Introduction:
Immune checkpoint inhibitors (ICI) target negative regulators of cellular activation and include CTLA-4 (inhibited by ipilimumab) and Programmed Cell Death-1 (PD-1; inhibited by pembrolizumab and nivolumab). The resulting dysregulated T cell activation enhances anti-tumour immunity, leading to disease regression and increased survival in patients with metastatic melanoma and a variety of other malignancies. While anti-CTLA-4 and anti-PD-1 therapies both independently improve survival in metastatic melanoma, the combination provides superior response rates and survival .
The augmented adaptive immunity induced by ICI comes at the cost of immune-related adverse events (irAE), most commonly affecting the skin, endocrine organs and gastrointestinal tract. Up to 44% of patients receiving combination anti-CTLA-4 and PD-1 inhibition develop colitis, higher than is seen with anti-PD-1 monotherapy . ICI-related colitis typically occurs within the first few treatment cycles (3-9 weeks) as a single-episode illness . If not recognised and managed promptly ICI-related colitis can cause significant morbidity, and remains the most common drug-related cause of death from combination immune checkpoint-inhibition .
Current guidelines advise corticosteroids as first-line treatment for ICI-related colitis, followed by intravenous steroids and anti TNFα therapy (infliximab) for steroid-refractory disease. The management of ICI-related colitis has evolved from more classical forms of inflammatory bowel disease (IBD), but it is unclear how analogous the newer disease is. It is anticipated that a greater understanding of the underlying mechanisms of ICI-related colitis may lead to more effective treatments.
The intestinal environment poses distinct immunological challenges, requiring tolerance to a diverse microbiome of commensals while mounting effective immunity to pathogens. While the pathogenesis of classical IBD is not fully elucidated, it is proposed that a breakdown of immune tolerance to bacterial species can contribute to disease development (reviewed in ). Propagation of intestinal inflammation is contributed to by multiple cell lineages including epithelial cells, intestinal macrophages, dendritic cells and innate lymphoid cells . A dysregulated T cell response is thought to be a key driver of the chronic inflammation of IBD, as supported by the success of T cell-directed therapies including vedolizumab (targeting the α4β7 integrin) and IL-12/23 p40 pathway inhibitors . A recent study of gut-isolated mononuclear cells (GMNC) by flow cytometry demonstrated that both Crohn’s Disease (CD) and ulcerative colitis (UC) were associated with a high proportion of CD4+ T cells and regulatory T cells (Treg) and lower proportions of CD8+ T cells compared with tissue from healthy donors, changes that reversed during remission .
There is limited data on the pathogenesis of ICI-related colitis. Much of the literature relates to early use of ipilimumab monotherapy and histopathological evaluation of colon biopsies. Endoscopically, ICI-colitis is a heterogenous entity that shares features of both CD and UC, with the sigmoid colon and rectum usually involved. Initial reports found an increased number of CD8+ T cells in biopsies from ipilimumab-associated colitis that positively correlated with disease activity , while others found an increased proportion of infiltrating CD4+ T cells . Upregulation of Th1 (IFNγ) and Th17 (interleukin (IL)-17) mRNA has been reported, as well as upregulation of regulatory factors FOXP3 and IL-10 . While an inflammatory cell infiltrate and cryptitis can be evident on histopathology of ipilimumab-related colitis , detailed phenotyping of peripheral blood mononuclear cells (PBMC) and GMNC in ICI-related colitis, including following combination ipilimumab and nivolumab, has not been studied to date.
Herein we characterised GMNC from patients treated with combination ipilimumab and nivolumab therapy who developed colitis (IN-COL) and compared these with those from patients who had no autoimmune adverse events (IN-NAE), patients during active UC flare and healthy volunteers. We additionally studied whether congruent changed could be detected in the peripheral blood. We hypothesised that IN-COL is associated with an increased proportion of activated T cells (as defined by the well-described co-expression of in vivo activation markers HLA-DR and CD38 ), and a low proportion of Treg (CD4+CD25+CD127-or CD4+25+FOXP3+ T cells) given the role of CTLA-4 in their function .
Mucosal-associated invariant T (MAIT) cells, innate like T cells enriched in the mucosa , were also of interest. Human MAIT cells express high levels of CCR6 and CD161 and a semi-invariant T cell receptor (TCR) α-chain (Vα7.2) . MAIT cells are restricted by the MR1 molecule that recognises vitamin B metabolites (e.g. riboflavin and folate) , but can also be activated by cytokines independent of their TCR . The role of MAIT cells in health and disease is still being established . They are predominantly effector cells that can produce high amounts of IFNγ, cytotoxic granules including granzyme B, IL-17, IL-22 with a mixed Th1/Th17 phenotype . There is data suggesting that MAIT cells play a regulatory role in some tissue but may be pathogenic in others (reviewed in ). Low circulating levels of MAIT cells have been reported in IBD , where MAIT cell numbers accumulate in inflamed bowel and undergo activation-induced cell death . MAIT cells are also depleted in the blood and gastrointestinal tissue of patients with celiac disease . MAIT cells (defined as CCR6+CD161+ or CD161+ TCR Vα7.2+) appear to play a protective role in graft versus-host disease (GVHD; a condition that commonly affects the gastrointestinal tract), where the circulating CD8+CD161+ T cell count is lower in patients with acute GVHD and the circulating MAIT cell count inversely correlates to disease grade . In mouse models a higher proportion of MAIT cells protects against colonic GVHD .
We sought to measure what perturbations in these immune-cell subtypes occurred in the gastrointestinal tissue and peripheral blood of patients with IN-COL and whether these changes were distinct from those seen in UC. We anticipate that a greater understanding of the cellular pathogenesis of ICI-associated colitis will eventually lead to improved and novel therapeutics.