## 1 Introduction  
Dendritic cell-specific intercellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN), a tetrameric transmembrane C-type lectin, which requires calcium for glycan recognition, is expressed in various types of human dendritic cells (DCs), including lymphoid DCs, peripheral blood monocyte-derived DCs (MoDCs), dermal DCs, and DCs in the mucosal epithelium [1; 2; 3; 4]. In recent years, DC-SIGN has emerged as a key pattern recognition receptor involved in host defenses against a wide variety of exogenous pathogens, including human immunodeficiency virus (HIV-1) [5], hepatitis C virus [6], Dengue virus [7], M. tuberculosis [8; 9; 10], and parasites [11]. Although DC-SIGN facilitates the CD8+ and CD4+ responses by promoting antigen uptake, processing, and presentation on MHC-J/MHC-II molecules, it also helps HIV and M. tuberculosis to subvert DC function and thus escape from intracellular degradation [8; 12]. Indeed, studies of CD209 promoter gene polymorphism demonstrated that the -336 G variant allele, which is linked to lower DC-SIGN expression, is associated with decreased risk of Dengue fever [7] and tuberculosis [13]. DC-SIGN reportedly activates the serine/threonine kinase Raf-1 upon ligand stimulation and suppresses Toll-like receptor (TLR)-induced immune responses [14; 15]. The interaction of DC-SIGN with pathogens is mediated by its carbohydrate recognition domain (CRD) in a Ca2+-dependent manner. DC-SIGN shows specificity to mannose- and fucose-containing glycans and high affinity to high-mannose and fucose-containing Lewis (Le) glycans [5; 16]. DC-SIGN recognizes pathogens that are heavily decorated with those glycans as non-self ligands. For example, lipoarabinomannan (ManLAM) is a major lipoglycan covering the cell wall of M. tuberculosis, with a structure that is not present in the host. DC-SIGN binds to ManLAM capped with dimeric and trimeric mannose residues but does not bind single mannose residues [10]. It has been proposed that the high binding affinity of DC-SIGN is achieved through tetramerization of DC-SIGN. Recognition of ManLAM by DC-SIGN inhibits DC maturation and induces strong upregulation of immune-inhibitory IL-10 production [15]. In addition to the recognition of foreign antigens, DC-SIGN is involved in the recognition of cancer cells through newly synthesized tumor-associated \"non-self\" glycans, as we and another group have demonstrated [16; 17; 18]. We found that DC-SIGN recognizes colon cancer cell lines (e.g., SW1116 and COLO205) and human colorectal cancer tissues based on Le (Le\({}^{\text{a}}\)/Le\({}^{\text{b}}\)) glycans [17; 18]. Conditioned medium from MoDC-COLO205 co-cultured cells blocked MoDC maturation and attenuated TLR4-mediated immune activation. Considering that the ligand recognition step carried out by DC-SIGN regulates the subsequent induction of immunosuppressive responses, elucidating the mechanism of the DC-SIGN glycan recognition system is critical for the development of anti-cancer therapeutics. To date, carcinoembryonic antigen (CEA), CEA-related cell adhesion molecule 1 (CEACAM-1), and mac-2 binding protein (Mac-2 BP) have been identified as endogenous DC-SIGN ligands carrying Le glycans [16; 17; 18]. In this study, we identified mosaic serine protease large-form (MSPL) and its alternative splicing variant transmembrane protease serine 13 (TMPRSS13) [19], which are type II transmembrane serine proteases [20; 21; 22], as novel DC-SIGN ligands. The binding of DC-SIGN to MSPL/TMPRSS13 was mediated by N-glycans of MSPL/TMPRSS13. Meanwhile, the soluble recombinant extracellular domain of DC-SIGN (DC-SIGN-ECD) was cleaved via MSPL/TMPRSS13 protease activity, indicating a different mode of recognition of DC-SIGN by MSPL/TMPRSS13. In the absence of Ca2+, MSPL digested DC-SIGN more efficiently, suggesting that the molecular association mediated by N-glycans impeded DC-SIGN digestion. Clarification of the dual recognition processes between DC-SIGN and MSPL/TMPRSS13 may lead to the development of a treatment that efficiently suppresses colorectal cancer.