Characterization of secretory component in amniotic fluid. Identification of new forms of secretory IgA

Amniotic fluid (AmF) contains low levels of IgA of fetal origin. Western blot analysis revealed that the major forms of IgA in human AmF contain secretory component (SC), but ranged in size between 170 and 200 kDa, unlike the 380-kDa size typical of previously described secretory (s)IgA. Preliminary characterization of these novel forms of sIgA suggests they may arise by reduction of selected disulfide bonds, rather than proteolytic cleavage, of 380-kDa sIgA. This study also shows that AmF contains SC in its free form. Free SC measured by ELISA in 30 AmF samples increased with gestational age of the fetus from 26 to 40 wk postconception. Late in gestation, the concentrations of free SC levels reached those of other external secretions. Both the fetal urogenital system and the amniotic membrane appear to contribute to both the free SC and sIgA in AmF. This report presents the initial description of a new form of sIgA and provides evidence that the AmF may be an early expression of the mucosal immune system in the developing fetus.