Major histocompatibility complex class I-dependent cell binding to isolated Ly-49A: evidence for high-avidity interaction

Ly-49A molecules negatively regulate a subset of mouse natural killer (NK) cells, preventing lysis of H-2D^d-expressing target cells. In the present report, we immunoaffinity-purified Ly-49A from the EL4 lymphoma using the A1 monoclonal antibody (mAb) and examined cell adhesion to immobilized Ly-49A. Adhesion was observed by cells expressing relatively high levels of H-2D^d, but not cells expressing very low or no cell surface D^d, while antibodies specific for D^d or Ly-49A inhibited the cell binding, indicating that D^d and Ly-49A mediate the observed adhesion. The density of immobilized Ly-49A was varied and confirmed by ELISA. Cell binding exhibited a threshold Ly-49A density requirement, and above this threshold, increases in Ly-49A density resulted in substantial increases in cell adhesion to a high maximum cell binding. The density of Ly-49A homodimers required to mediate cell adhesion was found to be quite low: 140–250 molecules/μm². These results suggest that the avidity of Ly-49A for D^d is relatively high and indicate that small changes in Ly-49A density near the threshold result in large changes in stable Ly-49A receptor engagement. The relatively sharp threshold and marked density dependence presented here for Ly-49A receptor engagement may explain the observation that relatively small differences in Ly-49A expression level on NK cells result in significant differences in functional outcome, i.e. whether a target cell expressing a low level of D^d is spared from lysis or not.