Activating Ly-49d and Inhibitory Ly-49a Natural Killer Cell Receptors Demonstrate Distinct Requirements for Interaction with H2-D^d

The activating Ly-49D receptor and the inhibitory Ly-49A receptor mediate opposing effects on natural killer (NK) cell cytotoxicity after interaction with the same major histocompatibility complex ligand, H2-D^d. To compare Ly-49D and Ly-49A interactions with H2-D^d, we created mutations in H2-D^d and examined the functional ability of these mutants to activate lysis through Ly-49D or to inhibit lysis through Ly-49A. Specific single amino acid changes in either the H2-D^d α₁ helix or the α₂ helix abrogated Ly-49D–mediated cytotoxicity, but these changes had no significant effect on Ly-49A–dependent inhibition. Each of three α₂ domain mutations in the floor of the peptide binding groove reduced functional recognition by either Ly-49D or Ly-49A, but all three were required to fully abrogate inhibition by Ly-49A. Our studies indicate that Ly-49D/H2-D^d interactions require distinct determinants compared with Ly-49A/H2-D^d interactions. These differences have important implications for the integration of activating and inhibitory signals in NK cells.