Preeclampsia has been suggested to be an immunologic disorder.1 The link between the immune system and preeclampsia is thought to be a maternal immune intolerance to fetoplacental antigen.2 In the normal pregnancy, despite the presence of maternal T lymphocytes specific for paternal histocompatibility antigen, the semi-allogenic fetus is protected from attack by its maternal immune system.3 It has recently been shown that the maternal immune system acquires a transient state of tolerance specific to paternal antigens by clonal deletion of activated maternal T lymphocytes, possibly through the Fas ligand present in the trophoblast.4,5
Fas, also called APO-1 or CD95, is a cell surface receptor that can induce apoptotic cell death in sensitive cells. The triggering of Fas can be achieved by monoclonal antibodies or upon binding by its natural ligand of Fas.6 Fas and Fas ligand-mediated apoptosis is involved in several regulatory functions within the immune system. An altered expression of trophoblastic Fas and Fas ligand in preeclamptic pregnancy was recently reported.7
Soluble Fas, an alternatively spliced product of the Fas gene, protects cells from apoptosis by antagonizing the binding between the membrane form of the Fas and the Fas ligand.8 Elevated serum-soluble Fas levels are found in patients with autoimmune disease,9 malignancy,10 hepatitis,11 congestive heart failure,12 and infection.13 However, soluble Fas level in preeclampsia has not been reported previously according to a MEDLINE search between 1966 and 2000 using the terms “soluble,” “Fas,” and “preeclampsia.” We sought to evaluate whether the concentrations of serum Fas was altered in preeclampsia.
Materials and Methods
This investigation was carried out at Yale-New Haven Hospital (New Haven, CT) with the approval of the institution's Human Investigational Committee. Preeclampsia was defined as proteinuric hypertension (blood pressure recorded on at least two occasions of at least 140/90 mmHg separated by at least 6 hours, and proteinuria [at least 2+ in a catheterized specimen, or at least 300 mg in a 24-hour urine sample]) after the 20th week of gestation. Severe preeclampsia was defined as blood pressure at least 160 mmHg systolic or 110 mmHg diastolic, proteinuria more than 5 g in 24 hours, oliguria (500 mL or less in 24 hours), pulmonary edema, eclampsia, or HELLP syndrome (presence of hemolysis, elevated liver enzymes [aspartate amnio-transferase greater than 70 IU/L], and low platelet count [less than 100,000 mm3]).
According to our preliminary observations (Am J Obstet Gynecol 1999;180:S44, Abstract #117), the difference of means of serum Fas between preeclamptic and normotensive women and the standard deviation within groups were 2.16 and 3.11 U/mL, respectively. We estimated that 34 subjects were needed in each group to detect this difference in soluble Fas levels with a power of 0.8 (β = 0.2) and α = 0.05 between women with and without preeclampsia. Thus, 34 pregnant women with singleton pregnancies complicated by preeclampsia and 34 normotensive pregnant women were studied. Subjects were matched as much as possible (not a one-to-one match) for gestational age, maternal age, parity, and race between the two groups.
After the subjects gave oral consent, we obtained blood samples from each woman. The samples were centrifuged at 2000g for 15 minutes at 4C. Serum was collected, aliquoted, and stored at −80C until the assay was performed. Serum soluble Fas level was measured by a two site enzyme-linked immunoassay (Oncogene Research Products, Cambridge, MA). The lowest limit detection (sensitivity) in this assay was 0.05 units/mL. The interassay and intra-assay coefficients of variation were less than 10%. Samples were not subjected to freeze-thaw cycles before analysis.
The two-tailed Student t test, χ2 test, Pearson correlation coefficients, and analysis of variance with post hoc test of Student–Newman Keuls test were used for statistical analyses. Data are expressed as mean ± standard error. P < .05 was considered statistically significant.
There were no significant differences in maternal age, parity, race, and gestational age because they were matched on these characteristics as possible before the soluble Fas assay between the two groups. However, pregnant women with preeclampsia had significantly higher serum soluble Fas levels than normotensive controls (10.59 ± 0.68 versus 5.65 ± 0.35 U/mL, P < .001). When preeclamptic women were further segregated into HELLP syndrome and non-HELLP preeclamptic pregnancy, serum soluble Fas levels in both groups were still significantly higher than that in nor-motensive controls (10.84 ± 0.93 versus 10.23 ± 1.02 versus 5.65 ± 0.35 U/mL, P < .001). However, there was no significant difference of serum soluble Fas between HELLP and non-HELLP preeclamptic women (10.84 ± 0.93 versus 10.23 ± 1.02 U/mL, P > .05).
There were no significant correlations between serum Fas level and maternal age, gestational age, parity, race, blood pressure, hematocrit, platelet count, proteinuria, serum uric acid, or serum creatinine levels.
