Neuroinflammation
Inflammation is the body’s natural response to injury, infection, or stress. In the brain and nervous system, this response is carried out primarily by resident immune cells like microglia and astrocytes, along with signals from the broader immune system. In the short term, inflammation can be protective: it helps clear debris, fight infection, and initiate repair.
The problem arises when inflammation becomes chronic or dysregulated. Instead of resolving damage, inflammation can sustain and amplify it. This shift from protective to harmful inflammation is a central feature of many neurological diseases.
Across conditions such as Alzheimer’s disease, Parkinson’s disease, ALS/FTD, multiple sclerosis, stroke, TBI, and even CIPN, inflammation is not just a reaction to damage. It becomes part of the disease process itself, contributing to ongoing injury and progression.
Normal Biology
In a healthy brain, immune activity is tightly regulated. Microglia continuously survey the environment, responding to signs of injury or abnormal activity. Astrocytes provide structural and metabolic support while also participate in immune signaling.
When damage occurs, these cells become activated. They release signaling molecules called cytokines and chemokines, which help coordinate the response. This can include recruiting additional immune cells, eating debris, and supporting tissue repair.
Importantly, inflammation is normally self-limited. Once the threat or abnormality is resolved, signaling pathways shift toward resolution, and the system returns to a baseline state. This balance between activation and resolution is critical for maintaining brain health.
Dysfunction
Inflammation becomes harmful when it is persistent, excessive, or improperly regulated.
One common issue is chronic microglial activation when, instead of returning to a resting state, microglia remain in an activated mode and continuously release inflammatory signals. This can damage nearby neurons and disrupt normal brain function.
Another problem is the breakdown of the blood-brain barrier (BBB). The BBB is a wall of cells that surround the blood vessels of the brain, that prevent toxins, pathogens, and even most therapeutic drugs from entering the brain. Its dysfunction can allow peripheral immune cells and inflammatory molecules to enter the brain. This can amplify local inflammation and create a more damaging environment.
Inflammation is also closely linked to other forms of cellular stress. Protein aggregates, damaged mitochondria, and dying cells can all trigger immune responses. When these signals persist, they create a feed-forward loop, where inflammation both responds to and drives further damage.
Disease Connections
Inflammation is a major component of many neurological diseases, though its role varies.
In multiple sclerosis, inflammation is a primary driver of disease. Immune-mediated attacks on myelin lead to demyelination and, over time, axonal loss.
In Alzheimer’s disease, microglial activation is closely associated with amyloid plaques and tau pathology. Emerging evidence, in fact, points to Alzheimer’s being caused by overactive immune response to protein aggregates rather than being caused by the aggregates themselves. Either way, inflammation contributes to synaptic loss and disease progression.
In Parkinson’s disease, inflammatory signaling is observed in affected brain regions and may contribute to the vulnerability of dopaminergic neurons.
In ALS/FTD, neuroinflammation is linked to both genetic and sporadic forms of disease and may interact with protein aggregation and neuronal stress.
In ischemic stroke and TBI, inflammation is triggered acutely by injury and persists afterwards, contributing to secondary damage and long-term outcomes. Additionally, TBIs and subconcussive head blows can disrupt the BBB trigger an immune response that way.
In CIPN, inflammation in peripheral nerves and surrounding tissues can contribute to pain and nerve dysfunction.
Across these conditions, inflammation often begins as a response to injury but becomes a sustained contributor to disease progression.
Molecular Consequences
At the molecular level, inflammation reshapes the neuronal environment through sustained activation of immune signaling pathways. Activated microglia and astrocytes release cytokines, chemokines, and reactive species that alter synaptic signaling and shift gene expression programs toward a stress-responsive state. Rather than being an acute response, chronic exposure drives persistent changes in neuronal function, including reduced synaptic plasticity and increased vulnerability to injury.
A key part of this process is the inflammasome, a protein complex that senses cellular stress and triggers maturation of pro-inflammatory cytokines such as IL-1β and IL-18. Inflammasome activation can also promote forms of inflammatory cell death, including pyroptosis, linking immune signaling directly to loss of neuronal and glial cells. Chronic inflammasome activation can also reinforce senescence-associated secretory phenotype (SASP), creating a self-perpetuating inflammatory environment that spreads dysfunction to neighboring cells.
Inflammation is tightly coupled to oxidative stress, as activated immune cells generate reactive oxygen and nitrogen species. These molecules damage lipids, proteins, and DNA, and further impair mitochondrial function, creating a cycle between inflammation and cellular energy failure.
Inflammatory signaling also disrupts proteostasis pathways, including autophagy and lysosomal function. This reduces the cell’s ability to clear misfolded or aggregated proteins, linking chronic inflammation to hallmark pathologies such as amyloid plaques, tau tangles, and other protein aggregates seen across neurodegenerative diseases.
Therapeutic Landscape
Targeting inflammation has been an appealing strategy in neurological disease, but results have been mixed.
One challenge is that inflammation is not purely harmful. It plays important roles in protection and repair, so broadly suppressing it can have negative consequences.
Some therapies aim to modulate rather than eliminate inflammation, shifting it toward a more protective state. In multiple sclerosis, for example, many disease-modifying therapies work by altering immune activity and reducing inflammatory attacks.
In other diseases, such as Alzheimer’s and Parkinson’s, anti-inflammatory approaches have shown more limited success so far. This may reflect the complexity of the inflammatory response and the need for more targeted interventions.
There is growing interest in therapies that target specific pathways, such as microglial activation states, cytokine signaling, or inflammasome activity.
Research Directions
Current research is focused on understanding the dual role of inflammation: when it is protective and when it becomes harmful. As part of this, one major goal is to better define microglial states, since these cells can adopt different functional roles depending on context. Understanding these states may allow for more precise therapeutic targeting.
Another area of interest is how inflammation interacts with other pathways, including mitochondrial dysfunction, ROS, protein aggregation, and autophagy. These interactions are likely critical in determining disease progression.
Researchers are also exploring how systemic factors, like aging and peripheral immune function, influence brain inflammation.
Finally, advances in imaging and molecular profiling are improving the ability to track inflammation in living patients, which may help guide treatment and monitor disease activity.
Sources
- Glass, C. K., Saijo, K., Winner, B., Marchetto, M. C., & Gage, F. H. (2010). Mechanisms underlying inflammation in neurodegeneration.
- Heneka, M. T., Carson, M. J., El Khoury, J., et al. (2015). Neuroinflammation in Alzheimer’s disease.
- Ransohoff, R. M. (2016). How neuroinflammation contributes to neurodegeneration.
- Prinz, M., & Priller, J. (2014). Microglia and brain macrophages in the molecular age: from origin to neuropsychiatric disease.
- Voet, S., Prinz, M., & van Loo, G. (2019). Microglia in central nervous system inflammation and multiple sclerosis pathology.