Protective autoimmunity functions by intracranial immunosurveillance to support the mind: The missing link between health and disease

Targeting brain-peripheral immune responses for secondary brain injury after ischemic and hemorrhagic stroke

The notion that the central nervous system is an immunologically immune-exempt organ has changed over the past two decades, with increasing evidence of strong links and interactions between the central nervous system and the peripheral immune system, both in the healthy state and after ischemic and hemorrhagic stroke. Although primary injury after stroke is certainly important, the limited therapeutic efficacy, poor neurological prognosis and high mortality have led researchers to realize that secondary injury and damage may also play important roles in influencing long-term neurological prognosis and mortality and that the neuroinflammatory process in secondary injury is one of the most important influences on disease progression. Here, we summarize the interactions of the central nervous system with the peripheral immune system after ischemic and hemorrhagic stroke, in particular, how the central nervous system activates and recruits peripheral immune components, and we review recent advances in corresponding therapeutic approaches and clinical studies, emphasizing the importance of the role of the peripheral immune system in ischemic and hemorrhagic stroke.

Mitochondria-Related Candidate Genes and Diagnostic Model to Predict Late-Onset Alzheimer's Disease and Mild Cognitive Impairment

Background

Late-onset Alzheimer's disease (LOAD) is the most common type of dementia, but its pathogenesis remains unclear, and there is a lack of simple and convenient early diagnostic markers to predict the occurrence.

Objective

Our study aimed to identify diagnostic candidate genes to predict LOAD by machine learning methods.

Methods

Three publicly available datasets from the Gene Expression Omnibus (GEO) database containing peripheral blood gene expression data for LOAD, mild cognitive impairment (MCI), and controls (CN) were downloaded. Differential expression analysis, the least absolute shrinkage and selection operator (LASSO), and support vector machine recursive feature elimination (SVM-RFE) were used to identify LOAD diagnostic candidate genes. These candidate genes were then validated in the validation group and clinical samples, and a LOAD prediction model was established.

Results

LASSO and SVM-RFE analyses identified 3 mitochondria-related genes (MRGs) as candidate genes, including NDUFA1, NDUFS5, and NDUFB3. In the verification of 3 MRGs, the AUC values showed that NDUFA1, NDUFS5 had better predictability. We also verified the candidate MRGs in MCI groups, the AUC values showed good performance. We then used NDUFA1, NDUFS5 and age to build a LOAD diagnostic model and AUC was 0.723. Results of qRT-PCR experiments with clinical blood samples showed that the three candidate genes were expressed significantly lower in the LOAD and MCI groups when compared to CN.

Conclusion

Two mitochondrial-related candidate genes, NDUFA1 and NDUFS5, were identified as diagnostic markers for LOAD and MCI. Combining these two candidate genes with age, a LOAD diagnostic prediction model was successfully constructed.

Changes in dendritic complexity and spine morphology following BCG immunization in APP/PS1 mice

Bacillus Calmette - Guerin (BCG) is an immune regulator that can enhance hippocampal synaptic plasticity in rats; however, it is unclear whether it can improve synaptic function in a mouse model with Alzheimer's disease (AD). We hypothesized that BCG plays a protective role in AD mice and investigated its effect on dendritic morphology. The results obtained show that BCG immunization significantly increases dendritic complexity, as indicated by the increased number of dendritic intersections and branch points, as well as the increase in the fractal dimension. Furthermore, the number of primary neurites and dendritic length also increased following BCG immunization, which increased the number of spines and promoted maturation. IFN-γ and IL-4 levels increased, while TNF-α levels decreased following BCG immunization; expression levels of p-JAK2, P-STAT3, SYN, and PSD-95 also increased. Therefore, this study demonstrates that BCG immunization in APP/PS1 mice mitigated hippocampal dendritic spine pathology, especially after the third round of immunization. This effect could possibly be attributed to; changes in dendritic arborization and spine morphology or increases in SYN and PSD-95 expression levels. It could also be related to mechanisms of BCG-induced increases in IFN-γ or IL-4/JAK2/STAT3 levels.

The industrial genomic revolution: A new era in neuroimmunology

Recent discoveries have highlighted the importance of understanding the complex interactions between the brain and immune systems in health and neurodegenerative disease. In this Primer, we outline single-cell multiomics approaches. Applied to patient samples with rich metadata, functional organoids, and animal models, single-cell studies will facilitate the next big leap in translational neuro-immune research. We believe this will pave the way for reshaping our understanding of the neuro-immune interplay: from descriptive to functional, from broad cell types to effective pathways, spatial organization, biomarkers, and targets, toward a comprehensive mechanistic understanding that will be the impetus for drug discovery and therapeutic breakthroughs. We envision that in the near future, single-cell multiomics technologies, along with the advances in immunotherapy development, will become a major driving force and fully integrated resource in the toolset for the development of therapeutic agents in neuroimmunology, which will revolutionize drug development for treating neurodegenerative diseases.

DCE-MRI of Brain Fluid Barriers: In Vivo Water Cycling at the Human Choroid Plexus

Metabolic deficits at brain-fluid barriers are an increasingly recognized feature of cognitive decline in older adults. At the blood-cerebrospinal fluid barrier, water is transported across the choroid plexus (CP) epithelium against large osmotic gradients via processes tightly coupled to activity of the sodium/potassium pump. Here, we quantify CP homeostatic water exchange using dynamic contrast-enhanced MRI and investigate the association of the water efflux rate constant (k_co) with cognitive dysfunction in older individuals. Temporal changes in the longitudinal relaxation rate constant (R₁) after contrast agent bolus injection were measured in a CP region of interest in 11 participants with mild cognitive dysfunction [CI; 73 ± 6 years] and 28 healthy controls [CN; 72 ± 7 years]. k_co was determined from a modified two-site pharmacokinetic exchange analysis of the R₁ time-course. K^trans, a measure of contrast agent extravasation to the interstitial space was also determined. Cognitive function was assessed by neuropsychological test performance. k_co averages 5.8 ± 2.7 s^-1 in CN individuals and is reduced by 2.4 s^-1 [ca. 40%] in CI subjects. Significant associations of k_co with global cognition and multiple cognitive domains are observed. K^trans averages 0.13 ± 0.07 min^-1 and declines with age [-0.006 ± 0.002 min^-1 yr^-1], but shows no difference between CI and CN individuals or association with cognitive performance. Our findings suggest that the CP water efflux rate constant is associated with cognitive dysfunction and shows an age-related decline in later life, consistent with the metabolic disturbances that characterize brain aging.

