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/mu m(2). 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.