Inhibition of lymphocyte transformation by pregnancy-associated plasma protein-A (PAPP-A)

A third trimester pregnancy plasma pool was divided into 2 aliquots, one of them was depleted of pregnancy-associated plasma protein-A (PAPP-A) by absorption onto a monospecific antiserum to PAPP-A. These plasmas as well as pure PAPP-A (isolated from pregnancy plasma) were tested for their ability to inhibit mitogen induced lymphocyte transformation. Pregnancy plasma containing PAPP-A exerted a significantly higher inhibition of phytohemagglutinin (PHA) induced lymphoblastogenesis than did pregnancy plasma devoid of PAPP-A. Pure PAPP-A added to normal human AB serum was also inhibitory. Since PAPP-A (pregnancy plasma with PAPP-A) is inhibitory even if it is preincubated in absence of PHA (and this mitogen added after washing the cells) one can conclude that the inhibitory effect of PAPP-A is specific and is not due to the binding of PHA to PAPP-A (glycoprotein). The fact that PAPP-A is active on prestimulated cells in absence of PHA seems to indicate that PAPP-A inhibits lymphoblastogenesis by acting on the secretion products of lymphocytes.