1 Department of Reproductive Medicine and Child Development, Division of Gynaecology and Obstetrics, 2 Stella Maris Foundation and 3 Institute of Immunohematology, University of Pisa, S. Chiara Hospital, Via Roma 55, 56100-Pisa, Italy
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Abstract |
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Key words: histocompatibility/HLA-DR antigens/homozygosity/pre-eclampsia
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Introduction |
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The placenta acts as an immunological barrier between the mother and the fetal `graft', allowing two antigenically different organisms to tolerate one another. During the course of pregnancy this barrier becomes extremely thin, <2 µm at term, which is only slightly greater than the pulmonary alveolar blood/air barrier (Jones and Fox, 1991). It is clear that any damage to this barrier from various ischaemic risk factors (metabolic, hormonal, genetic, immunological) may be responsible for lesions of the syncytiotrophoblast and villous vessel endothelial cells (Jones and Fox, 1980
). Therefore, mixing of maternal and fetal blood is possible, with triggering of a maternal rejection reaction (de Luca Brunori et al., 1994).
The concept that pre-eclampsia may be caused by an abnormal maternal immune response to antigenic challenge by the fetal allograft has been analysed by many authors (Jenkins et al., 1977; Stirrat, 1987
; El-Roeiy and Gleicher, 1988; Sibai, 1991
; Redman, 1992
; Vinatier and Monnier, 1995
). Following this immunological hypothesis, we previously undertook an immunohistochemical study, using HLA-DR monoclonal antibody in placentae from pre-eclamptic women. We observed intense and widespread expression of the HLA-DR antigens, absolutely absent in the control placentae (de Luca Brunori et al., 1994). This reaction is similar to that which precedes the hyperacute rejection reaction in transplanted human kidney and heart (Gille, 1980
; Caforio et al., 1990
).
To understand the mechanism at the basis of such an evident immunological reaction in pre-eclampsia, we undertook the present study to evaluate if pre-eclampsia, like transplant rejection, could be related to the immunological role of the HLA-DR antigens.
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Materials and methods |
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The group of pre-eclamptic women included 69 Italian Caucasian couples and one South-American couple. All women were singleton primigravidae, between the 26th and 35th week of gestation, aged 29 years on average; they had no history of hypertension, diabetes, renal diseases, autoimmune disorders or blood transfusions. No consanguineous marriages were included.
The control group included 70 normotensive healthy primigravidae women and their partners, selected after term delivery of live and healthy babies. This control group was made up of Italian Caucasian couples from the same geographical area as the couples in the study group to avoid any difference in ethnic composition between groups.
Aliquots of 9 ml of venous blood samples were taken in 1 ml adenine-citrate-dextrosium (ACD) solution tubes, and examined using the standard lymphocytotoxicity technique for HLA-DR typing (Terasaki and McLelland, 1964). Each couple included in the study and the control group was typed by means of two 72-well plates (Terasaki plates; One Lambda Inc., Canoga Park, CA, USA) differing in sera composition in order to use a very wide serological range.
Twenty pre-eclamptic and 10 control couples, in addition to serological Terasaki typing, were checked by polymerase chain reaction (PCR) and reverse dot blot hybridization technique (Buyse et al., 1993). INNO-LIPA (Milan, Italy) HLA-DRB genotyping at low resolution was used.
In order to make the DR typing data uniform, the results were reported as broad antigen specificities from 1 to 10.
The HLA-DR typing results of the pre-eclamptic and control groups were compared using the 2 test. All differences were considered to be statistically significant when P < 0.05. The relative risk (RR) was calculated by Woolf's method (Woolf, 1955
) with Haldane's modification (Haldane, 1956
).
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Results |
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The distribution of HLA-DR homozygosity in the study group (Table IV) was either in both partners, 26/70 cases (37.1%), or only in women, 22/70 cases (31.4%), or only in men, 20/70 cases (28.6%). No significant difference in homozygosity frequency was found between the partners in the couples of pre-eclamptic women.
Out of the two couples of pre-eclamptic heterozygous partners, the first was HLA-DR identical (female DR 6,7; male DR 6,7) while the second shared one DR specificity (female DR 2,5; male DR 4,5).
Pre-eclampsia was extremely severe for perinatal mortality and/or maternal endangering (eclampsia, pleural and pericardial effusion) in 24/26 couples in whom both partners were homozygous and also in the above mentioned couple of heterozygous but DR identical partners (female DR 6,7; male DR 6,7).
We have also analysed the antigenical variety (i.e. the number of different DR antigens in the couple) and the antigenical disparity (i.e. the DR specificities not shared by the partners) (Table V). From this table, it emerges that parents associated with pre-eclamptic pregnancies generally possess a smaller total number of different HLA-DR specificities than control couples.
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The analysis of the HLA-DR compatibility between partners, female to male (i.e. when the DR of the female is compatible with that of the male) and male to female (i.e. when the DR of the male is compatible with that of the female), showed a statistically significant increase in the female-to-male compatibility (RR 30.13, P = 0.0003) and a lower but significant increase of male-to-female compatibility (RR 4.6, P = 0.014) in comparison with controls (Table III).
In the 20 couples of pre-eclamptic women and 10 control couples checked by PCR in addition to the Terasaki method, the results of the biological molecular technique confirmed the serological data (Tables VI, VII).
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Discussion |
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In our study, we did not find any particular HLA-DR specificity associated with pre-eclampsia. However, we did find a highly significant increase in HLA-DR homozygosity compared to controls in both partners of the couples with pre-eclamptic pregnancies.
This finding is consistent with a previous observation (Redman et al., 1978) that led to the suggestion that increased HLA compatibility between couples might contribute to pre-eclampsia as a result of an impaired ability to generate a protective maternal immune response. This attractive hypothesis predicts increased male-to-female compatibility, but the data reported here are characterized by an exceptionally high degree of female-to-male compatibility. Moreover, our data are at variance with several other studies (reviewed by Kilpatrick et al., 1999).
Although different geographical populations were investigated and minor differences in disease definitions applied, the reason(s) for this discrepancy is not obvious and cannot readily be explained.
Nevertheless, from our data, two observations have emerged: (i) pre-eclampsia could be a couple's disease because of the occurrence of HLA-DR homozygosity in one or both partners of the couples associated with pre-eclamptic pregnancies; (ii) pre-eclampsia appears to occur more frequently in couples with a smaller total number of different HLA-DR specificities.
The discussion is still open and further studies are needed to clarify the enigma.
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Acknowledgments |
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Notes |
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References |
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Submitted on October 26, 1999; accepted on May 18, 2000.