Secretory component in human amniotic fluid and gestational tissues:: a potential endogenous phospholipase A2 inhibitor

OBJECTIVES: Prostaglandins (PGs) are essential mediators of labor during human pregnancy. Phospholipase A(2) (PLA(2)) provides the essential substrate for PG synthesis through the liberation of arachidonic acid from membrane phospholipid stores. Nonlaboring amniotic fluid (NL-AF) contains secretory component (SC)-like protein(s) that suppress in vitro PLA(2) activity. This study characterizes the biologic activity, identity, and tissue distribution of these protein(s) in NL-AF and gestational tissues. METHODS: Third-trimester NL-AF was collected by amniocentesis, fractionated by ammonium sulfate precipitation, and submitted to an in vitro PLA(2) assay. Identity of the PLA(2) inhibitor in NL-AF was confirmed by Western blot and antibody neutralization studies. Secretory component-immunoreactive proteins were purified by immunoaffinity chromatography and visualized by sodium dodecyl sulfate-gel electrophoresis. Tissue distribution of SC in gestational tissues was determined by immunohistochemistry. RESULTS: The 100% pellet and supernatant fractions of NL-AF suppressed PLA(2) activity, and this activity was neutralized by a polyclonal antibody to SC. Western blot studies revealed an SC-reactive protein in the 70-80-kD range in the 100% pellet fraction of NL-AF. Two SC-reactive proteins were detected in the 60-80-kD range in the eluate from the SC immunoaffinity column, along with minor proteins of 30 and greater than 100 kD. Immunohistochemical studies revealed SC in placental trophoblast, amniotic membranes, and decidual epithelium. CONCLUSIONS: These results demonstrate that proteins homologous to SC are present in human gestational tissues and possess anti-PLA(2) activity. These proteins may contribute to the maintenance of pregnancy by suppressing local PG production. (J Soc Gynecol Investig 1999;6:311-7) Copyright (C) 1999 by the Society for Gynecologic Investigation.