Stress and the Gut-Brain Axis: Implications for Cancer, Inflammation and Sepsis.

BACKGROUND: The gut-brain axis has been discussed, directly or indirectly, for centuries, with the ideas of the gut affecting anything from moods to overall physiology being discussed across the centuries. With a recent explosion in research that looks to the microbiota as a mechanistic link between the gut and the brain, one sees that the gut-brain axis has various means of communication, such as through the vagus nerve and the enteric nervous system and can use the metabolites in the gut to communicate to the brain. METHODS: The purpose of this review is to view the gut-brain axis through the lens of stress and how stress, from the prenatal period all the way through adulthood can impact the physiology of a human being. Studies have shown multiple mechanisms of measurable change with disruption in the microbiota that lead to behavioral changes. There are also effects of gut inflammation on the brain and the corresponding systemic response observed. CONCLUSION: The overall literature is encouraging that the more understanding of the gut-brain axis, the greater ability to wield that understanding for therapeutic benefits.

RNASeq analysis reveals upregulation of complement C3 in the offspring gut following prenatal stress in mice.

Dysregulated activation of inflammatory signaling by the immature neonatal immune system could lead to the development of many pediatric diseases including necrotizing enterocolitis (NEC). While the mechanism(s) of pathogenesis is unknown, NEC is believed to have multifactorial causes. Microbial dysbiosis and intestinal immaturity have been implicated as potential triggers for this disease. We hypothesized that psychological stress during pregnancy negatively impacts the development of intestinal tissues in offspring and contributes to development of NEC. Consistent with this hypothesis, we previously observed shorter villi and a decrease in total surface area in the small intestine of pups derived from mice that were chronically stressed during gestation. In this study, we performed RNASeq analysis to determine the gene expression changes in the offspring gut following prenatal stress in pregnant mice and identified several differentially expressed genes (DEGs) and biological pathways. Notably, C3 was upregulated in the small intestine and contributed to a higher tissue injury score in a mesenteric ischemia model compared to unstressed controls. We discuss the potential implications of these stress-induced genes expression changes and their contribution to development of intestinal inflammation.

Prenatal stress increases IgA coating of offspring microbiota and exacerbates necrotizing enterocolitis-like injury in a sex-dependent manner.

Necrotizing enterocolitis (NEC) is an intestinal inflammatory disease with high morbidity and mortality that affects almost exclusively premature infants. Breast milk feeding is known to substantially lower NEC incidence, and specific components of breast milk, such as immunoglobulin (Ig) A, have been identified as mediating this protective effect. On the other hand, accumulating evidence suggests dysbiosis of the neonatal intestinal microbiome contributes to NEC pathogenesis. In mice, neonates can inherit a dysbiotic microbiome from dams that experience stress during pregnancy. Here we show that while prenatal stress lowers fecal IgA levels in pregnant mice, it does not result in lower levels of IgA in the breast milk. Nevertheless, coating of female, but not male, offspring microbiota by IgA is increased by prenatal stress. Accordingly, prenatal stress was found to alter the bacterial community composition in female neonates but not male neonates. Furthermore, female, but not male, offspring of prenatally stressed mothers exhibited more severe colonic tissue damage in a NEC-like injury model compared to offspring with non-stressed mothers. Our results point to prenatal stress as a possible novel risk factor for NEC and potentially reveal new avenues in NEC prevention and therapy.

Depletion of dietary aryl hydrocarbon receptor ligands alters microbiota composition and function.

The intestinal microbiota is critical for maintaining homeostasis. Dysbiosis, an imbalance in the microbial community, contributes to the susceptibility of several diseases. Many factors are known to influence gut microbial composition, including diet. We have previously shown that fecal immunoglobulin (Ig) A levels are decreased in mice fed a diet free of aryl hydrocarbon receptor (AhR) ligands. Here, we hypothesize this IgA decrease is secondary to diet-induced dysbiosis. We assigned mice to a conventional diet, an AhR ligand-free diet, or an AhR ligand-free diet supplemented with the dietary AhR ligand indole-3-carbinol (I3C). We observed a global alteration of fecal microbiota upon dietary AhR ligand deprivation. Compared to mice on the conventional diet, family Erysipelotrichaceae was enriched in the feces of mice on the AhR ligand-free diet but returned to normal levels upon dietary supplementation with I3C. Faecalibaculum rodentium, an Erysipelotrichaceae species, depleted its growth media of AhR ligands. Cultured fecal bacteria from mice on the AhR ligand-free diet, but not the other two diets, were able to alter IgA levels in vitro, as was F. rodentium alone. Our data point to the critical role of AhR dietary ligands in shaping the composition and proper functioning of gut microbiota.

Skin-resident memory CD4+ T cells enhance protection against Leishmania major infection.

