In this study we investigated the effects of Marburg virus and Ebola virus (species Zaire and Reston) infections on freshly isolated suspended monocytes in comparison to adherent macrophages under culture conditions. Our data showed that monocytes are permissive for both filoviruses. As is the case in macrophages, infection resulted in the activation of monocytes which was largely independent of virus replication. The activation was triggered similarly by Marburg and Ebola viruses, species Zaire and Reston, as indicated by the release of the proinflammatory cytokines interleukin-1␤ (IL-1␤), tumor necrosis factor ␣, and IL-6 as well as the chemokines IL-8 and gro-␣. Our data suggest that infected monocytes may play an important role in the spread of filoviruses and in the pathogenesis of filoviral hemorrhagic disease.

Localization of VP40 in Marburg virus (MBGV)-infected cells was studied by using immunofluorescence and immunoelectron microscopic analysis. VP40 was detected in association with nucleocapsid structures, present in viral inclusions and at sites of virus budding. Additionally, VP40 was identified in the foci of virus-induced membrane proliferation and in intracellular membrane clusters which had the appearance of multivesicular bodies (MVBs). VP40-containing MVBs were free of nucleocapsids. When analyzed by immunogold labeling, the concentration of VP40 in MVBs was six times higher than in nucleocapsid structures. Biochemical studies showed that recombinant VP40 represented a peripheral membrane protein that was stably associated with membranes by hydrophobic interaction. Recombinant VP40 was also found in association with membranes of MVBs and in filopodia-or lamellipodia-like protrusions at the cell surface. Antibodies against marker proteins of various cellular compartments showed that VP40-positive membranes contained Lamp-1 and the transferrin receptor, confirming that they belong to the late endosomal compartment. VP40-positive membranes were also associated with actin. Western blot analysis of purified MBGV structural proteins demonstrated trace amounts of actin, Lamp-1, and Rab11 (markers of recycling endosomes), while markers for other cellular compartments were absent. Our data indicate that MBGV VP40 was able to interact with membranes of late endosomes in the course of viral infection. This capability was independent of other MBGV proteins. The family of Filoviridae comprises Marburg virus (MBGV)and Ebola virus (EBOV), which cause a severe and often fatal hemorrhagic disease in human and nonhuman primates. During the reported outbreaks, up to 80% of the cases had a fatal outcome. The recent outbreak of MBGV hemorrhagic fever in the Democratic Republic of the Congo underlines the emerging potential of this pathogen (68). Filoviral infections are pantropic, affecting almost every organ of the infected host. However, the major and primary targets are cells of the mononuclear phagocytic system (55).The enveloped MBGV particles are composed of seven structural proteins and the nonsegmented negative-strand RNA genome (7,15). The viral envelope is spiked with homotrimers of the glycoprotein GP (1,16,60). Four proteins are components of the nucleocapsid: the nucleoprotein NP (2,36,40,57), the L protein (46), VP35 (45), and VP30 (2). NP, VP35, and L are essential for viral replication and transcription (45); the function of VP30, an NP-binding phosphoprotein, is still unclear (44).Between the nucleocapsid and the viral envelope, MBGV particles contain two proteins, VP24 and the highly abundant VP40, whose function is not yet elucidated (2, 13). However, the position of VP40 in the genome (third gene), its hydrophobicity, and its abundance within the virions suggest that VP40 represents a homologue of the matrix proteins of other nonsegmented negative-strand RNA viruses.It is currently believed that matrix prot...

The pathogenetic mechanisms underlying viral hemorrhagic fevers are not fully understood, but hemorrhage, activation of coagulation, and shock suggest vascular instability. Here, we demonstrate that Marburg virus (MBG), a filovirus causing a severe form of hemorrhagic fever in humans, replicates in human monocytes/macrophages, resulting in cytolytic infection and release of infectious virus particles. Replication also led to intracellular budding and accumulation of viral particles in vacuoles, thus providing a mechanism by which the virus may escape immune surveillance. Monocytes/macrophages were activated by MBG infection as indicated by tumor necrosis factor alpha (TNF-␣) release. Supernatants of monocyte/macrophage cultures infected with MBG increased the permeability of cultured human endothelial cell monolayers. The increase in endothelial permeability correlated with the time course of TNF-␣ release and was inhibited by a TNF-␣specific monoclonal antibody. Furthermore, recombinant TNF-␣ added at concentrations present in supernatants of virus-infected macrophage cultures increased endothelial permeability in the presence of 10 M H 2 O 2. These results indicate that TNF-␣ plays a critical role in mediating increased permeability, which was identified as a paraendothelial route shown by formation of interendothelial gaps. The combination of viral replication in endothelial cells (H.-J. Schnittler, F. Mahner, D. Drenckhahn, H.-D. Klenk, and H. Feldmann, J. Clin. Invest. 19:1301-1309, 1993) and monocytes/macrophages and the permeability-increasing effect of virus-induced cytokine release provide the first experimental data for a novel concept in the pathogenesis of viral hemorrhagic fever.

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