Mol. Hum. Reprod. Advance Access originally published online on December 5, 2005
Molecular Human Reproduction 2005 11(10):699-710; doi:10.1093/molehr/gah185
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The soluble pool of HLA-G produced by human trophoblasts does not include detectable levels of the intron 4-containing HLA-G5 and HLA-G6 isoforms
1Institute of Cell Biology, Histology and Embryology/Center of Molecular Medicine, Medical University of Graz, Harrachgasse 21, A-8010 Graz, Austria, 2Institut für Immungenetik, Charité-Universitätsmedizin Berlin, Spandauer Damm 130, 14050 Berlin, Germany and 3Clinic of Obstetrics and Gynecology, Medical University of Graz, Auenbruggerplatz 14, A-8036 Graz, Austria
4 To whom correspondence should be addressed at: Institute of Cell Biology, Histology and Embryology/Center of Molecular Medicine, Medical University of Graz, Harrachgasse 21/7, A-8010 Graz, Austria. E-mail: gottfried.dohr{at}meduni-graz.at
In the context of implantation and pregnancy, several immunomodulating functions have been attributed to the different HLA-G isoforms. Increasing attention is now being addressed to the actively secreted soluble forms, because they might have a systemic function or could be useful as diagnostic tools. However, the cellular source of secretion, even during pregnancy, where HLA-G expression level is known to be highest, is still under debate. To elucidate the conflicting results, we investigated the isoform distribution in human first trimester and term placentas in situ and in vitro. Results obtained by applying immunohistochemistry, western blot, enzyme-linked immunosorbent assay (ELISA) and RT–PCR show that (1) all of the 
1 domain-containing HLA-G isoforms are restrictedly expressed in the extravillous cytotrophoblasts (EVCTs) and very few first-trimester syncytiotrophoblasts, which directly cover cell columns, whereas mesenchymal cells of the villous chorion do not express HLA-G; (2) as demonstrated in western blots, trophoblasts express only the HLA-G1 isoform; (3) HLA-G5 and -G6 transcripts could be detected in human term placenta and isolated first-trimester trophoblasts but levels are extremely low; and (4) conditioned media of primary first-trimester trophoblasts, and the chorion laeve-derived trophoblastic cell line AC1-M59 do contain HLA-G1 fragments shed from the cell surface. Our data provide substantial evidence that none of the intron 4-containing isoforms, the so-called actively secreted, soluble HLA-G5 or -G6, are produced by human trophoblasts in situ or in vitro.
Key words: HLA-G/human placenta/reproductive immunology/MHC class I/trophoblast
* The authors equally contributed to this work.
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