Neurogenesis after Spinal Cord Injury: State of the Art

Neurogenesis in the adult state is the process of new neuron formation. This relatively infrequent phenomenon comprises four stages: cell proliferation, cell migration, differentiation, and the integration of these cells into an existing circuit. Recent reports suggest that neurogenesis can be found in different regions of the Central Nervous System (CNS), including the spinal cord (SC). This process can be observed in physiological settings; however, it is more evident in pathological conditions. After spinal cord injury (SCI), the activation of microglial cells and certain cytokines have shown to exert different modulatory effects depending on the presence of inflammation and on the specific region of the injury site. In these conditions, microglial cells and cytokines are considered to play an important role in the regulation of neurogenesis after SCI. The purpose of this article is to present an overview on neural progenitor cells and neurogenic and non-neurogenic zones as well as the cellular and molecular regulation of neurogenesis. Additionally, we will briefly describe the recent advances in the knowledge of neurogenesis after SCI.

Dysregulation of Glutamate Transport Enhances Treg Function That Promotes VEGF Blockade Resistance in Glioblastoma

Anti-VEGF therapy prolongs recurrence-free survival in patients with glioblastoma but does not improve overall survival. To address this discrepancy, we investigated immunologic resistance mechanisms to anti-VEGF therapy in glioma models. A screening of immune-associated alterations in tumors after anti-VEGF treatment revealed a dose-dependent upregulation of regulatory T-cell (Treg) signature genes. Enhanced numbers of Tregs were observed in spleens of tumor-bearing mice and later in tumors after anti-VEGF treatment. Elimination of Tregs with CD25 blockade before anti-VEGF treatment restored IFNγ production from CD8⁺ T cells and improved antitumor response from anti-VEGF therapy. The treated tumors overexpressed the glutamate/cystine antiporter SLC7A11/xCT that led to elevated extracellular glutamate in these tumors. Glutamate promoted Treg proliferation, activation, suppressive function, and metabotropic glutamate receptor 1 (mGlutR1) expression. We propose that VEGF blockade coupled with glioma-derived glutamate induces systemic and intratumoral immunosuppression by promoting Treg overrepresentation and function, which can be pre-emptively overcome through Treg depletion for enhanced antitumor effects. SIGNIFICANCE: Resistance to VEGF therapy in glioblastoma is driven by upregulation of Tregs, combined blockade of VEGF, and Tregs may provide an additive antitumor effect for treating glioblastoma.

Cell Clearing Systems Bridging Neuro-Immunity and Synaptic Plasticity

In recent years, functional interconnections emerged between synaptic transmission, inflammatory/immune mediators, and central nervous system (CNS) (patho)-physiology. Such interconnections rose up to a level that involves synaptic plasticity, both concerning its molecular mechanisms and the clinical outcomes related to its behavioral abnormalities. Within this context, synaptic plasticity, apart from being modulated by classic CNS molecules, is strongly affected by the immune system, and vice versa. This is not surprising, given the common molecular pathways that operate at the cross-road between the CNS and immune system. When searching for a common pathway bridging neuro-immune and synaptic dysregulations, the two major cell-clearing cell clearing systems, namely the ubiquitin proteasome system (UPS) and autophagy, take center stage. In fact, just like is happening for the turnover of key proteins involved in neurotransmitter release, antigen processing within both peripheral and CNS-resident antigen presenting cells is carried out by UPS and autophagy. Recent evidence unravelling the functional cross-talk between the cell-clearing pathways challenged the traditional concept of autophagy and UPS as independent systems. In fact, autophagy and UPS are simultaneously affected in a variety of CNS disorders where synaptic and inflammatory/immune alterations concur. In this review, we discuss the role of autophagy and UPS in bridging synaptic plasticity with neuro-immunity, while posing a special emphasis on their interactions, which may be key to defining the role of immunity in synaptic plasticity in health and disease.

A Sentinel in the Crosstalk Between the Nervous and Immune System: The (Immuno)-Proteasome

The wealth of recent evidence about a bi-directional communication between nerve- and immune- cells revolutionized the traditional concept about the brain as an “immune-privileged” organ while opening novel avenues in the pathophysiology of CNS disorders. In fact, altered communication between the immune and nervous system is emerging as a common hallmark in neuro-developmental, neurodegenerative, and neuro-immunological diseases. At molecular level, the ubiquitin proteasome machinery operates as a sentinel at the crossroad between the immune system and brain. In fact, the standard proteasome and its alternative/inducible counterpart, the immunoproteasome, operate dynamically and coordinately in both nerve- and immune- cells to modulate neurotransmission, oxidative/inflammatory stress response, and immunity. When dysregulations of the proteasome system occur, altered amounts of standard- vs. immune-proteasome subtypes translate into altered communication between neurons, glia, and immune cells. This contributes to neuro-inflammatory pathology in a variety of neurological disorders encompassing Parkinson's, Alzheimer's, and Huntingtin's diseases, brain trauma, epilepsy, and Multiple Sclerosis. In the present review, we analyze those proteasome-dependent molecular interactions which sustain communication between neurons, glia, and brain circulating T-lymphocytes both in baseline and pathological conditions. The evidence here discussed converges in that upregulation of immunoproteasome to the detriment of the standard proteasome, is commonly implicated in the inflammatory- and immune- biology of neurodegeneration. These concepts may foster additional studies investigating the role of immunoproteasome as a potential target in neurodegenerative and neuro-immunological disorders.

