Viruses Flashcards
1
Q
Explain + draw viral structure in relation to influenza.
A
- Pleomorphic (spherical and filamentous forms)
- enveloped
- strand segmented RNA genome
- eight RNA molecules spanning 13.5 kilobases (encoding 11 proteins)
- RNA dependent RNA polymerase without proof reading function
- frequent mutations
- influenza types A,B,C (genera)
- febrile, respiratory illness with systemic symptoms
- There are four types of influenza viruses, A, B, C, and D. Influenza types A and B cause human infection annually during the epidemic season. Influenza A has several subtypes according to the combination of hemagglutinin (H) and the neuraminidase (N) proteins that are expressed on the surface of the viruses. There are 18 different hemagglutinin subtypes and 11 different neuraminidase subtypes (H1-18 and N1-11).
- Type A: Birds, mammals, humans
- Type A can jump between species
- Type B, C: mainly humans
2
Q
Outline the structure and function of hemagglutination and neurminidase proteins and how they’re important.
A
Hemagglutinin (HA) and neuraminidase (NA) are two major surface glycoproteins found on the influenza virus, and they play crucial roles in the virus’s life cycle and interaction with host cells.
-
Hemagglutinin (HA):
- Structure: Hemagglutinin is a trimeric glycoprotein with three identical subunits. Each subunit consists of two domains: the globular head and the stem region. The head contains the receptor-binding site, while the stem anchors the protein to the viral envelope.
- Function: Hemagglutinin’s primary function is to facilitate the entry of the virus into host cells. The receptor-binding site on the head region specifically recognizes and binds to sialic acid residues on the surface of host cells. This interaction allows the virus to attach to the host cell membrane.
- Importance: Hemagglutinin is a key determinant of host specificity and is a major target for the host’s immune response. The ability of hemagglutinin to bind to specific receptors on host cells influences the tissue tropism of the virus and its ability to cause infection. Additionally, it is a major antigenic determinant, and changes in the HA protein contribute to the seasonal variation of influenza viruses, requiring updates in the composition of influenza vaccines.
-
Neuraminidase (NA):
- Structure: Neuraminidase is a tetrameric glycoprotein with four identical subunits. Each subunit consists of a globular head attached to a slender stalk region that anchors the protein to the viral envelope.
- Function: Neuraminidase plays a crucial role in the release of newly formed virus particles from infected cells. After influenza viruses have replicated inside host cells, they bud from the cell surface, and neuraminidase cleaves the sialic acid residues on the host cell membrane. This enzymatic activity prevents newly formed virions from aggregating on the cell surface and facilitates their release.
- Importance: The neuraminidase activity is essential for the efficient spread of the virus within the host and the transmission of the virus to new hosts. Inhibition of neuraminidase by antiviral drugs, such as oseltamivir and zanamivir, can reduce the severity and duration of influenza infections.
Importance of HA and NA in Influenza:
- Viral Entry and Release: The interaction of HA with host receptors is essential for viral entry, while NA facilitates the release of viral progeny from infected cells.
- Antigenic Variation: The constant evolution of HA and NA through genetic changes, such as antigenic drift and shift, is a major factor contributing to the variability of influenza viruses and the need for regular updates in influenza vaccines.
- Antiviral Targets: Hemagglutinin and neuraminidase are important targets for antiviral drugs. Inhibitors targeting these proteins can help control influenza infections.
3
Q
Discuss the specific structure anf function of Neurminidase and haemagglutitin in influenza A.
A
1.Hemagglutinin (HA):
Structure:
- HA is a glycoprotein located on the surface of the influenza A virus.
- It forms a trimeric structure, with each monomer consisting of two subunits: HA1 and HA2.
- The HA1 subunit contains the receptor-binding site, crucial for the virus to attach to host cells.
- The HA2 subunit anchors the protein to the viral envelope.
Function:
- Viral Entry: The primary function of HA is to mediate the binding and entry of the virus into host cells. The receptor-binding site on HA recognizes and binds to sialic acid residues on the surface of host cells.
- Proteolytic Cleavage: The HA protein undergoes proteolytic cleavage during the viral life cycle, which is essential for its activation. The cleavage activates the fusion activity of HA2, allowing the virus to enter the host cell.
Proteolytic Cleavage:
- The proteolytic cleavage of HA is a crucial step for influenza A virus infectivity.
- In some strains of influenza A, the cleavage occurs extracellularly by host proteases (e.g., trypsin) and allows the virus to enter a wide range of cell types.
