Systemic immune-checkpoint blockade with anti-PD1 antibodies does not alter cerebral amyloid-β burden in several amyloid transgenic mouse models

Decoding the role of the CCL2/CCR2 axis in Alzheimer's disease and innovating therapeutic approaches: Keeping All options open

Alzheimer's disease (AD), as a neurodegenerative disorder, distresses the elderly in large numbers and is characterized by β-amyloid (Aβ) accumulation, elevated tau protein levels, and chronic inflammation. The brain's immune system is aided by microglia and astrocytes, which produce chemokines and cytokines. Nevertheless, dysregulated expression can cause hyperinflammation and lead to neurodegeneration. CCL2/CCR2 chemokines are implicated in neurodegenerative diseases exacerbating. Inflicting damage on nerves and central nervous system (CNS) cells is the function of this axis, which recruits and migrates immune cells, including monocytes and macrophages. It has been shown that targeting the CCL2/CCR2 axis may be a therapeutic option for inflammatory diseases. Using the current knowledge about the involvement of the CCL2/CCR2 axis in the immunopathogenesis of AD, this comprehensive review synthesizes existing information. It also explores potential therapeutic options, including modulation of the CCL2/CCR2 axis as a possible strategy in AD.

The path to next-generation disease-modifying immunomodulatory combination therapies in Alzheimer's disease

The cautious optimism following recent anti-amyloid therapeutic trials for Alzheimer's disease (AD) provides a glimmer of hope after years of disappointment. Although these encouraging results represent discernible progress, they also highlight the need to enhance further the still modest clinical efficacy of current disease-modifying immunotherapies. Here, we highlight crucial milestones essential for advancing precision medicine in AD. These include reevaluating the choice of therapeutic targets by considering the key role of both central neuroinflammation and peripheral immunity in disease pathogenesis, refining patient stratification by further defining the inflammatory component within the forthcoming ATN(I) (amyloid, tau and neurodegeneration (and inflammation)) classification of AD biomarkers and defining more accurate clinical outcomes and prognostic biomarkers that better reflect disease heterogeneity. Next-generation immunotherapies will need to go beyond the current antibody-only approach by simultaneously targeting pathological proteins together with innate neuroinflammation and/or peripheral-central immune crosstalk. Such innovative immunomodulatory combination therapy approaches should be evaluated in appropriately redesigned clinical therapeutic trials, which must carefully integrate the neuroimmune component.

PD-L1/PD-1 checkpoint pathway regulates hippocampal neuronal excitability and learning and memory behavior

Programmed death protein 1 (PD-1) and its ligand PD-L1 constitute an immune checkpoint pathway. We report that neuronal PD-1 signaling regulates learning/memory in health and disease. Mice lacking PD-1 (encoded by Pdcd1) exhibit enhanced long-term potentiation (LTP) and memory. Intraventricular administration of anti-mouse PD-1 monoclonal antibody (RMP1-14) potentiated learning and memory. Selective deletion of PD-1 in excitatory neurons (but not microglia) also enhances LTP and memory. Traumatic brain injury (TBI) impairs learning and memory, which is rescued by Pdcd1 deletion or intraventricular PD-1 blockade. Conversely, re-expression of Pdcd1 in PD-1-deficient hippocampal neurons suppresses memory and LTP. Exogenous PD-L1 suppresses learning/memory in mice and the excitability of mouse and NHP hippocampal neurons through PD-1. Notably, neuronal activation suppresses PD-L1 secretion, and PD-L1/PD-1 signaling is distinctly regulated by learning and TBI. Thus, conditions that reduce PD-L1 levels or PD-1 signaling could promote memory in both physiological and pathological conditions.

Beyond the amyloid hypothesis: how current research implicates autoimmunity in Alzheimer's disease pathogenesis

The amyloid hypothesis has so far been at the forefront of explaining the pathogenesis of Alzheimer's Disease (AD), a progressive neurodegenerative disorder that leads to cognitive decline and eventual death. Recent evidence, however, points to additional factors that contribute to the pathogenesis of this disease. These include the neurovascular hypothesis, the mitochondrial cascade hypothesis, the inflammatory hypothesis, the prion hypothesis, the mutational accumulation hypothesis, and the autoimmunity hypothesis. The purpose of this review was to briefly discuss the factors that are associated with autoimmunity in humans, including sex, the gut and lung microbiomes, age, genetics, and environmental factors. Subsequently, it was to examine the rise of autoimmune phenomena in AD, which can be instigated by a blood-brain barrier breakdown, pathogen infections, and dysfunction of the glymphatic system. Lastly, it was to discuss the various ways by which immune system dysregulation leads to AD, immunomodulating therapies, and future directions in the field of autoimmunity and neurodegeneration. A comprehensive account of the recent research done in the field was extracted from PubMed on 31 January 2022, with the keywords "Alzheimer's disease" and "autoantibodies" for the first search input, and "Alzheimer's disease" with "IgG" for the second. From the first search, 19 papers were selected, because they contained recent research on the autoantibodies found in the biofluids of patients with AD. From the second search, four papers were selected. The analysis of the literature has led to support the autoimmune hypothesis in AD. Autoantibodies were found in biofluids (serum/plasma, cerebrospinal fluid) of patients with AD with multiple methods, including ELISA, Mass Spectrometry, and microarray analysis. Through continuous research, the understanding of the synergistic effects of the various components that lead to AD will pave the way for better therapeutic methods and a deeper understanding of the disease.

