key: cord-266861-t5h133lp
authors: Moller-Tank, Sven; Maury, Wendy
title: Phosphatidylserine receptors: enhancers of enveloped virus entry and infection
date: 2014-11-01
journal: Virology
DOI: 10.1016/j.virol.2014.09.009
sha: 
doc_id: 266861
cord_uid: t5h133lp

A variety of both RNA and DNA viruses envelop their capsids in a lipid bilayer. One of the more recently appreciated benefits this envelope is incorporation of phosphatidylserine (PtdSer). Surface exposure of PtdSer disguises viruses as apoptotic bodies; tricking cells into engulfing virions. This mechanism is termed apoptotic mimicry. Several PtdSer receptors have been identified to enhance virus entry and we have termed this group of proteins PtdSer-mediated virus entry enhancing receptors or PVEERs. These receptors enhance entry of a broad range of enveloped viruses. Internalization of virions by PVEERs provides a broad mechanism of entry with little investment by the virus itself and may allow some viruses to attach to cells, thereby making viral glycoprotein/cellular receptor interactions more probable. Alternatively, other viruses may rely entirely on PVEERs for internalization into endosomes. This review provides an overview of PtdSer receptors that serve as PVEERs and the biology behind virion/PVEER interaction.

A variety of both RNA and DNA viruses envelop their capsids in a lipid bilayer. This outer membrane is obtained during virus budding from either plasma or organelle membranes. While reliance on an envelope sensitizes viruses to desiccation, detergents, and heat, these envelopes provide a number of benefits for the virus, including protection of viral structural proteins from immune recognition and neutralizing antibodies, a platform for displaying viral proteins, a barrier to enclose viral and cellular proteins necessary for early steps during infection, and a mechanism for virus egress without lysing infected cells. In addition, a more recently appreciated benefit is the incorporation of phospholipids into viral envelopes. Presentation of phosphatidylserine (PtdSer) on the outer leaflet of these membranes disguises viruses as apoptotic bodies, thereby conning cells into engulfing virions through cell clearance mechanisms. This mechanism of enhanced virus entry is termed apoptotic mimicry.

Over the past three years, receptors and receptor complexes have recently been identified that enhance entry of a diverse range of enveloped viruses. This group of viral receptors shares the ability to bind to PtdSer present on the viral envelope and, consequently, we have termed them PtdSer-mediated virus entry enhancing receptors or PVEERs. The broad expression of these receptors and their ability to interact with PtdSer on a wide array of enveloped viruses has huge potential implications for virus infection. Most importantly, PVEERs enhance virus binding to cells and facilitate internalization. These receptors also contribute to immune evasion through both anti-inflammatory signaling and a mechanism of entry that does not require extracellular exposure of key receptor-binding domain and fusion epitopes. In this review we summarize what is known about the identified PVEERs and their role in virus entry and identify important gaps in current knowledge.

Apoptotic mimicry was first hypothesized to be used by hepatitis B virus (Vanlandschoot and Leroux-Roels, 2003) , but was experimentally confirmed with vaccinia virus (Mercer and Helenius, 2008) . Inactivation of vaccinia virus by NP-40-mediated lipid depletion could be rescued by incubation with PtdSer liposomes (Oie, 1985) . However, the contribution of PtdSer to infection was not understood. Mercer et al. furthered these studies and determined that not only PtdSer is present on the surface of some vaccinia infectious particles, but Annexin V (AnxV), a PtdSer-binding protein, can bind to PtdSer on viral envelopes and inhibit vaccinia infection (Mercer and Helenius, 2008) . These results were confirmed by another study that found substitution with a non-biologically relevant isomer of PtdSer restored infectivity (Laliberte and Moss, 2009 ). In addition, a role for viral envelope PtdSer during infection was demonstrated for Pichinde virus and HIV-1 (Soares et al., 2008; Callahan et al., 2003) .

A protein complex composed of growth-arrest-specific 6 (Gas6) and the tyrosine kinase receptor, Axl, was the first set of cellular proteins to be implicated in PtdSer-binding enhancement of viral entry (Morizono et al., 2011) . It was shown that the soluble protein Gas6 binds to PtdSer on the virion surface and bridges virus to the cell surface via interaction with the tyrosine kinase receptor, Axl and formation of this complex is necessary for enhancement of virus entry. This study found in addition to vaccinia virus, the Gas6/Axl complex enhances binding and entry of lentiviruses pseudotyped with Ross River GP, baculovirus GP64, or Sindbis env, demonstrating for the first time that PtdSer binding can enhance entry of viral particles bearing a variety of different viral glycoproteins. More recently, a variety of additional PVEERs were identified, including T-cell immunoglobulin and mucin domain 1 and 4 (TIM-1 and 4) proteins and MFG-E8/integrin αvβ3 or αvβ5 complexes (Meertens et al., 2012; Jemielity et al., 2013; Moller-Tank et al., 2013; Morizono and Chen, 2014) . The details of virion interactions with different PVEERs are highlighted below.

Several recent studies have expanded our understanding of PVEERs. The group of viruses whose entry is enhanced by PVEERs now include members of the flavivirus, filovirus, New World arenavirus, baculovirus, and alphavirus families (Meertens et al., 2012; Jemielity et al., 2013; Moller-Tank et al., 2013; Morizono and Chen, 2014) . Between viruses within these families, the extent to which PVEERs enhance virus entry varies, but generally viruses do not appear to utilize one PVEER effectively and not another. This suggests that virion uptake mediated by the different PVEERs is mechanistically similar. Despite the strong evidence that PVEERs interact with virion associated PtdSer, the utilization of PVEERdependent virus entry is influenced by the viral glycoprotein presence on the virion. For instance, entry of viruses bearing the envelope proteins of Old World arenaviruses, coronaviruses, influenza A virus, vesicular stomatitis virus, or herpes simplex virus 1 is not enhanced by PVEER expression (Morizono et al., 2011; Meertens et al., 2012; Jemielity et al., 2013; Moller-Tank et al., 2013; Kondratowicz et al., 2011) . Thus, while PVEERs do not enhance entry all enveloped viruses, they do represent a potentially important class of receptors for a large and diverse collection of important human pathogenic viruses.

Six PVEERs have been identified to date: TIM-1, TIM-4, Gas6 or Protein S/Axl, Mer, and Tyro3, and MFG-E8/integrin αvβ3 and αvβ5. A summary of characteristics of these PVEERs is shown in Table 1 . The key property that these proteins or complexes utilize to enhance virus entry is their ability to bind PtdSer and a native function of all of these receptors is to bind and clear apoptotic bodies (Ravichandran, 2011) . Mutation of residues involved in PtdSer binding or complex formation results in inhibition of PVEER function (Morizono et al., 2011; Meertens et al., 2012; Moller-Tank et al., 2013; Morizono and Chen, 2014) . Further, competition with PtdSer liposomes, but not phosphatidylcholine liposomes, inhibits entry enhancement by PVEERs (Jemielity et al., 2013; Moller-Tank et al., 2013) . Phosphatidylethanolamine liposomes are also somewhat inhibitory, likely due to sharing similar structure to PtdSer (Jemielity et al., 2013) . In some cases, prebinding virus with the PtdSer-binding protein, Annexin V (AnxV), has been shown to inhibit PVEER enhancement of entry (Mercer and Helenius, 2008; Callahan et al., 2003; Meertens et al., 2012) , but not in others (Morizono and Chen, 2014) . Consistent with the ability of an array of PtdSer receptors to serve as PVEERS, an artificially generated PVEER containing the PtdSer-binding domain from AnxV was highly effective at mediating uptake of vesicular stomatitis virus (VSV) pseudovirions bearing a filovirus, alphavirus or baculovirus glycoprotein (Moller-Tank et al., 2014). (Kobayashi et al., 2007) Immune cells: Filovirus: Old World Arenavirus: Human: B cells (Sizing et al., 2007) , mast cells , T H 2 CD4 þ T cells (Meyers et al., 2005; Umetsu et al., 2005; Khademi et al., 2004) , and NKT cells Kim et al., 2013) Ebola• and Marburg (Jemielity et al., 2013; Moller-Tank et al., 2013; Kondratowicz et al., 2011) Lassa Virus, LCMV (Jemielity et al., 2013; Moller-Tank et al., 2013) Huh-7, ACHN, A498, 786-O, Caco-2, CAKI-1, TK-10, UO-3, Vero, A549, Cos-7 (Meertens et al., 2012; Kondratowicz et al., 2011) Epithelial cells:

Alphavirus: New World Arenavirus: Kidney Ichimura et al., 1998) and airway and eye mucosa Ross River•, Chikungunya, Sindbis•, Eastern Equine Encephalitis (Jemielity et al., 2013; Moller-Tank et al., 2013; Morizono and Chen 2014) Oliveros (Jemielity et al., 2013) Baculovirus:

Influenza A virus: Autographa californica multicapsid nucleopolyhedrovirus• (Moller-Tank et al., 2013) H7N1, H1N1• (Jemielity et al., 2013) Rhabdovirus:

Coronavirus: Vesicular stomatitis virus (Morizono and Chen, 2014) SARS (Jemielity et al., 2013) New World Arenavirus:

Rhabdovirus: Amapari, Tacaribe•, Junín, and Machupo virus (Jemielity et al., 2013) Vesicular stomatitis virus (Jemielity et al., 2013) Flavivirus:

Herpes simplex virus: West Nile Virus•, Dengue•, Yellow Fever virus• (Meertens et al., 2012; Jemielity et al., 2013) HSV-1• (Meertens et al., 2012) TIM-3