Role of Inflammation in Depression and Treatment Implications

Approximately one third of depressed patients fail to respond to currently available antidepressant therapies. Therefore, new conceptual frameworks are needed to identify pathophysiologic pathways and neurobiological targets for the development of novel treatment strategies. In this regard, recent evidence suggests that inflammation may contribute to symptoms relevant to a number of psychiatric disorders and particularly depression. Numerous studies (including meta-analyses) have found elevated peripheral and central inflammatory cytokines and acute phase proteins in depression. Chronic exposure to increased inflammation is thought to drive changes in neurotransmitters and neurocircuits that lead to depressive symptoms and that may also interfere with or circumvent the efficacy of antidepressants. Indeed, patients with high inflammation have been shown to exhibit poor response to conventional antidepressant therapies. Recent developments in our ability to understand and measure the effects of inflammation on the brain in patients have opened new doors for the testing of novel treatment strategies that target the immune system or its consequences on neurotransmitter systems. Such recent developments in the field of behavioral immunology and their translational implications for the treatment of depression are discussed herein.

Could a blood test for PTSD and depression be on the horizon?

INTRODUCTION

Depression and posttraumatic stress disorder (PTSD) are two complex and debilitating psychiatric disorders that result in poor life and destructive behaviors against self and others. Currently, diagnosis is based on subjective rather than objective determinations leading to misdiagnose and ineffective treatments. Advances in novel neurobiological methods have allowed assessment of promising biomarkers to diagnose depression and PTSD, which offers a new means of appropriately treating patients. Areas covered: Biomarkers discovery in blood represents a fundamental tool to predict, diagnose, and monitor treatment efficacy in depression and PTSD. The potential role of altered HPA axis, epigenetics, NPY, BDNF, neurosteroid biosynthesis, the endocannabinoid system, and their function as biomarkers for mood disorders is discussed. Insofar, we propose the identification of a biomarker axis to univocally identify and discriminate disorders with large comorbidity and symptoms overlap, so as to provide a base of support for development of targeted treatments. We also weigh in on the feasibility of a future blood test for early diagnosis. Expert commentary: Potential biomarkers have already been assessed in patients' blood and need to be further validated through multisite large clinical trial stratification. Another challenge is to assess the relation among several interdependent biomarkers to form an axis that identifies a specific disorder and secures the best-individualized treatment. The future of blood-based tests for PTSD and depression is not only on the horizon but, possibly, already around the corner.

CD4+ T Cells Have a Permissive Effect on Enriched Environment-Induced Hippocampus Synaptic Plasticity

Living in an enriched environment (EE) benefits health by acting synergistically on various biological systems including the immune and the central nervous systems. The dialog between the brain and the immune cells has recently gained interest and is thought to play a pivotal role in beneficial effects of EE. Recent studies show that T lymphocytes have an important role in hippocampal plasticity, learning, and memory, although the precise mechanisms by which they act on the brain remain elusive. Using a mouse model of EE, we show here that CD4⁺ T cells are essential for spinogenesis and glutamatergic synaptic function in the CA of the hippocampus. However, CD4⁺ lymphocytes do not influence EE-induced neurogenesis in the DG of the hippocampus, by contrast to what we previously demonstrated for CD8⁺ T cells. Importantly, CD4⁺ T cells located in the choroid plexus have a specific transcriptomic signature as a function of the living environment. Our study highlights the contribution of CD4⁺ T cells in the brain plasticity and function.

CD8+ T cells are essential for the effects of enriched environment on hippocampus-dependent behavior, hippocampal neurogenesis and synaptic plasticity

Enriched environment (EE) induces plasticity changes in the brain. Recently, CD4⁺ T cells have been shown to be involved in brain plasticity processes. Here, we show that CD8⁺ T cells are required for EE-induced brain plasticity in mice, as revealed by measurements of hippocampal volume, neurogenesis in the DG of the hippocampus, spinogenesis and glutamatergic synaptic function in the CA of the hippocampus. As a consequence, EE-induced behavioral benefits depend, at least in part, on CD8⁺ T cells. In addition, we show that spleen CD8⁺ T cells from mice housed in standard environment (SE) and EE have different properties in terms of 1) TNFα release after in vitro CD3/CD28 or PMA/Iono stimulation 2) in vitro proliferation properties 3) CD8⁺ CD44⁺ CD62L^low and CD62L^hi T cells repartition 4) transcriptomic signature as revealed by RNA sequencing. CD8⁺ T cells purified from the choroid plexus of SE and EE mice also exhibit different transcriptomic profiles as highlighted by single-cell mRNA sequencing. We show that CD8⁺ T cells are essential mediators of beneficial EE effects on brain plasticity and cognition. Additionally, we propose that EE differentially primes CD8⁺ T cells leading to behavioral improvement.

Contributions of the adaptive immune system to mood regulation: Mechanisms and pathways of neuroimmune interactions

Clinical and basic studies of functional interactions between adaptive immunity, affective states, and brain function are reviewed, and the neural, humoral, and cellular routes of bidirectional communication between the brain and the adaptive immune system are evaluated. In clinical studies of depressed populations, lymphocytes-the principal cells of the adaptive immune system-exhibit altered T cell subtype ratios and CD4+ helper T cell polarization profiles. In basic studies using psychological stress to model depression, T cell profiles are altered as well, consistent with stress effects conveyed by the hypothalamic-pituitary-adrenal axis and sympathetic nervous system. Lymphocytes in turn have effects on behavior and CNS structure and function. CD4+ T cells in particular appear to modify affective behavior and rates of hippocampal dentate gyrus neurogenesis. These observations force the question of how such actions are carried out. CNS effects may occur via cellular and molecular mechanisms whereby effector memory T cells and the cytokine profiles they produce in the blood interact with the blood-brain barrier in ways that remain to be clarified. Understanding the mechanisms by which T cells polarize and interact with the brain to alter mood states is key to advances in the field, and may permit development of therapies that target cells in the periphery, thus bypassing problems associated with bioavailability of drugs within the brain.

