Triggering and inhibitory molecules affecting target cell recognition by NK cells

Abstract: TRIGGERING AND INHIBITORY MOLECULESAFFECTING TARGET CELL RECOGNITION BY NK CELLS Margarita Salcedo Doctoral dissertation from the Microbiology and Tumor Biology Center KarolinskaInstitute, Stockholm, Sweden NK cells were described originally for their ability to Iyse certain tumor celllines without prior immunization. MHC class I molecules have been shown to delivera negative signal to NK cells. preventing killing of target cells. This reactionis mediated via specific receptors on NK cells, referred to as KIR in man and Ly49in mouse. In contrast to the inhibitory events, the receptor-ligand interactionsinitiating NK cell Iytic activity are still relatively poorly understood. The presentstudies describe the influence of MHC/peptide complexes on target cell sensitivityto NK cells and NK cell development, and a new role for costimulatory molecules inNK cell triggering. The first part of this thesis addresses the interaction of target MHC class Imolecules with NK cells. The first study demonstrates that resistance of NK cellmediated Iysis can be conferred by TAPI/2 genes in human antigen processing mutantcells. The second study addresses if NK cells are strictly protected by syngeneic("self") MHC class I molecules, or if also allogeneic class I moleculescould confer protection. It was observed that allogeneic MHC class I molecules exerteda (relative) protective function when exposed to NK effector cells, rather than beingtotally inert or triggering. In the third study, a new model was developed that alloweda more detailed analysis of H-2 allele specific protection from NK cell Iysis invitro, allowing detailed studies of the protective effects of syngeneic and allogeneicclass 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 complexesin the development of NK cells, and NK cell specificity in vivo. We analyzed whetheran altered MHC class I presented peptide repertoire would be sufficient to alterthe specificity of NK cells. By performing tumor rejection studies in (B6 X bm mutant)F1-hybridmice, it was demonstrated that absence of a specific MHC class I presented peptiderepertoire on grafted cells was not sufficient to mediate F1-hybrid anti parentalrejection responses. We also analysed the influence of MHC class I products on theexpression of different Ly49 receptors on NK cells from different MHC class I deficientmice. Ly49 receptor expression in MHC class I deficient mice was found to be alteredin at least two different ways: alteration of numbers of cells expressing a givenreceptor and alterations of the levels of receptor expression at the cell surface.The results suggest that Ly49 receptors "adjust" to self expression ofMHC 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 cellcytolytic machinery. It was observed that NK cell resistant tumor cell lines becamehighly sensitive to NK cell mediated killing upon transfection with the costimulatorymolecule B7. 1. The strength of the triggering effect imposed by B7. I was such thatit overcame the inhibitory signal delivered by target MHC class I molecules. Theresults suggested that B7. I may interact with a receptor on NK cells, being differentfrom CD28 or CTLA-4. Additionally, antigen presenting cells that are characterizedby their high expression of costimulatory molecules, such as activated macrophagesand dendritic cells, were found to be highly susceptible to NK cell rnediated Iysis.Altogether, these observations demonstrate that NK cell activation is regulated bya 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 constitutethe main mechanisms controlling the inhibition of NK cell cytolytic activity. Although,costimulatory molecules are powerful NK triggering structures, other structures ontarget cells may mediate this function as well. Keywords: NK cell, MHC class 1, NKinhibitory receptors, costimulatory molecules ISBN 9 1-628-2643-3 den