IgV Immune cells: Flavivirus: Alphavirus: Mouse: T H 1 and T H 17 CD4 þ T cells (Khademi et al., 2004; Hastings et al., 2009; Monney et al., 2002; , mast cells , DCs Anderson et al., 2007) , and monocytes (Anderson et al., 2007) West Nile Virus, Dengue Virus• (Meertens et al., 2012; Jemielity et al., 2013) Sindbis (Morizono and Chen, 2014) Raw264 IgV (Kobayashi et al., 2007; Santiago et al., 2007; Miyanishi et al., 2007) Immune cells: Filovirus: New World Arenavirus: Macrophages and mature DCs (Kobayashi et al., 2007; DeKruyff et al., 2010; Miyanishi et al., 2007; Mizui et al., 2008; Meyers et al., 2005) , and B-1 cells (Rodriguez-Manzanet et al., 2010) Ebola, Marburg (Jemielity et al., 2013; Moller-Tank et al., 2013) Oliveros (Jemielity et al., 2013) Tissues:

Alphavirus: Old World Arenavirus: Spleen, lymph node, and peritoneum (Miyanishi et al., 2007; Meyers et al., 2005) Sindbis, Ross River (Morizono and Chen, 2014) , Eastern Equine Encephalitis (Jemielity et al., 2013) LASV, LCMV (Jemielity et al., 2013) New World Arenavirus: Influenza A virus: Ebola, Marburg (Jemielity et al., 2013; Shimojima et al., 2006) LASV, LCMV (Jemielity et al., 2013) A549, Vero, Cos-7, HeLa, SNB19, and SN12C (Meertens et al., 2012; Shimojima et al., 2006; Brindley et al., 2011 ) Platelets (Angelillo-Scherrer et al., 2001 Rhabdovirus: New World Arenavirus: Immune cells:

Vesicular stomatitis virus (Morizono and Chen, 2014) Machupo (Jemielity et al., 2013) , (Seitz et al., 2007; Caraux et al., 2006; Behrens et al., 2003) Baculovirus:

Oliveros (Jemielity et al., 2013) Connective tissue: Autographa californica multicapsid nucleopolyhedrovirus (Morizono et al., 2011; Morizono and Chen, 2014) Rhabdovirus:

Bone marrow stromal cells (Caraux et al., 2006) Alphavirus: Vesicular stomatitis virus (Jemielity et al., 2013) 

Sindbis, Ross River (Morizono et al., 2011; Morizono and Chen, 2014) Influenza A virus:

Testes (Lu et al., 1999; Wang et al., 2005) , CNS (Stitt et al., 1995; Prieto et al., 2000) , retina (Prasad et al., 2006) , and foreskin fibroblasts (Hanayama et al., 2002; Finnemann and Rodriguez-Boulan, 1999; Antonov et al., 2004) , Immature DCs Albert et al., 1998) Ross River and Sindbis (Morizono and Chen, 2014) Vesicular stomatitis virus (Morizono and Chen, 2014) HEC-1A, Ishikawa, HEEC, K562, HL60 (Bocca et al., 2012; Kruger et al., 2000) ;

Baculovirus: Mouse: Mammary glands (Oshima et al., 1999; Stubbs et al., 1990) , spleen, lymph node, brain Garmy-Susini et al., 2007; Uehara and Uehara, 2014) , and vascular system (Silvestre et al., 2005; Brooks et al., 1994) Autographa californica multicapsid nucleopolyhedrovirus (Morizono and Chen, 2014) COMMA-1D, D1, P388D1 Thery et al., 1999; Oshima et al., 2002) CD300a IgV (Simhadri et al., 2012) Immune cells:

Alphavirus: Alphavirus: Human: CD8 þ T cells (Xu et al., 2012) , CD4 þ T cells (Clark et al., 2007; Simhadri et al., 2011; Narayanan et al., 2010) , B cells (Silva et al., 2011) , and NK cells (Lankry et al., 2010; Cantoni et al., 1999) Sindbis (Morizono and Chen, 2014) •• Ross River and Sindbis (Morizono and Chen, 2014) THP1, U937, REC-1, SUDHL5, 721.221 (Silva et al., 2011; Kim et al., 2012) Baculovirus: Autographa californica multicapsid nucleopolyhedrovirus (Morizono and Chen, 2014) Rhabdovirus: Vesicular stomatitis virus (Morizono and Chen, 2014) BAI1 Type 1 thrombospondin repeats (TSRs) (Park et al., 2007) Tissues: Alphavirus: Human: Brain (Shiratsuchi et al., 1997; Sokolowski et al., 2011 ), muscle(Hochreiter-Hufford et al., 2013 , bone marrow, and spleen (Park et al., 2007) Ross River and Sindbis (Morizono and Chen, 2014) J774 and RAW264.7 (Park et al., 2007) Immune cells:

Baculovirus: Macrophages (Park et al., 2007) Autographa (Akirav et al., 2012; Moser et al., 2007) , DCs (Dumitriu et al., 2005) , macrophages Sunahori et al., 2006) , and monocytes (Ohashi et al., 2010) Lassa (Moller-Tank et al., 2013) HaCaT , A549 (Nakano et al., 2006) Tissues: Filovirus: Smooth muscle Kamioka et al., 2011) , Ebola virus (Moller-Tank et al., 2013) cartilidge chondrocytes (Loeser et al., 2005) , skin keratinocytes , and vascular system (Pollreisz et al., 2010; Liu et al., 2010) Stabilin-1/-2 Epidermal growth factor-like domain Kim et al., 2010) Immune cells: Alphavirus: Human: Macrophages Martens et al., 2006) Ross River and Sindbis (Morizono and Chen, 2014) PMA stimulated THP1 ) Tissues:

Baculovirus: Mouse: Sinusoidal endothelial cells (Martens et al., 2006; Falkowski et al., 2003) , spleen, lymph node, liver, bone marrow, cornea, brain, heart, and kidney (Falkowski et al., 2003; Goerdt et al., 1991) Autographa californica multicapsid nucleopolyhedrovirus (Morizono and Chen, 2014) PMA stimulated P388D1 cells Rhabdovirus: Vesicular stomatitis virus (Morizono and Chen, 2014) •Confirmed using infectious virus. ••Enhances binding only, but not infection.

Recent studies have demonstrated that virus internalization into HEK 293T cells can occur entirely independently of the presence of a viral glycoprotein. This virion internalization is significantly enhanced by PVEER overexpression and is inhibited by competition with PtdSer liposomes (Jemielity et al., 2013; Moller-Tank et al., 2013) , providing compelling evidence that virion associated PtdSer/PVEER interactions are responsible for virion uptake, not just virus binding. While the cellular compartment(s) into which PVEERs deliver virions has yet to be identified, indirect evidence indicates that uptake of cargo is into endosomes (Kobayashi et al., 2007; Albacker et al., 2010) . Once the virion is internalized, the presence of a viral glycoprotein on the virion is necessary for fusion events with the endosomal membrane. An overview of each group of PVEERs is given below.

TIM-1 was first implicated in enveloped virus entry as a specific receptor for filoviruses . However, it was later discovered that both TIM-1 and family member, TIM-4, enhance virus entry as PVEERs through binding of PtdSer (Meertens et al., 2012; Jemielity et al., 2013; Moller-Tank et al., 2013) . The human TIM family members are type I, cell-surface glycoproteins that along with their murine counterparts, share a common structure. Their amino terminal immunoglobulin variable (IgV)-like domain extends from the plasma membrane by a heavily O-linked-glycosylated mucin-like domain (MLD), which is attached to the cell surface by a transmembrane domain followed by a cytoplasmic tail ( Fig. 1 ) (McIntire et al., 2004) . TIM-1 signals through phosphorylation of cytoplasmic tail tyrosines (Binne et al., 2007; de Souza et al., 2005 de Souza et al., , 2008 ; however, none of the TIM family members require their cytoplasmic domains for PVEER function (Meertens et al., 2012; Moller-Tank et al., 2014) .

TIM-1 and TIM-4 are unique among PVEERs as their PtdSerbinding domains are a portion of a transmembrane protein rather than an independent, small protein that binds to a plasma membrane receptor. The PtdSer-binding pocket is located between two loops of the IgV domain ( Fig. 2) (Kobayashi et al., 2007; Santiago et al., 2007; DeKruyff et al., 2010; Miyanishi et al., 2007) . Conserved aspartate and asparagine residues within the upper loop are involved in coordination of a cation that, in conjunction with residues of the lower loop, form hydrogen bonds with the phosphate and serine groups of PtdSer (Santiago et al., 2007) . This PtdSer binding is necessary for the native functions of TIM-1 and TIM-4: apoptotic body clearance and immune cell regulation (Albacker et al., 2010; Miyanishi et al., 2007; Lee et al., 2010; Kim et al., 2013; Ichimura et al., 2008; Rodriguez-Manzanet et al., 2010; Mizui et al., 2008; Wong et al., 2010) . Mutation of aspartate and/or asparagine or chelation of free cations with EGTA results in significant loss in PtdSer binding and subsequent PVEER efficacy (Meertens et al., 2012; Moller-Tank et al., 2013) . A recent study suggests TIM-4 is more sensitive than TIM-1 or -3 to differences in membrane-PtdSer concentrations due to additional PtdSerbinding residues outside of the pocket (Tietjen et al., 2014) . This altered sensitivity may have implications for virus binding.

While all human TIM family members can bind PtdSer and enhance the uptake of apoptotic bodies (Kobayashi et al., 2007; DeKruyff et al., 2010) , TIM-3 does not effectively enhance virus entry (o2 fold) (Meertens et al., 2012; Jemielity et al., 2013; Kondratowicz et al., 2011) . Though the IgV domain of TIM-3 has reduced affinity for PtdSer , replacing the TIM-1 or TIM-4 IgV domain with that of TIM-3 results in a functional PVEER (Moller-Tank et al., 2014) . Thus, the reduced efficacy is not due to the TIM-3 IgV domain. Rather, the inability of TIM-3 to function effectively as a PVEER is likely due to the short length of the TIM-3 MLD which is 2.5-fold shorter than that of TIM-1 and -4 ( Fig. 1 ). This is supported by studies showing that deletions within the MLD of TIM-1 reduce PVEER efficacy (Jemielity et al., 2013; Moller-Tank et al., 2014) .

