HLA-G antigen and parturition: maternal serum, fetal serum and amniotic fluid levels during pregnancy.

Objective: To determine whether soluble HLA-G1 (sHLA-G1) concentrations in maternal serum and in amniotic fluid are lower at term than in the second trimester.

Methods: In this prospective study amniotic fluid and maternal serum samples were aspirated from 21 pregnant women during genetic amniocentesis at 16-20 weeks' gestation, and from 19 women undergoing a cesarean section at term. In the latter group arterial umbilical cord blood was aspirated as well. sHLA-G1 levels were determined using ELISA assay. This assay included the anti-HLA-G monoclonal antibodies 87G and 16G1, both as capture antibodies and horseradish-peroxidase-labeled rabbit anti-human beta(2)-microglobulin antibodies, as the detection antibody. The relative concentrations of sHLA-G1 were measured from the absorbancy of the blue product at 650 nm. Student's t test was used for statistical analysis.

Results: sHLA-G1 levels in amniotic fluid were significantly lower at term than in the second trimester (0.160 +/- 0.05 vs. 0.272 +/- 0.150 OD units; p < 0.05). Levels of sHLA-G1 in maternal serum declined toward term, but the difference from the second trimester was not statistically significant (0.266 +/- 0.157 vs. 0.205 +/- 0.120 OD units; p = 0.193). There was a strong correlation of sHLA-G1 concentrations between cord serum and maternal serum (R(2) = 0.79; p < 0.001), but not between cord serum and amniotic fluid (R(2) = 0.00004) or amniotic fluid and maternal serum (R(2) = 0.02).

Conclusions: sHLA-G1 antigen expression is higher in amniotic fluid than in maternal-fetal compartments and significantly decreases toward term. We speculate that the declining amniotic fluid sHLA-G1 levels may stimulate a maternal immunological response against the fetus and contribute to the initiation of parturition.