In vitro inhibitory effect of human syncytiotrophoblast plasma membranes on the cytolytic activities of CTL and NK cells

In utero priming of fetal immune activation: Myths and mechanisms

Mechanisms of fetal immune system development in utero remain incompletely elucidated. Protective immunity, the arm of reproductive immunology concerned with the progressive education of the fetal immune system as pregnancy advances, allows for programming of the immune system and immune maturation in utero and provides a responsive system to respond to rapid microbial and other antigenic exposure ex utero. Challenges in studying fetal tissues, immune system development, and the contributions of various endogenous and exogenous factors to this process are difficult to study as a progressive sampling of fetal biological samples is impractical during pregnancy, and animal models are limited. This review provides a summary of mechanisms of protective immunity and how it has been shaped, from transplacental transfer of immunoglobulins, cytokines, metabolites, as well as antigenic microchimeric cells to perhaps more controversial notions of materno-fetal transfer of bacteria that subsequently organize into microbiomes within the fetal tissues. This review will also provide a quick overview of future direction in the area of research on fetal immune system development and discusses methods to visualize fetal immune populations and determine fetal immune functions, as well as a quick look into appropriate models for studying fetal immunity.

Placenta-Derived Exosomes as a Modulator in Maternal Immune Tolerance During Pregnancy

Exosomes are a subset of extracellular vesicles with an average diameter of ~100nm. Exosomes are released by all cells through an endosome-dependent pathway and carry nucleic acids, proteins, lipids, cytokines and metabolites, mirroring the state of the originating cells. The function of exosomes has been implicated in various reproduction processes, such as embryo development, implantation, decidualization and placentation. Placenta-derived exosomes (pEXO) can be detected in the maternal blood as early as 6 weeks after conception and their levels increase with gestational age. Importantly, alternations in the molecular signatures of pEXO are observed in pregnancy-related complications. Thus, these differentially expressed molecules could be the potential biomarkers for diagnosis of the pregnancy-associated diseases. Recent studies have demonstrated that pEXO play a key role in the establishment of maternal immune tolerance, which is critical for a successful pregnancy. To gain a better understanding of the underlying mechanism, we highlighted the advanced studies of pEXO on immune cells in pregnancy.

Disturbances in placental immunology: ready for therapeutic interventions?

Recent studies have provided new insight into aberrations in the immunological interplay between mother and fetus and their potential role in the development of recurrent fetal loss and preeclampsia. The action of anti-phospholipid antibodies in recurrent fetal loss is now proposed to involve the complement system, neutrophil activation and the production of TNFalpha by immune bystander cells. A clear involvement of the immune system is emerging in preeclampsia, involving mainly the innate arm, especially neutrophils. The activation of peripheral neutrophils by placentally released inflammatory debris triggers the induction of neutrophil extracellular traps (NETs), which may lead to an occlusion of the intervillous space, thereby further promoting a condition of placental hypoxia. It has, hence, been suggested that new therapeutic strategies be developed, including the possible use of TNFalpha antagonists in cases of recurrent miscarriage. These strategies need to be addressed with caution due to the possible induction of fetal congenital abnormalities.

Suppression of immunoglobulin secretion by soluble annexin II

Annexin II (AII) belongs to a family of glycoproteins that bind negatively charged phospholipids in the presence of calcium. The annexins exert various biological functions. We have previously shown that soluble AII suppresses mitogen-induced lymphoproliferation in vitro. In this study we address the question of whether soluble AII may also affect immunoglobulin secretion. Mononuclear cells from peripheral blood were stimulated with pokeweed mitogen in vitro and immunoglobulin-secreting cells were quantified using an ELISPOT assay. Retroplacental serum and soluble AII significantly inhibited secretion of IgG and IgM when added at concentrations that did not affect lymphoproliferation or cell viability. The inhibitory effect was dose- and time dependent. Significant suppression was observed when soluble AII was added at concentrations of 0.1 and 0.01 microg/ml. The strongest inhibition was observed when soluble AII or retroplacental serum was added initially. The data demonstrate that soluble AII can suppress immunoglobulin secretion in vitro. AII seems to be a potent immunosuppressive substance. The presence of high levels of soluble AII in retroplacental serum may indicate a possible immunomodulatory role in normal pregnancy.