Tyro3, Axl, and Mer (TAM) are members of the TAM family of receptor tyrosine kinases. These highly related proteins contain two N-terminal immunoglobulin-like domains, followed by two fibronectin type III domains, a single transmembrane domain, and a cytoplasmic protein tyrosine kinase (PTK) domain (Fig. 2) . TAM N-terminal interaction with Gas6 leads to activation of the TAM receptors and autophosphorylation of tyrosines within the PTK domain (Stitt et al., 1995) . Tyro3 and Mer, but not Axl, are similarly activated by binding Protein S (Stitt et al., 1995; Prasad et al., 2006) . Both Gas6 and Protein S consist of an N-terminal domain rich in γ-carboxyglutamic acid residues (Gla) that binds to PtdSer ( Fig. 1) (Huang et al., 2003; Ishimoto et al., 2000; Rajotte et al., 2008) , a loop region, four epidermal growth factor-like repeats, and two C-terminal laminin G-like domains forming a sex hormone-binding globulin-like structure that binds to the Ig-like domains of the TAM receptors (Sasaki et al., 2006 (Sasaki et al., , 2002 . Dimerization of TAM receptors occurs after binding of their ligands, resulting in a 2:2 complex, and is necessary for signaling (Sasaki et al., 2006) .

All TAM receptors in combination with their Gas6 or Protein S ligand have been shown to effectively enhance virus entry, although studies suggest that the relative effectiveness of the three TAM members as PVEERs varies (Morizono et al., 2011; Meertens et al., 2012; Jemielity et al., 2013; Bhattacharyya et al., 2013) . This may be due to differences in binding affinities between the TAM receptors and their ligands (Nagata et al., 1996) or relative expression. As the PtdSerbinding activity of PVEERs is essential for efficacy, in the case of the TAM receptors, this occurs through the Gla domain of Gas6/Protein S and removal of this domain eliminates PVEER function of the TAMs (Morizono et al., 2011; Meertens et al., 2012) . Similarly mutation of the Gas6 binding residues of Axl inhibits virus uptake by the complex (Meertens et al., 2012) . Thus, the PVEER efficacy of the TAM ligand/ receptor complexes requires both efficient binding of PtdSer by ligand and binding of ligand by receptor.

Contrary to the TIM family PVEERs where cytoplasmic tail signaling is not required for enhancing virus entry into cells (Moller-Tank et al., 2014) , signaling through the PTK domain of TAM receptors is essential for enhancement of viral infection (Meertens et al., 2012; Bhattacharyya et al., 2013) . While the Gas6/ Axl complex is still able to enhance WNV binding and internalization without kinase activity, subsequent infection is significantly impaired compared to infection of cells expressing WT Axl. This kinase activity is important in vivo for TAM-receptor-inhibition of DC activation and inflammation (Rothlin et al., 2007; Tibrewal et al., 2008) . Ligand binding by the receptors and assocation with the type I interferon receptor leads to activation of STAT1 and induction of suppressor of cytokine signaling (SOCS) proteins (Rothlin et al., 2007) . Viruses appear to utilize this anti-inflammatory signaling pathway to dampen the immune response and promote replication (Bhattacharyya et al., 2013) . While TAM signaling is triggered by interaction with ligand alone, signaling is significantly enhanced by the presence of virus which helps to facilitate the interaction of TAM ligands with their receptors (Bhattacharyya et al., 2013) . Thus, the TAM receptors enhance virus infection through PVEER functions by enhancing virus binding and internalization and non-PVEER functions by inhibiting innate immunity, both of which require binding to PtdSer on the virion surface and likely contribute to enhancing in vivo virus loads.

Milk fat globule-EGF factor 8 (MFG-E8), the most recently identified PVEER, is a secreted protein that contains two N-terminal EGF repeats followed by a C1 and C2 domain, of which the latter binds to PtdSer (Andersen et al., 2000; Ye et al., 2013) . Several key positively charged residues within the C2 domain are responsible for interacting with the phosphate and serine groups, while hydrophobic residues of the domain stabilize the positioning of PtdSer (Fig. 1) (Ye et al., 2013) . Like the Gas6, MFG-E8 bridges PtdSer containing membranes to cells through interaction with a membrane bound protein, either integrin αvβ3 or αvβ5 (Andersen et al., 2000; Hanayama et al., 2002) . This occurs through an Arg-Gly-Asp (RGD) motif present in the second EGF (Santiago et al., 2007) . The human CD300a IgV domain crystal structure (Dimasi et al., 2007) is shown with residues hypothesized for interaction (Simhadri et al., 2012) . The region of the RAGE IgV domain (3O3U) that corresponds to the binding pockets of CD300a and TIM-1 IgV domains is shown for comparison ; however, this domain is only hypothesized to interact with PtdSer and no residues have been implicated (Friggeri et al., 2011) . The thrombospondin type-1 repeats (TSRs) of BAI1 are represented by TSR domain 3 of human thrombospondin-1 (3R6B) (Klenotic et al., 2011) . Gas6 binds PtdSer via a Gla domain, represented by that of bovine prothrombin (1NL2) with residues identified that interact with PtdSer (red) (Huang et al., 2003) . For AnxV, the structure of one annexin repeat domain from rat Anx V is shown (1A8A) with residues identified that interact with PtdSer (red) (Swairjo et al., 1995) . The C2 domain of bovine MFG-E8 binds to PtdSer (3BN6) (Shao et al., 2008) and while the structure was not crystallized with PtdSer, key residues involved in interaction have been determined experimentally (Ye et al., 2013) . An EGF-like domain of stabilin-1 or -2 is represented by that from human heregulin alpha (1HRE) (Nagata et al., 1994). repeat of MFGE-8 and mutation of this motif inhibits PVEER efficacy (Morizono and Chen, 2014) .

Several aspects of MFG-E8 biology may contribute to infection. Similar to the Gas6/Axl complex, MFG-E8/integrin complexes have been shown to induce production of anti-inflammatory cytokines such as IL-10 (Aziz et al., 2009; Jinushi et al., 2009) , some of which are initiated by activation of STAT-3 (Brissette et al., 2012; Jinushi et al., 2011) . Interestingly, STAT-3 has also been identified as an upstream regulator of TIM-1 expression (Ajay et al., 2014) . However, currently MFG-E8 induced signaling activity has not yet been associated with enhancement of virus entry.

The presence of PVEERs in various tissues permissive to infection may contribute to the tropism of some viruses, particularly for flaviviruses and filoviruses, whose entries are most enhanced by PVEER expression. PVEERs are expressed in a variety of tissues and cell types (summarized in Table 1 ) that are key targets of infection by enveloped viruses. TIM-1, TIM-4, and Axl Dorfman et al., 2010; Bauer et al., 2012) may enhance uptake of Dengue virus into Langerhans dendritic cells (DCs), one of the first targets during flavivirus infection (Wu et al., 2000; Marovich et al., 2001) . TIM-1 is also present on the mucosal epithelia of the airway and eye , both of which may be routes of infection for filoviruses (Jaax et al., 1996) . Early during infection, flaviviruses and filoviruses replicate in antigen-presenting cell (APC) populations: DCs and macrophages (Geisbert et al., 2003a (Geisbert et al., , 2003b Jessie et al., 2004) . Similarly, alphaviruses infect DCs (Labadie et al., 2010; Gardner et al., 2000; Nishimoto et al., 2007; Shabman et al., 2007) and establish persistent infections in macrophages (Labadie et al., 2010; Linn et al., 1996) . Interestingly, these APCs express a variety of PVEERs including TIM-4 (Kobayashi et al., 2007; Rodriguez-Manzanet et al., 2010; Mizui et al., 2008; Meyers et al., 2005; Baghdadi et al., 2013) , TAM receptors (Seitz et al., 2007; Caraux et al., 2006; Behrens et al., 2003) , and MFG-E8/integrin αvβ3 or αvβ5 Miyasaka et al., 2004; Albert et al., 1998; Finnemann and Rodriguez-Boulan, 1999; Antonov et al., 2004) . However, little is known about whether these PVEERs play critical roles in entry of virus into APC populations. Knock out of TAM receptors from bone marrow-derived DCs significantly reduces infection by West Nile virus or lentiviruses pseudotyped with Ebola, Marburg, vesicular stomatitis, or murine leukemia virus entry proteins (Bhattacharyya et al., 2013) . In addition, PtdSer liposomes compete for uptake of EBOV virus-like particles (VLPs) in mouse peritoneal macrophages (Jemielity et al., 2013) . These data suggest these receptors are relevant for uptake into early infection targets.

PVEERs are also expressed in tissues that are later targets of virus spread. After infection by flaviviruses, Langerhans cells traffic to the lymph nodes (Johnston et al., 2000) . Once there, flaviviruses infect lymph node DCs (Balsitis et al., 2009; Kyle et al., 2007) , which have been shown to express TIM-4 (Kobayashi et al., 2007; Meyers et al., 2005) . The alphavirus, Chikungunya virus, and filoviruses also replicate in lymph nodes (Geisbert et al., 2003a (Geisbert et al., , 2003b Labadie et al., 2010) , where TIM-4, in conjunction with MFG-E8/integrin αvβ3 and αvβ5 Garmy-Susini et al., 2007) , may mediate infection. Infected APCs also traffic flaviviruses and filoviruses to the liver and spleen, two major sites of replication (Geisbert et al., 2003a; Jessie et al., 2004; Balsitis et al., 2009; Baskerville et al., 1985; Bhoopat et al., 1996) .

