(*PAPER IN PROGRESS -- Estimated Completion , Tues. Sep 30th, 2014)
The Ebola 2014 outbreak has been nothing short of unprecedented.
STEM Simulation of Ebola Virus 2014 West Africa Outbreak (Operon Labs, 2014).
Many medical doctors, virologists, epidemiologists, public health specialists, researchers, bioinformaticians, and other scientific experts are closely following ongoing events.
The recent epidemiological simulations from the CDC suggest Ebola cases have begun geometric expansion. Effective intervention is required should these assumptions prove correct.
The CDC shows Ebola virus infections above 1 million if interventions are not successful by Jan 2015. A reduction of 50% or more would be necessary to ensure the current outbreak does not become a global problem. The current response will be discussed in future updates.
With that said, the purpose of this post is to provide a background on EVD (Ebola Virus Disease) to interested scientists, researchers, or members of the public. The content is primarily geared towards scientists with an interest in Ebola, who may not specialize in filoviruses.
Ebola is an ancient disease. Evidence continues to mount which supports this claim. In fact, with the rise of bioinformatics, an entire discipline (Paleovirology) has emerged to evaluate such ideas.
Our problem is that Viruses generally do not make good fossils. Luckily for us, there are exceptions.
Sometimes, Viral genetic material can become incorporated into eukaryotic genomes through retrotransposons (L1 in humans). The resultant genetic 'trash' from viral infections is passed on to subsequent generations of a species, resulting in what are called EVEs -- Endogenous Viral Elements . EVEs are found in animals, plants, and fungi as 'relics' of viral infections. These viral fragments entered the host genome through long-deceased ancestors.
Viral replication strategies, endogenous viral elements, and the genomic fossil record.
A 2010 PLOS Genetic Analysis indeed found watermarks of viral genetic data in vertebrate retrotransposons -- to include humans and other mammals. To everyone's surprise, what they found were not the usual suspects (Rhinovirus, Coronavirus, etc).
This PLOS study ran data mining on 5666 viral genes (from all known non-retroviral ssRNA viruses at the time) and compared them to the complete genomes of 48 vertebrate species. The strongest statistical associations (right at the top of the list) were Ebolavirus and Bornavirus.
The result was so unexpected, the title of the paper started with "Unexpected Inheritance". In other words, no one thought Ebola would show up.
Phylogenetic tree of vertebrates that encode Bornavirus- and Filovirus- like proteins in their genomes.
Bornaviruses-related sequences are denoted by icosahedrons and Filoviruses-related sequences by triangles. Times of the viral gene integrations are approximate. (Belyi et al, 2010).
This suggests that Ebola (or an Ebola-like disease) has been an enemy of mankind (or at least fruit bats) for quite a long time.
So exactly how long have Ebola-like viruses been with us? Oh, at least 35 million years. As a lower estimate. This suggest Ebola-like viruses are much older than our frozen ancestor Lucy, and shows Ebola predates many species. Filovirus genetic fossils have been confirmed in many different animals, including primates. The chart below implies that human and macaque Ebola genetic fossils had a common ancestor over 40 million years ago. Note the blue column for filoviruses.
Timescaled phylogenetic tree of mammals screened in this study showing the known distribution of EVEs and of exogenous Borna-, Filo-, Circo-, and Parvoviruses. (Katzourakis et al, 2010)
To be fair, the studies quoted here did not find Filovirus 'fossils' in human genomes. But ancient Ebola virus fragments were found in microbat, wallaby, guinea pig, shrew, opossum, and tarsier. This suggests Filoviridae have been ubiquitous for millions of years, and are probably one of the oldest viruses conclusively known. The finding of Ebola-specific gene fragments in tarsier is highly significant, as the tarsier is a non-human primate (NHP) -- a common route from the probably Ebola reservoir species (bat) to human.
In the above annotated image, ancient Ebola was detected in non-human primates well over 35 million years ago -- probably much longer.