Current views on meningeal lymphatics and immunity in aging and Alzheimer's disease

Alzheimer's disease (AD) is an aging-related form of dementia associated with the accumulation of pathological aggregates of amyloid beta and neurofibrillary tangles in the brain. These phenomena are accompanied by exacerbated inflammation and marked neuronal loss, which altogether contribute to accelerated cognitive decline. The multifactorial nature of AD, allied to our still limited knowledge of its etiology and pathophysiology, have lessened our capacity to develop effective treatments for AD patients. Over the last few decades, genome wide association studies and biomarker development, alongside mechanistic experiments involving animal models, have identified different immune components that play key roles in the modulation of brain pathology in AD, affecting its progression and severity. As we will relay in this review, much of the recent efforts have been directed to better understanding the role of brain innate immunity, and particularly of microglia. However, and despite the lack of diversity within brain resident immune cells, the brain border tissues, especially the meninges, harbour a considerable number of different types and subtypes of adaptive and innate immune cells. Alongside microglia, which have taken the centre stage as important players in AD research, there is new and exciting evidence pointing to adaptive immune cells, namely T and B cells found in the brain and its meninges, as important modulators of neuroinflammation and neuronal (dys)function in AD. Importantly, a genuine and functional lymphatic vascular network is present around the brain in the outermost meningeal layer, the dura. The meningeal lymphatics are directly connected to the peripheral lymphatic system in different mammalian species, including humans, and play a crucial role in preserving a "healthy" immune surveillance of the CNS, by shaping immune responses, not only locally at the meninges, but also at the level of the brain tissue. In this review, we will provide a comprehensive view on our current knowledge about the meningeal lymphatic vasculature, emphasizing its described roles in modulating CNS fluid and macromolecule drainage, meningeal and brain immunity, as well as glial and neuronal function in aging and in AD.

N-acetylneuraminic acid links immune exhaustion and accelerated memory deficit in diet-induced obese Alzheimer's disease mouse model

Systemic immunity supports lifelong brain function. Obesity posits a chronic burden on systemic immunity. Independently, obesity was shown as a risk factor for Alzheimer's disease (AD). Here we show that high-fat obesogenic diet accelerated recognition-memory impairment in an AD mouse model (5xFAD). In obese 5xFAD mice, hippocampal cells displayed only minor diet-related transcriptional changes, whereas the splenic immune landscape exhibited aging-like CD4+ T-cell deregulation. Following plasma metabolite profiling, we identified free N-acetylneuraminic acid (NANA), the predominant sialic acid, as the metabolite linking recognition-memory impairment to increased splenic immune-suppressive cells in mice. Single-nucleus RNA-sequencing revealed mouse visceral adipose macrophages as a potential source of NANA. In vitro, NANA reduced CD4+ T-cell proliferation, tested in both mouse and human. In vivo, NANA administration to standard diet-fed mice recapitulated high-fat diet effects on CD4+ T cells and accelerated recognition-memory impairment in 5xFAD mice. We suggest that obesity accelerates disease manifestation in a mouse model of AD via systemic immune exhaustion.

Mesenchymal stromal cells for the treatment of Alzheimer’s disease: Strategies and limitations

Alzheimer’s disease (AD) is a major cause of age-related dementia and is characterized by progressive brain damage that gradually destroys memory and the ability to learn, which ultimately leads to the decline of a patient’s ability to perform daily activities. Although some of the pharmacological treatments of AD are available for symptomatic relief, they are not able to limit the progression of AD and have several side effects. Mesenchymal stem/stromal cells (MSCs) could be a potential therapeutic option for treating AD due to their immunomodulatory, anti-inflammatory, regenerative, antioxidant, anti-apoptotic, and neuroprotective effects. MSCs not only secret neuroprotective and anti-inflammatory factors to promote the survival of neurons, but they also transfer functional mitochondria and miRNAs to boost their bioenergetic profile as well as improve microglial clearance of accumulated protein aggregates. This review focuses on different clinical and preclinical studies using MSC as a therapy for treating AD, their outcomes, limitations and the strategies to potentiate their clinical translation.

The Role of Cytotoxic T-Lymphocyte Antigen 4 in the Pathogenesis of Multiple Sclerosis

Multiple sclerosis (MS) is an autoimmune neurodegenerative disorder of the central nervous system that presents heterogeneous clinical manifestations and course. It has been shown that different immune checkpoints, including Cytotoxic T-Lymphocyte Antigen 4 (CTLA-4), can be involved in the pathogenesis of MS. CTLA-4 is a critical regulator of T-cell homeostasis and self-tolerance and represents a key inhibitor of autoimmunity. In this scopingreview, we resume the current preclinical and clinical studies investigating the role of CTLA-4 in MS with different approaches. While some of these studies assessed the expression levels of CTLA-4 on T cells by comparing MS patients with healthy controls, others focused on the evaluation of the effects of common MS therapies on CTLA-4 modulation or on the study of the CTLA-4 blockade or deficiency in experimental autoimmune encephalomyelitis models. Moreover, other studies in this field aimed to discover if the CTLA-4 gene might be involved in the predisposition to MS, whereas others evaluated the effects of treatment with CTLA4-Ig in MS. Although these results are of great interest, they are often conflicting. Therefore, further studies are needed to reveal the exact mechanisms underlying the action of a crucial immune checkpoint such as CTLA-4 in MS to identify novel immunotherapeutic strategies for MS patients.

PD-1/PD-L Axis in Neuroinflammation: New Insights

The approval of immune checkpoint inhibitors (ICIs) by the Food and Drug Administration (FDA) led to an improvement in the treatment of several types of cancer. The main targets of these drugs are cytotoxic T-lymphocyte antigen 4 (CTLA-4) and programmed cell death protein-1/programmed death-ligand 1 pathway (PD-1/PD-L1), which are important inhibitory molecules for the immune system. Besides being generally safer than common chemotherapy, the use of ICIs has been associated with several immune-related adverse effects (irAEs). Although rare, neurological adverse effects are reported within the irAEs in clinical trials, particularly in patients treated with anti-PD-1 antibodies or a combination of both anti-CTLA-4 and PD-1 drugs. The observations obtained from clinical trials suggest that the PD-1 axis may play a remarkable role in the regulation of neuroinflammation. Moreover, numerous studies in preclinical models have demonstrated the involvement of PD-1 in several neurological disorders. However, a comprehensive understanding of these cellular mechanisms remains elusive. Our review aims to summarize the most recent evidence concerning the regulation of neuroinflammation through PD-1/PD-L signaling, focusing on cell populations that are involved in this pathway.