- In other strains, the cleavage occurs intracellularly after endocytosis by host proteases in the endosome, limiting the virus’s ability to infect specific cell types.
- Cleavage activation is a determinant of the virus’s pathogenicity and tissue tropism.
2. Neuraminidase (NA):
Structure:
- NA is another glycoprotein found on the surface of the influenza A virus.
- It forms a tetrameric structure, with each monomer consisting of a globular head and a slender stalk that anchors the protein to the viral envelope.
Function:
- Release of Viral Particles: Neuraminidase plays a crucial role in the release of newly formed virus particles from infected cells. After replication, viruses bud from the host cell surface, and NA cleaves sialic acid residues on the host cell membrane.
- Prevention of Viral Aggregation: The enzymatic activity of NA prevents the aggregation of newly formed virions on the cell surface, allowing their efficient release and spread.
Mucus Movement:
- In the context of mucus movement, neuraminidase has been associated with facilitating the penetration of mucus by the virus.
- Mucus contains sialic acid residues on its surface, and the neuraminidase activity helps the virus to cleave these sialic acids, reducing the viscosity of mucus and allowing the virus to move more freely.
Importance in Influenza A:
- Proteolytic cleavage of HA is a determinant of the virus’s infectivity and pathogenicity.
- Neuraminidase inhibitors (e.g., oseltamivir) are important antiviral drugs that block the enzymatic activity of NA, reducing the severity and duration of influenza infections.
4
Q
Disucss infulenza Pathogenesis and pathology.
A
- Virus spreads from person to person by airborne droplets or by contaminated hands or surfaces
- Avian virus spreads via droplets from birds
- A few cells of infected respiratory epithelium initiate virus replication
- Short incubation period (1-4 days) depending on virus dose
- Virus HA or NA specific secretory IgA or non specific secreted inhibitors (defensins) may prevent or slow down infection
- Progeny virus spreads to neighboring cells requiring NA, virus spread is slower in the presence of anti NA antibodies or NA inhibitors
- Virus shedding starts 1 day before the onset and last for 7 days
- Cellular destruction and desquamation of superficial layer of respiratory tract (epithelial cells and macrophages) but does not affect the basal layer
- Reparation of damage takes 1 month
5
Q
Explain the innate and adaptive immune response to infulenza.
A
INNATE
- Clilary Escalator: Thismovement of mucus moves at approximately 1mm per minute in peripheral airways, clearing the healthy lung in less than 24 hours. The mucociliary escalator is a major barrier against infection.
- Tissue Destruction: Influenza virus targets mainly airway and alveolar epithelial cells. Rapid replication of the virus in these cells disturbs the cellular function and damages tissue as well as causing production of a huge number of dead cells in the lung of the infected host.
- INNATE RESPONSE:
- Influenza infection elicits the production of interferons, pro-inflammatory cytokines and chemokines in the infected epithelial cells and in macrophages and dendritic cells
- Interferons protect neighboring cells by inducing an antiviraI state
- Interferons activate other innate and adaptive immune antiviral mechanisms
- Chemokines attract neutrophil granulocytes that eliminate virus and virus infected cells but amplify inflammation
- Pro-inflammatory cytokines induce local inflammation leading to pathogen and tissue destruction and elicit systemic effects
- Tissue damage further activates immune responses
- Overly activated responses cause fatal complications!
ADAPT
- Long lived subtype specific protective immunity by HA and NA antibodies. Resistance is mediated by anti HA, reduced severity by NA
- Secretory IgA is relevant
- Cell mediated immunity to type specific proteins clears infected cells in established infection
- No cross resistance between types or subtypes for antibodies, some by cellular responses within types
6
Q
Explain the antigenic changes of influenza virus.
A
Antigenic drift and antigenic shift are terms used to describe two different mechanisms by which influenza viruses undergo changes in their surface proteins, particularly hemagglutinin (HA) and neuraminidase (NA). These changes can impact the virus’s ability to infect and spread in populations and are crucial considerations for public health and vaccine development.
-
Antigenic Drift:
- Definition: Antigenic drift refers to the gradual accumulation of small, incremental changes in the influenza virus’s surface proteins over time.
- Mechanism: The genetic material of influenza viruses, particularly RNA, is prone to mutations during replication. As the virus replicates within a host, these mutations may lead to changes in the amino acid sequence of the HA and NA proteins.
