Although bacteria are often thought of as harmful, it is now recognized that the many bacteria species harbored by our intestines are essential for our well-being. Aside from their roles in eliminating toxins and extracting nutrients, there is much interest in understanding how the gut microflora might influence the development and function of our immune systems.
Building on previous work in which bacterial zwitterionic polysaccharides were shown to be presented as antigens in the activation of T cells, Mazmanian et al. observe that at least one such sugar--polysaccharide A (PSA)--can direct normal immune system development in the mouse. Reconstitution of germ-free mice with the bacterial commensal Bacteroides fragilis expanded T cell numbers and restored lymphoid structures that would otherwise have developed abnormally. Expression of PSA was sufficient and necessary for this activity and also reestablished balance in T helper 1 (TH1) and TH2 cell cytokine responses, through presentation of PSA by dendritic cells. The finding that a bacterial product can implement such direct governance over the mammalian immune system may explain how our microflora help maintain pathogen immunity while preventing unwanted inflammation and allergy. -- SJS
Cell 122, 107 (2005).
