Tolerance by TLR

Sci. Signal.  24 May 2011:
Vol. 4, Issue 174, pp. ec147
DOI: 10.1126/scisignal.4174ec147

Our bodies harbor more than a billion bacteria, the majority of which are commensal rather than pathogenic. Why does the immune system attack only pathogenic bacteria? Round et al. examined the underlying immune mechanisms that allow for the colonization of colonic mucosal tissue by the human commensal, Bacteroides fragilis. In germ-free mice and in mice colonized with only B. fragilis, the B. fragilis–secreted molecule, polysaccharide A, signaled through Toll-like receptor 2 (TLR2) to promote immune tolerance. This finding is rather unexpected because TLR2 signaling is typically associated with immune activation and bacterial clearance. Thus, commensal bacteria may produce factors that promote tolerance by signaling through classical innate immune response pathways.

J. L. Round, S. M. Lee, J. Li, G. Tran, B. Jabri, T. A. Chatila, S. K. Mazmanian, The Toll-like receptor 2 pathway establishes colonization by a commensal of the human microbiota. Science 332, 974–977 (2011). [Abstract] [Full Text]