Detection of the immunoregulator p43-placental isoferritin in the human embryo and fetus.

Human placental isoferritin (PLF) is known to exert an immunosuppressive activity in vitro and is involved in the down-regulation of the maternal immune system during pregnancy. We have investigated the presence of PLF in the human embryo and early fetus and its secretion into amniotic fluid (AF) and fetal blood. Immunohistochemistry was performed on 25 normal embryos and fetuses, at 7-22 weeks gestation, using the CM-H9 monoclonal antibody (mAb), generated specifically against the human p43-PLF protein. The amount of p43 was measured in AF of 81 fetuses at 11-22 weeks and in the blood of 19 fetuses at 15-22 weeks by means of an enzyme-linked immunosorbent assay with the same mAb. Positive p43-PLF immunostaining was found from 7 weeks gestation in proximal tubules of the primitive nephron and macrophages of the liver sinusoids, blood vessels and mesenchymal tissue. In the AF samples, p43-PLF was first detected at week 15 gestation and thereafter steadily increased with advancing gestation whereas in fetal blood, p43-PLF was below or just above the lower limit of the assay. The gap between the first appearance of 43-PLF in embryonic tissue and its secretion into the amniotic fluid is probably linked to maturation of the renal function. The detection of the p43-PLF immunomodulator protein in macrophages at a very early stage of embryonic development and its very low concentration in fetal blood suggests that its immunoregulatory role is limited to the feto-maternal interface.