Rat natural killer cell receptor systems and recognition of MHC class I molecules

Rat natural killer (NK) cells recognize MHC-I molecules encoded by both the classical (RT1-A) and non-classical (RT1-C/E/M) MHC class I (MHC-I) region s. We have identified a receptor, the STOK2 antigen, which belongs to the L y-49 family of killer cell lectin-like receptors, and we have localized the gene encoding it to the rat natural killer cell gene complex. We have also shown that it inhibits NK cytotoxicity when recognizing its cognate MHC-I ligand RT1-A1(c) on a target cell. This is the first inhibitory Ly-49-MHC-I interaction identified in the rat and highlights the great similarity betw een rat and mouse Ly-49 receptors and their MHC ligands. However, the mode of rat NK-cell recognition of target cells indicates that positive recognit ion of allo-MHC determinants, especially those encoded by the RT1-C/E/M reg ion, is a prevalent feature. NK cells recruited to the peritoneum as a cons equence of alloimmunization display positive recognition of allodeterminant s. In one case, NK cells activated in this way have been shown to be specif ic for the immunizing, non-classical class I molecule RT1-E-u. These findin gs show that allospecific NK cells sometimes show features reminiscent of t he adaptive immune response.