HLA-G expression in extravillous trophoblasts is an intrinsic property of cell differentiation: a lesson learned from ectopic pregnancies

Human leukocyte antigen (HLA)-G is a major histocompatibility gene expresse d almost exclusively in extravillous trophoblasts at the fetal-maternal int erface. HLA-G may play a role in protecting the fetus from attack by the ma ternal natural killer cells. The extravillous trophoblasts invade the decid ua and maternal spiral arteries. The factors which regulate the cell-specif ic expression of HLA-G are unknown. In this study we asked if HLA-G is expr essed in extravillous trophoblasts that develop outside of their normal cel lular environment, as in the case of ectopic pregnancies. Since all ectopic pregnancies implant in the absence of underlying decidua we also used a pl acenta accreta as an experimental control, We found that HLA-G mRNA and pro tein were expressed in the extravillous trophoblasts in the 13 ectopic spec imens studied. In a case of placenta accreta (which develops without decidu a basalis and is therefore adherent to the underlying myometrium), HLA-G mR NA and protein were also expressed. These results suggest that HLA-G expres sion is induced in a cell autonomous manner rather than determined by appro priate environmental cues.