Antiphospholipid antibodies increase the levels of mitochondrial DNA in placental extracellular vesicles: Alarmin-g for preeclampsia

Abstract

The pathogenesis of preeclampsia remains unclear but placental factors are known to play a crucial role causing maternal endothelial cell dysfunction. One potential factor is placental micro- and nano- vesicles. Antiphospholipid antibodies (aPL) increase the risk of preeclampsia ten-fold, in part by damaging the mitochondria in the syncytiotrophoblast. Since mitochondrial DNA (mtDNA) is a danger- associated molecular pattern (DAMP/alarmin) that may activate endothelial cells, the aims of the current study were to investigate whether aPL affect the number of placental vesicles extruded, their mtDNA content and their ability to activate endothelial cells. Exposure of first trimester human placental explants to aPL affected neither the number nor size of extruded micro- and nano- vesicles (n = 5), however their levels of mtDNA were increased (n = 6). These vesicles significantly activated endothelial cells (n = 5), which was prevented by blocking toll-like receptor 9 (TLR-9), a receptor for extracellular DNA. Thus, aPL may increase the risk of preeclampsia in part by increasing the amount of mtDNA associated with placental vesicles. That mitochondrial DNA is recognised as a DAMP by TLR-9 to cause endothelial cell activation, raises the possibility that placental vesicles or TLR-9 might be a target for pharmaceutical intervention to reduce the consequences of aPL in pregnancy.

Immunological effects of placental extracellular vesicles

“…The presence of DNA in placental EVs is important because: (1) the DNA is fetal in origin and is likely to be encapsulated and protected from circulating nucleases, increasing its potential to be used for noninvasive prenatal diagnosis; and (2) since fetal nuclear DNA is hypomethylated relative to adult nuclear DNA, this EV‐associated DNA may act as a danger‐associated molecular pattern to activate innate immune Toll‐like receptors in recipient cells. We have recently shown that mitochondrial DNA, present in placental micro‐ and nano‐EVs, activates TLR9 in endothelial cells, leading to endothelial cell activation, a hallmark of preeclampsia . In preeclampsia, an increase in DNA‐positive placental micro‐EVs in the maternal circulation has been reported …”

“…Similarly, macro‐EVs extruded by preeclamptic placentae can affect nitric oxide and calcium signaling in endothelial cells, leading to endothelial cell activation . These effects may be due to the presence of danger‐associated molecular patterns in placental EVs, such as HMGB1 or mitochondrial DNA …”

“…While some studies report that EV production is increased by preeclamptic placentae, circulating placental EV number alone does not seem to be predictive of preeclampsia in prospective studies . It is likely that qualitative changes in the placental EV cargo may be more important than quantitative differences, both for the pathogenesis and prediction of obstetric diseases . Furthermore standardization of the sample collection, isolation and characterization protocols will allow comparisons between studies to rapidly advance this field.…”

Endoplasmic reticulum stress occurs in association with the extrusion of toxic extracellular vesicles from human placentae treated with antiphospholipid antibodies

“…In addition, the export of both micro-EVs and nano-EVs containing misfolded proteins may harm the maternal cells that take up these vesicles [5,49]. We have previously shown that EVs from placentae treated with antiphospholipid antibodies contain increased amounts of various danger signals including mitochondrial DNA [50] and HMGB1 [51]. The increased load of misfolded proteins delivered to endothelial cells by EVs from antiphospholipid antibody-affected placentae may be an another factor that contributes to the endothelial cell activation that is a hallmark of preeclampsia, or in maternal organ damage.…”

Antibodies: The major participants in maternal–fetal interaction

“…Although there is no significance of raised APA in cases of infertility and recurrent abortions, and the positive of APA is not necessarily related to early spontaneous abortion, other studies including a multicenter study of 283 pregnancies still supported that APA may increase the risk of pre‐eclampsia in part by increasing the amount of mitochondrial DNA and confirmed that APA positivity is risk factors for adverse maternal and fetal outcomes …”