Recent Advances in the Treatment of Alzheimer’s Disease Using Nanoparticle-Based Drug Delivery Systems

Alzheimer’s disease (AD) is an irreversible and progressive neurodegenerative disorder. Most existing treatments only provide symptomatic solutions. Here, we introduce currently available commercial drugs and new therapeutics, including repositioned drugs, to treat AD. Despite tremendous efforts, treatments targeting the hallmarks of AD show limited efficacy. Challenges in treating AD are partly caused by difficulties in penetrating the blood–brain barrier (BBB). Recently, nanoparticle (NP)-based systems have shown promising potential as precision medicines that can effectively penetrate the BBB and enhance the targeting ability of numerous drugs. Here, we describe how NPs enter the brain by crossing, avoiding, or disrupting the BBB. In addition, we provide an overview of the action of NPs in the microenvironment of the brain for the treatment of AD. Diverse systems, including liposomes, micelles, polymeric NPs, solid-lipid NPs, and inorganic NPs, have been investigated for NP drug loading to relieve AD symptoms, target AD hallmarks, and target moieties to diagnose AD. We also highlight NP-based immunotherapy, which has recently gained special attention as a potential treatment option to disrupt AD progression. Overall, this review focuses on recently investigated NP systems that represent innovative strategies to understand AD pathogenesis and suggests treatment and diagnostic modalities to cure AD.

Programmed Cell Death Protein 1 Blockade Reduces Glycogen Synthase Kinase 3β Activity and Tau Hyperphosphorylation in Alzheimer's Disease Mouse Models

Alzheimer’s disease (AD) is a central nervous system degenerative disease, with no effective treatment to date. Administration of immune checkpoint inhibitors significantly reduces neuronal damage and tau hyperphosphorylation in AD, but the specific mechanism is unclear. Here, we found that programmed cell death-receptor 1 (PD1) and its ligand PDL1 were induced by an intracerebroventricular injection of amyloid-β; they were significantly upregulated in the brains of APP/PS1, 5×FAD mice and in SH-SY5Y-APP cell line compared with control. The PD1 and PDL1 levels positively correlated with the glycogen synthase kinase 3 beta (GSK3β) activity in various AD mouse models, and the PDL1-GSK3β immune complex was found in the brain. The application of PD1-blocking antibody reduced tau hyperphosphorylation and GSK3β activity and prevented memory impairments. Mechanistically, we identified PD1 as a critical regulator of GSK3β activity. These results suggest that the immune regulation of the PD1/PDL1 axis is closely involved in AD.

Alzheimer's disease modification mediated by bone marrow-derived macrophages via a TREM2-independent pathway in mouse model of amyloidosis

Microglia and monocyte-derived macrophages (MDM) are key players in dealing with Alzheimer's disease. In amyloidosis mouse models, activation of microglia was found to be TREM2 dependent. Here, using Trem2^-/-5xFAD mice, we assessed whether MDM act via a TREM2-dependent pathway. We adopted a treatment protocol targeting the programmed cell death ligand-1 (PD-L1) immune checkpoint, previously shown to modify Alzheimer's disease via MDM involvement. Blockade of PD-L1 in Trem2^-/-5xFAD mice resulted in cognitive improvement and reduced levels of water-soluble amyloid beta_1-42 with no effect on amyloid plaque burden. Single-cell RNA sequencing revealed that MDM, derived from both Trem2^-/- and Trem2^+/+5xFAD mouse brains, express a unique set of genes encoding scavenger receptors (for example, Mrc1, Msr1). Blockade of monocyte trafficking using anti-CCR2 antibody completely abrogated the cognitive improvement induced by anti-PD-L1 treatment in Trem2^-/-5xFAD mice and similarly, but to a lesser extent, in Trem2^+/+5xFAD mice. These results highlight a TREM2-independent, disease-modifying activity of MDM in an amyloidosis mouse model.

Microglial PD-1 stimulation by astrocytic PD-L1 suppresses neuroinflammation and Alzheimer's disease pathology

Chronic neuroinflammation is a pathogenic component of Alzheimer’s disease (AD) that may limit the ability of the brain to clear amyloid deposits and cellular debris. Tight control of the immune system is therefore key to sustain the ability of the brain to repair itself during homeostasis and disease. The immune‐cell checkpoint receptor/ligand pair PD‐1/PD‐L1, known for their inhibitory immune function, is expressed also in the brain. Here, we report upregulated expression of PD‐L1 and PD‐1 in astrocytes and microglia, respectively, surrounding amyloid plaques in AD patients and in the APP/PS1 AD mouse model. We observed juxtamembrane shedding of PD‐L1 from astrocytes, which may mediate ectodomain signaling to PD‐1‐expressing microglia. Deletion of microglial PD‐1 evoked an inflammatory response and compromised amyloid‐β peptide (Aβ) uptake. APP/PS1 mice deficient for PD‐1 exhibited increased deposition of Aβ, reduced microglial Aβ uptake, and decreased expression of the Aβ receptor CD36 on microglia. Therefore, ineffective immune regulation by the PD‐1/PD‐L1 axis contributes to Aβ plaque deposition during chronic neuroinflammation in AD.

