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J Exp Med. 2006 Dec 25;203(13):2853-63. Epub 2006 Dec 18.

A bacterial carbohydrate links innate and adaptive responses through Toll-like receptor 2.

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  • 1Channing Laboratory, Department of Medicine, Brigham and Women's Hospital, and Department of Microbiology and Molecular Genetics, Harvard Medical School, Boston, MA 02115, USA.


Commensalism is critical to a healthy Th1/Th2 cell balance. Polysaccharide A (PSA), which is produced by the intestinal commensal Bacteroides fragilis, activates CD4+ T cells, resulting in a Th1 response correcting the Th2 cell skew of germ-free mice. We identify Toll-like receptors as crucial to the convergence of innate and adaptive responses stimulated by PSA. Optimization of the Th1 cytokine interferon-gamma in PSA-stimulated dendritic cell-CD4+ T cell co-cultures depends on both Toll-like receptor (TLR) 2 and antigen presentation. Synergy between the innate and adaptive responses was also shown when TLR2-/- mice exhibited impaired intraabdominal abscess formation in response to B. fragilis. Commensal bacteria, using molecules like PSA, potentially modulate the Th1/Th2 cell balance and the response to infection by coordinating both the innate and adaptive pathways.

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