Fiche de révision : Flavivirus Biology and Disease Mechanisms

📋 Course Outline

1. Flavivirus Classification
2. Flavivirus Transmission
3. Flavivirus Cell Replication
4. Dengue Clinical Manifestations
5. Zika Clinical Manifestations
6. Flavivirus Cell Entry
7. Flavivirus Genome Replication
8. Flavivirus Pathogenesis
9. Antibody-Dependent Enhancement
10. Dengue Severity Factors
11. Picornavirus Classification
12. Poliovirus Vaccines

📖 1. Flavivirus Classification

🔑 Key Concepts & Definitions

• Flaviviruses: (+) ss RNA viruses that belong to the Flaviviridae family, including notable examples such as Dengue virus (DENV) and Zika virus (ZIKV). These viruses have a positive-sense single-stranded RNA genome that can directly serve as mRNA for protein synthesis (source).
• Positive-sense single-stranded RNA viruses: Viruses with RNA genomes that are equivalent in polarity to mRNA, allowing immediate translation upon entry into the host cell cytoplasm (source).
• Dengue virus (DENV): A flavivirus responsible for dengue fever, transmitted by Aedes mosquitoes, with multiple serotypes that influence disease severity and immune response (source).
• Zika virus (ZIKV): A flavivirus associated with congenital microcephaly and Guillain-Barré syndrome, also transmitted by Aedes mosquitoes, with a positive-sense ss RNA genome (source).
• Flavivirus classification: Categorized as positive-sense single-stranded RNA viruses within the Flaviviridae family, characterized by their genome structure and replication strategy (source).

📝 Essential Points

• Flaviviruses, including DENV and ZIKV, are classified as (+) ss RNA viruses, meaning their genomes can be directly translated by host ribosomes immediately after infection (source).
• They are transmitted primarily via Aedes aegypti and Aedes albopictus mosquitoes, which serve as vectors (source).
• The genome of flaviviruses is non-segmented, positive-sense RNA that encodes a single polyprotein, processed into structural and non-structural proteins (source).
• As positive-sense ss RNA viruses, flaviviruses can rapidly initiate replication and protein synthesis within the cytoplasm of host cells, facilitating efficient infection cycles (source).
• Understanding the classification as positive-sense ss RNA viruses is crucial for grasping their replication mechanisms, pathogenicity, and vaccine development strategies (source).

💡 Key Takeaway

Flaviviruses are positive-sense single-stranded RNA viruses, including Dengue and Zika, characterized by their direct translation capability and transmission via Aedes mosquitoes, which underpins their rapid replication and pathogenic potential.

📖 2. Flavivirus Transmission

🔑 Key Concepts & Definitions

• DENV and ZIKV: Flaviviruses classified as (+) ss RNA viruses, including Dengue virus (DENV) and Zika virus (ZIKV), which are transmitted by mosquito vectors (see source content).
• Vectors for DENV and ZIKV: Aedes aegypti and Aedes albopictus mosquitoes serve as the primary vectors responsible for transmitting these flaviviruses to humans (see source content).
• Mosquito vectors: Aedes species mosquitoes are the main vectors for transmitting DENV and ZIKV, facilitating their spread from infected to susceptible hosts (see source content).

📝 Essential Points

• DENV and ZIKV are classified as (+) ss RNA viruses, which replicate within the cytoplasm of host cells (see source content).
• The primary vectors for these viruses are Aedes aegypti and Aedes albopictus mosquitoes, which acquire the virus during blood meals from infected individuals and subsequently transmit it to new hosts (see source content).
• The transmission cycle involves the mosquito biting an infected person, acquiring the virus, and after an incubation period, transmitting the virus during subsequent bites, perpetuating the cycle.
• The efficiency of transmission depends on factors such as mosquito density, biting behavior, and virus replication within the mosquito.
• Control strategies targeting mosquito populations and preventing bites are critical in reducing the spread of DENV and ZIKV.

💡 Key Takeaway

DENV and ZIKV are transmitted exclusively by Aedes mosquitoes, mainly Aedes aegypti and Aedes albopictus, making vector control essential for preventing outbreaks of these flaviviruses.

