Yeast Gets Tough with Bad Hospital-acquired Bug

Beneficial bugs get tough and fight back when toxin-toting Clostridium difficile bacteria invade their territory. In a study published in the August Journal of Biological Chemistry, Ciaran Kelly, HMS associate professor of medicine at Beth Israel Deaconess Medical Center, first author Xinhua Chen, and colleagues found that the probiotic yeast Saccharomyces boulardii actively protects cells from a toxin produced by the pathogenic C. difficile, a common cause of nosocomial infection.

Enteritis averted. The pathogenic bacterium C. difficile releases Toxin A when it attacks intestinal cells. This prompts the release of cytokines, inducing intestinal damage, inflammation, and fluid secretion. S. boulardii, a probiotic yeast, halts this havoc, though researchers have yet to identify the specific factors conferring its protective effect. Illustration by Rachel Eastwood

The researchers noted S. boulardii’s decades-long history of clinical effectiveness in widespread use as a probiotic for treating intestinal disorders caused by infections, toxins, or antibiotics. “It seemed to work so broadly that we hypothesized it was acting on the host response, rather than the noxious agent itself,” said Kelly. Discovering exactly how S. boulardii—a close relative of brewer’s yeast or possibly even the same organism—exerts its beneficial effects could help treat Crohn’s disease, intestinal infections, and perhaps eventually inflammatory conditions in other parts of the body. “It’s very likely that there are specific factors produced by S. boulardii that, if identified and isolated, might be useful as medication,” said Kelly.

When C. difficile infects the intestine, it releases Toxin A, the primary cause of this bacterium’s injurious and potentially fatal effects. Toxin A activates several MAP kinases, triggering the release of numerous inflammatory cytokines. The researchers measured a representative of this group, IL-8, to quantify the proinflammatory reaction induced by C. difficile.

Using a human colon cell line, increases in IL-8 production could be induced either by applying Toxin A itself or IL-1 beta, a cytokine released in the initial inflammatory reaction. The addition of S. boulardii, however, exerted a protective effect, significantly reducing IL-8 production by the colon cells.

In an in vivo mouse model, Toxin A caused a 2.2-fold increase in fluid secretion from intestine and a 4.5-fold increase in the chemokine KC, the mouse equivalent of IL-8. Pretreatment with S. boulardii inhibited both of these damaging effects. To measure the degree of Toxin A–induced injury to the intestinal tissue, a pathologist, blinded to treatments, assessed epithelial necrosis, neutrophil infiltration, and mucosal edema. A 3.3-fold increase in injury compared with the control occurred with injection of Toxin A, but again, S. boulardii pretreatment prevented the damage.

Exposure of intestinal cells to Toxin A or IL-1 beta activates three major MAP kinase pathways. Interestingly, when looking at each of these pathways separately, researchers noted that S. boulardii inhibited two, the JNK and Erk MAP kinases, but did not affect the third, p38 MAP kinase. “There’s specific inhibition of certain pathways that appear to result in reduction in the injurious effects of the toxin,” said Kelly, but the mechanisms behind this selectivity remain unknown.

Kelly’s laboratory is now searching for the specific factors S. boulardii produces to defend against the likes of C. difficile. “Our ultimate goal is to understand, in finer detail, how these probiotics are effective so that we could increase their efficacy,” said Kelly.

—Kathleen Fink

Africa-derived Gene Region Raises Risk for Early Prostate Cancer

A gene on chromosome 8 is likely to be an important risk factor for prostate cancer, especially in African Americans, according to a study published online in Proceedings of the National Academy of Sciences on Aug. 21. Though prostate cancer is a complex disease that is known to be partially heritable, so far no genes have been clearly implicated.

The study, led by David Reich, assistant professor of genetics at HMS and the Broad Institute, used the admixture mapping technique, which he helped develop. The method takes advantage of the different ancestries of multiethnic populations to hunt for regions of the genome that confer risk of disease. Prostate cancer is a particularly promising test case for the technique because African-American men have about a 60 percent higher risk than other populations. The African-American population has only recently experienced mixture with European Americans. “As a result, the genome hasn’t had a lot of time to break up by recombination,” Reich said. Because of this, researchers can quickly scan a relatively small set of markers to locate regions of the genome that account for differences in the incidence of disease.

