Activation and selection of NK cells via recognition of an allogeneic, non-classical MHC class I molecule, RT1-E

Previous studies have established that NK cells express both inhibitory and activatory receptors. The inhibitory receptors have been shown to recognize major MHC class I molecules, but the physiological ligands for the activatory receptors have been only partly characterized. In this study we investigated whether NK cells could be activated by recognizing specific non-classical MHC class Ib molecules. NK cells from BN (RT1(n)) rats immunized in vivo with MHC-incompatible WF (RT1(u)) cells displayed cytolytic activity specific for product(s) of the MHC class Ib RT1-E(u) / C(u) region. These cells were shown to kill Rat2 fibroblast cells transfected with cDNA for RT1-E(u) but neither untransfected Rat2 nor a transfectant with the class Ia allele, RT1-A(u). Cytolysis of Rat2-RT1-E(u) was inhibited by the anti-RT1-E(u) antibody 70-3-C2. In addition, NK cells cytolytic against PVG (RT1(c)) targets, but not against WF (RT1(u)) or other allogeneic targets were activated after PVG immunization of BN rats. The generation of NK populations cytolytic for target cells of the same haplotype as the immunizing cells, but not for third-party targets, strongly suggests the existence of a selective NK-mediated response in vivo. We conclude that recognition of an allogeneic MHC class Ib RT1-E molecule activates NK cells and the specific cytolytic response could be regarded as adaptive.