Part 5 — Inflammation In The Brain Is The Common Thread
Step back and look at the papers so far.
- Anti-amyloid drugs work best in a genetically-defined subset of people with biomarker confirmation.
- Weight-loss and diabetes drugs seem to work upstream of full-blown Alzheimer’s, possibly through a direct effect on the brain.
- Metabolic improvement appears to drive much of the gut-microbiome signal, and the brain may benefit downstream.
- Long COVID brain fog is chronic brain inflammation with a candidate drug target.
Those four topics share one floor underneath all of them: neuroinflammation, meaning chronic, low-grade inflammation inside the brain. It’s the thread that connects genetics to air pollution to viral infections to metabolic dysfunction to hormonal transitions. In 2026, three more papers sharpened what neuroinflammation looks like in blood tests and in clinical decisions.
The air you breathe and the APOE interaction
Kimura and the Wisconsin Alzheimer’s Disease Research Center group, working with two of the most respected brain-biomarker scientists in the world (Blennow and Zetterberg), asked whether air pollution affects Alzheimer-related markers differently depending on your APOE status (Kimura 2026).
The answer was yes: people carrying the APOE ε4 gene variant showed bigger inflammation and synaptic-damage signals in response to the same air pollution exposure that non-carriers tolerated with much less change. This is gene-by-environment interaction made visible on standard blood and spinal fluid panels — not a theoretical risk model, a measurable one.
Here is the reframe that matters. APOE ε4 is less a direct cause of Alzheimer’s than a vulnerability amplifier. The ε4 variant tells you how hard your brain will react to the next insult — whether the insult is particulate air pollution, wildfire smoke, indoor mold, a reactivating herpesvirus, a bad diet, or a long stretch of metabolic inflammation. A non-carrier may tolerate exposures that an ε4 carrier cannot, and a homozygote (two copies) may struggle with things a heterozygote (one copy) handles. The genotype shapes the reaction to the insult rather than choosing the insult itself.
In Dr. Bredesen’s view, APOE ε4 had favorable characteristics hundreds of thousands of years ago when we didn’t have modern amenities such as refrigeration. Then, a strong inflammatory response could be lifesaving when it came to infected foods. That is a hypothesis, not consensus neuroscience, but it is clinically useful because it frames ε4 as a stronger inflammatory response, not simply a bad gene.
That framing shows up again in the second paper.
The viral hypothesis: mixed signals
Li Puma and colleagues continue a line of work linking herpes simplex virus type 1 (HSV-1, the common cold sore virus) to Alzheimer-like changes in the brain (Li Puma 2026). Their latest model showed that HSV-1 reactivation produces inflammation and synapse damage similar to what’s seen in Alzheimer’s. The synapse damage came before the obvious plaque and tangle pathology showed up. Which matches what patients describe in clinic (cognitive symptoms before imaging catches up), and matches a large 2025 paper in JAMA showing that shingles vaccination (another herpesvirus) is associated with lower dementia rates.
The chain is: infection → brain inflammation → synapse damage → cognitive symptoms. Ruth Itzhaki’s viral hypothesis of Alzheimer’s was 30 years old before most of neurology took it seriously. In 2026, the in vivo models and real-world epidemiology are strong enough that we do not ignore infection history, even though treatment remains the harder question.
A new paper from Richard Horowitz, MD, pushes this infection–neurodegeneration connection one step further by tying chronic Lyme disease to reversible changes in phosphorylated tau (p tau), a core Alzheimer’s biomarker. In a single patient case study published in Journal of Alzheimer’s Disease Reports, a 67 year old woman with longstanding Lyme and co infections showed markedly elevated plasma p tau 217 at baseline, consistent with Alzheimer type pathology, along with an adverse amyloid beta 42/40 ratio and elevated inflammatory markers. After nine weeks of double dose dapsone combination therapy targeting persistent and biofilm forms of Borrelia and co infections, her p tau 217 fell by roughly 63% into the normal range, her amyloid 42/40 ratio shifted toward a lower projected risk of cognitive impairment, and peripheral inflammatory markers normalized. We have seen similar patterns in a number of patients, but this is clinical experience layered on top of a case report, not randomized-trial evidence.
The viral hypothesis also took a hit in 2025. In the VALAD randomized clinical trial, 120 people with early symptomatic Alzheimer’s disease or biomarker-positive MCI, all seropositive for HSV-1 or HSV-2 by serum antibody testing, received valacyclovir 4 g/day or placebo for 78 weeks (Devanand 2025). Valacyclovir did not slow decline. On the primary cognitive endpoint, the valacyclovir group actually worsened more than placebo. That does not mean herpes viruses are irrelevant to brain health. It does mean that treating broad HSV antibody seropositivity with high-dose valacyclovir is not, by itself, a proven Alzheimer’s or MCI treatment.
