Natural killer cell inhibitory and activating receptors : regulatory role in effector functions against normal and tumor cells

Abstract

Natural killer (NK) cells can mediate effector functions as cytokine production and degranulation dependent cytotoxicity against cells that have become aberrant by mutagenesis or by infection. As part of the innate immunity, the NK cells are most important in the direct early defense against infections, and also in indirect shaping of the later adaptive response. NK cells monitor cells by the use of inhibitory and activating receptors which interact with MHC class I and other ligands, some of which can be induced during cellular stress or by infections. The net outcome of the signals transduced via the inhibitory and activating receptors will either lead to activation of the NK cells and delivery of effector functions, or if the inhibitory signals predominate the NK cell will leave the target cell unaffected and continue to screen other cells. The most important ligands for NK cell inhibitory receptors are the major histocompability complex (MHC) class I molecules. One important feature of the system is that the NK cell in its quest of seeking out and destroying aberrant cells does not kill or harm cells which are healthy, a mechanism that is referred to as NK cell self-tolerance. Tolerance is in part maintained by the expression of self MHC class I molecules on normal cells which NK cells interact with continuously. However, upon an infection, cellular mutagenesis or dysfunction, the MHC class I expression can be impaired, rendering cells susceptible to NK cell attack. This is referred to as missing self recognition. NK cells can also attack cells from other individuals, e.g. after haematopoetic transplantation. Even though allogeneic cells express MHC class I molecules, they often fail to present the self MHC class I molecules that the NK system has been educated to scan for. Many, but not all, tumors downregulate MHC class I expression or upregulate activating ligands for NK cells. Tumor cells are therefore often not recognized as aberrant cells by the NK cells.

The first studies in this thesis have utilized mice expressing different single MHC class I genes in order to characterize the quantitative and qualitative impact that different MHC class I alleles have on the education of missing self recognition by NK cells. In particular, these studies focused on the influence of host MHC on expression of inhibitory NK cell receptors in the Ly49 family. A second set of studies addressed whether, once tolerance has been established by MHC guided education, it is possible to break it by antibody blockade of inhibitory Ly49 receptors in such a way that tumor rejection is induced or enhanced, while tolerance is maintained towards healthy cells? The focus was then set on NK cell inhibitory receptors in interactions with dendritic cells (DCs). These studies addressed whether NK cells can discriminate between mature and naïve DCs, and in particular the role played by the non-classical MHC molecule Qa-1 on dendritic cells in interactions with NKG2A+ NK cells? A final series of studies addressed whether two main effector functions, IFN-gamma secretion and degranulation (associated with cytotoxicity), are coordinately regulated during NK cell maturation and under different conditions of stimulation.

Different MHC class I alleles exerted different educating impact for missing self recognition (the strength by which NK cells reject cells missing the relevant MHC gene). Furthermore, the observations suggested some rules for how this impact is affected when two or more MHC class I alleles are expressed together. The data also suggested that the educating impact of each MHC gene depends on the number of educated NK cells as well as the efficacy state that each NK cell has been educated to by the gene. Regarding attempts to interfere with NK cell tolerance, the studies demonstrated that blockade of the inhibitory NK cell Ly49C/I receptors in B6 mice induced NK cell mediated elimination of MHC class I expressing tumor cells without breaking of tolerance towards autologous healthy haematopoetic cells. This effect persisted during continuous receptor blockade for two week. Tolerance of NK cells towards mature autologous DCs depended on the interaction of the inhibitory receptor NKG2A and the ligand Qa-1 on the DCs. Last but not least, studies at the single cell level demonstrated that secretion of IFN-gamma and degranulation in response to different stimuli are not coordinately regulated within the total NK cell population. The effector response was influenced by activation status as well as maturation stage, the latter defined by different expression patterns of the surface markers CD27 and Mac-1.