A team of researchers at Childrens Hospital Los Angeles (CHLA), along with colleagues from the Keck School of Medicine of the University of Southern California and the Universite Auvergne, France, have discovered an important role for a small, naturally occurring protein called interleukin-10 (IL-10) in removing bacteria from the blood of infected mice and reversing damage to the brain.
This bacterium, Escherichia coli K1, is the most common cause of meningitis in premature infants and the second most common cause of the disease in newborns. "The ineffectiveness of antibiotics in treating newborns with meningitis and the emergence of antibiotic-resistant strains of bacteria require new strategies," explains Nemani V. Prasadarao, Ph. D, associate professor of infectious disease at Childrens Hospital Los Angeles.
Meningitis is the irritation of membranes covering the brain and spinal cord. This irritation can result from viral or bacterial infection. Bacterial meningitis can be very serious, possibly resulting in hearing loss, brain damage, or death, even when treated. Although the mortality rate can be decreased through use of antibiotics significant neurological consequences, like mental retardation, still occur in 30 - 40 percent of survivors.
"A recent surge in antibiotic-resistant strains of E. coli K1 is likely to significantly increase the rates of illness and death," said Prasadarao. "Also, the diagnosis of meningitis is difficult until the bacteria reach the cerebrospinal fluid. By that time, brain damage has begun. With large numbers of circulating bacteria, treatment with antibiotics can result in biochemical reactions that may cause septic shock and ultimately, organ failure. So identifying alternatives to antibiotic therapy is crucial."
One of a class of proteins known as cytokines, IL-10 is involved in immune function. "We found that during an episode of bacteremia, when a large number of bacteria are circulating in normally sterile blood, IL-10 acts to clear antibiotic-sensitive as well as antibiotic-resistant E. coli from the circulation of infected mice," said Rahul Mittal, Ph.D., lead author on the paper and a post-doctoral fellow in Prasadarao's lab.
They also determined that E. coli infection produced damage to the mouse brain comparable to that seen in humans. Three-dimensional imaging studies of infected animal and human infant brains showed similar gross morphological changes. "When we gave IL-10 to mice 48 hours after infection, those changes to the brain were reversed," said Mittal.
Tumor necrosis factor (TNF) is a cytokine active in producing inflammation. When the researchers replicated these experiments using antibiotic or anti-TNF, brain damage resulting from E. coli infection was not prevented.
The team also discovered a mechanism of action for IL-10 protection. In culture, using mouse and human white blood cells called neutrophils, they found that exposing these cells to IL-10 produced an increase in the number of a certain type of receptor on the surface of the neutrophils. An increase in the CR 3 receptor led to enhanced killing of bacteria.
Another white blood cell, called a macrophage, works to clear bacteria from the blood by engulfing or "eating" the pathogen. Similar to what was seen in neutrophils, macrophages treated with IL-10 showed an increase in CR 3 receptors that enhanced their ability to destroy invading bacteria.
To confirm that the CR 3 receptor is critical to the protective effect of IL-10 against E. coli, CR 3 expression was suppressed in a group of mice. Before exposing the animals to bacteria, white blood cells were examined and the CR 3 receptor was determined to be absent. These animals were exposed to E. coli and then treated with IL-10. The mice were found subsequently to have bacteria in the CSF and morphological changes indicating brain damage. The protective effect of IL-10 during bacteremia was absent in animals without CR 3 receptors. The researchers further concluded that the crucial increase in CR 3 receptor was a result of IL-10 suppressing an important inflammatory agent, prostaglandin E-2.
"Since diagnosing meningitis is difficult until bacteria reach the central nervous system, finding an agent that can clear the bacteria while also preventing or restoring the damaged brain is very exciting," said Mittal.
These studies provide a basis for exploring the use of IL-10 in newborns.
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