TLR9 Provokes Inflammation in Response to Fetal DNA: Mechanism for Fetal Loss in Preterm Birth and Preeclampsia

Abstract

Preterm birth, the major cause of neonatal mortality in developed countries, is associated with intrauterine infections and inflammation, although the exact mechanisms underlying this event are unclear. In this study, we show that circulating fetal DNA, which is elevated in pregnancies complicated by preterm labor or preeclampsia, triggers an inflammatory reaction that results in spontaneous preterm birth. Fetal DNA activates NF-κB, shown by IκBα degradation in human PBMCs resulting in production of proinflammatory IL-6. We show that fetal resorption and preterm birth are rapidly induced in mice after i.p. injection of CpG or fetal DNA (300 μg/dam) on gestational day 10–14. In contrast, TLR9−/− mice were protected from these effects. Furthermore, this effect was blocked by oral administration of the TLR9 inhibitor chloroquine. Our data therefore provide a novel mechanism for preterm birth and preeclampsia, highlighting TLR9 as a potential therapeutic target for these common disorders of pregnancy.

Sterile inflammation and pregnancy complications: a review

“…Additionally, RAGE and TLR4, two receptors with affinity for HMGB1, were found to have higher expression in placentas from pregnancies complicated with PE (Kim et al 2005, Chekir et al 2006. Pro-inflammatory effects of HMGB1 via TLR9 have been suggested to contribute to the pathophysiology of PE (Scharfe-Nugent et al 2012).…”

“…Evidence for the placental origin of cffDNA includes the following: (1) it is detected in anembryonic gestation (Alberry et al 2007); (2) it is still detected after therapeutic abortion in which placenta is incompletely removed, albeit undetectable after normal delivery (Lo et al 1999, Wataganara et al 2005; (3) it is detected in cases of invasive placenta, a postpartum pregnancy complication in which trophoblasts invade myometrium (Sekizawa et al 2002); and (4) it carries the placental genotype in patients with confined placental mosaicism (Masuzaki et al 2004). In contrast to maternal cellfree DNA, of which 32% of the fragments are >356 bp, cffDNA are short hypomethylated fragments (<313 bp) and potent inducer of sterile inflammation (Chan et al 2004, Scharfe-Nugent et al 2012, Schroeder et al 2013. The release of cffDNA is a physiological process present in all mammals, but its possible roles and implications in normal pregnancy (and more importantly parturition) remain poorly understood.…”

“…For these reasons, it is broadly investigated as a biomarker of PE (Martin et al 2014). As mentioned above, cffDNA can trigger inflammation and adverse pregnancy outcomes in mice via TLR9 (Scharfe-Nugent et al 2012). Similarly, mitochondrial DNA (mtDNA) is released by trophoblasts upon death and its levels in maternal circulation are increased in PE (and IUGR) (Lattuada et al 2008, Goulopoulou et al 2012, Qiu et al 2012; the immunostimulatory and pro-inflammatory role of mitochondrial DNA in PE has been reviewed elsewhere (McCarthy & Kenny 2016).…”

“…Importantly, this TLR9, NF-κB-dependent proinflammatory effect of cffDNA was shown in pregnant mice and is characterized by IL-6 production in human peripheral blood mononuclear cells (Scharfe-Nugent et al 2012). Classically, TLR9 is localized intracellularly in endoplasmic reticulum (ER), endosomes and lysosomes (Latz et al 2004).…”

“…In animals, sterile inflammation is sufficient to recreate major features of common reproductive diseases (Romero et al 1991, Scharfe-Nugent et al 2012, Gomez-Lopez et al 2016, whereas in humans, a significant part of patients suffering from preeclampsia, preterm labor or other inflammatory diseases during pregnancy display no clinical signs of infection. This has been extensively studied in the context of preterm birth; although observational, correlational and causal data accumulated for >30 years have linked infection to preterm labor, preterm birth without infection is more prevalent (Romero et al 2001).…”