Gut bacteria release bacterial extracellular vesicles (BEV), membrane-encapsulated communicative particles that contain a variety of active biomolecules through which they are recognized by the human immune system. The recent groundbreaking discovery that BEV are systemically present in cancer patients with impaired intestinal barrier integrity suggests that they may influence immune activity of the cancer patient, thereby modulating therapy response. This PhD project aims to investigate: (I) how circulating BEV affect immune activity in the patient by modulating immune cell compositions in organs and blood, (II) whether the presence of circulating BEV affects the tumor microenvironment and can lead to an antitumor response (III) whether the presence of circulating BEV can enhance the response to immunotherapy through their potentially antitumor activity.