Abstract 6681: Altered microbial community stability and increases in circulating B cells are associated with the development of severe immune related adverse events during ICI-immunotherapy

Abstract Background: Immune related adverse events (irAEs) are a major hurdle to the success of immune checkpoint inhibitor (ICI) immunotherapy. There is increasing evidence that the gut microbiota influences irAE development, however the microbe-immune crosstalk underpinning this remains cryptic. Methods: Paired pre- & post-treatment (baseline/week 6) stool samples from 129 Australian and Dutch melanoma patients treated on trial with neo-adjuvant combination anti-PD1/anti-CTLA4 immunotherapy were analyzed with 16S rRNA gene sequencing. Matched PBMCs at both timepoints were analyzed using mass cytometry (n=71). Results: ICI immunotherapy resulted in changes in the relative abundance of specific bacterial groups in the faecal microbiota, however the microbial community as a whole remained relatively stable. In particular enhanced oralization of the gut microbiome was observed with treatment, including a significant increase in the relative abundance of oral Streptococcus species in the gut (P=0.0004). These changes were particularly evident in patients whose microbial communities were Bacteroidaceae (Ba)-dominated at baseline, and those who developed severe (grade 3+) irAEs. Indeed, Ba-dominated microbiomes were linked with comparatively higher rates of severe irAEs compared to patients with Ruminococcaceae (Ru)-dominated gut microbiomes (P= 0.0440). Furthermore, Ba patients developed severe irAEs earlier during treatment, prior to week 6 (P=0.0016). A significant reduction in transitional B cells (CD24+ CD38hi CD27-) (P<0.0001) and an increase in activated B cells subsets (CD27- CD38- CD24- populations) (P=0.0022) were observed between baseline and week 6 across all patients who developed or went on to develop severe irAEs. Most notably, Ba patients who developed early, severe irAEs specifically had significantly higher frequencies of overall circulating B cells (P=0.0018) and lower frequencies of ICOS+ Tregs at week 6 post treatment (P=0.0005), which was not observed in Ru-dominated irAE patients. Conclusions: Higher risk of early, severe irAEs were associated with Bacteroidaceae-dominated gut microbiomes, reduced microbial community stability, and increases in activated circulating B cells during treatment. We postulate that differing baseline microbial community assemblages are associated with altered pre-established intestinal homeostasis influencing the likelihood of irAE development. Together, the data highlights a potential relationship between B cells, the gut microbiota and irAE development which may point to early steps in the induction of irAEs during ICI immunotherapy. Citation Format: Rebecca C. Simpson, Erin R. Shanahan, Ines P. Silva, Irene L. Reijers, Judith M. Versluis, Alexander M. Menzies, Gonzalez Maria, Umaimainthan Palendira, James S. Wilmott, Christian Blank, Richard A. Scolyer, Georgina V. Long. Altered microbial community stability and increases in circulating B cells are associated with the development of severe immune related adverse events during ICI-immunotherapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 6681.