Expression of MFG-E8/integrin αvβ5 and TIM-4 has been detected in spleen (Meyers et al., 2005; Uehara and Uehara, 2014) . In particular, TIM-4 is expressed on splenic macrophages (Kobayashi et al., 2007; Wong et al., 2010) , a target of flaviviruses (Balsitis et al., 2009 ). TIM-1 and integrin αvβ5 have been detected on hepatocellular, Huh7-derived cell lines (Meertens et al., 2012; Kondratowicz et al., 2011) and Kupffer cells (Wheeler et al., 2001) , respectively, and may contribute to flavivirus and filovirus infection of the liver (Geisbert et al., 2003b; Balsitis et al., 2009; Couvelard et al., 1999; Hall et al., 1991; Xiao et al., 2001; Ryabchikova et al., 1999) . Flaviviruses also infect bone marrow myeloid cells (Jessie et al., 2004; Balsitis et al., 2009) , which were recently shown to express TIM-4 after stimulation (Baghdadi et al., 2013) .

PVEER expression does not appear to affect the tropism of every virus for which they enhance entry. For instance, Ross River, Chikungunya, and Sindbis virus infect muscle cells, which do not express any currently identified PVEERs (Morrison et al., 2006; Johnson, 1965; Ozden et al., 2007) , suggesting that either other unidentified PVEERs exist or virus entry into muscle cells is independent of PVEER expression. Interestingly, the PtdSerbinding receptor BAI1 has been shown to be important for myoblast fusion (Hochreiter-Hufford et al., 2013) . Although BAI1 does not effectively enhance virus entry in vitro (Meertens et al., 2012) , it may contribute to virus entry in vivo in its native cell type and environment. This may also be the case for other PtdSer receptors that do not function as PVEERs in cell culture as the efficacy of these receptors was assessed after exogenous expression in cell types in which they are not endogenously expressed. Interestingly, PVEERs may also not function effectively in every cell population in which they are endogenously expressed as TIM-1 is present on some populations of T-cells (Meyers et al., 2005; Umetsu et al., 2005; Khademi et al., 2004) , but, T cells are refractory to EBOV infection (Wool-Lewis and Bates, 1998). However, this may also be due to defects in virion internalization machinery or subsequent steps in the viral life cycle and not PVEER activity. Vaccinia virus does infect activated T-cells (Chahroudi et al., 2005) and in this case TIM-1 may contribute to entry. Nonetheless, the effect of PtdSer binding protein expression on virus tropism has not been thoroughly explored in tissue culture and, as discussed below, has yet to be assessed in vivo.

While some PtdSer-binding receptors function effectively as PVEERs, this is not true for all. Receptor for advanced glycation end-products (RAGE), brain-specific angiogensis inhibitor 1 (BAI1), CD300a, Stabilin-1 and -2, and TIM-3 have all been shown to bind to phosphatidylserine and enhance engulfment of apoptotic cells (Fig. 3) Hochreiter-Hufford et al., 2013; Park et al., 2007; He et al., 2011; Friggeri et al., 2011; Nakahashi-Oda et al., 2012; Simhadri et al., 2012; ). However, expression of these proteins does not enhance virus entry (Meertens et al., 2012; Moller-Tank et al., 2013; Morizono and Chen, 2014) . Of these proteins, only the mechanism responsible for reduced PVEER efficacy of TIM-3 has been studied. The inability of TIM-3 to serve as a PVEER is due to the absence of a MLD stalk of sufficient length (Moller-Tank et al., 2014) . However, this may also explain the reduced efficacy of CD300a that, like TIM-3, binds PtdSer using an N-terminal IgV-like domain (Simhadri et al., 2012) and has a short stalk region between its IgV domain and the transmembrane domain.

Stabilin-1/-2 and BAI1 bind PtdSer through epidermal growth factor-like (EGF) domain repeats Kim et al., 2010; and type 1 thrombospondin repeats (TSRs), respectively (Park et al., 2007) (Fig. 2) , although the residues within these domains mediating PtdSer binding have yet to be identified. While it is unknown why these motifs are able to bind apoptotic cells, but not virions, none of the PtdSer binding receptors that function as PVEERs bind PtdSer using repeat regions. One explanation may be that the repeats require a larger or flatter surface area to bind PtdSer than a virion envelope allows. This biological conundrum requires further examination and may provide insight for identification of other PVEERs.

RAGE is the only PtdSer receptor listed above for which the PtdSer-binding-domain is unidentified. The extracellular portion of RAGE consists of an N terminal IgV domain followed by two IgC2 domains (Fig. 3) . The IgV domain of RAGE has been suggested to be responsible for PtdSer binding although this has not yet been experimentally shown (Fig. 2) (Friggeri et al., 2011) . The lack of a similar PtdSer-binding pocket to that of the CD300a and TIM IgVs and inability to functionally replace the IgV domain of TIM-1 (SMT, unpublished data) would suggest at least the IgV domain of RAGE cannot bind to PtdSer on viral envelopes. Additionally, the condensed structure of the RAGE stalk provided by the IgC2 domains likely also further reduces PVEER efficacy by reducing overall length (Moller-Tank et al., 2014) . Thus, both the absence of a clear PtdSer-binding pocket in the IgV domain and the short stalk likely explain the absent PVEER activity of RAGE.

The key factors required for proteins to function as PVEERs are PtdSer-binding activity and attachment to the plasma membrane (Morizono et al., 2011; Meertens et al., 2012; Jemielity et al., 2013; Moller-Tank et al., 2013) . While TIM-1 and TIM-4 individually perform both functions, the TAM receptors and αvβ3 or αvβ5 integrins provide membrane attachment and their ligands bind to PtdSer. In the case of the TIM family PVEERs, the spacer domain between the PtdSer-binding pocket and plasma membrane attachment is a MLD. MLDs from other molecules can substitute for the TIM MLDs, provided the MLD is of sufficient length, while the more compact structure of RAGE IgC2 domains is unable to substitute (Moller-Tank et al., 2014) . These studies led us to propose that an extended structure was needed in the spacer region, perhaps to extend the PtdSer binding-pocket above the extracellular matrix that surrounds the cell (Moller-Tank et al., 2014) . Thus, sufficient distance is necessary between the two essential functions of PVEERs for virus binding.

While we understand how PVEERs bind virus, much less is currently known about the pathways and/or mechanisms for PVEER mediated internalization of virus. There is no evidence for direct internalization. Ebola, baculo-, and vaccinia virus enter predominantly or at least partially through macropinocytosis (Mercer and Helenius, 2008; Aleksandrowicz et al., 2011; Nanbo et al., 2010; Kataoka et al., 2012; Saeed et al., 2010; Mulherkar et al., 2011) and these viruses are amongst those whose entry is most significantly enhanced by PVEER expression. However, entry of flaviviruses and alphaviruses is enhanced by PVEER expression, but primarily occurs through clathrin-mediated endocytosis (Chu and Ng, 2004; Peng et al., 2009; Acosta et al., 2009; Sourisseau et al., 2007; DeTulleo and Kirchhausen, 1998; Marsh et al., 1984) . Thus, if PVEERs directly mediate virus internalization, there are currently no data to support entry via a single pathway. It is certainly possible that different PVEERs mediate uptake through different pathways. Alternatively, in some cases, PVEERs may function only as attachment factors to facilitate interaction of viral proteins with other cell surface receptors that trigger internalization. This may be true for New World arenaviruses that are known to enter cells using transferrin receptor 1 (Radoshitzky et al., 2007) and whose entry is only moderately enhanced by PVEER expression (Jemielity et al., 2013) . However, this mechanism does not account for the ability of PVEERs to stimulate rapid uptake of viruses lacking a glycoprotein (Moller-Tank et al., 2013; Jemielity et al., 2013) .

Several viruses have been shown to expose PtdSer on the outer leaflet of their membrane (Soares et al., 2008; Morizono et al., 2011; Meertens et al., 2012; Moller-Tank et al., 2013) . However, it remains to be determined how PtdSer, which is normally present within the inner leaflet of the cell membrane, is exposed on the outer leaflet of viral membranes. On cellular membranes the asymmetrical distribution of PtdSer is maintained by the activity of flippases and floppases that transfer phospholipids unidirectionally from either the extracellular side to the cytosolic side or the reverse, respectively (Leventis and Grinstein, 2010) . Meanwhile, scramblases disrupt asymmetry by mediating random bidirectional transfer of phospholipids. In healthy cells, exposure of PtdSer can be induced by several mechanisms, including apoptosis and elevated levels of intracellular calcium (Leventis and Grinstein, 2010; Boon and Smith, 2002) . Some viruses encode proteins that increase cytosolic calcium levels such as Nef of HIV-1 (Manninen and Saksela, 2002) and p7 of hepatitis C virus (Griffin et al., 2003) , reviewed in (Zhou et al., 2009) , which may contribute to PtdSer exposure. Entry of flaviviruses into cells also increases intracellular calcium levels (Nour et al., 2013) . It is also possible that cellular stresses associated with virus infection trigger apoptosis as is seen with influenza A (Shiratsuchi et al., 2000) and flaviviruses (Su et al., 2002; Liu et al., 2014; Desprès et al., 1996) . There is evidence that West Nile, Sindbis, and Chikungunya virus actively activate apoptosis to their advantage (Yang et al., 2008; Krejbich-Trotot et al., 2011; Levine et al., 1993) . In contrast, however, apoptosis is not induced in cells that are infected by Ebola viruses (Geisbert et al., 2000; Olejnik et al., 2013) , although internalization of Ebola VLPs and infection of Ebola virus is still enhanced by TIM-1 expression (Jemielity et al., 2013; Moller-Tank et al., 2013; Kondratowicz et al., 2011) . Further, some viruses for which entry is enhanced by PVEERs also encode anti-apoptotic genes, such as NS1 of vaccinia virus (Maluquer de Motes et al., 2011; Cooray et al., 2007) and P35 of baculovirus (Bertin et al., 1996) . Thus, there is not a clear correlation between induction of apoptosis and incorporation of PtdSer on viral envelopes.

