NK cells and missing self recognition : genetic control, mhc class i dependent education and potential use in cancer therapy

Abstract

NK cells belong to the innate immune system and are important in the defense against virus infections and malignant cells. They mediate their effector functions via release of cytotoxic granules and by cytokine production which can influence the status of other (immune) cells. NK cells are regulated by germline encoded receptors, both activating and inhibitory, recognizing molecules that are induced upon infection or cellular stress and self ligands respectively. Ly49 receptors (Ly49r) make up the largest NK cell receptor family in mice. It contains both activating and inhibitory receptors most of which bind to major histocompatibility complex class I (MHC I) molecules. NK cells patrol tissues and inspect surrounding cells for alterations in activating ligands and MHC I expression, balancing the input for decision of response. If the activation exceeds the inhibition, the target cell is eliminated. This ability to sense loss of self MHC I is referred to as missing self recognition. It can be directed against virus infected cells and tumor cells which often downmodulate MHC I, while they upregulate activating ligands.

NK cells are educated via Ly49r-MHC I interactions to ensure self-tolerance and reactivity against aberrant cells. MHC I dependent education influences the NK cell population in at least two ways; modulation of responsiveness of each cell and skewing of the inhibitory receptor repertoire, i e the frequencies of NK cells expressing different combinations of Ly49r. The main aim of this thesis has been to study missing self recognition and MHC I dependent NK cell education and how these phenomena are influenced by different factors.

In paper I, we characterized a genetic defect leading to Impaired Missing Self Recognition, in a mouse strain that we have termed IMSR mice. These mice had originally been developed by targeting a non-classical MHC gene, but the defect and the IMSR defect segregated independently. The IMSR mice were found to have a normal number of NK cells, which retained some functions, while missing self rejection and some activation pathways were partly or completely impaired. This defect was found to be NK cell intrinsic; it was not due to total lack of inhibitory receptors function, nor lack of MHC dependent education.

In paper II and III we investigated how NK cells respond to altered inhibitory input from the environment in the host. Antibody mediated inhibitory receptor blockade was used as a tool to reduce the inhibitory input, which led to two different effects on the targeted NK cell populations 1) increased in vivo elimination of MHC I+ tumor cells without breaking tolerance towards normal healthy cells (paper II) and 2) induction of hyporesponsiveness i e reduced in vitro responsiveness or reduced capacity to eliminate MHC I- spleen cells. Importantly, elimination of MHC I- tumor cells was maintained. This was also investigated in an adoptive transfer model where the NK cell responsiveness could be either increased or reduced, depending on the MHC I expression in the recipient host (paper III). In conclusion, we found that NK cells can retune their responsiveness upon altered inhibitory input, but that responsiveness levels are adapted to healthy cells, still allowing efficient killing of tumor cells of the same missing self phenotype.

In paper IV, we investigated whether skewing of the inhibitory receptor repertoire occurs already during NK cell development, before they reach the blood and the spleen. We found that the process leading to overrepresentation of NK cells expressing only one self MHC receptor is initiated during in the bone marrow already at the first NK cell developmental stage where inhibitory Ly49 receptors are expressed. This is most probably influenced both by selective proliferation and apoptosis.