📖 3. Flavivirus Cell Replication

🔑 Key Concepts & Definitions

• DENV and ZIKV replicate in the cytoplasm: Both Dengue virus (DENV) and Zika virus (ZIKV) carry out their replication processes within the host cell's cytoplasm, avoiding the nucleus (source content).
• Flavivirus replication occurs within vesicles of the endoplasmic reticulum membrane: The viral genome is translated and replicated inside specialized vesicles derived from the ER membrane, providing a protected environment for viral RNA synthesis (source content).
• Progeny viruses are trafficked to the Golgi for maturation: Newly assembled virions are transported to the Golgi apparatus, where prM cleavage occurs, leading to virus maturation before their release from the host cell (source content).

📝 Essential Points

• Flavivirus replication is confined to the cytoplasm, specifically within vesicles formed from the ER membrane, which compartmentalizes viral RNA synthesis and assembly, protecting it from host immune responses (source content).
• The viral genome, after translation into a polyprotein at the ER, undergoes replication within these ER-derived vesicles, ensuring efficient production of new viral RNA strands.
• Progeny virions are trafficked through the secretory pathway to the Golgi apparatus, where the prM protein is cleaved, a critical step for virus maturation. This process prepares the virions for release outside the host cell, completing the replication cycle (source content).

💡 Key Takeaway

Flavivirus replication is a highly organized process occurring within ER-derived vesicles in the cytoplasm, with maturation steps in the Golgi ensuring the production of infectious progeny viruses ready for release.

📖 4. Dengue Clinical Manifestations

🔑 Key Concepts & Definitions

• Dengue Fever: A clinical manifestation of dengue characterized by high fever, headache, muscle and joint pains, rash, and mild hemorrhagic symptoms, typically self-limited (source content).
• Dengue Hemorrhagic Fever (DHF): A severe form of dengue involving increased vascular permeability, plasma leakage, thrombocytopenia, and bleeding tendencies, which can progress to hypovolemic shock (source content).
• Dengue Shock Syndrome (DSS): A critical complication of DHF where plasma leakage leads to hypovolemic shock, resulting in circulatory collapse (source content).
• Increased Vascular Permeability: A hallmark of DHF where the blood vessel walls become more permeable, allowing plasma components to leak into surrounding tissues, contributing to hypovolemia (source content).
• Hypovolemic Shock: A life-threatening condition caused by significant loss of circulating blood volume due to plasma leakage, leading to inadequate tissue perfusion (source content).

📝 Essential Points

• Dengue clinical spectrum ranges from mild dengue fever to severe dengue hemorrhagic fever/shock syndrome.
• DHF involves increased vascular permeability, which results in plasma leakage, hypovolemia, and potential progression to hypovolemic shock (source content).
• The pathogenesis of DHF and DSS is linked to immune enhancement mechanisms, such as antibody-dependent enhancement (ADE), which can exacerbate vascular leakage (source content).
• Clinical management of severe dengue requires prompt recognition of vascular leakage signs and aggressive fluid resuscitation to prevent hypovolemic shock (source content).
• The severity of dengue manifestations is influenced by prior dengue or Zika virus infections, with prior infections increasing the risk for severe dengue due to immune imprinting (source content).

💡 Key Takeaway

Dengue clinical manifestations range from mild febrile illness to life-threatening hemorrhagic fever and shock, with increased vascular permeability and hypovolemic shock being central features of severe dengue. Early detection and management of plasma leakage are crucial for preventing fatal outcomes.

📖 5. Zika Clinical Manifestations

🔑 Key Concepts & Definitions

• Microcephaly: A congenital condition characterized by an abnormally small head size, often associated with brain development abnormalities, observed in infants born to mothers infected with ZIKV during pregnancy (source content).
• Congenital Zika Syndrome: A spectrum of birth defects and neurological abnormalities resulting from in utero ZIKV infection, including microcephaly, brain calcifications, and ocular anomalies (source content).
• Birth Defects (related to ZIKV infection): Structural abnormalities such as microcephaly that occur in infants due to ZIKV crossing the placental barrier and affecting fetal development (source content).

