MGL‐mediated internalization and antigen presentation by dendritic cells: A role for tyrosine‐5

Abstract

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...

The C-Type Lectin Macrophage Galactose-Type Lectin Impedes Migration of Immature APCs

“…For some Ags the migratory DCs seem to act as simple ferries, while efficient CD8 ϩ T cell priming requires the transfer of Ag to LN-resident potentially MGL ϩ DCs (33,34). Furthermore, we demonstrated that MGL can also function as an genuine Ag uptake receptor that facilitates Ag presentation in MHC class II (35). Thus, the MGL-sinus interaction would certainly retain the immature DCs at their strategic localization.…”

The Macrophage Galactose-Type Lectin Can Function as an Attachment and Entry Receptor for Influenza Virus

“…Alternatively, MGL1 itself could act as both an attachment and entry receptor for IAV. Therefore, we generated an MGL1 construct with 45 nucleotides (corresponding to 15 amino acids, including the putative internalization motif YENL [27,32,45]) deleted from its cytoplasmic domain (Fig. 7A).…”

“…The MGL1 internalization assay was adapted from the protocol used by Van Vliet et al to examine internalization of MAb by human MGL (32). Briefly, cells were detached and incubated with 0.1 mg/ml biotin-labeled anti-mouse MGL MAb (clone ER-MP23; AbD Serotec) in TBS containing 10 mM CaCl 2 (TBS-Ca) for 30 min on ice.…”

Targeting of macrophage galactose‐type C‐type lectin (MGL) induces DC signaling and activation

“…MGL1 is highly specific for Lewis X and Lewis A structures, while MGL2, similar to the hMGL, recognizes N-GalNAc and galactose, including O-linked Tn-antigen, TFantigen, and core 2 structures [10]. hMGL is equipped with a partial dileucine zipper and with YENF internalization motifs [11]. This receptor is involved in the recognition of a large plethora of pathogens by DCs [12][13][14][15] and in the maintenance of T-cell homeostasis [16].…”