An ELISA for the detection of maternal anti-trophoblast antibodies in human pregnancy

Unexplained intrauterine fetal death is accompanied by activation of complement

OBJECTIVE

Activation of the complement system has recently been implicated in the mechanisms of fetal loss in the antiphospholipid syndrome. It is, however, possible that complement activation is also involved in other causes of fetal death in the second and third trimesters of pregnancy. We therefore conducted a study to determine whether fetal death is associated with changes in the maternal plasma concentrations of complement split products or anaphylatoxins (C3a, C4a and C5a).

STUDY DESIGN

A cross-sectional study was designed to include normal pregnant women (n=60) and patients with fetal death (n=60). Patients with fetal death were classified according to the cause of fetal demise into: a) unexplained (n=44); b) associated with preeclampsia (n=8); and c) associated with chromosomal abnormalities or major congenital fetal anomalies (n=8). The plasma concentrations of C3a, C4a and C5a were measured using sensitive and specific ELISAs. Non-parametric statistics were used for analysis. A P value of <0.05 was considered significant.

RESULTS

1) The median plasma concentration of C5a was higher in patients with fetal death than in normal pregnant women [median 16 ng/mL (range 4.5-402.5) vs. median 11.6 ng/mL (range 1.2-87.1), respectively; P<0.001]; 2) patients with an unexplained fetal death and those associated with preeclampsia had a higher median plasma C5a concentration than normal pregnant women (P=0.002 and P<0.001, respectively); 3) no differences were observed in the maternal plasma concentrations of C3a and C4a among the study groups.

CONCLUSIONS

Unexplained fetal death is associated with evidence of complement activation.

Cytotoxic cells directed against placental cells detected in human habitual abortions by an in vitro terminal labeling assay

PROBLEM

From the clinical point of view, it has been proposed that an immunological imbalance between the mother and the fetus might exist in one of the mechanisms for human habitual abortion. However, in the definition of habitual abortion, we have no distinct immunological criteria for this clinical entity at the moment.

METHOD

We employed aborted placental cells as the target cells in an in vitro terminal labeling (IVTL) assay system, in which the cytotoxic activity of maternal peripheral blood mononuclear cells (PBMCs) against the placental cell was examined.

RESULTS

Our results showed that the cytotoxic activity of maternal PBMCs was significantly higher in the habitual aborters (the mean target cell destruction: %TCD = 34.9%, N = 14) than that in the women with a normal first trimester (the mean % TCD = 8.9%, N = 14, P < .01). The results from the IVTL assay did not correlate with other assays using paternal lymphocytes as the target cells. The surface marker analyses revealed that CD16+ cells, CD14+ cells, and CD5+ cells were involved in the cytotoxic response against the placental cells in various degrees among the cases.

CONCLUSIONS

The above evidence suggests that a variety of cytotoxic cells participate in the phenomenon of human habitual abortion.

Antibodies to trophoblast in normal pregnant and secondary aborting women

Enzyme-linked immunosorbent assays (ELISA) with the use of chorionic villous plasma membranes prepared from first trimester and term placentae were employed to detect antibodies to trophoblast in normal primigravid women. Normal pregnant women were found to produce IgG antibodies to trophoblast. These antibodies could be eluted from first trimester placentae. This antibody response was observed in the first trimester and gradually decreased as pregnancy progressed. IgM antibody responses were observed only in the third trimester. Antibodies in some primigravid women and secondary recurrent aborters showed allotypic reactivity with individual trophoblast membranes. This finding was confirmed by immunoblotting experiments in which antibodies from some normal pregnant women were shown to recognize the same trophoblast antigens as those recognized by antibodies from secondary recurrent spontaneous aborters.

An ELISA for anti-thymocyte autoantibody in NZB mice. Comparison with the cytotoxic assay

An ELISA is described for the detection of anti-thymocyte antibody found in the serum of NZB autoimmune mice. This assay avoids the disadvantages of previous assay techniques which require carefully selected complement sources and/or radioactive labelled target cells. Thymocytes from a variety of strains of mice can be used as the antigenic targets, and the plates stored as long as 1 month. The lymphoma line EL4 can be used as the antigenic target in this assay, but it not as sensitive to most serum pools as thymocytes.

