Innate Immune Mechanisms in Alzheimer's Disease

Alzheimer’s disease is characterised by the accumulation of misfolded amyloid-β and tau proteins, but innate immune cells such as microglia and peripheral monocytes play a central role in modulating disease onset and progression. Resident microglia survey the neural parenchyma, responding to protein aggregates through pattern recognition receptors and scavenger systems that trigger phagocytosis, cytokine release and synaptic remodelling. In early stages, microglial clearance of small amyloid oligomers may be protective, yet chronic exposure to plaques and persistent activation can induce functional impairment, dystrophic morphology and an aberrant inflammatory state that compromises neuronal networks.

Peripheral monocytes also contribute to the neuroimmune landscape. Under conditions of blood–brain barrier dysfunction or endothelial activation, monocytes infiltrate the parenchyma, adopt microglia-like phenotypes and engage with plaques. Complement pathways, cytokine gradients and cell-surface receptors including integrins and scavenger molecules govern the balance between effective debris removal and propagation of neuroinflammation. Age-related decline in innate immune competence, combined with systemic inflammatory signals, can further skew microglia and macrophage responses towards a maladaptive phenotype, exacerbating synaptic loss and cognitive decline. Understanding these dualistic roles offers new avenues for immunomodulatory therapies aimed at restoring homeostatic clearance without provoking collateral damage.

Research from Nature Portfolio

Recent studies have illuminated new mechanisms by which peripheral monocytes interface with cerebral amyloid. One investigation found large amyloid-β aggregates circulating in plasma of individuals with mild cognitive impairment bind strongly to complement receptor 4, are preferentially phagocytosed by CD18-rich monocytes and promote lysosomal activation in stem cell-derived microglia, suggesting a complement-mediated axis for monocyte recruitment to cortical capillaries. A complementary line of work has demonstrated that blocking the PD-1/PD-L1 immune checkpoint fosters homing of monocyte-derived macrophages into the brain, where they express scavenger receptors such as MSR1, reshaping the neuroinflammatory milieu and ameliorating cognitive deficits in models of tauopathy.

Innate Immune Mechanisms in Alzheimer's Disease publication trend

The graph below shows the total number of articles in innate immune mechanisms in alzheimer's disease across all publications each year (not limited to Nature Index journals).

Technical terms

Innate immunity: Immediate, non-specific defence mediated by cells such as microglia and monocytes.

Microglia: Resident myeloid cells in the central nervous system that survey and clear extracellular debris.

Monocytes: Circulating myeloid cells that can infiltrate tissues and differentiate into macrophages.

Phagocytosis: Cellular process of engulfing and degrading particulate matter, including amyloid aggregates.

Complement receptor 4 (CR4): A cell-surface integrin that binds complement-opsonised targets, including amyloid fibrils.

Chimeric antigen receptor (CAR) macrophages: Genetically engineered macrophages expressing synthetic receptors to target specific antigens.

PD-1/PD-L1 checkpoint blockade: Therapeutic inhibition of an immune regulatory pathway that enhances monocyte-derived macrophage recruitment and activity.

References

1. Amyloid-β aggregates activate peripheral monocytes in mild cognitive impairment. Nature Communications (2024).
2. Chimeric antigen receptor macrophages target and resorb amyloid plaques. JCI Insight (2024).
3. Monocyte-derived cells invade brain parenchyma and amyloid plaques in human Alzheimer’s disease hippocampus. Acta Neuropathologica Communications (2023).
4. The effects of high plasma levels of Aβ1-42 on mononuclear macrophage in mouse models of Alzheimer’s disease. Immunity & Ageing (2023).
5. Clearance of cerebral Aβ in Alzheimer’s disease: reassessing the role of microglia and monocytes. Cellular and Molecular Life Sciences (2017).
6. Functional Impairment of Microglia Coincides with Beta-Amyloid Deposition in Mice with Alzheimer-Like Pathology. PLOS ONE (2013).
7. The dynamics of monocytes and microglia in Alzheimer’s disease. Alzheimer's Research & Therapy (2015).
8. Distinct inflammatory phenotypes of microglia and monocyte‐derived macrophages in Alzheimer's disease models: effects of aging and amyloid pathology. Aging Cell (2016).
9. PD-1/PD-L1 checkpoint blockade harnesses monocyte-derived macrophages to combat cognitive impairment in a tauopathy mouse model. Nature Communications (2019).
10. Accelerated microglial pathology is associated with Aβ plaques in mouse models of Alzheimer's disease. Aging Cell (2014).
11. Microglial Scavenger Receptors and Their Roles in the Pathogenesis of Alzheimer’s Disease. International Journal of Alzheimer's Disease (2012).