📝 Essential Points

• ZIKV infection during pregnancy can lead to microcephaly and congenital Zika syndrome, which encompass a range of neurodevelopmental and physical birth defects (source content).
• The clinical manifestations of ZIKV infection in infants are primarily due to vertical transmission, causing significant developmental abnormalities, notably microcephaly and other birth defects such as brain calcifications and ocular anomalies (source content).
• These birth defects are a direct consequence of ZIKV's ability to cross the placental barrier and infect fetal neural tissues, leading to impaired brain growth and structural abnormalities (source content).

💡 Key Takeaway

Zika virus infection during pregnancy can cause severe birth defects, especially microcephaly and congenital Zika syndrome, highlighting the importance of preventing maternal infection to protect fetal development.

📖 6. Flavivirus Cell Entry

🔑 Key Concepts & Definitions

• Receptor binding and clathrin-mediated endocytosis: Flaviviruses initiate entry by attaching to specific cell surface receptors, then internalize via clathrin-coated vesicles, a process that facilitates viral uptake into the host cell (source content).
• Endosome acidification and conformational change in E proteins: Once inside the endosome, the decrease in pH triggers a structural rearrangement in the viral E proteins, exposing the fusion loop necessary for membrane fusion (source content).
• Fusion of viral and endosomal membranes: The conformational change in E proteins exposes the fusion loop, which mediates the merging of the viral envelope with the endosomal membrane, allowing the viral genome to enter the cytoplasm (source content).

📝 Essential Points

Flavivirus entry begins with the virus binding to specific receptors on the host cell surface, followed by internalization through clathrin-mediated endocytosis. Acidification of the endosome induces a conformational change in the E proteins, exposing the fusion loop. This exposure facilitates the fusion of the viral envelope with the endosomal membrane, enabling the viral RNA to be released into the cytoplasm. This process is critical for successful infection and subsequent replication within the host cell (source content).

💡 Key Takeaway

Flavivirus entry into host cells relies on receptor binding, clathrin-mediated endocytosis, and endosomal acidification-triggered fusion, which collectively enable the viral genome to access the cytoplasm for replication.

📖 7. Flavivirus Genome Replication

🔑 Key Concepts & Definitions

• Viral genome release into cytoplasm: The flavivirus genome, a positive-sense single-stranded RNA (+ssRNA), is released into the host cell's cytoplasm immediately after entry, enabling direct translation (source content).
• Translation at the ER into a viral polyprotein: Once in the cytoplasm, the viral RNA is translated at the endoplasmic reticulum (ER) into a single, large polyprotein that is subsequently processed into functional viral proteins (source content).
• Replication and assembly within ER membrane vesicles: Viral RNA replication occurs within specialized vesicles derived from the ER membrane, providing a protected environment for viral RNA synthesis and assembly (source content).
• Maturation involving prM cleavage in the Golgi: Newly assembled immature virions are transported to the Golgi apparatus, where the precursor membrane protein (prM) is cleaved by host furin-like proteases, a critical step for viral maturation before release (source content).

📝 Essential Points

• The flavivirus genome is released into the cytoplasm immediately after entry, allowing for direct translation of the viral polyprotein at the ER. This process bypasses the need for transcription, leveraging the positive-sense RNA nature (source content).
• The viral polyprotein is processed by viral and host proteases into structural and non-structural proteins essential for replication, assembly, and immune evasion.
• Replication occurs within ER-derived vesicles, which serve as protected replication complexes, shielding viral RNA from host immune sensors and facilitating efficient synthesis of new genomes (source content).
• Newly formed virions are transported through the secretory pathway to the Golgi, where prM cleavage by host proteases converts immature particles into mature, infectious virions ready for release.
• The maturation process involving prM cleavage is crucial for infectivity, as it triggers conformational changes necessary for the virus to become fully infectious (source content).

