Objectives: The interleukin-1 receptor antagonist is a newly discovered cytokine that blocks the biologic effects of interleukin-1 in vitro and in vivo. This cytokine is a physiologic component of amniotic fluid and is considered to be of critical importance in the homeostasis of the cytokine network. This study was undertaken to systematically examine the bioavailability of interleukin-1 receptor antagonist in the maternal, fetal, and amniotic fluid compartments during term and preterm parturition in women with and without microbial invasion of the amniotic cavity.
Study design: The patient population consisted of (1) pregnant women in the midtrimester (n = 42), (2) patients who underwent cordocentesis for diagnostic purposes (n = 39), (3) patients with preterm labor (n = 126), (4) women with term gestation (n = 102), and (5) healthy nonpregnant women (n = 8). Amniotic fluid was cultured for aerobic and anaerobic bacteria, as well as Mycoplasma sp. Interleukin-1 receptor antagonist concentrations were determined by enzyme-linked immunoassay in maternal and fetal plasma, amniotic fluid, and neonatal urine. Microbial invasion of the amniotic cavity was defined as the presence of a positive amniotic fluid culture for microorganisms.
Results: (1) Interleukin-1 receptor antagonist was normally present in fetal plasma samples obtained by cordocentesis, and its concentration increased with advancing gestational age (n = 39; r = 0.61, p < 0.001). (2) Patients at term not in labor had higher amniotic fluid interleukin-1 receptor antagonist concentrations than patients in the midtrimester (median 40.1 ng/ml, range 5.7 to 213.1 vs median 16.2 ng/ml, range 3.2 to 62.2, respectively, p < 0.001). (3) Amniotic fluid and cord plasma interleukin-1 receptor antagonist concentrations were significantly higher in patients with preterm labor and microbial invasion of the amniotic cavity than in those without microbial invasion of the amniotic cavity (amniotic fluid: median 219.9 ng/ml, range 35.4 to 504 vs median 80.6 ng/ml, range 24.3 to 399, respectively, p < 0.001; umbilical cord plasma: median 4.8 ng/ml, range 0.3 to 167.0 vs median 1.0 ng/ml, range 0 to 276.0, respectively, p < 0.05). In contrast, these differences were not found in patients with term labor either with or without microbial invasion of the amniotic cavity. (4) In both term and preterm patients the amniotic fluid and neonatal urine concentrations of interleukin-1 receptor antagonist were significantly higher in female fetuses than in male fetuses (amniotic fluid, preterm: median 191.9 ng/ml, range 51.6 to 504.0 vs median 61.1 ng/ml, range 11.5 to 284.9, respectively, p < 0.001; amniotic fluid, term: median 58.7 ng/ml, range 25.5 to 264.0 vs median 33.9 ng/ml, range 3.4 to 132.4, respectively, p < 0.001; neonatal urine: median 317 ng/ml, range 59.0 to 440.8 vs median 12.2 ng/ml, range 2.5 to 61.6, respectively, p < 0.005).
Conclusions: (1) Interleukin-1 receptor antagonist is physiologically present in the fetal, maternal, and amniotic fluid compartments; (2) microbial invasion of the amniotic cavity in the preterm gestation is associated with a significant increase in the concentrations of this cytokine in the fetal and amniotic fluid compartments but not in maternal plasma; (3) fetal urine is a source of amniotic fluid interleukin-1 receptor antagonist; (4) fetal plasma interleukin-1 receptor antagonist concentrations increase with gestational age; (5) there is a significant effect of fetal gender in amniotic fluid and neonatal urine concentrations of interleukin-1 receptor antagonist.