Long before the body gets around to making antibodies, the innate immune system is doing heavy lifting against bacterial invaders.
And research published in the Feb. 21, 2006, issue of Current Biology suggested a way to help - by blocking a DNA-cutting enzyme that lets bacteria escape the white blood cells that are the innate immune system’s first line of defense.
Neutrophils are a type of white blood cells, and it was discovered a few years back that they use extracellular traps, now called NETs, to capture and destroy their foes, the bacterial bad guys.
"It was always assumed that neutrophils gobble up bacteria," said Victor Nizet, associate professor of pediatrics at the University of California at San Diego medical school. "But then it was discovered that even if you block neutrophils’ ability to take up bacteria, they are still pretty good at killing them."
The neutrophil nets consist of DNA and histone proteins; Nizet told BioWorld Today that they are "sticky, like a trap or a spider web."
Ever resourceful, some bacteria have acquired a DNA-cutting enzyme, or DNAse, that allows them to snip away that web and get back onto the body’s open road, the bloodstream. Nizet and his colleagues at UCSD and the Veterans Affairs Medical Center in Memphis, Tenn., wanted to see how important the nets are to the immune response. They used a mix of gain-of-function and loss-of function experiments, adding the DNAse gene to a strain of strep in some experiments, and deleting it from a strain that had it in others.
They found that adding the gene led to strep’s ability to degrade DNA, while taking it away destroyed that ability. Furthermore, when a strep strain with the DNAse lost only the DNAse gene, "it became very susceptible to our immune system," Nizet said. When mice were injected with both strains, one on each leg, only the DNAse-containing strain led to spreading infections.
When the scientists used the antibiotic G-actin to inhibit DNAse activity, the white blood cells were more easily able to get rid of strep bacteria in vitro, and strep virulence was reduced in vivo, though Nizet stressed that G-actin’s activity against strep was only a "proof of concept," and not sufficient to provide an off-the-shelf treatment.
The scientists also conducted real-time imaging studies to directly test whether the DNAse-degrading ability was the cause of infectious ability. "We wanted to see what was going on in real time, as the DNAse and the neutrophils were facing off against each other," Nizet said. The imaging confirmed that white blood cells rapidly made nets after exposure to bacteria and that strep strains with a DNAse gene destroyed those nets.
Nizet pointed out that a clinical approach targeting the DNAse would not kill the bacteria but block their virulence, and that that might be an advantage for avoiding drug resistance. "Every antibiotic we make tries to kill bacteria, so there is tremendous pressure to evolve resistance," he said. "Maybe the next generation will try to disarm them instead."
Other approaches to reducing virulence, like quorum sensing, have sometimes met with unexpected roadblocks. (See BioWorld Today, Feb 21, 2006.)
But Nizet was optimistic that blocking the bacterial DNAse will not face similar travails. "Quorum sensing is pivotal to everything the bacterium does," he explained, so there is strong pressure on the bacteria to find a workaround when quorum sensing is blocked. "The DNAse, as far as we know, is not critical to bacterial survival. Many strains of strep don’t make a DNAse, yet they are very successful."
He added that next-generation products are "pretty urgent, given the types of problems we’re seeing," such as multidrug resistance and an increasing need for hospitalization to treat infections. "I’m a physician, too, and it’s really a nightmare a lot of the time," he said.
To bring such advances into the clinic, Nizet co-founded in 2005 a company, HypoxyGen Inc., in San Diego, which currently is living off angel funding. Nizet said HypoxyGen seeks to commercialize technology related to boosting the innate immune system "by making white blood cells kill bacteria better." The technology was discovered in the labs of Nizet and his UCSD colleagues Randall Johnson and Emmanuel Theodorakis.
Toll-like receptors, which also control the innate immune system, have been the focus of significant industry interest recently, with multiple companies getting into the game. (See BioWorld Today, Sept. 16, 2005.)
But Hypoxygen co-founder Robert Bohrer told BioWorld Today that most companies working on Toll-like receptors "have very targeted approaches to activation of innate immunity, via the interferon pathway, with different drugs for different targets, at least as far as I understand their programs. Our approach does not operate via the interferon pathway, and is very broad spectrum, potentially including antiviral, antibiotic and antifungal responses."
Though Bohrer dryly noted that the founders "all still have day jobs," he also said that the company expects to secure larger funding within the next six months. While government grants for bioterrorism research are "a possibility," the main focus is on securing private-sector funding.