Efficient Leukocyte Ig-like Receptor Signaling and Crystal Structure of Disulfide-linked HLA-G Dimer

Abstract

HLA-G is a nonclassical major histocompatibility complex class I (MHCI) molecule, which is expressed in trophoblasts and confers immunological tolerance in the maternal-fetal interface by binding to leukocyte Ig-like receptors (LILRs, also called as LIR/ILT/CD85) and CD8. HLA-G is expressed in disulfide-linked dimer form both in solution and at the cell surface. Interestingly, MHCI dimer formations have been involved in pathogenesis and T cell activation. The structure and receptor binding characteristics of MHCI dimers have never been evaluated. Here we performed binding studies showing that the HLA-G dimer exhibited higher overall affinity to LILRB1/2 than the monomer by significant avidity effects. Furthermore, the cell reporter assay demonstrated that the dimer formation remarkably enhanced the LILRB1-mediated signaling at the cellular level. We further determined the crystal structure of the wild-type dimer of HLA-G with the intermolecular Cys 42 -Cys 42 disulfide bond. This dimer structure showed the oblique configuration to expose two LILR/CD8-binding sites upward from the membrane easily accessible for receptors, providing plausible 1:2 (HLA-G dimer:receptors) complex models. These results indicated that the HLA-G dimer conferred increased avidity in a proper structural orientation to induce efficient LILR signaling, resulting in the dominant immunosuppressive effects. Moreover, structural and functional implications for other MHCI dimers observed in activated T cells and the pathogenic allele, HLA-B27, are discussed.During pregnancy, the fetus can be the allogenic object for the maternal immune system, and thus a special system of immune tolerance is necessary for escaping from maternal immune surveillance to achieve a successful pregnancy. However, knowledge of the molecular mechanism of the maternal-fetal immune tolerance is still limited. In the maternal-fetal interface, the fetal extravillous cytotrophoblasts do not express major histocompatibility complex class I molecules (MHCIs), 5 HLA-A or -B, on the cell surface but do express minor classical MHCI, HLA-C, and nonclassical MHCIs, HLA-E and -G (1). Toward T cells, the classical MHCIs present 8 -10 amino acid peptides processed inside cells (e.g. proteasome) to induce peptide-specific T cell immune responses. Thus, the loss of HLA-A and -B expression significantly suppresses maternal T cell responses. Although HLA-C and -E are expressed in normal cells, the expression of HLA-G is restricted to a few tissues as follows: extravillous trophoblasts, thymus epithelial cells, and some tumors (1). In contrast with polymorphic classical MHCIs, HLA-G shows the limited polymorphism, suggesting that HLA-G may potentially have a role as a common ligand for generic immunosuppressive receptors in the protection of fetus cells from the maternal immune cells. Recently, several immunologically relevant cell-surface receptors were found to mediate the negative regulation of immune cells through binding to classical and nonclassical MHCIs. The receptors of HLA-G rep...

Structural basis for recognition of the nonclassical MHC molecule HLA-G by the leukocyte Ig-like receptor B2 (LILRB2/LIR2/ILT4/CD85d)

“…Previous binding studies and structural reports of HLA-G proposed that the hydrophobic amino acids Phe-195 and Tyr-197 form hydrophobic interactions with LILRB1 and LILRB2 (4,5,14). The current structure clearly showed that Phe-195 is located close to the hydrophobic faces of Thr-48 and Ile-47 of LILRB2 and that Tyr-197 directly interacts with Arg-36 and Tyr-38 of LILRB2 ( Fig.…”

“…In addition, HLA-G forms a disulfidelinked dimer in solution and at the cell surface (2,3). This dimer mediates much more efficient inhibitory signals than HLA-G monomers by binding to LILRB1 and LILRB2, as shown by cellular and biochemical studies (2)(3)(4). The recent crystal structures of the HLA-G monomer (5) and dimer (4) provided the structural basis for efficient signaling.…”

Novel MHC Class I Structures on Exosomes

“…However, HLA-G is unique among MHC I molecules, as it possesses an unpaired Cys 42 residue, situated on an external loop of the ␣1 domain, which allows it to form dimers (46). Such HLA-G dimers have been shown to be up to 100 times more efficient at LILRB1 signaling (47). Thus, it is possible that the exosomal MHC class I dimers we describe in this study may be potent ligands for recognition by NK lineage receptors biased toward HLA-A and -B molecules.…”

HLA-G homodimer-induced cytokine secretion through HLA-G receptors on human decidual macrophages and natural killer cells

“…CD8 also binds to HLA-G and competes with LILRB for binding to this site. These sites are readily accessible to receptors, explaining the increased avidity (26). However, no crystal structure of an HLA-G homodimer complexed with a LILRB protein has been published, although one for the HLA-G monomer-LILRB2 complex was reported (28).…”

“…Previous reports clearly demonstrated that LILRB1 and LILRB2 preferentially bind to HLA-G relative to other classical class I MHC proteins such as HLA-A2 (25). HLA-G has a much higher affinity for LILRB1 than LILRB2 (26,27). A recent structural study showed that the HLA-G dimer binds with greater avidity to LILRB1/2 and signals more strongly through LILRB than monomeric HLA-G and other class I MHC proteins (25).…”