After examining more than 1,500 samples from African-American patients with prostate cancer, the team identified a region that confers a significantly higher risk of the disease in men who have inherited DNA from their African rather than their European ancestors at that section of chromosome 8. Reich noted that if there was a way to intervene medically to remove the added risk of carrying the Africa-derived variant, it would cut the rate of the disease in African Americans under the age of 72 by about half.

Significantly, the variant was associated with earlier onset of disease, which may explain why the higher risk of prostate cancer is more pronounced in younger African-American men.

A recent study from deCODE Genetics in Iceland identified the same region on chromosome 8 as a major risk factor for prostate cancer, primarily using Europe-derived populations. This study also went further in identifying a specific genetic variant as a risk factor. The HMS paper confirms the deCODE study in four different ethnic groups, but also finds that the variant identified by deCODE, while important, does not account for the vast majority of the added risk to African Americans.

That means a significant cancer gene remains to be uncovered in this 3.8-megabase stretch of DNA. First author Matthew Freedman, an HMS instructor in medicine at the Dana–Farber Cancer Institute, said the team is now investigating the most obvious candidate, the oncogene Myc, but “there’s a lot of other real estate there we need to cover.”

—Courtney Humphries

Dendritic Cells May Be Itinerant Teachers of Immune Tolerance

Dendritic cells are among the rarest and most powerful cells in the immune system. Scattered in isolated lairs in the tissues and blood, these spider-shaped cells are on the constant lookout for invaders. Once the pathogen is caught, they chew it up and travel to the lymph nodes, where they present the resulting foreign antigen to cytotoxic T cells. It now appears that the lymph nodes are not their only stop. Roberto Bonasio and colleagues have spotted immature dendritic cells entering the thymus and educating T cell precursors about what cells to attack and avoid—a feat once thought unlikely.

“The paradigm has been, what happens in the periphery stays in the periphery,” said Ulrich von Andrian, the Edward Mallinckrodt Jr. professor of immunopathology at HMS. “This new study suggests that the paradigm has to be at least softened, if not reconsidered.” The findings by Bonasio, HMS graduate student in immunology, von Andrian, and colleagues appeared online Sept. 3 in Nature Immunology.

Though dispersed sparsely throughout the tissues and the blood, dendritic cells have an enormous impact once they arrive in the lymph nodes. A single cell can exchange information with 5,000 lymphocytes in an hour. In 2005, Bonasio and colleagues observed dendritic cells migrating to the bone marrow. They thought the cells might also be traveling to the thymus. To find out, they infused green fluorescent–stained dendritic cells into mice. Sure enough, the cells homed to the thymus.

To see what they were doing there, the researchers introduced dendritic cells once again, this time loaded with a particular antigen, into the blood of mice. The thymuses of these mice were engineered to contain two distinct populations of nascent T cells, one recognizing the antigen and the other not. Only the antigen-sensitive thymocytes were deleted, suggesting the dendritic cells had homed to the thymus, where they deleted the antigen-bearing cells.

Still, it was not clear that dendritic cells migrate to the thymus in vivo. In a final and telling experiment, Bonasio and colleagues painted the skin of mice, which is relatively rich in dendritic cells, with a fluorescent substance. Days later, they could see fluorescent dendritic cells in the spleen and thymus of the animals. During their experiment, the researchers identified two homing molecules used by dendritic cells on their journey to the thymus. They blocked one of these with antibodies, and the fluorescent dendritic cells never showed up in the thymus.

The findings could help explain a longstanding enigma. “How is it possible to maintain tolerance to antigens that are present in the periphery, but not expressed in the thymus?” said Bonasio. “Also, there are innocuous antigens not encoded in the genome that are part of the body, such as commensal bacteria in the intestines.” The findings suggest that dendritic cells may be traveling back to the thymus with the crucial information.

—Misia Landau

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