Ebola Virus Exploits a Monocyte Differentiation Program To Promote Its Entry

Abstract

Antigen-presenting cells (APCs) are critical targets of Ebola virus (EBOV) infection in vivo.However, the susceptibility of monocytes to infection is controversial. Studies indicate productive monocyte infection, and yet monocytes are also reported to be resistant to EBOV GP-mediated entry. In contrast, monocyte-derived macrophages and dendritic cells are permissive for both EBOV entry and replication. Here, freshly isolated monocytes are demonstrated to indeed be refractory to EBOV entry. However, EBOV binds monocytes, and delayed entry occurs during monocyte differentiation. Cultured monocytes spontaneously downregulate the expression of viral entry restriction factors such as interferon-inducible transmembrane proteins, while upregulating the expression of critical EBOV entry factors cathepsin B and NPC1. Moreover, these processes are accelerated by EBOV infection. Finally, ectopic expression of NPC1 is sufficient to rescue entry into an undifferentiated, normally nonpermissive monocytic cell line. These results define the molecular basis for infection of APCs and suggest means to limit APC infection. Zaire Ebola virus (EBOV) is an emerging zoonotic pathogen that has caused outbreaks of viral hemorrhagic fever in humans with fatality rates approaching 90% (1). EBOV tropism toward antigen-presenting cells (APCs) is thought to play an important role in viral pathogenesis, contributing to the establishment of infection and to the development of hemorrhagic fever (2). EBOV productively infects APCs, including monocytes, macrophages, and dendritic cells (DCs), in vitro (3-9), and tissue sections from EBOV-infected humans and nonhuman primates contain APCs positive for EBOV antigen/nucleic acid, demonstrating APC infection in vivo (10-16). Although APCs serve as sites of virus amplification, their infection also deregulates APC function (2,17,18). This deregulation may contribute to the development of an ineffective antiviral immune response, as well as an intense and deregulated inflammatory response (4,19).Although monocytes, the most abundant blood-borne APCs, likely contribute to EBOV pathogenesis, an apparent discrepancy exists in our understanding of monocyte infection by EBOV (18). Specifically, although EBOV productively infects human bloodderived monocytes (4,20), several other studies demonstrate limited or restricted EBOV GP-mediated entry into monocytes (21-23).EBOV entry, which includes attachment and penetration into the target cell cytoplasm, is mediated by the surface glycoprotein (GP) (24). GP likely mediates viral attachment through the receptor-binding domain (RBD) located at its N terminus (25-28). Subsequent viral uptake likely occurs via macropinocytosis (29)(30)(31)(32). A number of cell surface molecules, including C-type lectins, have been identified as attachment or entry factors (33-38). However, none of these factors appears to function as an essential cell surface receptor. Additional host factors have also been implicated as regulating entry, including components of the homotypic fusio...

Ebola Virus Disease Is Characterized by Poor Activation and Reduced Levels of Circulating CD16 + Monocytes

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Enhancement of Ebola Virus Infection via Ficolin-1 Interaction with the Mucin Domain of GP Glycoprotein

“…7A), which might suggest that, under certain circumstances, cell surface ficolin-1 could have a protective effect against viral infection. Cell surface expression of endogenous ficolin-1 has been shown to be restricted to monocytes and granulocytes, which seem to be refractory to specific EBOV GP-mediated entry (71,72). Interestingly, M-ficolin expression is silenced in macrophages and dendritic cells (56,73), which are major target cells of EBOV.…”

Molecular Basis for Ebolavirus VP35 Suppression of Human Dendritic Cell Maturation

“…In vivo, DCs are target cells of infection (2,3). In vitro, DCs support productive virus replication (4)(5)(6). Experimental infection of human monocytederived DCs (MDDCs) results in impaired alpha/beta interferon (IFN-␣/␤) and proinflammatory cytokine expression, abnormal upregulation of costimulatory markers, and inhibition of MDDCmediated activation of naive T cells (4,5,7).…”