The Brain’s Immune Cells Hold the Switch Between Dementia and Resilience

The study identified six distinct tissue domains corresponding to different phases of disease development, with the most dramatic cellular shifts occurring precisely at the transition from amyloid-β plaque-associated pathology to tau-driven neurodegeneration . It is this microglial state switch—not simply the accumulation of plaques or tangles—that appears to be the decisive event.

Resilience in Octogenarians: Containment at the Plaque Perimeter

The first resilience mechanism emerged from cognitively healthy older adults who carried substantial Alzheimer’s pathology but showed no signs of dementia. In these individuals, microglia did something remarkably effective: they built better walls around amyloid plaques.

Compared to people with clinical dementia, these resilient octogenarians exhibited enhanced microglial dynamics in the immediate plaque microenvironment. They had more microglia surrounding each plaque, more compact plaque cores, and less accumulation of filamentous plaques . The consequence was a dramatic reduction in tau seeding activity—the toxic spread of tau protein that is tightly linked to neuron death. In fact, tau seeding activity in these resilient brains was comparable to that found in completely healthy brains .

In other words, their microglia did not eliminate amyloid plaques. They contained them. By compacting plaques and limiting their surface area, these microglia starved the pathological cascade of its next step: tau aggregation. The protective role of TREM2-mediated microglial activation at this exact intersection of amyloid and tau pathologies has been confirmed in experimental models, where loss of TREM2 dramatically enhances tau seeding and spreading around plaques .

Resilience in Centenarians: Clearance Through Neprilysin

Centenarians told a different story. Rather than merely containing amyloid, their microglia appeared to actively degrade it.

The researchers found that centenarian-derived microglia produced strikingly elevated levels of neprilysin, one of the brain’s primary amyloid-β-degrading enzymes . Normally, neprilysin levels decline with age, which is thought to contribute to the gradual buildup of amyloid in the aging brain. But in microglia from select centenarians and cognitive "superagers," neprilysin levels were significantly higher than in controls . Functional experiments confirmed that these centenarian microglia were more efficient at clearing amyloid, providing a biochemical resilience mechanism that attacks the problem at its root.

This distinction is biologically significant. Octogenarian resilience is about containment—locking amyloid away so it cannot seed tau pathology. Centenarian resilience is about clearance—destroying amyloid before it can accumulate to dangerous levels. Both strategies preserve cognition, but they operate through distinct molecular programs. The centenarian mechanism, driven by neprilysin, offers an especially attractive therapeutic target because it suggests that boosting this single enzyme could tip the balance toward amyloid clearance even in aging brains .

Therapeutic Implications: TREM2 Agonists as Microglial Reprogrammers

These findings provide a direct biological rationale for therapies that target TREM2, a receptor expressed on microglia that governs their activation state and function. TREM2 is essential for driving microglia toward the protective, plaque-compacting states seen in resilient individuals . The early protective microglial response identified at the Aβ–Tau inflection point is enriched for TREM2-related signaling—the very pathway that Muna Therapeutics aims to activate with its drug candidate .

MNA-001 is an oral small-molecule TREM2 agonist that, in preclinical studies, demonstrated the ability to significantly reduce neurotoxic amyloid burden and reprogram microglia into protective states . The drug is designed to push microglia toward the resilient phenotypes—whether that means enhancing the plaque-compacting response seen in octogenarians or activating amyloid-clearing pathways like neprilysin that dominate in centenarians.

Muna Therapeutics launched the Phase 1 trial of MNA-001 with first subjects dosed in November 2025 . The study is actively recruiting and evaluating safety, tolerability, pharmacokinetics, and pharmacodynamic effects on biomarkers of TREM2 target engagement and activation, with topline data expected in mid-2026 . In January 2026, the Alzheimer’s Association awarded Muna a $1 million research grant to support the trial and to validate translational biomarkers of TREM2 function . CEO Rita Balice-Gordon noted that the grant would help shift treatment paradigms toward early intervention by enhancing the brain’s intrinsic microglial defense mechanisms .

The implications extend beyond any single drug. The study proposes that the transition from amyloid pathology to tau pathology is not inevitable—it is modifiable. Cognitively intact individuals maintain brain health either by avoiding the harmful microglial state transition altogether or by decoupling microglial activation from tau accumulation . A therapy that locks microglia into their early protective mode, or that reactivates amyloid-clearing programs like neprilysin after they have been silenced by age, could intercept Alzheimer’s disease at the tipping point before cognitive decline begins.

Whether MNA-001 or other TREM2-targeting therapies succeed in the clinic remains an open question to be answered by the ongoing trial data. Which specific downstream pathways TREM2 agonism engages—containment, clearance, or both—also remains an active area of investigation . But the biological logic is now clearer than ever: microglia are not passive witnesses to Alzheimer’s disease. They are the arbiters of its fate.