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Brain Response to Immune Challenge

Principal Investigator: Jarred Younger, Ph.D.
University of Alabama at Birmingham
(January 2020 – project delayed by COVID)

“The key piece for understanding fibromyalgia is the concept of sensitized or “primed” microglia (the brain’s immune cells), which are operating on a hair-trigger,” says Younger. “Primed microglia react with full force to events that would not normally activate them. If the microglia in individuals with fibromyalgia are primed, a tiny trigger may set them off and produce low-level inflammation in the brain.”

Ordinarily the microglia are in a resting state (M0), quietly surveying the environment in the brain. When an infectious agent invades the body, the microglia rapidly transition to an activated state (M1) to neutralize the threat by producing pro-inflammatory cytokines, excitatory chemicals, and neurotoxic factors. All of these substances interact with the neurons to contribute to flu-like symptoms, such as pain, fatigue, cognitive dysfunction, and sleep disturbances.

While fibromyalgia resembles the flu, the process of microglia morphing into an activated state during an infection occurs in everyone. In fact, once the threat is resolved, these cells go back to their resting state and all flu-like symptoms disappear in healthy people. But what if there is something else going on in fibromyalgia patients to keep more of their microglia in an active, pro-inflammatory state?

If the microglia are already sensitized, they could easily become activated by small daily triggers that would normally go unnoticed. Persistently active microglia could overwhelm the energy demands of the brain, causing metabolites to stockpile and lead to the chronic symptoms of fibromyalgia. It may also explain why infections make the disease worse and the recovery time is prolonged. But the hows and whys for these events remain hazy.

What could be sensitizing the microglia in people with fibromyalgia so that these cells become “stuck” in an activated state? Perhaps more important, how can it be shown that a very mild infection causes these brain cells to go ballistic while it doesn’t trigger a response in healthy people?

Addressing the initial issue of sensitized microglia, Younger says, “One cause could be abnormal crosstalk between peripheral immune signals and microglia.” In other words, there is something in the blood that is sending exaggerated distress alarms to the brain. Younger suspects an appetite-regulating hormone called leptin is the likely culprit.

Why leptin? For starters, blood levels of this hormone correlate with body pain in otherwise healthy women. But there is more. Leptin readily crosses from the blood into the brain, so it can directly influence the microglia. In addition, Younger has preliminary data to implicate this hormone’s role specifically in people with fibromyalgia. The diagram below illustrates how the normally “resting” microglia might be in a predominantly hypersensitive or primed state. In turn, this would lead to an abundance of activated M1 cells.

Putting together Younger’s hypothesis, fibromyalgia patients must have two important steps occurring simultaneously, such that they feed off one another:

  1. A substance in the blood, perhaps leptin, communicates with the microglia to keep them sensitized and ready to charge into action.
  2. An exaggerated immune and inflammatory response in the brain, as reflected by increased metabolites, occurs any time patients are exposed to a mild pathogen … something that would not trigger a response in healthy people.

Scanning the brain for metabolites linked to microglia activation is expected to show an accumulation of undesirable chemicals, but this alone won’t explain why the microglia are misbehaving. Younger will use a three-part protocol to demonstrate that the above two processes are taking place in fibromyalgia patients but not healthy controls. First, metabolites are measured. Second, the study participants are injected with a challenge. Third, metabolites are measured again. Symptoms and blood levels of leptin, as well as other candidates, will also be analyzed.

The first step involves using molecular resonance spectroscopic (MRS) imaging of the brain. MRS scans can measure levels of various metabolic end products. Normally these chemicals are whisked away by the blood. However, metabolites can accumulate if the brain’s processes are ramped up, as would be expected with increased microglia activation. The temperature throughout the brain will also be measured because activated microglia burn more energy and generate more heat.

Younger has performed this initial metabolite detection phase in a group of 15 chronic fatigue syndrome patients, but only two of them met the widespread pain criteria for fibromyalgia. Given that the symptoms of these two patient groups greatly overlap, it’s important to note that Younger found brain areas of elevated metabolites and increased temperature.

For the current project, the MRS data collected in the study’s first step should be viewed as a baseline measure for the 15 fibromyalgia patients and 15 healthy controls. Why? Subjects are excluded if they have recently undergone a trigger known to make fibromyalgia symptoms worse. These include a viral or bacterial infection, a surgery, or a vaccination. In fact, participants cannot even be taking an anti-inflammatory medication for a pulled muscle or headache. In addition, multiple tests must confirm that the immune system in the blood is just quietly humming along and not fending off infections.

In the second step of the study, all subjects are injected with lipopolysaccharide (LPS) to produce a mild immune stimulus. Bacterial cell walls contain LPS, so when it is injected into subjects, it can trigger an immune response. Of course, it’s just a short-lived trick to activate the immune system because no bacteria are actually present.

One can’t simply grind up bacteria and inject this concoction into study subjects. Safety measures are in place to prevent causing a “live” infection. Researchers must obtain purified LPS from the National Institutes of Health after they have received FDA approval for its use in their study.

The goal of this second phase of the project is to use a mild strength of LPS that would not be expected to cause a measurable immune response (or any symptoms) in healthy people. To achieve this, Younger will be significantly diluting the LPS dose. If his hypothesis that the microglia in fibromyalgia patients are already hypersensitized (or primed), the microglia should respond by going “off the rails.”

The third step of this project involves putting all study participants back into the MRS scanner two hours after they have been injected with LPS. Metabolite levels and temperatures throughout the brain will be measured again. Compared to the first scan, everything should look a lot worse after the LPS injection in the patient group, but not in the healthy controls. Leptin, and perhaps other substances in the blood, should also respond in an exaggerated fashion in the patients, but there should be no change in the healthy subjects.

Preliminary evidence by Younger shows that leptin production is highly abnormal in fibro individuals compared to healthy people. Are there other substances that might respond errantly to an LPS challenge in fibro? What about changes in the brain? Does the LPS exaggerate the metabolite levels or temperature hot spots? Are there specific regions in the brain that are more affected by the LPS? Given that MRS scans are expensive, are there any indicators in the blood that might reflect what is happening in the brain? These are all questions that Younger hopes to address.

“The detection of abnormal reactivity in the fibromyalgia brain would be a substantial step forward, opening up new avenues for diagnostics and treatments,” says Younger. But exactly how would this work? By combining the three-step protocol, he adds: “We will for the first time be able to trigger an abnormal brain immune response and monitor it. This model will also be useful to test new fibromyalgia treatments to determine if they reduce the brain’s immune hyperactive response.”