BACKGROUND & AIMS:The pathogenesis of immune checkpoint inhibitor (ICI)-colitis remains incompletely understood. We sought to identify key cellular drivers of ICI-colitis and their similarities to idiopathic ulcerative colitis, and to determine potential novel therapeutic targets. METHODS: We used a cross-sectional approach to study patients with ICI-colitis, those receiving ICI without the development of colitis, idiopathic ulcerative colitis, and healthy controls. A subset of patients with ICI-colitis were studied longitudinally. We applied a range of methods, including multiparameter and spectral flow cytometry, spectral immunofluorescence microscopy, targeted gene panels, and bulk and single-cell RNA sequencing. RESULTS: We demonstrate CD8 þ tissue resident memory T (T RM ) cells are the dominant activated T cell subset in ICIcolitis. The pattern of gastrointestinal immunopathology is distinct from ulcerative colitis at both the immune and Gastroenterology 2021;161:1229-1244 BASIC AND TRANSLATIONAL AT epithelial-signaling levels. CD8 þ T RM cell activation correlates with clinical and endoscopic ICI-colitis severity. Single-cell RNA sequencing analysis confirms activated CD8 þ T RM cells express high levels of transcripts for checkpoint inhibitors and interferon-gamma in ICI-colitis. We demonstrate similar findings in both anti-CTLA-4/PD-1 combination therapy and in anti-PD-1 inhibitor-associated colitis. On the basis of our data, we successfully targeted this pathway in a patient with refractory ICIcolitis, using the JAK inhibitor tofacitinib. CONCLUSIONS: Interferon gamma-producing CD8 þ T RM cells are a pathological hallmark of ICI-colitis and a novel target for therapy.

Human blood monocytes are subclassified as classical, intermediate and nonclassical. In this study, it was shown that conventionally defined human intermediate monocytes can be divided into two distinct subpopulations with mid- and high-level surface expression of HLA-DR (referred to as DR and DR intermediate monocytes). These IM subpopulations were phenotypically and functionally characterized in healthy adult blood by flow cytometry, migration assays and lipoprotein uptake assays. Their absolute numbers and proportions were then compared in blood samples from obese and nonobese adults. DR and DR intermediate monocytes differentially expressed several proteins including CD62L, CD11a, CX3CR1 and CCR2. Overall, the DR intermediate monocytes surface profile more closely resembled that of classical monocytes while DR intermediate monocytes were more similar to nonclassical. However, in contrast to classical monocytes, DR intermediate monocytes migrated weakly to CCL2, had reduced intracellular calcium flux following CCR2 ligation and favored adherence to TNFα-activated endothelium over transmigration. In lipid uptake assays, DR intermediate monocytes demonstrated greater internalization of oxidized and acetylated low-density lipoprotein than DR intermediate monocytes. In obese compared to nonobese adults, proportions and absolute numbers of DR , but not DR intermediate monocytes, were increased in blood. The results are consistent with phenotypic and functional heterogeneity within the intermediate monocytes subset that may be of specific relevance to lipoprotein scavenging and metabolic health.