Soluble HLA-G release by the human embryo: an interesting artefact?

Several publications have recently claimed that early human embryos release soluble human leukocyte antigen-G (HLA-G), and that there is a direct positive relationship between the quantity of this protein secreted and embryo quality and viability. These publications report a detection limit of 1 or 10 ng/ml of medium, and the quantity of material released was of the order of magnitude from 3 to 80 ng per day and per embryo. The protein content of a human preimplantation embryo can be estimated at 45-50 ng. This means that the HLA-G release reported could be from 10 to 100% and above the total protein content of the embryo. In recent decades, bio-industry has intensively engineered Chinese hamster ovary cells and optimized their gene expression in order to achieve extremely high yields of recombinant protein secretions reaching 50 pg per cell and per day. This is 100 times lower than what is proposed for soluble HLA-G release by the embryo. It is obvious that the embryo releases signals, but these are unlikely to be secreted at the order of magnitude of the concentrations that have been so far estimated.