Microbes in the tumor microenvironment: Bacterial influences on host immunity in colorectal cancer

Mounting evidence suggests that a prominent T cell response in the colorectal cancer (CRC) tumor microenvironment is a valuable prognostic indicator, independent of stage at diagnosis. However, the strength and quality of host immune responses are highly variable across patients, and the factors that influence this variability are not well understood. The unique microbial communities present in the CRC tumor microenvironment may contribute to the modulation of local host immune responses, but prior investigations have been limited and focused on candidate microbes. Here, we investigated the association between the CRC tumor bacterial microbiome and the quantity and clonality of the T cell infiltrate. This investigation included 51 CRC from the Molecular Epidemiology of Colorectal Cancer (MECC) study, a population-based case-control study in northern Israel. Genomic DNA was extracted from fresh frozen tumors using microbiome-optimized techniques and sequenced using a PCR amplicon-mediated workflow on the Illumina MiSeq. Sequences from the V1-V3 hypervariable regions of the bacterial 16S rRNA gene region were clustered into operational taxonomic units (OTUs), a proxy for species, and taxonomic identification was determined using the Ribosomal Database Project. In addition, T cell repertoires were measured using the DNA-based immunoSEQ assay (Adaptive Biotechnologies), and T cell fraction (i.e. proportion of rearranged T cells relative to all nucleated cells) and clonality (i.e. T cell receptor diversity) were calculated. Bacterial sequencing was successful in 48 (94%) of samples, with a total of 868 OTUs identified. Klebsiella and Escherichia/Shigella were the most prevalent genera, detected in approximately 90% of tumors, followed by Propionibacterium, Pelomonas, and Veillonella, with each detected in approximately 50% of tumors. Fusobacterium, a genus of oral anaerobic bacteria believed to accelerate tumor progression and enable immune evasion in CRC, was detected in 20% of tumors. When present, Bacteroides and Fusobacterium were the most abundant, comprising 10-15% of all bacteria within each tumor, on average. ImmunoSEQ analysis identified a mean productive T cell fraction of 0.08 (SD: 0.16) and a mean productive clonality of 0.14 (SD: 0.06). Analysis of molecular variance (AMOVA) of the Theta YC measure of dissimilarity showed a statistically significant difference in bacterial community structure (i.e. composition and relative abundance) between samples in the highest versus lower three quartiles of T cell fraction (P=0.03) and clonality (P=0.01). These preliminary findings suggest a potential relationship between microbial community structure and the quantity and diversity of the CRC T cell infiltrate. Additional analyses are underway to better understand this association as well as the independent contributions of the microbiome to CRC prognosis. Citation Format: Christine M. Pierce, Bo-young Hong, Hannah J. Hoehn, Maria F. Gomez, Marilena Melas, Kevin McDonnell, Youngchul Kim, Erica Sodergren, George Weinstock, Hedy S. Rennert, Thomas Giordano, Joel Greenson, Gad Rennert, Stephen B. Gruber, Stephanie L. Schmit. Microbes in the tumor microenvironment: Bacterial influences on host immunity in colorectal cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4746.