Serum programmed cell death proteins in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a multifactorial, multisystem pro-inflammatory neuromuscular disorder. Activation of programmed cell death-1 (PD-1), and its ligands, programmed cell death-ligand 1 and 2 (PD-L1/L2), leads to immune suppression. Serum soluble forms of these proteins, sPD-1/sPD-L1/sPD-L2, inhibit this suppression and promote pro-inflammatory responses. The purpose of this study was to determine if sPD-1, sPD-L1, and sPD-L2 were increased in sera of patients with ALS. sPD-1 and sPD-L2 were elevated in sera of patients and accurately reflected patients’ disease burdens. Increased sera levels of programmed cell death proteins reinforce the concept that peripheral pro-inflammatory responses contribute to systemic inflammation in patients with ALS.

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Immune regulatory checkpoint pathways play important roles in maintaining the homeostasis of the immune system.

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ALS and tumor pathobiologies may be thought of as opposite ends of a detrimental versus beneficial spectrum of pro-inflammatory immune responses.

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Programmed cell death-1 (PD-1) protein, and its ligands, programmed cell death-ligand 1 and 2 (PD-L1/PD-L2), three proteins involved in these regulatory pathways that suppress activation of immune cells.

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Serum soluble forms of these proteins, sPD-1/sPD-L1/sPD-L2, inhibit this suppression.

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sPD-1 levels were elevated in the sera of patients with ALS.

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sPD-1 was only elevated in sera from fast progressing patients with ALS.

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sPD-L1 was not increased in patients with ALS.

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sPD-L2 was increased in the sera of patients with ALS.

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sPD-L2 was increased in fast and slowly progressing patients with ALS.

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Sera sPD-1 and sPD-L2 positively correlated with patients’ disease burdens.

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sPD-1 positively correlated with sPD-L2.

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Sera sPD-1 and sPD-L2 positively correlated with patients’ sera LBP levels.

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Increased sera levels of programmed cell death proteins reinforce the concept that peripheral pro-inflammatory responses contribute to systemic inflammation in patients with ALS.

Application of immune checkpoint targets in the anti-tumor novel drugs and traditional Chinese medicine development

Immune checkpoints are the crucial regulators of immune system and play essential roles in maintaining self-tolerance, preventing autoimmune responses, and minimizing tissue damage by regulating the duration and intensity of the immune response. Furthermore, immune checkpoints are usually overexpressed in cancer cells or noninvasive cells in tumor tissues and are capable of suppressing the antitumor response. Based on substantial physiological analyses as well as preclinical and clinical studies, checkpoint molecules have been evaluated as potential therapeutic targets for the treatment of multiple types of cancers. In the last few years, extensive evidence has supported the immunoregulatory effects of traditional Chinese medicines (TCMs). The main advantage of TCMs and natural medicine is that they usually contain multiple active components, which can act on multiple targets at the same time, resulting in additive or synergistic effects. The strong immune regulation function of traditional Chinese medicine on immune checkpoints has also been of great interest. For example, Astragalus membranaceus polysaccharides can induce anti-PD-1 antibody responses in animals, and these antibodies can overcome the exhaustion of immune cells under tumor immune evasion. Furthermore, many other TCM molecules could also be novel and effective drug candidates for the treatment of cancers. Therefore, it is essential to assess the application of immune checkpoints in the development of new drugs and TCMs. In this review, we focus on research progress in the field of immune checkpoints based on three topics: (1) immune checkpoint targets and pathways, (2) development of novel immune checkpoint-based drugs, and (3) application of immune checkpoints in the development of TCMs.

Emerging Role of PD-1 in the Central Nervous System and Brain Diseases

Programmed cell death protein 1 (PD-1) is an immune checkpoint modulator and a major target of immunotherapy as anti-PD-1 monoclonal antibodies have demonstrated remarkable efficacy in cancer treatment. Accumulating evidence suggests an important role of PD-1 in the central nervous system (CNS). PD-1 has been implicated in CNS disorders such as brain tumors, Alzheimer's disease, ischemic stroke, spinal cord injury, multiple sclerosis, cognitive function, and pain. PD-1 signaling suppresses the CNS immune response via resident microglia and infiltrating peripheral immune cells. Notably, PD-1 is also widely expressed in neurons and suppresses neuronal activity via downstream Src homology 2 domain-containing protein tyrosine phosphatase 1 and modulation of ion channel function. An improved understanding of PD-1 signaling in the cross-talk between glial cells, neurons, and peripheral immune cells in the CNS will shed light on immunomodulation, neuromodulation, and novel strategies for treating brain diseases.

VISTA regulates microglia homeostasis and myelin phagocytosis, and is associated with MS lesion pathology

V-type immunoglobulin domain-containing suppressor of T-cell activation (VISTA) is a negative checkpoint regulator (NCR) that is involved in T-cell quiescence, inhibition of T-cell activation, and in myeloid cells regulates cytokine production, chemotaxis, phagocytosis, and tolerance induction. In the central nervous system (CNS), VISTA is expressed by microglia, the resident macrophage of the parenchyma, and expression is decreased during neuroinflammation; however, the function of VISTA in microglia is unknown. Here, we extensively analyzed VISTA expression in different MS lesion stages and characterized the function of VISTA in the CNS by deleting VISTA in microglia. VISTA is differentially expressed in distinct MS lesion stages. In mice, VISTA deletion in Cx3Cr1-expressing cells induced a more amoeboid microglia morphology, indicating an immune-activated phenotype. Expression of genes associated with cell cycle and immune-activation was increased in VISTA KO microglia. In response to LPS and during experimental autoimmune encephalomyelitis (EAE), VISTA KO and WT microglia shared similar transcriptional profiles and VISTA deletion did not affect EAE disease progression or microglia responses. VISTA KO in microglia in vitro decreased the uptake of myelin. This study demonstrates that VISTA is involved in microglia function, which likely affects healthy CNS homeostasis and neuroinflammation.