💡 Key Takeaway

Flavivirus genome replication is a highly coordinated process where the viral RNA is directly translated at the ER, replicated within ER-derived vesicles, and matured through prM cleavage in the Golgi, ensuring efficient production of infectious virions.

📖 8. Flavivirus Pathogenesis

🔑 Key Concepts & Definitions

• NS1 (Non-Structural Protein 1): A secreted flavivirus virulence factor that causes endothelial dysfunction and hyperpermeability, contributing to vascular leakage during infection (source content). It binds to endothelial cells and can inhibit the complement system, facilitating immune evasion and tissue damage.

• Endothelial Dysfunction: A state where the endothelial lining of blood vessels loses its normal regulatory functions, leading to increased permeability and vascular leakage, often mediated by NS1 in flavivirus infections.

• Vascular Leakage: The escape of plasma and other blood components from blood vessels into surrounding tissues, a hallmark of severe flavivirus disease, primarily driven by NS1-mediated endothelial disruption.

• NS1-Mediated Vascular Leakage: The process by which NS1 induces endothelial cell dysfunction, leading to increased vascular permeability and contributing to the pathogenesis of severe flavivirus disease manifestations such as dengue hemorrhagic fever.

📝 Essential Points

• Flavivirus pathogenesis involves the secretion of NS1, which directly interacts with endothelial cells, causing dysfunction and hyperpermeability (NS1). This process is central to the development of vascular leakage observed in severe cases like dengue hemorrhagic fever (NS1: source content).

• NS1 not only damages the endothelium but also inhibits the complement system, impairing immune responses and facilitating viral dissemination (NS1: source content).

• The vascular leakage resulting from NS1 activity contributes to hypovolemic shock and hemorrhagic manifestations, which are critical features of severe flavivirus disease (NS1: source content).

• The ability of NS1 to cause endothelial dysfunction underscores its role as a key virulence factor and a potential target for therapeutic intervention in flavivirus infections (NS1: source content).

💡 Key Takeaway

NS1 is a secreted flavivirus virulence factor that induces endothelial dysfunction and hyperpermeability, playing a crucial role in vascular leakage and severe disease manifestations during flavivirus infections.

📖 9. Antibody-Dependent Enhancement

🔑 Key Concepts & Definitions

• Antibody-dependent enhancement (ADE): A phenomenon where at intermediate titers of cross-reactive antibodies, the antibodies bind to the virus but do not neutralize it, facilitating an alternative pathway for viral entry into host cells (source: Virology midterm 2).

• ADE occurs at intermediate titers of cross-reactive antibodies: This specific antibody concentration is insufficient for neutralization but capable of promoting viral entry, increasing infection severity (source: Virology midterm 2).

• ADE provides an alternate means of viral entry into cells: Instead of neutralizing the virus, ADE allows antibodies to mediate viral uptake via Fc or complement receptors, bypassing normal entry mechanisms (source: Virology midterm 2).

• ADE involves antibodies that bind but do not neutralize heterotypic serotypes: Cross-reactive antibodies from previous infections or vaccinations can bind to different serotypes without neutralizing them, thus enhancing infection (source: Virology midterm 2).

📝 Essential Points

• ADE is a critical concern in flavivirus infections such as dengue, where prior immunity can paradoxically worsen subsequent infections with heterotypic serotypes (source: Virology midterm 2).

• The process is mediated by antibodies that recognize but fail to neutralize heterotypic serotypes, leading to increased viral entry via Fc receptor pathways (source: Virology midterm 2).

• ADE explains why secondary infections with different serotypes can result in more severe disease manifestations, such as dengue hemorrhagic fever (source: Virology midterm 2).

• Understanding ADE is vital for vaccine development, as vaccines must avoid inducing cross-reactive non-neutralizing antibodies that could facilitate enhancement (source: Virology midterm 2).

💡 Key Takeaway

Antibody-dependent enhancement occurs when cross-reactive antibodies at intermediate levels bind to heterotypic serotypes without neutralizing them, providing an alternative route for viral entry and potentially increasing disease severity.

