Severe Fever with Thrombocytopenia Virus Glycoproteins Are Targeted by Neutralizing Antibodies and Can Use DC-SIGN as a Receptor for pH-Dependent Entry into Human and Animal Cell Lines

Abstract

f Severe fever with thrombocytopenia syndrome virus (SFTSV) is a novel bunyavirus that recently emerged in China. Infection with SFTSV is associated with case-fatality rates of up to 30%, and neither antivirals nor vaccines are available at present. Development of antiviral strategies requires the elucidation of virus-host cell interactions. Here, we analyzed host cell entry of SFTSV. Employing lentiviral and rhabdoviral vectors, we found that the Gn/Gc glycoproteins (Gn/Gc) of SFTSV mediate entry into a broad range of human and animal cell lines, as well as human macrophages and dendritic cells. The Gn/Gc proteins of La Crosse virus (LACV) and Rift Valley Fever Virus (RVFV), other members of the bunyavirus family, facilitated entry into an overlapping but not identical range of cell lines, suggesting that SFTSV, LACV, and RVFV might differ in their receptor requirements. Entry driven by SFTSV Gn/Gc was dependent on low pH but did not require the activity of the pH-dependent endosomal/lysosomal cysteine proteases cathepsins B and L. Instead, the activity of a cellular serine protease was required for infection driven by SFTSV and LACV Gn/Gc. Sera from convalescent SFTS patients inhibited SFTSV Gn/Gc-driven host cell entry in a dose-dependent fashion, demonstrating that the vector system employed is suitable to detect neutralizing antibodies. Finally, the C-type lectin DC-SIGN was found to serve as a receptor for SFTSV Gn/Gc-driven entry into cell lines and dendritic cells. Our results provide initial insights into cell tropism, receptor usage, and proteolytic activation of SFTSV and will aid in the understanding of viral spread and pathogenesis.

<b>The amino acid at position 624 in the glycoprotein of SFTSV (severe fever with thrombocytopenia virus) plays a critical role in low-pH-dependent cell fusion </b><b>activity </b>

“…3C). Similar to other phleboviruses, it was reported that SFTSV infection was inhibited by treatment with lysosomotropic agents and that formation of syncytia was induced under acidic conditions (12,26). We also confirmed that SFTSV strain YG1 induced the formation of syncytia under acidic conditions and we then established subclones based on their ability to facilitate cell fusion (20).…”

“…The L segment encodes an RNA-dependent RNA polymerase; the M segment encodes a glycoprotein precursor (GPC) that is co-translationally cleaved into Gn and Gc, which then form a spike complex known as the envelope glycoprotein (GP); and the S segment encodes a nucleocapsid protein and a non-structural protein (3, 29). As is the case for other viruses in the family Bunyaviridae, Gn and Gc of SFTSV are involved in virus attachment and entry (6,12,25). SFTSV attaches to host cell receptors and is endocytosed in a receptor-mediated manner.…”

Characterization of Glycoprotein-Mediated Entry of Severe Fever with Thrombocytopenia Syndrome Virus

“…Virus entry into target cells is initiated by the binding of GP to appropriate cell surface receptors. Recently, it was reported that a C-type lectin, dendritic cell-specific intercellular adhesion molecule 3-grabbing nonintegrin (DC-SIGN), is an initial binding receptor for SFTSV (5), as was previously reported for other phleboviruses (6). It was also reported that nonmuscle myosin heavy chain IIA is critical for the cellular entry of SFTSV (7).…”

“…Furthermore, this system also allows the life cycle of highly pathogenic viruses to be studied outside biosafety level 3 (BSL-3) or BSL-4 containment. SFTSV is categorized as a BSL-3 pathogen in Japan, and the characteristics of cell entry and virus neutralization have thus far been investigated only through the use of pseudotype viruses bearing the GP of SFTSV (SFTSV-GP) (5). Although pseudotype viruses bearing specific glycoproteins can authentically mimic the entry process of the native virus, experiments using the native virus will be required to confirm that the entry process is adequately recapitulated by the pseudotype viruses.…”

Nonmuscle Myosin Heavy Chain IIA Is a Critical Factor Contributing to the Efficiency of Early Infection of Severe Fever with Thrombocytopenia Syndrome Virus

“…Rift Valley fever and Uukuniemi viruses bind to DC-SIGN through high-mannose Nglycans of the viral glycoproteins, whereas subsequent penetration of the host cell depends on endocytic internalization (15). Recently, pseudotypes of vesicular stomatitis virus (VSV) bearing SFTSV Gn/Gc envelopes were also shown to utilize DC-SIGN to enter human monocyte-derived dendritic cells naturally expressing DC-SIGN and Raji B cells transfected with DC-SIGN (16). However, importantly, most of the cell types that are susceptible to SFTSV infection do not express DC-SIGN (1), indicating that SFTSV might also use another receptor(s) that is more broadly expressed in human cells.…”