Human NK cells express two types of receptors that bind to HLA class I molecules on target cells (1
). The killer cell Ig-like receptors (KIRs)1
form a family including members that inhibit NK cells upon recognition of specific classical HLA class I molecules, in particular HLA-C and HLA-B. In addition, the lectin-like CD94/NKG2A heterodimer is an inhibitory receptor specific for the nonclassical class I molecule HLA-E (2
). Another nonclassical HLA class I molecule, HLA-G, has been reported to inhibit NK cells (5
). However, the protection of HLA-G–expressing target cells from NK-mediated lysis can be explained by the recognition of HLA-E on these cells by the CD94/NKG2A receptor. Cell surface expression of HLA-E depends on the binding of a specific peptide derived from the leader sequence of other class I molecules, including HLA-G (9
The KIR and NKG2 families include members that activate, rather than inhibit, NK cells (1
). These activating receptors, such as KIR2DS and NKG2C, have short cytoplasmic tails that lack immunoreceptor tyrosine-based inhibition motifs (ITIMs). In addition, a lysine residue in their transmembrane domain contributes to their association with DAP12 (12
), a homodimer of a 12-kD molecule with a cytoplasmic immunoreceptor tyrosine-based activation motif (ITAM). In contrast, the inhibitory receptors, such as KIR2DL, KIR3DL, and NKG2A, have long cytoplasmic tails with two ITIM sequences. In their case, inhibition is achieved by the recruitment of the tyrosine phosphatase SHP-1 to phosphorylated ITIMs. Expression of the different KIR and NKG2 family members, including activating and inhibitory forms, is clonally distributed among NK cells. Individual NK cells express several such receptors at once, in no particular combination. Besides the few inhibitory KIRs with established specificities for HLA class I, all other KIR family members have poorly defined ligand specificities.
HLA-G is expressed only on fetal trophoblast cells that invade the maternal decidua (13
). These invading trophoblast cells are surrounded by maternal stromal cells and NK cells that are abundant in the decidua basalis during early pregnancy. It has been proposed that HLA-G on trophoblast cells interacts with a receptor on NK cells (13
). Such an interaction may serve to inhibit NK cytotoxicity, but may also provide signals that result in a positive NK response.
In a search for an HLA-G–specific receptor, soluble forms of several KIRs were produced and tested for binding to transfected cells expressing HLA-G. One of the KIRs of unknown function, called KIR2DL4, is structurally unique among the KIR family members. Complementary DNA clones encoding KIR2DL4 predict a molecule with two Ig domains in a unique D0–D2 configuration (14
). Other KIRs have either two or three Ig domains in the order D1–D2 or D0–D1–D2, respectively. The predicted transmembrane region that includes an arginine residue and the long cytoplasmic tail with a single ITIM are also unique. Therefore, KIR2DL4 has features typical of both activating and inhibitory receptors, leaving some uncertainty as to what type of signal it may deliver to NK cells. In this study, we describe expression of KIR2DL4 at the surface of all NK cells and identify it as an HLA-G–specific receptor.