Loss of chromosome Y (LOY) in blood cells is associated with increased risk for disease and mortality in aging men

Recent discoveries have shown that harboring cells without the Y chromosome in the peripheral blood is associated with increased risk for all-cause mortality and disease such as different forms of cancer, Alzheimer’s disease, as well as other conditions in aging men. In the entire world, the life expectancy of men is shorter compared to women, a sex difference that has been known for centuries, but the underlying mechanism(s) are not well understood. As a male-specific genetic risk factor, an increased risk for pathology and mortality associated with mosaic loss of chromosome Y (LOY) in blood cells could help to explain that men on average live shorter lives compared to women. This review primarily focuses on observed associations between LOY in blood and various diseases in aging men. Other topics covered are known risk factors for LOY, methods to detect LOY, and a discussion regarding mechanisms such as immunosurveillance, that could possibly explain how an acquired mutation in blood cells can be associated with disease processes in other organs.

Evolution, immunity and the emergence of brain superautoantigens

While some autoimmune disorders remain extremely rare, others largely predominate the epidemiology of human autoimmunity. Notably, these include psoriasis, diabetes, vitiligo, thyroiditis, rheumatoid arthritis and multiple sclerosis. Thus, despite the quasi-infinite number of "self" antigens that could theoretically trigger autoimmune responses, only a limited set of antigens, referred here as superautoantigens, induce pathogenic adaptive responses. Several lines of evidence reviewed in this paper indicate that, irrespective of the targeted organ (e.g. thyroid, pancreas, joints, brain or skin), a significant proportion of superautoantigens are highly expressed in the synaptic compartment of the central nervous system (CNS). Such an observation applies notably for GAD65, AchR, ribonucleoproteins, heat shock proteins, collagen IV, laminin, tyrosine hydroxylase and the acetylcholinesterase domain of thyroglobulin. It is also argued that cognitive alterations have been described in a number of autoimmune disorders, including psoriasis, rheumatoid arthritis, lupus, Crohn's disease and autoimmune thyroiditis. Finally, the present paper points out that a great majority of the "incidental" autoimmune conditions notably triggered by neoplasms, vaccinations or microbial infections are targeting the synaptic or myelin compartments. On this basis, the concept of an immunological homunculus, proposed by Irun Cohen more than 25 years ago, is extended here in a model where physiological autoimmunity against brain superautoantigens confers both: i) a crucial evolutionary-determined advantage via cognition-promoting autoimmunity; and ii) a major evolutionary-determined vulnerability, leading to the emergence of autoimmune disorders in Homo sapiens. Moreover, in this theoretical framework, the so called co-development/co-evolution model, both the development (at the scale of an individual) and evolution (at the scale of species) of the antibody and T-cell repertoires are coupled to those of the neural repertoires (i.e. the distinct neuronal populations and synaptic circuits supporting cognitive and sensorimotor functions). Clinical implications and future experimental insights are also presented and discussed.

Insights into CCL21's roles in immunosurveillance and immunotherapy for gliomas

Chemokine (C-C) motif ligand 21 (CCL21) is involved in immunosurveillance and has recently garnered the attention of neuro-oncologists and neuroscientists. CCL21 contains an extended C-terminus, which increases binding to lymphatic glycosaminoglycans and provides a mechanism for cell trafficking by forming a stationary chemokine concentration gradient that allows cell migration via haptotaxis. CCL21 is expressed by endothelial cells of the blood-brain barrier in physiologic and pathologic conditions. CCL21 has also been implicated in leukocyte extravasation into the central nervous system. In this review, we summarize the role of CCL21 in immunosurveillance and explore its potential as an immunotherapeutic agent for the treatment of gliomas.

Therapeutic Implications of Brain-Immune Interactions: Treatment in Translation

A wealth of data has been amassed that details a complex, yet accessible, series of pathways by which the immune system, notably inflammation, can influence the brain and behavior. These data have opened the window to a diverse array of novel targets whose potential efficacy is tied to specific neurotransmitters and neurocircuits as well as specific behaviors. What is clear is that the impact of inflammation on the brain cuts across psychiatric disorders and engages dopaminergic and glutamatergic pathways that regulate motivation and motor activity as well as the sensitivity to threat. Given the ability to identify patient populations with increased inflammation, the precision of interventions can be further tuned, in conjunction with the ability to establish target engagement in the brain through the use of multiple neuroimaging strategies. After a brief overview of the mechanisms by which inflammation affects the brain and behavior, this review examines the extant literature on the efficacy of anti-inflammatory treatments, while forging guidelines for future intelligent clinical trial design. An examination of the most promising therapeutic strategies is also provided, along with some of the most exciting clinical trials that are currently being planned or underway.

Inflammatory Response in the CNS: Friend or Foe?

Inflammatory reactions could be both beneficial and detrimental to the brain, depending on strengths of their activation in various stages of neurodegeneration. Mild activation of microglia and astrocytes usually reveals neuroprotective effects and ameliorates early symptoms of neurodegeneration; for instance, released cytokines help maintain synaptic plasticity and modulate neuronal excitability, and stimulated toll-like receptors (TLRs) promote neurogenesis and neurite outgrowth. However, strong activation of glial cells gives rise to cytokine overexpression/dysregulation, which accelerates neurodegeneration. Altered mutual regulation of p53 protein, a major tumor suppressor, and NF-κB, the major regulator of inflammation, seems to be crucial for the shift from beneficial to detrimental effects of neuroinflammatory reactions in neurodegeneration. Therapeutic intervention in the p53-NF-κB axis and modulation of TLR activity are future challenges to cope with neurodegeneration.

