Staphylococcus aureus Bacteria Turns Immune System Against Itself
News Dec 05, 2013
Around 20 percent of all humans are persistently colonized with Staphylococcus aureus bacteria, a leading cause of skin infections and one of the major sources of hospital-acquired infections, including the antibiotic-resistant strain MRSA.
University of Chicago scientists have recently discovered one of the keys to the immense success of S. aureus-the ability to hijack a primary human immune defense mechanism and use it to destroy white blood cells. The study was published Nov. 15 in Science.
“These bacteria have endowed themselves with weapons to not only anticipate every immune defense, but turn these immune defenses against the host as well,” said Olaf Schneewind, professor and chair of microbiology and senior author of the paper.
Neutrophils, a type of white blood cell that ensnares invaders in neutrophil extracellular traps (NETs), a web-like structure of DNA and proteins, is one of the first lines of defense in the human immune response.
Captured bacteria are then destroyed by amoeba-like white blood cells known as macrophages. However, S. aureus infection sites are often marked by an absence of macrophages, indicating the bacteria somehow defend themselves against the immune system.
To reveal how these bacteria circumvent the human immune response, Schneewind and his team screened a series of S. aureus possessing mutations that shut down genes thought to play a role in infection. They looked to see how these mutated bacteria behaved in live tissue, and identified two strains that were unable to avoid macrophage attack.
When these mutations-to the staphylococcal nuclease (nuc) and adenosine synthase A (adsA) genes respectively-were reversed, infection sites were free of macrophages again.
Looking for a mechanism of action, the researchers grew S. aureus in a laboratory dish alongside neutrophils and macrophages. The white blood cells were healthy in this environment and could clear bacteria. But the addition of a chemical to stimulate NET formation triggered macrophage death.
Realizing that a toxic product was being generated by S. aureus in response to NETs, the team used high performance liquid chromatography and mass spectrometry techniques to isolate the molecule.
They discovered that S. aureus were converting NETs into 2’-deoxyadenosine (dAdo), a molecule that is toxic to macrophages. This effectively turned NETs into a weapon against the immune system.
“Sooner or later almost every human gets some form of S. aureus infection. Our work describes for the first time the mechanism that these bacteria use to exclude macrophages from infected sites,” Schneewind said. “Coupled with previously known mechanisms that suppress the adaptive immune response, the success of these organisms is almost guaranteed.”
S. aureus bacteria are found on the skin or in the respiratory tracts of colonized humans and commonly cause skin infections in the form of abscesses or boils. Normally not dangerous, severe issues arise when the bacteria enter the bloodstream, where they can cause diseases such as sepsis and meningitis. Antibiotic-resistant strains, such as methicillin-resistant S. aureus (MRSA), are difficult to treat and have plagued healthcare systems around the world.
Schneewind and his team hope to leverage their findings toward therapies against S. aureus infections. But both genes and the dAdo molecule are closely related to important human physiological mechanisms, and Schneewind believes targeting these in bacteria, without harming human function, could be difficult.
“In theory you could build inhibitors of these bacterial enzymes or remove them,” Schneewind said. “But these are untested waters and the pursuit of such a goal requires a lot more study.”
Gilead Company Wins FDA Approval for CAR T Cancer Therapy for Treatment of Adult LymphomaNews
Kite’s Yescarta™ (axicabtagene ciloleucel) becomes first CAR T therapy approved by the FDA for the treatment of adult patients with relapsed or refractory large B-cell lymphoma after two or more lines of systemic therapy.READ MORE
SYGNIS AG Announces Completion of Relocation of its Innova Biosciences UnitNews
Move forms part of integration into SYGNIS Group.READ MORE
Mechanism Behind Inflammatory Response to Cytoplasmic DNA IdentifiedNews
A team led by LMU‘s Veit Hornung has elucidated the mechanism by which human cells induce inflammation upon detection of cytoplasmic DNA. Notably, the signal network involved differs from that used in the same context in mice.READ MORE