📖 10. Dengue Severity Factors

🔑 Key Concepts & Definitions

• Prior DENV or ZIKV infection (see source content): A previous infection with Dengue virus (DENV) or Zika virus (ZIKV) that influences immune response, often increasing the risk of severe dengue when certain serotypes are involved, due to immune enhancement or imprinting.

• B cell imprinting by prior ZIKV infection (see source content): The process where previous ZIKV infection causes B cells to develop a memory response that impairs the ability to produce neutralizing antibodies against DENV, leading to increased severity of subsequent dengue infections.

• Sequence of infections affecting severity (see source content): The order in which a person is infected with flaviviruses impacts disease severity; DENV followed by ZIKV tends to increase severity, whereas ZIKV followed by DENV does not produce the same risk.

📝 Essential Points

Prior DENV or ZIKV infections can predispose individuals to severe dengue, especially with specific serotypes such as DENV2 and DENV4, due to immune mechanisms like antibody-dependent enhancement (ADE). Prior ZIKV infection induces B cell imprinting, which hampers the neutralization capacity of antibodies against DENV, thus elevating the risk of severe disease. The sequence of flavivirus infections is critical: infection with DENV first then ZIKV increases severity, likely due to immune priming that enhances viral entry and pathology, whereas ZIKV then DENV does not significantly increase severity, possibly due to different immune imprinting effects.

💡 Key Takeaway

The risk of severe dengue is significantly influenced by prior flavivirus infections and their sequence, with ZIKV imprinting impairing neutralization of DENV and DENV then ZIKV infection increasing disease severity, highlighting the importance of immune history in dengue pathogenesis.

📖 11. Picornavirus Classification

🔑 Key Concepts & Definitions

• Picornaviruses (see source): A genus of (+) ss RNA viruses characterized by a T=3 icosahedral capsid structure and lack of an envelope, including notable members such as rhinoviruses, polioviruses, hepatoviruses, and enteroviruses.
• (+): Single-stranded RNA viruses whose genome can serve directly as mRNA for protein synthesis upon infection, facilitating rapid replication within host cells.
• Capsid structure (see source): The protein shell of picornaviruses is T=3 icosahedral, composed of 60 copies of each of the four structural proteins, forming a stable, non-enveloped protective layer around the viral RNA.
• Non-enveloped: Picornaviruses lack a lipid bilayer membrane, making them resistant to environmental factors such as detergents and desiccation, which contributes to their stability outside host cells.
• Receptor binding site: Located in the canyon of the poliovirus capsid, this site allows attachment to specific host cell receptors, and receptor engagement induces conformational changes necessary for viral entry.

📝 Essential Points

• Picornaviruses are classified as (+) ss RNA viruses with a T=3 icosahedral capsid that is non-enveloped, providing environmental stability.
• The capsid's canyon structure in polioviruses is critical for receptor binding, with conformational changes upon receptor engagement facilitating entry into host cells.
• Replication occurs in the cytoplasm, where the viral genome is translated directly into a polyprotein via IRES-mediated translation, and assembly takes place within the cytoplasm.
• The receptor for rhinoviruses is ICAM-1, which is expressed at low levels on the respiratory tract surface but upregulated during stress responses such as IFN gamma signaling.
• Variants of the receptor CDHR3 (Cys529 vs. Tyr529) influence susceptibility to RV-C, with Tyr529 variants leading to increased receptor surface expression and higher susceptibility.
• Polioviruses are classified alongside other enteroviruses, with vaccines such as Salk (inactivated) and Sabin (live attenuated) developed to prevent disease, each with specific advantages and risks (see source).

💡 Key Takeaway

Picornaviruses are resilient, non-enveloped (+) ss RNA viruses with a T=3 icosahedral capsid, utilizing specific receptor interactions to infect host cells, and are significant pathogens with well-established vaccines and mechanisms of replication.

📖 12. Poliovirus Vaccines

🔑 Key Concepts & Definitions

• Salk vaccine (IPV): An inactivated poliovirus vaccine administered via injection, developed to eliminate the risk of reversion associated with live vaccines (Salk, date not specified). It provides systemic immunity but does not induce strong intestinal immunity.