Neonatal hepatitis B vaccination impaired the behavior and neurogenesis of mice transiently in early adulthood

The immune system plays a vital role in brain development. The hepatitis B vaccine (HBV) is administered to more than 70% of neonates worldwide. Whether this neonatal vaccination affects brain development is unknown. Newborn C57BL/6 mice were injected intraperitoneally with HBV or phosphate-buffered saline. HBV induced impaired behavioral performances and hippocampal long-term potentiation at 8 weeks (w) of age without influence at 4 or 12w. At 6w, there was decreased neurogenesis, M1 microglial activation and a neurotoxic profile of neuroimmune molecule expression [increased tumor necrosis factor-α and reduced interferon (IFN)-γ, brain-derived neurotrophic factor and insulin-like growth factor-1] in the hippocampus of the HBV-vaccinated mice. In the serum, HBV induced significantly higher levels of interleukin (IL)-4, indicating a T helper (Th)-2 bias. Moreover, the serum IFN-γ/IL-4 ratio was positively correlated with the levels of neurotrophins and neurogenesis in the hippocampus at the individual level. These findings suggest that neonatal HBV vaccination of mice results in neurobehavioral impairments in early adulthood by inducing a proinflammatory and low neurotrophic milieu in the hippocampus, which follows the HBV-induced systemic Th2 bias.

Therapeutic effects of stress-programmed lymphocytes transferred to chronically stressed mice

Our group has recently provided novel insights into a poorly understood component of intercommunication between the brain and the immune system by showing that psychological stress can modify lymphocytes in a manner that may boost resilience to psychological stress. To demonstrate the influence of the adaptive immune system on mood states, we previously showed that cells from lymph nodes of socially defeated mice, but not from unstressed mice, conferred anxiolytic and antidepressant-like effects and elevated hippocampal cell proliferation when transferred into naïve lymphopenic Rag2(-/-) mice. In the present study, we asked whether similar transfer could be anxiolytic and antidepressant when done in animals that had been rendered anxious and depressed by chronic psychological stress. First, we demonstrated that lymphopenic Rag2(-/-) mice and their wild-type C57BL/6 mouse counterparts had similar levels of affect normally. Second, we found that following chronic (14days) restraint stress, both groups displayed an anxious and depressive-like phenotype and decreased hippocampal cell proliferation. Third, we showed that behavior in the open field test and light/dark box was normalized in the restraint-stressed Rag2(-/-) mice following adoptive transfer of lymph node cells from green fluorescent protein (GFP) expressing donor mice previously exposed to chronic (14days) of social defeat stress. Cells transferred from unstressed donor mice had no effect on behavior. Immunolabeling of GFP+ cells confirmed that tissue engraftment had occurred at 14days after transfer. We found GFP+ lymphocytes in the spleen, lymph nodes, blood, choroid plexus, and meninges of the recipient Rag2(-/-) mice. The findings suggest that the adaptive immune system may play a key role in promoting recovery from chronic stress. The data support using lymphocytes as a novel therapeutic target for anxiety states.

Depression as a microglial disease

Despite decades of intensive research, the biological mechanisms that causally underlie depression are still unclear, and therefore the development of novel effective antidepressant treatments is hindered. Recent studies indicate that impairment of the normal structure and function of microglia, caused by either intense inflammatory activation (e.g., following infections, trauma, stroke, short-term stress, autoimmune or neurodegenerative diseases) or by decline and senescence of these cells (e.g., during aging, Alzheimer's disease, or chronic unpredictable stress exposure), can lead to depression and associated impairments in neuroplasticity and neurogenesis. Accordingly, some forms of depression can be considered as a microglial disease (microgliopathy), which should be treated by a personalized medical approach using microglial inhibitors or stimulators depending on the microglial status of the depressed patient.

Mosaic Loss of Chromosome Y in Blood Is Associated with Alzheimer Disease

Men have a shorter life expectancy compared with women but the underlying factor(s) are not clear. Late-onset, sporadic Alzheimer disease (AD) is a common and lethal neurodegenerative disorder and many germline inherited variants have been found to influence the risk of developing AD. Our previous results show that a fundamentally different genetic variant, i.e., lifetime-acquired loss of chromosome Y (LOY) in blood cells, is associated with all-cause mortality and an increased risk of non-hematological tumors and that LOY could be induced by tobacco smoking. We tested here a hypothesis that men with LOY are more susceptible to AD and show that LOY is associated with AD in three independent studies of different types. In a case-control study, males with AD diagnosis had higher degree of LOY mosaicism (adjusted odds ratio = 2.80, p = 0.0184, AD events = 606). Furthermore, in two prospective studies, men with LOY at blood sampling had greater risk for incident AD diagnosis during follow-up time (hazard ratio [HR] = 6.80, 95% confidence interval [95% CI] = 2.16–21.43, AD events = 140, p = 0.0011). Thus, LOY in blood is associated with risks of both AD and cancer, suggesting a role of LOY in blood cells on disease processes in other tissues, possibly via defective immunosurveillance. As a male-specific risk factor, LOY might explain why males on average live shorter lives than females.

Ly6C(hi) Monocytes Provide a Link between Antibiotic-Induced Changes in Gut Microbiota and Adult Hippocampal Neurogenesis

Antibiotics, though remarkably useful, can also cause certain adverse effects. We detected that treatment of adult mice with antibiotics decreases hippocampal neurogenesis and memory retention. Reconstitution with normal gut flora (SPF) did not completely reverse the deficits in neurogenesis unless the mice also had access to a running wheel or received probiotics. In parallel to an increase in neurogenesis and memory retention, both SPF-reconstituted mice that ran and mice supplemented with probiotics exhibited higher numbers of Ly6C(hi) monocytes in the brain than antibiotic-treated mice. Elimination of Ly6C(hi) monocytes by antibody depletion or the use of knockout mice resulted in decreased neurogenesis, whereas adoptive transfer of Ly6C(hi) monocytes rescued neurogenesis after antibiotic treatment. We propose that the rescue of neurogenesis and behavior deficits in antibiotic-treated mice by exercise and probiotics is partially mediated by Ly6C(hi) monocytes.