Viruses may not utilize cellular functions to induce PtdSer flipping, but rather, concentrate the limited PtdSer present on the outer leaflet. PtdSer may be incorporated into the outer leaflet of viral envelopes during budding for promotion of favorable membrane curvature (Graham and Kozlov, 2010) . Alternatively, viruses may associate with PtdSer in the inner leaflet due to its anionic charge as has been suggested for the matrix proteins of Ebola virus (Adu-Gyamfi et al., 2013) and human immunodeficiency virus-1 (Chukkapalli et al., 2013; Vlach and Saad, 2013) . The VSV matrix has also been shown to associate with PtdSer enriched domains (Luan et al., 1995) . Thus, the inner leaflet would be enriched with PtdSer and, after budding, the absence of flippases that normally regulate PtdSer asymmetry may lead to equilibration of PtdSer between the outer and inner leaflets. Nonetheless, the expression of the matrix protein alone results in sufficient PtdSer accumulation on the outer leaflet of Ebola virus VLPs to allow for TIM-1 enhancement of internalization (Jemielity et al., 2013; Moller-Tank et al., 2013) . These data suggest that regardless of the mechanism, activity of the matrix protein is sufficient for PtdSer incorporation into viral envelopes.

Assuming PVEER enhancement of virus entry occurs through misrecognition of viruses as apoptotic bodies, it stands to reason that the pathways of PVEER-mediated virus internalization overlap with those involved in apoptotic body uptake. Initially it was believed that apoptotic bodies were internalized through macropinocytosis Ogden et al., 2001) which involves the ruffling of membrane to form large cups that engulf fluid (Swanson, 1989) . Macropinocytosis can be stimulated by receptor signaling induced by a variety of ligands including growth factors (Racoosin and Swanson, 1989) or PMA (Swanson, 1989) . While spontaneous ruffling occurs (Sallusto et al., 1995) , only signaling-induced macropinocytosis leads to complete closure of the macropinosomes (Li et al., 1997) . Contrary to what was previously thought, studies using SEM revealed that while necrotic cells are taken up through macropinocytosis, apoptotic bodies are phagocytosed (Krysko et al., 2006) . Unlike macropinocystosis, which does not specifically engulf cargo, phagocytosis involves direct interaction between ligands and receptors. Targets to be engulfed must be entirely covered in ligand and become enclosed as binding between receptor and ligand facilitates the binding of adjacent receptors to ligand (Griffin et al., 1976) . This mechanism is appropriately compared to a zipper (Griffin et al., 1975) .

In theory, phagocytosis would best explain the mechanism of PVEER-mediated internalization with PVEERs sequentially binding PtdSer on the virion surface. This is particularly fitting for TIM-1and TIM-4-mediated internalization of apoptotic bodies, as they have been shown to form phagocytic cups around cargo (Ichimura et al., 2008; Wong et al., 2010) . However, both PtdSer and VLPs have been shown to induce macropinocytosis (Aleksandrowicz et al., 2011; Hoffmann et al., 2001) . Additionally, amiloride and its derivative 5-(N-Ethyl-N-isopropyl) amiloride (EIPA), considered specific inhibitors of macropinocytosis (West et al., 1989) and not phagocytosis (Fukushima et al., 1996) , inhibit Ebola virus uptake into and infection of Vero cells or SNB19 cells that express TIM-1 and Axl respectively Saeed et al., 2010; Mulherkar et al., 2011; Hunt et al., 2011) . Thus, there exists disparity between mechanisms of apoptotic body and virus uptake. Some of this confusion may be attributed to many components being shared between phagocytosis and macropinocytosis. Both require phosphoinositide 3-kinase (PI3K) signaling (Araki et al., 1996) , phospholipase Cγ (Amyere et al., 2000; Cheeseman et al., 2006) , Rac1 (Caron and Hall, 1998; Ridley et al., 1992) , and dynamin (Liu et al., 2008; Cao et al., 2007; Gold et al., 1999; Tse et al., 2003) , making differentiation between the two processes by targeting components difficult. The only macropinocytosis specific inhibitor, EIPA, functions by deregulating intracellular pH and thus may disrupt additional aspects of the viral life cycle that contribute to inhibition of virus infection (Koivusalo et al., 2010) .

Neither phagocytosis nor macropinocytosis have been studied in the context of PVEERs. Phagocytosis is associated with phagocytic cells such as macrophages and dendritic cells (Rabinovitch, 1995) , while all cells can initiate macropinocytosis. It is possible that both mechanisms are being used and are cell-type dependent. For example Ebola virus initially infects macrophages and dendritic cells, which may occur through phagocytosis, and subsequently spreads and enters a broad variety of tissues and cell types, perhaps by macropinocytosis (Feldmann and Geisbert, 2011) . Interestingly, receptors that induce either phagocytosis or macropinocytosis require phosphorylation and recruitment of kinases and adapter proteins (Sobota et al., 2005; Swanson, 2008) , but several PVEERs have been shown not to require signaling through cytoplasmic tails for internalization of virus (Meertens et al., 2012; Moller-Tank et al., 2014; Bhattacharyya et al., 2013) .

Neither phagocytosis nor macropinocytosis of have been shown to be directly elicited by PVEERs. While Axl has been associated with macropinocytosis in some cells , it is unknown whether Axl triggers macropinocytosis. There are, however, several studies that show PtdSer receptors coordinate to induce uptake signaling. In macrophages there is evidence that Mer induces internalization of apoptotic bodies, but requires TIM-4 for initial attachment due to a higher binding affinity for PtdSer (Nishi et al., 2014) . Mer has also been shown to work synergistically with integrin αvβ5 (Wu et al., 2005) . Similarly, TIM-4 and MFG-E8 have been implicated as partners for uptake of apoptotic bodies in which MFG-E8/integrin signaling triggers uptake (Toda et al., 2012) . Several details of these mechanisms remain to be elucidated, such as whether or not TIM-4 actively associates with signaling partners. TIM-4 has also been shown to interact with adenosine monophosphate activating kinase (AMPK) (Baghdadi et al., 2013) , a protein important for macropinocytosis of Ebola and vaccinia virus Moser et al., 2010) . However, this interaction is believed to occur after phagocytosis and does not explain how initial internalization events are triggered. This is also complicated by evidence that the cytoplasmic tail of TIM-4, the only domain accessible to AMPK, is unnecessary for internalization ). Further, these mechanisms do not explain why entry of viruses that utilize clathrin-coated pits is enhanced by PVEER expression. Thus, additional studies are required to determine if and how PVEERs contribute directly to internalization.

As mentioned above, PVEER expression does not enhance entry of all enveloped viruses into every cell type. In the case of arenaviruses, there exists an interesting dichotomy between the ability of PVEERs to enhance entry of New World but not Old World arenaviruses. Unlike for filo-, baculo-, alpha-, vaccinia, and flaviviruses, definitive cellular receptors have been identified for both Old World and New World arenaviruses: α-dystroglycan (Cao et al., 1998) and transferrin receptor 1 (Radoshitzky et al., 2007; Abraham et al., 2009) , respectively. These viruses also use different entry pathways as New World arenaviruses such as Junín virus enter using clathrin coated pits while Old World arenaviruses such as Lassa and lymphocytic choriomeningitis virus use an unknown pathway that is clathrin, caveolin, and dynamin independent (Rojek et al., 2008a (Rojek et al., , 2008b . PVEER-mediated enhancement of some New World arenaviruses entry into TIM-1 expressing HEK 293T cells is more modest than that found with filoviruses and flaviviruses, but still significant (Jemielity et al., 2013; Moller-Tank et al., 2013) . These New World arenaviruses can bind to host cells and mediate efficient entry through interaction with transferrin receptor 1, and this can clearly be supplemented by PVEER expression. For Old World arenaviruses, expression of and/or affinity to α-dystroglycan may be sufficiently robust that PVEER expression does not notably enhance entry further. Indeed, in cells lacking α-dystroglycan, other attachment factors, including Axl, can enhance Old World arenavirus entry (Shimojima et al., 2012) . However, PVEERs are likely not biologically relevant for Old World arenavirus entry as α-dystroglycan expression is ubiquitous (Cao et al., 1998; Sullivan et al., 2013) .

When carrying its native glycoprotein, entry of VSV is in most cases, like the Old World arenaviruses, not enhanced by PVEER expression (Table 1) . However, in a few cases we (unpublished data) and others (Jemielity et al., 2013) have observed a slight increase in VSV G-mediated entry into cells expressing PVEERs. The modest effect of PVEER expression on VSV entry is not due to an absence of PtdSer on the viral envelope as PtdSer can be readily detected on VSV pseudovirions (Moller-Tank et al., 2013) . Instead, as we propose above for LASV, the utilization by VSV of PVEERs would be anticipated to be dependent upon the relative availability of its native receptor. If the native glycoprotein of VSV is sufficient for optimal virus binding to and entry into cells, PVEER expression would make little to no contribution to VSV entry.

