The transmembrane sequence of human histocompatibility leukocyte antigen (HLA)-C as a determinant in inhibition of a subset of natural killer cells

Molecular interactions with the extracellular domains of class I major hist ocompatibility complex proteins are major determinants of immune recognitio n that have been extensively studied both physically and biochemically. How ever, no immunological function has yet been placed on the transmembrane or cytoplasmic amino acid sequences of these proteins despite strict conserva tion of unique features within each class I major histocompatibility comple x locus. Here we report that lysis by a subset of natural killer (NK) cells inhibited by target cell expression of human histocompatibility leukocyte antigen (HLA)-Cw6 or -Cw7 was not inhibited by expression of chimeric prote ins consisting of the extracellular domains of HLA-C and the COOH-tenninal portion of HLA-G. Assays using transfectants expressing a variety of HLA-Cw 6 mutants identified the transmembrane sequence and, in particular, cystein e at position 309 as necessary for inhibition of 68% (25/37) of NK cell lin es and 23% (33/145) of NK clones tested. Moreover, these NK clones inhibite d by target cell expression of HLA-Cw6 and dependent upon the transmembrane sequence were found not to express or to only dimly express NK inhibitory receptors (NKIR1) that are EB6/HP3E4-positive. Furthermore, assays using mo noclonal antibody blocking suggest that an NK receptor other than NKIR1 or CD94 is responsible for recognition dependent upon the transmembrane sequen ce of HLA-Cw6.