The known animal viruses are remarkably diverse in the
ways in which they sustain themselves in nature, occa-
sionally wreaking havoc in their preferred or accidental
hosts. The immune system is central in this battle for
survival, which is exemplified here by the cases of Ebola
and Marburg virus (EBOV and MARV, respectively)
infections in primates. In healthy but unvaccinated mon-
keys or humans infected with the most virulent strains of
EBOV and MARV, the immune system loses the battle
in nearly 90% of encounters, not slowly and inexorably
as with some viruses, but in a matter of days (BOX 1). The
reasons seem to reside in a complex series of interrelated
viral and immune events.
As will be discussed, it seems that EBOV and MARV
relentlessly infect cells of the monocyte–macrophage
lineage, accelerating the release of pro-inflammatory
cytokines, including tumour-necrosis factor (TNF)
and interferon-γ (IFNγ), which in turn can disrupt the
architecture of the vascular endothelium1,2 and other
tissues (BOX 1; FIG. 1). The triggering of neutrophils and
perhaps other polymorphonuclear leukocytes by inter-
actions between viral particles and triggering receptors
expressed on myeloid cells (TREMs) or Toll-like recep-
tors (TLRs) provides another source of the copious
amounts of inflammatory mediators produced as the
viral burden increases during the course of infection.
Along with fever, diarrhoea is a frequent symptom,
and both dehydration and septicaemia become risks.
Overshadowing everything is the inexorable replication
of the virus, beginning in dendritic cells (DCs), mono-
cytes and various types of macrophage, but proceed-
ing to a more pantropic profile in which only certain
cells — notably lymphocytes — are spared. In the case
of MARV, hepatocytes are particularly susceptible,
elevated liver enzymes are among the first telling signs of
disease and liver damage seems to account for much
of the coagulopathy3,4. Coincident with and underlying
the unchecked viral replication, filoviruses deceive
and dysregulate both innate and adaptive immune
responses. Here, we describe recent findings pertinent
to the clash between virus and host defences, summa-
rize and briefly highlight the progress in the develop-
ment of filovirus vaccines and therapeutics (BOX 2) and
attempt to illuminate crucial areas in which questions
and opportunities remain.
First encounters: receptor-mediated tropisms
The observed distribution of filoviruses in animals in
the first days of infection5–7, as well as various in vitro
experiments, show that DCs, monocytes and macro-
phages are early targets of infection. This distribution
occurs in a larger context in which filoviruses show
diverse cell tropism and can eventually be found in
numerous non-lymphocytic cell types in later stages of
There seem to be several cellular receptors for filo-
viruses, and to the extent that they have been identified,
they appear to be relatively nonspecific or pattern based
(FIG. 2). Therefore, C-type lectins, such as DC-specific
intercellular adhesion molecule 3 (ICAM3)-grabbing
non-integrin (DC-SIGN; also known as CD209) or liver/
lymph node-SIGN (L-SIGN; also known as CLEC4M),
are sufficient to confer permissive filovirus glycoprotein-
mediated infection9. Moreover, human macrophage
C-type lectin specific for galactose/N-acetylgalactos-
amine (hMGL) that is expressed by cells of monocytic
origin (such as immature DCs) also promotes filoviral
entry10. Another C-type lectin, the asialoglycoprotein
*US Army Medical Research
Institute for Infectious
‡Johns Hopkins University
School of Medicine,
Baltimore, Maryland, USA.
Correspondence to M.M.
Tending towards the capacity
to infect a wide range of cells
Refers to a group of conditions
of the blood-clotting system in
which bleeding is prolonged
A virus’s affinity for or
tendency towards preferential
infection of certain cells.
How Ebola and Marburg viruses
battle the immune system
Mansour Mohamadzadeh*‡, Lieping Chen‡, and Alan L. Schmaljohn*
Abstract | The filoviruses Ebola and Marburg have emerged in the past decade from
relative obscurity to serve now as archetypes for some of the more intriguing and daunting
challenges posed by such agents. Public imagination is captured by deadly outbreaks of
these viruses and reinforced by the specter of bioterrorism. As research on these agents has
accelerated, it has been found increasingly that filoviruses use a combination of familiar and
apparently new ways to baffle and battle the immune system. Filoviruses have provided
thereby a new lens through which to examine the immune system itself.
556 | JULY 2007 | VOLUME 7
© 2007 Nature Publishing Group
Report Documentation Page
OMB No. 0704-0188
Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and
maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information,
including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington
VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it
does not display a currently valid OMB control number.
1. REPORT DATE
1 APR 2006
2. REPORT TYPE
3. DATES COVERED
4. TITLE AND SUBTITLE
How Ebola and Marburg viruses battle the immune system. Nature
Reviews of Immunology 7:556-567
5a. CONTRACT NUMBER
5b. GRANT NUMBER
5c. PROGRAM ELEMENT NUMBER
Mohamadzadeh, M Ghen, L Schmaljohn, AL
5d. PROJECT NUMBER
5e. TASK NUMBER
5f. WORK UNIT NUMBER
7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)
United States Army Medical Research Institute of Infectious Diseases,
Fort Detrick, MD
8. PERFORMING ORGANIZATION
9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)
10. SPONSOR/MONITOR’S ACRONYM(S)
11. SPONSOR/MONITOR’S REPORT
12. DISTRIBUTION/AVAILABILITY STATEMENT
Approved for public release, distribution unlimited
13. SUPPLEMENTARY NOTES
The original document contains color images.
The filoviruses Ebola and Marburg have emerged in the past decade from relative obscurity to serve now
as archetypes for some of the more intriguing and daunting challenges posed by such agents. Public
imagination is captured by deadly outbreaks of these viruses and reinforced by the specter of bioterrorism.
As research on these agents has accelerated, it has been found increasingly that filoviruses use a
combination of familiar and apparently new ways to baffle and battle the immune system. Filoviruses have
provided thereby a new lens through which to examine the immune system itself.
15. SUBJECT TERMS
filovirus, Ebola, Marburg, review, glycoprotein, inflammatory responses, therapeutics, cytokines
16. SECURITY CLASSIFICATION OF:
17. LIMITATION OF
19a. NAME OF
c. THIS PAGE
Standard Form 298 (Rev. 8-98)
Prescribed by ANSI Std Z39-18
Plasmacytoid dendritic cells
(pDCs). A distinct subset of DCs
that differ from conventional
myeloid DCs in their capacity
to produce copious amounts of
interferon-γ in response to
many viruses and Toll-like-
Invariant natural killer T cell
(iNKT cell). A cell type thought
to be particularly important in
bridging between innate and
adaptive immunity. iNKT cells
are typified by a capacity for
self-recognition and rapid
release of cytokines such as
receptor that is expressed by hepatocytes, has specific
affinity for the N-linked sugar chains with terminal
galactose residues of MARV glycoproteins11 and therefore
initiates filoviral cell infectivity by facilitating viral cell
entry. Conversely, the binding of filoviruses to C-type
lectin molecules is significantly inhibited by the carbo-
hydrate mannan, which indicates that the binding is due
to the interaction between carbohydrate motifs on the
surface of host cells and N-linked carbohydrate structures
of glycoprotein on the filoviral surface. Recently, we also
observed that a class of immunorecognition receptors
known as TREMs, which are implicated in the ampli-
fication of septic shock and cell activation, might have
a crucial early role in the interaction of filoviruses with
neutrophils12, monocytes and mature DCs.
Collectively, these observations about the binding
of filoviruses to the C-type lectin and TREM receptors
on cells of myeloid lineage are consistent with in vivo
observations. Nevertheless, as filoviruses are pantropic
and other infected cell types, such as endothelial and
epithelial cells, do not express C-type lectins or TREMs,
there might be other ubiquitous molecules, such as
heparin-sulphate proteoglycan (HSPG; also known
as SDC2)13 and folate receptor-α (REF. 14), involved in
viral- and host-receptor-mediated entry.
If this complexity were not enough, it has been shown
that filoviral glycoproteins are glycosylated differently,
depending on the cells in which they are grown15, and
that they bud from lipid rafts16 (a process in which viral
particles might passively acquire host proteins that
influence viral cell tropism). Further compounding
the specificities of early interactions between viruses
and cells, we recently observed that lactoferrin — an
antimicrobial and immunoregulatory protein product
that is released rapidly from preformed granules after
the binding of filoviruses to neutrophils — can act to
enhance the uptake of filoviruses by immature DCs17.
This observation was in direct contrast to a previous
report in which lactoferrin inhibited the transmission
of HIV in vitro18, but illustrates a larger principle: soluble
products that accompany infection and inflammation
in vivo might affect viral cell tropisms in ways that could
be either helpful or damaging to the host.
Other molecules to consider in the engagement of
MARV and EBOV with neutrophils and monocytes are
TLRs, which are often central in the ability of innate
immune cells to detect various pathogenic agents and
to establish adaptive immune responses. So far, our pre-
liminary data indicate that the expression of TLR1, the
signalling of which converges with that of TLR2, was
significantly increased on EBOV- and MARV-activated
neutrophils, whereas the expression of other TLRs was
not modulated17. Further studies are required to estab-
lish the pathogenic importance and the accessibility to
intervention of these many early interactions between
filoviruses and innate immune cells.
