neuroinflammation

Introduction

Neuroinflammation represents a complex and multifaceted physiological response of the central nervous system (CNS) to injury, infection, toxic protein aggregation, or disease ^{1Neuroinflammation and Alzheimer's disease: Unravelling the molecular mechanisms.2025 · J Alzheimers Dis · DOI 10.1177/13872877251374353 · PMID 40938771Open reference}. While acute neuroinflammation serves as an essential protective mechanism facilitating tissue repair and pathogen clearance, chronic neuroinflammation evolves into a self-perpetuating pathological driver in neurodegenerative including Alzheimer’s disease (AD), Parkinson’s disease (PD), amyotrophic lateral sclerosis (ALS), Huntington’s disease (HD), and multiple sclerosis (MS) ^{2Glycosylation in neuroinflammation: mechanisms, implications, and therapeutic strategies for neurodegenerative diseases.2025 · Transl Neurodegener · DOI 10.1186/s40035-025-00506-4 · PMID 40983949Open reference}. This pathway page provides a comprehensive overview of the molecular and cellular underlying neuroinflammation across neurodegenerative conditions, integrating current understanding with therapeutic implications ^{3Neuroinflammation across the Spectrum of Neurodegenerative Diseases: Mechanisms and Therapeutic Frontiers.2025 · Neuroimmunomodulation · DOI 10.1159/000548021 · PMID 40931498Open reference}. ^{4Artificial β-propeller protein-based hydrolases.2019 · Chemical communications (Cambridge, England) · DOI 10.1039/c9cc04388h · PMID 31321399Open reference}

The fundamental paradox of neuroinflammation lies in its dual nature: temporally limited, acute neuroinflammation represents an essential defense mechanism that facilitates tissue repair and pathogen clearance, whereas sustained, chronic neuroinflammation becomes self-perpetuating and drives progressive neuronal loss through sustained pro-inflammatory cytokine production, complement-mediated synaptic elimination, and oxidative stress. Understanding the molecular switches that transition neuroinflammation from protective to pathological is critical for developing effective therapeutic interventions that preserve the beneficial aspects while blocking pathological cascades. ^{5Human skin wounds: a major and snowballing threat to public health and the economy.2010 · Wound repair and regeneration : official publication of the Wound Healing Society [and] the European Tissue Repair Society · DOI 10.1111/j.1524-475X.2009.00543.x · PMID 19903300Open reference}

Overview

The neuroinflammatory cascade involves multiple coordinated cellular and molecular responses that collectively determine disease outcomes: ^{6Hypoxic pulmonary hypertension is prevented in rats with common bile duct ligation.2005 · Journal of applied physiology (Bethesda, Md. : 1985) · DOI 10.1152/japplphysiol.00556.2004 · PMID 15516365Open reference}

• Microglial activation: The primary resident immune cells of the brain, undergoing dramatic phenotypic transformations in response to pathological stimuli. Microglia originate from yolk sac progenitors during embryogenesis and maintain self-renewal throughout life, representing a unique immune population distinct from peripheral macrophages.

• Astrocytic reactivity: Supporting cells that adopt inflammatory phenotypes and contribute to the neurotoxic A1 astrocyte subtype through secretion of complement components and inflammatory mediators.

• Peripheral immune infiltration: Both adaptive and innate immune cells entering the CNS through a compromised blood-brain barrier, including monocytes, T lymphocytes, and occasionally B cells.

