The overall goal of this proposal is to better understand how natural killer (NK) cells are tolerant to self in order to gain insight into how NK cells control tumors and infections. The major guiding principle for NK cell tolerance is the missing-self hypothesis which postulates that NK cells survey all tissues for normal expression of MHC class I (MHC-I) molecules which prevents NK cell attack. In a pathologic event, MHC-I is down- regulated and NK cells kill the affected target cell. This is due to inhibitory NK cell receptors that recognize target cell MHC-I molecules and block NK cell activation. However, a caveat with the missing-self hypothesis is that MHC-I-deficient hosts do not exhibit auto-reactive NK cells. Instead the NK cells do not function normally, demonstrating diminished capacity to produce cytokines when triggered through their activation receptors. The applicant's laboratory has provided previous evidence indicating that NK cells are educated through self-MHC-I recognition by their MHC-specific inhibitory receptors, in a process termed licensing. In the current application, they provide additional evidence that require modification of the missing-self hypothesis. They show that when NK cells are unrestrained to attack MHC-I-deficient cells, they target only certain cells, suggesting that missing-self applies only to certain tissues. Therefore, the specific aims of this proposal are to study: 1) Tests of missing-self in parabiotic mice; 2) Tissue-specific constraints of missing-self; 3) Mechanisms by which NK cells are licensed by self-MHC. These studies will provide significant insight into the NK cells and self-tolerance which will inform clinical studies designed to harness NK cells against tumors.