The Blood-Brain Barrier in Neuroimmune Interactions and Pathological Processes

The blood-brain barrier (BBB) is a kind of filter, highly selective in relation to various types of substances. The BBB supports the immune status of the brain and is an important regulator of neuroimmune interactions. Some of the molecular and cellular features of the BBB, as well as the five main pathways of neuroimmune communication mediated by the BBB, are analyzed in this article. The functions of the BBB in neuroimmune interactions in various diseases are discussed: multiple sclerosis and Alzheimer's and Parkinson's diseases. The latest data on BBB dysfunction in COVID-19 coronavirus infection caused by the SARS-CoV-2 virus are considered.

Gateway reflexes, neuronal circuits that regulate the gateways for autoreactive T cells in organs that have blood barriers

Gateway reflexes are neural circuits that maintain homeostasis of the immune system. They form gateways for autoreactive T cells to infiltrate the central nervous system in a noradrenaline-dependent manner despite the blood-brain barrier. This mechanism is critical not only for maintaining organ homeostasis but also for inflammatory disease development. Gateway reflexes can be regulated by environmental or artificial stimuli including electrical stimulation, suggesting that the infiltration of immune cells can be controlled by bioelectronic medicine. In this review, we describe the discovery of gateway reflexes and their future directions with special focus on bioelectronic medicine.

IL-6 in inflammation, autoimmunity and cancer

IL-6 is involved both in immune responses and in inflammation, hematopoiesis, bone metabolism and embryonic development. IL-6 plays roles in chronic inflammation (closely related to chronic inflammatory diseases, autoimmune diseases and cancer) and even in the cytokine storm of corona virus disease 2019 (COVID-19). Acute inflammation during the immune response and wound healing is a well-controlled response, whereas chronic inflammation and the cytokine storm are uncontrolled inflammatory responses. Non-immune and immune cells, cytokines such as IL-1β, IL-6 and tumor necrosis factor alpha (TNFα) and transcription factors nuclear factor-kappa B (NF-κB) and signal transducer and activator of transcription 3 (STAT3) play central roles in inflammation. Synergistic interactions between NF-κB and STAT3 induce the hyper-activation of NF-κB followed by the production of various inflammatory cytokines. Because IL-6 is an NF-κB target, simultaneous activation of NF-κB and STAT3 in non-immune cells triggers a positive feedback loop of NF-κB activation by the IL-6-STAT3 axis. This positive feedback loop is called the IL-6 amplifier (IL-6 Amp) and is a key player in the local initiation model, which states that local initiators, such as senescence, obesity, stressors, infection, injury and smoking, trigger diseases by promoting interactions between non-immune cells and immune cells. This model counters dogma that holds that autoimmunity and oncogenesis are triggered by the breakdown of tissue-specific immune tolerance and oncogenic mutations, respectively. The IL-6 Amp is activated by a variety of local initiators, demonstrating that the IL-6-STAT3 axis is a critical target for treating diseases.

Neural stimulations regulate the infiltration of immune cells into the CNS

The systemic regulation of immune reactions by the nervous system is well studied and depends on the release of hormones. Some regional regulations of immune reactions, on the other hand, depend on specific neural pathways. Better understanding of these regulations will expand therapeutic applications for neuroimmune and organ-to-organ functional interactions. Here, we discuss one regional neuroimmune interaction, the gateway reflex, which converts specific neural inputs into local inflammatory outputs in the CNS. Neurotransmitters released by the inputs stimulate specific blood vessels to express chemokines, which serve as a gateway for immune cells to extravasate into the target organ such as the brain or spinal cord. Several types of gateway reflexes have been reported, and each controls distinct CNS blood vessels to form gateways that elicit local inflammation, particularly in the presence of autoreactive immune cells. For example, neural stimulation by gravity creates the initial entry point to the CNS by CNS-reactive pathogenic CD4+ T cells at the dorsal vessels of fifth lumbar spinal cord, while pain opens the gateway at the ventral side of blood vessels in the spinal cord. In addition, it was recently found that local inflammation by the gateway reflex in the brain triggers the activation of otherwise resting neural circuits to dysregulate organ functions in the periphery including the upper gastrointestinal tract and heart. Therefore, the gateway reflex represents a novel bidirectional neuroimmune interaction that regulates organ functions and could be a promising target for bioelectric medicine.

Neural activity regulates autoimmune diseases through the gateway reflex

The brain, spinal cord and retina are protected from blood-borne compounds by the blood-brain barrier (BBB), blood-spinal cord barrier (BSCB) and blood-retina barrier (BRB) respectively, which create a physical interface that tightly controls molecular and cellular transport. The mechanical and functional integrity of these unique structures between blood vessels and nervous tissues is critical for maintaining organ homeostasis. To preserve the stability of these barriers, interplay between constituent barrier cells, such as vascular endothelial cells, pericytes, glial cells and neurons, is required. When any of these cells are defective, the barrier can fail, allowing blood-borne compounds to encroach neural tissues and cause neuropathologies. Autoimmune diseases of the central nervous system (CNS) and retina are characterized by barrier disruption and the infiltration of activated immune cells. Here we review our recent findings on the role of neural activity in the regulation of these barriers at the vascular endothelial cell level in the promotion of or protection against the development of autoimmune diseases. We suggest nervous system reflexes, which we named gateway reflexes, are fundamentally involved in these diseases. Although their reflex arcs are not completely understood, we identified the activation of specific sensory neurons or receptor cells to which barrier endothelial cells respond as effectors that regulate gateways for immune cells to enter the nervous tissue. We explain this novel mechanism and describe its role in neuroinflammatory conditions, including models of multiple sclerosis and posterior autoimmune uveitis.

Connection between Systemic Inflammation and Neuroinflammation Underlies Neuroprotective Mechanism of Several Phytochemicals in Neurodegenerative Diseases

Oxidative damage, mitochondrial dysfunction, and neuroinflammation are strongly implicated in the pathogenesis of neurodegenerative diseases including Alzheimer's disease (AD) and Parkinson's disease (PD), and a substantial portion of elderly population at risk of these diseases requires nutritional intervention to benefit health due to lack of clinically relevant drugs. To this end, anti-inflammatory mechanisms of several phytochemicals such as curcumin, resveratrol, propolis, polyunsaturated fatty acids (PUFAs), and ginsenosides have been extensively studied. However, correlation of the phytochemicals with neuroinflammation or brain nutrition is not fully considered, especially in their therapeutic mechanism for neuronal damage or dysfunction. In this article, we review the advance in antioxidative and anti-inflammatory effects of phytochemicals and discuss the potential communication with brain microenvironment by improved gastrointestinal function, enhanced systemic immunity, and neuroprotective outcomes. These data show that phytochemicals may modulate and suppress neuroinflammation of the brain by several approaches: (1) reducing systemic inflammation and infiltration via the blood-brain barrier (BBB), (2) direct permeation into the brain parenchyma leading to neuroprotection, (3) enhancing integrity of disrupted BBB, and (4) vagal reflex-mediated nutrition and protection by gastrointestinal function signaling to the brain. Therefore, many phytochemicals have multiple potential neuroprotective approaches contributing to therapeutic benefit for pathogenesis of neurodegenerative diseases, and development of strategies for preventing these diseases represents a considerable public health concern and socioeconomic burden.