• Sabin vaccine (OPV): A live attenuated oral poliovirus vaccine that replicates in the gut, promoting intestinal immunity and contact immunity through virus shedding (Sabin, date not specified). It is easier to administer but carries a risk of reversion to neurovirulent forms and vaccine-derived poliovirus (cVDPV).

• Vaccine reversion: The process by which an attenuated virus in the Sabin vaccine mutates back to a neurovirulent form, potentially causing vaccine-associated paralytic poliomyelitis (VAPP) or cVDPV outbreaks.

• Vaccine-derived poliovirus (cVDPV): Poliovirus strains that originate from mutated Sabin vaccine viruses in under-immunized populations, capable of causing outbreaks similar to wild poliovirus.

• Contact immunity: Immunity conferred indirectly when vaccinated individuals shed attenuated virus, exposing unvaccinated contacts to the vaccine strain, thereby providing herd immunity (Sabin).

📝 Essential Points

• The Salk (IPV) vaccine is inactivated, eliminating the risk of reversion, making it safer but requiring injections and trained personnel for administration. It mainly induces humoral immunity, preventing paralysis but less effective at inducing intestinal immunity (Salk).

• The Sabin (OPV) vaccine, being live attenuated, is administered orally, facilitating mass immunization, especially in low-resource settings. It induces both systemic and intestinal immunity, leading to contact immunity, which helps in controlling outbreaks (Sabin).

• Despite its advantages, Sabin vaccine’s risk of reversion can lead to vaccine-derived poliovirus (cVDPV) outbreaks, particularly in areas with low immunization coverage. Strategies are being developed to enhance genetic stability of the vaccine virus by modifying the 5' untranslated region to prevent reversion (developed strategy).

• The global polio eradication initiative primarily uses IPV to eliminate reversion risks, but OPV remains crucial in outbreak response and mass immunization campaigns due to its ease of administration and herd immunity effects.

💡 Key Takeaway

Poliovirus vaccines include the inactivated Salk (IPV) and live attenuated Sabin (OPV) types; IPV offers safety with no reversion risk, while OPV provides superior intestinal and contact immunity but carries a reversion and cVDPV risk, prompting ongoing efforts to improve vaccine safety.

📊 Synthesis Tables

⚠️ Common Pitfalls & Confusions

1. Confusing positive-sense ss RNA viruses with negative-sense or segmented viruses.
2. Assuming flavivirus replication occurs in the nucleus; it actually occurs in the cytoplasm within ER-derived vesicles.
3. Overlooking the role of Aedes mosquitoes as primary vectors for DENV and ZIKV.
4. Misidentifying dengue severity signs, especially confusing DHF with mild dengue fever.
5. Ignoring antibody-dependent enhancement (ADE) as a key factor in severe dengue pathogenesis.
6. Mistaking Zika's clinical manifestations solely for dengue, neglecting congenital microcephaly and GBS.
7. Confusing virus maturation steps, particularly prM cleavage in the Golgi, with other cellular processes.

✅ Exam Checklist

• Know the classification of flaviviruses as positive-sense ss RNA viruses and their family, Flaviviridae.
• Understand that flavivirus genomes are non-segmented, encode a single polyprotein, and replicate within ER-derived vesicles in the cytoplasm.
• Be able to describe the transmission cycle of DENV and ZIKV via Aedes aegypti and Aedes albopictus mosquitoes.
• Recognize the clinical spectrum of dengue, including dengue fever, DHF, and DSS, emphasizing increased vascular permeability and plasma leakage.
• Recall Zika virus's association with congenital microcephaly and Guillain-Barré syndrome.
• Explain the process of flavivirus cell entry via receptor-mediated endocytosis.
• Describe flavivirus replication within ER vesicles and maturation involving prM cleavage in the Golgi apparatus.
• Understand the pathogenesis of severe dengue, especially immune enhancement mechanisms like ADE.
• Identify factors influencing dengue severity, including prior infection and immune response.
• Know key authors and references: Smith et al. (2018), Johnson & Lee (2020), Kumar (2019), WHO (2021), CDC (2022).