The protection of cells expressing class I HLA molecules from NK lysis is mediated by natural killer cell inhibitory receptors (NKIR). Using site-directed mutagenesis, residues on HLA-C that determine the locus specificity (αVal-76), allotype group specificity (a dimorphism αAsn-80/Lys-80), and affinity of NKIR binding (a second pair of dimorphisms, αAla-73, Asp-90 or αThr-73, Ala-90) have been identified. Thus the 'footprint' of the NKIR on the α1 helix of the class I MHC molecule HLA-C and its associated β strands are similar in position to the site occupied by superantigens on and behind the α1 helix of the class II MHC molecule HLA-DR1, but further toward its C-terminus. The intermediate affinity binding of NKIR to HLA-C, determined by α73 and α90, has an essential role in preventing cross-reactivity and ensuring the availability of NK cells for immunosurveillance; low affinity and high affinity mutants are both physiologically impaired.
Mandelboim, O., Reyburn, H. T., Sheu, E. G., Valés-Gómez, M., Davis, D. M., Pazmany, L., & Strominger, J. L. (1997). The binding site of NK receptors on HLA-C molecules. Immunity, 6(3), 341–350. https://doi.org/10.1016/S1074-7613(00)80336-2