The study by Katzourakis et al had this to say about Filoviruses like Ebola in Paleovirology:
Filoviruses EVEs were identified not only in North American bats (M. lucifugus) and Asian primates (tarsier), but also in insectivores, rodents, and in both South American and Australian mammals (Figure 6). In concordance with the recent identification of Ebola Reston in swine , this unexpected result indicates that the distribution of filoviruses is likely much broader than has previously been recognized (Katzourakis et al, 2010).
Ebola is very old. So far we have prevailed, which is the good news.
Ebola Virus Structure: Overview
The following image (an NSF contest runner-up) visualizes the Ebola Virus Structural Biology:
Image Credit: Ivan Konstantinov, Yury Stefanov, Alexander Kovalevsky, Anastasya Bakulina, Visual Science
The above excellent artistic image above shows how an infectious Ebola viral proteins are assembled into a 3D model. The current 2014 outbreak possess changes in multiple genes, both ncRNA and coding regions. The 2014 genetic changes will be the topic of a follow up discussion. For the moment, let's continue with our Ebola review...
The above artistic image shows how the proteins in the Ebola virus are assembled (along with it's genetic material, -ssRNA) to make a complete viral particle. The Ebola virus is made out of seven distinct proteins (nine proteins if we include secreted products). The seven structural proteins are formed from seven transcriptional units on the RNA genome.
The Ebola secreted proteins are an additional two soluble products which can be made from Ebola's seven transcriptional-unit mRNA genome. The remaining two proteins (sGP and ssGP) are secreted into the extracellular fluid, and are not virus structural components.
The purpose of Ebola's secreted soluble proteins sGP and ssGP are unclear at this time, although they appear to play a role in acting as 'decoys' to subvert the host immune system, by absorbing anti-GP antibodies (Mohan, 2012). We will not discuss the Ebola secreted proteins in further detail, as they are still poorly understood, and are not critical to viral replication.
Regarding the Ebola virus gene products themselves, they are characterized as follows:
Ebola Virus Genome
||-Essential Component of Nucleocapsid.
-Transcription & Assembly.
into non-functional helical structures (~20nm)
||Virion Protein 35
-Essential Component of Nucleocapsid.
-L Polymerase Cofactor
-Silences host RIG-1 dsRNA binding
-Inhibits host IRF-3 phosphorylation.
-Targets Innate immune response.
-Interacts with cellular kinases IKKε & TBK-1.
-Can disrupt IKKε and IPS-1 interactions.
||Virion Protein 40
/ "Matrix Protein"
|-Maintains virion structural integrity
-Required for viral cellular egress
-Associated with late endosomes
|-Can form 'budding' viral-like particles
in absence of all other Ebola proteins
-VP40 + GP VLPs can enter cells
-May exploit COPII transfer system to
reach host cell internal membrane for egress
||-'Spikes' on virion outer membrane
-Required for viral cellular entry
-Broad tissue tropism
-GP is Ebola's 'cell fusion' protein
-GP1/GP2 complex is heterodimer
-Outer GP1/2 is a trimer of heterodimers
|-GP trimer forms 10nm long spikes
-GP is Post-Tr Cleaved into GP1/GP2
-GP1/GP2 linked via S-S bond
-GP is RNA-edited and Golgi-proc'd
-Dimeric sGP product is secreted
-sGP assists in viral cloaking ('decoy')
||Virion Protein 30
||-Essential Component of Nucleocapsid.
-Transcription Activation & Reinitiation
|-Transcription will not proceed without VP30
-Contains Zn-binding Cys-His Motif
-Overall, poorly understood at present.
||Virion Protein 24 /
"2nd Matrix Protein"
|-Required for fully-functional Nucleocapsid
-Associated with Matrix Protein (VP40)
-Blocks IFN-α/β + IFN-γ signaling
|-Located between capsid & envelope
-Possibly associated with capsid assembly
-Competes with STAT1 for karyopherin
-Viral transcription inhibitor
-May play regulatory role in switch from
transcription <--> translation
||-Essential Component of Nucleocapsid.
-Genome Transcription & Translation
-Copies Viral -ssRNA into segmented
and capped viral mRNAs for translation.
