DNA of good bacteria drives intestinal response to infection

Oct 02, 2008

A new study shows that the DNA of so-called "good bacteria" that normally live in the intestines may help defend the body against infection.

The findings, available Oct. 2 online in the journal Immunity, are reported by Yasmine Belkaid, Ph.D., and her colleagues in the Laboratory of Parasitic Diseases at the National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health.

A person normally has 300 to 500 species of beneficial bacteria, known as commensals, in their intestines. These bacteria are not harmful and, in fact, help an individual maintain his or her digestive health. Typically, the immune system does not attack gut commensals, even though they are bacteria.

"Within the body of a healthy adult, microbial cells vastly outnumber human cells. Research to understand these microbial communities is an exciting scientific frontier," says Anthony S. Fauci, MD, NIAID director. "Among many opportunities related to the so-called 'microbiome,' targeting beneficial bacteria may offer new avenues for therapy against infectious and immune-mediated diseases."

Just how commensals protect against harmful bacteria, known as pathogens, is a complex question. "Pathogens often behave similarly to gut commensals," Dr. Belkaid says. Because the body needs commensals but also has to rid itself of disease-causing microbes, the immune system must distinguish the good bugs from the bad ones.

One mechanism of protection is through the interaction between the commensals and certain immune cells in the intestines. This interaction occurs through the binding of the commensals to receptors on the T cells known as Toll-like receptors (TLRs).

In healthy individuals, some intestinal T cells (known as Tregs) play a regulatory role, recognizing commensals and keeping the immune system from attacking them. During an infection, however, T cells shift into attack mode to fight the infection. The factors controlling this shift from defense to offense have not been well understood.

Dr. Belkaid's team describes a novel way in which the Tregs are regulated to facilitate an immune response to a pathogen. They found that during an infection, the DNA of the body's beneficial bacteria binds to a specific receptor on the intestinal immune cells, called TLR9. The binding of commensal DNA to TLR9 in the presence of a pathogen prevents the generation of Tregs in favor of the generation of protective T cells. These protective T cells can then clear the body of the invading pathogen.

In effect, the commensal DNA acts as a natural adjuvant by boosting the activity of T cells so they can destroy the invading pathogen.

"There is a balance of regulatory immune signals in the body," notes Dr. Belkaid. "During an infection, we've found that commensals can break this balance in favor of an infection-fighting response."

While the immune system must react to invading pathogens to maintain health, an immune response to commensals can cause problems. For example, certain inflammatory bowel diseases, such as Crohn's disease, are thought to be caused in part by immune reactions against commensal bacteria.

Understanding how commensals interact with the immune system opens up the possibility of using beneficial bacteria as targets for future oral therapies against infections or autoimmune diseases.

Source: National Institute of Allergy and Infectious Diseases

Explore further: US scientists make embryonic stem cells from adult skin

add to favorites email to friend print save as pdf

Related Stories

Compound in Apples Inhibits E. coli O157:H7

Dec 16, 2011

A compound that is abundant in apples and strawberries inhibits the highly pathogenic E. coli O157:H7 biofilms while sparing a beneficial strain of E. coli that also forms biofilms in the human gut, according to a paper in ...

Learning to tolerate our microbial self

Apr 21, 2011

(PhysOrg.com) -- The human gut is filled with 100 trillion symbiotic bacteria—ten times more microbial cells than our own cells—representing close to one thousand different species. "And yet, if ...

Recommended for you

Leeches help save woman's ear after pit bull mauling

Apr 18, 2014

(HealthDay)—A pit bull attack in July 2013 left a 19-year-old woman with her left ear ripped from her head, leaving an open wound. After preserving the ear, the surgical team started with a reconnection ...

New pain relief targets discovered

Apr 17, 2014

Scientists have identified new pain relief targets that could be used to provide relief from chemotherapy-induced pain. BBSRC-funded researchers at King's College London made the discovery when researching ...

User comments : 0

More news stories

Filipino tests negative for Middle East virus

A Filipino nurse who tested positive for the Middle East virus has been found free of infection in a subsequent examination after he returned home, Philippine health officials said Saturday.

Study says we're over the hill at 24

(Medical Xpress)—It's a hard pill to swallow, but if you're over 24 years of age you've already reached your peak in terms of your cognitive motor performance, according to a new Simon Fraser University study.

NASA's space station Robonaut finally getting legs

Robonaut, the first out-of-this-world humanoid, is finally getting its space legs. For three years, Robonaut has had to manage from the waist up. This new pair of legs means the experimental robot—now stuck ...

Ex-Apple chief plans mobile phone for India

Former Apple chief executive John Sculley, whose marketing skills helped bring the personal computer to desktops worldwide, says he plans to launch a mobile phone in India to exploit its still largely untapped ...

Egypt archaeologists find ancient writer's tomb

Egypt's minister of antiquities says a team of Spanish archaeologists has discovered two tombs in the southern part of the country, one of them belonging to a writer and containing a trove of artifacts including reed pens ...

Airbnb rental site raises $450 mn

Online lodging listings website Airbnb inked a $450 million funding deal with investors led by TPG, a source close to the matter said Friday.