Inhibition of human monocyte Fc receptor and HLA-DR antigen expression by pregnancy alpha-2 glycoprotein.

The precise factor responsible for the altered immunological reactivity and diminished inflammatory responses occurring in pregnancy is unknown. Recent observations implicate the pregnancy-associated alpha-2 glycoprotein (PAG). Using rosette assays for the detection of cells carrying Fc receptors and for the demonstration of surface HLA-DR antigens, we demonstrate that a serum protein fraction rich in PAG inhibits these surface markers on human monocytes. Brief exposure of normal peripheral blood monocytes to physiological concentrations of PAG led to almost total loss of Fc receptor expression, an effect that was concentration-related. Using a monoclonal anti-HLA-DR antibody linked to red cells, physiological concentrations of PAG also significantly inhibited the detection of HLA-DR on monocytes. As controls, we used a protein fraction isolated in identical fashion from male sera devoid of PAG, and an alpha-2 macroglobulin isolate. These findings suggest that PAG, by masking monocyte surface markers, could be responsible for the suppression of some cell functions during pregnancy.