Does the Microbiome Affect Immune Responses During Pregnancy?

Based on: Changes in intestinal gene expression and microbiota composition during late pregnancy are mouse strain dependent

Throughout pregnancy, women undergo numerous hormonal changes to support fetal development. The mother must adapt to tolerate the presence and growth of the fetus as the paternal components of the embryo are "foreign" to the mother. Typically, the immune system functions to eliminate or destroy foreign entities.  It has been shown that pregnancy-induced hormonal changes initiate immunological modifications to allow for fetal tolerance. There is a maternal increase in regulatory T cells (Tregs) to avoid the fetal destruction [1]. A Treg is a type of white blood cell, a T cell, that regulates other immune cells and suppresses immune responses to foreign entities.

Source: http://www.jimmunol.org/content/192/11/4949

Gray and gold circle represents the maternal (individual A) embryonic material, and the red circle represents the paternal (individual B) embryonic material. 

Studies have shown that the microbiome is linked to immunological modifications. The microbiome is composed of a diverse range of bacteria, viruses, and other microbes that reside in the gut. It possesses many functions including digestion, vitamin production, and maintaining immunity [2]. Some bacterial species have been shown to promote Treg accumulation. The main investigative question of this paper is: is there is a relationship between the microbiome and immunological modifications at the end of pregnancy in mice?

The authors studied two main concepts: the effect of pregnancy on microbiome composition and the effect of pregnancy of abundance of immune cells in pregnant and non-pregnant C5BL/6 (B6) and BALB/c mice. These mice strains have known differences in microbiome composition and in intestinal immunological responses.

The authors determined the microbiome composition and thus determined the richness (number of unique species) and Shannon diversity (calculation between richness and evenness/distribution). The authors determined that pregnancy decreased microbiome diversity and richness.

A microbiome with low diversity and richness can be associated with obesity, insulin resistance, and high cholesterol, for example [4]. As it is evidently disadvantageous for a host to undergo these changes, it is unknown why this decrease occurs during pregnancy.

Interestingly, the changes in microbiota composition were mainly observed in the BALB/c mouse strain. Pregnant BALB/c mice had a significantly higher relative abundance of Lactobacillus paracasei et rel., Roseburia intestinalis et rel., and Eubacterium hallii et rel. compared to non-pregnant BALB/c mice. These bacteria species are associated with immune tolerance and anti-inflammatory responses- crucial to a successful pregnancy [6, 7]. There was little change in microbiota composition in the B6 mouse strain. This may be a due to non-pregnant B6 females' higher abundance of these bacterial species than the non-pregnant BALB/c females. Thus, pregnancy affected intestinal microbiota composition in a strain-dependent manner.

To determine if pregnancy influenced intestinal immunological pathways, the authors performed a DNA microarray on colonic tissues to determine the genes that were expressed at different levels between non-pregnant and pregnant mice. The authors determined pregnancy significantly affected the expression of genes involved with immunological pathways. However, the effect was more prominent in the B6 mouse strain.

Next, to determine if there was a relationship between the microbiome bacterial species and the  differentially expressed intestinal immunological genes, the authors combined the microbiota and colonic tissue gene expression data from non-pregnant and pregnant BALB/c mice. They only used this strain because the B6 mouse strain demonstrated no significant change in microbiome composition during pregnancy. They determined there to be 6 main clusters of maternal colonic genes that correlated either negatively or positively with 3 main clusters of bacteria species.

Knowing there are immunological genes that are differentially expressed during pregnancy, the authors next looked at the abundance of certain immune cell subsets in two locations: the mediastinal lymph node, a specific type of lymph node, and the spleen. They used the MLN as they are the lymph node associated with the intestines and used the spleen as a reference point for overall immunity.  They focused on T cell subsets because many of the implicated genes have been shown to be involved with T cells. They looked at percentages of CD4+ T cells, CD8+ T cells, and Tregs. CD4+ T cells are "helper" T cells that help other white blood cells perform their immunological functions through the secretion of small toxic particles called cytokines [8]. CD8+ T cells are "cytotoxic" T cells that destroy infected cells [8]. Tregs mediate the CD4+ and CD8+ T cell response.

The authors found that in both the MLN and the spleen there was an increase in CD4+ T cell abundance and no change in CD8+ T cell abundance in both strains. However, there was a strain-dependent effect of Treg abundance. In the MLN, there was an increase in Treg abundance in both strains. In the spleen, there was only an increase in Treg abundance in the pregnant B6 mice, not the pregnant BALB/c mice. 

                                                    MLN                                               Spleen

This is an interesting finding in that there was little change in microbiota composition during pregnancy in the B6 mouse strain. This indicates that the spleen Treg induction is caused by factors outside of microbiome composition.

These findings are some of the first describing the microbiome's role in maternal-fetal tolerance. Some future directions of this research may include determining the other factors that influence the increased Treg abundance in pregnant B6 mice. Researchers may also want to look at earlier stages of pregnancy. There may be interesting findings concerning infertility with regard to the microbiome's influence on the beginning of pregnancy. Additionally, research could be directed towards determining what specific bacteria species are involved in the differential gene expression.

[1] Leber, A.; Teles, A.; Zenclussen, A. C. Regulatory T Cells and Their Role in Pregnancy. Am. J. Reprod. Immunol. 2010, 63 (6), 445–459. https://doi.org/10.1111/j.1600-0897.2010.00821.x.
[2] Shreiner, A. Be.; Kao, J. Y.; Young, V. B. The Gut Microbiome in Health and in Disease. Curr Opin Gastroenterol 2015, 31 (1), 69–75. https://doi.org/10.1097/MOG.0000000000000139.
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[5] Elderman, M.; Hugenholtz, F.; Belzer, C.; Boekschoten, M.; de Haan, B.; de Vos, P.; Faas, M. Changes in Intestinal Gene Expression and Microbiota Composition during Late Pregnancy Are Mouse Strain Dependent. Scientific Reports 2018, 8 (1), 10001. https://doi.org/10.1038/s41598-018-28292-2.
[6] Smelt, M. J. et al. L. plantarum, L. salivarius, and L. lactis attenuate Th2 responses and increase Treg frequencies in healthy mice in a strain dependent manner. PLoS One 7, e47244 (2012).
[7] Duncan, S. H., Hold, G. L., Barcenilla, A., Stewart, C. S. & Flint, H. J. Roseburia intestinalis sp. nov., a novel saccharolytic, butyrate-producing bacterium from human faeces. Int. J. Syst. Evol. Microbiol. 52, 1615–1620 (2002).
[8] https://en.wikipedia.org/wiki/T_cell