The main goal of this prospective non-interventional exploratory study is to characterize the tumor micro-environment of advanced NSCLC in single-cell resolution, prior to immune checkpoint blockade exposure, and correlate the findings to clinical outcome. This approach will allow to generate new hypotheses regarding mechanism of action of ICI and (primary) resistance mechanisms. The long-term goal is that these novel mechanistic insights will be translated to a clinical setting to develop better biomarkers of ICI efficacy. Importantly, since the investigators will also sequentially profile the immune composition of peripheral blood, this research offers an opportunity to develop circulating (non-invasive) biomarkers. A second aim is to characterize the immune cell composition of bronchoalveolar lavage (BAL) fluid from these ICI-treated cancer patients if they would develop ICI-pneumonitis. These mechanistic insights can directly lead to putative diagnostic biomarkers and therpeutic targets. Since single-cell profiling of blood samples will also be performed, circulating biomarkers of ICI toxicity can also be identified, making non-invasive diagnosis feasible.