It has been estimated that 50–70% of human conceptions fail and that 1–3% of women in the United States suffer recurrent miscarriages (1
). Moreover, intrauterine growth restriction (IUGR) occurs in up to 10% of infants born in the United States (2
), and growth-restricted fetuses have higher mortality and morbidity rates than fetuses with weights greater than the 10th
percentile. Although the insult to the fetus occurs in utero, the deleterious influence of IUGR contributes to long-term developmental delay, and a suboptimal intrauterine environment has been linked to metabolic disorders during adult life, including coronary artery disease, hypertension, hyperlipidemia, and insulin resistance (3
Although the causes of recurrent miscarriages and IUGR are poorly understood, an immune mechanism involving inappropriate and subsequently injurious recognition of the conceptus by the mother's immune system has been proposed. This misdirected immune response to fetus, placenta, or both could yield a wide range of phenotypes, including miscarriages early in pregnancy and IUGR later in pregnancy. Pregnancy constitutes a major challenge to the maternal immune system because it requires tolerance of fetal alloantigens encoded by paternal genes. Local factors at the maternal–fetal interface are required to maintain such tolerance and assure normal development of semiallogenic concepti (4
). The fetus is protected from maternal immune responses through mechanisms such as expression of HLA-G (5
), inhibitory T cell costimulatory molecules (6
), and complement regulatory proteins by trophoblasts (7
), and by local maternal regulatory T cells (8
) and production of the immunosuppressive enzyme indoleamine 2,3 dioxygenase (9
A growing body of evidence now indicates that the adaptive immune response is regulated by the innate immune system; yet once engaged, adaptive immune responses can commandeer innate effectors to induce injury. Indeed, some perturbations of innate immune responses are associated with “spontaneous” abortion. Complement activation, in particular, has emerged as a common event in recurrent pregnancy loss (10
), but whether it also contributes to abnormal placental development is unknown. We have previously identified complement as a critical early effector in antiphospholipid antibody-induced pregnancy loss and IUGR (11
). Although activation of complement and recruitment of inflammatory cells within decidual tissue are necessary intermediary steps in antibody-mediated pregnancy complications, the downstream pathogenic mediators of placental and fetal damage have not been defined. It is likely that neutrophils and monocytes stimulated by complement activation products directly damage the developing embryo. Alternatively, these armed effector cells may release factors that cause placental dysfunction and compromise fetal growth, a mechanism that we investigated and report on in this study.
Satisfactory development of the fetomaternal vasculature is required for successful embryonic growth, and insufficient placental vascularization has been associated with early embryonic mortality, preeclampsia, and IUGR (13
). Normal placental development requires coordinated expression of angiogenic growth factors, vascular endothelial growth factor (VEGF) and placenta growth factor, as well as expression of their respective receptors on invasive trophoblasts (14
). VEGF promotes placental development and invasiveness primarily through interaction with the high-affinity type III receptor tyrosine kinases: VEGF receptor 1 (VEGFR-1, also known as Flt-1) and VEGFR-2 (15
). Alternative splicing of VEGFR-1 results in production of the secreted protein, soluble VEGFR-1 (sVEGFR-1, also known as sFlt-1), which lacks the cytoplasmic and transmembrane domains but retains the ligand-binding domain (16
). The production of sVEGFR-1, a potent anti-angiogenic molecule that sequesters circulating VEGF and placenta growth factor and prevents their interaction with endogenous receptors (17
), may be regulated independently of cell-bound VEGFR-1 (18
). Placental trophoblasts exposed to stress, such as hypoxia, release large amounts of sVEGFR-1 into the maternal circulation (20
). Excess sVEGFR-1 has been shown to inhibit placental cytotrophoblast differentiation and invasion (21
) and is thought to play a direct role in the pathogenesis of abnormal placentation associated with preeclampsia and IUGR (14
). Indeed, sVEGFR-1 levels have been shown to be increased in the placenta and blood of women with preeclampsia (for review see reference 14
). Although considerable evidence indicates that proteins and receptors in the VEGF family are involved in the pathogenesis of placental disorders, the relationship between inflammation and dysregulation of angiogenic factors in pregnancy is unexplored.
A link between inflammation and angiogenesis has been established in the pathogenesis of other disease processes. Inflammatory mediators have been shown to stimulate resident cells to produce VEGF and promote angiogenesis and tissue damage: joint destruction in rheumatoid arthritis and choroidal neovascularization in age-related macular degeneration (24
). Under certain conditions, however, inflammatory cells may interrupt or prevent VEGF-induced angiogenesis by secreting sVEGR-1, which sequesters VEGF (18
). Given that functional VEGF deficiency leads to abnormal placental development, we considered the possibility that inflammation at the maternal–fetal interface triggers sVEFGR-1 production, leading to angiogenic factor imbalance and poor pregnancy outcomes.
Here we show that complement activation is a required intermediary event in the pathogenesis of fetal injury in an antibody-independent model of spontaneous miscarriage and IUGR, these pregnancy complications are associated with elevated levels of the anti-angiogenic factor sVEGFR-1, and products of the complement cascade directly trigger release of sVEGFR-1 from monocytes. We provide the first evidence that complement activation alters the balance of angiogenic factors in pregnancy and identify a novel mechanism for immune-triggered pregnancy complications.