CLEC9A is a recently discovered C-type lectin receptor involved in sensing necrotic cells. In humans, this receptor is selectively expressed by BDCA3 ؉ myeloid dendritic cells (mDCs), which have been proposed to be the main human cross-presenting mDCs and may represent the human homologue of murine CD8 ؉ DCs. In mice, it was demonstrated that antigens delivered with antibodies to CLEC9A are presented by CD8 ؉ DCs to both CD4 ؉ and CD8 ؉ T cells and induce antitumor immunity in a melanoma model. Here we assessed the ability of CLEC9A to mediate antigen presentation by human BDCA3 ؉ mDCs, which represent < 0.05% of peripheral blood leukocytes. We demonstrate that CLEC9A is only expressed on immature BDCA3 ؉ mDCs and that cell surface expression is lost after TLR-mediated maturation. CLEC9A triggering via antibody binding rapidly induces receptor internalization but does not affect TLR-induced cytokine production or expression of costimulatory molecules. More importantly, antigens delivered via CLEC9A antibodies to BDCA3 ؉ mDCs are presented by both MHC class I (cross-presentation) and MHC class II to antigen-specific T cells. We conclude that CLEC9A is a promising target for in vivo antigen delivery in humans to increase the efficiency of vaccines against infectious or malignant diseases. (Blood. 2012; 119(10):2284-2292)
IntroductionDendritic cells (DCs) are central players in the induction of adaptive immune responses. 1 They reside in peripheral tissues, where they are positioned to capture antigens. In the immature state, DCs continuously sample their environment by receptormediated endocytosis, pinocytosis, and phagocytosis. Once DCs also encounter danger signals, such as those present in pathogens, endogenous danger molecules, inflammatory cytokines, or immune complexes, they become activated and migrate to the lymph nodes and differentiate into mature DCs, which is accompanied by stabilization of peptide-MHC complexes on the cell surface, up-regulation of costimulatory molecules, and cytokine release. These alterations contribute to optimal antigen presentation to T lymphocytes. DCs have the unique capacity to process extracellular antigens for cross-presentation via MHC class I. This feature allows DCs to induce CD8 T-cell responses against dying cells, tumor cells, and viruses that do not replicate in DCs.In human peripheral blood, 2 main populations of DCs can be distinguished: CD11c-positive myeloid DCs (mDCs) and CD11c-negative plasmacytoid DCs (pDCs). 2,3 Human mDCs can be subdivided further on the basis of differential surface expression of BDCA1 (CD1c), CD16, and BDCA3 (CD141). 4,5 Because the frequency of circulating mDCs in human blood is very low (Ͻ 2% of the peripheral blood leukocytes), many studies exploit in vitro-generated monocyte-derived DCs (moDCs) as "surrogate mDCs" because of the relative ease of obtaining large quantities of these cells. DC subsets are heterogeneous in the expression of cell surface markers and pathogen-recognition receptors, cytokine production after stimulation, as w...