The mechanism of immunologic tolerance between the semi-allogenic fetus and maternal immune system remains largely unknown. The outer layer of trophoblast, the syncytiotrophoblast, is directly bathed in maternal blood in the uterine sinusoid, and is entirely devoid of both class I and class II antigens of the major histocompatibility complex. Thus, the lack of any recognizable foreign antigenic signals offers a plausible explanation as to how the syncytiotrophoblast can be protected from attack by the hostile immune cells in the maternal blood.
Recently, it has been suggested that trophoblasts can block the response of activated maternal T lymphocytes or natural killer cells through the expression of Fas ligand on its surface.5 Allaire et al7 recently reported that trophoblastic FasL ligand expression was significantly lower and Fas expression significantly greater in patients with preeclampsia than those without preeclampsia. As maternal immune cells bear the Fas receptor, soluble Fas could protect them from apoptosis by Fas ligand expressed on trophoblasts. In addition to the altered expression of Fas and Fas ligand in trophoblasts, we speculate that preeclampsia might be a disorder of maternal immune intolerance partly associated with elevated circulating levels of soluble Fas that protect maternal immune cells from apoptosis and consequently lead to an increased apoptosis in trophoblasts. However, the limitation of our study is that the measurement of soluble Fas level in our subjects was somewhat late in the disease course. A further study for the measurement of soluble Fas level in the early stage of preeclampsia is warranted.
The source for the elevation of serum levels of soluble Fas in patients with preeclampsia remains to be determined. We speculate that abnormal secretion of soluble Fas by activated trophoblast, maternal immune cells, or abnormal shedding from injured vascular endothelium could contribute to the elevation of soluble Fas in preeclamptic pregnancy.
As almost all of the preeclamptic women in this study were categorized as severely preeclamptic, we could not correlate the serum levels of soluble Fas with the severity of this disease. HELLP syndrome is currently classified as severe preeclampsia, and serum levels of soluble Fas have been reported to be elevated in patients with hepatitis,11 we further analyzed the serum levels of soluble Fas in patients with HELLP syndrome. Abnormal secretion from activated maternal immune cells, hepatocytes, or shedding from vascular endothelium might consequently contribute to marked elevation of serum-soluble Fas observed in patients with HELLP syndrome and non-HELLP preeclamptic pregnancy.
1. Need JA. Immunological phenomena in preeclamptic toxemia. Clin Obstet Gynecol 1979;6:443–60.
2. Dekker GA, Robillard PY, Hulsey TC. Immune maladaptation in the etiology of preeclampsia: A review of corroborative epidemiologic studies. Obstet Gynecol Surv 1998;53:377–82.
3. Saji F, Matsuaki N, Azuma C, Koyama M, Ohashi K, Tanizawa O. The fetus as an allograft: Immunobiologic role of human trophoblasts for fetal survival. Am J Obstet Gynecol 1992;167:251–6.
4. Mor G, Gutierrez LS, Mariel E, Kahyaoglu F, Arici A. Fas-Fas ligand system-induced apoptosis in human placenta and gestational trophoblastic disease. Am J Reprod Immunol 1998;40:89–94.
5. Runic R, Lockwood CJ, Yuehong MA, Dipasquale B, Guller S. Expression of Fas ligand by human cytotrophoblasts: Implications in placentation and fetal survival. J Clin Endocrinol Metab 1996; 81:3119–22.
6. Alderson MR, Tough TW, Davis-Smith T, Braddy S, Falk B, Schooley KA, et al. Fas ligand mediates activation-induced death in human T lymphocytes. J Exp Med 1995;181:71–7.
7. Allaire AD, Ballenger KA, Wells SR, McMahon MJ, Lessey BA. Placental apoptosis in preeclampsia. Obstet Gynecol 2000;96:271–6.
8. Cheng J, Zhou T, Liu C, Shaprio JP, Brauer MJ, Kiefer MC, et al. Protection from Fas-mediated apoptosis by a soluble form of the Fas molecule. Science 1994;263:1759–62.
9. Knipping E, Krammer PH, Onel KB, Lehman TJ, Mysler E, Elkon KB. Soluble Fas/APO1/CD95 in systemic lupus erythematosus and juvenile rheumatoid arthritis. Arthritis Rheum 1995;38:1735–7.
10. Midis GP, Shen Y, Owen-Schaub LB. Elevated soluble Fas (sFas) levels in nonhematopoietic human malignancy. Cancer Res 1996; 56:3870–4.
11. Iio S, Hayashi N, Mita E, Ueda K, Mochizuki K, Hiramatsu N, et al. Serum levels of soluble Fas antigens in chronic hepatitis C patients. J Hepatol 1998;29:517–23.
12. Okuyama M, Yamaguchi S, Nozaki N, Yamaoka M, Shirakabe M, Tomoike H. Serum levels of soluble form of Fas molecule in patients with congestive heart failure. Am J Cardiol 1997;79:1698–701.
13. Kamihira S, Yamada Y, Hiragata Y, Yamaguchi T, Izumikawa K, Matsuo Y, et al. Serum levels of soluble Fas/APO-1 receptor in human retroviral infection and associated diseases. Intern Med 1997;36:166–70.