Neuroinflammation in Alzheimer's Disease

Alzheimer’s disease (AD) is a neurodegenerative disease associated with human aging. Ten percent of individuals over 65 years have AD and its prevalence continues to rise with increasing age. There are currently no effective disease modifying treatments for AD, resulting in increasingly large socioeconomic and personal costs. Increasing age is associated with an increase in low-grade chronic inflammation (inflammaging) that may contribute to the neurodegenerative process in AD. Although the exact mechanisms remain unclear, aberrant elevation of reactive oxygen and nitrogen species (RONS) levels from several endogenous and exogenous processes in the brain may not only affect cell signaling, but also trigger cellular senescence, inflammation, and pyroptosis. Moreover, a compromised immune privilege of the brain that allows the infiltration of peripheral immune cells and infectious agents may play a role. Additionally, meta-inflammation as well as gut microbiota dysbiosis may drive the neuroinflammatory process. Considering that inflammatory/immune pathways are dysregulated in parallel with cognitive dysfunction in AD, elucidating the relationship between the central nervous system and the immune system may facilitate the development of a safe and effective therapy for AD. We discuss some current ideas on processes in inflammaging that appear to drive the neurodegenerative process in AD and summarize details on a few immunomodulatory strategies being developed to selectively target the detrimental aspects of neuroinflammation without affecting defense mechanisms against pathogens and tissue damage.

Neuroinflammation in Prion Disease

Neuroinflammation, typically manifest as microglial activation and astrogliosis accompanied by transcriptomic alterations, represents a common hallmark of various neurodegenerative conditions including prion diseases. Microglia play an overall neuroprotective role in prion disease, whereas reactive astrocytes with aberrant phenotypes propagate prions and contribute to prion-induced neurodegeneration. The existence of heterogeneous subpopulations and dual functions of microglia and astrocytes in prion disease make them potential targets for therapeutic intervention. A variety of neuroinflammation-related molecules are involved in prion pathogenesis. Therapeutics targeting neuroinflammation represents a novel approach to combat prion disease. Deciphering neuroinflammation in prion disease will deepen our understanding of pathogenesis of other neurodegenerative disorders.

Influenza vaccine combined with moderate-dose PD1 blockade reduces amyloid-β accumulation and improves cognition in APP/PS1 mice

Immune dysfunction is implicated in Alzheimer's disease (AD), whereas systemic immune modulation may be neuroprotective. Our previous results have indicated immune challenge with Bacillus Calmette-Guerin attenuates AD pathology in animal models by boosting the systemic immune system. Similarly, independent studies have shown that boosting systemic immune system, by blocking PD-1 checkpoint pathway, modifies AD. Here we hypothesized that influenza vaccine would potentiate function of moderate dose anti-PD-1 and therefore combining them might allow reducing the dose of PD-1 antibody needed to modify the disease. We found that moderate-dose PD-1 in combination with influenza vaccine effectively attenuated cognitive deficit and prevented amyloid-β pathology build-up in APP/PS1 mice in a mechanism dependent on recruitment of peripheral monocyte-derived macrophages into the brain. Eliminating peripheral macrophages abrogated the beneficial effect. Moreover, by comparing CD11b⁺ compartments in the mouse parenchyma, we observed an elevated subset of Ly6C⁺ microglia-like cells, which are reportedly derived from peripheral monocytes. In addition, myeloid-derived suppressor cells are strongly elevated in the transgenic model used and normalized by combination treatment, indicating restoration of brain immune homeostasis. Overall, our results suggest that revitalizing brain immunity by combining IV with moderate-dose PD-1 inhibition may represent a therapeutic immunotherapy for AD.

Repositioning of Immunomodulators: A Ray of Hope for Alzheimer's Disease?

Alzheimer's disease (AD) is the most common age-related neurodegenerative disorder characterized by cognitive decline and by the presence of amyloid β plaques and neurofibrillary tangles in the brain. Despite recent advances in understanding its pathophysiological mechanisms, to date, there are no disease-modifying therapeutic options, to slow or halt the evolution of neurodegenerative processes in AD. Current pharmacological treatments only transiently mitigate the severity of symptoms, with modest or null overall improvement. Emerging evidence supports the concept that AD is affected by the impaired ability of the immune system to restrain the brain's pathology. Deep understanding of the relationship between the nervous and the immune system may provide a novel arena to develop effective and safe drugs for AD treatment. Considering the crucial role of inflammatory/immune pathways in AD, here we discuss the current status of the immuno-oncological, immunomodulatory and anti-TNF-α drugs which are being used in preclinical studies or in ongoing clinical trials by means of the drug-repositioning approach.

PD-1 Regulates GABAergic Neurotransmission and GABA-Mediated Analgesia and Anesthesia

The immune checkpoint inhibitor programmed cell death protein 1 (PD-1) plays a critical role in immune regulation. Recent studies have demonstrated functional PD-1 expression in peripheral sensory neurons, which contributes to neuronal excitability, pain, and opioid analgesia. Here we report neuronal expression and function of PD-1 in the central nervous system (CNS), including the spinal cord, thalamus, and cerebral cortex. Notably, GABA-induced currents in spinal dorsal horn neurons, thalamic neurons, and cortical neurons are suppressed by the PD-1-neutralizing immunotherapeutic Nivolumab in spinal cord slices, brain slices, and dissociated cortical neurons. Reductions in GABA-mediated currents in CNS neurons were also observed in Pd1^−/− mice without changes in GABA receptor expression. Mechanistically, Nivolumab binds spinal cord neurons and elicits ERK phosphorylation to suppress GABA currents. Finally, both GABA-mediated analgesia and anesthesia are impaired by Pd1 deficiency. Our findings reveal PD-1 as a CNS-neuronal inhibitor that regulates GABAergic signaling and GABA-mediated behaviors.

