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

Targeting CD38 immunometabolic checkpoint improves metabolic fitness and cognition in a mouse model of Alzheimer's disease

Protective immunity, essential for brain maintenance and repair, may be compromised in Alzheimer's disease (AD). Here, using high-dimensional single-cell mass cytometry, we find a unique immunometabolic signature in circulating CD4+ T cells preceding symptom onset in individuals with familial AD, featured by the elevation of CD38 expression. Using female 5xFAD mice, a mouse model of AD, we show that treatment with an antibody directed to CD38 leads to restored metabolic fitness, improved cognitive performance, and attenuated local neuroinflammation. Comprehensive profiling across distinct immunological niches in 5xFAD mice, reveals a high level of disease-associated CD4+ T cells that produce IL-17A in the dural meninges, previously linked to cognitive decline. Targeting CD38 leads to abrogation of meningeal TH17 immunity and cortical IL-1β, breaking the negative feedback loop between these two compartments. Taken together, the present findings suggest CD38 as an immunometabolic checkpoint that could be adopted as a pre-symptomatic biomarker for early diagnosis of AD, and might also be therapeutically targeted alone or in combination with other immunotherapies for disease modification.

Aging and neurodegeneration: when systemic dysregulations affect brain macrophage heterogeneity

Microglia, the major population of brain resident macrophages, differentiate from yolk sac progenitors in the embryo and play multiple nonimmune roles in brain organization throughout development and life. Various microglia subtypes have been described by transcriptomic and proteomic signatures, involved metabolic pathways, morphology, intracellular complexity, time of residency, and ontogeny, both in development and in disease settings. Such macrophage heterogeneity increases with aging or neurodegeneration. Monocytes' infiltration and differentiation into monocyte-derived macrophages (MDMs) in the brain contribute to this diversity. Microbiota's role in brain diseases has been recently highlighted, revealing how microbial signals, such as metabolites, influence microglia and MDMs. In this brief review, we describe how these signals can influence microglia through their sensome and shape MDMs from their development in the bone marrow to their differentiation in the brain. Monocytes could then be a crucial player in the constitution of a dysbiotic gut-brain axis in neurodegenerative diseases and aging.

Monocyte-derived macrophages act as reinforcements when microglia fall short in Alzheimer's disease

The central nervous system (CNS) is endowed with its own resident innate immune cells, the microglia. They constitute approximately 10% of the total cells within the CNS parenchyma and act as 'sentinels', sensing and mitigating any deviation from homeostasis. Nevertheless, under severe acute or chronic neurological injury or disease, microglia are unable to contain the damage, and the reparative activity of monocyte-derived macrophages (MDMs) is required. The failure of the microglia under such conditions could be an outcome of their prolonged exposure to hostile stimuli, leading to their exhaustion or senescence. Here, we describe the conditions under which the microglia fall short, focusing mainly on the context of Alzheimer's disease, and shed light on the functions performed by MDMs. We discuss whether and how MDMs engage in cross-talk with the microglia, why their recruitment is often inadequate, and potential ways to augment their homing to the brain in a well-controlled manner.

Peripheral and central neuroimmune mechanisms in Alzheimer's disease pathogenesis

Alzheimer's disease (AD) poses a growing global health challenge as populations age. Recent research highlights the crucial role of peripheral immunity in AD pathogenesis. This review explores how blood-brain barrier disruption allows peripheral immune cells to infiltrate the central nervous system (CNS), worsening neuroinflammation and disease progression. We examine recent findings on interactions between peripheral immune cells and CNS-resident microglia, forming a self-perpetuating inflammatory cycle leading to neuronal dysfunction. Moreover, this review emphasizes recent developments in the dysregulation of immune factors from both the periphery and CNS, and their impact on AD progression. With ongoing research and development of new therapeutic strategies, this review underscores the importance of modulating interactions between the peripheral immune system and CNS in AD therapy.

Immune mechanisms and shared immune targets in neurodegenerative diseases

The immune system plays a major part in neurodegenerative diseases. In some, such as multiple sclerosis, it is the primary driver of the disease. In others, such as Alzheimer disease, amyotrophic lateral sclerosis and Parkinson disease, it has an amplifying role. Immunotherapeutic approaches that target the adaptive and innate immune systems are being explored for the treatment of almost all neurological diseases, and the targets and approaches are often common across diseases. Microglia are the primary immune cells in the brain that contribute to disease pathogenesis, and are consequently a common immune target for therapy. Other therapeutic approaches target components of the peripheral immune system, such as regulatory T cells and monocytes, which in turn act within the CNS. This Review considers in detail how microglia, monocytes and T cells contribute to the pathogenesis of multiple sclerosis, Alzheimer disease, amyotrophic lateral sclerosis and Parkinson disease, and their potential as shared therapeutic targets across these diseases. The microbiome is also highlighted as an emerging therapeutic target that indirectly modulates the immune system. Therapeutic approaches being developed to target immune function in neurodegenerative diseases are discussed, highlighting how immune-based approaches developed to treat one disease could be applicable to multiple other neurological diseases.

