'Knockout' Surprise: Mice More Likely To Survive Malaria When Key Gene Is Disabled In White Blood Cell

One-quarter of mice in which the alpha D gene was knocked out survived a severe form of malaria, compared with zero survivors in a group that had functioning alpha D genes. The study confirms the role of alpha D in the body’s immune response to infection, but also indicates the molecule can cause uncontrolled inflammation that makes malaria, and possibly other systemic infections, even more deadly.

“The finding was contrary to what we expected,” said Guy A. Zimmerman, M.D., professor of internal medicine, director of the University of Utah’s Program in Human Molecular Biology and Genetics, and the study’s corresponding author. “We thought knocking out alpha D would make the mice more susceptible to death.”

Alpha D belongs to a family of receptor proteins called integrins, which are found on cell surfaces and allow them to receive and integrate outside information and adhere to other cells. Alpha D and another protein called Beta 2 bind to form a complete integrin, named Alpha D Beta 2 or Alpha D for short. One of Alpha D’s specialized functions is to help white blood cells called macrophages recognize the presence of pathogens and systemic infections in the blood, such as malaria and Salmonella. Because of this role, the researchers thought alpha D would protect mice from the effects of a severe type of malaria.

But working with two groups of mice that had malaria, the researchers found those with alpha D experienced 100 percent mortality within 30 days of infection. The other group, in which the molecule had been knocked out, suffered 75 percent mortality—or 25 percent survival.

Those results don’t translate into a new therapy to help humans survive malaria, but given the 2 million annual deaths from the disease, most of them in children, Zimmerman finds the results encouraging. More than 90 percent of malaria deaths occur in sub-Saharan Africa, but the disease is found worldwide in countries with tropical climates. Symptoms of malaria include fever, chills, and violent shaking. Severe forms of the disease can cause fatal anemia, coma, and lung inflammation. If the results of the study could be replicated in people, preventing 25 percent of deaths from malaria would save 500,000 lives a year.

The study’s co-authors include a group from Brazil, where more than 500,000 cases of malaria are reported each year, which totals more than half of all malaria cases in Central and South America. The Brazil collaborators developed the mouse model of malaria used in the study.

Inflammation, a process in which macrophages and other white blood cells activate in response to infection or trauma, is a protective reaction of the body; but if it goes unchecked, inflammation damages tissue in organs such as the brain, lungs, and heart. Malaria attacks the brain, which might explain the higher death rate in the mice with alpha D, according to Zimmerman. He suspects the increased mortality may have occurred because alpha D mistakenly transmitted signals that caused uncontrolled inflammation in the mice. The investigators are now trying to determine if alpha D on brain macrophages cause this injurious effect.

Although alpha D plays an injurious role in a systemic infection such as malaria, it protects against other systemic infections. The researchers studied two other groups of mice with Salmonella, a bacterial blood infection, and found that those in which alpha D had been knocked out experienced 100 percent mortality from the infection; but those with functioning alpha D experienced up to 15 percent fewer deaths.

The study clarifies the role of alpha D in inflammation and the body’s response to infection and trauma. The researchers suspected it is involved in fighting infection after characterizing its expression in wild mice and finding it on macrophages in bone marrow, the thymus (an organ that helps regulate immune responses), and in an area of the spleen that clears the body of infected red blood cells and removes foreign material from the blood. 

The dual role of alpha D illustrates the complex balance of the body’s immune response system, according to Zimmerman. “The immune system has evolved to defend people from infection and injury,” he said. “But if the system becomes unbalanced (such as with uncontrolled activation of macrophages that express alpha D) it can contribute to inflammatory disease.”

The researchers report in the Dec. 21 issue of the Journal of Immunology online. Collaborators on the study include researchers from the University of Utah Program in Human Molecular Biology and Genetics; the Instituto Oswaldo Cruz, Rio de Janeiro, Brazil; Tokyo Medical and Dental University; Lerner Institute, Cleveland; and Oklahoma Medical Research Foundation, Oklahoma City.