Proteolytic cleavage in an endolysosomal compartment is required for activation of Toll-like receptor 9

Abstract

TLR9 binds unmethylated CpG DNA and sends signals from endolysosomes. Ploegh and colloeagues find that cleavage mediated by endolysosomal cathepsins is required for TLR9 activation. 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.