Using state-of-the-art brain imaging technology, scientists at the National Institutes of Health filmed what happens in the brains of mice that developed cerebral malaria (CM). The results reveal the processes that lead to fatal outcomes of the disease and suggest an antibody therapy that may treat it.
Efforts in recent years have shown a decline in malaria infections and deaths. Yet, with drug resistance on an exponential incline and increases in global temperatures threatening to expand the territory of mosquito-borne diseases, understanding the molecular mechanism that underlies this devastating parasitic infection is critical now more than ever.
Now, scientists at the National Institutes of Health (NIH) have utilized state-of-the-art brain imaging technology to record what occurs in the brains of infected mice that developed cerebral malaria (CM). The findings from this new study, which were published recently in PLOS Pathogens in an article entitled “CD8+ T Cells Induce Fatal Brainstem Pathology during Cerebral Malaria via Luminal Antigen-Specific Engagement of Brain Vasculature,” reveal the processes that lead to fatal outcomes of the disease and provide a hypothesis for an antibody therapy that may be able to treat it.
“By looking into the living brain, we were able to watch the chain of events that cause cerebral malaria to kill thousands of people every year,” explained senior study author Dorian McGavern, Ph.D., a senior investigator at the NIH's National Institute of Neurological Disorders and Stroke (NINDS). “Our study also suggests there may be a simple treatment available to stop this deadly disease.”
In 2015, there were more than 200 million cases of malaria worldwide and 400,000 deaths from the disease, mainly in children under 5 years old. For some of the infected individuals, the parasite affects the brain and causes cerebral malaria, which kills between 15% to 30% of patients with that form of the disease. Moreover, patients who survive cerebral malaria often experience long-term neurological symptoms, including cognitive impairment and limb paralysis. The cause of death from cerebral malaria is often due to brain swelling and bleeding, but the mechanisms leading to these outcomes have not been completely understood.
However, previous work in the rodent model of CM indicated that CD8+ T cells played a key role in the development of the disease, so the NIH team decided to focus its cameras on those cells. The investigators peered inside the brains of mice infected with a parasite that causes CM, using an imaging technology known as intravital microscopy, which allowed them to watch cells in action.
“Using the animal model of experimental cerebral malaria (ECM), we sought mechanistic insights into the pathogenesis of CM,” the authors wrote. “Fatal disease was associated with alterations in tight junction proteins, vascular breakdown in the meninges/parenchyma, edema, and ultimately neuronal cell death in the brainstem, which is consistent with cerebral herniation as a cause of death. At the peak of ECM, we revealed using intravital two-photon microscopy that myelomonocytic cells and parasite-specific CD8+ T cells associated primarily with the luminal surface of CNS blood vessels. Myelomonocytic cells participated in the removal of parasitized red blood cells (pRBCs) from cerebral blood vessels, but were not required for the disease.”
The researchers were able to show that as pRBCs adhere to cerebral blood vessels (a hallmark of CM), the immune system attempts to clean them off. Despite these efforts, endothelial cells making up the walls of cerebral blood vessels shed bits of the parasite, which CD8+ T cells recognize, causing those immune cells to attach to and attack the vessels. Once the CD8+ T cells amassed on the surface of brain blood vessels, the vessels began to leak. The subsequent leaking led to swelling and increased pressure in the brain, which was fatal. Results also showed that the CD8+ T cells interacted preferentially with blood vessels in the brain and not in other parts of the body.
To determine which parts of the brain were affected preferentially by these events, the researchers injected mice with dyes that marked dead cells and blood vessel leakage. Interestingly, the results indicated that the brain regions with the most damaged vessels and cell death were the olfactory bulb and more crucially, the brainstem—which controls such vital functions as breathing and heart rate.
“These movies show us a terrible side effect sometimes associated with malaria,” Dr. McGavern noted. “The parasite can fool the body's immune system into attacking the blood vessels within its own brain.”
Additionally, the NIH team tested a potential therapy to see if it could be used to remove the CD8+ T cells from vessel walls. Initially, they watched as CD8+ T cells began to interact with the cerebral blood vessels in the CM mice. Then, they treated the mice with two FDA-approved, intravenous drugs that block the molecules that CD8+ T cells use to attach to blood vessels. Within 30 minutes of the treatment, the CD8+ T cells broke off from the blood vessels and could not stick to them, preventing the fatal brain swelling in all the treated mice. These findings suggest that the interactions between CD8+ T cells and blood vessels lead to death from CM and preventing that binding may increase survival from the disease.