How does the immune system impact brain development? The exciting and somewhat unexpected relationship between the immune system and the brain has become one of the most fascinating topics in neuroscience. Even though the immune system was initially implicated in resolving viral and bacterial threats, it is now becoming more evident that it also plays a role in processes in the brain, both under healthy and pathological conditions. This novel role of the immune system in brain health has been implicated in various psychopathologies where neurodevelopment, stress and mood are central. In particular, its role in healthy brain development is becoming more evident, and understanding neuroimmune communication is becoming crucial in treating neurodevelopmental and mood disorders in later life. In the brain, glia function as part of the innate immune system and are programmed to respond to pathogens and physical injury. They also play an important role in neuronal development and pruning. These cells communicate with and respond to chemical signals, such as cytokines and chemokines, which can then initiate or downregulate inflammatory responses. Finally, the trillions of microbes residing in the gut can also stimulate cytokine and chemokine responses in the periphery and play an important role in both immunity and brain development.
A growing body of preclinical and clinical evidence supports a relationship between the complexity and diversity of the microorganisms that inhabit our gut (human gastrointestinal microbiome) and health status. These microbes can influence centrally regulated emotional behaviour through mechanisms including microbially derived bioactive molecules, mucosal immune and enteroendocrine cell activation, as well as vagal nerve stimulation. Changes to the microbial environment, as a consequence of illness, stress or injury can lead to a broad spectrum of local physiological and behavioural effects including a decrease in gut barrier integrity, altered gut motility, inflammatory mediator release, as well as nociceptive and distension receptor sensitization. Impacts at a central level include alterations in the hypothalamic-pituitary-adrenal axis, neuroinflammatory events and concomitant changes to neurotransmitter systems. Thus, both central and peripheral pathways associated with pain manifestation and perception are altered as a consequence of the microbiome-gut-brain axis imbalance. The dogmatic approach of antibiotic treatment in the latter century, for the treatment of many diseases and conditions, has undergone a radical change. We are 90% microbe, and pragmatism suggests that we manipulate this ecosystem for the treatment of various ailments, stress dysfunction and affective disorders, including the alleviation of visceral pain.