• Inflammatory mediators: A complex network of cytokines, chemokines, complement , and reactive oxygen/nitrogen species that create feedback loops amplifying or resolving inflammation.

graph TD
 A[" CNS Injury / Pathogen / Protein Aggregation "] --> B[" Microglial Activation "]
 B --> C[" Release of Pro-inflammatory Mediators "]
 C --> D[" Cytokine Storm: IL-1beta, TNF-alpha, IL-6 "]
 C --> E[" Chemokine Production: CCL2, CXCL8 "]
 C --> F[" Complement Activation: C1q, C3 "]
 D --> G[" Neuronal Dysfunction "]
 E --> H[" Peripheral Immune Recruitment "]
 F --> I[" Synaptic Pruning "]
 G --> J[" Blood-Brain Barrier Breakdown "]
 H --> J
 I --> J
 J --> K[" Chronic Neuroinflammation "]
 K --> L[" Neurodegeneration "]

Molecular Triggers of Neuroinflammation

Protein Aggregates as Endogenous Danger Signals

Pathological protein aggregates serve as endogenous triggers of neuroinflammation, acting as damage-associated molecular patterns (DAMPs) that activate innate immune responses through multiple receptor systems:

Amyloid-beta (Aβ) 1

• Activates microglia via TREM2(//trem2) receptor signaling, engaging the DAP12/TYROBP adaptor complex

• Aβ oligomers and fibrils trigger robust inflammatory responses through multiple receptor systems including CD36, TLR4, and RAGE

• The TREM2-APOE pathway drives the microglial phenotypic transition from homeostatic to disease-associated states

• Aβ-mediated inflammation involves NLRP3 inflammasome activation and subsequent IL-1β release

Alpha-synuclein (α-syn) 2

• Stimulates NLRP3 inflammasome in microglia through TLR2 and TLR4 activation

• Extracellular α-syn aggregates are recognized by microglia and trigger the release of pro-inflammatory cytokines including IL-1β and IL-18

• Post-translational modifications of α-syn (phosphorylation, nitration) enhance its immunogenic properties

• α-syn propagation between neurons and microglia creates a cyclical inflammatory response

Tau protein 3

• Released from neurons in association with exosomes, promotes microglial activation through the TREM2 axis

• Hyperphosphorylated tau binds to microglia receptors and triggers complement activation

• Extracellular tau fibrils act as DAMPs, engaging TLR2 and TLR4 to trigger NF-κB activation

• Tau pathology spread correlates with microglial activation patterns in human AD brain

TDP-43

• Aggregate in ALS and frontotemporal dementia, activates innate immune responses through RNA sensing pathways

• Misfolded TDP-43 triggers interferon responses through cGAS-STING pathway activation

• ALS-associated mutations in TDP-43 alter its immunogenic properties

Mutant SOD1

• Triggers neuroinflammation in familial ALS through activation of microglia and astrocyte NADPH oxidase

• SOD1 aggregates are recognized by pattern recognition receptors, driving pro-inflammatory cytokine production

DAMPs (Damage-Associated Molecular Patterns)

Endogenous molecules released from damaged cells serve as potent activators of neuroinflammation, providing endogenous “danger signals” that amplify immune responses:

Extracellular ATP 4

• P2X7 receptor activation on microglia triggers potassium efflux and NLRP3 inflammasome assembly

• Under pathological conditions, extracellular ATP levels increase dramatically due to cellular damage from necrosis, apoptosis, or neurotransmitter release

• Purinergic signaling represents a critical link between neuronal activity and microglial surveillance

• P2X7 receptor blockade protects against dopaminergic neuron loss in experimental PD

High Mobility Group Box 1 (HMGB1)

• Binds TLR4 and RAGE receptors, amplifying inflammatory responses

• Released from necrotic neurons, HMGB1 acts as a late mediator of neuroinflammation

• HMGB1 translocation from nucleus to cytoplasm precedes its extracellular release

• Anti-HMGB1 antibodies show neuroprotective effects in experimental models

**S100 calcium-binding **

• S100A8/A9 and S100A12 are released by damaged astrocytes

• Activate RAGE and TLR4 signaling cascades

• S100A8/A9 forms a calprotectin complex with potent pro-inflammatory properties

Nucleic acids 5

• Mitochondrial DNA damage releases mtDNA into the cytosol, activating cGAS-STING pathway and type I interferon responses

• Cytosolic DNA sensing through cGAS triggers STING-dependent inflammation

• Nuclear DNA damage also contributes to cytosolic DNA accumulation

Uric acid

• Crystallizes in chronic neuroinflammation, directly activating NLRP3 inflammasome

• Elevated uric acid levels correlate with inflammatory markers in neurodegenerative

Microglial Activation States

Microglia exhibit remarkable phenotypic plasticity, transitioning between distinct activation states in response to environmental cues. The traditional M1/M2 classification has evolved into a more nuanced understanding of microglial heterogeneity.