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Pd1 mRNA and PD-1 protein are widely expressed in spinal cord and brain neurons

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GABA-induced currents in CNS neurons are suppressed by PD-1 blockade with Nivolumab

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Nivolumab binds neuronal PD-1 to induce ERK activation and GABAergic inhibition

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GABA-mediated pain inhibition and anesthesia is impaired after Pd1 deficiency

Exploring the VISTA of microglia: immune checkpoints in CNS inflammation

Negative checkpoint regulators (NCR) are intensely pursued as targets to modulate the immune response in cancer and autoimmunity. A large variety of NCR is expressed by central nervous system (CNS)-resident cell types and is associated with CNS homeostasis, interactions with peripheral immunity and CNS inflammation and disease. Immunotherapy blocking NCR affects the CNS as patients can develop neurological issues including encephalitis and multiple sclerosis (MS). How these treatments affect the CNS is incompletely understood, since expression and function of NCR in the CNS are only beginning to be unravelled. V-type immunoglobulin-like suppressor of T cell activation (VISTA) is an NCR that is expressed primarily in the haematopoietic system by myeloid and T cells. VISTA regulates T cell quiescence and activation and has a variety of functions in myeloid cells including efferocytosis, cytokine response and chemotaxis. In the CNS, VISTA is predominantly expressed by microglia and macrophages of the CNS. In this review, we summarize the role of NCR in the CNS during health and disease. We highlight expression of VISTA across cell types and CNS diseases and discuss the function of VISTA in microglia and during CNS ageing, inflammation and neurodegeneration. Understanding the role of VISTA and other NCR in the CNS is important considering the adverse effects of immunotherapy on the CNS, and in view of their therapeutic potential in CNS disease.

Inhibition of TLR4 Induces M2 Microglial Polarization and Provides Neuroprotection via the NLRP3 Inflammasome in Alzheimer's Disease

Accumulating evidence has indicated that activation of microglia and neuroinflammation reaction play a prominent role in Alzheimer’s disease (AD). Inhibition of toll-like receptor 4 (TLR4) has been shown to be associated with immune responses and brain damage, but its effects on AD remain unclear. This study mainly aimed to investigate the protective effect of TAK-242 (TLR4-specific inhibitor) on microglial polarization and neuroprotection in an AD mouse model and the underlying mechanisms. We found that APP/PS1 transgenic AD mice exhibited a dramatic increase in TLR4 levels concomitant with a significantly higher expression of inflammatory microglia compared to C57BL/6 wild-type mice. Furthermore, inhibition of TLR4 by TAK-242 administration significantly improved neurological function, decreased the level of Bax, and caused a significant reduction in the levels of M1-markers (iNOS and TNFα), while the expressions of M2-phenotype markers (Trem-2 and Arg-1) were increased both in vivo and in vitro. Furthermore, TAK-242 treatment enhanced BV2 microglial phagocytosis. Moreover, Aβ_25–35 caused the upregulation of inflammatory cytokine production, MyD88, NF-kappaB-p65, and NLRP3, which could be ameliorated by NLRP3-siRNA or TAK-242. These findings indicated that TLR4 inhibition provided neuroprotection and promoted a microglial switch from the inflammatory M1 phenotype to the protective M2 phenotype in AD. The mechanism involved may be related to modulation of the MyD88/NF-kappaB/NLRP3 signaling pathway.

Neuroprotective effect of tormentic acid against memory impairment and neuro‑inflammation in an Alzheimer's disease mouse model

Cognitive impairment and neuro-inflammatory responses are the distinctive characteristics of Alzheimer's disease (AD). Tormentic acid (TA) is one of the major active components of Potentilla chinensis and has been demonstrated to have anti-inflammatory properties. However, the potential effects of TA on neuro-inflammatory responses and memory impairment in AD remain unknown. The present study investigated the therapeutic effect of TA on neuro-inflammation, as well as learning and memory impairment in AD mice. In addition, the effects of TA treatment were also examined in a co-culture system of microglia and primary neurons. Intraperitoneal administration of TA attenuated memory deficits in amyloid β precursor protein/presenilin 1 transgenic mice, with a marked decrease in amyloid plaque deposition. TA also reduced microglial activation and decreased the secretion of pro-inflammatory factors in AD mice. Furthermore, pre-treatment with TA suppressed the production of pro-inflammatory markers, as well as the nuclear translocation of nuclear factor-κB (NF-κB) p65 induced by Aβ exposure in BV2 cells. TA also reduced inhibited neurotoxicity and improved neuron survival in a neuron-microglia co-culture system. Taken together, these findings suggested that TA could attenuate neuro-inflammation and memory impairment, which may be closely associated with regulation of the NF-κB pathway.

Learning from Principles of Evidence-Based Medicine to Optimize Nonclinical Research Practices

Thousands of pharmacology experiments are performed each day, generating hundreds of drug discovery programs, scientific publications, grant submissions, and other efforts. Discussions of the low reproducibility and robustness of some of this research have led to myriad efforts to increase data quality and thus reliability. Across the scientific ecosystem, regardless of the extent of concerns, debate about solutions, and differences among goals and practices, scientists strive to provide reliable data to advance frontiers of knowledge. Here we share our experience of current practices in nonclinical neuroscience research across biopharma and academia, examining context-related factors and behaviors that influence ways of working and decision-making. Drawing parallels with the principles of evidence-based medicine, we discuss ways of improving transparency and consider how to better implement best research practices. We anticipate that a shared framework of scientific rigor, facilitated by training, enabling tools, and enhanced data sharing, will draw the conversation away from data unreliability or lack of reproducibility toward the more important discussion of how to generate data that advances knowledge and propels innovation.

