Friendly Bacteria

(Ivanhoe Newswire) -- New studies offer insight into the constant dialogue between intestinal microbes and the human immune system, pointing to a major role for a class of microbes known as segmented filamentous bacteria (SFB) in shaping the human immune response.

"It's the first example of a commensal bacteria that can induce accumulation in the gut of a highly specific branch of the immune system," Dan Littman of the Howard Hughes Medical Institute and the New York University School of Medicine, who led the study reported in Cell, was quoted as saying. "We're headed into an exciting new area, and we hope more pieces of how the microbial-host interaction contributes to health will begin to fall into place."

"Our study provides the surprising result that among the hundreds of bacterial species composing the gut microbiota -- only a very small number, the prototype of which is SFB -- can efficiently stimulate the post-natal physiologic maturation of the immune barrier," Valérie Gaboriau-Routhiau of INSERM in France, who led the Immunity report, was quoted as saying. Most notably, the studies show that the SFBs stimulate particular types of helper T cells, known as Th17 cells.

The findings by Littman's group were an accidental discovery. Researchers were studying T cells in the intestine and were getting some inconsistencies that could be traced to differences in the gut floras of mice in the presence or absence of SFB.

They found that introduction of SFB, but not other bacteria, stimulated the production of Th17 cells in mice that were otherwise deficient. The bacteria also set in motion a pro-inflammatory gene program. The SFB-induced immune response protected the mice from becoming ill with an intestinal pathogen, suggesting that the SFBs played a role in setting up the intestine's immunity barrier.

Gaboriau-Routhiau found in similar studies of mice that colonization of the gut induced a broad spectrum of pro-inflammatory and T cell responses, including the emergence of Th17 cells. That function appeared limited to a restricted number of bacteria, her team reported, the prototype of which is the SFB.

"For us, it was first a surprise to observe so little redundancy in the role of commensal bacteria on stimulating immune responses," Gaboriau-Routhiau said. "One striking feature of SFB, which makes it very different from the vast majority of the members of the microbiota, is its capacity to adhere to epithelial cells notably in the ileum, a property normally more the prerogative of pathogens." The ileum is the final section of the small intestine and has many folds, giving it a very large surface area.

The studies also suggest how such bacteria might go from beneficial inhabitants, helping to fend off nasty bugs, to ones that may tip the balance of the immune system toward the development of inflammatory autoimmune disease, such as Crohn's disease, psoriasis and even arthritis, according to the researchers. Indeed, the Th17 cells observed in the new studies have been noted in recent years because of their importance in autoimmune diseases, Littman explained.

"Th17 cells make cytokines that can be highly protective in the case of infection," he said. "At the same time, in the wrong context or in the wrong amount [they can lead to disease]. You need to have the right balance."

Given the bacterial diversity within our guts, the studies show how much there is to learn about this important aspect of the immune system. While probiotic products on the market today do not have the benefit of such a thorough understanding, said Littman, there is little doubt that down the road we may be able to manipulate our immune system in beneficial ways with microbes.

SOURCE: Cell and Immunity, October 16, 2009