Homeostatic Microglia

In the healthy brain, microglia maintain surveillance through specific receptor systems that keep them in a quiescent, monitoring state:

• P2RY12 receptors for ATP sensing, enabling detection of cellular damage and providing information about neuronal activity

• CX3CR1 signaling from neurons via the fractalkine (CX3CL1) pathway, maintaining anti-inflammatory phenotype

• TREM2 expression at low levels, providing baseline surveillance for apoptotic cells and cellular debris

• CD200R engagement with CD200 on neurons, delivering inhibitory signals that prevent inappropriate activation

• TGF-β signaling from neurons and astrocytes, maintaining microglial quiescence and promoting tissue homeostasis

The microglial surveillance state involves constant process motility scanning the brain parenchyma, with processes extending toward sites of injury or neuronal activity. This surveillance function positions microglia as critical sentinels of brain homeostasis.

Disease-Associated Microglia (DAM)

In neurodegenerative conditions, microglia transition through defined stages characterized by distinct transcriptional programs 6:

graph LR
 A["Homeostatic Microglia"] --> B["Stage 1: Intermediate"]
 B --> C["Stage 2: DAM"]
 C --> D["Chronic Activation"]

 A -->|"CX3CR1 signaling"| A
 B -->|"TREM2 independent"| B
 C -->|"TREM2/DAP12 activation"| C
 
 B -->|"Downregulation of P2RY12"| B
 C -->|"Upregulation of ApoE, TREM2"| C
 C -->|"Phagocytosis of debris"| C

Stage 1 DAM (TREM2-independent)

• Triggered by initial neuronal damage signals

• Downregulation of homeostatic genes including P2RY12, CX3CR1

• Upregulation of some inflammatory genes

• Cells remain relatively quiescent in phagocytic activity

Stage 2 DAM (TREM2-dependent)

• Requires functional TREM2 signaling for transition

• Upregulation of lipid metabolism genes (ApoE, Lipg)

• Enhanced phagocytic capacity for protein aggregates and cellular debris

• Expression of complement components (C1q, C3)

• Pro-inflammatory cytokine production

The TREM2-APOE pathway represents a critical regulatory axis driving the microglial response in AD, with TREM2 risk variants impairing the DAM response and reducing clearance of Abeta 7.

Key Microglial Receptors and Their Disease Relevance

Astrocytic Responses

Astrocytes undergo reactive transformation in response to neuroinflammation, adopting distinct phenotypic programs that profoundly affect neuronal survival.

A1 vs A2 Reactive Astrocytes

The binary A1/A2 classification provides a useful but oversimplified framework for understanding astrocyte heterogeneity 8:

A1 (Neurotoxic) Astrocytes

• Induced by microglial IL-1α, TNF, and C1q signaling

• Lose supportive functions (glutamate uptake, potassium buffering)

• Gain toxic properties through complement component release

• Release factors that trigger synaptic elimination

• Produce neurotoxic cytokines including IL-6 and CCL2

• Characteristic markers: C3 (core marker), Serping1, H2-D1, Lyz2

A2 (Neuroprotective) Astrocytes

• Induced by ischemia and CNS injury

• Upregulate neurotrophic factors including BDNF and GDNF

• Promote tissue repair and angiogenesis

• Maintain glutamate uptake and water homeostasis

• Characteristic markers: S100A10, PTX3, TM4SF1

The balance between A1 and A2 astrocytes critically influences disease outcomes. In AD and PD, the balance shifts dramatically toward A1 astrocytes, contributing to synaptic loss and neuronal dysfunction.