Dysregulation of initial innate immune activation
For the initiation of an effective antiviral immune
response, early events are crucial in establishing an
appropriate balance of stimulatory and inhibitory sig-
nals that act in conjunction with antigen recognition.
Early innate immunity may be conceived in two general
forms: first, as a relatively unspecific response involv-
ing pattern-recognition molecules, pro-inflammatory
cytokines and cell-based antiviral functions; and
second, as an orchestrated preparation for an antigen-
specific response involving T cells and antibodies. In
this early balance, filoviruses cause several significant
perturbations (FIGS 1–3).
Recently, filoviruses were found to be capable of disa-
bling at least some of the host IFN pathway: the filoviral
protein VP35 prevents the production of type I IFNs
(that is, IFNα and IFNβ)19,20 and VP24 interferes with
the ability of IFNα, IFNβ and IFNγ to induce an anti-
viral state in cells21 (FIG. 2). Furthermore, because IFNs
are crucial factors that are secreted abundantly by DCs
of the monocytic lineage and plasmacytoid DCs (pDCs),
and which are responsible for the efficient activation of
natural killer (NK) cells, invariant NKT cells (iNKT cells)
and T cells22, the effects of IFN antagonism are impli-
cated not only in the high viral burdens late in the course
of disease, but also in much of the early dysregulation of
innate immunity. Another factor that suggests additional
complexities of IFN gene regulation in response to viral
infection is that the overall IFN response is not ablated
in vivo — elevated levels of IFN are frequently noted in
the blood during acute infection.
For T-cell responses, and therefore also for T-cell-
dependent antibody responses, DCs are pivotal as
antigen-presenting cells (APCs) — in addition to pre-
senting peptide antigen on MHC molecules, DCs can
provide either co-stimulatory or co-inhibitory signals
through various ligands. The first indications of filovirus-
mediated dysfunctions of DCs were typified by a gen-
eralized failure of EBOV- or MARV-infected human
Box 1 | Filovirus-disease basics
The filamentous single-stranded negative-sense RNA viruses belonging the family now
called Filoviridae were first recognized only in 1967 (Marburg virus (MARV)) and 1976
(Zaïre ebolavirus (ZEBOV) and Sudan ebolavirus (SEBOV)). Studies of ZEBOV have
identified bats as potentially being among the natural hosts for ebolaviruses87, and
epidemiological evidence for MARV is also consistent with bats being the wildlife
reservoirs of these viruses. The impact of these viruses on individual bats and bat
populations is relatively unknown, but the consequences of filoviral infections in
humans and non-human primates can be devastating: an acute disease with human
mortality rates as high as 89% for sizeable outbreaks of either ZEBOV88 or MARV89.
The viruses are apparently even more deadly in non-human primates than humans,
with mortality rates approaching 100% in most non-human primate species, including
the ebolavirus species for which human disease is rare (Ivory Coast ebolavirus) or has
not been observed at all (Reston ebolavirus)90.
In humans and monkeys, the hallmark of filoviral disease is unchecked viral growth
that coincides with a relatively wide range of possible disease manifestations including
fever, malaise, diarrhoea and vomiting, severe liver damage and various coagulation
deficits that cause filoviruses to be categorized among the viral haemorrhagic fevers.
The worst of the symptoms, including haemorrhage in a few individuals3,91, seem to
flow from a ‘cytokine storm’, a profuse release of pro-inflammatory cytokines52 (FIG. 1).
In addition to cytokine effects on vascular permeability, causes of excessive bleeding
can include plummets in platelet numbers, severe liver damage and the activation of
tissue factor in monocytes and macrophages92. The time from infection to death is
generally 1–2 weeks, with some variability depending on the virus and host species,
as well as on initial dose93. For survivors, recovery is a lengthy process.
NATURE REVIEWS | IMMUNOLOGY
VOLUME 7 | JULY 2007 | 557
© 2007 Nature Publishing Group
• No IFNα, IFNβ
• No TNF, IL-6
• ↑ Co-inhibition
• ↓ Co-stimulation
Endothelial leakageRash, haemorrhage
• No IFNα, IFNβ
• ↑ TNF, IL-6
• ↑ Tissue factor
• Early (↑ immunity,
↑ signal to adaptive immunity)
• Late (fever, malaise, gastrointestinal
distress, possible septicaemia, LPS?)
B cell (?)
CD8+ T cell
CD4+ T cell
• High viraemia
• Many cells and tissues
infected late in disease
• ↑ Viral burden, direct
• ↓ Immune response
• ↑ Inflammation
DCs to make the transition from the immature to the
mature antigen-presenting DC stage, and a concomit-
ant failure to produce an array of pro-inflammatory
cytokines required for T-cell signalling20,23. This selec-
tive loss of DC capabilities contrasted with earlier
reports showing that monocytes and macrophages, like
DCs, were susceptible to productive virus infection and
were rendered incapable of IFN production, but, unlike
DCs, responded by producing TNF and other pro-
inflammatory cytokines1,24,25. Therefore, a virus-induced
deficit in the co-stimulatory properties of infected DCs
was implied, and was supported by the observation of a
diminished capacity of such DCs to stimulate allogeneic
Interestingly, NK cells, which are an effective early
defence against filoviruses in mice26, might be among
the host cells that are indirectly affected by the overall
diminished synthesis of IFNs. Pre-treatment of animals
with IFNs or IFN inducers before EBOV infection
is considerably more effective in mice than in non-
human primates27; this indicates a decisive difference
in the susceptibilities of different species to the effects
Figure 1 | System overview of filoviral pathogenesis. Initially, productive infection (that is, that which results in more
viral progeny) occurs primarily in dendritic cells (DCs), monocytes and macrophages. All infected cells can be at least
partially impaired in interferon (IFN) production, but some important differences have been described between DCs and
monocytes: monocytes respond with the production of pro-inflammatory cytokines, whereas DCs conspicuously lack such
a response. Neutrophils are not productively infected, nor are lymphocytes, but neutrophils are activated by interaction
with viral particles with resultant degranulation and shedding of triggering receptor expressed on myeloid cells 1
(TREM1). As viral burden increases, lymphocyte apoptosis and a generalized failure of specific immune responsiveness are
observed; we propose these to be rooted in virally induced upregulation of co-inhibitory molecules (such as B7-H1) on
DCs and monocytes, followed by interaction with programmed death 1 (PD1) receptors on T and B cells. Infection spreads
to many cells including liver hepatocytes, and the increasing release of pro-inflammatory cytokines crosses a threshold
from beneficial to potentially harmful inflammation, also degrading vascular epithelium. As repeated cycles of viral
replication overwhelm and outpace a dysregulated adaptive immune response, elements of innate immunity and
inflammation that are potentially helpful early in the response only add to the spiralled dysfunction as they collide with
high viral burdens. Dysfunctions in DCs, monocytes and macrophages are particularly important for their secondary
effects on innate and adaptive immune responses, inflammation and vascular integrity.
558 | JULY 2007 | VOLUME 7
© 2007 Nature Publishing Group
of IFN antagonism that results from filoviral infection.
Moreover, the effects of VP35, VP24 and perhaps other
filoviral proteins might prove even more pleiotropic
— not only IFN-related genes but also multiple cellular
genes were perturbed by filoviral infection of hepato-
cytes, many of these genes were differently dysregulated
depending on the filovirus species28. Further, it was
shown previously that interleukin-8 (IL-8; also known
as CXC-chemokine ligand 8 (CXCL8)), an α-chemokine
produced abundantly during filoviral infection, antago-
nizes the antiviral activity of IFNα by inhibiting the
function of 2′,5′-oligoadenylate synthetase (OAS)
in encephalomyocarditis virus (EMCV) infection29.
Accordingly, it may be worthwhile to explore the role of
IL-8 in filoviral infections.
Further contributing to the initial immune response,
viral replication in monocytes and macrophages contin-
ues unabated, enabled by the filoviral IFN-antagonist
proteins and accompanied by the secretion of other
non-inhibited pro-inflammatory cytokines. Viral par-
ticles, in turn, are able to trigger polymorphonuclear
leukocytes (such as neutrophils) to expel the contents of
their preformed granules and enter a state of activation12.
Such early inflammatory responses are not extraordinary
and might even be helpful in shaping normal immune
responses30, but might also exacerbate an underlying
early immune response dysfunction that originates
principally in DCs.
Also potentially important are the roles of various
cellular proteases (for example, cathepsins, furins and
sheddases) (FIG. 2). By affecting either viral entry31,32 or
exit33,34, these and other proteases could influence viral
cellular tropisms and perhaps interactions between glyco-
protein domains and antibodies (FIGS 2,3). Moreover,
Barrientos and Rollin reported recently that endosomal
proteolytic enzymes, including cathepsins, were released
from EBOV-infected cells, that this release was more
pronounced in cells infected with a more lytic variant
of EBOV and that cytopathicity was diminished by
the cathepsin inhibitor E64 (REF. 35); from this, they
inferred a possible role of proteases in pathogenic
events including vascular leakage.
Related to all these observations is the abundance of
all known cathepsins, especially cathepsin D, cathepsin E
and cathepsin S, in monocytes, macrophages and DCs36,
which are cells that are important owing to their great
susceptibility to filoviral infection and also owing to the
role of their proteases in antigen processing. Interestingly,
a variant of EBOV that is resistant to furin cleavage (the
same variant noted as being more cytopathic in vivo) was
recently reported to be similar to wild-type EBOV in its
virulence to non-human primates37.