-Copies -ssRNA to +ssRNA template
-Positive ssRNA template used to make
new neg ssRNAs for packaging into viral progeny
-Ebola RNA Replicase is complex
-Can engage in RNA editing (many ORFs)
-Has interactions in relation to ORFs
-Upstream repeat ORFs can regulate L
-Cellular stress can regulate L
The above table should give a high-level overview of the various genes in the Ebola virus, and their functions within both EBOV structural and functional biology. The life-cycle of the Ebola virus is complex, but closely resembles other filoviruses as well as well-studied NNS viruses such as VSV.
As described above, there are seven main Ebola structural proteins, as in the 3D model presented above. Let's review the virus diagram from the outside inward.
Ebola Virus Structure: Cellular Entry via GP
A notable feature of Ebola is that underneath the GP spikes, there is a lipid-bilayer (grey in above diagram), making Ebola an enveloped virus. The lipid bilayer is derived from infected cell membranes as the virus buds from the cell. Thus, Ebola membranes contain not only 'normal' lipid bilayer, they also contain a fair amount of other proteins and lipid rafts which were derived from the virus's previous host cell. A viral envelope (as seen in Ebola) can help many viruses evade the immune system, because most of the virus's antigenic components besides GP are not exposed to scrutiny (they are 'cloaked' by a real human-cell derived lipid bilayer). The video to the right gives a good background on encapsulated viral fusion and entry.
||Notice that the Ebola virus outer viral 'fusion proteins' (GP) spikes resemble that of influenza. This is expected as both are believed to be Class I fusion proteins...Both are also homotrimers and undergo pH-dependent conformational change in the late endosome. The interaction of fusion proteins with the endosome is how these viruses 'trick' their way out of the late endosome, which in practice means these viruses pop out of the cellular 'trash can' and into the cytoplasm. (SIB, 2014)
The surface proteins on the Ebola virus bind to a target cell (mostly monocytes, fibroblasts, and endothelial cells) and triggers a process called macropinocytosis. The cell unwittingly brings the virus inside, enclosed in a digestive and sorting compartment called the endosome.
The endosome is processed , and eventually becomes a 'late endosome' where acidification of its viral payload occurs. This triggers the activity of endosome enzymes Cathepsin B and L.
The process is: (1) An interaction between Ebola GP1,2 and cell receptors trigger macropinocytosis into an endosome, (2) The endosome pH drops, activating cathepsin proteases, (3) Endosomal Cathepsin B and L sequentially cleave the Ebola GP1,2 into a 'primed' form, ready for fusion, (4) The primed Ebola GP1,2 is reduced and interacts with NPC1 receptor, (5) An unidentified event occurs, triggering primed GP1,2 to fuse with NPC1 and the endosomal membrane, liberating the Ebola virus contents into the host cell cytoplasm. (Hoffman-Winkler, 2012)
This is a detailed view of the Ebola virus fusion process in the late endosome. First, Cathepsin L cleaves GP1 into a smaller 20kDa GP1 subunit. Next, Cathepsin B cleaves the 20kDa GP1 into a 19kDa form that is now 'primed'. Finally, an additional unidentified event occurs (possibly involving reduction and NPC1 interaction), triggering fusion with the endosome compartment and viral escape into the cell cytoplasm. (White, 2012)
"All enveloped viruses penetrate into host cells using a viral membrane fusion protein. Class I fusion proteins are trimers of three identical units (see the figure; the initial and final protein depictions are based on X‑ray structures of several class I fusion proteins). For most of these proteins, including Ebola virus (EBOV) glycoprotein, influenza virus haemagglutinin and retroviral Env proteins, each monomeric unit consists of a receptor-binding (rb) and a fusion (f) subunit, which are initially present in a single polypeptide chain. Priming by proteolytic cleavage (generally between the receptor-binding and fusion subunits) converts the protein from a fusion-incompetent to a fusion-competent (metastable) form that can respond to a fusion trigger. Triggering exposes and repositions the previously hidden (or tacked-down) fusion loop (or fusion peptide), which then binds hydrophobically to the target membrane." (White, 2012)
Ebola Virus Structure: Internal Proteins & Nucleocapsid
|Beneath Ebola's GP spikes and lipid bilayer exists the Ebola virus matrix protein (VP40 aka "major matrix protein", and VP24 aka "minor matrix protein). Both of these proteins have a structural role to maintain the shape of the viral particle, but they also have additional functions such as suppressing the host immune response, regulating cellular trafficking to the benefit of the virus, etc.
