In 2017, we witnessed further rapid growth in the vast interdisciplinary area of neuroendocrine immunology. It became a tradition to reflect on the new significant developments and conceptual trends in the field that were presented at BrainImmune. We will retrace some of our commentaries, revisiting the studies on BrainImmune we believe are illustrative, stimulating and of general interest. Below is a highly condensed outline of the most interesting research and emerging concepts as covered by BrainImmune.
NEW EVOLVING CONCEPTS
Brain ‘Superautoantigens’ and Autoimmunity
According to Serge Nataf only a limited range of the human ‘self’ antigen library is actually targeted by pathological autoimmunity. Thus, in the context of autoimmunity, “not all ‘self’ antigens are equal with regard to immunogenicity and what could be called ‘superautoantigens’ are more prone to be targeted under pathological conditions”.
Nataf proposes a theoretical model where in humans “a majority of autoantigens that are targeted in common non-CNS autoimmune diseases are indeed brain autoantigens belonging to the synaptic or myelin compartments”.
As per the ‘brain superautoantigens’ theory – the major autoantigens targeted in non-CNS autoimmune disorders belong to the synaptic compartment. One such example is GAD65 (also known as GAD2, glutamate decarboxylase, an enzyme of the GABAergic system) – the main autoantigen in diabetes type 1.
Interestingly, this model also proposes that mechanisms allowing a proper exposure of brain superautoantigens to the immune system have exerted a major leverage evolutionary force. Importantly, the model also admits that the immune and neural repertoires are mutually nourishing throughout the whole life of an individual. This may also suggests that a “pathophysiological re-assessment of conditions usually considered as purely immunological (autoimmune disorders) or purely neurological (autism, schizophrenia, neurodegenerative diseases)” should be considered.
Pro-Inflammatory and Highly Pathogenic Th17 Cells Are Glucocorticoid Resistant
The paradigm of T helper (Th)1 and Th2 cells regulating cellular and humoral immunity dominated until 2000-2005. Shortly after that, however, with the discovery of IL-23 and the third T-cell subset, known as Th17 cells, they received the major attention, primarily because they appear to be the principal players in several autoimmune and inflammatory disorders.
Yet, more recent research indicates that not all Th17 cells are inflammatory.
Elisabetta Profumo outlined the emerging concept that Th17 cells are not a homogenous population but rather consist of different subsets of Th17 cells with distinct phenotype and functions. This reflects their pro- or anti-inflammatory activities and glucocorticoid (GC)-sensitivity.
Thus, the PATHOGENIC PRO-INFLAMMATORY Th17 cells express high levels of IL-23 receptor and integrin avβ3 on their surface, produce the pro-inflammatory cytokines granulocyte-macrophage colony-stimulating factor (GM-CSF) and interferon (IFN)-γ and express the Th1 transcription factor T-bet, while the PROTECTIVE Th17 cells produce the anti-inflammatory cytokine IL-10.
Recent evidence also indicates that the high PATHOGENIC Th17 cells are GC-RESISTANT. PRO-INFLAMMATORY Th17 cells, producing both IL-17 and (IFN)-γ, express the molecule multi-drug resistance type 1 (MDR1), also known as P-glycoprotein, and these cells are refractory to GC-mediated T cell suppression. Th17 resistance to GCs is also associated with high levels of the anti-apoptotic factor BCL-2, the transcription factor RORγt, STAT3 and high levels of IL-6.
Importantly, the glucocorticoid-resistant pathogenic Th17 cells appear to be the major players driving chronic inflammatory process. Of note, long-term glucocorticoid treatment may deplete GC-sensitive T cells and promote the expansion of GC resistant subsets, thus sustaining the inflammatory process. Interestingly new research suggests that the GC-resistant Th17 are most likely sensitive to the calcineurin inhibitor cyclosporine A.
The ‘Selfish Brain and Immune Systems’ – Energy Deficiency Link
Rainer Straub argues that the selfishness of the brain and immune systems provoke energy deficiency in aging and chronic inflammation. Thus, several chronic inflammation-related signs and symptoms result from a highly active, energy-consuming immune system. But, the same signs and symptoms also are present in elderly individuals, and in chronic inflammatory diseases in remission, without much inflammation.
