<i>Bifidobacterium animalis</i> subsp. <i>Lactis</i> BL‐99 Improves Maternal and Fetal Immune Responses and Pregnancy Outcomes in Pregnant Antibiotics‐Treated Mice

The maternal gut microbiome is involved in adapting immune responses to the presence of the semiallogeneic foetus. We have previously shown that antibiotics-induced gut dysbiosis, alterations in the maternal immune response and decreased foetal and placental weight. Here, we tested whether Bifidobacterium animalis subsp. lactis BL-99 (BL-99) could improve antibiotics-induced gut dysbiosis, maternal immune responses and foetal and placental weight. To do so, pregnant mice received antibiotics in their drinking water (day 9-16) and BL-99 via oral gavage (day 9-18). After sacrifice (day 18) immune responses were measured using flowcytometry. BL-99 increased placental weight in antibiotics-treated pregnant mice. BL-99 did not significantly change the maternal microbiome, but improved maternal immune responses by decreasing splenic Th1 cells and Treg cells, and increasing FoxP3/RoRγT double-positive cells in the Peyer's patches to levels observed in control pregnant mice. BL-99 also improved monocyte subsets and activation status. Additionally, BL-99 changed foetal monocyte subsets and activational status and increased foetal splenic Th cells. We thus showed that the effect of antibiotics treatment on immune cells and placental weight was mitigated by supplementation of BL-99. We suggest that pregnancy complications associated with a disturbed microbiome and immune responses, such as preeclampsia or obese pregnancies, could benefit from BL-99 supplementation. This should be tested in future studies.