A recent review published in the Journal of Clinical Investigation highlights on Apr. 16 how the brain's immune system can both protect against and contribute to neurodegenerative diseases such as Alzheimer's and Parkinson's.
This topic is important because understanding how immune responses impact these diseases could influence future treatments. The review points out that timing, cell type, and disease context are critical factors for developing therapies targeting the immune system.
The article explains that during neurodegeneration, neurons release signals that activate microglia and astrocytes—cells responsible for responding to damage in the central nervous system (CNS). These cells then recruit additional immune cells from outside the brain. Activation of pattern recognition receptors like Toll-like receptor 2 (TLR2) and TLR4 leads to increased production of cytokines, which can either help clear harmful proteins or worsen inflammation.
Genetic studies have found certain variants in the TREM2 gene—highly expressed by microglia—are risk factors for Alzheimer's disease comparable to known genetic risks such as APOE ε4. Other molecules like RAGE are also linked with faster cognitive decline when overexpressed. The review discusses how CD4+ T cells may play protective or harmful roles depending on their specificity: some help prevent cognitive decline while others promote inflammation. CD8+ T cells have been found both protecting against amyloid plaques and correlating with neuronal death, showing diverse effects depending on context.
Factors such as aging, repetitive head injury, and viral infections can disrupt normal CNS immunity and increase vulnerability to neurodegeneration. For example, traumatic brain injury has been associated with higher risks of Alzheimer's and Parkinson's later in life due to changes in microglial activity.
The findings suggest that a complex mix of environmental exposures and biological processes shape how the immune system influences these diseases. The authors emphasize that "appreciating the cell-intrinsic states and temporal dynamics of immune cells is crucial for understanding immune signatures and determining when immunomodulatory interventions will be beneficial."
