Immunosuppressive activity of rhesus monkey pregnancy serum

Normal pregnancy serum from the rhesus monkey was found to have immunosuppressive activity. Using two-way stimulation, the mixed lymphocyte response was suppressed as much as 80%. Control serum from nonpregnant females was not suppressive. The inhibiting factor was found to have the following characteristics: (1) it was nonspecific in activity; (2) it inhibited the mixed lymphocyte response 20% at in vitro concentrations of 1%; (3) it was heat stable (56 degrees C for 30 minutes) and nondialyzable; (4) it was present in both the IgM- and IgG-containing fractions of pregnancy serum; (5) it was detected in postpartum and second and third trimester serum; and (6) it was at low levels or absent from the serum of two pregnancies which terminated in unexplained stillbirths.