The role of the microbiota in acute stress-induced myeloid immune cell trafficking

Authors: VAN DE WOUW, MARCEL - University College Cork; Lyte, Joshua - Josh; BOEHME, MARCUS - University College Cork; SICHETTI, MARZIA - University College Cork; MOLONEY, GERARD - University College Cork; GOODSON, MICHAEL - University College Cork; KELLEY-LOUGHNANE, NANCY - University College Cork; CLARKE, GERARD - University College Cork; DINAN, TIMOTHY - University College Cork; CRYAN, JOHN - University College Cork

Submitted to: Brain Behavior and Immunity
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/2/2019
Publication Date: 2/14/2020
Citation: Van De Wouw, M., Lyte, J.M., Boehme, M., Sichetti, M., Moloney, G., Goodson, M.S., Kelley-Loughnane, N., Clarke, G., Dinan, T.G., Cryan, J.F. 2020. The role of the microbiota in acute stress-induced myeloid immune cell trafficking. Brain Behavior and Immunity. 84:209-217. https://doi.org/10.1016/j.bbi.2019.12.003.
DOI: https://doi.org/10.1016/j.bbi.2019.12.003

Interpretive Summary: Short amounts of stress (e.g. acute stress) can cause changes in how different types of cells of the body's immune system move around the body, and this can have long-term impact on health. During times of stress, immune cells can move into sites that typically encounter foreign organisms, such as bacteria. Moreover, the gut microbiota, which is the population of bacteria and other microorganisms that reside in the gut, is recognized to influence a person's or animal's immune system. Therefore, we sought to understand how the gut microbiota may influence how stress affects the body's immune cell movement. A greater understanding of this scientific question will help inform future strategies that aim to use the gut microbiota to improve human or animal health. In this study, we hypothesized that the gut microbiota could influence how the immune cells of the body move around the body following an acute stress. To accomplish our study we utilized animals that are called germ-free and colonized germ-free; germ-free means raised in a sterile environment and without any microbiota; colonized germ-free means germ-free animals that were brought into a non-sterile environment and allowed to gain what may be considered a normal microbiota. We found that the movement of immune cells in germ-free animals following acute stress was, in several ways, different compared to normal and colonized germ-free animals. This accomplishment helps clarify the role of the gut microbiota in affecting the immune system following stress.

Technical Abstract: There has been a growing recognition of the involvement of the gastrointestinal microbiota in the development of stress-related disorders. Acute stress and activation of the innate immune system are crucial for this process, which has both been demonstrated to be affected by the microbiota. Nonetheless, much is still unclear about the role of the gastrointestinal microbiota in acute stress-induced immune activation. In this study, we investigated whether the microbiota influences acute stress-induced changes in innate immunity using mice devoid of any microbiota (i.e. germ-free, GF), and colonized GF mice (CGF). In tandem, we explored novel kinetics of stress-induced immune cell mobilization in the blood, the spleen and mesenteric lymph nodes (MLNs). Mice were either sacrificed prior to stress or underwent restraint stress, were allowed to recover and were then sacrificed at various time points (i.e. 0, 45- and 240-minutes post-stress). Plasma adrenalinadrenaline and noradrenalinadrenaline levels were analysed using ELISAs and immune cell levels were quantified using flow cytometry. GF mice had increased levels of adrenalinadrenaline and noradrenalinadrenaline, of which adrenalinadrenaline was alleviated in CGF mice. In tandem, GF mice had decreased granulocytes, LY6Chi and LY6Cmid, CCR2+ monocytes, but not LY6C-, CX3CR1+ monocytes, which were normalized in CGF mice. Acute stress decreased blood LY6Chi and LY6Cmid, CCR2+ monocytes while increasing granulocyte levels in all groups 45 minutes post-stress. However, only GF mice showed stress-induced changes in LY6Chi monocytes and granulocytes 240 minutes post-stress, indicating impairments in the recovery of acute stress-induced innate immune activation. LY6C-, CX3CR1+ monocytes remained unaffected by stress, indicating that acute stress impacts systemic innate immunity in a cell-type-specific manner. Overall, these data show that the microbiota influences the priming and recovery of the innate immune system to an acute stressor and may inform future microbiota-targeted therapeutics aimed at modulating stress-induced immune activation in stress-related disorders.