Astrocyte Contributions to Neuroinflammation

Astrocytes amplify neuroinflammation through multiple :

1. Chemokine production: CCL2, CXCL1, CXCL10 recruit peripheral immune cells

2. Cytokine secretion: IL-6, IL-1β, TNF-α contributing to cytokine storm

3. Complement synthesis: C3, C4 components participating in opsonization

4. Blood-brain barrier modulation: VEGF and MMP-9 production affecting barrier integrity

5. Ion homeostasis disruption: Dysregulated glutamate clearance causing excitotoxicity

6. Water balance dysregulation: AQP4 mislocalization affecting glymphatic clearance

Inflammatory Signaling Pathways

NF-κB Pathway

The NF-κB transcription factor serves as the master regulator of neuroinflammation, controlling the expression of virtually all pro-inflammatory genes 9:

Activation triggers:

• TLR signaling (TLR2, TLR4, TLR9)

• TNF receptor engagement

• IL-1R signaling

• ROS and oxidative stress

• ATP and purinergic signaling

Canonical pathway:

• IKK complex phosphorylates IκBα

• Ubiquitination targets IκBα for proteasomal degradation

• p65/p50 NF-κB dimer translocates to nucleus

• Gene transcription activation

Target genes:

• Pro-inflammatory cytokines: IL-1β, TNF-α, IL-6, IL-8

• Chemokines: CCL2, CCL5, CXCL8, CXCL10

• Adhesion molecules: ICAM-1, VCAM-1, E-selectin

• Enzymes: COX-2, iNOS, MMP-9

• Complement components: C1q, C3, Factor B

Negative regulators:

• IκBα (NFKBIA) - feedback inhibitor

• A20 (TNFAIP3) - deubiquitinating enzyme

• SOCS - JAK/STAT inhibitors

• Pellino - negative regulators

MAPK Pathways

Three major MAPK cascades contribute to neuroinflammation with distinct downstream effects:

ERK1/2 Pathway

• Primarily involved in cell survival and proliferation

• Contributes to COX-2 and MMP expression

• Activated by growth factors and cellular stress

• Cross-talk with NF-κB pathway

JNK Pathway

• Key mediator of stress-induced inflammation

• Activates AP-1 transcription factor

• Promotes expression of pro-inflammatory genes

• Involved in excitotoxicity and neuronal death

p38 Pathway

• Critical for cytokine production, particularly IL-1β and TNF-α synthesis

• Regulates mRNA stability for inflammatory mediators

• Multiple inhibitors have been developed

• Clinical translation limited by toxicity 10

NLRP3 Inflammasome

The NLRP3 inflammasome represents a critical molecular hub integrating multiple danger signals into inflammatory responses 11:

Components:

• NLRP3 sensor protein

• ASC adaptor protein (PYCARD)

• Caspase-1 effector protease

Activation signals:

• Priming signal: NF-κB-mediated upregulation of NLRP3 and pro-IL-1β

• Activation signal: ATP, uric acid crystals, ROS, mitochondrial DNA, calcium influx

Outputs:

• Mature IL-1β cytokine (potent pro-inflammatory mediator)

• IL-18 cytokine

• Gasdermin D-mediated pyroptotic cell death

Disease relevance:

• NLRP3 activation by Aβ in AD

• NLRP3 activation by α-syn in PD

• Genetic variants affect disease risk

• Pharmacological inhibition protects neurons in preclinical models

Peripheral Immune System Cross-Talk

Monocyte Recruitment

Peripheral monocytes infiltrate the CNS in neurodegenerative through a well-characterized mechanism:

Mechanism:

• CCL2 (MCP-1) production by activated microglia and astrocytes

• CCR2 expression on circulating monocytes

• Migration across compromised blood-brain barrier

Fate:

• Some monocytes differentiate into microglia-like cells

• Others become inflammatory macrophages

• Contribution to disease pathology varies with timing and context

Dual role:

• Protective: clearance of cellular debris and protein aggregates

• Pathogenic: production of pro-inflammatory cytokines, ROS

T Cell Involvement

Adaptive immunity participates in neuroinflammation through multiple T cell subsets:

CD4+ T cells:

• Th1 cells produce IFN-γ, activating microglia and promoting inflammation

• Th17 cells produce IL-17, contributing to blood-brain barrier disruption

• Th2 cells may have protective roles through anti-inflammatory cytokine production

CD8+ T cells:

• Cytotoxic T cells can directly target neurons

• Perforin and Fas-FasL mediated cytotoxicity

• Accumulate insubstantia nigra in PD

Regulatory T cells (Tregs):

• Normally suppress neuroinflammation through IL-10 and TGF-β

• Their dysfunction contributes to autoimmunity

• Treg deficiency correlates with disease progression

Disease-Specific Mechanisms

Alzheimer’s Disease

Neuroinflammation in AD involves multiple interconnected pathways:

1. Aβ-triggered microglial activation: Aβ binds TREM2, CD36, and TLRs, triggering pro-inflammatory cytokine release

2. Tau-mediated inflammation: Extracellular tau aggregates activate microglia through TLR2/4

3. Complement-mediated synapse elimination: C1q and C3 tag synapses for microglial phagocytosis

4. Chronic cytokine exposure: Elevated IL-1β, TNF-α, IL-6 contribute to synaptic dysfunction and memory impairment

Genetic evidence supports the critical role of neuroinflammation:

• TREM2 variants (R47H, R62H) increase AD risk by 3-4 fold 7

• ABCA7, CLU (clusterin), CR1 complement receptor 1 are established inflammation-related risk loci 12

• MS4A gene cluster variants affect microglial signaling and AD risk

Parkinson’s Disease

Neuroinflammation in PD follows distinct patterns:

1. α-Synuclein-induced inflammation: Misfolded α-syn activates NLRP3 inflammasome in microglia

2. Microglial NADPH oxidase activation: Excessive ROS production damages dopaminergic neurons

3. Peripheral inflammation: Elevated peripheral cytokines correlate with disease progression

4. Blood-brain barrier dysfunction: Allows peripheral immune cell infiltration

Post-mortem studies demonstrate extensive microglial activation in the substantia nigra of PD patients, with high HLA-DR expression on microglia 13. PET imaging using TSPO ligands confirms chronic neuroinflammation in living patients, particularly in the substantia nigra and striatum.

Amyotrophic Lateral Sclerosis

Neuroinflammation in ALS exhibits unique characteristics:

1. Microglial activation: Rapidly progresses from neuroprotective to toxic phenotype

2. Astrogliosis: Reactive astrocytes surround motor neurons

3. T cell infiltration: CD4+ and CD8+ T cells accumulate in spinal cord

4. Mutant SOD1 effects: Activates microglia through nitric oxide and ROS production

The timing of neuroinflammation in ALS suggests it may be secondary to initial neuronal dysfunction in some cases, but rapidly becomes self-perpetuating and drives disease progression once established.

Non-cell-autonomous glial pathways play a critical role in ALS progression. Astrocytes and microglia from ALS patients release toxic factors that kill motor neurons, creating a neuroinflammation-neuron death cycle.

The relationship between TDP-43 pathology and neuroinflammation in ALS is bidirectional:

• TDP-43 aggregation in microglia and astrocytes triggers pro-inflammatory responses

• Chronic neuroinflammation promotes further TDP-43 mislocalization and aggregation

• This creates a self-amplifying loop that drives progressive motor neuron degeneration

Neuroinflammation in ALS connects to genetic risk factors including TREM2 variants that influence microglial responses.

Therapeutic Targets

Current Approaches and Clinical Status

The repeated failure of anti-inflammatory approaches in clinical trials suggests that timing and target selection are critical. Interventions may need to occur before chronic neuroinflammation becomes established.