As the brain requires similar amounts of energy as the immune system, thus, we should also consider here, the energy expenditure of the selfish and active brain.
The new model, suggested by Rainer Straub, takes into consideration “that energy shortage in these groups of individuals depends on increasing energy expenditure caused by high levels of psychomotor activity (brain and muscles). For example, pain, psychological stress, sleeping alterations and anxiety increase energy expenditure”.
Importantly, in the short term, these changes may represent “highly favorable coping mechanisms used by the body in situations of short-lived energy under-supply”. In the long-term, however, the use of these mechanisms may become maladaptive, and in turn, contribute to unfavorable complications.
Thus, as per the author, in chronic inflammatory diseases, physicians and pharmaceutical companies should take an integrative approach and look for solutions beyond immunosuppression.
β2-Adrenoceptor Agonists – Candidates as Novel Non-Conventional Anti-Parkinson’s Drugs
A study published in Science indicated that the β2-adrenoreceptor (β2AR) is a regulator of the α-synuclein gene (SNCA), and may drive the risk of Parkinson’s disease (PD). In 11 years of follow-up in 4 million Norwegians, Shuchi Mittal et al. found that the β2AR agonist salbutamol, a brain-penetrant asthma medication, was associated with reduced risk of developing PD.
In a recent commentary at BrainImmune Marco Cosentino et al. argued that “the study by Mittal et al. (2017) has the merit to attract novel attention on the possibility to repurpose as antiparkinson drugs the β2-AR agonists, which are well established as bronchodilators in asthma”. Importantly, Cosentino et al. discuss 3 relevant studies where salbutamol improved the therapeutic response to levodopa and the motor performance, and possibly increased muscle mass in PD patients.
The authors of this commentary also address this issue from a different perspective – discussing the role of peripheral adaptive immunity, and the link to neuroinflammation ultimately leading to neurodegeneration.
Thus, T lymphocytes occur in the substantia nigra of parkinsonian brains and T cells from PD patients recognize α-syn peptides. CD4+ T cells may acquire proinflammatory phenotypes, and animal models of PD suggests that Th1 and Th17 cells may be detrimental, whereas Th2 and Treg cells may be protective.
Of note, β2-AR agonists might be beneficial, in this regard, as it is now known that they inhibit Th1 responses but potentiate Th2 cells and cytokines; and perhaps, also enhancing Treg suppressive functions.
Thus, overall, these drugs affect neuroinflammation and neurodegeneration, and favorably affect Th1, Th2 and Treg cells, which are involved in regulation of these processes. Hence, Cosentino et al. suggest that the “β2-AR agonists might be ideal candidates to be tested as novel nonconventional antiparkinson drugs”.
SOME NEW DEVELOPMENTS
Heart electrical conduction – modulated by resident macrophages!
Innate immune cells such as macrophages are present in the heart, but their role in electrical conduction and the heart rhythm is unknown. In 2017, Maarten Hulsmanet et al., provided the first evidence that resident macrophages located at the atrioventricular (AV) node modulate heart electrical conduction and activity of cardiomyocytes.
The AV node, composed by cardiomyocytes conducts electrical impulses between atria and ventricles. This specialized conducting tissue (cardiac, not neural in origin) slows the impulse conduction, thus allowing sufficient time for complete atrial depolarization and contraction (systole) prior to ventricular depolarization and contraction.
Hulsmanet et al. found that in mice, AV macrophages express ion channels and exchangers, and genes associated to electrical conduction. These cells also interact with cardiomyocytes through gap juntion proteins and increase cardiomyocyte resting membrane potentials, suggesting their role in cardiomyocyte repolarization.
As per the authors, myocardial infarction, heart failure, diabetes or inflammatory diseases of the heart such as Chagas, Lyme, sarcoid and myocarditis may contribute to changes in the cardiac macrophages’ phenotype and numbers, which in turn may result in arrhythmias and conduction abnormalities.
Brain IL-6 and Depression
Some inflammatory cytokines profoundly affect brain functions, mood and behavior. Thus, inflammatory mechanisms might be involved in the etiology and pathophysiology of neuropsychiatric disorders such as depression.
A study published in Molecular Psychiatry indicates that the depressive symptoms in inflammatory conditions might be linked to an increase in central IL-6 concentration.