Entry of SARS corona-, influenza A, and herpes simplex 1 viruses is not enhanced by PVEER expression (Meertens et al., 2012; Jemielity et al., 2013) . Influenza A and SARS-CoV have viral envelope proteins that bind effectively to sialic acid (Weis et al., 1988) and ACE2 (Li et al., 2003) respectively. Expression of TIM-1 on HEK 293T cells enhances internalization of pseudovirions bearing either envelope protein, but does not result in enhancement of transduction (Jemielity et al., 2013) . These results indicate that PVEER-mediated or -enhanced internalization results in unproductive infection by these viruses. This could occur due to virions being delivered to compartments with incompatible conditions for fusion (i.e. lacking correct pH or fusion triggers). An alternative possibility is that PVEERs enhance internalization of defective SARS or influenza virions with low or negligible levels of glycoprotein on the viral envelope. These particles would not effectively bind cellular receptors or be able to fuse, and thus their enhanced internalization would not contribute to infection. Internalization into endosomes may similarly inhibit entry of herpes simplex virus 1, which fuses at the cell surface (Akhtar and Shukla, 2009 ). However, as the effects of PVEER expression on herpes simplex virus 1 internalization have not been tested, it is possible that, in a manner similar to that proposed for Old World arenaviruses and VSV, strong association with cellular receptors masks any enhancement. Interestingly in all cases, PVEER overexpression on permissive cells does not appear to inhibit overall virus entry, suggesting that either a majority of these viruses either escapes these compartments or enters through natural productive routes.

A major question remaining regarding PVEER-mediated enhancement of virus entry is its relevancy in vivo. The efficacy of PVEERs on wild-type infectious virus has been demonstrated in vitro with virus harvested from both cell culture and mice (Jemielity et al., 2013; Moller-Tank et al., 2013) . Additionally, entry of Ebola virus VLPs into mouse peritoneal macrophages is inhibited by PtdSer liposomes, providing evidence of PVEER importance for filovirus entry into relevant primary cell populations (Jemielity et al., 2013) . However, to date only limited studies have been done in mice to determine the effect of PtdSer-binding inhibitors and none using PVEER knock down or knock out. Difficulty of testing in vivo relevancy of PVEERs using knockout mice arises from the potential for compensation by other PVEERs. Additionally, many PVEERs play critical roles in regulating adaptive immunity and single or combination knockout of genes could make interpretation of results difficult and/or lead to development of autoimmunity (Rodriguez-Manzanet et al., 2010; Miyanishi et al., 2012; Xiao et al., 2012; . Nonetheless, in a single study, a chimeric antibody that recognizes PtdSer was shown to inhibit infection by the New World arenavirus, Pichinde (Soares et al., 2008) . Future studies will benefit from the development of a broad PtdSer-binding inhibitor that can target multiple PVEERs.

Utilization of PtdSer-binding proteins by viruses to enhance internalization provides a broad mechanism of viral entry with little investment by the virus itself. This mechanism may allow some viruses to attach to cells, thereby making viral glycoprotein/cellular receptor interactions more probable. Alternatively, other viruses may rely entirely on PVEERs for internalization into endosomes. This latter mechanism would eliminate the need of viruses to expose sensitive viral epitopes extracellularly, thereby protecting critical receptor binding or membrane fusion motifs from neutralizing antibodies. Once virions are internalized into endosomes, glycoprotein structural alterations that lead to membrane fusion can occur unhindered by antibodies. Additionally, the broad expression of PVEERs might contribute to the extensive tropism of viruses such as flaviviruses and alphaviruses that infect a broad array of insect and mammalian hosts.

Many questions still remain regarding PVEER-mediated entry. Although there is preliminary evidence as discussed above, the most important question is what is the in vivo significance of PVEERs and their relative contribution to infection? If PVEER utilization by viruses in vivo proves to be important, a model for testing inhibitors of PtdSer binding by PVEERs would be valuable and could lead to development of a single antiviral or cocktail capable of inhibiting multiple human viruses. However, as PVEER expression does not enhance entry of all enveloped virus families, targeting of PVEERs as an antiviral therapeutic may be limited to those such as the filoviruses and flaviviruses. Nonetheless, these viral families include highly pathogenic viruses for which limited antivirals are available. Second, how is PtdSer being incorporated into viral envelopes and does it matter which membrane the envelope is derived from? Third, what is the mechanism mediating PVEER-dependent entry? Is there a mechanism of direct internalization that results in productive infection? Is any cell signaling involved in these events? Finally, are there additional unidentified PVEERs that contribute to this newly appreciated means of viral infection? This may include potential PtdSer receptors that have not been tested as PVEERs. CD36, for example, has been shown to bind oxidized PtdSer for uptake of apoptotic cells into macrophages (Driscoll et al., 2013; Greenberg et al., 2006) . HEK 293 T cells do not express a detectable amount of the identified PVEERs on their surface by surface staining, yet support low but detectable levels of transduction of filoviruses, flaviviruses, and alphaviruses. Certainly one possibility is that there are additional undiscovered PVEERs mediating entry. Alternatively, while studies detecting expression of Tyro3 and Mer on HEK 293T cells have been contradictory, low levels of expression may allow for entry (Morizono et al., 2011; Bhattacharyya et al., 2013; Shimojima et al., 2006) . Further research on these receptors will elucidate these gaps in knowledge and determine feasibility of broad-spectrum antivirals to target them.

Host-species transferrin receptor 1 orthologs are cellular receptors for nonpathogenic new world clade B arenaviruses

Alternative infectious entry pathways for dengue virus serotypes into mammalian cells

The Ebola virus matrix protein penetrates into the plasma membrane: a key step in viral protein 40 (VP40) oligomerization and viral egress

A bioinformatics approach identifies signal transducer and activator of ranscription-3 and checkpoint kinase 1 as upstream regulators of kidney injury molecule-1 after kidney injury

Viral entry mechanisms: cellular and viral mediators of herpes simplex virus entry

RAGE expression in human T cells: a link between environmental factors and adaptive immune responses

TIM-4, a receptor for phosphatidylserine, controls adaptive immunity by regulating the removal of antigen-specific T cells

Immature dendritic cells phagocytose apoptotic cells via alphavbeta5 and CD36, and cross-present antigens to cytotoxic T lymphocytes

Ebola virus enters host cells by macropinocytosis and clathrin-mediated endocytosis

Constitutive macropinocytosis in oncogene-transformed fibroblasts depends on sequential permanent activation of Phosphoinositide 3-Kinase and Phospholipase C

Functional analyses of two cellular binding domains of Bovine Lactadherin †

Promotion of tissue inflammation by the immune receptor Tim-3 expressed on innate immune cells

Deficiency or inhibition of Gas6 causes platelet dysfunction and protects mice against thrombosis

Regulation of macrophage foam cell formation by αvβ3 integrin: potential role in human atherosclerosis

A role for phosphoinositide 3-kinase in the completion of macropinocytosis and phagocytosis by macrophages

MFG-E8 attenuates intestinal inflammation in murine experimental colitis by modulating osteopontindependent αvβ3 Integrin signaling

TIM-4 glycoprotein-mediated degradation of dying tumor cells by autophagy leads to reduced antigen presentation and increased immune tolerance

Tropism of dengue virus in mice and humans defined by viral nonstructural protein 3-specific immunostaining

Ultrastructural pathology of experimental ebola haemorrhagic fever virus infection

Identification of Axl as a downstream effector of TGF-beta1 during Langerhans cell differentiation and epidermal homeostasis

The mer receptor tyrosine kinase: expression and function suggest a role in innate immunity

Apoptotic suppression by baculovirus P35 involves cleavage by and inhibition of a virus-induced CED-3/ICE-like protease

Enveloped viruses disable innate immune responses in dendritic cells by direct activation of TAM receptors

Immunohistochemical characterization of a new monoclonal antibody reactive with dengue virus-infected cells in frozen tissue using immunoperoxidase technique. Asian Pac

Human TIM-1 associates with the TCR complex and up-regulates T cell activation signals

Milk fat globule epidermal growth factor 8 (MFG-E8): A novel protein in the mammalian endometrium with putative roles in implantation and placentation

Chemical control of phospholipid distribution across bilayer membranes

Tyrosine kinase receptor Axl enhances entry of Zaire ebolavirus without direct interactions with the viral glycoprotein

MFG-E8 released by apoptotic endothelial cells triggers antiinflammatory macrophage reprogramming

Requirement of vascular integrin alpha v beta 3 for angiogenesis

Phosphatidylserine on HIV envelope is a cofactor for infection of monocytic cells

Molecular and functional characterization of IRp60, a member of the immunoglobulin superfamily that functions as an inhibitory receptor in human NK cells

Dynamin 2 mediates fluid-phase micropinocytosis in epithelial cells

Identification of alphadystroglycan as a receptor for lymphocytic choriomeningitis virus and Lassa fever virus

Natural killer cell differentiation driven by Tyro3 receptor tyrosine kinases

Identification of two distinct mechanisms of phagocytosis controlled by different Rho GTPases

Vaccinia virus tropism for primary hematolymphoid cells is determined by restricted expression of a unique virus receptor

Targeting of protein kinase C-A during Fcγ receptor-dependent phagocytosis requires the ϵC1B domain and phospholipase C-γ1

Infectious entry of West Nile virus occurs through a clathrinmediated endocytic pathway

Evidence in support of rnamediated inhibition of phosphatidylserine-dependent HIV-1 Gag membrane binding in cells

Novel human CD4þ T lymphocyte subpopulations defined by CD300a/c molecule expression

Functional and structural studies of the vaccinia virus virulence factor N1 reveal a Bcl-2-like anti-apoptotic protein

Report of a fatal case of dengue infection with hepatitis: demonstration of dengue antigens in hepatocytes and liver apoptosis

TIM-1) is expressed on in vivo-activated T cells and provides a costimulatory signal for T cell activation

T cell Ig and mucin domain-1-mediated T cell activation requires recruitment and activation of phosphoinositide 3-kinase

