Stromal modulation through gastrointestinal microbial dysbiosis

The overall use of antibiotics has increased significantly in recent years. Besides fighting infections, antibiotics also alter the gut microbiota. Commensal bacteria in the gastrointestinal tract are crucial to maintain immune homeostasis, and microbial imbalance or dysbiosis affects disease susceptibility and progression. We hypothesized that antibiotic-induced dysbiosis of the gut microbiota would suppress cytokine profiles in the host, thereby leading to changes in the tumor microenvironment. Indeed, dysbiosis resulted in alterations in bacterial abundance, composition and diversity. On the host side, antibiotic-induced dysbiosis caused elongated small intestines and ceca, and B16-F10 melanoma and Lewis Lung carcinoma progressed more quickly than in orthobiotic control mice. Mechanistic studies revealed that this progression was mediated by suppressed TNF-a levels, both locally and systemically, resulting in reduced expression of tumor endothelial adhesion molecules, particularly intercellular adhesion molecule-1 (ICAM-1) and a subsequent decrease in the number of activated CD8+ T-cells in the tumor. However, suppression of ICAM-1 or its binding site, the alpha subunit of lymphocyte function-associated antigen-1, was not seen in the spleen or thymus during dysbiosis. TNF-a supplementation in dysbiotic mice was able to increase ICAM-1 expression and leukocyte trafficking into the tumor. Overall, these results demonstrate the importance of commensal bacteria in supporting anti-cancer immune surveillance, define an important role of tumor endothelial cells within this process, and suggest adverse consequences of antibiotics on distal cancer development. Significance: Antibiotic-induced dysbiosis enhances distal tumor progression by altering host cytokine levels resulting in suppression of leukocyte trafficking via modulation of tumor endothelial adhesion molecules.