Toll-like receptors (TLRs) activate the innate immune system in response to pathogens. Here we show that TLR9 proteolytic cleavage is a prerequisite for TLR9 signaling. Inhibition of lysosomal proteolysis rendered TLR9 inactive. The carboxy-terminal fragment of TLR9 thus generated included a portion of the TLR9 ectodomain, as well as the transmembrane and cytoplasmic domains. This cleavage fragment bound to the TLR9 ligand CpG DNA and, when expressed in Tlr9-/- dendritic cells, restored CpG DNA-induced cytokine production. Although cathepsin L generated the requisite TLR9 cleavage products in a cell-free in vitro system, several proteases influenced TLR9 cleavage in intact cells. Lysosomal proteolysis thus contributes to innate immunity by facilitating specific cleavage of TLR9.
Bibliographical noteFunding Information:
We thank G.-P. Shi (Brigham and Women’s Hospital and Harvard Medical School) for the selective inhibitors of cathepsins S, L and K and for mice deficient in cathepsins K, S and L; S. Akira (Osaka University), A. Marshak-Rothstein (Boston University) and K. Kiefer (Boston University) for Tlr9–/– mice; S.K. Dougan and C. Schlieker for critical reading of the manuscript; and T. DiCesare for graphic design. Supported by the National Institutes of Health (H.L.P.), Novartis (H.L.P.), the Charles A. King Trust, Bank of America (M.M.B.) and the Whitehead Institute for Biomedical Research, Landon T. Clay (B.R.).
All Science Journal Classification (ASJC) codes
- Immunology and Allergy