Single-cell transcriptomics identifies the common perturbations of monocyte/macrophage lineage cells in inflammaging of bone marrow

Background

Bone marrow inflammaging is a low-grade chronic inflammation that induces bone marrow aging. Multiple age-related and inflammatory diseases involve bone marrow inflammaging. Whether common pathological pathways exist in bone marrow inflammaging remains unclear.

Methods

We collected bone marrow from telomerase-deficient mice (telomerase RNA component, TERCko/ko), 5 × FAD mice and Dmp1 Cre -DTA ki/wt mice and High-fat diet-fed mice (HFD), and lumbar 5 nerve compression mice. We performed scRNA-Seq analysis on bone marrow obtained from these mouse models to investigate the potential shared pathway of bone marrow inflammation.

Results

We identified the monocyte/macrophage lineage was activated via the App-Cd74 axis in multiple aging and inflammatory mouse models. Increased expression of CD38 and Ly6a, and decreased expression of Col1a and Lif in macrophages serve as shared changes in different mouse models. The activated macrophages, interacting with other cells, control the expansion of B cells via the CD52-Siglec-G axis. The Ccl6-Ccr2 and Ccl9-Ccr1 ligand-receptor pairs, along with Fn1 and C3-related pathways in macrophages, were associated with immune cell activation and the recruitment of lymphocytes. Interactions with mesenchymal cells were enriched for integrins (Itga4), Fn1, and adhesion molecules (Vcam1).

Conclusion

Our study demonstrates that monocyte/macrophage lineage stimulation is a key event in bone marrow inflammaging. We identified common differentially expressed genes and activated pathways in this lineage, suggesting potential targets for future interventions.

The translational potential of this article

Our study revealed shared genes and ligand-receptor pairs in the activated monocyte/macrophage lineage within inflammaging bone marrow. These findings offer potential therapeutic targets for cell-specific anti-inflammatory treatments.

Lipid-associated macrophages between aggravation and alleviation of metabolic diseases

Lipid-associated macrophages (LAMs) are phagocytic cells with lipid-handling capacity identified in various metabolic derangements. During disease development, they locate to atherosclerotic plaques, adipose tissue (AT) of individuals with obesity, liver lesions in steatosis and steatohepatitis, and the intestinal lamina propria. LAMs can also emerge in the metabolically demanding microenvironment of certain tumors. In this review, we discuss major questions regarding LAM recruitment, differentiation, and self-renewal, and, ultimately, their acute and chronic functional impact on the development of metabolic diseases. Further studies need to clarify whether and under which circumstances LAMs drive disease progression or resolution and how their phenotype can be modulated to ameliorate metabolic disorders.

Boosting peripheral immunity to fight neurodegeneration in the brain

Reciprocal communication between the brain and the immune system is essential for maintaining lifelong brain function. This interaction is mediated, at least in part, by immune cells recruited from both the circulation and niches at the borders of the brain. Here, we describe how immune exhaustion and senescence, even if not primary causative factors, can accelerate neurodegenerative diseases. We emphasize the role of a compromised peripheral immune system in driving neurodegeneration and discuss strategies for harnessing peripheral immunity to effectively treat neurodegenerative diseases, including the underlying mechanisms and opportunities for clinical translation. Specifically, we highlight the potential of boosting the immune system by blocking inhibitory checkpoint molecules to harness reparative immune cells in helping the brain to fight against neurodegeneration.

Identification of senescent, TREM2-expressing microglia in aging and Alzheimer's disease model mouse brain

Alzheimer's disease (AD) and dementia in general are age-related diseases with multiple contributing factors, including brain inflammation. Microglia, and specifically those expressing the AD risk gene TREM2, are considered important players in AD, but their exact contribution to pathology remains unclear. In this study, using high-throughput mass cytometry in the 5×FAD mouse model of amyloidosis, we identified senescent microglia that express high levels of TREM2 but also exhibit a distinct signature from TREM2-dependent disease-associated microglia (DAM). This senescent microglial protein signature was found in various mouse models that show cognitive decline, including aging, amyloidosis and tauopathy. TREM2-null mice had fewer microglia with a senescent signature. Treating 5×FAD mice with the senolytic BCL2 family inhibitor ABT-737 reduced senescent microglia, but not the DAM population, and this was accompanied by improved cognition and reduced brain inflammation. Our results suggest a dual and opposite involvement of TREM2 in microglial states, which must be considered when contemplating TREM2 as a therapeutic target in AD.