Disordering of adaptive immunity
Dysfunctional antigen presentation. Generally, properly
activated DCs are tailored to evoke optimal activation of
T cells; however during filoviral infection, the functions
of DCs might be reversed, as they are first dependant on
sensing pathogens through pattern-recognition receptors,
and second they are dependant on the signals delivered to
them by the viruses. Accordingly, filoviruses have been
shown to silence active co-stimulatory molecules in DCs
(such as CD40, CD86 and IL-12)20,23, which indicates that
infected DCs enter a stage of cellular dysfunction. In a
basic outline, the adaptive immune response can be char-
acterized by the four-part sequence of initial activation,
antigen-specific expansion (proliferation), contraction
(downsizing) and establishment of immune memory.
Filoviruses seem to foster dysregulation of the first three
stages and even the fourth might prove problematic for
filovirus vaccine development.
Here, we consider that the effects of filoviruses on
DCs and monocytes seem to be even more pernicious
than those that would result from just the failure to
deliver regulated positive immune signals, and that
these effects are possibly a result of active co-inhibition
of the immune response. The first suggestions of this
concept came from the observed clinical manifestations
of advanced filoviral diseases in humans38 and mon-
keys39, in which there were pronounced indications of
apoptotic death of lymphocyte populations in peripheral
blood and lymph nodes. Because contraction — the loss
of some 95% of cells that had proliferated in response
to antigenic stimulation — is a normal part of immune
homeostasis40, it was unclear at first the degree to which
these observations were exaggerated in filoviruses
compared with other acute infections. However, the
observation aligns with the emerging understanding of
co-inhibitory immune signalling, and gives mechanistic
support to the hypothesis that filoviruses are among the
viruses that can, at least in some species, upset and dis-
order the natural processes of immune-cell proliferation
Role of co-inhibitory molecules in infection. Ordinarily,
T-cell co-signalling pathways are stimulated shortly
after viral infection — for example, through inter actions
between the well-characterized co-stimulatory molecule
CD28, which is expressed by T cells, and B7-family
ligands CD80 and CD86, which are induced on func-
tioning and mature DCs — and they are responsible for
activating what is at first a relatively rare population of
antigen-specific T cells. Conversely, these molecules can
Box 2 | Therapies for filovirus infection
In simplest terms, therapeutic strategies are intended to slow the viral infection, treat
symptoms and rely on immune clearance. Therefore, the clash between filoviruses
and immune systems is expected to remain important even if effective therapies are
developed. Currently, no effective therapies are available for human use, not even
treatments useful shortly after exposure (and before disease symptoms develop),
nor prophylactic measures. However, prospects for such therapies are encouraging,
based on various preliminary successes in animal models (reviewed in detail
elsewhere17,66,77,93). In brief, these include the possibility of therapy with virus-specific
antibodies; mitigation of virus-induced coagulation deficits using recombinant
nematode anticoagulant protein c2; antisense compounds or small interfering RNAs
to inhibit viral genes; inhibitors of putative specific or nonspecific viral receptors on
susceptible cells; interferons; and the possibility of using specific peptides to mitigate
adverse consequences of interactions between filoviruses and neutrophils. Other
potential vulnerabilities of the viruses, such as the viral fusion domain, cellular
pathways required for viral exit or proteolytic cascades required for entry, are also
open to therapeutic exploitation; even post-exposure vaccination shows promise.
NATURE REVIEWS | IMMUNOLOGY
VOLUME 7 | JULY 2007 | 559
© 2007 Nature Publishing Group
GP on viral surface
• As binding moiety?
• Cell proteins
(GP1 and cleaved
GP from non-edited
GP gene; EBOV only)
Receptors, ligands and co-receptors for viral entry
Stepwise proteolysis of GP
• Involves endosomal
cathepsins, changes in pH
• Partial removal of GP1
• Exposure of the N-terminal
Membrane fusion, release of RNA
Nuclear accumulation of
Inhibition of IFN responsiveness
Inhibition of IFNα
and IFNβ production
Figure 2 | Viral and cellular events in filoviral infection. Central to filoviral disease is the rapidly increasing viral
burden, which can peak at more than 109 plaque-forming units (PFUs) in tissues 7–10 days after infection of non-human
primates. Several possible cellular receptors for viral entry have been described; these studies have suggested that
there is a redundancy in the receptors used for viral entry and that there are partial biases towards the infection of
certain cells. Viral entry involves the stepwise proteolysis of viral glycoprotein followed by membrane fusion and
disassembly of the elongated matrix–nucleoprotein complex, which contains the other six proteins encoded by the
viral genome. In the transcription of mRNAs from the negative-strand genome template, the glycoprotein gene of
Ebola virus (EBOV; but not Marburg virus (MARV)) undergoes editing to produce soluble variants of glycoprotein as
well as the membrane-bound form, which trimerizes to form the viral envelope spikes. Additional forms of soluble
glycoprotein arise post-translationally from proteolysis and/or failure to establish disulphide bonds between naturally
occurring glycoprotein fragments GP1 and GP2; the role of soluble forms of glycoprotein in confounding the immune
response remains hypothetical. The unchecked replication in primate cells can be explained in large part by the recently
described capacity of VP35 to inhibit interferon (IFN)-regulatory factor 3 (IRF3) and thereby prevent infected cells from
synthesizing type I IFNs19, and the ability of VP24 to interrupt the nuclear accumulation of tyrosine-phosphorylated
STAT1 (signal transducer and activator of transcription 1), rendering cells insensitive to the antiviral effects of
interferons21. EBOV and MARV are cytopathic in most cell types, but direct viral damage to cells is slow enough that
host-cell pathways, including protein synthesis, continue to function for many hours, albeit abnormally. After completing
the cycle and initiating another, filamentous virions assemble at areas at the plasma membrane called lipid rafts,
which are rich in a subset of host-cell proteins. APC, antigen-presenting cell; DC-SIGN, dendritic-cell-specific
intercellular adhesion molecule 3 (ICAM3)-grabbing non-integrin; GP, glycoprotein; TLR, Toll-like receptor.
560 | JULY 2007 | VOLUME 7
© 2007 Nature Publishing Group
B-cell production of
and inhibition of virion
attachments by GP-specific
• Ineffective with MARV
• Depends on target cells?
Receptors, ligands and
co-receptors for viral entry
I or II
Disruption of viral egress
processing (any or
all viral proteins)
The condition of functionally
T cells, typified by elevated
surface expression of PD1.
interact with cytotoxic T-lymphocyte antigen 4 (CTLA4)
and thereby induce T-cell silencing. Such co-stimulatory
and co-inhibitory signals have also been ascribed to
other newly described B7-family molecules that are pos-
tulated to be crucial in regulating the dialogue between
APCs and T cells. This newly described class of the B7
family (including B7-DC (also known as PD-L2 and
PDCD1LG2), B7-H1 (also known as PD-L1 and CD274),
B7-H2 (also known as ICOSLG), B7-H3 (also known as
CD276) and B7-H4 (also known as VTCN1)) and their
receptors (such as programmed cell death 1 (PD1; also
known as PDCD1) and inducible T-cell co-stimulator
(ICOS)) control T-cell responses in both a positive and
a negative manner in cancer and infectious diseases41,42.
In general, co-inhibitory pathways are considered to
be among the host mechanisms required to institute or
restore homeostasis, minimizing harmful self recogni-
tion or re-balancing the system after a robust response
to foreign antigen. However, cancer, infectious diseases
or autoimmune diseases such as rheumatoid arthritis
exploit these processes, and this results in immune eva-
sion, autoreactivity or dysfunctions of immunity, such as
anergy and exhaustion41.
How might viral-mediated dysfunction of DCs and
monocytes be manifested not only as poor positive signal-
ling but as active inhibition of immune responsiveness?
An interaction that typifies such inhibition is one
that occurs between B7-H1, a co-inhibitory molecule
expressed on DCs and monocytes (and many other
cells), and its ligand PD1, expressed on activated T and
Figure 3 | Effects of filoviral infection on immunological events. Both antibody and T-cell responses (as well as innate
effectors such as natural killer (NK) cells) are relevant to immunity and act together to promote viral clearance. Whereas
high total antibody levels against glycoprotein are predictive of immunity, serum antibodies of the kind that prevent viral
entry (that is, the most familiar type of neutralizing antibody) have proved difficult to measure, especially against Marburg
virus (MARV), have been poor correlates of immune protection in vaccination and passive transfer studies. Antibodies
might also suppress viral burden by other means, for example, by directing lysis of glycoprotein-expressing cells or
disrupting virion formation and release at the cell surface. Hypothetically, soluble forms of glycoprotein are detrimental,
having the capacity to sequester antibodies that would otherwise have antiviral effects. T-cell responses are impaired in
infected humans and monkeys. Possibly explaining some of the immune deficit, infected human and monkey dendritic
cells (DCs) are not only disabled in their interferon (IFN) pathways but are impaired in their capacities to differentiate,
express co-stimulatory molecules and produce a normal array of cytokines. Simultaneously, infected DCs and monocytes
may express more of the co-inhibitory molecule B7-H1, prompting an increase in its apoptosis-linked receptor
programmed cell death 1 (PD1) on T cells. In the resulting model, the adaptive immune response to filoviruses in
unvaccinated animals is baffled not only by IFN antagonism in DCs and monocytes but by the viral dysregulation of pro-
inflammatory cytokines, co-stimulatory and co-inhibitory molecules. APC, antigen-presenting cell; CD40L, CD40 ligand;
CTLA4, cytotoxic T-lymphocyte antigen 4; GP, glycoprotein; IL-12, interleukin-12, NP, nucleoprotein; TCR, T-cell receptor.
