Triggering and inhibitory molecules affecting target cell recognition by NK cells

NK cells were described originally for their ability to lyse certain tumor cell lines without prior immunization. MHC class I molecules have been shown to deliver a negative signal to NK cells preventing killing of target cells. This reaction is mediated via specific receptors on NK cells, referred to as KIR in man and Ly49 in mouse. In contrast to the inhibitory events, the receptor-ligand interactions initiating NK cell Iytic activity are still relatively poorly understood. The present studies describe the influence of MHC/peptide complexes on target cell sensitivity to NK cells and NK cell development, and a new role for costimulatory molecules in NK cell triggering.

The first part of this thesis addresses the interaction of target MHC class I molecules with NK cells. The first study demonstrates that resistance of NK cell mediated Iysis can be conferred by TAPI/2 genes in human antigen processing mutant cells. The second study addresses if NK cells are strictly protected by syngeneic ("self") MHC class I molecules, or if also allogeneic class I molecules could confer protection. It was observed that allogeneic MHC class I molecules exerted a (relative) protective function when exposed to NK effector cells, rather than being totally inert or triggering. In the third study, a new model was developed that allowed a more detailed analysis of H-2 allele specific protection from NK cell Iysis in vitro, allowing detailed studies of the protective effects of syngeneic and allogeneic class I molecules, and in vitro studies of F1-anti-parental reactions.

The second part of this thesis focuses on the influence of host MHC/peptide complexes in the development of NK cells, and NK cell specificity in vivo. We analyzed whether an altered MHC class I presented peptide repertoire would be sufficient to alter the specificity of NK cells. By performing tumor rejection studies in (B6 X bm mutant) F1-hybrid mice, it was demonstrated that absence of a specific MHC class I presented peptide repertoire on grafted cells was not sufficient to mediate F1-hybrid anti parental rejection responses. We also analysed the influence of MHC class I products on the expression of different Ly49 receptors on NK cells from different MHC class I deficient mice. Ly49 receptor expression in MHC class I deficient mice was found to be altered in at least two different ways: alteration of numbers of cells expressing a given receptor and alterations of the levels of receptor expression at the cell surface. The results suggest that Ly49 receptors "adjust" to self-expression of MHC class I molecules assuring a self-tolerant, yet functional, NK cell repertoire.

The third part of this thesis relates to aspects of the activation of the NK cell cytolytic machinery. It was observed that NK cell resistant tumor cell lines became highly sensitive to NK cell mediated killing upon transfection with the costimulatory molecule B7.1. The strength of the triggering effect imposed by B7.1 was such that it overcame the inhibitory signal delivered by target MHC class I molecules. The results suggested that B7.1 may interact with a receptor on NK cells, being different from CD28 or CTLA-4. Additionally, antigen presenting cells that are characterized by their high expression of costimulatory molecules, such as activated macrophages and dendritic cells, were found to be highly susceptible to NK cell mediated Iysis. Altogether, these observations demonstrate that NK cell activation is regulated by a delicate balance of negative and positive signals resulting from cell-cell interactions. Binding of MHC class I molecules to specific receptors on NK cells, irrefutably constitute the main mechanisms controlling the inhibition of NK cell cytolytic activity. Although, costimulatory molecules are powerful NK triggering structures, other structures on target cells may mediate this function as well.