Pregnant women show significant immune system changes linked to gut microbiome

In a recent study published in Clinical Microbiology, a group of researchers investigated how gut microbiota and metabolite changes relate to immune function during pregnancy by comparing the gut microbiota, fecal and plasma metabolites, and cytokines in pregnant and non-pregnant women.

Study: Multi-omics analysis reveals the associations between altered gut microbiota, metabolites, and cytokines during pregnancy. Image Credit: Ground Picture/Shutterstock.com

Background 

Pregnancy induces significant changes in hormonal levels, body structure, and immune function, essential for fetal development.

Initially, the maternal immune system is pro-inflammatory, becoming anti-inflammatory, then shifting back to pro-inflammatory to initiate labor.

The role of the gut microbiome in immune regulation during pregnancy is increasingly recognized, with studies showing its influence on pregnancy outcomes and conditions like preeclampsia through changes in the microbial composition and interactions with immune cells.

However, the detailed mechanisms remain unclear. Metabolites, rather than direct microbial interactions, predominantly mediate the relationship between the microbiome and the immune system, highlighting areas for further research to understand pregnancy-related immune adjustments and develop new therapeutic strategies.

About the study 

In the study conducted at the First Affiliated Hospital of Jinan University between February 2019 and August 2020, 30 pregnant and 15 non-pregnant women were recruited to explore the interactions between gut microbiota, metabolites, and immune function.

Eligible pregnant participants were between 18 and 34 years old, had naturally conceived singleton pregnancies, and had pre-pregnancy body mass index (BMI) of 18.5 to 21.9 kg/m², excluding those with pregnancy complications or immune disorders.

The control group consisted of healthy females matching the pregnant group in age and BMI, with neither group using probiotics or antibiotics in the six months before the study.

Fecal and blood samples were collected from pregnant women in the late third trimester and non-pregnant women on the 14th day of their menstrual cycle.

Fecal samples were obtained using sterile techniques and stored at -80°C, while blood samples were processed to separate the serum for storage under the same conditions.

The study employed 16S ribosomal ribonucleic acid (rRNA) gene sequencing to analyze the gut microbiota, with deoxyribonucleic acid (DNA) extracted and sequenced to identify microbial species.

A combination of random forest analysis and Weighted Gene Co-expression Network Analysis (WGCNA) was used to differentiate microbial profiles between groups.

Untargeted liquid chromatography-tandem mass spectrometry (LC-MS/MS) metabolomics analyzed the fecal and plasma samples to identify metabolic changes, employing quality control measures to ensure data reliability.

The metabolomic data were processed and analyzed to determine significant differences and map them to biological pathways using the Kyoto Encyclopedia of Genes and Genomes (KEGG) database.

Plasma cytokines were measured using a multiplex bead assay to assess immune function differences between the groups.

Finally, using statistical and visualization tools, multi-omics analysis integrated the data from microbial, metabolomic, and cytokine analyses to explore potential correlations and mediation effects. 

Study results 

The study found no significant differences between average age or pre-pregnancy BMI between the two groups. Immune profiling revealed that pregnant participants had lower levels of pro-inflammatory cytokines and higher levels of certain anti-inflammatory cytokines than the controls, indicating a shift towards immunosuppression during pregnancy.

The study also investigated gut microbiota, finding distinct microbial compositions between pregnant and non-pregnant women.

Pregnant women had a higher diversity of operational taxonomic units (OTUs) and differences in the abundance of specific bacterial phyla.

Using random forest models and WGCNA methods, the researchers identified microbial modules that correlated negatively with pro-inflammatory cytokines, suggesting that certain gut bacteria groups have a consistent impact on reducing inflammation during pregnancy.

Notably, bacteria like Bifidobacterium and Ruminococcus, known for their anti-inflammatory properties, were more abundant in pregnant women and were negatively correlated with pro-inflammatory cytokines.

Metabolomic analysis of fecal and plasma samples using untargeted LC-MS revealed significant metabolic alterations during pregnancy. Differential metabolites, particularly lipids and bile acids, were identified, with many showing downregulation in pregnant women.

These included arachidonic acid and various bile acids, known to be associated with inflammation. The analysis highlighted the significant role of bile acid metabolism during pregnancy.

Correlation studies between metabolites and cytokines indicated that certain metabolites enriched in pregnant women were negatively correlated with pro-inflammatory cytokines, suggesting their involvement in modulating immune responses.

The study further explored the associations between gut microbiota, metabolites, and cytokines, finding that metabolites might mediate the relationship between microbiota and the immune system.

Directional mediation analysis identified specific linkages among microbes, metabolites, and cytokines, suggesting that certain gut microbes could influence cytokine levels by modulating metabolite concentrations.

For example, Ruminococcus callidus and other bacteria could decrease pro-inflammatory cytokines by affecting specific metabolites like deoxycholic acid and arachidonic acid levels.