Brain border-associated macrophages: common denominators in infection, aging, and Alzheimer's disease?

Mammalian brain border-associated macrophages (BAMs) are strategically positioned to support vital properties and processes: for example, the composition of the brain's perivascular extracellular matrix and cerebrospinal fluid flow via the glymphatic pathway. BAMs also effectively restrict the spread of infectious microbes into the brain. However, while fighting infections, BAMs sustain long-term transcriptomic changes and can be replaced by inflammatory monocytes, potentially leading to a gradual loss of their beneficial homeostatic functions. We hypothesize that by expediting the deterioration of BAMs, multiple infection episodes might be associated with accelerated brain aging and the putative development of neurodegenerative diseases. Our viewpoint is supported by recent studies suggesting that rejuvenating aged BAMs, and counterbalancing their detrimental inflammatory signatures during infections, might hold promise in treating aging-related neurological disorders, including Alzheimer's disease (AD).

Towards early diagnosis of Alzheimer's disease: advances in immune-related blood biomarkers and computational approaches

Alzheimer's disease has an increasing prevalence in the population world-wide, yet current diagnostic methods based on recommended biomarkers are only available in specialized clinics. Due to these circumstances, Alzheimer's disease is usually diagnosed late, which contrasts with the currently available treatment options that are only effective for patients at an early stage. Blood-based biomarkers could fill in the gap of easily accessible and low-cost methods for early diagnosis of the disease. In particular, immune-based blood-biomarkers might be a promising option, given the recently discovered cross-talk of immune cells of the central nervous system with those in the peripheral immune system. Here, we give a background on recent advances in research on brain-immune system cross-talk in Alzheimer's disease and review machine learning approaches, which can combine multiple biomarkers with further information (e.g. age, sex, APOE genotype) into predictive models supporting an earlier diagnosis. In addition, mechanistic modeling approaches, such as agent-based modeling open the possibility to model and analyze cell dynamics over time. This review aims to provide an overview of the current state of immune-system related blood-based biomarkers and their potential for the early diagnosis of Alzheimer's disease.

Vital role for primary healthcare providers: urgent need to educate the community about daily nutritional self-care to support immune function and maintain health

The importance of self-care to improve health and social well-being is well recognised. Nevertheless, there remains a need to encourage people to better understand how their body works, and how to keep it healthy. Because of its important role, part of this understanding should be based on why the immune system must be supported. This highly complex system is essential for defending against pathogens, but also for maintaining health throughout the body by preserving homeostasis and integrity. Accordingly, the immune system requires active management for optimal functioning and to reduce the risk of chronic diseases. In addition to regular exercise, healthy sleeping patterns, cultivating mental resilience, adequate nutrition through healthy and diverse dietary habits is key to the daily support of immune function. Diet and the immune system are closely intertwined, and a poor diet will impair immunity and increase the risk of acute and chronic diseases. To help elucidate the roles of primary healthcare providers in supporting individuals to engage in self-care, an international group of experts reviewed the evidence for the roles of the immune system in maintaining health and for nutrition in daily immune support, and discussed implications for population health and clinical practice.

Skull bone marrow channels as immune gateways to the central nervous system

Decades of research have characterized diverse immune cells surveilling the CNS. More recently, the discovery of osseous channels (so-called 'skull channels') connecting the meninges with the skull and vertebral bone marrow has revealed a new layer of complexity in our understanding of neuroimmune interactions. Here we discuss our current understanding of skull and vertebral bone marrow anatomy, its contribution of leukocytes to the meninges, and its surveillance of the CNS. We explore the role of this hematopoietic output on CNS health, focusing on the supply of immune cells during health and disease.

Cholesterol 24-hydroxylase at the choroid plexus contributes to brain immune homeostasis

The choroid plexus (CP) plays a key role in remotely controlling brain function in health, aging, and disease. Here, we report that CP epithelial cells express the brain-specific cholesterol 24-hydroxylase (CYP46A1) and that its levels are decreased under different mouse and human brain conditions, including amyloidosis, aging, and SARS-CoV-2 infection. Using primary mouse CP cell cultures, we demonstrate that the enzymatic product of CYP46A1, 24(S)-hydroxycholesterol, downregulates inflammatory transcriptomic signatures within the CP, found here to be elevated across multiple neurological conditions. In vitro, the pro-inflammatory cytokine tumor necrosis factor α (TNF-α) downregulates CYP46A1 expression, while overexpression of CYP46A1 or its pharmacological activation in mouse CP organ cultures increases resilience to TNF-α. In vivo, overexpression of CYP46A1 in the CP in transgenic mice with amyloidosis is associated with better cognitive performance and decreased brain inflammation. Our findings suggest that CYP46A1 expression in the CP impacts the role of this niche as a guardian of brain immune homeostasis.

