How Immune System Dysfunction Drives Alzheimer's Disease Progression

Alzheimer's disease has long been framed as a brain-centric disorder, but this landmark 2026 review from Harvard-affiliated researchers reframes it as a systemic immune disease with lifelong origins. Drawing on GWAS, mouse models, single-cell RNA sequencing, and human cohort data, the authors argue that peripheral immune dysfunction both predisposes the brain to AD and accelerates its progression once established.

The review traces immune-brain crosstalk across the entire lifespan. During development, CD4+ T cells and B cells support oligodendrogenesis, myelination, and microglial maturation; immunodeficient mice show impaired hippocampal neurogenesis reversible by T cell transfer. Maternal immune activation via pro-inflammatory cytokines (IL-6, IL-1β, IL-17A) can imprint AD-like vulnerability before birth. In adulthood, IFN-γ from Th1 cells sustains synaptic plasticity and cognitive resilience, while chronic IL-1β elevation drives peripheral myeloid infiltration and synaptic injury. With aging, immunosenescence—marked by naive lymphocyte depletion, exhausted T cell accumulation, and myeloid hyperactivation—lowers the threshold for AD pathology. Transplanting young bone marrow into aged AD model mice restores youthful immune gene profiles and reduces pathology, underscoring that an old immune system actively contributes to neurodegeneration.

Microglia occupy center stage in the CNS immune response to AD. The review catalogues multiple disease-associated microglial states: MGnD/DAM (TREM2-driven, phagocytic), interferon-responsive microglia (IRM), activated response microglia (ARM, enriched for AD risk genes), terminally inflammatory microglia (TIM, associated with APOE4 and immune exhaustion), and monocyte-derived putative disease-inflammatory macrophages (DIM). The balance among these states is context-dependent—MGnD activity limits plaque burden, but its impairment in APOE4 carriers (partly via IL-17RA upregulation) worsens pathology. Pharmacological IL-17RA inhibition restored the MGnD response and reduced AD pathology in female mice.

Peripheral lymphocytes also play nuanced roles. IFN-γ-secreting Th1 cells and Aβ-reactive CD8 T cells can promote microglial plaque clearance, but exhausted or regulatory T cell subsets accumulate in AD and suppress protective responses. B cell-derived autoantibodies against AD-related antigens are elevated in patients, and regulatory B cells modulate neuroinflammation. Neutrophils infiltrate AD brains, occlude capillaries, exacerbate oxidative stress, and their depletion in mouse models reduces pathology. Monocytes/macrophages display epigenetic reprogramming and metabolic dysfunction (mitochondrial dysfunction, altered glycolysis and cholesterol handling) that impair their phagocytic and anti-inflammatory capacity.

On the therapeutic front, the review evaluates immune checkpoint inhibitors (targeting PD-1/PD-L1, LAG-3, TIM-3), cytokine modulators (anti-IL-17A, IFN-γ delivery), and strategies to rejuvenate or reprogram peripheral immune cells. The authors emphasize that most mechanistic insights derive from mouse models and call urgently for human-derived data. They advocate for personalized approaches integrating genetic risk (APOE, TREM2, CLU, BIN1, PLCG2 variants), immune profiling, and aging metrics to stratify patients and guide immunotherapy. Key knowledge gaps include the timing and sequence of immune events, sex differences in immune responses, and the long-term safety of immune modulation in elderly patients.