Cytoplasmic microvesicular form of Fas ligand in human early placenta: switching the tissue immune privilege hypothesis from cellular to vesicular level

Abstract

The local immune privilege of the fetus is created by the placenta. Fas ligand (FasL) expression in trophoblast has been implied as one of the mechanisms of fetal tolerance. However, the expression of membranal FasL by trophoblast has failed to explain this role of FasL. Two objections can be raised: (1) there have been contradictions considering which trophoblast cells, syncytiotrophoblast (ST) or cytotrophoblast, express FasL; (2) in vivo and in vitro studies have shown that the membranal form of FasL evokes inflammatory response and thus may promote fetal rejection. Using different assays and the FasL-specific antibody G247-4 we demonstrate beyond doubt that in vivo, (1) FasL is produced by and stored in the first trimester human ST only and (2) the human ST lacks surface membranal FasL. Instead, FasL, loaded in microvesicles, is stored in cytoplasmic granules. These results complement the recent in vitro studies of the microvesicular form of FasL secretion by cultured trophoblast cells, and suggest that placental FasL is synthesized by villous ST, stored in microvesicular form and secreted as exosomes. Secretion of the exosome-associated form of FasL may be one mechanism by which the placenta promotes a state of immune privilege. Additionally, FasL expression in Hofbauer cells is also demonstrated.

Pregnancy-Associated Exosomes and Their Modulation of T Cell Signaling

“…In a recent report (36), cytoplasmic microvesicular forms of FasL were demonstrated in syncytiotrophoblasts, and although not able to demonstrate release, these investigators postulated the release of these intracellular microvesicles in the form of exosomes. Their hypothesis was supported by the demonstration of secreted full-length, biologically active FasL in microvesicular form in first and third trimester pregnancies (21,37).…”

“…Investigations have implicated placenta-derived exosomes as a key component in establishing the placenta as an immune-privileged site (36,37). These past studies have not addressed qualitative, quantitative, or functional characteristics of placental exosomes in normal term pregnancies or in pregnancy complications.…”

B cell activation regulates exosomal HLA production

“…On the other hand, exosomes secreted from cancer cells or in malignant effusions possess Fas ligands and mediate apoptosis of T cells [44][45][46]. In addition, placenta-derived exosomes of pregnant women [47][48][49][50] and IL-10-treated dendritic cell-derived exosomes [51] have immunoregulating functions. Based on our observations, the exosomes derived from EBVtransformed B cells showed an immunoregulatory function in vitro, although it remains to be determined whether trace amounts of unknown factors included in our exosomal fractions exert their effect.…”

Human Placenta Expresses and Secretes NKG2D Ligands via Exosomes that Down-Modulate the Cognate Receptor Expression: Evidence for Immunosuppressive Function

“…The placenta is not only a source of nourishment and pregnancy hormones but also actively produces and releases various immunomodulatory factors to prevent rejection of the fetal semiallograft by the innate and adaptive maternal immune system (5,6). In previous reports, we and other researchers have shown that human placenta releases bioactive Fas ligand (FasL) 3 by exosomes that could induce suppression of intracellular signaling and apoptosis of T cells (4,(7)(8)(9). Exosomes are 30-to 100-nm-sized microvesicles with a defined morphology and phenotype that are formed and released through the late endosomal compartment of a variety of cells (10), including syncytiotrophoblast (STB) of human placenta (2,8).…”

“…In previous reports, we and other researchers have shown that human placenta releases bioactive Fas ligand (FasL) 3 by exosomes that could induce suppression of intracellular signaling and apoptosis of T cells (4,(7)(8)(9). Exosomes are 30-to 100-nm-sized microvesicles with a defined morphology and phenotype that are formed and released through the late endosomal compartment of a variety of cells (10), including syncytiotrophoblast (STB) of human placenta (2,8). Exosomes can carry membrane-bound and cytosolic proteins, mRNA, and bioactive lipids to target cells and also transfer infectious agents between cells.…”