With the increasing use of ICIs, irAEs, such as enteritis, are becoming more frequent. While the mechanisms underlying irAE enteritis remain elusive, emerging studies have suggested the involvement of the gut microbiota. Our analysis focused on longitudinal changes in the gut microbiota of a patient who developed irAE enteritis and achieved remission through treatment.
Following the onset of irAE enteritis, the symptoms improved swiftly with steroid administration, and endoscopic and histological evaluations confirmed a reduction in inflammation. Fecal calprotectin has also been used as a marker of enteritis. Fecal calprotectin and lactoferrin are recognized as valuable markers of enteritis. The cutoff values for endoscopic remission and histological remission are reported to be less than 116 mg/kg and 80 mg/kg, respectively [5]. In our case, fecal calprotectin levels decreased from 1150 mg/kg at onset to 211 mg/kg at remission, paralleling the endoscopic findings and suggesting an improvement in enteritis.
We also examined cytokine profile changes from onset to remission of irAE enteritis. Initially, an increase in BTLA suggested enhanced inhibitory signaling, which might prevent the overactivation of CD8 + T cells. Along with enteritis improvement, a significant reduction in GITR levels indicated the potential recovery of regulatory T cells (Tregs), underlining GITR's role in the pathogenesis and resolution of irAE enteritis.
Moreover, a slight increase in TLR-2 expression corresponded with enteritis recovery, likely reflecting the immune system's response to gut microbiota diversity, and contributing to the resolution of inflammation.
The expression of PD-1 and CTLA-4, the key treatment targets, was compared during enteritis onset and remission. PD-1 levels decreased as enteritis recovered, likely because anti-PD-1 antibodies activated lymphocytes, causing PD-1 cleavage, and potentially increasing soluble PD-1 levels in the bloodstream, which then gradually declined. Conversely, CTLA-4 was minimally detectable throughout the study period.
Overall, these observations underscore the complex interplay among immune system activation, regulation, and microbial pattern recognition during the development and resolution of irAE enteritis.
Recent reports have increasingly suggested a role of the gut microbiome in irAEs, including enteritis. Gut microbial diversity is known to affect not only the exacerbation of irAE enteritis but also the responsiveness of tumors to ICI therapy [2]. Methods to evaluate microbial diversity within a single sample include the estimated total number of bacterial species, the Chao1 index, and the Shannon diversity index, which emphasize species richness and evenness. In this study, the Chao1 index decreased with the improvement in irAE enteritis, whereas the Shannon diversity index increased, suggesting that species evenness recovery is also crucial for recovery from irAE enteritis-induced dysbiosis.
Studies on the specific types of bacteria present before ICI administration have often investigated patients prone to developing irAEs. For instance, the abundance of microbes such as Bacteroidetes and Bifidobacterium has been associated with a reduced risk of gastrointestinal irAEs [6], whereas patients who developed gastrointestinal irAEs were reported to have an abundance of Faecalibacterium and Firmicutes prior to ICI administration. However, studies examining the changes in the gut microbiome before and after the onset of irAE enteritis are rare. A transition of the gut microbiome in pancreatic irAEs has been reported, with a decreased Bacteroidetes/Firmicutes ratio at the onset of irAE pancreatitis [7]. In our case, a very high prevalence of Streptococcus was observed at the onset of irAE enteritis, which decreased as the enteritis improved. In studies on patients with inflammatory bowel disease, the Streptococcus genus has been suggested to be an intestinal pathogen possibly originating from the oral cavity [8], and its increase has also been reported in radiation enteritis.
Other bacteria whose prevalence recovered with improvement in enteritis included Prevotella and Fusobacterium, which can degrade fiber and produce short-chain fatty acids, and Faecalibacterium and Bifidobacterium, which have anti-inflammatory effects. Recovery of the prevalence of these bacterial species is considered to have a positive effect on the normalization of the intestinal environment. In this case, the proportion of Bacteroidetes was low at the onset of irAE enteritis and recovered upon remission of the enteritis with prednisolone treatment.
Within the Bacteroidetes species, Bacteroides fragilis has been reported to be involved in the downregulation of inflammatory Th17 cells [9], and Bacteroides has been reported to have a protective role against colitis [10]. Additionally, Bacteroidetes can decompose complex carbohydrates to produce simpler sugars and metabolites that can be utilized by other bacteria. Fusobacterium can grow using these metabolic products. Thus, the activity of Bacteroides may support an increase in Fusobacterium. Bacteroides fragilis produces polysaccharide A, which is involved in the regulation of the immune system and maintaining the balance of immune responses by activating T cells, particularly Tregs, in the gut. Thus, Bacteroides may play a significant role in the development of irAE enteritis.
Understanding gut microbiome and immunological changes at the onset of irAE enteritis is important for establishing future strategies for the prevention and risk assessment of irAE.
The findings from this case report demonstrate a significant shift in the gut microbiota profile during the occurrence of irAE enteritis, marked by an increase in the prevalence of specific bacteria and a recovery of diversity following treatment. These observations underscore the importance of the gut microbiome in the context of immune responses and irAE enteritis, suggesting that monitoring and potentially modulating the gut microbiota could serve as integral components of effective treatment strategies.