The challenge in these conditions is finding the right testing that tightens the link between chronic infections and brain issues. We have been using T-cell testing through Infectolab, a specialty lab that looks for immune-cell evidence of infection activity, as one way of doing this. PCR and FISH testing of DNA/RNA from the suspected infectious organism is another. The VALAD result is a reminder that seropositivity alone, without evidence of active reactivation, is probably not enough to guide antiviral treatment decisions.
In our clinic, we still run HSV-1 and HSV-2 blood tests when the clinical picture supports it, especially in APOE ε4 carriers with cognitive symptoms. We discuss antiviral treatment only when the broader picture points toward reactivation, not simply because an antibody test is positive. The evidence base for that decision is not yet large-randomized-trial level for Alzheimer’s outcomes, but it is tightening.
A new blood marker worth watching
A third paper from Yoshida and colleagues (Yoshida 2026) found that a specific molecule called miR-155-5p — a small regulatory RNA that’s a master switch for inflammation — works as a blood marker of early Alzheimer-related changes in people with mild cognitive impairment. And, again, the link was stronger in APOE ε4 carriers. Same pattern: ε4 amplifies the inflammatory response to an upstream trigger.
miR-155-5p is not clinically available today. Neither is the pollution-response biomarker panel from the Wisconsin group. Both are likely to become available within the next few years. They’re the near future of what we measure.
What this means for you
The 2024 Lancet Commission on Dementia said that about 40% of dementia risk comes from 14 modifiable factors — things like diet, exercise, sleep, education, hearing, blood pressure, alcohol, smoking, social contact, and environmental exposures. That number is big for a reason. Inflammation in the brain is the common thread that links most of those factors to the cognitive endpoint. Each modifiable factor is an input. APOE ε4, if you carry it, is a multiplier on the input.
This has practical implications for what to do.
If you carry APOE ε4 (especially two copies), what you breathe, eat, sleep on, and are exposed to matters more than it does for people without that variant. Specifically, it’s worth thinking about:
- Traffic proximity and air quality at home. Living near a busy highway, frequent wildfire smoke exposure, indoor air quality.
- Occupational exposures. Dust, solvents, pesticides, metals.
- Mold history. Past water damage in homes or workplaces.
- Sleep quality and any history of concussion or traumatic brain injury.
- Chronic subclinical infections. Including dental infections, gut infections, and viral reactivation patterns.
What this means in our clinic
Our intake for all patients with MCI and patients who are here for preventive brain health asks specifically about outdoor air quality, occupational dust, indoor air quality, and mold history. We ask not because any single factor is diagnostic, but because the cumulative environmental load in a carrier is a different problem than in a non-carrier, and we treat it accordingly: mitigation, HEPA air filtration, serial blood-marker monitoring, and earlier, more aggressive use of the other levers in a precision medicine approach.
References — Part 5
- Kimura K, Driscoll I, Cook N, Shahzad S, Betthauser TJ, Johnson SC, Asthana S, Gallagher CL, Hermann BP, Sager MA, Blennow K, Zetterberg H, Carlsson CM, Kollmorgen G, Okonkwo OC. Associations between air pollution and markers of neuroinflammation, synaptic dysfunction and core Alzheimer’s disease pathology vary by APOE genotype. Neurotoxicity Research. 2026;44(2):19. doi: 10.1007/s12640-026-00786-2
- Li Puma DD, Boni G, Puliatti G, Bandiera B, Rinaudo M, Lerose F, De Chiara G, Palamara AT, Piacentini R, Grassi C. Neuroinflammatory responses and synaptic impairment in a Herpes simplex virus type 1 model of sporadic Alzheimer’s disease. Neural Regeneration Research. 2026. doi: 10.4103/NRR.NRR-D-25-01175
- Yoshida K, Saito N, Takahashi R, Ando S, Kashibayashi T, Fujita J, Takano K, Kowa H, Hashiramoto A. Inflammation-related miR-155-5p as an APOE ε4-modulated biomarker for amyloid pathology in mild cognitive impairment. Journal of Alzheimer’s Disease. 2026. doi: 10.1177/13872877261437135
- Devanand DP, Wisniewski T, Razlighi Q, Qian M, Wei R, Andrews HF, Acosta EP, Bell KL, Pelton GH, Deliyannides D, Perrin AC, Caccappolo E, Gershon AA, Prasad KM, Kreisl WC, Mintz A, Huey ED. Valacyclovir Treatment of Early Symptomatic Alzheimer Disease: The VALAD Randomized Clinical Trial. JAMA. 2026;335(6):511-522. doi: 10.1001/jama.2025.21738
- Horowitz RI, Freeman PR. Improving biomarkers of inflammation including phosphorylated tau with dapsone combination therapy in a patient with Lyme disease and cognitive impairment: A case report. Journal of Alzheimer’s Disease Reports. 2026;X(X):1–10. doi:10.1177/25424823261445434.