NATURE REVIEWS | IMMUNOLOGY
VOLUME 7 | JULY 2007 | 561
© 2007 Nature Publishing Group
The impaired or absent ability
of an immune cell to respond
to specific antigens.
B cells. Overexpression of B7-H1 typically precedes
the upregulation of PD1 on activated T cells. PD1 then
transmits elevated negative signals into T cells, which, in
collaboration with unregulated inflammatory cytokines
(such as TNF and IFNγ), may effectively establish
exhaustion, anergy, and subsequent suppression and
apoptosis of these cells40,41,43. Indeed, we have previously
shown that B7-H1 not only controls CD8+ T-cell homeo-
stasis, but also contributes selectively to the deletion of
intra hepatic CD8+ T cells44. Iwai et al. also showed cor-
relatively that PD1-deficient mice were more resistant to
acute hepatic damage owing to adenovirus infection45.
Moreover, Barber et al. recently showed that although
most lymphocytic choriomeningitis virus (LCMV)-
specific CD8+ T cells in persistently infected mice
express activation molecules (such as CD69 and CD44),
these cells are functionally exhausted and therefore
do not exert their antiviral effector function46. Barber
et al. further showed that PD1 is expressed on impaired
LCMV-specific CD8+ T cells and that blockage of the
PD1 interaction with B7-H1 during chronic LCMV
infection reanimates these cells and induces clear-
ance of LCMV46. These trends have been confirmed in
human studies showing that elevated levels of B7-H1
(REF. 47) and PD1 (REFS 48,49) are associated with T-cell
exhaustion and depletion in HIV, and T-cell func-
tion can be restored in vitro by using B7-H1-specific
antibody to antagonize the co-inhibitory interaction
between B7-H1 and PD1. The commonality between
these and other earlier observations of the importance
of co-inhibition mediated by B7-H1–PD1 interactions
is that they involve chronic conditions.
Returning to the particular case of filoviruses and
the acute diseases that they cause, recent investigations
in individuals and non-human primates infected with
filoviruses have demonstrated the gradual disappear-
ance and deletion of CD8+ T cells and NK cells38,50. That
little or no immune response to the virus is observed in
humans and non-human primates before they succumb
to disease4,6,51 is consistent with the incapacity of infected
DCs to regulate their CD40 and CD86 molecules, and
the coincident failure of DCs to secrete IL-12 and other
cytokines20,23. The naturally increased expression of B7-H1
in liver macrophages and parenchymal cells — thought
to explain, at least in part, the poor immune responses to
a wide range of antigens typically observed in the liver44
— is particularly intriguing in view of the hepatotropic
manifestation of filoviral disease (FIG. 1, BOX 1). For now,
because immune dysfunction that is directly attributable
to the rapid triggering of co-inhibitory pathways is only
beginning to be implicated in acute viral infections 42,
the importance of this pathway in filovirus pathogenesis
Paradoxically, the role of IFNγ, which is secreted
copiously in the plasma of infected individuals and
non-human primates late in filovirus infection8,52,
might be more detrimental than helpful, as any regu-
latory effects of IFNγ on CD8+ T cells must be proxi-
mal to initial activation; otherwise, this cytokine might
induce pro-apoptotic effects that result in cell-population
contraction and T-cell apoptosis53. Although CD8+
T cells have mechanisms to evade this cytokine by
downregulating the IFNγ receptor subunit 2 to com-
plete their cellular expansion54, the rapid filoviral
invasion might induce other unknown signalling
events that might close the window of opportunity to
promote functional CD8+ T cells. Moreover, what are
the roles of other regulatory and inflammatory mol-
ecules (such as IL-10, transforming growth factor-β1
(TGFβ1) and IL-17), which are abundantly secreted
during the induction phase of filoviral infection?
Answers will come as future studies shed light on
additional aspects of filoviral infection and disease.
Other immune-component cells. In addition to
NK cells, filoviruses might also trigger CD1d-restricted
iNKT cells55,56, and even IFN-producing killer DCs
(IKDCs), a newly described DC subset in which the
cells have characteristics of both DCs and NK cells57.
The resulting signalling cascade would involve massive
detrimental inflammation, dysfunction of DCs, reten-
tion of DCs in an immature stage and abolition of the
co-stimulatory function of DCs; therefore, NK cells
or iNKT cells would not be able to exercise effective
responses to invading filoviruses. Worse, on receiving
co-inhibitory signals, these cells might contribute to
the production of copious amounts of IFNγ, which can
induce more B7-H1 (REF. 58) and thus facilitate further
cellular anergy and subsequent NK-cell deletion38,50.
Conversely, as filovirally infected DCs retain an imma-
ture phenotype, these cells could be eliminated by the
NK-cell-dependent NKp30 trigger receptor59. Further
fundamental questions remain to be addressed to
understand what mediates the impaired cytotoxicity of
NK cells, iNKT cells or even IKDCs, and to discern the
factors that are induced by dysfunctional DCs and the
strong inflammatory microenvironment that occurs
early during the infection, wherein these cells reside.
Collectively, therefore, much remains to be clarified
with respect to the relative importance of new or dys-
regulated pathways in the innate and adaptive immune
responses to filoviruses. However, recent studies point
towards what might be reversible dysfunctions and
therefore to new therapeutic rationales to restore balance
and favour an animal’s survival.
Inferences from vaccines and therapies
Filovirus antigens. Filoviruses belong to the order of
Mononegavirales, which includes respiratory syncytial,
measles, mumps, parainfluenza and rabies viruses.
As antigens, filoviruses have a uniquely filamentous
structure, but at a glance otherwise seem to be unex-
ceptional among similar viral genera. That is, from a
single 19-kilobase strand of negative-sense RNA, the
viral replicase complex makes mRNA to encode rela-
tively few (seven or more) proteins (FIG. 2). Of these, only
one (the glycoprotein) is known with certainty to appear
on virion surfaces (and also on the surface of infected
cells), where it can serve as a target for antibodies, some
of which might mediate protection. In theory, any of
these viral proteins could be recognized by T cells;
indeed, in the case of a mouse-adapted variant of
562 | JULY 2007 | VOLUME 7
© 2007 Nature Publishing Group
Zaïre ebolavirus (ZEBOV), six of the seven existing
proteins (the replicase protein L was not examined)
were observed to evoke protective responses in at least
one strain of mouse, with demonstrable T-cell responses
associated with survival, and, in some cases, effective on
adoptive transfer60,61. However, in guinea pigs challenged
with MARV, only viral glycoprotein, nucleoprotein and
VP35 afforded some measure of protection when used
as vaccines in a replicon-based vector system4. In guinea
pigs vaccinated and then challenged with ZEBOV, viral
glycoprotein was a superior protective antigen, with
nucleoprotein providing no62 or equivocal63 protec-
tion. In the case of non-human primates that had been
vaccinated and then challenged with either MARV or
ZEBOV, viral glycoprotein has proved to be necessary
and sufficient as a vaccine component. Nucleoprotein
alone provided incomplete protection in monkeys
infected with MARV, whereas viral glycoprotein by
itself (or admixed with nucleoprotein) prevented illness
and death in six out of six infected animals4. Similarly,
the first successes in protecting non-human primates
against ZEBOV were with a vaccine containing glyco-
protein; subsequent studies showed glycoprotein by
itself to be sufficient64,65, and there are no published data
to indicate that there are any other proteins capable of
protecting non-human primates against ZEBOV in the
absence of glycoprotein.
It is frequently observed that filoviral vaccines and
therapies that succeed in rodents are far less successful in
non-human primates4,8,66–68. Therefore, the collective and
empirical experience with vaccines tested for the capa-
city to protect against either MARV or ZEBOV in the
most susceptible and perhaps most relevant (to humans)
host — non-human primates — has revolved entirely
around vaccines that contain glycoprotein (TABLE 1).
Whether filovirus vaccines for non-human primates can
be improved by removing some regions of glycoprotein
remains to be shown.
Humoral immunity against filovirus infection.
Mechanistically, most of the available evidence indicates
that both antibody and T-cell responses of appropriate
kind and magnitude are required for robust protection
against filoviruses, especially in non-human primates.
