Modification of protective mechanisms against Listeria monocytogenes during pregnancy.

Protective mechanisms against Listeria monocytogenes were examined at different stages of pregnancy in C3H/He mice. Bacterial growth at an early phase of infection was depressed from day 10 of gestation to day 5 postpartum, whereas bacterial growth at a late phase of infection was enhanced on day 10 of gestation. Spleen cells from nonimmunized mice on day 10 of gestation suppressed the expression of acquired resistance to infection in immunized mice. We conclude that the augmented resistance during pregnancy at an early phase of primary infection is due to the activation of nonimmune macrophages, which compensates for the depressed cell-mediated immunity at a late phase of infection that may be due to the function of nonspecific suppressor cells detectable in the spleen of the pregnant mice.