1924 Richard Apps et al. Eur. J. Immunol. 2007. 37: 1924–1937 A homodimeric complex of HLA-G on normal trophoblast cells modulates antigen-presenting cells via LILRB1 Richard Apps, Lucy Gardner, Andrew M. Sharkey, Nick Holmes and Ashley Moffett Department of Pathology, Cambridge University, Cambridge, UK In healthy individuals, the non-classical MHC molecule HLA-G is only expressed on fetal trophoblast cells that invade the decidua during placentation. We show that a significant proportion of HLA-G at the surface of normal human trophoblast cells is present as a disulphide-linked homodimer of the conventional b2m-associated HLA-I complex. HLA-G is a ligand for leukocyte immunoglobulin-like receptors (LILR), which bind much more efficiently to dimeric HLA-G than to conventional HLA-I molecules. We find that a LILRB1-Fc fusion protein preferentially binds the dimeric form of HLA-G on trophoblast cells. We detect LILRB1 expression on decidual myelomonocytic cells; therefore, trophoblast HLA-G may modulate the function of these cells. Co-culture with HLA-G+ cells does not inhibit monocyte-derived dendritic cell up-regulation of HLA-DR and costimulatory molecules on maturation, but did increase production of IL-6 and IL-10. Furthermore, proliferation of allogeneic lymphocytes was inhibited by HLA-G binding to LILRB1/2 on responding antigen-presenting cells (APC). As HLA-G is the only HLA-I molecule that forms b2m-associated dimers with increased avidity for LILRB1, this interaction could represent a placental-specific signal to decidual APC. We suggest that the placenta is modulating maternal immune responses locally in the uterus through HLA-G, a trophoblast-specific, monomorphic signal present in almost every pregnancy. See accompanying commentary: http://dx.doi.org/10.1002/eji.200737515 Received 17/1/07 Revised 20/3/07 Accepted 4/5/07 [DOI 10.1002/eji.200737089] Key words: Allogeneic fetal survival Decidual leukocytes HLA-G Human trophoblast Leukocyte immuno- globulin-like receptors Introduction HLA-G is a non-classical HLA class I (HLA-I) molecule, selectively expressed in healthy individuals by fetal placental trophoblast cells that invade maternal uterine tissues during placentation [1–4]. HLA-G is encoded in the MHC on chromosome 6 and displays a typical HLA-I structure, including association with b2m and peptide. Correspondence: Ashley Moffett, Department of Pathology, Tennis Court Rd., Cambridge CB2 1QP, UK Fax: +44-1223-765065 e-mail: am485@cam.ac.uk Abbreviations: EVT: extravillous trophoblast KIR: killer immunoglobulin-like receptor LILR: leukocyte immuno- globulin-like receptor MDDC: monocyte-derived dendritic cell f 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Unusual features of HLA-G include absence of an endosomal recycling motif, leading to a prolonged cell surface half life [5]. Minimal polymorphism [6, 7], an endoplasmic reticulum retrieval mechanism [8], and the structure of the peptide-binding groove [9] contribute to a restricted peptide repertoire in vivo [10]. These characteristics argue against a role for HLA-G as a conventional HLA-I molecule presenting pathogen-derived peptides to T cells. The functions of HLA-G are not clear, although restricted expression to extravillous trophoblast (EVT) cells at the maternal- fetal interface suggests a role in normal pregnancy. Fetal EVT cells invading the decidua and myometrium come into close contact with maternal uterine leuko- cytes that, in early gestation, are predominantly composed of NK cells, as well as myelomonocytic cells and some T cells [11]. www.eji-journal.eu