Gut Microbiota as a Mediator of Host Neuro-Immune Interactions: Implications in Neuroinflammatory Disorders

The dynamic population of microbes that reside in the gastrointestinal tract plays a pivotal role in orchestrating several aspects of host physiology and health, including but not limited to nutrient extraction and metabolism, as well as the regulation of intestinal epithelial barrier integrity. Gut microbes interact with the host in a bi-directional manner as the microbiota can support the development and education of the innate and adaptive immune systems, thereby conferring protection against pathogens and harmful stimuli while training the host to maintain a homeostatic tolerance towards commensal symbiotics. Recent advances in the field have highlighted the importance of the host-microbiota relationship in neurodevelopment and behaviour, with relevant implications for the onset and progression of brain disorders of inflammatory origin. Microbial modulation of brain function is achieved throughout complex neuro-immune-endocrine pathways of the microbiome-gut-brain axis. Changes in the composition of the gut microbiota or perturbation in microbial-derived metabolites and neuroactive compounds are sensed by the afferent branches of the sympathetic and vagal innervation and transmitted to the central nervous system, which in turn produces behavioural responses. Here, we focus on how the crosstalk between the gut microbiota and the immune system modulates the development and function of the peripheral and central nervous systems. Specific attention is afforded to the involvement of host-microbe neuroimmune interactions in the pathogenesis of neuro-psychiatric and neuroinflammatory disorders such as autism spectrum disorders, anxiety, and depression, as well as Parkinson's and Alzheimer's diseases.