Myeloid Cells As a Promising Target for Brain-Bone Degenerative Diseases from a Metabolic Point of View

Brain and bone degenerative diseases such as Alzheimer's disease and osteoporosis are common in the aging population and lack efficient pharmacotherapies. Myeloid cells are a diverse group of mononuclear cells that plays important roles in development, immune defense, and tissue homeostasis. Aging drastically alters the expansion and function of myeloid cells, which might be a common pathogenesis of the brain-bone degenerative diseases. From this perspective, the role of myeloid cells in brain-bone degenerative diseases is discussed, with a particular focus on metabolic alterations in myeloid cells. Furthermore, targeting myeloid cells through metabolic regulation via drugs such as metformin and melatonin is proposed as a potential therapy for the clinical treatment of brain-bone diseases.

Rescue of Alzheimer's disease phenotype in a mouse model by transplantation of wild-type hematopoietic stem and progenitor cells

Alzheimer's disease (AD) is the most prevalent cause of dementia; microglia have been implicated in AD pathogenesis, but their role is still matter of debate. Our study showed that single systemic wild-type (WT) hematopoietic stem and progenitor cell (HSPC) transplantation rescued the AD phenotype in 5xFAD mice and that transplantation may prevent microglia activation. Indeed, complete prevention of memory loss and neurocognitive impairment and decrease of β-amyloid plaques in the hippocampus and cortex were observed in the WT HSPC-transplanted 5xFAD mice compared with untreated 5xFAD mice and with mice transplanted with 5xFAD HSPCs. Neuroinflammation was also significantly reduced. Transcriptomic analysis revealed a significant decrease in gene expression related to "disease-associated microglia" in the cortex and "neurodegeneration-associated endothelial cells" in the hippocampus of the WT HSPC-transplanted 5xFAD mice compared with diseased controls. This work shows that HSPC transplant has the potential to prevent AD-associated complications and represents a promising therapeutic avenue for this 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.

The effects of enhancing angiotensin converting enzyme in myelomonocytes on ameliorating Alzheimer's-related disease and preserving cognition

This review examines the role of angiotensin-converting enzyme (ACE) in the context of Alzheimer's disease (AD) and its potential therapeutic value. ACE is known to degrade the neurotoxic 42-residue long alloform of amyloid β-protein (Aβ42), a peptide strongly associated with AD. Previous studies in mice, demonstrated that targeted overexpression of ACE in CD115+ myelomonocytic cells (ACE10 models) improved their immune responses to effectively reduce viral and bacterial infection, tumor growth, and atherosclerotic plaque. We further demonstrated that introducing ACE10 myelomonocytes (microglia and peripheral monocytes) into the double transgenic APPSWE/PS1ΔE9 murine model of AD (AD+ mice), diminished neuropathology and enhanced the cognitive functions. These beneficial effects were dependent on ACE catalytic activity and vanished when ACE was pharmacologically blocked. Moreover, we revealed that the therapeutic effects in AD+ mice can be achieved by enhancing ACE expression in bone marrow (BM)-derived CD115+ monocytes alone, without targeting central nervous system (CNS) resident microglia. Following blood enrichment with CD115+ ACE10-monocytes versus wild-type (WT) monocytes, AD+ mice had reduced cerebral vascular and parenchymal Aβ burden, limited microgliosis and astrogliosis, as well as improved synaptic and cognitive preservation. CD115+ ACE10-versus WT-monocyte-derived macrophages (Mo/MΦ) were recruited in higher numbers to the brains of AD+ mice, homing to Aβ plaque lesions and exhibiting a highly Aβ-phagocytic and anti-inflammatory phenotype (reduced TNFα/iNOS and increased MMP-9/IGF-1). Moreover, BM-derived ACE10-Mo/MΦ cultures had enhanced capability to phagocytose Aβ42 fibrils, prion-rod-like, and soluble oligomeric forms that was associated with elongated cell morphology and expression of surface scavenger receptors (i.e., CD36, Scara-1). This review explores the emerging evidence behind the role of ACE in AD, the neuroprotective properties of monocytes overexpressing ACE and the therapeutic potential for exploiting this natural mechanism for ameliorating AD pathogenesis.

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.

Targeting the immune system towards novel therapeutic avenues to fight brain aging and neurodegeneration

The incidence of age-related dementia is growing with increased longevity, yet there are currently no disease-modifying therapies for these devastating disorders. Studies over the last several years have led to an evolving awareness of the role of the immune system in supporting brain maintenance and repair, displaying a diverse repertoire of functions while orchestrating the crosstalk between the periphery and the brain. Here, we provide insights into the current understanding of therapeutic targets that could be adopted to modulate immune cell fate, either systemically or locally, to defeat brain aging and neurodegeneration.

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.