Support for the involvement of antibodies takes several
forms. Foremost, passive immunotherapy with conva-
lescent sera or monoclonal antibodies has been shown
to mitigate or prevent MARV and ZEBOV disease in
Table 1 | Marburg- and Ebola-virus vaccines containing glycoprotein
Early vaccine efforts and recent proofs of concept; inadequate efficacy‡
in non-human primates
Only as proofs of concept with natural or passaged viruses; high risk
could theoretically be mitigated by reverse-genetics approach
Safety§; potency¶; observed disease
Live vaccine; balance of safety and
potency incomplete attenuation or
Live vaccine balance of safety and
potency; vector immunity#
Potency, adjuvant requirement; altered
Vector immunity; safety at doses high
enough to achieve potency
Proof of concept, deprioritized along with other live pox-vectored
Incomplete efficacy in guinea pigs, no reported efficacy in non-human
Excellent efficacy in rodents, first demonstration of efficacy against
MARV in non-human primates, minimum protective dose about 108 IU
in non-human primates
Adequate in rodents; incomplete non-human-primate efficacy with
MARV and none reported with EBOV; touted for immunological
Excellent efficacy if doses 1010 IU or higher. First demonstration of non-
human-primate efficacy with EBOV, including one-dose protection
Good rodent efficacy; safety and possible potency advantage
compared with killed virus particles
Excellent rodent and non-human-primate efficacy with both MARV and
EBOV; single-shot vaccine, rapid immunity; no overt illness from live
vaccine itself; in recombinant vaccine, filovirus glycoprotein replaces
Good efficacy against Ebola virus in guinea pigs and non-human
primates; contains both parainfluenza virus and EBOV glycoproteins
*Vaccines are listed in approximate chronological order of their first public descriptions of significant efficacy; the first four genetic vaccines (vaccinia through
DNA) became known roughly simultaneously in 1996. ‡Here, vaccine efficacy for filoviruses is operationally defined by prevention of disease in a susceptible animal
inoculated with an amount of virus (usually 102 to 103 plaque-forming units ≈ LD50) associated with ordinary exposure. §Safety concerns might arise in manufacture
itself (as with killed viruses), but more often refer to anticipated difficulties in achieving manufacture suitable for regulated human trials, and then an expectation
of adequate safety in large trials and diverse populations. ¶Potency is interpreted as the capacity to elicit a protective immune response as defined by a quantitative
assay that correlates with protection. #For live and replication-defective recombinant vaccines, vector immunity refers to pre-existing immunity to the vaccine
vector, which can arise naturally or by successive vaccination with the same vector, and acts to diminish the effective dose and therefore potency of vaccine
delivered. **Environmental release is an additional concern for new replicating vaccines, requiring assessment of possible consequences of vaccine transmission to
humans or other animals, including arthropods. EBOV, Ebola virus; IU, infectious units for replication-defective vectors that infect cells without further spread;
MARV, Marburg virus; VEE, Venezuelan equine encephalitis virus; VSV, vesicular stomatitis virus.
Vector immunity, safety at doses high
enough to achieve potency
Potency; adjuvant requirement
Live vaccine; balance of safety and
potency; environmental release**
Live vaccine; balance of safety and
potency; environmental release
NATURE REVIEWS | IMMUNOLOGY
VOLUME 7 | JULY 2007 | 563
© 2007 Nature Publishing Group
guinea pigs and mice69–72, delay viraemia and death in
non-human primates27 and possibly improve clinical
outcomes in humans73,74. Moreover, B-cell-deficient mice
were impaired in their ability to eliminate ZEBOV75.
Further, the empirical observation of glycoprotein as
the most effective protective antigen in non-human
primates argues indirectly that humoral immunity is
relevant, and protection has been associated with total
On the other hand, antibody alone has not yet been
shown to be sufficient to protect non-human primates
from lethal infection with more than a few infectious
viruses27,76, and the human clinical data were equivocal
because treatments were performed in uncontrolled
emergency situations. It has been confounding to some
researchers that neutralization of filoviruses by antibod-
ies — as measured by a reduction in viral infectivity
after admixture with antibodies — has proved to be a
completely unreliable in vitro predictor of antibody or
vaccine effectiveness4,64,70,76. Others researchers point to a
rich history of antibodies shown to protect in ways other
than what is popularly understood as neutralization77.
Some of the mechanisms by which such non-neutralizing
antibodies might control viral burden are shown in FIG. 3
and are briefly reviewed elsewhere77. It remains unclear
whether monoclonal antibodies can be found to act by
themselves therapeutically in non-human primates,
whether other strategies will prove more effective (BOX 2),
or whether combination therapies, perhaps including
antibodies, will be required.
Cellular immunity to filovirus infection. The role of T cells
in successful vaccination against filoviruses is similarly
persuasive, yet insufficient to attribute this protection
wholly to cellular immunity. T-cell involvement has been
documented most clearly in the mouse model of ZEBOV,
in which cytotoxic T cells specific for nucleoprotein were
protective on adoptive transfer61, CD8+ T-cell-deficient
mice were more susceptible to lethal challenge75 and at
least five cytoplasmic ZEBOV proteins (presumably
inaccessible to antibodies) were independently capable of
eliciting both T cells and protective immunity60. Internal
nucleoprotein and VP35 were capable of inducing signifi-
cant protection in MARV-infected guinea pigs4, and the
nucleoprotein-specific protection in non-human primates
was probably due to T cells4. In an effort to discern how
mice resist ZEBOV, a case was made that not only CD8+
T cells but also CD4+ T cells are crucial in viral clear-
ance75, a finding that would be among those expected if
glycoprotein-specific antibodies were important. Reliable
quantification of T-cell responses in a way that correlates
with protection has not been achieved with filoviruses in
guinea pigs or non-human primates, but when serious
efforts to measure T-cell responses have been undertaken,
it has been routinely observed that filovirus-antigen-
specific T cells can be found in animals that are shown
subsequently to be immune to lethal infection78.
A collective analysis of the vaccine studies in TABLE 1
indicates that the magnitude of the total glycoprotein-
specific antibody response is associated with protection,
a measure which presumably correlates not only with
types of antibody that are protective but also with T cells
for which suitable assays remain elusive. Put most sim-
ply, higher doses of vaccine seem to evoke greater total
immune responses, along with a protection that is more
rapidly acquired and more complete than that obtained
with replication-defective vaccines given at much lower
doses. A recent report with a recombinant adenovirus
that expresses the ZEBOV glycoprotein, in which the
minimal protective dose of 1010 particles is described
as a ‘low dose’, affirms that protection is dependent on
the vaccine dose65, and might explain why replicon-
based vaccines given at much lower doses have not been
uniformly successful67 and DNA vaccines have been
only partially effective79. Live vesicular stomatitis virus
(VSV)64 or parainfluenza virus80,81 recombinants, which
show considerable promise in protecting non-human pri-
mates from filoviral infection, presumably generate high
amounts of viral glycoprotein antigen in vivo. Herein
resides a most familiar challenge of vaccine licensure:
the achievement of a suitable balance between vaccine
potency and vaccine safety.
Mechanistically, additional immunological and
virological puzzles have been exposed by recent dem-
onstrations of protective effects of vaccinating rodents
or primates with replication-competent vaccines shortly
before or shortly after infection with MARV82 or EBOV83.
Surmising that protective mechanisms in this exceptional
circumstance could be ‘multifactorial’, Feldmann et al.
proposed as possible explanations not only antibod-
ies and T cells, but also NK cells, innate immune
responses and viral interference (attenuated-vaccine
virus out-competing virulent filoviruses at the cellular
level)83. Hypothetically, well-balanced innate and adap-
tive immune responses against the replicating vaccine
(here in a VSV vector) occur in parallel with otherwise
dysfunctional responses to filoviruses, suppressing and
ultimately eliminating both viruses. Whether replication-
defective filovirus vaccines may have similar protective
effects if given shortly before or after filoviral infection
is not yet clear.
Difficulties presented to the immune system by filoviral
glycoprotein antigens. If it were not difficult enough
for the immune system to repel filoviral attack, viral
glycoprotein presents an array of deceptions and chal-
lenges. Summarized in TABLE 2, the confounding traits
of viral glycoprotein influence the rational design and
improvement of vaccines. Moreover, although several
experimental vaccine approaches have been shown to
be capable of eliciting the robust immune responses
necessary to protect non-human primates against either
EBOV or MARV disease, it remains to be determined
whether appropriate immunological memory (as
defined by long-term protection) will be established by
the same vaccines. As for the immune systems of the
unvaccinated primates exposed to glycoprotein for the
first time, and in the context of all the aforementioned
advantages held by the infectious virus, it is perhaps
most impressive that the immune response occasionally
prevails, and many individuals and a few non-human
primates do survive.
564 | JULY 2007 | VOLUME 7
© 2007 Nature Publishing Group
Concluding hypotheses and prospects
It is becoming progressively clearer how EBOV and
MARV overwhelm host defences and cause disease by
dysregulating and defeating first the innate and then
the adaptive immune systems of primates and humans
(FIGS 1–3). Each of several properties of the viruses are
probably required to account for their profound viru-
lence, and it has already been observed that changes in
the IFN antagonists VP35 and VP24, the glycoprotein,
the viral replicase or the nucleoprotein can diminish
filovirus virulence in the mouse model84. It remains to be
explained why the profound virulence, characteristic of
all members of the diverse family Filoviridae, is appar-
ently limited to primates85, which are presumably acci-
dental hosts for the viruses. Nonetheless, our increased
understanding of the events involved in filoviral infection
points towards new possibilities in shifting the advantage
to the immune system. In general, the many therapeutic
approaches designed to slow viral replication can allow
the immune system to gain additional time to mount an
effective defence, but such intervention might be required
early, even before symptoms develop.