Chronic PD-1 Checkpoint Blockade Does Not Affect Cognition or Promote Tau Clearance in a Tauopathy Mouse Model

Programmed cell death protein 1 (PD-1) checkpoint blockade with an antibody has been shown to reduce amyloid-β plaques, associated pathologies and cognitive impairment in mouse models. More recently, this approach has shown effectiveness in a tauopathy mouse model to improve cognition and reduce tau lesions. Follow-up studies by other laboratories did not see similar benefits of this type of therapy in other amyloid-β plaque models. Here, we report a modest increase in locomotor activity but no effect on cognition or tau pathology, in a different more commonly used tauopathy model following a weekly treatment for 12 weeks with the same PD-1 antibody and isotype control as in the original Aβ- and tau-targeting studies. These findings indicate that further research is needed before clinical trials based on PD-1 checkpoint immune blockage are devised for tauopathies.

Gene delivery of a modified antibody to Aβ reduces progression of murine Alzheimer's disease

Antibody therapies for Alzheimer’s Disease (AD) hold promise but have been limited by the inability of these proteins to migrate efficiently across the blood brain barrier (BBB). Central nervous system (CNS) gene transfer by vectors like adeno-associated virus (AAV) overcome this barrier by allowing the bodies’ own cells to produce the therapeutic protein, but previous studies using this method to target amyloid-β have shown success only with truncated single chain antibodies (Abs) lacking an Fc domain. The Fc region mediates effector function and enhances antigen clearance from the brain by neonatal Fc receptor (FcRn)-mediated reverse transcytosis and is therefore desirable to include for such treatments. Here, we show that single chain Abs fused to an Fc domain retaining FcRn binding, but lacking Fc gamma receptor (FcγR) binding, termed a silent scFv-IgG, can be expressed and released into the CNS following gene transfer with AAV. While expression of canonical IgG in the brain led to signs of neurotoxicity, this modified Ab was efficiently secreted from neuronal cells and retained target specificity. Steady state levels in the brain exceeded peak levels obtained by intravenous injection of IgG. AAV-mediated expression of this scFv-IgG reduced cortical and hippocampal plaque load in a transgenic mouse model of progressive β-amyloid plaque accumulation. These findings suggest that CNS gene delivery of a silent anti-Aβ scFv-IgG was well-tolerated, durably expressed and functional in a relevant disease model, demonstrating the potential of this modality for the treatment of Alzheimer’s disease.

Interferons: A molecular switch between damage and repair in ageing and Alzheimer's disease

Alzheimer's disease was first described over 100 years ago, yet it remains incurable and affects 44 million people worldwide. Traditionally, research has largely focused on the amyloid cascade hypothesis, but interest in the importance of inflammation in the progression of the disease has recently been increasing. Interferons, a large family of cytokines that trigger the immune system, are believed to play a crucial role in the pathology of Alzheimer's disease. This review focuses on how interferons affect the brain during ageing and whether they could be candidate therapeutic targets for the treatment of Alzheimer's disease.

Microglial Drug Targets in AD: Opportunities and Challenges in Drug Discovery and Development

Alzheimer’s disease (AD) is a large and increasing unmet medical need with no disease-modifying treatment currently available. Genetic evidence from genome-wide association studies (GWASs) and gene network analysis has clearly revealed a key role of the innate immune system in the brain, of which microglia are the most important element. Single-nucleotide polymorphisms (SNPs) in genes predominantly expressed in microglia have been associated with altered risk of developing AD. Furthermore, microglia-specific pathways are affected on the messenger RNA (mRNA) expression level in post-mortem AD tissue and in mouse models of AD. Together these findings have increased the interest in microglia biology, and numerous scientific reports have proposed microglial molecules and pathways as drug targets for AD. Target identification and validation are generally the first steps in drug discovery. Both target validation and drug lead identification for central nervous system (CNS) targets and diseases entail additional significant obstacles compared to peripheral targets and diseases. This makes CNS drug discovery, even with well-validated targets, challenging. In this article, we will illustrate the special challenges of AD drug discovery by discussing the viability/practicality of possible microglia drug targets including cluster of differentiation 33 (CD33), K_Ca3.1, kynurenines, ionotropic P2 receptor 7 (P2X7), programmed death-1 (PD-1), Toll-like receptors (TLRs), and triggering receptor expressed in myeloid cells 2 (TREM2).

Neuroimmune nexus of depression and dementia: Shared mechanisms and therapeutic targets

Dysfunctional immune activity is a physiological component of both Alzheimer's disease (AD) and major depressive disorder (MDD). The extent to which altered immune activity influences the development of their respective cognitive symptoms and neuropathologies remains under investigation. It is evident, however, that immune activity affects neuronal function and circuit integrity. In both disorders, alterations are present in similar immune networks and neuroendocrine signalling pathways, immune responses persist in overlapping neuroanatomical locations, and morphological and structural irregularities are noted in similar domains. Epidemiological studies have also linked the two disorders, and their genetic and environmental risk factors intersect along immune-activating pathways and can be synonymous with one another. While each of these disorders individually contains a large degree of heterogeneity, their shared immunological components may link distinct phenotypes within each disorder. This review will therefore highlight the shared immune pathways of AD and MDD, their overlapping neuroanatomical features, and previously applied, as well as novel, approaches to pharmacologically manipulate immune pathways, in each neurological condition. LINKED ARTICLES: This article is part of a themed section on Therapeutics for Dementia and Alzheimer's Disease: New Directions for Precision Medicine. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.18/issuetoc.

PD-1/PD-L1 checkpoint blockade harnesses monocyte-derived macrophages to combat cognitive impairment in a tauopathy mouse model

Alzheimer's disease (AD) is a heterogeneous disorder with multiple etiologies. Harnessing the immune system by blocking the programmed cell death receptor (PD)-1 pathway in an amyloid beta mouse model was shown to evoke a sequence of immune responses that lead to disease modification. Here, blocking PD-L1, a PD-1 ligand, was found to have similar efficacy to that of PD-1 blocking in disease modification, in both animal models of AD and of tauopathy. Targeting PD-L1 in a tau-driven disease model resulted in increased immunomodulatory monocyte-derived macrophages within the brain parenchyma. Single cell RNA-seq revealed that the homing macrophages expressed unique scavenger molecules including macrophage scavenger receptor 1 (MSR1), which was shown here to be required for the effect of PD-L1 blockade in disease modification. Overall, our results demonstrate that immune checkpoint blockade targeting the PD-1/PD-L1 pathway leads to modification of common factors that go awry in AD and dementia, and thus can potentially provide an immunotherapy to help combat these diseases.

