Tubal versus uterine placentation: similar HLA-G expressing extravillous cytotrophoblast invasion but different maternal leukocyte recruitment.

The nonclassical HLA-G class I gene is expressed by extravillous cytotrophoblast that invades decidua in uterine pregnancy, suggesting that it may contribute to the immunological mechanisms that protect the fetus against maternal alloimmune response and/or pathogen infections. We first addressed the question of whether HLA-G expression was dependent on maternal tissue environment by comparing uterine and ectopic tubal pregnancies. Using HLA-G-specific mAb on placental cryosections, we found by immunohistochemistry that all subtypes of extravillous cytotrophoblast similarly expressed HLA-G in pregnant tubes, demonstrating that its expression was independent of the site of implantation. We next compared by immunohistochemistry the phenotype of maternal leukocytes recruited in both pregnant tissues. In contrast to decidua, pregnant tubes were characterized firstly, by the lack of natural killer (NK) cells and of cells expressing CD94 receptor specific for HLA-E, secondly, by a prominent increase of CD8+ T cells, dendritic cells, and macrophages, the latter co-expressing the LIR1/ILT2 killer immunoglobulin-like receptor (KIR), and finally, by the presence of cells expressing LIR2/ILT4 KIR or BY55 NK receptors, known to bind to HLA-G. Such cell types may favor a unique innate defense in pregnant tubes. These observations also suggest that trophoblast HLA-G expression does not influence the recruitment of particular maternal leukocytes in pregnant tissues.