Table 2 | Evasion and deception strategies available to filovirus glycoprotein
Variability between isolates and species is
concentrated in the central ‘mucin-like’ portion of GP,
which, after cleavage to GP1–GP2 becomes the distal
aspect of the GP spike
Concentrated in mucin-like domain; variations in
glycosylation, some due to cell type
Negligible cross-reactivities of antibodies and no
cross-protection between viral species, limited cross-
reactivities of antibodies among disparate isolates of
Epitope masking; phenotypic variation; viral tropism
for cells having lectin-like receptors (such as DCs and
Soluble antigens compete for antibodies otherwise
effective against viruses or infected cells
Abundant N- and
GP shedding Failure of disulfide-bond formation results in GP1
shedding; cell proteases cause an alternative form of
shedding of GP1–GP2 ectodomain
Observed in EBOV, not MARV; genome encodes
mRNA for a truncated, frame-shifted version of GP
Key functional domains are proximal to membrane,
exposed only transiently upon entry
GP gene editing
Similar to GP shedding, but with added twists of
truncation, frame-shift and different glycosylation of
Antibodies underrepresented and possibly ineffective
against important functional regions
Structural masking of
GP apparently binds multiple cell receptors with
varying efficiencies; filamentous viral structure might
favour effectiveness of low-affinity interactions
GP on viral particles is sufficient to trigger
intracellular cascades, granule release, cytokines
Might be less of a problem in vivo than with in vitro
Tenuous observation, with importance still neither
confirmed nor refuted
EBOV, Ebola virus; GP, glycoprotein; MARV, Marburg virus; sGP, soluble GP.
Problematic for antibodies to block initial interactions
between viruses and a variety of cells and receptors
Triggering of cytokine
Toxicity of GP for cells
Disorienting milieu for initiation of balanced immune
Could affect performance or manufacture of genetic
Remains a possible explanation for some
dysregulations now attributed to whole GP
Other interventions, intended to manage disease
symptoms, might also provide the immune system
with additional time. One possibility that has emerged
is an intentional intervention in the early inflamma-
tion and co-inhibitory dialogue between DCs and
T cells and even other immune-cell types, for exam-
ple, with specific tolls (such as B7-H1-specific anti-
body) to prevent exaggerated regulatory signals from
inducing T-cell exhaustion; this hypothetical means
to control viral clearance has precedence in cancer,
HIV and, more recently, in a mouse model of chronic
lymphocytic choriomeningitis virus infection41,43,46,86.
Improvements in filoviral vaccine development strat-
egies might follow from the ongoing identification
and exploitation of glycoprotein components that
evoke protective immunity. On the whole, the efforts
to provide medical countermeasures for these deadly
but relatively rare viruses seem poised to tell us a great
deal not only about these particular viruses, but more
generally about the immune responses to acute viral
infections, and how to shift the battle in favour of
immunity and survival.
Feldmann, H. et al. Filovirus-induced endothelial
leakage triggered by infected monocytes/
macrophages. J. Virol. 70, 2208–2214 (1996).
Wahl-Jensen, V. et al. Role of Ebola virus secreted
glycoproteins and virus-like particles in activation of
human macrophages. J. Virol. 79, 2413–2419 (2005).
Slenczka, W. G. The Marburg virus outbreak of
1967 and subsequent episodes. Curr. Top. Microbiol.
Immunol. 235, 49–75 (1999).
Hevey, M., Negley, D., Pushko, P., Smith, J. &
Schmaljohn, A. Marburg virus vaccines based
upon alphavirus replicons protect guinea pigs
and nonhuman primates. Virology 251, 28–37
This reference provides a seminal
demonstration of glycoprotein as a
necessary and sufficient protective antigen,
and of a vaccine capable of providing robust
immunity against filovirus disease in non-human
Ryabchikova, E. I., Kolesnikova, L. V. & Luchko, S. V.
An analysis of features of pathogenesis in two
animal models of Ebola virus infection. J. Infect. Dis.
179 (Suppl. 1), 199–202 (1999).
Ignatyev, G. M. Immune response to filovirus
infections. Curr. Top. Microbiol. Immunol.
235, 205–217 (1999).
NATURE REVIEWS | IMMUNOLOGY
VOLUME 7 | JULY 2007 | 565
© 2007 Nature Publishing Group
Geisbert, T. W. et al. Pathogenesis of Ebola
hemorrhagic fever in cynomolgus macaques: evidence
that dendritic cells are early and sustained targets of
infection. Am. J. Pathol. 163, 2347–2370 (2003).
This paper reports distinctive in vivo studies in
which early viral events in non-human primates
were tracked in serial sampling experiments.
Feldmann, H., Jones, S., Klenk, H. D. & Schnittler, H. J.
Ebola virus: from discovery to vaccine. Nature Rev.
Immunol. 3, 677–685 (2003).
Simmons, G. et al. DC-SIGN and DC-SIGNR bind Ebola
glycoproteins and enhance infection of macrophages
and endothelial cells. Virology 305, 115–123 (2003).
10. Takada, A. et al. Human macrophage C-type lectin
specific for galactose and N-acetylgalactosamine
promotes filovirus entry. J. Virol. 78, 2943–2947
11. Becker, S., Spiess, M. & Klenk, H. D. The
asialoglycoprotein receptor is a potential liver-
specific receptor for Marburg virus. J. Gen. Virol.
76, 393–399 (1995).
12. Mohamadzadeh, M. et al. Activation of triggering
receptor expressed on myeloid cells-1 on human
neutrophils by Marburg and Ebola viruses. J. Virol.
80, 7235–7244 (2006).
13. Bobardt, M. D. et al. Syndecan captures, protects, and
transmits HIV to T lymphocytes. Immunity 18, 27–39
14. Chan, S. Y. et al. Folate receptor-α is a cofactor
for cellular entry by Marburg and Ebola viruses.
Cell 106, 117–126 (2001).
15. Feldmann, H., Nichol, S. T., Klenk, H. D., Peters, C. J.
& Sanchez, A. Characterization of filoviruses based on
differences in structure and antigenicity of the virion
glycoprotein. Virology 199, 469–473 (1994).
16. Bavari, S. et al. Lipid raft microdomains: a gateway for
compartmentalized trafficking of Ebola and Marburg
viruses. J. Exp. Med. 195, 593–602 (2002).
17. Mohamadzadeh, M., Chen, L., Olinger, G. G.,
Pratt, W. D. & Schmaljohn, A. Filoviruses and the
balance of innate, adaptive, and inflammatory
responses. Viral Immunol. 19, 602–612 (2006).
18. Groot, F. et al. Lactoferrin prevents dendritic
cell-mediated human immunodeficiency virus type 1
transmission by blocking the DC-SIGN–gp120
interaction. J. Virol. 79, 3009–3015 (2005).
19. Cardenas, W. B. et al. Ebola virus VP35 protein binds
double-stranded RNA and inhibits α/β interferon
production induced by RIG-I signaling. J. Virol.
80, 5168–5178 (2006).
Along with reference 21, this paper provides
recent mechanistic data and an overview on IFN
antagonists encoded by filoviruses.
20. Bosio, C. M. et al. Ebola and Marburg viruses
replicate in monocyte-derived dendritic cells
without inducing the production of cytokines
and full maturation. J. Infect. Dis. 188, 1630–1638
21. Reid, S. P. et al. Ebola virus VP24 binds karyopherin
α1 and blocks STAT1 nuclear accumulation. J. Virol.
80, 5156–5167 (2006).
22. Marrack, P., Kappler, J. & Mitchell, T. Type I
interferons keep activated T cells alive. J. Exp. Med.
189, 521–530 (1999).
23. Mahanty, S. et al. Cutting edge: impairment of
dendritic cells and adaptive immunity by Ebola and
Lassa viruses. J. Immunol. 170, 2797–2801 (2003).
24. Gupta, M., Mahanty, S., Ahmed, R. & Rollin, P. E.
Monocyte-derived human macrophages and
peripheral blood mononuclear cells infected with
Ebola virus secrete MIP-1α and TNF-α and inhibit
poly-IC-induced IFN-α in vitro. Virology 284, 20–25
25. Ströher, U. et al. Infection and activation of
monocytes by Marburg and Ebola viruses. J. Virol.
75, 11025–11033 (2001).
26. Warfield, K. L. et al. Role of natural killer cells in
innate protection against lethal Ebola virus infection.
J. Exp. Med. 200, 169–179 (2004).
27. Jahrling, P. B. et al. Evaluation of immune globulin
and recombinant interferon-α2b for treatment of
experimental Ebola virus infections. J. Infect. Dis.
179 (Suppl 1), 224–234 (1999).
28. Kash, J. C. et al. Global suppression of the host
antiviral response by Ebola- and Marburgviruses:
increased antagonism of the type I interferon
response is associated with enhanced virulence.
J. Virol. 80, 3009–3020 (2006).
29. Khabar, K. S. et al. The α chemokine, interleukin 8,
inhibits the antiviral action of interferon α.
J. Exp. Med. 186, 1077–1085 (1997).
30. Nathan, C. Neutrophils and immunity: challenges
and opportunities. Nature Rev. Immunol. 6, 173–182
This is a timely and highly readable review of the
sometimes neglected importance of neutrophils
in innate and adaptive immunity.
31. Chandran, K., Sullivan, N. J., Felbor, U., Whelan, S. P.
& Cunningham, J. M. Endosomal proteolysis of the
Ebola virus glycoprotein is necessary for infection.
Science 308, 1643–1645 (2005).
32. Schornberg, K. et al. Role of endosomal cathepsins
in entry mediated by the Ebola virus glycoprotein.
