DC-SIGN (CD209)
DC-SIGN (also called CD209) specifically expressed on the surface of DCs, is a C-type lectin-like cell-surface receptor with multiple functions. Its expression is also found in gastric and intestinal mucosa and other epithelial cells. Initially, DC-SIGN was proved to only be a pattern recognition receptor (PRR) and adhesion molecule for dendritic cells to recognize pathogenic infections and participate in innate immunity of organisms. Accumulating evidence has indicated that DC-SIGN also act as the receptor of many viruses for infecting hosts and the mediator of virus immune escape. Therefore, DC-SIGN has increasingly become a research hotspot in the field of virology. Importantly, DC-SIGN, together with other PRRs, can identify and capture viruses, further swallow and store viruses to evade lysosome degradation, then participate in antigen presentation, thereby achieving the mediation of virus infection and dissemination in vivo. For instance, compared with natural 3T3 cells, transfection of 3T3 cells with plasmids expressing DC-SIGN renders the infection of infectious bronchitis virus (IBV) to be significantly enhanced, while pretreatment with anti-DC-SIGN monoclonal antibody inhibits IBV infection (Zhang, Buckles, & Whittaker). Therefore, it has been speculated that the mannose carbohydrate residues on the surface of coronavirus spike protein can bind to DC-SIGN receptor and play an important role in the process of coronavirus infection (Z. Y, E, & GR, 2012). Human aminopeptidase N (hAPN) has been confirmed to be a cell receptor of HCoV-229E which is a common coronavirus of upper respiratory tract. Similarly, a series of experiments have also confirmed that HCoV-229E can use CD209L as one of its receptors (Jeffers, Hemmila, & Holmes, 2006). Based on the similarity of the sequence of HCoV-229E with that of PEDV, CD209L may be a receptor of PEDV as well. However, substantial evidence is required to confirm this concept in the future.