Suppressive effect on lymphoproliferation in vitro by soluble annexin II released from isolated placental membranes

PROBLEM

Syncytiotrophoblast microvillous plasma membranes (StMPM) are potent suppressors of lymphoproliferation in vitro. We have previously shown that soluble annexin II (AII) is present at higher levels in retroplacental serum (RPS) than in peripheral serum, and that soluble AII has an immunosuppressive effect. The aims of this study were to determine whether AII can be released from StMPM and whether soluble AII from StMPM exerts any immunosuppressive effect.

METHOD OF STUDY

Isolated StMPM were incubated in growth medium for 18 hr and supernatants were prepared by ultracentrifugation. Soluble AII was detected by immunoblotting. StMPM, StMPM supernatant, and affinity-purified AII were analysed in a lymphoproliferation assay for immunomodulating activity.

RESULTS

AII heavy chain and its p11 light chain were detected both in StMPM supernatant and in RPS after removal of StMPM particles by ultracentrifugation. StMPM, StMPM supernatant, and purified AII suppressed lymphoproliferation in a dose-dependent manner. Absorption of AII from StMPM supernatant reduced the suppressive activity. The suppressive effect of StMPM supernatant and purified AII was completely reversed by heating at 100 degrees C for 30 min or by adding recombinant interleukin-2 at 100 units/ml. Although StMPM and affinity-purified AII suppressed the proliferation of lymphocytes from all donors tested, StMPM supernatant suppressed the proliferation of lymphocytes from 12 of 23 donors. Six of eight female non-suppressed donors were multiparae, whereas five of five female suppressed donors were nulliparae.

CONCLUSIONS

Annexin II is released by isolated placental membranes in vitro and is present in RPS, indicating in vivo release of AII at the fetomaternal interface, probably as AII heterotetramer. AII has immunosuppressive activity and may be important in fetal allograft survival.

Human placental Fc gamma-binding proteins in the maternofetal transfer of IgG

Annexin II, a member of the annexin family of Ca2+ and phospholipid binding proteins, is present in human placenta. Placental annexin II has low affinity FcR activity, and is present as a heterotetramere on syncytiotrophoblast apical cell membrane extracellular surface. In addition to annexin II, transmembraneous leukocyte FcRIII is present on syncytiotrophoblast apical membrane. Either one, or both molecules may mediate the binding of IgG and thereby facilitate its transport through the syncytiotrophoblast layer. However, the presence of other maternal plasma proteins in syncytiotrophoblasts that are not transported to the human fetus is suggestive of nonspecific fluid phase endocytosis. The MHC class I like FcR, similar to the receptor found in neonatal rodent intestine, FcRn, is present intracellularly in human syncytiotrophoblasts, as is its light chain beta 2-microglobulin. The hFcRn is not detected on the apical plasma membrane. The placental hFcRn co-localizes with IgG in syncytiotrophoblast granules. It is likely that hFcRn binds and transcytoses IgG through the syncytiotrophoblast. Protected transfer of IgG may occur within syncytiotrophoblast endocytotic vesicles prior to release in the villous stroma and subsequent translocation into the lumen of fetal stem vessels by uptake and transport in endothelial caveolae.

Surface annexin II on placental membranes of the fetomaternal interface

PROBLEM

The phospholipidbinding membrane protein annexin II has been demonstrated to possess FcR activity for IgG and has been localized to the outer part of the syncytiotrophoblast cell layer. The question has arisen whether annexin II is exposed on the surface of syncytiotrophoblast cells thus enabling it to take part in the transport of IgG across the maternal barrier.

METHOD

Syncytiotrophoblast microvillous plasma membranes were analyzed by flow cytometry for annexin II as well as established surface molecules. Fresh, fixed placental tissue was preincubated with antibodies to annexin II or known trophoblast surface molecules, and analyzed by confocal laser scanning microscopy.

RESULTS

Annexin II and its subunit p11 were expressed on the surface of the syncytiotrophoblast microvillous plasma membranes as were other established surface proteins (CD46, CD59, placental alkaline phosphatase), using both flow cytometry and confocal microscopy. Annexin was not detected on the surface of viable cultured trophoblast cells.

CONCLUSION

Annexin II is exposed on the surface of syncytiotrophoblast cells as a heterotetramer together with its light chain p11. It is exposed to maternal blood and may be instrumental in IgG transport across the placental barrier by binding.

