risk of ipilimumab-induced enterocolitis[53]. Little data is available on the risk of immune-related colitis in patients with Crohn’s disease and ulcerative colitis.

Colonoscopy, with an exam of the terminal ileum and biopsies of the colon and ileal mucosa, is the gold standard diagnostic test for ICI-mediated colitis in patients with persistent grade 2 or higher diarrhea. Patients with upper GI symptoms such as nausea or vomiting should also undergo EGD with biopsies. A normal appearance of the mucosa on endoscopic examination does not exclude enterocolitis, and mucosal biopsies must always be attained[33].

Some patients with immune-mediated diarrhea or colitis may demonstrate ulcerations, but others may demonstrate erosions, erythema, loss of vascular pattern, or even grossly normal appearing mucosa (Figure 1)[60]. Ipilimumab-induced colitis most often involves lesions of the rectum and sigmoid, so flexible sigmoidoscopy is usually sufficient for diagnosis. A majority of patients, however, also have endoscopic lesions proximal to the sigmoid. In more extensive colitis, inflammation can be either patchy or continuous[53,61]. Studies have shown no correlation between diarrheal grade or severity of abdominal pain and endoscopic appearance, but bloody stools have been correlated with higher endoscopic Mayo scores[56,62].

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Figure 1 Patient who experienced immune-mediated colitis 4 wk after ipilimumab therapy. Colonoscopy showed a sigmoid colon with ulcers, diffuse erythema, and loss of vascularity.

ICI colitis presents with an array of histologic findings ranging from focal active colitis with patchy crypt abscesses to diffuse mucosal inflammation[53,63]. Biopsies most commonly demonstrate features of acute colitis such as increased cellularity of the lamina propria with mononuclear cells, intraepithelial neutrophilic infiltrates or crypt abscesses, and an increased number of apoptotic cells in crypts[56].

Histologic findings may precede the onset of diarrhea or colitis. One study of asymptomatic patients who underwent colonoscopy 1 wk-2 wk following ipilimumab induction demonstrated that inflammatory changes were already present prior to symptoms (which occurred 3 wk later in the majority of patients)[63]. Biopsies most frequently showed focal neutrophilic cryptitis and neutrophilic infiltration in the lamina propria, and a subset of patients had excess plasma cells in the lamina propria and lymphocytic cryptitis. When a subset of these patients did eventually present with symptomatic diarrhea or colitis within 24 wk of induction, their biopsies showed more severe infiltration of the lamina propria with mixed inflammatory cells (neutrophils, lymphocytes, plasma cells, and eosinophils), neutrophilic cryptitis and crypt abscesses, as well as glandular destruction and mucosal erosions or ulcers. This study did not show a significant increase in the number of intraepithelial lymphocytes, apoptotic activity, or histologic evidence of chronicity, such as crypt architectural distortion, basal plasmacytosis, the presence of granulomas, Paneth cell metaplasia, or pyloric metaplasia. As several other studies have also shown a relative lack of chronic inflammation on histology, the pathogenesis of ICI-induced enterocolitis is generally considered distinct from that of IBD, although both diseases have similar clinical presentations.

However, the finding of chronic inflammation changes in some ICI-treated patients has caused speculation that their presence is consistent with the progression of acute inflammation or the development of IBD. More recent studies have shown that some patients do exhibit intraepithelial lymphocytes or basal lymphocytes and crypt architecture distortion, which is more consistent with findings in chronic colitis[62]. One study found features of chronic colitis in three out of nine patients with endoscopic or histological inflammation on biopsies that were taken several months after the onset of enterocolitis[53].

Interestingly, one recent study demonstrated distinct immunological characteristics of colonic biopsies taken from patients with IMC and IBD that were analyzed by immunohistochemistry and flow cytometry. The lamina propria and epithelium of anti-PD-1-induced colitis was predominantly characterized by CD8⁺ T-cells, whereas the lamina propria in anti-CTLA-4-induced colitis was predominantly characterized by CD4⁺ T-cells and high mucosal TNFα concentrations. In IBD or anti-PD-1-induced colitis, Treg cells were predominant, whereas in anti-CTLA-4-induced colitis more conventional CD4⁺ cells were found[63].

Similarities between IMC and IBD still persist though these differences are helpful. As histologic features may overlap, clinical factors must be utilized to differentiate these distinct clinical entities, including disease onset soon after initiation of ICI or other associated symptoms. IMC is one of the most common irAEs, but patients often exhibit other concomitant irAEs such as hepatitis, hypophysitis, and hypothyroidism; if any of these also coincide in a patient with diarrhea, ICI colitis is much more likely than IBD.

Identification of serologic markers specific to IMC has been an important focus of research. Fecal calprotectin is a marker that has high sensitivity and specificity for intestinal inflammation. Calprotectin is a calcium-binding protein derived primarily from neutrophils and activated macrophages[64]. While calprotectin can be used to distinguish inflammatory from noninflammatory diarrhea, it is not specific for ICI-induced colitis[65]. Further, while fecal calprotectin can be elevated in patients receiving ipilimumab, indicating active bowel inflammation, it does not predict the onset of colitis. There are few reports regarding the use of new novel markers to diagnose IMC. In one report by Callahan at al[66] patients with melanoma treated with ipilimumab who developed colitis had higher on-treatment serum concentrations of interleukin 17 compared with those without colitis. Another study reported that an increase in the number of eosinophils from baseline after treatment was associated with the advent of irAEs[67]. Whether these are predictors of who will develop colitis on treatment or just markers of active inflammation is still unclear. Plasma CRP and albumin may be helpful but again can be affected by other systemic inflammatory processes[68]. ICI-induced enterocolitis is one of the most commonly occurring irAEs, but the differential diagnosis for diarrhea in ICI-treated patients should also include other irAEs that manifest as diarrhea, such as ICI-induced hyperthyroidism or celiac disease.

In one recent study, four out of six patients who had an additional infusion of ipilimumab after going into enterocolitis remission relapsed[53]. Among them, three patients required a new steroid course, including one patient who had a severe steroid-refractory relapse requiring infliximab infusion. Reintroduction of anti-CTLA4 in patients, who had previously experienced enterocolitis, poses a high risk of relapse and should be discussed on an individual basis.

To date, there have been no studies that demonstrate effective measures to prevent ICI-induced colitis. One randomized, double-blind, placebo-controlled study of 115 patients receiving ipilimumab investigated the efficacy of concomitant budesonide as prophylaxis against colitis. The authors found that prophylactic steroids did not show a benefit in the tolerability of ipilimumab and did not alter the frequency of grade ≥ 2 diarrhea compared to placebo[65,84].

While rare, bowel perforation is an irAE with fatal consequences. Smith et al[85] noted that in a cohort of 22 patients treated with ipilimumab followed by high-dose IL-2 therapy, 3 patients experienced perforation and subsequently required emergency laparotomy. This was found to be more significant compared to 8 perforations among 1797 patients treated with IL-2 alone and compared to 4 perforations among 198 patients treated with ipilimumab alone[85]. To reduce the risk of this complication, the authors recommended that patients who have received prior anti-CTLA4 therapy, and who plan to receive treatment with IL-2, undergo diagnostic colonoscopy before initiating IL-2 to rule out chronic active colitis even in the absence of symptoms[85].

Current theories suggest that increased irAEs predict improved response to checkpoint inhibitors and improved overall survival[62]. One study of 117 patients treated with ICIs who experienced diarrhea found that diarrhea is an independent predictor of improved survival regardless of treatment requirement, and that immunosuppressive treatment for diarrhea did not significantly affect overall survival[13].