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Exosomes secreted by human placenta carry functional Fas ligand and TRAIL molecules and convey apoptosis in activated immune cells, suggesting exosome-mediated immune privilege of the fetus
Umeå University, Faculty of Medicine, Department of Clinical Microbiology, Clinical Immunology.
Umeå University, Faculty of Medicine, Department of Clinical Microbiology, Clinical Immunology.
Umeå University, Faculty of Medicine, Department of Clinical Microbiology, Clinical Immunology.
Umeå University, Faculty of Medicine, Department of Clinical Microbiology, Clinical Immunology.
2013 (English)In: Journal of Immunology, ISSN 0022-1767, E-ISSN 1550-6606, Vol. 191, no 11, 5515-5523 p.Article in journal (Refereed) Published
Abstract [en]

Apoptosis is crucially important in mediating immune privilege of the fetus during pregnancy. We investigated the expression and in vitro apoptotic activity of two physiologically relevant death messengers, the TNF family members Fas ligand (FasL) and TRAIL in human early and term placentas. Both molecules were intracellularly expressed, confined to the late endosomal compartment of the syncytiotrophoblast, and tightly associated to the generation and secretion of placental exosomes. Using immunoelectron microscopy, we show that FasL and TRAIL are expressed on the limiting membrane of multivesicular bodies where, by membrane invagination, intraluminal microvesicles carrying membranal bioactive FasL and TRAIL are formed and released in the extracellular space as exosomes. Analyzing exosomes secreted from placental explant cultures, to our knowledge, we demonstrate for the first time that FasL and TRAIL are clustered on the exosomal membrane as oligomerized aggregates ready to form death-inducing signaling complex. Consistently, placental FasL-and TRAIL-carrying exosomes triggered apoptosis in Jurkat T cells and activated PBMC in a dose-dependent manner. Limiting the expression of functional FasL and TRAIL to exosomes comprise a dual benefit: 1) storage of exosomal FasL and TRAIL in multivesicular bodies is protected from proteolytic cleavage and 2) upon secretion, delivery of preformed membranal death molecules by exosomes rapidly triggers apoptosis. Our results suggest that bioactive FasL-and TRAIL-carrying exosomes, able to convey apoptosis, are secreted by the placenta and tie up the immunomodulatory and protective role of human placenta to its exosome-secreting ability.

Place, publisher, year, edition, pages
The American Association of Immunologists , 2013. Vol. 191, no 11, 5515-5523 p.
National Category
Immunology in the medical area
Identifiers
URN: urn:nbn:se:umu:diva-84107DOI: 10.4049/jimmunol.1301885ISI: 000327180600019OAI: oai:DiVA.org:umu-84107DiVA: diva2:680481
Available from: 2013-12-18 Created: 2013-12-16 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Immunomodulation during human pregnancy: placental exosomes as vehicles of immune suppression.
Open this publication in new window or tab >>Immunomodulation during human pregnancy: placental exosomes as vehicles of immune suppression.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The mammalian pregnancy comprises a challenge to the maternal immune system since the fetus is semi-allogeneic and could thus be rejected. Pregnancy success is associated with the placenta that is not only essential for oxygen supply, nourishment and pregnancy hormones but also plays a role in the protection of the fetus against maternal immunologic attack. The aim of the current studies was to elucidate the role of human placenta as an immunomodulatory organ with a special focus on placental exosomes as vehicles for establishment of maternal tolerance to the fetus.

We discovered that the syncytiotrophoblast in human normal pregnancy constitutively produces and secretes exosomes. Exosomes are 30-100 nanometer-sized membrane vesicles of endosomal origin that convey intercellular communication. Exosomes are produced and released through the endosomal compartment and reflect the type and the activation state of the cells that produce and secrete them. They carry cytosolic and membrane-bound proteins and nucleic acids and can influence and re-program recipient cells. Depending on their interactions with cells of the immune system they can be divided into immunostimulatory or immunosuppressive.

We developed methods for isolation and culture of trophoblast and placental explants from human normal first trimester pregnancy and isolated exosomes from the culture supernatants.  These exosomes were characterized biochemically and functionally regarding mechanisms with potential importance in the establishment of maternal tolerance towards the fetus. The following aspects were studied: 1) exosomal modulation of the NKG2D receptor-ligand system, a major cytotoxic pathway for NK- and cytotoxic T cells and thus potentially dangerous to the fetus; 2) placental exosome-mediated apoptosis of activated immune effector cells; and 3) Foxp3-expressing T regulatory cells in human pregnant uterine mucosa, the decidua.

Using immuno electron microscopy we show that human early syncytiotrophoblast constitutively expresses the stress-inducible NKG2D ligands MICA/B and ULBP1-5, and the apoptosis inducing molecules FasL and TRAIL. While MICA/B were expressed both on the cell surface and intracellularly on the limiting membrane of multivesicular bodies (MVB) and on exosomes, the ULBP1-5, FasL and TRAIL  were solely  processed through the MVB of the endosomal compartment and secreted on exosomes. The NKG2D ligand-expressing placental exosomes were able to internalize the cognate receptor from the cell surface of activated NK- and T cells thus down regulating their cytotoxic function. In our studies of apoptosis we found that placental exosomes carry the proapoptotic ligands FasL and TRAIL in their active form as a hexameric complex of two homotrimeric molecules, required for triggering of the apoptotic signaling pathways. This finding was supported by the ability of isolated placental FasL/TRAIL expressing exosomes to induce apoptosis in activated peripheral blood mononuclear cells (PBMC) and Jurkat T cells. Additionally, we studied Foxp3-expressing T regulatory (Treg) cells in paired human decidual and blood samples from pregnant women compared to non-pregnant controls. The CD4+CD25+Foxp3+ Treg cells were 10 fold enriched in the decidual mucosa compared to peripheral blood of pregnant women and non-pregnant controls. We discovered a pool of Foxp3-expressing, CD4+CD25- cells in human decidua, a phenotype consistent with naïve/precursor Foxp3+ Treg cells. These results suggest local enrichment of Treg cells in decidua of normal pregnancy. Furthermore, we have results indicating that the exosomes, isolated from placental explant cultures, carry PD-L1 and TGFβ on their surface, molecules known to promote induction of Treg cells. Taken together, our results provide evidence that placental exosomes are immunosuppressive and underline their role in the maternal immune modulation during pregnancy. The constitutive production and secretion of immunosuppressive placental exosomes create a protective exosomal gradient in the blood surrounding the feto-placental unit. This “cloud of immunosuppressive exosomes” conveys immunologic privilege to the developing fetus and thus contributes to the solution of the immunological challenge of mammalian pregnancy. 

Place, publisher, year, edition, pages
Umeå: Umeå Universitet, 2014. 72 p.
Series
Umeå University medical dissertations, ISSN 0346-6612 ; 1640
Keyword
exosomes, microvesicles, pregnancy, placenta, syncytiotrophoblast, immune privilege, apoptosis, cytotoxicity, T regulatory cells, NKG2D, MICA/B, FasL, TRAIL
National Category
Immunology in the medical area
Research subject
Immunology
Identifiers
urn:nbn:se:umu:diva-87566 (URN)978-91-7601-034-1 (ISBN)
Public defence
2014-04-25, A103 - Astrid Fagraeus-salen, byggnad 6A, Norrlands universitetssjukhus, Umeå, 13:00 (English)
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Supervisors
Available from: 2014-04-04 Created: 2014-04-03 Last updated: 2014-04-04Bibliographically approved

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