Cumulative inhibition of NK cells and T cells resulting from engagement of multiple inhibitory Ly49 receptors.

Inhibitory receptors specific for MHC class I molecules are expressed on partially overlapping subpopulations of NK cells and memory T cells. A central question pertinent to NK cell development and function is how the combinatorial expression of different receptors with distinct class I specificities affects functional recognition. We therefore studied the quantitative effects resulting from class I engagement of multiple inhibitory Ly49 receptors. We used a transgenic mouse model in which all NK cells and T cells express two different Ly49 receptors with shared class I specificity. Comparisons of cells from these mice with cells from single transgenic mice and wild-type mice revealed that Ly49 receptors cumulatively inhibit lymphocyte effector functions. Multiple Ly49 interactions also had a cumulative impact on NK cell development. The findings suggest that the interactions of inhibitory receptors with class I are interpreted quantitatively rather than as on/off switches. They have intriguing implications concerning NK cell tolerance and reactivity toward cells with extinguished expression of a limited number of class I molecules.