Humans differ from other hominids in lacking an activating NK cell receptor that recognizes the C1 epitope of MHC class I

Abstract

Modulation of human NK cell function by killer cell Ig-like receptors (KIR) and MHC class I is dominated by the bipartite interactions of inhibitory lineage III KIR with the C1 and C2 epitopes of HLA-C. In comparison, the ligand specificities and functional contributions of the activating lineage III KIR remain poorly understood. Using a robust, sensitive assay of KIR binding and a representative panel of 95 HLA class I targets, we show that KIR2DS1 binds C2 with ~50% the avidity of KIR2DL1, whereas KIR2DS2, KIR2DS3, and KIR2DS5 have no detectable avidity for C1, C2, or any other HLA class I epitope. In contrast, the chimpanzee has activating C1- and C2-specific lineage III KIR with strong avidity, comparable to those of their paired inhibitory receptors. One variant of chimpanzee Pt-KIR3DS2, the activating C2-specific receptor, has the same avidity for C2 as does inhibitory Pt-KIR3DL4, and a second variant has ~73% the avidity. Chimpanzee Pt-KIR3DS6, the activating C1-specific receptor, has avidity for C1 that is ~70% that of inhibitory Pt-KIR2DL6. In both humans and chimpanzees we observe an evolutionary trend toward reducing the avidity of the activating C1- and C2-specific receptors through selective acquisition of attenuating substitutions. However, the extent of attenuation has been extreme in humans, as exemplified by KIR2DS2, an activating C1-specific receptor that has lost all detectable avidity for HLA class I. Supporting such elimination of activating C1-specific receptors as a uniquely human phenomenon is the presence of a high-avidity activating C1-specific receptor (Gg-KIR2DSa) in gorilla.

Fig. 1. Binding of human activating lineage III KIR to C2⁺ HLA-C and to C1⁺ HLA-B and -C

C2 binding is normalized to that of inhibitory KIR2DL1; C1 binding is normalized to that of inhibitory KIR2DL3.

Fig. 2. Binding of chimpanzee and gorilla activating lineage III KIR to C1 and C2

Panels A and B show the binding of chimpanzee KIR with C1 and C2 specificity, respectively. Data is normalized as described in the legend to Fig. 1. Panel C compares the binding of gorilla Gg-KIR2DSa and human KIR2DL3 to HLA-A, B and C allotypes.

Fig. 3. The genomic organization of the chimpanzee and human KIR loci

Shown are composite haplotypes containing all known KIR genes. Inhibitory and activating lineage III genes are colored red and blue, respectively. Conserved framework genes are colored green, other genes and the 2DP1 pseudogene are white. Double-headed arrows connect the paired receptors. †, indicates loss of reactivity by 2DS2.

Fig.4. Functional analysis of chimpanzee activating KIR

Panel A shows the results of cytotoxic assays in which the target cells are 221 cells expressing either C1+ or C2+ MHC class I and the effector cell are NKL cells expressing inhibitory chimpanzee KIR or chimeric proteins carrying the D0, D1 and D2 domains of activating KIR with the transmembrane and cytoplasmic domains of the paired inhibitory receptor. Shown is the capacity of the C2 (left) and C1 (right) epitopes to engage the chimeric receptors and prevent NKL cell mediated lysis of 221 target cells. Panel B shows the results of cytotoxic assays in which the target cells are P815 cells and the effector cells are NKL cells and NKL cell transduced with activating Pt-KIR3DS2 or inhibitory Pt-KIR3DL4 that react with the anti-KIR antibody, NKVSF1. Assays were carried out in the presence or absence of monoclonal antibodies directed against KIR, NKG2D and 2B4.