Leishmaniasis causes a significant disease burden worldwide. Although Leishmania-infected patients become refractory to reinfection after disease resolution, effective immune protection has not yet been achieved by human vaccines. Although circulating Leishmania-specific T cells are known to play a critical role in immunity, the role of memory T cells present in peripheral tissues has not been explored. Here, we identify a population of skin-resident Leishmania-specific memory CD4+ T cells. These cells produce IFN-γ and remain resident in the skin when transplanted by skin graft onto naive mice. They function to recruit circulating T cells to the skin in a CXCR3-dependent manner, resulting in better control of the parasites. Our findings are the first to demonstrate that CD4+ TRM cells form in response to a parasitic infection, and indicate that optimal protective immunity to Leishmania, and thus the success of a vaccine, may depend on generating both circulating and skin-resident memory T cells.

Environmental-mediated intestinal homeostasis in neonatal mice.

BACKGROUND: Immunoglobulin A (IgA) plays a key role in coating luminal antigens and preventing translocation of harmful bacteria. The aryl hydrocarbon receptor (AhR) is a basic helix-loop-helix transcription factor that when stimulated activates factors important for barrier function and intestinal homeostasis. We hypothesize that AhR signaling is critical for establishment of intestinal homeostasis in neonates. MATERIAL AND METHODS: Mice: C57BL/6 (B6) AhR+/+ wild type (WT), B6.AhR-/- Aryl-hydrocarbon receptor knockout (KO), and B6.AhR+/+ raised on an AhR ligand-free diet (AhR LF). Enzyme-linked immunosorbent assay was used to measure fecal and serum IgA levels. Bacterial translocation was measured by culturing the mesenteric lymph nodes. RESULTS: Two week old KO mice had significantly less fecal IgA compared with WT (and AhR LF, P value = 0.0393. The amount of IgA from the gastric contents of 2-wk-old mice was not significantly different. At age 8 wk, AhR LF mice had significantly less fecal IgA than WT and KO P value = 0.0077. At 2 wk, KO mice had significantly higher levels of bacterial translocation and at 8 wk AhR LF had significantly higher levels of bacterial translocation compared with WT. CONCLUSIONS: In neonatal mice, the lack of AhR signaling is associated with loss of intestinal homeostasis, evidenced by decreased levels of IgA and increased bacterial translocation. In adult mice, exogenous AhR ligand and not receptor signaling is necessary for maintenance of intestinal integrity.

Development of CD27+ marginal zone B cells requires GALT.

In species other than mouse, little is known about the origin and development of marginal zone (MZ) B cells. Using cross-reactive antibodies, we identified and characterized splenic MZ B cells in rabbits as CD27(+) CD23(-). In rabbits in which organized gut-associated lymphoid tissue (GALT) was surgically removed at birth, we found only CD23(+) follicular (FO) B cells and almost no CD27(+) MZ B cells in the spleen, indicating that GALT is required for the development of splenic MZ B cells. These findings lead us to suggest that commensal microbiota contribute to the development of MZ B cells.

Somatically diversified and proliferating transitional B cells: implications for peripheral B cell homeostasis.

The peripheral B cell compartment in mice and humans is maintained by continuous production of transitional B cells in the bone marrow. In other species, however, including rabbits, B lymphopoiesis in the bone marrow abates early in life, and it is unclear how the peripheral B cell compartment is maintained. We identified transitional B cells in rabbits and classified them into T1 (CD24(high)CD21(low)) and T2 (CD24(high)CD21(+)) B cell subsets. By neutralizing B cell-activating factor in vivo, we found an arrest in peripheral B cell development at the T1 B cell stage. Surprisingly, T1 B cells were present in GALT, blood, and spleen of adult rabbits, long after B lymphopoiesis was arrested. T1 B cells were distinct from their counterparts in other species because they are proliferating and the Ig genes are somatically diversified. We designate these newly described cells as T1d B cells and propose a model in which they develop in GALT, self renew, continuously differentiate into mature B cells, and thereby maintain peripheral B cell homeostasis in adults in the absence of B lymphopoiesis.

Requirement for BAFF and APRIL during B cell development in GALT.

The effects of B cell-activating factor belonging to the TNF family (BAFF) on B cell maturation and survival in the mouse are relatively well understood. In contrast, little is known about the role of BAFF in B cell development in other mammals, such as rabbits, that use GALT to develop and maintain the B cell compartment. We examined the expression and requirement of BAFF and a proliferation-inducing ligand (APRIL) during peripheral B cell development in young rabbits. By neutralizing BAFF and APRIL in neonates with a soluble decoy receptor, transmembrane activator calcium modulator and cyclophilin ligand interactor-Fc, we found a marked reduction in the number of peripheral B cells, but found no change in the bone marrow (BM) compartment. In the appendix, the size and number of proliferating B cell follicles were greatly reduced, demonstrating that although BAFF/APRIL is dispensable for B cell development in BM, it is required for B cell development in GALT. We found that all rabbit B cells expressed BAFF receptor 3, but did not bind rBAFF, suggesting that the BAFF-binding receptors (BBRs) are bound by endogenous soluble BAFF. Further, we found that B cells themselves express BAFF, suggesting that the soluble BAFF bound to BBRs may be endogenously produced and stimulate B cells in an autocrine fashion. Additionally, we propose that this chronic occupancy of BBRs on B cells may provide a tonic and/or survival signal for the maintenance of peripheral B cells in adults after B lymphopoiesis is arrested in BM.