Molecular mechanisms of D-cycloserine in facilitating fear extinction: insights from RNAseq

D-cycloserine (DCS) has been shown to be effective in facilitating fear extinction in animal and human studies, however the precise mechanisms whereby the co-administration of DCS and behavioural fear extinction reduce fear are still unclear. This study investigated the molecular mechanisms of intrahippocampally administered D-cycloserine in facilitating fear extinction in a contextual fear conditioning animal model. Male Sprague Dawley rats (n = 120) were grouped into four experimental groups (n = 30) based on fear conditioning and intrahippocampal administration of either DCS or saline. The light/dark avoidance test was used to differentiate maladapted (MA) (anxious) from well-adapted (WA) (not anxious) subgroups. RNA extracted from the left dorsal hippocampus was used for RNA sequencing and gene expression data was compared between six fear-conditioned + saline MA (FEAR + SALINE MA) and six fear-conditioned + DCS WA (FEAR + DCS WA) animals. Of the 424 significantly downregulated and 25 significantly upregulated genes identified in the FEAR + DCS WA group compared to the FEAR + SALINE MA group, 121 downregulated and nine upregulated genes were predicted to be relevant to fear conditioning and anxiety and stress-related disorders. The majority of downregulated genes transcribed immune, proinflammatory and oxidative stress systems molecules. These molecules mediate neuroinflammation and cause neuronal damage. DCS also regulated genes involved in learning and memory processes, and genes associated with anxiety, stress-related disorders and co-occurring diseases (e.g., cardiovascular diseases, digestive system diseases and nervous system diseases). Identifying the molecular underpinnings of DCS-mediated fear extinction brings us closer to understanding the process of fear extinction.

Neonatal BCG vaccination of mice improves neurogenesis and behavior in early life

Bacillus Calmette-Guérin (BCG) is administered to neonates worldwide, but it is still unknown whether this neonatal vaccination affects brain development during early postnatal life, despite the close association of the immune system with the brain. Newborn C57BL/6 mice were injected subcutaneously with BCG or phosphate-buffered saline (PBS) and their mood status and spatial cognition were observed at four and eight weeks (w) old. The mice were also subjected to tests at 2 and 6 w to examine BCG's effects on neurogenesis, the hippocampal microglia phenotype and number, and the expression of hippocampal neuroimmune molecules and peripheral cytokines. The BCG-injected mice showed better behavioral performances at 4 w. We observed elevated neurogenesis, M2 microglial activation and a neurotrophic profile of neuroimmune molecules [more interferon (IFN)-γ, interleukin (IL)-4, transforming growth factor (TGF)-β, brain-derived neurotrophic factor (BDNF) and insulin-like growth factor (IGF)-1 and less tumor necrosis factor (TNF)-α and IL-1β] in the hippocampus of the 2-w-old BCG-mice. In the periphery, BCG induced a T helper (Th)-1 serum response. At the individual level, there were positive correlations between the serum IFN-γ/IL-4 ratio and the levels of neurotrophins and neurogenesis in the hippocampus. These findings suggest that neonatal BCG vaccination improved neurogenesis and mouse behavior in early life by affecting the neuroimmune milieu in the brain, which may be associated with a systemic Th1 bias.

Immune mediators in the brain and peripheral tissues in autism spectrum disorder

Increasing evidence points to a central role for immune dysregulation in autism spectrum disorder (ASD). Several ASD risk genes encode components of the immune system and many maternal immune system-related risk factors--including autoimmunity, infection and fetal reactive antibodies--are associated with ASD. In addition, there is evidence of ongoing immune dysregulation in individuals with ASD and in animal models of this disorder. Recently, several molecular signalling pathways--including pathways downstream of cytokines, the receptor MET, major histocompatibility complex class I molecules, microglia and complement factors--have been identified that link immune activation to ASD phenotypes. Together, these findings indicate that the immune system is a point of convergence for multiple ASD-related genetic and environmental risk factors.

Activated regulatory T cell regulates neural stem cell proliferation in the subventricular zone of normal and ischemic mouse brain through interleukin 10

Recent studies have demonstrated that the depletion of Regulatory T cells (Tregs) inhibits neural progenitor cell migration after brain ischemia. However, whether Tregs affect neural stem/progenitor cell proliferation is unclear. We explored the effect of Tregs on neurogenesis in the subventricular zone (SVZ) after ischemia. Tregs were isolated and activated in vitro. Adult male C57BL/6 mice underwent 60 min transient middle cerebral artery occlusion (tMCAO). Then Tregs (1 × 10⁵) were injected into the left lateral ventricle (LV) of normal and ischemic mouse brain. Neurogenesis was determined by immunostaining. The mechanism was examined by inhibiting interleukin 10 (IL-10) and transforming growth factor (TGF-β) signaling. We found that the number of BrdU⁺ cells in the SVZ was significantly increased in the activated Tregs-treated mice. Double immunostaining showed that these BrdU⁺ cells expressed Mash1. Blocking IL-10 reduced the number of Mash1⁺/BrdU⁺ cells, but increased the amount of GFAP⁺/BrdU⁺ cells. Here, we conclude that activated Tregs enhanced neural stem cell (NSC) proliferation in the SVZ of normal and ischemic mice; blockage of IL-10 abolished Tregs-mediated NSC proliferation in vivo and in vitro. Our results suggest that activated Tregs promoted NSC proliferation via IL-10, which provides a new therapeutic approach for ischemic stroke.