Beneath the major and minor matrix proteins is what is called the Nucleocapsid. The Ebola virus nucleocapsid is a complex of four proteins (NP, VP35, VP30, and L) and the viral genome (ssRNA). The NP protein forms a sort of helix which allows the Ebola ssRNA to be wound and packed tightly. RNA is negatively charged (thus self-repels, like two North poles) so it is necessary for the virus to overcome (or at least minimize) the repulsive energy of its RNA. So the Ebola RNA is wound onto the protein complex of NP (nucleoprotein), VP35, VP30, and L.
The nucleocapsid proteins alone are sufficient for self-assembling Viral-like Particles (VLPs), which spontaneously form structures almost identical in size and shape to Ebola viral particles... The nucleocapsid complex (when wound with proper viral ssRNA) is , quite remarkably, sufficient for transcription and replication. To be specific, the VP30 protein is the necessary factor for transcription initiation -- and so can be thought of as a protein critical in transcription initiation in the L polymerase.
However, the four protein nucleocapsid complex alone , as described above, is not sufficient for a fully-functional virus -- viral functions such as cellular ingress and egress are impaired to non-existent. Recombinant non-pathogenic Ebola viruses (VLPs) often consist of Ebola Matrix Protein (VP40) with other Nucleocapsid genes , and are often used for study of viral behavior under non-BSL4 conditions.
Ebola Virus Structure: Immune Suppression
An additional interesting property of the Ebola nucleocapsid complex (NP, VP35, VP30, and L) is it also has 'multi-functional' roles for the proteins involved. For example, the VP35 protein has a secondary function whereby it is partially responsible for 'shutting off' the host cell's immune response.
The Ebola VP35 protein suppresses the activation of a host enzyme RIG-I. Ebola binds to RIG-I, and prevents induction of downstream Interferon genes.
When a cell is infected by an RNA virus like Ebola, the presence of short ssRNA or dsRNA in the cytoplasm alerts the cell's pattern-recognition enzymes that something is amiss.
dsRNA is not normally present in large amounts in eukaryotic cytoplasm, so when it is detected, an cellular enzyme called RIG-I binds to the dsRNA and induces a signal cascade.
Essentially, RIG-I 'tattles' to the nucleus that a virus might be in the cell. In response to detection of a virus (for example, a signal from RIG-I), the cell upregulates a constellation of antiviral genes, the most important of which are Type I Interferons.
Ebola's VP35 gene prevents this process from happening.
The VP35 protein has the dual-role with the ability to bind to RIG-I, thus preventing the host cell from detecting the Ebola dsRNA that's amplifying in the cytoplasm. An analogy here is that the VP35 protein is a cloaking mechanism that makes Ebola 'invisible' to RIG-I... What happens is that RIG-I becomes 'clogged up' with Ebola VP35 protein instead of viral ssRNA. The end result is that no antiviral signaling occurs via the RIG-I pathway. This prevents induction of cellular Interferon.
Interferon is categorically one of the most important 'master' antiviral genes that exists. Interferon is the 'switch' for a whole constellation of gene products (called Interferon Stimulated Genes), most all of which cause the cell to 'double check' and 'inspect' everything, making it nearly impossible for a virus to properly replicate. The power of Interferon is why Ebola (and other dangerous viruses) target it so ruthlessly. Ebola must absolutely destroy and disable the host Interferon response in order to be able to succeed in it's prime directive -- replicate at all costs.