J. Virol. 80, 4174–4178 (2006).
33. Dolnik, O. et al. Ectodomain shedding of the
glycoprotein GP of Ebola virus. EMBO J.
23, 2175–2184 (2004).
34. Sanchez, A. et al. Biochemical analysis of the
secreted and virion glycoproteins of Ebola virus.
J. Virol. 72, 6442–6447 (1998).
35. Barrientos, L. G. & Rollin, P. E. Release of cellular
proteases into the acidic extracellular milieu
exacerbates Ebola virus-induced cell damage.
Virology 358, 1–9 (2007).
36. Mohamadzadeh, M., Mohamadzadeh, H.,
Brammer, M., Sestak, K. & Luftig, R. B. Identification
of proteases employed by dendritic cells in the
processing of protein purified derivative (PPD).
J. Immune Based Ther. Vaccines 2, 8 (2004).
37. Neumann, G. et al. Proteolytic processing of the
Ebola virus glycoprotein is not critical for Ebola
virus replication in nonhuman primates. J. Virol.
81, 2995–2998 (2007).
38. Baize, S. et al. Defective humoral responses and
extensive intravascular apoptosis are associated
with fatal outcome in Ebola virus-infected patients.
Nature Med. 5, 423–426 (1999).
39. Geisbert, T. W. et al. Apoptosis induced in vitro and
in vivo during infection by Ebola and Marburg viruses.
Lab. Invest. 80, 171–186 (2000).
40. Haring, J. S., Badovinac, V. P. & Harty, J. T. Inflaming
the CD8+ T cell response. Immunity 25, 19–29
41. Chen, L. Co-inhibitory molecules of the B7-CD28
family in the control of T-cell immunity. Nature Rev.
Immunol. 4, 336–347 (2004).
This review describes co-signalling molecules,
a class of cell-surface glycoproteins that direct,
modulate and fine-tune the TCR, and summarizes
data indicating that T-cell functions can be
promoted or suppressed on the basis of the
functional outcome of co-stimulatory or
co-inhibitory signals. The author emphasizes the
appropriate time and location of these molecules
directing either positive or negative signals to
control priming, growth, differentiation and
functional maturation of a T-cell immune response.
42. Sharpe, A. H., Wherry, E. J., Ahmed, R. &
Freeman, G. J. The function of programmed cell
death 1 and its ligands in regulating autoimmunity
and infection. Nature Immunol. 8, 239–245
This authoritative recent review highlights the
crucial functions of PD1 and its ligands (PDL1 and
PDL2) in regulating antimicrobial and self-reactive
T-cell immune responses, and discusses potential
therapeutic strategies in manipulating the
immunological outcomes of such interactions.
43. Carreno, B. M., Carter, L. L. & Collins, M. Therapeutic
opportunities in the B7/CD28 family of ligands and
receptors. Curr. Opin. Pharmacol. 5, 424–430
44. Dong, H. et al. B7-H1 determines accumulation
and deletion of intrahepatic CD8+ T lymphocytes.
Immunity 20, 327–336 (2004).
45. Iwai, Y., Terawaki, S., Ikegawa, M., Okazaki, T. &
Honjo, T. PD-1 inhibits antiviral immunity at the
effector phase in the liver. J. Exp. Med. 198, 39–50
46. Barber, D. L. et al. Restoring function in exhausted
CD8 T cells during chronic viral infection. Nature
439, 682–687 (2006).
47. Trabattoni, D. et al. B7-H1 is up-regulated in HIV
infection and is a novel surrogate marker of disease
progression. Blood 101, 2514–2520 (2003).
48. Trautmann, L. et al. Upregulation of PD-1 expression
on HIV-specific CD8+ T cells leads to reversible
immune dysfunction. Nature Med. 12, 1198–1202
49. Day, C. L. et al. PD-1 expression on HIV-specific T cells
is associated with T-cell exhaustion and disease
progression. Nature 443, 350–354 (2006).
50. Reed, D. S., Hensley, L. E., Geisbert, J. B., Jahrling, P. B.
& Geisbert, T. W. Depletion of peripheral blood
T lymphocytes and NK cells during the course of
Ebola hemorrhagic fever in cynomolgus macaques.
Viral Immunol. 17, 390–400 (2004).
51. Leroy, E. M. et al. Human asymptomatic Ebola
infection and strong inflammatory response.
Lancet 355, 2210–2215 (2000).
52. Baize, S. et al. Inflammatory responses in Ebola virus-
infected patients. Clin. Exp. Immunol. 128, 163–168
53. Badovinac, V. P. & Harty, J. T. Programming,
demarcating, and manipulating CD8+ T-cell memory.
Immunol. Rev. 211, 67–80 (2006).
54. Haring, J. S., Badovinac, V. P., Olson, M. R., Varga, S. M.
& Harty, J. T. In vivo generation of pathogen-specific
Th1 cells in the absence of the IFN-γ receptor.
J. Immunol. 175, 3117–3122 (2005).
55. Kronenberg, M. Toward an understanding of NKT cell
biology: progress and paradoxes. Annu. Rev. Immunol.
23, 877–900 (2005).
56. Taniguchi, M., Seino, K. & Nakayama, T. The NKT cell
system: bridging innate and acquired immunity.
Nature Immunol. 4, 1164–1165 (2003).
57. Taieb, J. et al. A novel dendritic cell subset involved
in tumor immunosurveillance. Nature Med.
12, 214–219 (2006).
58. Lee, S. J. et al. Interferon regulatory factor-1 is
prerequisite to the constitutive expression and
IFN-γ-induced upregulation of B7-H1 (CD274).
FEBS Lett. 580, 755–762 (2006).
59. Vitale, M. et al. NK-dependent DC maturation
is mediated by TNFα and IFNγ released upon
engagement of the NKp30 triggering receptor.
Blood 106, 566–571 (2005).
60. Wilson, J. A., Bray, M., Bakken, R. & Hart, M. K.
Vaccine potential of Ebola virus VP24, VP30, VP35,
and VP40 proteins. Virology 286, 384–390 (2001).
61. Wilson, J. A. & Hart, M. K. Protection from Ebola virus
mediated by cytotoxic T lymphocytes specific for the
viral nucleoprotein. J. Virol. 75, 2660–2664 (2001).
62. Pushko, P. et al. Recombinant RNA replicons derived
from attenuated Venezuelan equine encephalitis virus
protect guinea pigs and mice from Ebola hemorrhagic
fever virus. Vaccine 19, 142–153 (2000).
63. Xu, L. et al. Immunization for Ebola virus infection.
Nature Med. 4, 37–42 (1998).
64. Jones, S. M. et al. Live attenuated recombinant
vaccine protects nonhuman primates against Ebola
and Marburg viruses. Nature Med. 11, 786–790
65. Sullivan, N. J. et al. Immune protection of nonhuman
primates against Ebola virus with single low-dose
adenovirus vectors encoding modified GPs. PLoS Med.
3, e177 (2006).
66. Bray, M. & Paragas, J. Experimental therapy of
filovirus infections. Antiviral Res. 54, 1–17 (2002).
67. Geisbert, T. W. et al. Evaluation in nonhuman primates
of vaccines against Ebola virus. Emerg. Infect. Dis.
8, 503–507 (2002).
68. Ignatyev, G. M., Agafonov, A. P., Streltsova, M. A. &
Kashentseva, E. A. Inactivated Marburg virus elicits a
nonprotective immune response in Rhesus monkeys.
J. Biotechnol. 44, 111–118 (1996).
69. Hevey, M., Negley, D., Geisbert, J., Jahrling, P. &
Schmaljohn, A. Antigenicity and vaccine potential of
Marburg virus glycoprotein expressed by baculovirus
recombinants. Virology 239, 206–216 (1997).
70. Wilson, J. A. et al. Epitopes involved in antibody-
mediated protection from Ebola virus. Science
287, 1664–1666 (2000).
This is the strongest of several demonstrations
of antibody-mediated prevention of lethal filoviral
disease in rodent models; here, monoclonal
antibodies of differing specificities and in vitro
functions are shown to protect in a mouse model
of Ebola virus. Evidence that filovirus-specific
antibodies (either monoclonal or polyclonal) can
prevent disease in non-human primates is still
lacking (for example, reference 76).
71. Gupta, M., Mahanty, S., Bray, M., Ahmed, R. &
Rollin, P. E. Passive transfer of antibodies protects
immunocompetent and imunodeficient mice
against lethal Ebola virus infection without complete
inhibition of viral replication. J. Virol. 75, 4649–4654
72. Parren, P. W., Geisbert, T. W., Maruyama, T.,
Jahrling, P. B. & Burton, D. R. Pre- and postexposure
prophylaxis of Ebola virus infection in an animal
model by passive transfer of a neutralizing human
antibody. J. Virol. 76, 6408–6412 (2002).
566 | JULY 2007 | VOLUME 7
© 2007 Nature Publishing Group
73. Martini, G. A. & Siegert, R. Marburg virus disease.
(Springer-Verlag Berlin, New York, 1971).
This is a classic monograph that contains chapters
on the clinical presentation, treatment efforts and
discovery of causative agent in the first recognized
filovirus outbreak in Europe in 1967. Because
subsequent Marburg virus outbreaks have
occurred in African sites poorly served by advanced
medical care, this remains the best description of
Marburg-virus disease in humans, and is updated
by one of the key 1967 scientists in reference 3.
