ABSTRACT. The intestinal microflora affects inflammation and immunity, not only locally at the mucosal level but also systemically, raising the question of whether the microflora affects inflammatory processes that contribute to cancer and its therapy. Prebiotics have also been found to play an antitumor role that is not limited to the gut. We investigated the antitumor roles of the intestinal microbiota using the Lewis lung cancer mouse model. In mice treated with cisplatin combined with ABX (an antibiotic cocktail of vancomycin, ampicillin, and neomycin), which can destroy the host commensal microflora, the tumor size was larger than in mice on a single treatment of cisplatin. Moreover, the survival rate of mice treated with cisplatin combined with ABX was significantly reduced. In contrast, mice treated with cisplatin combined with Lactobacillus bacteria had smaller tumors and an improved survival rate. Further study on gene expression indicated that ABX can partially impair the function of cisplatin by upregulating the expression of VEGFA and downregulating the expression of BAX 5643Commensal microbiota contributes to lung cancer immunity ©FUNPEC-RP www.funpecrp.com.br Genetics and Molecular Research X (X): XXX-XXX (2015) and CDKN1B. The expression of IFN-γ, GZMB, and PRF1 in the CD8 + T cells of these mice was reduced by ABX, indicating an immunoenhancement role of commensal microbiota. Conversely, Lactobacillus co-treatment mice showed an enhanced antitumor response with upregulated IFN-γ, GZMB, and PRF1 expression. We conclude that the commensal microbiota contributes to the anti-lung cancer response and probiotics co-treatment can enhance the antigrowth and proapoptotic effects of cisplatin.