Mechanisms regulating Natural Killer Cell Cytotoxicity

Abstract: Over the last 50 years, cancer survival rates have steadily improved thanks to earlier detection and novel treatment regimens. For example, drugs that act on the immune system, so-called immunotherapy, have drastically increased the prospect of survival for patients suffering from some of the most aggressive cancer types including advanced malignant melanoma. Nevertheless, many patients still do not respond to neither traditional treatments or immunotherapy. Expanding the knowledge of immune cell function, and of how the immune system is dysregulated in cancer, will hence be fundamental for the development of more efficient treatment strategies.Natural Killer (NK) cells, a type of cytotoxic innate immune cell, have been identified as a target for immunotherapy due to their capacity to recognize and destroy cancer cells. Tumor-infiltrating NK cells often display reduced functionality, and therapies targeting NK cells therefore aim at enhancing their anti-tumor activity. However, the responses of individual NK cells are highly heterogeneous, and although some cells efficiently destroy harmful targets, others do not.Both functionality and phenotype are most efficiently studied using single-cell approaches, as these can resolve differences within heterogenous populations. In immunology, flow cytometry has been the golden standard for single-cell assessment, but this method has limited applicability when studying dynamic processes. For this purpose, imaging-based methods are a superior alternative, especially when combined with spatial confinement of single cells.In this thesis, I describe the development and application of microscopy-based approaches for the study of single NK cell functional responses. Specifically, I have sought to increase our understanding of the mechanisms regulating NK cell cytotoxicity. In Paper 1, we developed a plastic microwell chip for the investigation of the function of NK cells at the single-cell level. The platform was used to examine how NK cell cytotoxicity is negatively affected by several factors present in the tumor microenvironment, including limited glucose and glutamine availability. In Paper 2, we explored the cytotoxic mechanisms deployed by individual NK cells during sequential killing. We showed that single NK cells switch from almost exclusively applying degranulation in early killing events, to using death ligand engagement for their final kill. In Paper 3, we further investigated the factors prohibiting NK cells from continued killing, during both natural ligand and antibody-mediated cytotoxicity. We discovered that most NK cells retain a large pool of granzyme B-positive lytic granules after they have ceased killing, but calcium signaling is maintained only in NK cells that are capable of sequential killing. In Paper 4, we explored ways of improving the therapeutic efficacy of in vitro-activated immune cells by avoiding their rejection by the host immune system. We showed that the combined deletion of CD54 and CD58 in grafted cells resulted in a reduced recognition by host NK cells, thereby improving graft survival.In summary, the work presented in this thesis demonstrates the importance of single-cell methods for characterizing immune cell function. Such advancements are crucial for the continued development of immunotherapies and will hopefully contribute to further improved chances for cancer patients in the future.