Neonatal vaccination with bacillus Calmette-Guérin and hepatitis B vaccines modulates hippocampal synaptic plasticity in rats

Immune activation can exert multiple effects on synaptic transmission. Our study demonstrates the influence of neonatal vaccination on hippocampal synaptic plasticity in rats under normal physiological conditions. The results revealed that neonatal BCG vaccination enhanced synaptic plasticity. In contrast, HBV hampered it. Furthermore, we found that the cytokine balance shifted in favour of the T helper type 1/T helper type 2 immune response in BCG/HBV-vaccinated rats in the periphery. The peripheral IFN-γ:IL-4 ratio was positively correlated with BDNF and IGF-1 in the hippocampus. BCG raised IFN-γ, IL-4, BDNF and IGF-1 and reduced IL-1β, IL-6, and TNF-α in the hippocampus, whereas, HBV triggered the opposite effects.

Alterations in immune cells and mediators in the brain: it's not always neuroinflammation!

Neuroinflammation was once a clearly defined term denoting pathological immune processes within the central nervous system (CNS). Historically, this term was used to indicate the four hallmarks of peripheral inflammaton that occur following severe CNS injuries, such as stroke, injury or infection. Recently, however, the definition of neuroinflammation has relaxed to the point that it is often now assumed to be present when even only a single classical hallmark of inflammation is measured. As a result, a wide range of disorders, from psychiatric to degenerative diseases, are now assumed to have an integral inflammatory component. Ironically, at the same time, research has revealed unexpected nonclassical immune actions of immune mediators and cells in the CNS in the absence of pathology, increasing the likelihood that homeostatic and adaptive immune processes in the CNS will be mistaken for neuroinflammation. Thus, we suggest reserving the term neuroinflammation for contexts where multiple signs of inflammation are present to avoid erroneously classifying disorders as inflammatory when they may instead be caused by nonimmune etiologies or secondary immune processes that serve adaptive roles.

Immune and neurotrophin stimulation by electroconvulsive therapy: is some inflammation needed after all?

A low-grade inflammatory response is commonly seen in the peripheral blood of major depressive disorder (MDD) patients, especially those with refractory and chronic disease courses. However, electroconvulsive therapy (ECT), the most drastic intervention reserved for these patients, is closely associated with an enhanced haematogenous as well as neuroinflammatory immune response, as evidenced by both human and animal studies. A related line of experimental evidence further shows that inflammatory stimulation reinforces neurotrophin expression and may even mediate dramatic neurogenic and antidepressant-like effects following exposure to chronic stress. The current review therefore attempts a synthesis of our knowledge on the neurotrophic and immunological aspects of ECT and other electrically based treatments in psychiatry. Perhaps contrary to contemporary views, we conclude that targeted potentiation, rather than suppression, of inflammatory responses may be of therapeutic relevance to chronically depressed patients or a subgroup thereof.

Lymphocytes from chronically stressed mice confer antidepressant-like effects to naive mice

We examined whether cells of the adaptive immune system retain the memory of psychosocial stress and thereby alter mood states and CNS function in the host. Lymphocytes from mice undergoing chronic social defeat stress or from unstressed control mice were isolated and adoptively transferred into naive lymphopenic Rag2(-/-) mice. Changes in affective behavior, hippocampal cell proliferation, microglial activation states, and blood cytokine levels were examined in reconstituted stress-naive mice. The mice receiving lymphocytes from defeated donors showed less anxiety, more social behavior, and increased hippocampal cell proliferation compared with those receiving no cells or cells from unstressed donors. Mice receiving stressed immune cells had reduced pro-inflammatory cytokine levels in the blood relative to the other groups, an effect opposite to the elevated donor pro-inflammatory cytokine profile. Furthermore, mice receiving stressed immune cells had microglia skewed toward an anti-inflammatory, neuroprotective M2-like phenotype, an effect opposite the stressed donors' M1-like pro-inflammatory profile. However, stress had no effect on lymphocyte surface marker profiles in both donor and recipient mice. The data suggest that chronic stress-induced changes in the adaptive immune system, contrary to conferring anxiety and depressive behavior, protect against the deleterious effects of stress. Improvement in affective behavior is potentially mediated by reduced peripheral pro-inflammatory cytokine load, protective microglial activity, and increased hippocampal cell proliferation. The data identify the peripheral adaptive immune system as putatively involved in the mechanisms underlying stress resilience and a potential basis for developing novel rapid-acting antidepressant therapies.

Current perspectives on the link between neuroinflammation and neurogenesis

The link between neuroinflammation and neurogenesis is an area of intensive research in contemporary neuroscience. The burgeoning amount of evidence accumulated over the past decade has been incredible, and now there remains the figuring out of minutia to give us a more complete picture of what individual, synergistic, and antagonistic events are occurring between neurogenesis and neuroinflammation. An intricate study of the inflammatory microenvironment influenced by the presence of the various inflammatory components like cytokines, chemokines, and immune cells is essential for: 1) understanding how neurogenesis can be affected in such a specialized niche and 2) applying the knowledge gained for the treatment of cognitive and/or motor deficits arising from inflammation-associated diseases like stroke, traumatic brain injury, Alzheimer's disease, and Parkinson's disease. This review is written to provide the reader with up-to-date information explaining how these inflammatory components are effecting changes on neurogenesis.

