By attaching a microparticle "backpack" to important inflammatory cells called macrophages, researchers have significantly reduced the size of lesions and inflammation caused by brain trauma. This new approach works with biology, rather than against it, and has the potential to be an effective way to treat debilitating conditions.
In 2019, there were 27.16 million new cases of traumatic brain injury worldwide, and there are currently 48.99 million patients with traumatic brain injury. While neuroinflammation is important for promoting cell and tissue regeneration immediately after traumatic brain injury, long-term inflammation can lead to secondary injury. Activation of brain-resident macrophages—white blood cells that can switch between pro- and anti-inflammatory states—and other immune cells can expand lesions and increase the likelihood of complications such as depression, sensorimotor and memory impairments, and dementia.
Now, researchers at Harvard University's Wyss Institute have recruited macrophages to help treat traumatic brain injuries. Equipped with a microparticle "backpack" full of anti-inflammatory molecules, these macrophages significantly reduced local brain inflammation, lesion size and bleeding in pigs with traumatic brain injury.
Samir Mitragotri, one of the study's corresponding authors, said: "Millions of people suffer traumatic brain injuries every year, but currently there are no treatments other than controlling symptoms. We applied cell backpack technology - which we have previously used to improve the inflammatory response of macrophages to cancer tumors - to deliver localized anti-inflammatory treatment in the brain, which helps reduce the cascade of runaway inflammation that leads to tissue damage and death in relevant models in humans."
When brain cells die from a traumatic impact, they release a cocktail of pro-inflammatory cytokines that attract immune cells to repair the damage. But these cytokines can also disrupt the blood-brain barrier, causing blood to leak into the brain, causing swelling, oxygen supply impairment, and increased inflammation. It's a vicious cycle of bleeding and damage that leads to more cell death.
Based on their previous work outfitting macrophages with backpacks, the researchers believe these backpacks may be effective against traumatic brain injuries.
Rick Liao, co-first author of the study, said: "It is widely believed that anti-inflammatory therapies can be effective in treating traumatic brain injury, but so far, no therapy has been clinically proven to be effective. Our previous research on macrophages showed that we can use backpack technology to effectively guide their behavior when they arrive at the site of injury. Since these cells already play an active role in the body's natural immune response to traumatic brain injury, we had a hunch that we could enhance this pre-existing biological property to reduce the initial damage."
The "cellular backpack" consists of two outer layers of dexamethasone (an anti-inflammatory steroid drug) and a middle layer of IL-4 (an immunomodulatory cytokine). The outer layer of dexamethasone is polylactic acid (PLGA) and the middle layer of IL-4 is polyvinyl alcohol (PVA). Combining dexamethasone and IL-4 produces a synergistic anti-inflammatory effect. The backpacks have an average diameter of 8.2 microns and a thickness of 914 nanometers, and are designed to adhere to the surface of macrophages.
Porcine macrophages were cultured from bone marrow and backpacks were attached to the macrophage surface to form backpacks-macrophage complexes. Intravenous injection was given to traumatic brain injury model pigs with injuries to the outermost cortex of the brain tissue. Seven days later, a higher density of backpacking macrophages was observed at the site of injury than in other brain regions.
Compared with pigs injected with saline, treatment with backpack macrophages reduced the total volume of large lesions by 56% and the amount of bleeding was also significantly reduced (73 cubic millimeters vs. 21 cubic millimeters). The amount of bleeding is positively correlated with the volume of the lesion, indicating that the amount of bleeding during the development of the lesion has a great influence on it. Piggybacking macrophages reduced minimal lesion volume (a measure of permanent tissue damage) by 47%. Pigs treated with backpacks also had fewer large lesions.
The researchers analyzed microglia (brain injury repair cells) in the lesions of the treated pigs and found a decrease in the pro-inflammatory marker CD80 compared with the saline group. Peripheral inflammatory biomarkers were also reduced in pigs treated with backpack macrophages. Twenty-four hours after injury, the treated pigs had 82.7% serum tumor necrosis factor alpha (TNF-237A), a major regulator of the inflammatory response, compared with 117.5% in the control group. Seven days after injury, serum glial fibrillary acidic protein (GFAP), a diagnostic and prognostic biomarker for traumatic brain injury and neuroinflammation, was lower in the treatment group (75.2%) than in the control group (158.4%).
The incidence of adverse events in treated pigs did not differ from animals receiving saline. No symptoms of toxicity due to piggyback macrophage therapy were observed in the spleen, liver, kidneys, and lungs.
Donald Ingber, founding director of the Wyss Institute, said: "The susceptibility of macrophages to local environmental influences has historically hindered scientists from fully harnessing their immunomodulatory capabilities. This impressive study describes a truly novel and potentially powerful macrophage-based therapy that can treat inflammation, the root cause of so many diseases in humans, in an effective and non-invasive way, working with biology rather than against it."
The research was published in the journal Proceedings of the National Academy of Sciences (PNAS Nexus).