Conformation of human leucocyte antigen‐C molecules at the surface of human trophoblast cells

Summary Human leucocyte antigen (HLA)‐C is expressed at lower levels than other classical HLA‐I molecules on somatic cells. Surface HLA‐C proteins can occur as conventionally β 2 ‐microglobulin (β2m)‐associated complexes or as open conformers dissociated from peptide and/or β 2 m. We investigated the conformation of HLA‐C molecules on normal human trophoblast cells, which invade the maternal decidua during placentation. A panel of monoclonal antibodies to different conformations of HLA‐I molecules was used in flow cytometry and surface immunoprecipitation experiments. On the surface of trophoblast cells only β 2 m‐associated complexes of HLA‐C molecules were detected. In contrast, both open conformers and β 2 m‐associated HLA‐C could be detected on other cells from the decidua, HLA‐C‐transfectants and cell lines. The levels of HLA‐C expressed on primary trophoblast cells could be detected by antibodies specific to non‐β 2 m‐associated conformations because binding was seen after acid‐induced denaturation of surface proteins. In contrast to HLA‐G molecules on trophoblasts, we found no evidence for the presence of disulphide‐linked multimers of HLA‐C complexes. These results show that most HLA‐C molecules present at the trophoblast cell surface are in the conventional β 2 m‐associated conformation. These findings have implications regarding the stability of trophoblast HLA‐C molecules and how they interact with receptors on decidual leucocytes during placentation.