Emerging Therapeutic Strategies

1. TREM2 agonists: Antibodies enhancing TREM2 signaling (ADCS-036, AL002) aim to enhance the protective microglial response to Aβ 14

2. Microglial repopulation: CSF1R antagonists (pexidartinib) eliminate disease-associated microglia and allow replacement with healthy cells

3. NLRP3 inhibitors: Small molecule inhibitors (MCC970, Dapansutrile) block inflammasome activation at the source

4. Pro-resolving mediators: Specialized pro-resolving mediators (SPMs) including resolvins and protectins promote inflammation resolution

5. Autophagy enhancement: mTOR inhibitors and other approaches clearing protein aggregates reduce inflammatory triggers

Conclusion

Neuroinflammation represents a central pathological process in neurodegenerative , acting as both a consequence of protein aggregation and a driver of progressive neuronal loss. The complex interplay between microglia, astrocytes, and peripheral immune cells creates self-perpetuating inflammatory cascades that resist natural resolution. Understanding the molecular underlying chronic neuroinflammation provides critical opportunities for therapeutic intervention. Targeting specific nodes in the inflammatory network—particularly TREM2, NLRP3, and pro-inflammatory cytokines—offers promise for disease-modifying therapies across multiple neurodegenerative conditions. The failures of broad-spectrum anti-inflammatory drugs highlight the need for targeted approaches that preserve the beneficial aspects of neuroinflammation while blocking pathological cascades.

See Also

• Microglia — Primary resident immune cells of the CNS

• Alzheimer’s Disease — Neuroinflammation in AD

• Parkinson’s Disease — Neuroinflammation in PD

• TREM2 — AD risk gene and microglial receptor

• ALS Disease Page

• Non-Cell-Autonomous Glial Pathways in ALS

• TDP-43 Pathology in ALS

• Mitochondrial Dysfunction in ALS-FTD

• Cytokines in Neurodegeneration — Inflammatory signaling molecules

• Complement System — Synaptic pruning mechanisms

• Disease-Associated Microglia — DAM pathway

Neuroinflammation in Aging

Aging represents the strongest risk factor for neurodegenerative , and neuroinflammation plays a critical role in age-related brain changes:

Inflammaging

• Chronic low-grade inflammation in the aging brain

• Elevated baseline levels of IL-6, TNF-α, CRP in elderly individuals

• Microglial priming: enhanced inflammatory responses to secondary challenges

• Decreased microglial surveillance and process motility

Age-related microglial changes

• Dystrophic morphology with shortened processes

• Increased soma size and irregular shapes

• Loss of homeostatic markers (P2RY12, CX3CR1)

• Reduced capacity for surveillance and process extension

Blood-brain barrier aging

• Increased permeability with age

• Pericyte degeneration

• Reduced endothelial tight junction integrity

• Enhanced peripheral immune cell infiltration

Sex Differences in Neuroinflammation

Sex differences in neurodegenerative disease prevalence may relate to neuroinflammation:

Microglial sex differences

• Female microglia exhibit higher baseline inflammatory gene expression

• Estrogen modulates microglial responses through estrogen receptors

• Males show greater microglial density in certain brain regions

Clinical implications

• Women have higher AD risk post-menopause

• PD affects more men than women

• ALS shows male predominance

Clinical Trial Data

Anti-neuroinflammatory therapies in clinical trials for neurodegenerative diseases:

Biomarker Connections

Neuroinflammation biomarkers for diagnosis and trial enrichment:

Patient Impact

Disease-modifying potential:

• Neuroinflammation is upstream of neurodegeneration - early intervention may prevent neuronal loss

• Targeting microglial activation could slow disease progression in AD, PD, and ALS

• Combination therapy with anti-amyloid/anti-tau may enhance efficacy

Therapeutic challenges:

• BBB penetration: Many anti-inflammatory drugs don’t cross the BBB

• Dose timing: Early intervention likely critical - trials in advanced disease may fail

• Target specificity: Broad immunosuppression risks infections

• Biomarker stratification: Patients with elevated neuroinflammation biomarkers may respond better

Clinical practice integration:

• TSPO PET for patient selection in clinical trials

• CSF YKL-40 for disease progression monitoring

• Monitoring for infections during immunosuppressive therapy