Inhibition of induced lymphocyte proliferation by lipid and protein components of the syncytiotrophoblast plasma membrane

PROBLEM

The aim of this work was to define the respective responsibilities of the lipid and protein components of syncytiotrophoblast plasma membranes on the inhibition of lymphocyte proliferation induced in vitro.

METHOD

A fractionation method using octyl-beta-D-glucopyranoside enabled lipoprotein, lipid, and protein fractions to be isolated from the membrane.

RESULTS

The lipid fraction was shown nonspecifically to inhibit lymphocyte proliferation, to a lower extent compared with the native membrane. Alternatively, the protein fraction used as a proteoliposome contained the totality of the cytostatic effect of the native fraction.

CONCLUSION

These results are discussed generally in the context of the immunoregulatory role of membrane lipids and proteins and in relation to the local properties of syncytiotrophoblast plasma membrane components in fetal graft tolerance.

Inhibitory effect of human syncytiotrophoblast plasma membrane vesicles on Jurkat cells activated by phorbol ester and calcium ionophore

The effects of syncytiotrophoblast plasma membrane vesicles (STPM) on stimulated Jurkat leukemic T cells have been investigated. STPM inhibited IL-2 production and the expression of protein P55 of the IL-2 receptor (IL-2R P55), when Jurkat cells were stimulated by a combination of calcium ionophore A23187 (CaI) + phorbol 12-myristate 13-acetate (PMA). STPM also inhibited IL-2R P55 when cells were stimulated by PMA alone, a situation in which IL-2 production is negligible. On the other hand, STPM had no effect on the sustained mobilization of intracellular Ca2+ induced by CaI nor on the PKC-dependent CD3 down regulation induced by PMA. Finally STPM had no effect on intracellular cAMP levels. These results show that (i) the inhibitory effect of STPM on IL-2R P55 expression is independent of the inhibition of IL-2 production, and (ii) the inhibitory effects of STPM are at least partially independent of phosphatidylinositol 4,5-bisphosphate hydrolysis. They suggest that STPM affect a signaling pathway activated by PMA but possibly PKC independent.

The inhibitory effect of human syncytiotrophoblast plasma membrane vesicles on in vitro lymphocyte proliferation is associated with reduced interleukin 2 receptor expression

The mechanisms by which vesicles of syncytiotrophoblast plasma membranes (STPM) prepared from full-term human placentas inhibit lymphocyte proliferation have been investigated. In the presence of STPM, IL-2 secretion and the expression of protein P55 (IL-2R P55) from its receptor were examined in two models of PBMC proliferation: induced by PHA in 3-day-old cultures, and induced by IL-2 in 6-day-old cultures. In the case of PHA stimulation, STPM strongly inhibited IL-2 (but not IL-1) secretion and IL-2R P55 expression at a concentration where lymphocyte proliferation was also blocked. In these conditions, the addition of excess recombinant IL-2 (rIL-2) only partially restored proliferation and IL-2R P55 expression. In addition, STPM inhibited proliferation and IL-2R P55 expression when resting PBMC were stimulated by a high concentration of rIL-2. These results suggest that STPM inhibit lymphocyte proliferation by affecting one or several events occurring in the synthesis and/or expression of IL-2R P55 by a mechanism which is at least partially independent of its inhibitory effect on IL-2 secretion. The significance of these results is discussed in the context of the survival of the fetal allograft.

Inhibition of natural killer cell activity by retroplacental sera as compared to peripheral sera. Lack of influence of immune complexes

A decrease of natural killer cell activity (NKCA) during human pregnancy might contribute to the acceptance of the allogeneic fetus by the maternal host. The inhibition of NKCA might be due to serum factors derived from the trophoblast. We focused especially on the role of immune complexes, as it has already been described that these complexes depress NKCA and as they are found frequently in retroplacental serum. We have compared the influence of 19 paired retroplacental and peripheral blood sera on NKCA of normal donors. One peripheral and eight retroplacental sera contained immune complexes. Normal donor mononuclear cells were incubated with carboxyfluorescein-labeled K562 cells in the presence of retroplacental serum or peripheral serum. NKCA was measured on a FACS Analyzer. Ten of 19 retroplacental sera inhibited NKCA significantly in comparison to the corresponding peripheral serum (P = 0.003). There was no correlation between NKCA and the immune complex level. We conclude that, as compared to peripheral serum taken at delivery, there is a retroplacental serum-induced inhibition of NKCA, which is not correlated with the presence of immune complexes.