Maternal alloantibody responses during early pregnancy detected by a cellular enzyme-linked immunospecific assay

Using a cellular enzyme-linked immunospecific assay (CELISA), we have examined sera from nulliparous women and women in the first trimester of a first or subsequent pregnancy for the presence of antibodies directed to surface determinants on peripheral blood lymphocytes from unrelated donors. Maternal antibody activity was found in sera from 1/13 nulliparae, 19/37 primigravidae, and 8/12 multigravidae. Cytotoxic antibody activity was present in 3/12 multigravidae but in no other group. Absorption with packed, pooled platelets did not remove the antibody activity from three of the primigravid sera; unabsorbed sera, however, bound equally well to T and B lymphocytes. These data suggest that the antibody detected by CELISA is not directed to any of the classical HLA antigen series (-A, -B, -C, or -DR) but may be directed to the HLA linked non-class I HT antigen system.

The separation of the antibody and antigen components of immune complexes formed during normal human pregnancy

By the manipulation of the fractionation conditions of a series of protein A affinity columns it was possible to separate the antibody and antigen components of the immune complexes formed in the serum during normal human pregnancy. The two components were partially characterised in order to determine that they were separate biological entities. The IgG immunoglobulin component possessed antibody activity directed against the outer layer of the placenta, the syncytiotrophoblast. The antigen component was of apparent syncytiotrophoblast origin and consisted of a protein with two major polypeptides of Mr 63 kDaltons and 31 kDaltons.

Hypothesis: blocking factors and human pregnancy: an alternative explanation for the success of lymphocyte transfusion therapy in abortion-prone women

Recent evidence has demonstrated that the non-specific immunosuppression that develops during pregnancy is not due to the de novo synthesis of immunosuppressive factors (ISF) but is a result of the activation of pre-existing molecules. The latter are present in normal sera as inactive complexes with a molecule which is specifically depleted following conception resulting in the activation of the ISF; the depleted molecule is termed the pregnancy-depleted immunoregulatory factor (pdIRF). Since non-specific ISF are detected in various diseases, it is proposed that the ISF-pdIRF relationship observed in pregnancy is a universal phenomenon and that the depletion of pdIRF and the activation of ISF can be achieved by a variety of stimuli. It is further proposed that abortion-prone women fail to adequately deplete their pdIRF levels so that the ISF are not activated. However, owing to the universal nature of the system, lymphocyte transfusion therapy supplies the necessary stimuli so that the pdIRF levels are depleted and the ISF activated which ensures a successful outcome to the pregnancy.

Identification of selectively solubilised syncytiotrophoblast plasma membrane proteins as potential antigenic targets during normal human pregnancy

Syncytiotrophoblast plasma membranes prepared from term placentae were selectively solubilised in non-ionic detergents. The solubilised proteins and the insoluble residue were tested in an ELISA assay for their ability to function as antigenic targets for anti-trophoblast antibodies present in normal first trimester pregnancy sera. The soluble proteins were fractionated by gel filtration and four major antigen forms were identified. The antigens were reactive with affinity purified anti-trophoblast antibody isolated from maternal sera and hence were termed maternally-recognised trophoblast antigens (MRTA); these were designated MRTA-I (Mr = 400,000 D), MRTA-II (Mr = 142,000), MRTA-III (Mr = 50,000) and MRTA-IV (Mr = 13,000). The relationship between MRTA-I, II, III and IV and antigens identified in maternal sera in the form of immune complexes is discussed.

Anti-trophoblast antibody responses during normal human pregnancy

During the course of a normal uncomplicated human pregnancy the mother generates an antibody response directed against determinants present on the plasma membrane of the outer fetal layer of the term placenta, the syncytiotrophoblast. The response, measured by an ELISA that utilises syncytiotrophoblast plasma membrane as the antigenic target, is predominantly IgG in nature, but with a minor contribution from IgM molecules. Maximum responses were observed during the first trimester and the levels gradually declined as the pregnancy progressed. On a population basis, this antibody response profile was mainly restricted to first and second pregnancies, although anti-trophoblast antibody responses could be detected in multiparous women but with a greatly reduced incidence compared with primipara. Mechanisms to account for these observations are discussed. Throughout, the anti-trophoblast antibody levels detected in pregnancy sera were compared with the background levels which were observed in sera obtained from males and nulliparous non-pregnant females.