Characterization of the Interactions Between the Gut Microbiome and the Immune System in Mouse Models of Early Life Microbiome Assembly and Multiple Sclerosis
Creators: Archer, Dale
Publisher: name: University of Alberta Library
Year: 2024

<p>The gut microbiome interacts with its host in a symbiotic relationship and makes critical contributions to the development and function of the central nervous and immune systems. It has therefore been hypothesized that microbiome alterations from industrialization and the loss of certain microbes, including Limosilactobacillus and Helicobacter species, have contributed to the increase in non-communicable chronic diseases (NCDs) in Western societies. Diet and lifestyle factors in industrialized societies have altered host-microbiome interactions in mothers during pregnancy and in infants during early-life microbiome assembly with long-lasting consequences for the offspring. Host-microbiome disruption during the perinatal period may contribute to alterations to host development and an increased risk of developing NCDs. However, while the maternal and neonatal gut microbiomes have important roles in host development, the relative importance of microbial signals in the prenatal and postnatal periods is unclear. Additionally, it is unknown whether VANISH species contribute to host development and if they can influence the outcomes of chronic autoimmune diseases, such as multiple sclerosis (MS). The overall objectives of this research were to assess the importance of the timing of microbial signals in early life for the development of the host and to determine whether and how VANISH species modulate host development and autoimmunity. These objectives were addressed through three aims.</p>

<p>We first aimed to determine the importance of the timing of gut microbiome colonization in host development and the ability of Limosilactobacillus reuteri to restore host development when gut microbiome colonization is delayed. To address this, we used germ-free mice to control the timing of gut microbiome colonization in early life and characterized immune cell populations at adulthood. Compared to conventional mice, germ-free mice conventionalized at birth (EC mice) showed few differences in immune cell populations at adulthood, explaining only 2.36% of the</p>

<p>variation in immune phenotypes. By contrast, delaying conventionalization to 28 days of age (DC mice) affected several splenic immune cell populations at adulthood, including regulatory T cells (Tregs) and dendritic cells, explaining 29.01% of the variation in immune phenotypes. Treatment of DC mice with L. reuteri immediately after birth restored splenic dendritic cells and Tregs to levels observed in EC mice, with strain-specific effects on the proportions of splenic CD4+ T cells, CD8+ T cells, and mononuclear phagocytes.</p>

<p>Our second aim was to determine the mechanisms by which L. reuteri modulates disease pathology in the relapsing-remitting experimental autoimmune encephalomyelitis (RR-EAE) mouse model of MS and the role of aryl hydrocarbon receptor (AhR) activation in the effects of this bacterium. Treatment of RR-EAE mice with L. reuteri R2lc, which potently activates the AhR through the pks gene cluster, resulted in severe pathology with 70% mortality. In contrast, treatment with an isogenic R2lcΔpks mutant and a pks-negative strain (PB-W1) showed reduced pathology and 14% and 0% mortality, respectively. Differences in pathology occurred in parallel with a pks-dependent downregulation of the AhR pathway, reduced gene expression of occludin, and elevated IFN 𝛾 in the forebrain, and reduced proportions and counts of Tregs and Th17 cells in secondary lymphoid organs.</p>

<p>Our third aim was to evaluate whether Helicobacter macacae and Helicobacter pylori could beneficially modulate disease pathology in the RR-EAE model and the mechanisms underlying their effects. H. pylori had detrimental effects in RR-EAE pathology through an expansion of activated microglia/macrophages in the hindbrain, and an increase in the proportions and counts of CD4+ and CD8+ T effector/effector memory cells and Th1 cells in the spleen. Conversely, treatment with H. macacae resulted in mild disease, with no disease in males, and promoted an expansion of Tregs, Th2 cells, and plasma cells in secondary lymphoid organs.</p>

<p>Our findings indicate that the use of microbial products in the suckling period, more so than during pregnancy, will have the greatest impact on immune development and could be used to reduce the risk of developing immune-mediated diseases. In the RR-EAE model L. reuteri PB-W1 and H. macacae showed protective effects. However, the strain- and species-specific effects of L. reuteri and Helicobacter species urge caution when developing VANISH species as therapeutics for NCDs that are more common in Western societies. Preclinical models of disease are therefore essential to determine the safety and efficacy of VANISH species before their application to humans.</p>