|Placental exosomes at the maternal-fetal interface|
Reproduced with permission from Mincheva-Nilsson and Baranov Am J Reprod Immunol 2014
(c) 2014 John Wiley & Sons A/S
A brand new review by Lucia Mincheva-Nilsson and Vladimir Baranov (here) summarizes several mechanisms that might be involved and suggest "the placenta is surrounded by a cloud of exosomes that creates a beneficient and protective mileau for its existence." Among these mechanisms (summarized in the Figure) are reduced NK-cell cytotoxicity (through down regulation of the NKG2D receptor), impaired T-cell signalling, apoptosis of activated lymphocytes and effects mediated by TGF-beta that might include recruitment of regulatory T-cells (previous post).
There is more. The syncytiotrophoblast also releases much larger particles (0.2-2 micrometers) often referred to as STBM (for syncytiotrophoblast-derived microparticles). They are not carefully assembled as are exosomes but resemble a form of cellular debris. Importantly, they are pro-inflammatory with the potential to activate the immune system. Mincheva-Nilsson and Baranov hypothesize that placental exosome production may counterbalance the deleterious effects of STBM. They argue that determination of a normal range for the STBM/exosome ratio should be a research priority as it could lead to development of new diagnostic tools.