The Type I Interferons put the cell into a highly antiviral state -- I like to think of the cellular result of Interferon is a sort of 'stasis' where non-essential cell machinery gets shut down, and all non-essential staff go home. The call calls in the engineers, and the engineers go back and 'double-checks' the cell's machinery to ensure that everything is okay. If it's a false alarm, the cell returns to business as usual after about 6h to 48h. However, if a virus is indeed present, Interferon stimulated genes will take action. Nearby cells to alerted to 'be on the lookout' for a virus. If the cell cannot 'clear' the virus, it will actually undergo programmed cell death (often with help from immune cells) to destroy the virus within it.
In addition to inhibiting RIG-I, Ebola VP35 also targets IKKε, TBK1, IPS-1, and possibly TRAF3 interactions. All these host proteins are downstream signals of RIG-I. Thus, Ebola VP35 targets enzymes throughout the entire host antiviral signaling pathway.
Ebola is so intent on shutting off all host antiviral signalling, that targeting four to five proteins is not enough.
VP35 protein actually has a final target for Interferon suppression: VP35 inhibits the phosphorylation of IRF-3. Phosphorylation of IRF-3 is the last step prior to antiviral signals reaching the cell's nucleus (for production of Interferon Beta).
A second Ebola protein (VP24) has even more anti-interferon activity. Ebola's VP24 protein inhibits the nuclear import of phosphorylated STAT1 through competitive binding for karyopherin (KPNA5) (Xu et al, 2014) .
To summarize, using the diagram on the right: VP35 targets RIG-I, IKKε, IPS-1, TRAF3, TBK1, and IRF-3. VP24 targets nuclear importation of STAT1.
In summary, Ebola inhibits multiple antiviral pathways as shown in this diagram, by acting through synergystic antiviral signal blockades.
It is quite remarkable that a Ebola virus protein (VP35) has (at least) four secondary and distinct anti-Interferon functions... All of VP35's targets involve the production of cellular Interferon or ISG. The Ebola virus as a whole actually targets both the first (RIG-I) and final steps (IRF-3 / STAT1) of the host anti-viral signal cascade. The specificity is actually remarkable, if it weren't so deadly and hostile.
The end result is that Ebola has multiple ways to 'cloak' itself from immune responses. . . Ebola thus relies on suppressing host antiviral signalling to allow early-phase replication to proceed in the host undetected by the innate immune response (NK-Cells, Interferon, ISG, etc). By the time the host realizes that it's been tricked, the Ebola virus has already undergone extreme amplification undetected, and has claimed the Liver, Spleen, and Lymphatic system as it's turf.
Ebola Virus Genome
Filoviridae like Ebola have the longest genomes of all the Mononegaviruses, clocking in at almost 20 kbp.
Filoviridae are also curious in that they have long non-translated sequences that flank the coding regions (after the stop signals). These regions are of unknown purpose, but may be involved in auto-control of viral gene expression.
There are seven genes in the Ebola virus; The following comprise their order within the negative-sense (-) ssRNA non-segmented virion.
The overall Ebola viral gene products have decreasing expression (as read from 3' to 5') , as in other NNS (nonsegmented negative-strand) viruses, due to the activity of the L-polymerase (via polymerase entry, initiation, and termination). Put another way... Ebola has only a single promoter at the 3' end of it's genome for the RNA Polymerase to bind. The RNA Polymerase can only load at one site. As the RNA Polymerase moves down the genome, it tends to 'fall off' (and start over at 3') as it hits the 'bumps' of translation start and stop signals of the seven individual viral genes. Thus, genes located near the promoter (3' end) are expressed at much higher levels than genes toward the 5' end.
Another consequence of this is that it makes things complex in terms of making full-length positive-sense RNA. The polymerase needs to (sooner or later) override or ignore the start and stop signals it encounters in order to make a full-length positive sense template (polymerase read-through) . The resulting complete 19kb +ssRNA serves as a master copy for production of full-length 19kb negative-sense RNAs. The replicated -ssRNAs are copied to be incorporated into progeny viral genomes (after success of +ssRNA/-ssRNA read-through translation, mRNA gene transcription, and ribosomal/Golgi production of viral gene products). The result of this process leads accumulation of new full-genome +/-ssRNAs in the cytoplasm, the self-assembly of the nucleocapsid complex with 19kb -ssRNAs, full virion assembly, and ultimately the budding functional new viral particles.
