Professional antigen-presenting cells are essential for the initiation of adaptive immune responses; however, they also play a vital role in the maintenance of tolerance towards self-antigens. C-type lectins can function as antigen receptors by capturing carbohydrate ligands for processing and presentation. Here, we focused on the dendritic cell (DC)-expressed macrophage galactose-type lectin (MGL), a C-type lectin with a unique specificity for terminal GalNAc residues, such as the tumor-associated Tn antigen. Soluble model antigens are efficiently internalized by MGL and subsequently presented to responder CD4 + T cells. The tyrosine-5 residue in the YENF motif, present in the MGL cytoplasmic domain, was essential for the MGL-mediated endocytosis in CHO cells. In conclusion, MGL contributes to the antigen processing and presentation capacities of DC and may provide a suitable target for the initiation of anti-tumor immune responses.
IntroductionDendritic cells (DC) efficiently capture antigens for processing and presentation to naive T cells [1]. In an immature state, DC line all peripheral tissues, where they continuously inspect their surroundings for incoming microbes or changes in local conditions. Upon pathogen encounter DC undergo maturation, consisting of a short enhanced wave of antigen uptake, followed by the up-regulation of chemokine receptors, MHC molecules and costimulatory molecules [2, 3]. Mature DC show enhanced migration towards afferent lymph nodes for initiation of naive T cell responses. However, even under steady-state conditions, semi-mature DC continuously migrate to lymph nodes to present selfantigens to T cells, thereby actively inducing T cell tolerance towards harmless self-antigens [4].DC express many different types of molecules, such as Fc receptors, scavenger receptors and the C-type lectins, that participate in the scavenging of antigens. Several C-type lectins, which recognize carbohydrate ligands in a Ca 2+ -dependent manner, have been shown to function as efficient antigen receptors [5][6][7]. Specialized internalization motifs in the C-type lectin cytoplasmic domain facilitate endosomal/lysosomal targeting, although the pathways for entry vary among the different C-type lectins studied [5, 8,9]. The mannose receptor (MR) utilizes a di-aromatic sequence (YF) for endosomal sorting, whereas DC-SIGN-mediated internalization is regulated by a dileucine motif (LL) [5,10]. Strikingly, the internalization process is equally influenced by the ligand. The intracellular trafficking of Dectin-1 depends on the size of the interacting b-glucan [8]. Although DC-SIGN carbohydrate ligands are effectively targeted and degraded in the lysosomal compartment after binding [11], some pathogens are partially able block this pathway [12,13].The uptake of exogenous ligands via C-type lectins such as DC-SIGN, MR and Dectin-1 results in present- [20][21][22]. In skin MGL is a marker for the CD1a + dermal DC, a cell type with enhanced ability to stimulate naive T cells compared to other dermal APC subs...