It has been proposed that either excessive inflammation or an imbalance in angiogenic factors cause pre-eclampsia. In the present review, the arguments for and against the role of inflammation and/or angiogenic imbalance as the cause of pre-eclampsia are discussed on the basis of the Bradford–Hill criteria for disease causation. Although both angiogenic imbalance and systemic inflammation are implicated in pre-eclampsia, the absence of temporality of inflammatory markers with pre-eclampsia challenges the concept that excessive inflammation is the cause of pre-eclampsia. In contrast, the elevation of anti-angiogenic factors that precede the clinical signs of pre-eclampsia fulfils the criterion of temporality. The second most important criterion is the dose–response relationship. Although such a relationship has not been proven between pro-inflammatory cytokines and pre-eclampsia, high levels of anti-angiogenic factors have been shown to correlate with increased incidence and disease severity, hence satisfying this condition. Finally, as the removal of circulating sFlt-1 (soluble Fms-like tyrosine kinase receptor-1) from pre-eclamptic patients significantly improves the clinical outcome, it fulfils the Hill's experiment principle, which states that removal of the cause by an appropriate experimental regimen should ameliorate the condition. In contrast, treatment with high doses of corticosteroid fails to improve maternal outcome in pre-eclampsia, despite suppressing inflammation. Inflammation may enhance the pathology induced by the imbalance in the angiogenic factors, but does not by itself cause pre-eclampsia. Development of therapies based on the angiogenic and cytoprotective mechanisms seems more promising.
Keywords: cytokine, inflammation, pre-eclampsia, soluble endoglin (sEng), soluble Fms-like tyrosine kinase receptor-1 (sFlt-1), vascular endothelial growth factor (VEGF)
Abbreviations: eNOS, endothelial nitric oxide synthase; ET-1, endothelin-1; HELLP, haemolysis, elevated liver enzymes and low platelets; IFNγ, interferon γ; IL, interleukin; PlGF, placenta growth factor; sEng, soluble endoglin; sFlt-1, soluble Fms-like tyrosine kinase receptor-1; TNFα, tumour necrosis factor α; VEGF, vascular endothelial growth factor; VEGFR, VEGF receptor