Filovirus Tropism: Cellular Molecules for Viral Entry

Takada, Ayato

C-type lectins are a family of Ca 2+ -dependent carbohydraterecognition proteins that play crucial roles in innate immunity. It has been demonstrated that membrane-anchored cellular C-type lectins facilitate filovirus infection in vitro by binding to glycans focused on the MLR (Figure 6) . The asialoglycoprotein receptor, a C-type lectin found exclusively in hepatocytes, initially proposed as a receptor for Marburg virus (Becker et al., 1995) , recognizes GPs displaying N-linked sugar chains with terminal galactose residues on the GP molecule and enhances filovirus infectivity. It was subsequently shown that carbohydrate chains on filovirus GP, especially on the MLR, are recognized by other cellular C-type lectins such as dendritic cell-and liver/lymph node-specific ICAM-3-grabbing non-integrin (DC/L-SIGN) (Alvarez et al., 2002; Lin et al., 2003; Simmons et al., 2003a; Marzi et al., 2004; Gramberg et al., 2008) , human macrophage galactose-type C-type lectin (hMGL) (Takada et al., 2004; Matsuno et al., 2010a) , and liver and lymph node sinusoidal endothelial cell C-type lectin (LSECtin) (Gramberg et al., 2005; Dominguez-Soto et al., 2007; Powlesland et al., 2008) . Though these C-type lectins show different specificities, depending on the structures of target glycans, and thus MLR may not be the only binding site for the lectins, all have been reported to promote filovirus entry. It should be noted that C-type lectins enhance filovirus infectivity when expressed on the target cell-surface, but are unlikely to act as functional receptors mediating both attachment and membrane fusion (Simmons et al., 2003a; Marzi et al., 2007; Matsuno et al., 2010b) . The fact that interaction between the GP MLR and C-type lectins is not essential for viral entry into cells lacking C-type lectins (e.g., Vero E6 cells) may also suggest that C-type lectins facilitate viral attachment but not infectious entry.