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.