Outcome of concurrent treatment with a-CTLA4 and metronidazole in murine model of colon adenocarcinoma.

Abstract

e14566 Background: Immune checkpoint blockade (ICB) therapy transformed clinical oncology by inducing durable responses and increasing survival rates in many types of cancers. However, ICB is effective in only in a subset of patients. Recent studies delineated the role of gut microbiome as both a biomarker and a therapeutic in ICB responsiveness. We aimed to increase understanding of the microbiome-immune system axis in ICB therapy by using antibiotics to knock out certain components of gut microbiome. Methods: We treated MC38 colon adenocarcinoma-bearing mice with a widely-used antibiotic, metronidazole, that is effective against anaerobic and protozoal infections including Clostridioides difficile – a major mediator of colitis. Metronidazole was administered via oral gavage or mixed in drinking water before and after tumor injections. Mice received twice weekly treatment with a-CTLA4 ICB. Results: Metronidazole treatment alone slowed the growth rate of MC38 tumors, consistent with the current literature regarding colon cancer murine models. When metronidazole treatment was combined with a-CTLA4 therapy, we found ̃90% complete tumor regression. In the metronidazole and a-CTLA4 combination group, we also observed an increase in the number of CD103+ type 1 conventional dendritic cells (cDC1s) in colon lamina propria, which suggests enhanced antigen sampling from lumen. Also, in the mesenteric lymph nodes (mLN), we detected upregulation of CD80 and CD86 co-stimulatory molecule expression on CX3CR1+ antigen presenting cells. Multiplex analysis of colon cytokines and colon pathology evaluation was comparable among groups, which implies a non-inflammatory environment in colon. Analysis of tumor-draining lymph nodes eight days after tumor injections showed higher expression of CD86 on CD103+ cDC1s implying superior anti-tumor immunity. 16S rRNA gene sequencing analysis of fecal samples revealed loss of Lachnospiraceae and enrichment of Bifidobacteriaceae and Sutterellaceae families in metronidazole-treated mice. Conclusions: Shifting microbiome composition with metronidazole treatment elicits a favorable anti-tumor immune response to a-CTLA4 treatment in murine colon adenocarcinoma.