Abstract
Recognition of microbial components is critical for activation of TLRs, subsequent innate immune signaling, and directing adaptive immune responses. The DNA sensor TLR9 traffics from the endoplasmic reticulum to endolysosomal compartments where it is cleaved by resident proteases to generate a competent receptor. Activation of TLR9 by CpG-motif containing oligodeoxynucleotides (CpG ODNs) is preceded by agonist endocytosis and delivery into the endolysosomes. The events that dictate this process remain largely unknown; furthermore, it is unclear whether the receptors involved in mediating uptake of exogenous DNA are conserved for both naturally derived pathogenic DNA and synthetic ODNs. In this study, we report that peritoneal macrophages from a wild-derived inbred mouse strain, MOLF/Ei, are hyporesponsive to CpG ODN but are fully responsive to bacterial DNA, thus implying that microbial recognition is not fully recapitulated by a synthetic analog. To identify the gene responsible for the CpG ODN defect, we have performed genome-wide linkage analysis. Using N2 backcross mice, we mapped the trait with high resolution to a single locus containing Mrc1 as the gene conferring the trait. We show that mannose receptor 1 (MRC1; CD206) is involved in CpG ODN uptake and trafficking in wild-derived MOLF/Ei peritoneal macrophages. Furthermore, we show that other strains of wild-derived mice also require MRC1 for CpG-induced cytokine responses. These findings reveal novel functions for MRC1 and demonstrate that wild-derived mice are important and indispensable model for understanding naturally occurring regulators of inflammatory responses in innate immune pathways.