Bacterium proves essential to immune system development

In the July 15, 2005 Cell, a team led by Dennis Kasper, the William Ellery Channing Professor of Medicine at Brigham and Women's Hospital and professor of microbiology and molecular genetics at Harvard Medical School (HMS), and Sarkis Mazmanian, HMS instructor in microbiology and molecular genetics, both at the Channing laboratory, reports that Bacteriodes fragilis aids immune system development.

Mammals contain approximately a thousand species, and one trillion cells, of bacteria to every gram of intestinal contents. The team studied germ-free mice and found that these mice have fewer CD4+ T cells in their immune system. When B. fragilis colonized the mice, their CD4+ T cell levels were restored.

Another team in Kasper's lab previously announced that T cells could recognize certain bacterial carbohydrates as antigens. Kasper and Mazmanian found that if the mice were colonized with a strain of B. fragilis that lacked the carbohydrate polysaccharide A (PSA), the bacteria could no longer restore T cell levels in the mice.

The team found that PSA induces the Th1 subset of T cells. The immune system relies on a balance between these cell-mediated responses and antibody-mediated, or Th2, responses. Kasper said that mice and humans raised in sterile environments have immune systems skewed toward Th2 responses. If bacterial factors like PSA are necessary for development of the Th1 arm of the immune system, it would reinforce that bacteria is essential for immune function.