The feto-maternal lymphocyte interaction in preeclampsia and in uncomplicated pregnancy

Pathogenesis of preeclampsia: a comprehensive model

The objective of this review was to provide a comprehensive and practical concept on the pathogenesis of preeclampsia on the basis of the currently available scientific evidence. A MEDLINE search was performed of English-language articles published between 1966 and 1997, supplemented with references cited in relevant research articles. Using our data sources, we developed a scheme describing the sequence of events between implantation and the time of manifest clinical disease characterized by generalized endothelial cell dysfunction. A yet unidentified toxic circulating factor released by the ischemic placenta, is held responsible for the impaired endothelial cell function. Particularly, epidemiological studies point to a concept in which immune maladaptation to the fetal allograft plays a key role in causing defective placentation leading to placental ischaemia. The incidence of preeclampsia in sisters and daughters of women who had had preeclampsia is raised. Disease states with vascular involvement, like chronic hypertension and diabetes mellites, are associated with an increased risk for preeclampsia. Recently subclinical abnormalities in hemostasis, metabolism and volume homeostasis have been described in patients with a history of preeclampsia. Placental ischemia secondary to defective placentation, a prerequisite for the development of preeclampsia, has a multifactorial origin consisting of three major components: immune maladaptation, genetic predisposition, and vascular mediated factors. Probably, a summation of these factors will determine whether a pregnant woman is to develop the syndrome. The recently described subclinical abnormalities in hemostasis, metabolism, and vascular function in patients with a history of preeclampsia might give the clinician the opportunity to reduce the recurrence risk by pharmacotherapeutic intervention.

Pre-eclampsia: physiology and immunological aspects

Pre-eclampsia is a frequent, unpredictable syndrome which is dangerous for both mother and foetus. The concept of placental ischemia has gained wide acceptance among the numerous theories put forward to explain the illness. The setting up of preeclampsia seems to be scheduled in two steps: (1) an absolute or relative placental ischemia due to vascular diseases or hypertrophic placenta, or most often secondary to implantation defect, particularly anomaly with the invasive trophoblast; (2) a diffuse endothelial disease. The connection between these two steps is incompletely disclosed. The authors demonstrate that the maternal immune system which is strongly stressed during all the stages of normal gestation is implicated in pre-eclampsia. Its role is probably not univocal. Foeto-trophoblastic antigens could be poorly recognised. This defect of recognition could lead to the abnormalities of trophoblastic invasion observed in pre-eclampsia. Pre-eclampsia does not seem to be accompanied by an immunological rejection of the foetus. Some genetically predisposed patients do not have a sufficiently competent immune system to neutralise one or more of the toxic products released by the ischemic placenta. Certain types of pre-eclampsia could be auto-immune, with the auto-antibodies directed against certain types of phospholipids or trophoblastic constituents. A disequilibrium between oxidation and anti-oxidation mechanisms involving neutrophils could lead to aggression of the endothelium which is observed in pre-eclampsia. Pre-eclampsia could represent a form of immuno-dystrophy, with the excessive production of adverse cytokines locally, directed against the trophoblast. Without directly implicating the immune system as the trigger of pre-eclampsia, it seems that its role is unclear. In some cases it develops protective mechanisms which, when overwhelmed or inadequate, allows pre-eclampsia to occur. In other cases it can form part of the cascade of aggressions leading to the abnormalities encountered. The integration of these abnormalities in the pathophysiological models, could help improve the classification of pre-eclampsia. This attempt will lead to a more adapted preventive and therapeutic management of pre-eclampsia.

Immunological aspects of pre-eclampsia

The first pregnancy preponderance and apparent partner specificity of pre-eclampsia suggest that it might have an immune aetiology. The pathogenesis of pre-eclampsia is undefined although it is clear that it is a placental disorder. The maternal syndrome appears to be mediated by placental ischaemia secondary to spiral artery insufficiency. This leads to a hypothesis that pre-eclampsia is a two-stage disease. The first comprises processes that limit the size of the spiral arteries (poor placentation) or obstruct them (acute atherosis). Either or both may have immunological causes although there is no direct evidence. Factors limiting placentation could involve maternal immune intolerance of the fetal allograft, which in their most extreme expression could lead to immunologically mediated abortion. Thus pre-eclampsia may be part of a wider spectrum of pregnancy loss secondary to poor maternal immune accommodation of her genetically disparate fetus. The second stage involves the consequences of the ensuing placental ischaemia. The syndrome is currently tentatively ascribed to diffuse maternal endothelial dysfunction. There is less reason to invoke immunological mechanisms in the second stage although neutrophil activation could explain generalized endothelial damage. It should be clear that these conclusions are provisional and that the greatest need is for more investigation to eliminate the uncertainty which clouds our concepts.

Lymphocyte subsets in normal and pre-eclamptic pregnancies

Peripheral blood lymphocyte subsets in normal and preeclamptic pregnancies have been studied by using the monoclonal antibodies OKT3 (T cells), OKT4 (helper/inducer T cells) and OKT8 (suppressor/cytotoxic T cells). In addition the numbers of mononuclear cells bearing Ia and monocyte antigens have been assessed by using the monoclonal antibodies OKIa1 and OKM1. No significant differences were found between 10 normal pregnant and 10 non-pregnant subjects. Ten preeclamptic patients were studied and showed an increase in OKT4-positive helper cells. This was significant in terms of percentage of mononuclear cells but not the absolute numbers or the helper/suppressor (OKT4/OKT8) ratio.

Cellular hyperreactivity to placenta in toxemia of pregnancy

It is generally assumed that maternal immune response may be involved in some pathological conditions of pregnancy, such as toxemia. The activity of cell-mediated immunity to placenta was studied in women with gestosis. The leucocyte inhibition technique was employed to test reactivity of peripheral blood leucocytes from postpartum women. The results indicated that leucocyte migration was significantly inhibited in the presence of placental microsomal fraction when blood from toxemic women was used. Thus, about 87% out of 30 studied cases of toxemia showed positive results. At the same time, in only 22% of women with uncomplicated pregnancy was such a positive leucocyte migration inhibition test found. There was no correlation between the severity of the toxemic symptoms and the extent of inhibition. In further experiments the leucocyte migration inhibition test with the microsomal fraction antigens from cadaver kidneys was performed; only 1 out of 6 patients showed positive leucocyte migration inhibition. These findings may suggest that the normal status of the mother is altered towards cellular hyperreactivity to placental antigens in toxemic pregnancy.