Light and electron microscope immunocytochemical studies of the distribution of pregnancy associated glycoproteins (PAGs) throughout pregnancy in the cow: possible functional implications

Pregnancy associated glycoproteins (PAGs) comprise a large group of placental antigens of the aspartic proteinase family. Phylogenetic analysis indicated that the PAGs form two distinct groups, one of ancient origin and one produced by a more recent series of gene duplications. This paper summarises the molecular biological and biochemical studies which have been used to purify and raise antibodies against specific PAGs and groups of related PAGs and their use in light and electron microscope immunocytochemistry to demonstrate that the ancient PAG group has a similar distribution at the placental fetomaternal interface (microvillar junction, MVJ) in cows and pigs. This localization suggests either a possible role in binding the surfaces together and/or in establishment of an immunological barrier. The more recently evolved PAG group, absent in the pig, exhibited no significant localization to the MVJ but was restricted to the trophoblast binucleate cell (BNC) granules in the cow. Furthermore, these PAGs bind to newly formed maternal uterine connective tissue to which they are delivered by BNC migration and fusion with uterine epithelial cells. At this location in the developing maternal villi of the placentomes, they are ideally positioned to manipulate the maternal immune system to facilitate a successful pregnancy.