Synergistic polymorphism at two positions distal to the ligand-binding site makes KIR2DL2 a stronger receptor for HLA-C than KIR2DL3.

Interactions between HLA-C ligands and inhibitory killer cell Ig-like receptors (KIR) control the development and response of human NK cells. This regulatory mechanism is usually described by mutually exclusive interactions of KIR2DL1 with C2 having lysine 80, and KIR2DL2/3 with C1 having asparagine 80. Consistent with this simple rule, we found from functional analysis and binding assays to 93 HLA-A, HLA-B, and HLA-C isoforms that KIR2DL1003 bound all C2, and only C2, allotypes. The allotypically related KIR2DL2001 and KIR2DL3001 interacted with all C1, but they violated the simple rule through interactions with several C2 allotypes, notably Cw0501 and Cw0202, and two HLA-B allotypes (B4601 and B7301) that share polymorphisms with HLA-C. Although the specificities of the "cross-reactions" were similar for KIR2DL2001 and KIR2DL3001, they were stronger for KIR2DL2001, as were the reactions with C1. Mutagenesis explored the avidity difference between KIR2DL2001 and KIR2DL3001. Recombinant mutants mapped the difference to the Ig-like domains, where site-directed mutagenesis showed that the combination, but not the individual substitutions, of arginine for proline 16 in D1 and cysteine for arginine 148 in D2 made KIR2DL2001 a stronger receptor than KIR2DL3001. Neither residue 16 or 148 is part of, or near to, the ligand-binding site. Instead, their juxtaposition near the flexible hinge between D1 and D2 suggests that their polymorphisms affect the ligand-binding site by changing the hinge angle and the relative orientation of the two domains. This study demonstrates how allelic polymorphism at sites distal to the ligand-binding site of KIR2DL2/3 has diversified this receptor's interactions with HLA-C.