Are pregnancy-associated serum proteins responsible for the inhibition of lymphocyte transformation by pregnancy serum?

Certain pregnancy-associated serum proteins, namely pregnancy-associated alpha 2-glycoprotein, pregnancy-specific beta 1-glycoprotein, alpha-foetoprotein, human placental lactogen and human chroionic gonadotrophin, have been proposed as immunosuppressive factors. A pregnancy serum was constructed from a number of such sera to produce high serum levels of these compounds. Each of the proteins was then removed sequentially from the serum by affinity chromatography and the remaining materials examined for inhibitory activity on lymphocyte transformation. Only the removal of pregnancy-associated alpha 2-glycoprotein decreased suppression by the serum. However, a large proportion of the serum inhibitory activity could not be accounted for, indicating the presence of other suppressor factors.