Receptor/ligand avidity determines the capacity of Ly49 inhibitory receptors to interfere with T-cell receptor-mediated activation.

The specificity and the relative affinity of many Ly49 receptors for major histocompatibility complex class I ligands have been studied in detail in various adhesion and binding assays. However, how the level of cell surface expression of a given Ly49 receptor and its ligand affinity influence the strength of the inhibition signal is not well documented. To address this issue, we developed a series of human Jurkat T-cell transfectants expressing the whole range of Ly49A and Ly49C levels found in vivo on natural killer and T cells and evaluated their capacity to alter superantigen-induced NF-AT activation and interleukin-2 production. We show that the strength of the inhibition induced by Ly49A/H-2Dd interaction correlates with Ly49A density up to a certain level after which increasing expression does not further inhibit significantly the T-cell receptor-induced activation. This system also represents a valuable tool for the determination of the relative strength of the inhibitory signals of Ly49 receptors following their interactions with different ligands. Even at high levels of expression there was no evidence that engagement of Ly49A with H-2b class I molecules provided an inhibitory signal. Moreover, we showed that functional inhibitory interactions of Ly49C with H-2b class I molecules were only the result of H-2Kb and that H-2d represent lower affinity ligands for Ly49C than H-2b. Therefore, depending on the relative affinity of Ly49 receptors for their ligands, the modulation of their expression level will be determinant for the functional outcome of activated T cells.