Immature dendritic cells capture antigens by phagocytosis, macropinocytosis or via interaction with a variety of cell surface receptors and endocytosis. The most prevalent antigen receptors expressed by dendritic cells include members of the C-type lectin family. For example, DEC-205, a type I C-type lectin containing multiple calcium-dependent binding domains and a unique cytoplasmic tail, may function in directing captured antigens to specialized antigen-processing compartments within dendritic cells.
Additional type II C-type lectins considered important for receptor-mediated antigen uptake include dendritic cell immunoreceptor (DCIR), dendritic cell-associated C-type lectin-2 (dectin-2) and C-type lectin receptor 1 (CLEC-1). DCIR contains a cytoplasmic immunoreceptor tyrosine-based inhibitory motif (ITIM) and can bind glycosylated ligands. The macrophage mannose receptor (MMR) and Fc receptors for immunoglobulins, Fc-gamma R, are also involved in antigen handling by immature dendritic cells. DCIR, MMR, and Fc-gamma R are all down-regulated upon dendritic cell maturation, further emphasizing their specific roles in antigen uptake in immature dendritic cells.