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 anti-tumor immunity in a melanoma model. Here we assessed the ability of CLEC9A to mediate antigen presentation by human BDCA3+ mDCs, which represent less than 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 Toll-like receptor-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.
Schreibelt, G., Klinkenberg, L. J. J., Cruz, L. J., Tacken, P. J., Tel, J., Kreutz, M., Adema, G. J., Brown, G. D., Figdor, C. G., & de Vries, I. J. M. (2012). The C type lectin receptor CLEC9A mediates antigen uptake and (cross-)presentation by human blood BDCA3+ myeloid dendritic cells. Blood, 119(10), 2284-2292. https://doi.org/10.1182/blood-2011-08-373944