74. Mupapa, K. et al. Treatment of Ebola hemorrhagic fever
with blood transfusions from convalescent patients.
International Scientific and Technical Committee.
J. Infect. Dis. 179 (Suppl. 1), 18–23 (1999).
75. Gupta, M. et al. Persistent infection with Ebola
virus under conditions of partial immunity.
J. Virol. 78, 958–967 (2004).
76. Oswald, W. B. et al. Neutralizing antibody fails to
impact the course of Ebola virus infection in monkeys.
PLoS Pathog. 3, e9 (2007).
77. Burnett, J. C., Henchal, E. A., Schmaljohn, A. L. &
Bavari, S. The evolving field of biodefence: therapeutic
developments and diagnostics. Nature Rev. Drug
Discov. 4, 281–297 (2005).
78. Sullivan, N. J., Sanchez, A., Rollin, P. E., Yang, Z. Y. &
Nabel, G. J. Development of a preventive vaccine
for Ebola virus infection in primates. Nature
408, 605–609 (2000).
This reference provides the earliest demonstration
of a genetic vaccine capable of preventing Ebola-
virus disease in non-human primates. The authors’
subsequent studies showed glycoprotein antigen
expressed in replication-defective adenovirus to be
79. Riemenschneider, J. et al. Comparison of individual
and combination DNA vaccines for B. anthracis, Ebola
virus, Marburg virus and Venezuelan equine
encephalitis virus. Vaccine 21, 4071–4080 (2003).
80. Bukreyev, A. et al. A single intranasal inoculation with
a paramyxovirus-vectored vaccine protects guinea
pigs against a lethal-dose Ebola virus challenge.
J. Virol. 80, 2267–2279 (2006).
81. Bukreyev, A. et al. Successful topical respiratory
tract immunization of primates against Ebola virus.
J. Virol. 81, 6378–6388 (2007).
82. Daddario-DiCaprio, K. M. et al. Postexposure
protection against Marburg haemorrhagic fever
with recombinant vesicular stomatitis virus vectors
in non-human primates: an efficacy assessment.
Lancet 367, 1399–1404 (2006).
83. Feldmann, H. et al. Effective post-exposure treatment
of Ebola infection. PLoS Pathog. 3, e2 (2007).
84. Ebihara, H. et al. Molecular determinants of Ebola
virus virulence in mice. PLoS Pathog. 2, e73 (2006).
85. Swanepoel, R. et al. Experimental inoculation of
plants and animals with Ebola virus. Emerg. Infect.
Dis. 2, 321–325 (1996).
86. Yao, S. & Chen, L. Reviving exhausted T lymphocytes
during chronic virus infection by B7-H1 blockade.
Trends. Mol. Med. 12, 244–246 (2006).
87. Leroy, E. M. et al. Fruit bats as reservoirs of Ebola
virus. Nature 438, 575–576 (2005).
88. Feldmann, H., Slenczka, W. & Klenk, H. D.
Emerging and reemerging of filoviruses.
Arch. Virol. Suppl. 11, 77–100 (1996).
89. Towner, J. S. et al. Marburgvirus genomics and
association with a large hemorrhagic fever
outbreak in Angola. J. Virol. 80, 6497–6516
90. Jahrling, P. B. et al. Experimental infection of
cynomolgus macaques with Ebola-Reston filoviruses
from the 1989–1990 U.S. epizootic. Arch. Virol.
Suppl. 11, 115–134 (1996).
91. Bwaka, M. A. et al. Ebola hemorrhagic fever
in Kikwit, Democratic Republic of the Congo:
clinical observations in 103 patients. J. Infect. Dis.
179 (Suppl. 1), 1–7 (1999).
This is a good overview, by an international
team, of the clinical presentation of human
Ebola-virus disease in a sizeable and deadly
92. Geisbert, T. W. et al. Treatment of Ebola virus
infection with a recombinant inhibitor of factor
VIIa/tissue factor: a study in rhesus monkeys.
Lancet 362, 1953–1958 (2003).
93. Borio, L. et al. Hemorrhagic fever viruses as biological
weapons: medical and public health management.
JAMA 287, 2391–2405 (2002).
94. Hevey, M. et al. Marburg virus vaccines: comparing
classical and new approaches. Vaccine 20, 586–593
95. Chepurnov, A. A. et al. Attempts to develop a vaccine
against Ebola fever. Vopr. Virusol. 40, 257–260
(1995) (in Russian).
96. Bray, M., Davis, K., Geisbert, T., Schmaljohn, C. &
Huggins, J. A mouse model for evaluation of
prophylaxis and therapy of Ebola hemorrhagic
fever. J. Infect. Dis. 178, 651–661. (1998).
This paper describes derivation of a mouse-
adapted variant of ZEBOV and provides evidence
of many similarities between mouse and primate
disease. That several subsequent studies have
demonstrated differences between mouse and
monkey disease manifested in the greater difficulty
in protecting monkeys with vaccines and therapies,
the selected use of the mouse model has not been
97. Volchkov, V. E. et al. Recovery of infectious Ebola virus
from complementary DNA: RNA editing of the GP
gene and viral cytotoxicity. Science 291, 1965–1969
98. Mellquist-Riemenschneider, J. L. et al. Comparison of
the protective efficacy of DNA and baculovirus-derived
protein vaccines for Ebola virus in guinea pigs.
Virus. Res. 92, 187–193 (2003).
99. Vanderzanden, L. et al. DNA vaccines expressing
either the GP or NP genes of Ebola virus protect
mice from lethal challenge. Virology 246, 134–144
100. Kobinger, G. P. et al. Chimpanzee adenovirus
vaccine protects against Zaire Ebola virus.
Virology 346, 394–401 (2006).
101. Swenson, D. L. et al. Virus-like particles exhibit
potential as a pan-filovirus vaccine for both Ebola and
Marburg viral infections. Vaccine 23, 3033–3042
102. Hevey, M., Negley, D. & Schmaljohn, A.
Characterization of monoclonal antibodies to Marburg
virus (strain Musoke) glycoprotein and identification
of two protective epitopes. Virology 314, 350–357
103. Takada, A. et al. Identification of protective epitopes
on Ebola virus glycoprotein at the single amino acid
level by using recombinant vesicular stomatitis
viruses. J. Virol. 77, 1069–1074 (2003).
104. Jeffers, S. A., Sanders, D. A. & Sanchez, A. Covalent
modifications of the Ebola virus glycoprotein. J. Virol.
76, 12463–12472 (2002).
105. Volchkov, V. E., Volchkova, V. A., Dolnik, O.,
Feldmann, H. & Klenk, H. D. Polymorphism of
filovirus glycoproteins. Adv. Virus Res. 64, 359–381
106. Sanchez, A., Trappier, S. G., Mahy, B. W., Peters, C. J.
& Nichol, S. T. The virion glycoproteins of Ebola
viruses are encoded in two reading frames and are
expressed through transcriptional editing. Proc. Natl
Acad. Sci. USA 93, 3602–3607 (1996).
107. Manicassamy, B., Wang, J., Jiang, H. & Rong, L.
Comprehensive analysis of Ebola virus GP1 in viral
entry. J. Virol. 79, 4793–4805 (2005).
108. Weissenhorn, W., Carfi, A., Lee, K. H., Skehel, J. J. &
Wiley, D. C. Crystal structure of the Ebola virus
membrane fusion subunit, GP2, from the envelope
glycoprotein ectodomain. Mol. Cell 2, 605–616
109. Kuhn, J. H. et al. Conserved receptor-binding
domains of Lake Victoria marburgvirus and Zaire
ebolavirus bind a common receptor. J. Biol. Chem.
281, 15951–15958 (2006).
110. Yang, Z. Y. et al. Identification of the Ebola virus
glycoprotein as the main viral determinant of vascular
cell cytotoxicity and injury. Nature Med. 6, 886–889
111. Simmons, G., Wool-Lewis, R. J., Baribaud, F.,
Netter, R. C. & Bates, P. Ebola virus glycoproteins
induce global surface protein down-modulation
and loss of cell adherence. J. Virol. 76, 2518–2528
112. Bukreyev, A., Volchkov, V. E., Blinov, V. M. &
Netesov, S. V. The GP-protein of Marburg virus
contains the region similar to the ‘immunosuppressive
domain’ of oncogenic retrovirus P15E proteins.
FEBS Lett. 323, 183–187 (1993).
The views, opinions and/or findings contained in this report
are those of the authors and should not be construed as an
official Department of the Army or Johns Hopkins University
position, policy, or decision unless designated by other docu-
mentation. This work was supported in part by the Defense
Threat Reduction Agency.
Competing interests statement
The authors declare no competing financial interests.
The following terms in this article are linked online to:
Entrez Gene: http://www.ncbi.nlm.nih.gov/entrez/query.
B7-H1 | CD40 | CD86 | DC-SIGN | ICAM3 | IFNα | IFNβ | IFNγ |
IL-12 | L-SIGN
Entrez Genome: http://www.ncbi.nlm.nih.gov/sites/
MARV | SEBOV | ZEBOV
Access to this links box is available online.
NATURE REVIEWS | IMMUNOLOGY
VOLUME 7 | JULY 2007 | 567
© 2007 Nature Publishing Group