Role of the immune system in HIV-associated neuroinflammation and neurocognitive implications

Individuals living with HIV who are optimally treated with combination antiretroviral therapy (cART) can now lead an extended life. In spite of this remarkable survival benefit from viral suppression achieved by cART in peripheral blood, the rate of mild to moderate cognitive impairment remains high. A cognitive decline that includes impairments in attention, learning and executive function is accompanied by increased rates of mood disorders that together adversely impact the daily life of those with chronic HIV infection. The evidence is clear that cells in the brain are infected with HIV that has crossed the blood-brain barrier both as cell-free virus and within infected monocytes and T cells. Viral proteins that circulate in blood can induce brain endothelial cells to release cytokines, invoking another source of neuroinflammation. The difficulty of efficient delivery of cART to the central nervous system (CNS) contributes to elevated viral load in the CNS, resulting in a persistent HIV-associated neurocognitive disorders (HAND). The pathogenesis of HAND is multifaceted, and mounting evidence indicates that immune cells play a major role. HIV-infected monocytes and T cells not only infect brain resident cells upon migration into the CNS but also produce proinflammatory cytokines such as TNF and IL-1ß, which in turn, further activate microglia and astrocytes. These activated brain resident cells, along with perivascular macrophages, are the main contributors to neuroinflammation in HIV infection and release neurotoxic factors such as excitatory amino acids and inflammatory mediators, resulting in neuronal dysfunction and death. Cytokines, which are elevated in the blood of patients with HIV infection, may also contribute to brain inflammation by entering the brain from the blood. Host factors such as aging and co-morbid conditions such as cytomegalovirus co-infection and vascular pathology are important factors that affect the HIV-host immune interactions in HAND pathogenesis. By these diverse mechanisms, HIV-1 induces a neuroinflammatory response that is likely to be a major contributor to the cognitive and behavior changes seen in HIV infection.

Effector memory CD4(+) T cells are associated with cognitive performance in a senior population

Objective:

Immunosenescence and cognitive decline are common markers of the aging process. Taking into consideration the heterogeneity observed in aging processes and the recently described link between lymphocytes and cognition, we herein explored the possibility of an association between alterations in lymphocytic populations and cognitive performance.

Methods:

In a cohort of cognitively healthy adults (n = 114), previously characterized by diverse neurocognitive/psychological performance patterns, detailed peripheral blood immunophenotyping of both the innate and adaptive immune systems was performed by flow cytometry.

Results:

Better cognitive performance was associated with lower numbers of effector memory CD4⁺ T cells and higher numbers of naive CD8⁺ T cells and B cells. Furthermore, effector memory CD4⁺ T cells were found to be predictors of general and executive function and memory, even when factors known to influence cognitive performance in older individuals (e.g., age, sex, education, and mood) were taken into account.

Conclusions:

This is the first study in humans associating specific phenotypes of the immune system with distinct cognitive performance in healthy aging.

Mesenchymal stem cells can prevent alterations in behavior and neurogenesis induced by Aß25-35 administration

Mesenchymal stem cells (MSCs) are known to enhance neurogenesis in the dentate gyrus, as well as to modulate immune cell activity and inflammation. Easily obtained and expanded from the bone marrow and other tissues, MSCs have been proposed as candidates for stem cell therapy in various neurodegenerartive diseases. In the present study, we sought to explore these therapeutic properties of MSC on Aß25-35-induced pathology when coadministered together. Apparently, coadministration of MSC prevented mild cognitive deficits observed following Aß administration alone, by promoting microglial activation and rapid clearance of injected Aß aggregates. Surprisingly, increased hippocampal neurogenesis was observed in the Aß-injected animals and was normal in MSC-coadministered animals just as in control animals. The observed increase in neurogenesis can be explained as a compensating mechanism responsible for the mild and temporary cognitive deficits observed in the Morris water maze assay in Aß-injected animals. Interestingly, MSC engrafted not only to the hippocampus but were also detected in the choroid plexus. We thus conclude that MSC may act in multiple pathways to protect the CNS from Aß pathology, while neurogenesis is a possible compensating mechanism; it is not always activated by MSC, which in turn may interact with local immune cells to regulate Aß accumulation.

Probiotics normalize the gut-brain-microbiota axis in immunodeficient mice

The gut-brain-microbiota axis is increasingly recognized as an important regulator of intestinal physiology. Exposure to psychological stress causes activation of the hypothalamic-pituitary-adrenal (HPA) axis and causes altered intestinal barrier function, intestinal dysbiosis, and behavioral changes. The primary aim of this study was to determine whether the effects of psychological stress on intestinal physiology and behavior, including anxiety and memory, are mediated by the adaptive immune system. Furthermore, we wanted to determine whether treatment with probiotics would normalize these effects. Here we demonstrate that B and T cell-deficient Rag1(-/-) mice displayed altered baseline behaviors, including memory and anxiety, accompanied by an overactive HPA axis, increased intestinal secretory state, dysbiosis, and decreased hippocampal c-Fos expression. Both local (intestinal physiology and microbiota) and central (behavioral and hippocampal c-Fos) changes were normalized by pretreatment with probiotics, indicating an overall benefit on health conferred by changes in the microbiota, independent of lymphocytes. Taken together, these findings indicate a role for adaptive immune cells in maintaining normal intestinal and brain health in mice and show that probiotics can overcome this immune-mediated deficit in the gut-brain-microbiota axis.

A phase-specific neuroimmune model of clinical depression

Immune dysfunction and pro-inflammatory states in particular have been implicated in the aetiology and pathogenesis of depression. Whilst the onset of an episode and certain symptoms of depression appear well explained by this inflammatory model, the underpinnings of the episodic and progressive nature, as well as relapse and remission status in depression require attention. In this review it is suggested that additional immune factors beyond pro- and anti-inflammatory cytokines may effectively contribute to the understanding of the neurobiology of clinical depression. Considering neurobiological effects of immunomodulatory factors such as T cells, macrophages, microglia and astrocytes relevant to depression, we suggest a neuroimmune model of depression underpinned by dynamic immunomodulatory processes. This perspective paper then outlines a neuroimmune model of clinical phases of depression in an attempt to more adequately explain depression-like behaviours in pre-clinical models and the dynamic nature of depression in clinical populations. Finally, the implications for immunomodulatory treatments of depression are considered.