Molecular Mechanisms of TLR2-Mediated Antigen Cross-Presentation in Dendritic Cells

Abstract

Cross-presentation is a key function of dendritic cells (DCs), which present exogenous Ags on MHC class I molecules to prime CTL responses. The effects of TLR triggering on the cross-presentation of exogenous Ags by DCs remain unclear. In this study, we used synthetic dipalmitoylated peptides and TLR2 agonist–conjugated peptides as models to elucidate the mechanisms of TLR2-mediated cross-presentation. We observed that the internalization of dipalmitoylated peptides by bone marrow–derived DCs was facilitated by TLR2 via clathrin-mediated endocytosis. The administration of these dipalmitoylated peptide-pulsed bone marrow–derived DCs eliminated established tumors through TLR2 signaling. We further demonstrated that the induction of Ag-specific CTL responses and tumor regression by dipalmitoylated peptides was TAP independent. In addition, presentation of dipalmitoylated peptides by MHC class I molecules was blocked in the presence of an endosomal acidification inhibitor (chloroquine) or a lysosomal degradation inhibitor (Z-FL-COCHO). The endocytosed dipalmitoylated peptide also passed rapidly from early endosome Ag-1–positive endosomes to RAS-related GTP-binding protein 7 (Rab7)–associated late endosomes compared with their nonlipidated counterparts. Furthermore, we found that dipalmitoylated peptide–upregulated Rab7 expression correlated with Ag presentation via the TLR2/MyD88 pathway. Both JNK and ERK signaling pathways are required for upregulation of Rab7. In summary, our data suggest that TLR2-mediated cross-presentation occurs through the upregulation of Rab7 and a TAP-independent pathway that prime CTL responses.

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“…Since in the vacuolar pathway the presented peptides must be generated in the endocytic compartment, this pathway may operate in more catabolic endocytic compartments. This idea is consistent with findings that the route of antigen uptake and/or the activation state of the APC may promote vacuolar XPT, as targeting antigen to TLR2 results in peptide-loading of MHC I molecules in later, rab7+ LAMP1+ vesicles (142). …”

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