This exploratory study aims to gain insight in gut immune system phenotypes before and after immunotherapy. After informed consent is obtained, sigmoidoscopies at baseline and during treatment with immunotherapy will be performed, During the endoscopies, biopsies from the sigmoid and rectum will be obtained. Subsequent immune cell analyses in these biopsies will be performed. Parallel to the sigmoidoscopies, venous blood samples will be obtained to monitor inflammatory markers.