T cell/transmembrane, Ig, and mucin-3 allelic variants differentially recognize phosphatidylserine and mediate phagocytosis of apoptotic cells

The clathrin endocytic pathway in viral infection

Human isolates of dengue type 1 virus induce apoptosis in mouse neuroblastoma cells

Molecular analysis and solution structure from small-angle X-ray scattering of the human natural killer inhibitory receptor IRp60 (CD300a)

The phosphatidylserine receptors, T cell immunoglobulin mucin proteins 3 and 4, are markers of histiocytic sarcoma and other histiocytic and dendritic cell neoplasms

Macrophage ADAM17 deficiency augments CD36-dependent apoptotic cell uptake and the linked anti-inflammatory phenotype

Release of high mobility group box 1 by dendritic cells controls t cell activation via the receptor for advanced glycation end products

Expression of stabilin-2, a novel fasciclin-like hyaluronan receptor protein, in murine sinusoidal endothelia, avascular tissues, and at solid/liquid interfaces

Ebola haemorrhagic fever

Macrophage and retinal pigment epithelium phagocytosis: apoptotic cells and photoreceptors compete for alphavbeta3 and alphavbeta5 integrins, and protein kinase C regulates alphav-beta5 binding and cytoskeletal linkage

Participation of the receptor for advanced glycation end products in efferocytosis

Naþ /Hþ exchange activity during phagocytosis in human neutrophils: role of Fcgamma receptors and tyrosine kinases

Infection of human dendritic cells by a sindbis virus replicon vector is determined by a single amino acid substitution in the E2 glycoprotein

Methods to study lymphatic vessel integrins

Apoptosis induced in vitro and in vivo during infection by Ebola and Marburg viruses

Pathogenesis of Ebola hemorrhagic fever in primate models: evidence that hemorrhage is not a direct effect of virus-induced cytolysis of endothelial cells

Pathogenesis of Ebola Hemorrhagic fever in Cynomolgus Macaques: evidence that dendritic cells are early and sustained targets of infection

Identification of a novel high molecular weight protein preferentially expressed by sinusoidal endothelial cells in normal human tissues

Dynamin 2 is required for phagocytosis in macrophages

Interplay of proteins and lipids in generating membrane curvature

Oxidized phosphatidylserine-CD36 interactions play an essential role in macrophage-dependent phagocytosis of apoptotic cells

Studies on the mechanism of phagocytosis. I. Requirements for circumferential attachment of particle-bound ligands to specific receptors on the macrophage plasma membrane

Studies on the mechanism of phagocytosis. II. The interaction of macrophages with anti-immunoglobulin IgG-coated bone marrow-derived lymphocytes

The p7 protein of hepatitis C virus forms an ion channel that is blocked by the antiviral drug, amantadine

Demonstration of yellow fever and dengue antigens in formalin-fixed paraffin-embedded human liver by immunohistochemical analysis

Identification of a factor that links apoptotic cells to phagocytes

Autoimmune disease and impaired uptake of apoptotic cells in mfg-e8-deficient mice

Expression of developmental endothelial locus-1 in a subset of macrophages for engulfment of apoptotic cells

TIM-3 is expressed on activated human CD4 þ T cells and regulates Th1 and Th17 cytokines

Receptor for advanced glycation end products binds to phosphatidylserine and assists in the clearance of apoptotic cells

Phosphatidylserine receptor BAI1 and apoptotic cells as new promoters of myoblast fusion

Phosphatidylserine (PS) induces PS receptor-mediated macropinocytosis and promotes clearance of apoptotic cells

Structural basis of membrane binding by Gla domains of vitamin Kdependent proteins

The Tyro3 receptor kinase Axl enhances macropinocytosis of Zaire ebolavirus

Kidney injury molecule-1 (KIM-1), a putative epithelial cell adhesion molecule containing a novel immunoglobulin domain, is up-regulated in renal cells after injury

Kidney injury molecule-1 is a phosphatidylserine receptor that confers a phagocytic phenotype on epithelial cells

Promotion of the uptake of PS liposomes and apoptotic cells by a product of growth arrest-specific gene, gas6

Lethal experimental infection of rhesus monkeys with Ebola-Zaire (Mayinga) virus by the oral and conjunctival route of exposure

TIM-family proteins promote infection of multiple enveloped viruses through Virion-associated phosphatidylserine

Localization of dengue virus in naturally infected human tissues, by immunohistochemistry and in situ hybridization

Milk fat globule epidermal growth factor-8 blockade triggers tumor destruction through coordinated cell-autonomous and immune-mediated mechanisms

Tumor-associated macrophages regulate tumorigenicity and anticancer drug responses of cancer stem/initiating cells

Virus invasion of the central nervous system: a study of sindbis virus infection in the mouse using fluorescent antibody

Langerhans cells migrate to local lymph nodes following cutaneous infection with an arbovirus

Involvement of membrane type 1-matrix metalloproteinase (MT1-MMP) in RAGE activation signaling pathways

Baculovirus GP64-mediated entry into mammalian cells

T Cell Ig-and mucin-domain-containing molecule-3 (TIM-3) and TIM-1 molecules are differentially expressed on human Th1 and Th2 cells and in cerebrospinal fluid-derived mononuclear cells in multiple sclerosis

CD300a and CD300f differentially regulate the MyD88 and TRIF-mediated TLR signalling pathways through activation of SHP-1 and/or SHP-2 in human monocytic cell lines

T-cell immunoglobulin and mucin domain 1 deficiency eliminates airway hyperreactivity triggered by the recognition of airway cell death

The conserved histidine in epidermal growth factor-like domains of stabilin-2 modulates pH-dependent recognition of phosphatidylserine in apoptotic cells

Expression, purification and structural characterization of functionally replete thrombospondin-1 type 1 repeats in a bacterial expression system

TIM-1 and TIM-4 glycoproteins bind phosphatidylserine and mediate uptake of apoptotic cells

Amiloride inhibits macropinocytosis by lowering submembranous pH and preventing Rac1 and Cdc42 signaling

T-cell immunoglobulin and mucin domain 1 (TIM-1) is a receptor for Zaire Ebolavirus and Lake Victoria Marburgvirus

AMP-activated protein kinase is required for the macropinocytic internalization of ebolavirus

Chikungunya virus mobilizes the apoptotic machinery to invade host cell defenses

Expression of human milk fat globulin proteins in cells of haemopoietic origin

Macrophages use different internalization mechanisms to clear apoptotic and necrotic cells

Dengue virus infects macrophages and dendritic cells in a mouse model of infection

Chikungunya disease in nonhuman primates involves long-term viral persistence in macrophages

Appraising the apoptotic mimicry model and the role of phospholipids for poxvirus entry

Expression and function of CD300 in NK cells

Apoptotic cells activate NKT cells through T cell Ig-like mucin-like-1 resulting in airway hyperreactivity

Immunobiology of the TAM receptors

The distribution and function of phosphatidylserine in cellular membranes

Conversion of lytic to persistent alphavirus infection by the bcl-2 cellular oncogene

Uncoupling of membrane ruffling and pinocytosis during ras signal transduction

Expression and anatomical distribution of TIM-containing molecules in Langerhans cell sarcoma

Systemic autoimmunity in TAM triple knockout mice causes inflammatory brain damage and cell death

Angiotensin-converting enzyme 2 is a functional receptor for the SARS coronavirus

Antibody-dependent enhancement and persistence in macrophages of an arbovirus associated with arthritis

AGEs increased migration and inflammatory responses of adventitial fibroblasts via RAGE, MAPK and NF-κB pathways

Dengue virus subgenomic RNA induces apoptosis through the Bcl-2-mediated PI3k/Akt signaling pathway

Isoform and splice-variant specific functions of Dynamin-2 revealed by analysis of conditional Knock-Out cells

Articular chondrocytes express the receptor for advanced glycation end products: potential role in osteoarthritis

Tyro-3 family receptors are essential regulators of mammalian spermatogenesis

Formation of membrane domains created during the budding of vesicular stomatitis virus. A model for selective lipid and protein sorting in biological membranes

Inhibition of apoptosis and NF-kappaB activation by vaccinia protein N1 occur via distinct binding surfaces and make different contributions to virulence

HIV-1 Nef interacts with Inositol Trisphosphate receptor to activate calcium signaling in T cells

Human dendritic cells as targets of dengue virus infection

Semliki forest virus entry and the endocytic pathway

Differential expression of a gene signature for scavenger/lectin receptors by endothelial cells and macrophages in human lymph node sinuses, the primary sites of regional metastasis

Characterization of Junín arenavirus cell entry

TIM-1, a novel allergy and asthma susceptibility gene

The TIM and TAM families of phosphatidylserine receptors mediate dengue virus entry

Vaccinia virus uses macropinocytosis and apoptotic mimicry to enter host cells

TIM-4 is the ligand for TIM-1, and the TIM-1-TIM-4 interaction regulates T cell proliferation

Identification of Tim4 as a phosphatidylserine receptor

Synergistic effect of Tim4 and MFG-E8 null mutations on the development of autoimmunity

Expression of milk fat globule epidermal growth factor 8 in immature dendritic cells for engulfment of apoptotic cells

Bimodal regulation of T cell-mediated immune responses by TIM-4

Role of the phosphatidylserine receptor TIM-1 in enveloped-virus entry

Characterizing functional domains for TIM-mediated enveloped virus entry

Th1-specific cell surface protein Tim-3 regulates macrophage activation and severity of an autoimmune disease

The role of phosphatidylserine receptors in enveloped virus infection

The soluble serum protein Gas6 bridges virion envelope phosphatidylserine to the TAM receptor tyrosine kinase Axl to mediate viral entry

Characterization of Ross river virus Tropism and virus-induced inflammation in a mouse model of viral Arthritis and Myositis

