The role of an exposed loop in the alpha2 domain in the mouse MHC class I H-2Dd molecule for recognition by the monoclonal antibody 34-5-8S and the NK-cell receptor Ly49A.

Natural killer (NK) cells express major histocompatibility complex (MHC) class I-specific inhibitory receptors. The region mediating the protective effect of the MHC class I molecule H-2D(d) (D-d), recognized by the inhibitory receptor Ly49A, has been mapped to the alpha(1)/alpha(2) domains. Here we have focused on an exposed loop in the N-terminal part of the alpha(2) domain, which constitutes a major structural motif that differs between D-d (strong binding to Ly49A) and D-b (weak binding to Ly49A at best). We mutated the residues 103, 104 and 107 in D-d to the corresponding amino acids in D-b. The D-d mutant molecule retained the ability to be stabilized by a D-d-binding peptide. However, the mutation totally abolished the recognition by the conformational dependent monoclonal antibody (MoAb) 34-5-8S, known to inhibit the interaction between D-d and Ly49A. In addition, there was a marked impairment of the binding to Ly49A as evaluated by the ability of tetramers of the D-d mutant molecule to bind to Ly49A-transfected reporter cells and spleen cells. These results demonstrate that the introduced changes at positions 103, 104 and 107 directly or indirectly affect the epitopes for the MoAb 34-5-8S and the Ly49A receptor.