Harald Engler and Manfred Schedlowski’s team from the University Hospital Essen, Germany found that in healthy male volunteers the intravenous administration of low-dose endotoxin resulted in a robust and selective increase of IL-6 in the cerebrospinal fluid (CSF).
Of note, the endotoxin-induced increase of IL-6 correlated with the severity of mood impairment, where larger increases in IL-6 CSF concentrations contributed to a greater deterioration in mood.
These results imply IL-6 as an important messenger transferring the inflammatory signal from the body’s periphery to the brain. Thus, the appearance of depressive symptoms in inflammatory conditions might be causally and primarily linked to an increase in brain IL-6 concentrations, identifying IL-6 as a potential therapeutic target in mood disorders.
High IFN-γ and TNF-α versus low IL-10 Levels in Generalized Anxiety Disorder
Generalized anxiety disorder typically includes an excessive, uncontrollable and often irrational worry about events or activities, and is affecting almost 7 million adults or about 3% of the U.S. Population.
BrainImmune covered a study published in Brain, Behavior, and Immunity showing that anxiety is associated with a peripheral imbalance of pro- and anti-inflammatory cytokines.
Ruihua Hou and colleagues, from the University of Southampton, UK found that patients with generalized anxiety had high levels of interferon (IFN)-γ and tumor necrosis factor (TNF)-α, but low IL-10 serum levels when compared to healthy control subjects.
IFN-γ and TNF-α belong to the group of major pro-inflammatory cytokines, whereas IL-10 is a potent anti-inflammatory cytokine. As per the authors of this study, the cytokine ‘signature’ or profile in generalized anxiety patients is specific for this type of pathology.
Major depression is the principal comorbidity of generalized anxiety disorder. Hou R et al. propose that the pro-inflammatory state contribute to the altered activity of an enzyme involved in the metabolism of tryptophan, leading to degradation of serotonin in patients with generalized anxiety, via a mechanism similar to the one seeing in major depression.
Afferent Vagus Stimulation, the Sympathetic Nervous System-Synovial Axis and Joint Inflammation
Vagus nerve stimulation is known to affect immunity and inflammation through a mechanism that involves the peripheral efferent vagus nerve connection to the immune system. Yet, new research suggests that the afferent vagal stimulation also is able to inhibit inflammation, but by unknown brain and spinal cord axis pathway.
A study published in the Brain, Behavior, and Immunity provides perhaps the first evidence that stimulation of vagal afferents or specific brain areas modulate joint inflammation, and improves local inflammatory response in experimental arthritis. Bassi and colleagues, from the Medicine College of Ribeirão Preto, Brazil, showed that this new central pathway involves the activation of the sympathetic nervous system, synovial adrenergic innervation and stimulation of local synovial β-adrenoceptors.
The mechanism that reduced experimental joint arthritis, however, was not dependent on the integrity of the spleen, adrenal glands, the celiac vagus or lymphocyte activity. Importantly, the integrity of the brain locus coeruleus (LC), but not that of paraventricular nucleus (PVN), was critical for vagal regulation of the arthritic joint inflammation.
This study show the existence of potential brain immunomodulatory structures (immunological homunculus), and a new central neuroimmune anti-inflammatory pathway dependent on specific sympathomodulatory brain areas, and local sympathetic-synovial-β-adrenergic mechanisms. This pathway may drive anti-inflammatory effects in the body’s periphery.
Hypertension and the Brain-Splenic Nerve Axis
In the past, hypertension was regarded mostly as a dysfunction of the autonomic or sympathetic nervous systems. Recently, however, new evidence suggests that dysfunction at the brain-immune system connection may also contribute to hypertension.
A Nature Communications report indicates the existence of a new sympathetic mechanism in hypertension involving activation of the brain-splenic nerve axis. In other words, and as discussed by the authors of this study, hypertension is a condition where “sympathetic overactivity has effects beyond the kidney and baroreflexes”.
Daniela Carnevale et al. found that the experimental angiotensin II-induced hypertension is driven, to a great extent, by a splenic sympathetic nerve discharge, and that this mechanism is mediated by a vagal-coeliac-splenic connection.
Notably, in the experimental hyperthensive model, splenic denervation prevented the splenic T-cells egress to the systemic circulation and reduced the number of activated T cells to infiltrate into the aorta and kidneys.
