Human inhibitory receptors Ig-like transcript 2 (ILT2) and ILT4 compete with CD8 for MHC class I binding and bind preferentially to HLA-G

^aDivision of Structural Biology, Medical Institute of Bioregulation, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan; ^bDepartment of Biomolecular Engineering, Graduate School of Engineering, Tohoku University, Aoba-yama 07, Sendai 980-8579, Japan; ^cStructural Biology Center, National Institute of Genetics, 1111 Yata, Mishima, Shizuoka 411-8540, Japan; ^dDepartment of Pathology and Immunology, Washington University School of Medicine, Box 8118, 660 South Euclid Avenue, St. Louis, MO 63110; ^eNuffield Department of Clinical Medicine, University of Oxford, John Radcliffe Hospital, Oxford OX3 9DU, United Kingdom; ^gCancer Research U.K. Receptor Structure Group, Wellcome Trust Centre for Human Genetics, Roosevelt Drive, Headington, Oxford OX3 7BN, United Kingdom; and ^hSir William Dunn School of Pathology, University of Oxford, Oxford OX1 3RE, United Kingdom

Edited by Max D. Cooper, University of Alabama at Birmingham, Birmingham, AL, and approved May 20, 2003 (received for review February 23, 2003)

Abstract

Ig-like transcript 4 (ILT4) (also known as leukocyte Ig-like receptor 2, CD85d, and LILRB2) is a cell surface receptor expressed mainly on myelomonocytic cells, whereas ILT2 (also known as leukocyte Ig-like receptor 1, CD85j, and LILRB1) is expressed on a wider range of immune cells including subsets of natural killer and T cells. Both ILTs contain immunoreceptor tyrosine-based inhibitory receptor motifs in their cytoplasmic tails that inhibit cellular responses by recruiting phosphatases such as SHP-1 (Src homology 2 domain containing tyrosine phosphatase 1). Although these ILTs have been shown to recognize a broad range of classical and nonclassical human MHC class I molecules (MHCIs), their precise binding properties remain controversial. We have used surface plasmon resonance to analyze the interaction of soluble forms of ILT4 and ILT2 with several MHCIs. Although the range of affinities measured was quite broad (K _d = 2–45 μM), some interesting differences were observed. ILT2 generally bound with a 2- to 3-fold higher affinity than ILT4 to the same MHCI. Furthermore, ILT2 and ILT4 bound to HLA-G with a 3- to 4-fold higher affinity than to classical MHCIs, suggesting that ILT/HLA-G recognition may play a dominant role in the regulation of natural killer, T, and myelomonocytic cell activation. Finally, we show that ILT2 and ILT4 effectively compete with CD8 for MHCI binding, raising the possibility that ILT2 modulates CD8⁺ T cell activation by blocking the CD8 binding as well as by recruiting inhibitory molecules through its immunoreceptor tyrosine-based inhibitory receptor motif.

Footnotes

↵ i To whom correspondence should be addressed at: Division of Structural Biology, Medical Institute of Bioregulation, Kyushu University, 3-1-1 Maidashi, Higashi-ku, Fukuoka 812-8582, Japan. E-mail: kmaenaka{at}bioreg.kyushu-u.ac.jp.
↵ f Present address: Institut de Pharmacologie Moleculaire et Cellulaire, Centre National de la Recherche Scientifique, Sophia Antipolis 06560, France.
This paper was submitted directly (Track II) to the PNAS office.
Abbreviations: ILT, Ig-like transcript; KIR, killer cell Ig-like receptor; NK, natural killer; TCR, T cell antigen receptor; MHCI, MHC class I molecule; Dn, N-terminal domain n; β₂m, β₂-microglobulin; SPR, surface plasmon resonance.
Note Added in Proof. The conclusion that site 2 residues are well positioned to contribute to ILT binding is supported by a recently determined crystal structure of LIR-1 (ILT2) bound to a classical MHCI (B.W., L. M. Thomas, and P. J. Bjorkman, unpublished data).