Identification of an inhibitory MHC receptor on alloreactive rat natural killer cells

Studies of allogeneic lymphocyte cytotoxicity have shown that the rat NK allorecognition repertoire is controlled by genetic elements in both the MHC (RT1) and the NK gene complex (NKC). DA rats, possessing NK cells that are unable to lyse allogeneic lymphoblasts, were immunized with alloreactive NK cells from MHC-matched PVG.1AV1 rats, and two mAb, STOK1 and STOK2, were generated. STOK1 and STOK2 stained identical subsets of NKR-P1⁺ T and NK cells from certain strains of rats. Relative numbers varied markedly in a panel of MHC congenic strains, however, implicating a role for self MHC genes in their development. Both STOK1 and STOK2 immunoprecipitated a 110-kDa disulfide-linked homodimeric molecule, with extensive N-linked glycosylations, encoded by a gene that mapped to the NKC. NK cells expressing this glycoprotein displayed an increased ability to lyse allogeneic lymphoblasts, while syngeneic targets were spared. However, blockade of the STOK2 Ag with F(ab')₂ of STOK2 permitted the NK lysis of syngeneic targets, but did not affect NK allorecognition. These results indicate that mAb STOK1 and STOK2 identify an NKC-encoded MHC receptor in the rat that acts as a negative regulator of cytotoxicity.