Abstract
A successful pregnancy depends on not only the tolerance of the fetal immune system by the mother but also resistance against the threat of hazardous microorganisms. Infection with pathogenic microorganisms during pregnancy may lead to premature delivery, miscarriage, growth restriction, neonatal morbidity, and other adverse outcomes. Moreover, the host also has an intact immune system to avoid these adverse outcomes. It is important to note the presence of normal bacteria in the maternal reproductive tract and the principal role of the maternal-placental-fetal interaction in antimicrobial immunity. Previous studies mainly focused on maternal infection during pregnancy. However, this review summarizes the new views on the study of the maternal microbiome and expounds the innate immune defense mechanism of the maternal vagina and decidua as well as how cytotrophoblasts and syncytiotrophoblasts recognize and kill bacteria in the placenta. Fetal immune systems, thought to be weak, also exhibit an immune defense function that is indispensable for maintaining the safety of the fetus. The skin, lungs, and intestines of the fetus during pregnancy constitute the main immune barriers. These findings will provide a new understanding of the effects of normal microbial flora and how the host resists harmful microbes during pregnancy. We believe that it may also contribute to the reference on the clinical prevention and treatment of gestational infection to avoid adverse pregnancy outcomes.
Highlights
In recent years, research on the relationship between intestinal flora and diseases has become an interesting field, mainly focusing on the relationships between intestinal flora and liver disease, tumor, encephalopathy, autoimmune diseases, reproductive health and so on [1,2,3], and studies in this field imply that transplanting gut bacteria could be a new way to prevent and treat some diseases [4]
Nasioudis et al evaluated relative abundance of bacteria in the vaginal microbiome in firsttrimester pregnant women, and the results showed that L. crispatus was the numerically most abundant bacterium in 76.4% of women with a first conception, 50.0% with only a prior spontaneous or scheduled abortion, and 22.2% with a prior birth; L. iners was the most abundant bacterium in 3.8% of women with a first conception as compared to 19.2 and 20.8% in those with a prior abortion or birth, respectively; Gardnerella as the most abundant bacterial genus increased from 3.8% in women with a first conception to 15.4 and 14.3% in those with a prior abortion or birth, respectively [24]
The most abundant cells in the decidua, the natural killer (NK) cells, mainly produce interferon (IFN)γ to participate in the following immune response mechanisms: [1] macrophages and neutrophil cells were stimulated by IFNγ to induce the production of antimicrobial peptides; [2] IFN-γ can increase the secretion of IL-1 from T cells to play a role in anti-inflammation; and [3] IFN-γ can stimulate B lymphocytes to differentiate into plasma cells and secrete antibodies [58]
Summary
Research on the relationship between intestinal flora and diseases has become an interesting field, mainly focusing on the relationships between intestinal flora and liver disease, tumor, encephalopathy, autoimmune diseases, reproductive health and so on [1,2,3], and studies in this field imply that transplanting gut bacteria could be a new way to prevent and treat some diseases [4]. Few studies have been conducted on normal microbiome, and the protective mechanism of the maternal, placental, and fetal immune systems against microbial infection during pregnancy is unclear.
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