Immunohistochemical localization of murine alpha 1-pregnancy-associated protein (alpha 1-PAP) in pregnant mice: relationship between serum alpha 1-PAP levels and incidence of positive cells.

Using an electroimmunoassay, the murine pregnancy-associated protein alpha 1-PAP was detected in the sera of virgin MF1 but not C57BL/10 female mice. During pregnancy, alpha 1-PAP levels rose in both strains, although concentrations were higher in the latter and in both fell towards term. Using the unlabelled peroxidase anti-peroxidase (PAP) staining procedure, alpha 1-PAP was detected within mononuclear leukocytes, the majority resembling macrophages, in the small and large intestinal mucosae, Peyer's patches and hepatocytes of virgin MF1 but not C57BL/10 females. During pregnancy, alpha 1-PAP positive cells were observed in each of these sites and in the decidua and placenta of both strains. Quantitative studies revealed that the incidence of alpha 1-PAP positive cells in the gut and associated lymphoid tissues reflected the circulating levels of the protein. In the placenta, the frequency and intensity of alpha 1-PAP positive staining was also reduced towards parturition. In contrast, hepatocyte staining remained constant throughout gestation in both strains. Our observations suggest that there may be at least two types of alpha 1-PAP synthesis operative and that circulating levels of the protein in female mice are influenced by strain, pregnancy and stage of gestation. These findings are discussed in relation to the cell types involved, their contribution to serum levels and the possible role of alpha 1-PAP in fetal allograft survival.