Receptor for advanced glycation end products expression on T cells contributes to antigen-specific cellular expansion in vivo

A kinome RNAi screen identified AMPK as promoting poxvirus entry through the control of actin dynamics

The Ebola virus glycoprotein mediates entry via a non-classical dynamin-dependent macropinocytic pathway

Solution structure of the epidermal growth factor-like domain of heregulin-alpha, a ligand for p180erbB-4

Identification of the product of growth arrest-specific gene 6 as a common ligand for Axl, Sky, and Mer receptor tyrosine kinases

Phenotypic differences between Th1 and Th17 cells and negative regulation of Th1 cell differentiation by IL-17

TIM-1 and TIM-3 enhancement of Th2 cytokine production by mast cells

Identification of phosphatidylserine as a ligand for the CD300a immunoreceptor

Association of advanced glycation end products with A549 cells, a human pulmonary epithelial cell line, is mediated by a receptor distinct from the scavenger receptor family and RAGE

Ebolavirus is internalized into host cells via macropinocytosis in a viral glycoprotein-dependent manner

Human Th1 cells that express CD300a are polyfunctional and after stimulation up-regulate the T-box transcription factor eomesodermin

Tim4-and MerTK-mediated engulfment of apoptotic cells by mouse resident peritoneal macrophages

Restricted and selective tropism of a Venezuelan equine encephalitis virus-derived replicon vector for human dendritic cells

Viral membrane fusion and nucleocapsid delivery into the cytoplasm are distinct events in some flaviviruses

C1q and Mannose binding lectin engagement of cell surface Calreticulin and cd91 initiates Macropinocytosis and uptake of Apoptotic cells

Advanced glycation end products enhance monocyte activation during human mixed lymphocyte reaction

Reversible inactivation and reactivation of vaccinia virus by manipulation of viral lipid composition

Ebola virus does not block apoptotic signaling pathways

Lactation-dependenteExpression of an mRNA splice variant with an Exon for a multiplyOglycosylated domain of mouse milk fat globule glycoprotein MFG-E8

Secretion of a peripheral membrane protein, MFG-E8, as a complex with membrane vesicles

Human muscle satellite cells as targets of Chikungunya virus infection

The phosphatidylserine receptor TIM-4 does not mediate direct signaling

The 1.5 A crystal structure of human receptor for advanced glycation endproducts (RAGE) ectodomains reveals unique features determining ligand binding

Epidermal growth factor-like domain repeat of stabilin-2 recognizes Phosphatidylserine during cell corpse clearance

Rapid cell corpse clearance by stabilin-2, a membrane phosphatidylserine receptor

Stabilin-1 mediates phosphatidylserine-dependent clearance of cell corpses in alternatively activated macrophages

Entry of dengue virus serotype 2 into ECV304 cells depends on clathrindependent endocytosis, but not on caveolae-dependent endocytosis

Receptor for advanced glycation endproducts mediates proatherogenic responses to periodontal infection in vascular endothelial cells

TAM receptor function in the retinal pigment epithelium

Expression of the receptor protein-tyrosine kinases Tyro-3, Axl, and mer in the developing rat central nervous system

Professional and non-professional phagocytes: an introduction

Macrophage colony-stimulating factor (rM-CSF) stimulates pinocytosis in bone marrow-derived macrophages

Transferrin receptor 1 is a cellular receptor for New World haemorrhagic fever arenaviruses

Gas6-mediated signaling is dependent on the engagement of its gamma-carboxyglutamic acid domain with phosphatidylserine

Beginnings of a good apoptotic meal: the find-me and eatme signaling pathways

The small GTPbinding protein rac regulates growth factor-induced membrane ruffling

T and B cell hyperactivity and autoimmunity associated with niche-specific defects in apoptotic body clearance in TIM-4-deficient mice

Different mechanisms of cell entry by human-pathogenic Old World and New World arena viruses

Cellular entry of lymphocytic choriomeningitis virus

TAM receptors are pleiotropic inhibitors of the innate immune response

An analysis of features of pathogenesis in two animal models of Ebola virus infection

Cellular entry of Ebola virus involves uptake by a macropinocytosis-like mechanism and subsequent trafficking through early and late endosomes

Dendritic cells use macropinocytosis and the mannose receptor to concentrate macromolecules in the major histocompatibility complex class II compartment: downregulation by cytokines and bacterial products

Structures of T cell immunoglobulin mucin protein 4 show a metal-Ion-dependent ligand binding site where phosphatidylserine binds

Crystal structure of a C-terminal fragment of growth arrest-specific protein Gas6. Receptor tyrosine kinase activation by laminin Glike domains

Structural basis for Gas6-Axl signalling

Macrophages and dendritic cells use different Axl/Mertk/Tyro3 receptors in clearance of apoptotic cells

Differential induction of type I interferon responses in myeloid dendritic cells by mosquito and mammalian-cell-derived alphaviruses

Crystal structure of lactadherin C2 domain at 1.7 A resolution with mutational and computational analyses of its membrane-binding motif

Tyro3 family-mediated cell entry of Ebola and Marburg viruses

Identification of cell surface molecules involved in dystroglycan-independent Lassa virus cell entry

Phosphatidylserinemediated Phagocytosis of influenza A virus-infected cells by mouse peritoneal macrophages

Cloning and characterization of BAI2 and BAI3, novel genes homologous to brain-specific angiogenesis inhibitor 1 (BAI1)

CD300a is expressed on human B cells, modulates BCR-mediated signaling, and its expression is down-regulated in HIV infection

Lactadherin promotes VEGFdependent neovascularization

Differential expression of CD300a/c on human TH1 and TH17 cells

Human CD300a binds to phosphatidylethanolamine and phosphatidylserine, and modulates the phagocytosis of dead cells

Epitope-dependent effect of anti-murine TIM-1 monoclonal antibodies on T cell activity and lung immune responses

Targeting inside-out phosphatidylserine as a therapeutic strategy for viral diseases

Binding of IgG-Opsonized particles to FcγR is an active stage of phagocytosis that involves receptor clustering and phosphorylation

Brain-specific angiogenesis inhibitor-1 expression in astrocytes and neurons: Implications for its dual function as an apoptotic engulfment receptor

Characterization of reemerging Chikungunya virus

The anticoagulation factor protein S and its relative, Gas6, are ligands for the Tyro 3/Axl family of receptor tyrosine kinases

cDNA cloning of a mouse mammary epithelial cell surface protein reveals the existence of epidermal growth factor-like domains linked to factor VIII-like sequences

Japanese Encephalitis virus infection initiates endoplasmic reticulum stress and an unfolded protein response

Is the TAM receptor Axl a receptor for lymphocytic choriomeningitis virus?

Increased expression of receptor for advanced glycation end products by synovial tissue macrophages in rheumatoid arthritis

Ca(2 þ )-bridging mechanism and phospholipid head group recognition in the membrane-binding protein annexin V

Phorbol esters stimulate macropinocytosis and solute flow through macrophages

Shaping cups into phagosomes and macropinosomes

Molecular characterization of dendritic cell-derived exosomes. Selective accumulation of the heat shock protein hsc73

Autophosphorylation docking site Tyr-867 in Mer receptor tyrosine kinase allows for dissociation of multiple signaling pathways for phagocytosis of apoptotic cells and down-modulation of Lipopolysaccharide-inducible NF-κB transcriptional activation

Molecular mechanism for differential recognition of membrane phosphatidylserine by the immune regulatory receptor Tim4

Two-step engulfment of apoptotic cells

Differential role of Actin, Clathrin, and Dynamin in Fcγ receptormediated Endocytosis and Phagocytosis

Integrin αvβ5 in endothelial cells of rat splenic sinus: an immunohistochemical and ultrastructural study

TIM-1 induces T cell activation and inhibits the development of peripheral tolerance

Viral apoptotic mimicry: an immune evasion strategy developed by the hepatitis B virus?

Trio engagement via plasma membrane phospholipids and the myristoyl moiety governs HIV-1 matrix binding to bilayers

Immunoexpression of Tyro 3 family receptors-Tyro 3, Axl, and Mer-and their ligand Gas6 in postnatal developing mouse testis

Structure of the influenza virus haemagglutinin complexed with its receptor, sialic acid

Distinct endocytotic pathways in epidermal growth factor-stimulated human carcinoma A431 cells

Adenoviral gene delivery can inactivate Kupffer cells: role of oxidants in NF-κB activation and cytokine production

Phosphatidylserine receptor Tim-4 is essential for the maintenance of the homeostatic state of resident peritoneal macrophages

Characterization of Ebola virus entry by using pseudotyped viruses: identification of receptor-deficient cell lines

Human skin Langerhans cells are targets of dengue virus infection

A role for Mer tyrosine kinase in alphavbeta5 integrin-mediated phagocytosis of apoptotic cells

Defect in regulatory Bcell function and development of systemic autoimmunity in T-cell Ig mucin 1 (Tim-1) mucin domain-mutant mice

Experimental yellow fever virus infection in the Golden hamster (Mesocricetus auratus). II. pathology

Crystal structure of the extracellular segment of Integrin αVβ3 in complex with an Arg-Gly-Asp ligand

Peripheral CD300a þ CD8þ T lymphocytes with a distinct cytotoxic molecular signature increase in pregnant women with chronic chorioamnionitis

West Nile virus capsid protein induces p53-mediated apoptosis via the sequestration of HDM2 to the nucleolus

NMR solution structure of C2 domain of MFG-E8 and insights into its molecular recognition with phosphatidylserine

Anti-receptor for advanced glycation end products therapies as novel treatment for abdominal aortic aneurysm

Viral calciomics: interplays between Ca2 þ and virus

Involvement of RAGE, MAPK and NF-κB pathways in AGEs-induced MMP-9 activation in HaCaT keratinocytes