Abstract 3463: Longitudinal microbiome-immune dynamics in melanoma patients treated with immune checkpoint inhibitor immunotherapy

Abstract Background: Immune checkpoint inhibitor (ICI) immunotherapies have revolutionized the treatment of melanoma, although drug resistance or concurrent immune-related adverse events (irAEs) still impact many patients. There is growing evidence that immunotherapy outcomes are influenced by the gut microbiome. This includes differential associations for key bacterial groups such as Bacteroidaceae and Ruminococacceae. However, how gut microbes interact with the immune system to influence the success of ICIs and/or the development of irAEs remains unclear. Methods: We longitudinally profiled the gut microbiome and circulating immune cell subsets in stage III melanoma patients from Australia and the Netherlands (n=126) treated on trial with combination anti-PD-1/anti-CTLA-4 immunotherapy in the neo-adjuvant setting (NCT02977052). Paired pre- and post-treatment (baseline/week 6) stool samples were analyzed using 16S rRNA gene sequencing (n=126) and matched PBMCs were comprehensively profiled using mass cytometry (n=71). Results: Treatment with anti-PD-1/anti-CTLA-4 immunotherapy was associated with an increase in the frequency (% total CD45+) of CD8+ effector memory T cells (Tem) (CD45RA- CCR7-) (P= 0.0096) and a reduction in regulatory T cells (Tregs) (CD25+FoxP3+) (P= 0.0029) between baseline and week 6. The reduction in Tregs was dominantly observed in responders (P= 0.0016) compared to non-responders (P= 0.9515). However, patients who developed severe irAEs were also particularly deficient in Tregs at week 6 compared to patients with no or mild toxicities (P= 0.0293). Notably, in patients where circulating Tregs were reduced, the degree of reduction in Tregs positively correlated with the baseline relative abundance of Bacteroides (P< 0.0001). Higher pre-treatment Bacteroides abundance was also linked to the development of more severe irAEs (P= 0.0472) and overall steroid use (P= 0.0345) prior to week 6. Thus, while Bacteriodes here is linked to a response-associated parameter (reduction in Tregs), it is also linked to severe, early irAEs. In contrast, a higher abundance of Ruminococcaceae was protective, and overall a greater proportion of patients with Ruminococcaceae dominated microbiomes were responders, highlighting a role for both bacterial groups in shaping ICI outcomes. Conclusions: While response to ICI therapy was associated with a decrease in circulating Tregs, excessive reduction in Tregs was observed in patients that develop severe irAEs. Together, this highlights the delicate balance between immune activation and regulation in facilitating anti-tumor immune responses whilst avoiding irAEs. Our data suggests that the gut microbiome may have a role in establishing immune tone prior to and during immunotherapy and may influence this “tipping point” and the risk of developing irAEs. Citation Format: Rebecca C. Simpson, Erin R. Shanahan, Marcel Batten, Ines P. Silva, Irene L. Reijers, Judith M. Versluis, Alexander M. Menzies, Maria Gonzalez, Umaimainthan Palendira, Andrew Holmes, James S. Wilmott, Christian U. Blank, Richard A. Scolyer, Georgina V. Long. Longitudinal microbiome-immune dynamics in melanoma patients treated with immune checkpoint inhibitor immunotherapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 3463.