Immuno- pathogenesis of Ebola virus disease DR. MD. MAMUNUR RASHID Assistant Professor Clinical Tropical Medicine Department BITID, Fouzderhat, Ctg.

PATHOGENESIS  Due to the difficulty of performing clinical studies under outbreak conditions, almost all data on the pathogenesis of Ebola virus diseases have been obtained from laboratory experiments employing on mice, guinea pigs, and a variety of nonhuman primates. Whatever the point of entry into the body - The first line cells to be affected are- macrophages & dendritic cells. Other cells to be affected are- monocytes, endothelial cells, fibroblasts, hepatocytes, adrenal cortical cells & epithelial cells.

Viral Entry Figure-1. Macropinocytosis.

Upon membrane fusion, the capsid moves into the cell cytoplasm at a site where replication proceeds optimally. The Ebola virus glycoprotein (GP) is synthesized in a secreted (sGP) or full- length transmembrane form & each gene product has distinct biochemical & biological properties.

Replication of Ebola virus Viral RNA polymerase, encoded by the L gene Promoter Transcription Synthesis of mRNA Translation Synthesis of newer protein New virus formation

Viral Budding Figure-2. Budding of newly assembled Ebolavirus virions is mediated by VP40.

Spread of virus to regional lymph nodes from ECF Further rounds of replication Dissemination of virus to dendritic cells and fixed and mobile macrophages in the liver, spleen, thymus, and other lymphoid tissues. Aided by virus-induced suppression of type I interferon Rapid systemic spread

Progression of the disease Extensive tissue damage Systemic spread   Progression of the disease Infection spread to hepatocytes, adrenal cortical cells , fibroblasts & many other cells Extensive tissue damage Induction of systemic inflammatory syndrome by releasing cytokines like TNF-α, IL-6, IL-8, NO, MCP-1

Fever, inflammation & loss of vuscular integrity   Loss of vascular integrity causes further synthesis of viral GP Reduces specific integrins responsible for cell adhesion to the inter cellular structures Damage of different organs of body causing coagulopathy, hypotension, vasodilation, increased vascular permeability, shock etc.

Pathophysiology Figure-3.Pathogenesis schematic

macrophages TF + Cytokines Coagulation defects Ebola virus-infected macrophages synthesize cell-surface tissue factor (TF) Cytokine induced macrophages TF + Cytokines Triggering the extrinsic coagulation pathway   Extensive consumptive coagulopathy leading to DIC like features . The simultaneous occurrence of these two stimuli helps to explain the early appearance, rapid development, and ultimate severity of the coagulopathy in ebola virus infection causing bleeding.

Blood samples from Ebola-infected monkeys contain D-dimers within 24 hours after virus challenge, and D-dimers are also present in the plasma of humans with Ebola hemorrhagic fever . In macaques, activated protein C is decreased on day two, but the platelet count does not begin to fall until days three or four, suggesting that activated platelets are adhering to endothelial cells. As the disease progresses, hepatic injury may also cause a decline in plasma levels of certain coagulation factors which also induces bleeding.

Ebola virus causes destabilization of the vascular endothelium leading to hemorrhage. Figure-4

IMMUNE RESPONSE TO EBOLA VIRUS INFECTION Ebola virus act both directly and indirectly to disable antigen-specific immune responses. Dendritic cells, which have primary responsibility for the initiation of adaptive immune responses, are a major site of filoviral replication.

Ebola virus Infection to immune related cells Apoptotic death of   Apoptotic death of Dendritic cell Lymphocytes Loss of Innate Failure of initiation of immunity adaptive immune response

Figure-5 Antibody Dependent Enhancement of Ebolavirus Infection. (A) Human kidney cells infected with Zaire virus in the presence or absence of purified mouse antibodies (B) Human kidney cells were infected with the Zaire virus or VSV pseudotyped with the Zaire GP following incubation with convalescent human plasma or serum (Takada et al. 2003).

Figure-6 C1q-mediated Antibody Dependent Enhancement of Ebolavirus Infection (Model). Ebola virus initiates infection by binding to its specific receptors (top panel). C1q enables binding between the virus-antibody complex and C1q ligands on the cell surface, promoting interaction between the virus and its receptor (bottom panel). Binding of the virus via the C1q molecule increases the likelihood of viral attachment to the cell surface. (Takada et al. 2003).

Therefore, while antibodies normally protect the body, this virus is able to use them for faster and easier attachment to target cells. In addition being an essential cofactor for the viral RNA polymerase complex, VP35 produced in response to viral infection has been identified as an inhibitor of multiple components of the interferon (IFN) pathways, which exerts anti viral, cell growth inhibitory & immunoregulatory activities.

In summary, an understanding of the mechanisms underlying Ebola virus-induced cytopathic effects has facilitated the process of vaccine and antiviral therapy development, which has in turn provided new information about pathogenesis and the immune response. Ebola virus does not exhibit the high degree of variability that other enveloped viruses may employ to evade host immunity, but Ebola virus GP alters target-cell function and exemplifies a novel strategy for immune evasion that may have arisen through the evolution of Ebola virus with its natural host. The cytotoxic effects of GP on macrophage and endothelial cell disrupt inflammatory cell function and the integrity of the vasculature.

In addition, by altering the cell surface expression of adhesion proteins and immune recognition molecules, Ebola virus may disrupt processes critical to immune activation and cytotoxic-T-cell function. These phenomena likely account for the dysregulation of the inflammatory response and the vascular dysfunction characteristic of lethal Ebola virus infection, providing a rationale for focusing on GP as a target for a preventative vaccine and other clinical interventions. Another target focus may be the viral entry pathway to stop replication.

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