On the whole, these experimental data suggests that the cholinergic-sympathetic drive, operating through the vagus-splenic nerve connection, may in turn activate the T cells to migrate to target organs and contribute to blood pressure regulation.
Spinal cord injury – induced immunosuppression linked to an interruption of neuroendocrine reflex involving the sympathetic nervous system and adrenal glands
Spinal cord injury is known to be related to immunosuppression and increased occurrence of infections. Yet, it is unknown whether these changes are due to humoral (via corticosteroids) or neural (via the sympathetic nervous system) disruptions.
A Nature Neuroscience report indicates that life-threatening infections in spinal cord injury patients may be driven by an interruption of the physiological interactions within a neuroendocrine reflex involving the sympathetic nervous system and adrenal glands.
Harald Prüss and colleagues from the Harvard medical school, Boston, Massachusetts found that the experimental thoracic spinal cord transection decreased peripheral norepinephrine (noradrenaline) levels and increased corticosterone levels in mice without activating the hypothalamus–pituitary–adrenal (HPA) axis. The spontaneous pneumonia was associated with decreased norepinephrine and increased corticosterone levels – not dependent on the integrity of the HPA axis – but dependent from a “maladaptive sympathetic-neuroendocrine reflex involving the adrenal glands”.
As per the authors, a two-step reflex mechanism drives these changes, where disruption of the tonic control of the adrenal glands by spinal cord efferents is followed by systemic effects that include low catecholamine levels and increased glucocorticoid release. This may explain the immunosuppressive effects observed after spinal cord injury.
Fibromyalgia and IL-8
Both neuroendocrine dysfunctions, e.g. relentless sympathetic hyperactivity or immune and cytokine, e.g. IL-8 and IL-17 abnormalities may participate in the pathogenesis of fibromyalgia.
A Journal of Pain Research study documents the presence of high levels of the chemokines interleukin-8 (IL-8) and CX3CL1 (also known as fractalkine) in fibromyalgia patients.
Emmanuel Bäckryd and colleagues from the Linköping University, Sweden applied the multiplex proximity extension assay, where 92 proteins were simultaneously analyzed in cerebrospinal fluid (CSF) from fibromyalgia patients. The authors found elevated CSF and plasma IL-8 levels, but high levels of CX3CL1 were monitored only in CSF fibromyalgia samples.
Whether this reflects pathophysiology in fibromyalgia, or a risk factor present prior to the development of chronic pain in these patients, remains to be clarified. Interestingly, CX3CL1 is “found throughout the brain, its receptor present on microglial cells”, whereas “IL-8 is the first mediator to be identified as evoking hyperalgesia involving the sympathetic nervous system”.
As “fibromyalgia & related disorders are heterogenous conditions…a continuum with one end a purely peripherally driven painful condition and the other end….where pain is purely centrally driven”, a stratification of FM patients in terms of CX3CL1 and IL-8 levels may, in fact, be existing.
Neuropathic Pain and IL-17
Neuropathic pain or chronic pain is a disease syndrome caused by injury to peripheral nerves, the spinal cord or the brain.
In this condition, the microglial/astrocyte activation or infiltration of macrophages and T cells contribute to central sensitization. Also, pronociceptive factors such as cytokines and chemokines can sensitize neurons of the first pain synapse.
A study in Molecular Medicine Reports indicates that interleukin (IL)-17 drives neuropathic pain via astrocytes activation and secretion of proinflammatory cytokines.
Caixia Sun and colleagues from the Jiangsu University Zhenjiang, China monitored high IL-17 levels in the spinal cord of sciatic nerve-injured rats. This was related to local infiltration of CD4/IL‑17+ cells and increased astrocyte activity.
These cells were associated with an up-regulation of IL‑17, IL‑1β and IL‑6 mRNA expression and high IL-17 protein levels in the spinal cord. Moreover, in vitro, IL‑17 stimulated resting astrocytes to produce IL-1β and IL-6 that may be linked to pain hypersensitivity.
Thus, according to the authors IL‑17 may participate in the pathogenesis of neuropathic pain by promoting astrocytes activation and proliferation, and upregulating proinflammatory cytokines release in the spinal nerve ligation‑induced model of neuropathic pain.
