EXPRESSION OF EOSINOPHIL-GRANULE MAJOR BASIC-PROTEIN MESSENGER-RIBONUCLEIC-ACID IN PLACENTAL-X CELLS

BACKGROUND: The human eosinophil-granule major basic protein (MBP) is a 13.8-kilodalton cationic polypeptide constituting the core of the eo sinophil granule. MBP is cytotoxic to parasites and numerous mammalian cells and is a potent secretagogue for platelets, basophils, mast cel ls, and neutrophils. Concentrations of a molecule immunochemically sim ilar to eosinophil granule MBP are present in maternal plasma, and MBP has been localized by immunofluorescence to placental X cells. EXPERI MENTAL DESIGN: To determine whether X cells produce MBP, the expressio n of MBP messenger RNA (mRNA) was investigated in placentas by Norther n blot analyses and by in situ hybridization with S-35-labeled RNA pro bes. RESULTS: Northern blot analyses of RNA from placental septa and v illi showed the existence of a 1.0-kb RNA band that hybridized with th e MBP anti-sense probe; no MBP mRNA was detected in whole blood of nor mal or pregnant women or in cord blood. Analyses of placentas by in si tu hybridization showed MBP mRNA in X cells of placental septa and anc horing villi, but not in other cellular elements such as syncytiotroph oblasts, cytotrophoblasts, villous stromal cells, and fetal endothelia l cells. RNase pretreatment abolished X-cell hybridization signals; tr eatment of sections with an excess of nonradiolabeled anti-sense RNA a lso blocked binding of the S-35-labeled antisense RNA probe. Additiona l evidence supporting the production of MBP by X cells was obtained us ing a combination of in situ hybridization and immunofluorescence, whi ch showed colocalization of MBP and its mRNA. CONCLUSIONS: The presenc e of MBP mRNA and MBP protein in placental X cells indicates that X ce lls synthesize this biologically active molecule.