A diagram of NNS (ie Ebola virus) replication (transcription/translation) scheme (source)
Another visual example of the Ebola virus -ssRNA (NNS) genome
All NNS viruses use a genetic regulation mechanism analogous to Ebola, which is best characterized as follows:
"The levels of gene expression [in NNS viruses] are primarily regulated by their position on the genome. The promoter proximal gene is transcribed in greatest abundance and each successive downstream gene is synthesized in progressively lower amounts due to attenuation of transcription at each successive gene junction" (Barr JN, Whelan SP, Wertz GW., 2002)
Again, to put it another way, Ebola virus gene expression is regulated by a transcriptional gradient from the negative-sense 3' to 5' end, with gene products towards the 3' end (NP mRNA) transcribed at much higher concentrations than those at the 5' end (L mRNA). (Shabman, et al, 2013). The Ebola mRNA levels are roughly characterized as follows: NP > VP35 > VP40 > GP > VP30 > VP24 > L, but are time-dependent. This would be characterized as the study of Ebola proteomics.
The following diagram shows the Ebola virus transcriptional gradient -- shown are mRNAs at 6h, 12h, and 24h post infection (Shabman, et al, 2013).
"Representative mRNA levels for each EBOV mRNA [...] at 6, 12, and 24 hpi. Each bar corresponds to a different EBOV mRNA."
source: An Upstream Open Reading Frame Modulates Ebola Virus Polymerase Translation and Virus Replication (Shabman, et al, 2013)
Notice that NP (nucleoprotein) is expressed at the highest levels, with VP40 (matrix protein) at third-highest levels, while L (RNA Polymerase) is usually expressed at the lowest levels.
Ebola Virus: Life Cycle Overview
The main process could be summarized as follows:
(White & Schornberg, 2012).
|Ebola virus (EBOV) binds to attachment factors and receptors on the cell surface through the viral spike protein, glycoprotein (GP) (step 1)
||The virus is then internalized into a macropinosome (step 2).
||The virus is trafficked to an endosomal compartment containing the cysteine proteases cathepsin B (CatB) and CatL. These proteases digest GP to a 19 kDa form. (step 3)
||Primed GP Initiates fusion between the viral and endosomal membranes (step 4).
||The viral nucleocapsid is released into the cytoplasm, where the genome is replicated (step 5).
||The viral genome is transcribed with the aid of the viral proteins VP35, VP30 and L (step 6).
||Viral mRNAs are then translated (step 7).
||mRNAs encoding GP are brought to the endoplasmic reticulum (ER), where GP is synthesized, modified with N-linked sugars and trimerized (step 8).
||GP is further modified in the Golgi and delivered to the plasma membrane in secretory vesicles (step 9).
||At the plasma membrane the ribonucleoprotein complex (RNA plus nucleoprotein (NP)) and associated viral proteins assemble with the membrane-associated proteins (matrix proteins VP24 and VP40, and GP), and the resultant virions bud from the cell surface (step 10).
||Non-structural forms of GP, including soluble GP (sGP), are also secreted (step 11).
Ebola Virus: Tissue Tropism
The Ebola virus first enters a human or NHP host through mucus membranes, small skin abrasions, or via parenteral administration. Historically, EVD exposure is generally through direct contact or contaminated needles.
Infectious Ebola virus particles in sick individuals are found on skin, mucus membranes, in bodily fluids, and nasal secretions in non-human primates (NHP). Exposure generally occurs through direct contact with an infected individual, and may also occur through viral shedding and transmission via fomites. Oral consumption of infectious particles can also result in disease. Aerosol transmission of infectious particles have been demonstrated in animal models in a laboratory setting, though this finding remains controversial due to methodology. Aerosol transmission may need to be re-evaluated in the context of the 2014 outbreak, especially considering the high numbers of medical personnel that have become infected.
