Lactobacillus rhamnosus GG Orchestrates an Antitumor Immune Response.

ABSTRACT Background and Aims In colorectal cancer, about 95% of patients are refractory to immunotherapy due to low anti-tumor immune responses. Therefore, these is an exigent need to develop treatments that increase anti-tumor immune responses and decrease tumor burden to enhance immunotherapy. Methods The gut microbiome has been described as a master modulator of immune responses. We administered the human commensal, Lactobacillus rhamnosus GG (LGG) to mice and characterized the changes in the gut immune landscape. Because the presence of lactobacilli in the gut microbiome has been linked with decreased tumor burden and anti-tumor immune responses, we also supplemented LGG to a genetic and a chemical model of murine intestinal cancer. For clinical relevance, we therapeutically administered LGG after tumors had formed. We also tested for the requirement of CD8 T-cells in LGG-mediated modulation of gut tumor burden. Results We detected elevated colonic CD8 T-cell responses specifically in LGG supplemented mice. The CD8 T-cell induction was dependent on dendritic cell activation mediated via Toll-like receptor-2 (TLR2), thereby describing a novel mechanism where a member of the human microbiome induces an intestinal CD8 T-cell response. We also show that LGG decreased tumor burden in the murine gut cancer models by a CD8 T-cell-dependent manner. Conclusions These data support the potential use of LGG to augment anti-tumor immune responses in colorectal cancer patients and ultimately for increasing the breadth and efficacy of immunotherapy.