Decreased opsonic activity for Staphylococcus aureus in neonatal and late gestation maternal sera

Heat-killed Staphylococcus aureus (ATCC strain 25923) and Escherichia coli K-12 were used as target microorganisms for opsonization by serum from neonates or mothers at various stages of pregnancy or postpartum. The level of opsonic activity was evaluated by titrating serum for the ability to coat bacteria for recognition by normal human PMN. Recognition of the organisms was quantitated by measuring the PMN superoxide anion generation response to opsonized organisms. Studies show that opsonic activity for S. aureus was markedly decreased in serum from mothers in their second and third trimesters of pregnancy, in cord blood, and from infants at two weeks of age. Decreased activity was not observed in maternal serum in the first trimester of pregnancy and returned to control levels shortly after delivery. No significant difference was seen in opsonic activity for E. coli in these sera. The reduction in serum opsonic activity in mothers during the second and third trimesters of pregnancy suggests a systemic suppression of maternal immune responses during fetal development. This decreased activity is also observed in neonatal sera.