Too soon to conclude radical progress
GLASGOW – Targeting circulating parasitic histones may hold promise for patients with cerebral malaria (CM), according to investigators.
A retrospective study, involving over 300 individuals, compared parasitic histone concentrations among patients with various forms of malaria and non-malarial illnesses, in addition to healthy controls, finding that elevated histone levels were associated with malarial disease severity and death, reported Simon Abrams, PhD, of the University of Liverpool, UK, a coauthor of the study. He noted that this research could guide the development of treatment strategies for hundreds of thousands of patients each year, particularly children.
“Cerebral malaria is the most severe form of Plasmodium falciparum infection, and despite effective anti-malarial therapy, between 10% and 20% of children that develop cerebral malaria die,” Dr. Abrams said during his presentation at the annual meeting of the British Society for Haematology. “This accounts for a huge amount of deaths per annum. Around 400,000 malarial deaths are in children in subSaharan Africa, and death typically occurs within 24 hours of hospital admission.”
In CM, the blood-brain barrier deteriorates, leading to brain swelling, hemorrhaging, clot formation, and in many cases, death, Dr. Abrams said. CM patients with the worst outcomes typically have retinal abnormalities on fundic exam, granting the disease subtype “retinopathy-positive.”
Aided by colleagues in Malawi, the investigators gathered over 300 patient samples for analysis. They found that patients with retinopathy-positive CM had higher mean extracellular histone levels than retinopathy-negative CM patients and healthy controls (22.6 mcg/ml, 6.31 mcg/ml, and 0.33 mcg/ml, respectively). In addition, retinopathy-positive CM patients who died had significantly higher levels of circulating histones, compared with similar patients who survived (35.7 mcg/ml vs. 21.6 mcg/ml).
These findings translated to predictive capability, as the investigators showed that patients with CM who had elevated histones when admitted to the hospital were at a higher risk of death than those with normal histone levels (P = .04). Unlike patients with CM, patients with uncomplicated malaria had relatively low histone levels (0.57 mcg/ml), as did patients with mild non-malarial febrile illness (1.73 mcg/ml) and non-malarial coma (1.73 mcg/ml).
During his presentation, Dr. Abrams elaborated on the origins of these histones and how they contribute to poor outcomes in patients.
“Histones are small positively charged proteins that bind to negatively charged DNA,” Dr. Abrams said. “Typically, they are found within the cell nucleus, where they are involved in the packaging of DNA. However, during cell death and cell damage, histones are released from the nucleus, extracellularly, and we find that they are very much elevated in critically ill patients that have undergone huge amounts of cell death and damage.”
Once in circulation, histones can make a bad situation even worse.
“Work by ourselves and others around the globe have found that when circulating histones are elevated in these critically ill patients, they’re extremely toxic,” Dr. Abrams said. “Histones can induce endothelial damage and vascular permeability.” In addition, he pointed out that histones are pro-inflammatory and pro-coagulant. “If you bring all of these phenomena together,” he pointed out, “histones induce organ injury and mortality in critically ill patients.”
“The current hypothesis is that if you’re treating patients with these antimalarials, and it’s killing off the parasite, it may cause the histones to be released, which is actually worse for certain patients,” Dr. Abrams explained.
Based on this hypothesis, the investigators developed an anti-histone therapy.
“We’ve got a small peptide that we use to bind to the histones that reduces their toxicity,” Dr. Abrams said. “If we coincubate the serum of [CM] patients with our anti-histone reagent and then put this onto a monolayer of endothelial cells, we see that this toxicity is inhibited. Therefore, this is suggestive that a major toxic factor within these patients are the extracellular histones.”
Providing additional support for the role of histones in cerebral toxicity, postmortem brain tissue from patients with CM showed localization of histones to the endothelium, which has been tied with increased permeability of vascular tissue. In addition, “we are seeing co-localization between the histones and the sequestration of the malarial parasite itself,” Dr. Abrams said.
Concluding his presentation, he looked to the future.
“It’s difficult to get an animal model for malaria,” but he and his associates are currently working with other investigators to develop one. Once developed, the investigators plan on testing concurrent administration of anti-malarial therapy with antihistone therapy.
“What we’re hoping is that sometime in the future, maybe we’d be able to target circulating histones in this patient cohort to improve the survival of these patients,” Dr. Abrams said.
The investigators declared no conflicts of interest.
SOURCE: Moxon et al. BSH 2019. Abstract OR-034.