VISTA expression by microglia decreases during inflammation and is differentially regulated in CNS diseases

V-type immunoglobulin domain-containing suppressor of T-cell activation (VISTA) is a negative checkpoint regulator (NCR) involved in inhibition of T cell-mediated immunity. Expression changes of other NCRs (PD-1, PD-L1/L2, CTLA-4) during inflammation of the central nervous system (CNS) were previously demonstrated, but VISTA expression in the CNS has not yet been explored. Here, we report that in the human and mouse CNS, VISTA is most abundantly expressed by microglia, and to lower levels by endothelial cells. Upon TLR stimulation, VISTA expression was reduced in primary neonatal mouse and adult rhesus macaque microglia in vitro. In mice, microglial VISTA expression was reduced after lipopolysaccharide (LPS) injection, during experimental autoimmune encephalomyelitis (EAE), and in the accelerated aging Ercc1 Δ/- mouse model. After LPS injection, decreased VISTA expression in mouse microglia was accompanied by decreased acetylation of lysine residue 27 in histone 3 in both its promoter and enhancer region. ATAC-sequencing indicated a potential regulation of VISTA expression by Pu.1 and Mafb, two transcription factors crucial for microglia function. Finally, our data suggested that VISTA expression was decreased in microglia in multiple sclerosis lesion tissue, whereas it was increased in Alzheimer's disease patients. This study is the first to demonstrate that in the CNS, VISTA is expressed by microglia, and that VISTA is differentially expressed in CNS pathologies.

Targeting neuro-immune communication in neurodegeneration: Challenges and opportunities

Deczkowska and Schwartz describe how harnessing the immune system in a well-controlled manner can be used as a universal therapeutic approach to neurodegeneration.

Immune cells patrol the brain and can support its function, but can we modulate brain–immune communication to fight neurological diseases? Here, we briefly discuss the mechanisms orchestrating the cross-talk between the brain and the immune system and describe how targeting this interaction in a well-controlled manner could be developed as a universal therapeutic approach to treat neurodegeneration.

Peripheral immune system in aging and Alzheimer's disease

Alzheimer's disease (AD) represents an urgent public health mandate. AD is no longer considered a neural-centric disease; rather, a plethora of recent studies strongly implicate a critical role played by neuroinflammation in the pathogeneses of AD and other neurodegenerative conditions. A close functional connection between the immune system and central nervous system is increasingly recognized. In late-onset AD, aging represents the most significant risk factor. Here, from an immunological perspective, we summarize the prominent molecular and cellular changes in the periphery of aging individuals and AD patients. Moreover, we review the knowledge gained in the past several years that implicate specific arms of the peripheral immune system and other types of immune responses in modulating AD progression. Taken together, these findings collectively emphasize a dynamic role of a concert of brain-extrinsic, peripheral signals in the aging and degenerative processes in the CNS. We believe that a systematic view synthesizing the vast amounts of existing results will help guide the development of next-generation therapeutics and inform future directions of AD investigation.

Lymphocyte activation gene 3 (Lag3) expression is increased in prion infections but does not modify disease progression

Prion diseases, Alzheimer’s disease and Parkinson’s disease (PD) are fatal degenerative disorders that share common neuropathological and biochemical features, including the aggregation of pathological protein conformers. Lymphocyte activation gene 3 (Lag3, also known as CD223) is a member of the immunoglobulin superfamily of receptors expressed on peripheral immune cells, microglia and neurons, which serves as a receptor for α-synuclein aggregates in PD. Here we examined the possible role of Lag3 in the pathogenesis of prion diseases. Through quantitative real-time PCR and RNA-sequencing, we found that the expression levels of Lag3 were relatively low in the adult mouse brains, yet its expression was increased after prion infection. However, we failed finding significant differences regarding the incubation time, PrP^Sc load, neurodegeneration, astrocyte and microglia reactions and inflammatory gene expression between the Lag3 knockout mice and wild-type littermate controls after prion infection. We conclude that loss of Lag3 has no significant influence on prion disease pathogenesis. Considering that Lag3 is an immune checkpoint receptor, our results suggest that immune checkpoint inhibition (an increasingly prevalent therapeutic modality against many types of cancer) might not exert positive or negative effects on the progression of prion diseases.

PD-1 deficiency is not sufficient to induce myeloid mobilization to the brain or alter the inflammatory profile during chronic neurodegeneration

Innate immune activation is a major driver of neurodegenerative disease and immune regulatory pathways could be potential targets for therapeutic intervention. Recently, Programmed cell death-1 (PD-1) immune checkpoint inhibition has been proposed to mount an IFN-γ-dependent systemic immune response, leading to the recruitment of peripheral myeloid cells to the brain and neuropathological and functional improvements in mice with Alzheimer's disease-like β-amyloid pathology. Here we investigate the impact of PD-1 deficiency on murine prion disease (ME7 strain), a model of chronic neurodegeneration. Although PD-1 was found to be increased in the brain of prion mice, the absence of PD-1 did not cause myeloid cell infiltration into the brain or major changes in the inflammatory profile. However, we observed a slight exacerbation of the behavioural phenotype of ME7 mice upon PD-1 deficiency. These results do not support the possibility of using immune checkpoint blockade as a therapeutic strategy in neurodegenerative disease.