**4.2.3 The C-type lectin receptors and WNV cell entry**

The superfamily of proteins containing lectin-like domains is a large group of extracellular metazoan proteins with diverse functions (Zelensky and Gready, 2005). The main function of the calcium-dependent 'C'-type lectins is to interact with pathogen-associated molecular patterns (PAMPs) which are well conserved and expressed by various microorganisms and to internalize these pathogens for processing and antigen presentation (Drickamer, 1995). For instance, C-type lectins contain Carbohydrate Recognition Domains (CRDs) capable of binding to conserved oligosaccharides that are commonly found on the surface glycoproteins of viruses (Cambi et al., 2005). C-type lectins are secreted as soluble proteins or produced as transmembrane proteins. The Mannan Binding Lectin (MBL), present in the plasma, is a member of the collectin family and is an example of soluble protein. MBL is

Apoptosis following viral infection is a robust line defence mechanism, since the cell auto destruction appears one of the best ways to limit virus production and spreading (Griffin and Hardwick, 1997). Unequivocally, the expected purpose of this cell suicide in response to infection is to short-circuit the viral expansion and to promote the clearance of dead infected cells by professional phagocytes (Fujimoto et al., 2000; Hashimoto et al., 2007). However, most viruses deal with this defensive reaction and have evolved strategies to evade or delay

Under normal cellular conditions, exposure PS serves as a recognition signal for internalization of apoptotic cells or debris. PS-mediated apoptotic clearance is a highly conserved mechanism, occurring in both professional and nonprofessional phagocytes (Henson et al., 2001). Importantly, this process does not elicit an inflammatory response (Savill et al., 2002). "Apoptotic mimicry" or the exposure of PS on the surface of a pathogen provides a signal for virus uptake and may represent a mechanism for viruses to stunt the host inflammatory response and evade immune recognition. Some viruses like CMV or HSV-1 and HSV-2, which acquire their envelopes from intracellular organelles, have externalized PS (Pryzdial and Wright, 1994; Sutherland et al., 1997). A previous study investigated the early apoptotic events in real time in intact live ND7 neuron-like cells following HSV-1 infection (Gautier et al., 2003). The authors demonstrated that infection of differentiated ND7 cells by HSV-1 triggers detectable alterations including PS exposure

Furthermore, control of the apoptotic process by the viruses is key, either to establish a permanent infection when they are able to block apoptosis, or to facilitate their dissemination and prevent inflammatory and immune response when they are able to control and activate the late phase of apoptosis (Chen et al., 2006). Dissemination of viral particles through the production of apoptotic blebs from host cells could confer protection from the immune system (Chen et al., 2006; Everett and McFadden, 1999). Conversely, it has been proposed that phagocytosis of these pathogen (Sindbis for example) -enriched apoptotic blebs by antigen presenting cells (e.g. dendritic cells) could contribute to a robust and uncontrolled adaptive immune response leading to autoimmunity against self-antigens contained within the blebs (Rosen et al., 1995). In the case of CHIKV, it has recently been evidenced that completion of the apoptotic process is an important element for efficient virus propagation (Krejbich-Trotot et al., 2011). Indeed, CHIKV was revealed to exploit, may be in parallel to classical ways of entry and cell exit, an ingenious way of camouflage in apoptotic blebs to enhance viral spreading to neighbouring cells while being shielded from

The superfamily of proteins containing lectin-like domains is a large group of extracellular metazoan proteins with diverse functions (Zelensky and Gready, 2005). The main function of the calcium-dependent 'C'-type lectins is to interact with pathogen-associated molecular patterns (PAMPs) which are well conserved and expressed by various microorganisms and to internalize these pathogens for processing and antigen presentation (Drickamer, 1995). For instance, C-type lectins contain Carbohydrate Recognition Domains (CRDs) capable of binding to conserved oligosaccharides that are commonly found on the surface glycoproteins of viruses (Cambi et al., 2005). C-type lectins are secreted as soluble proteins or produced as transmembrane proteins. The Mannan Binding Lectin (MBL), present in the plasma, is a member of the collectin family and is an example of soluble protein. MBL is

indicative of physiological changes associated with early stages of apoptosis.

apoptosis (Teodoro and Branton, 1997).

the immune system.

**4.2.3 The C-type lectin receptors and WNV cell entry** 

expressed by monocytes/microglia and possesses a globular carboxy terminal sequence that binds to virus proteins containing mannose molecules. The transmembrane lectins include the Mannose Membrane Receptor (MMR also known as CD206) and the Dentritic Cell-Specific ICAM-3 Grabbing Non-integrin (DC-SIGN also known as CD209) receptor that recognizes "self " intercellular adhesion molecule 2 (ICAM2) and ICAM3 (van Kooyk and Geijtenbeek, 2003). MMR contains eight CRD (Weis et al., 1998). DC-SIGN is a type II transmembrane protein which possesses a single globular-structured CRD. DC-SIGN's CRD is separated from the transmembrane region by a neck domain involved in oligomerization and which regulates carbohydrate specificity. Finally, a cytoplasmic tail is present, which includes internalization motifs and an incomplete immunoreceptor tyrosine-based activation motif (ITAM). Microglia, perivascular macrophages and dendritic cells express MMR and DC-SIGN (Burudi et al., 1999; Linehan et al., 1999; Mukhtar et al., 2002; Sallusto et al., 1995; Schwartz et al., 2002). The expression of DC-SIGN on brain microvascular endothelial cells could be advantageous for viral entry into the CNS (Mukhtar et al., 2002). Two major lineages of WNV, L1 and L2, have been identified after the phylogenetic analyses

of glycoprotein E (Lanciotti et al., 1999). L1 strains have a large distribution and have been found in Africa, Europe, the Middle-East, North and Central America whereas the L2 strain is mainly localized in Africa and Madagascar. WNV human infection is usually asymptomatic, however life-threatening neurological disease, including encephalitis or meningitis, generally occurs in older or immunocompromised individuals (Campbell et al., 2002; Granwehr et al., 2004). A recent study investigated the relative contribution of DC-SIGN in infection by glycosylated L1 and non-glycosylated L1 and L2 strains (Martina et al., 2008). The authors showed that in contrast to a non-glycosylated L1 strain, the glycosylated L1 strains use DC-SIGN as an attachment receptor on dendritic cells, leading to enhanced infection.
