1. Supplementary Materials and Methods Table S1. List of the antibodies used for flow cytometry 2. Supplementary Figures Supplementary Fig. S1. Impact of antibiotic treatment on trastuzumab efficacy and the tumor microenvironment. Supplementary Fig. S2. Depletion of commensal microbiota by antibiotic cocktail (ABX) treatment and fecal microbiota transplantation. Supplementary Fig. S3. Impact of vancomycin on trastuzumab efficacy in Î"16HER2 transgenic FVB mice. Supplementary Fig. S4. Differentially abundant bacteria in the gut microbiota of antibiotic-treated mice. Supplementary Fig. S5. Short-chain fatty acids (SCFAs) quantification in fecal samples from antibiotic-treated mice. Supplementary Fig. S6. Analysis of intratumor and stromal cell staining in tumors of antibiotic-treated mice. Supplementary Fig. S7. Impact of antibiotic treatment on tumor immune infiltrate. Supplementary Fig. S8. Analysis of intratumor and stromal cell staining in tumors from FMT mice. Supplementary Fig. S9. Impact of vancomycin treatment on intestinal and systemic immune features. Supplementary Fig. S10. Impact of IL12p70 and CD4+ cell depletion on trastuzumab antitumor efficacy and on tumor immune infiltrate. Supplementary Fig. S11. Causal role of human commensal bacteria in immune-mediated trastuzumab antitumor efficacy. 3. Supplementary References