We have recently reported that human pregnancy is characterized by a 10- to 20-fold elevation of eosinophil major basic protein (MBP) immunoreactivity in maternal blood. Here we show, by immunofluorescence, that placental tissue specifically binds antibody to MBP in and around the placental X cells and placental-site giant cells and, using thin plastic sections, that placenta has no infiltrating eosinophils. The X cells line the inner aspects of placental septal cysts, and the cyst fluid, obtained by aspiration, contains immunoreactive MBP at concentrations of 100 micrograms/ml, a sixfold greater concentration than the highest levels measured in maternal blood. The soluble MBP immunoreactivities in placental homogenates and in maternal serum chromatograph identically on Sephadex G-50, and both these gestational MBP molecules migrate as though substantially larger than the MBP found in serum from patients with hypereosinophilic syndrome or purified from the eosinophil granule. Our inability to demonstrate eosinophils in maternal blood or placental tissue, coupled with the large quantities of immunoreactive MBP highly localized in placental cysts and the chromatographic behavior of this molecule, suggest that the MBP detected in human gestation is produced by placenta.

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