Route of infection affects clinical course and disease outcome. IV or IP administration of Ebola is almost invariably fatal (CFR=100%). Direct Contact transmission results in lower fatality rates (80%). Furthermore, IV/IP infection of Ebola was calculated to have an incubation period of 6.3 days, while contact exposures had a significantly longer incubation period of 9.5 days. This fact may also be of relevance to the 2014 outbreak, perhaps if there are longer-than-expected generation times or incubation periods (i.e. oral vs nasopharyngeal infection).
An infectious Ebola virus particle can result in ultimate organ titers as high as 10 million to 100 million (10^7 to 10^8) PFU / g, which correlates with very high viral amplification in hosts. Viral load has also been demonstrated to correlate with mortality.
Ebola infection will be divided into 'conceptual' phases based on available information. These are provided for purposes of a conceptual framework, rather than the ultimate word on disease progression. The following diagram shows the overall process, with all disease 'phases' superimposed.
NOTE: THE FOLLOWING SECTION IS IN PROGRESS -- TO BE COMPLETED , Tue Sep 30th, 2014
A 'first-draft' conceptual model is presented below is provided as a repository of the author's knowledge to be refined and updated. This model is not an experimentally verified framework , but simply a conceptual way of organizing information. It will be refined and updated as time permits.
Ebola Infection: Phase 1:
Ebola infection begins when an individual comes into contact with the viral particles at an endothelial or epithelial site. These can include mucus membranes like the nose, mouth, or eyes, or can include the nasopharynx or gastrointestinal tract. Regardless, a substantial amount of infectious viral particles have been deposited on an endothelial surface. The virus infects local cells immediately -- preferentially, it infects monocyte-derived white blood cells (WBCs) -- including monocytes, macrophages, and dendritic cells (DCs). If such cells are not readily available, the virus will infect endothelial cells or fibroblastic cells at the site of infection.
The only cells the Ebola virus cannot infect other WBCs like NK Cells, T-cells or B-cells; a substantial subset of non-monocytic WBCs are innately immune to Ebola infection. Ebola has broad tissue tropism otherwise. But Ebola can generally only infect three main types of White Blood Cell -- Monocyte, Macrophage, and Dendritic Cell (DC).
Transient Local inflammation results from Ebola primary replication and progeny release at site of initial infection. This results in a muted and understated immune response and suppression of most all Interferon and ISGs. Cytokines may be induced, but Interferon is switched off. Monocyte-derived WBC arrive at the infection site , and are themselves quickly infected by the Ebola virus. The virus continues to infect monocyte-derived cells in the vicinity. Some of these infected monocyte-derived WBCs migrate to local lymph nodes, carrying the Ebola virus as a payload. Once in a local lymph node, the Ebola virus rapidly infects more and more monocytes, macrophages, and dendritic cells. A secondary target is Fibroblastic Reticular Cells of the FRC Conduit. A tertiary target is endothelial cells.
At a certain point, Ebola infected monocyte-derived WBCs probabilistically cross over into blood circulation in a concentraiton sufficient to infect the Liver and Spleen. At this point, this virus enters Phase 2.
Ebola Infection: Phase 2:
Once Ebola begins to appear in the bloodstream , originating from the site of entry and nearby lymph nodes, the virus specifically exhibits tissue tropism for the Liver and Spleen (as well as Adrenal Glands). Blood containing Ebola-infected WBCs is filtered by the liver and spleen, where infected Ebola cells and/or viral particles are removed and deposited in these organs.
<REMAINING SECTIONS IN PROGRESS>
Death occurs due to hypovolemic shock or multiple organ failure.
|"Lymph node of an infected African Green Monkey. The location of the Ebola Zaire antigen is indicated by the red stain. The large ovoid structure at the center of the picture is a high endothelial venule (HEV) that is infected with Ebola. The viral replication in the fibroblastic cells that control the HEV's structure has almost totally destroyed the HEV" (Dr. Tom Geisbert)
||"Photomicrograph of the lip of an African green monkey. Ebola virus (red) is penetrating between the epithelial cells of the lip overlying a lymphoid aggregate in the submucosa. This immunohistochemistry preparation was done by Keith Steele." (Dr. Tom Geisbert)
This process is under study and review.