- Impact: These subtle changes in the virus’s surface proteins can result in alterations to the virus’s antigenic properties. Over time, this accumulation of mutations may lead to a strain that is antigenically different from previously circulating strains.
- Consequences: Antigenic drift is a common phenomenon in influenza viruses and is responsible for the seasonal variation in flu strains. It is one reason why new flu vaccines are developed regularly to match the circulating strains.
-
Antigenic Shift:
- Definition: Antigenic shift refers to a major, sudden change in the influenza virus, resulting in a novel subtype with surface proteins that are markedly different from those of previously circulating strains.
- Mechanism: Antigenic shift occurs when two different influenza viruses infect the same host cell, allowing their genetic material to reassort. This reassortment can lead to the emergence of a new influenza virus subtype with a combination of surface proteins not previously seen.
- Impact: Because the new virus has a significantly different antigenic profile, it may evade existing immunity in the population. This can lead to the rapid spread of the new strain and potentially cause pandemics.
- Consequences: Antigenic shift is responsible for more dramatic and unpredictable changes in influenza viruses. Pandemics, such as the 2009 H1N1 pandemic, often result from antigenic shift events.
7
Q
Briefly outline historically known influenza pandemics.
A
Spanish(H1N1); occurred from 1918-1920, 40-100 million deaths.
Hong Kong(H3N2); occurred from 1968-1969, 1 million deaths.
Russian(H1N1); occurred 1977, 0.75 million deaths.
8
Q
How many herpes viruses have been identified. Be detailed.
A
- Historically, single herpesvirus family historically based on virion morphology.
- Growth characteristics and cellular tropism defined three subfamilies:
- α – neurotropic viruses
- β – diverse cellular tropic viruses
- γ – lymphotropic/endotheliotropic viruses
- Based on genetic relatedness, alloherpesviruses (amphibian and fish) and single malacoherpesvirus (bivalve molluscs) are new separate families, related by structure but not gene or protein sequence.
- Currently 8 human herpesviruses have been identified:
- α – (3) Herpes Simplex 1 & 2 and varicella zoster virus
- β – (3) Cytomegalovirus (CMV) and herpesvirus 6 & 7.
- γ – (2) Kaposi’s Sarcoma Herpesvirus and Epstein Barr virus (EBV).
9
Q
How do herpesviruses replicate?
A
- Herpesvirus virion is composed of >30 different proteins.
- Virions contain multiple glycoproteins (gB, gL and gH) that mediate attachment and entry.
- Heparan sulfate is a common attachment receptor, but other receptors are less clearly defined.
- Capsid transports DNA to the nucleus and uncoats with linear genome circularizing.
- Capsids are assembled in the nucleus and packed with a single ds genome copy.
- Two distinct envelopment steps (1°at nuclear membrane; 2° at distal assembly compartment – multivesicular bodies, MVBs).
- Budding of MVBs with plasma membrane release progeny virus.
- Herpesvirus genes expressed in kinetic waves Immediate Early (IE), Early (E) and Late (L).
- Viral DNA replication is required for L gene expression.
- IE: Transcription factors, immune response modifiers.
- E: DNA replication machinery, and more host response modifiers.
- L: Virion structural proteins.
10
Q
What are some replication strategies employed by herpesviruses.
A
- All herpesviruses persist for the life of the host.
- Following acute infection, latency or chronic low level replication.
- Large coding capacity and dedicate >25% of their genome to encode modifiers of the host system.
DRAW MIND MAP (site and silence, stealth and sabotage)
11
Q
Discuss ‘site and silence’ and ‘stealth and sabatoge’
A
Site and silence
- All herpesviruses to a lesser or greater degree establish latency in specific cell types.
- α-herpesviruses establish latency in neurons (dorsal root ganglia), and reactivate periodically.
- For VZV, reactivation in dorsal root ganglia infected during chicken pox as a child causes shingles in older age.
- Low level persistent replication is an additional characteristic of β and γ herpesviruses.
DRAW FROM NOTION
Stealth and sabatoge
- Evasoion of CD8+ T cells is critical for superinfection by CMV
- Novel capacity of CMV to reinfect the CMV-immune host.
- Deletion of 4 separate MHC-I down-modulators prevents re-infection.
- Only affects re-infection ‘window’. Once re-infection has occurred absence of MHC-I downmodulators does not affect persistence.
12
Q
What are the three phases of immune responses against herpes? Explain in depth and try to use figures.
A
-
Innate Immune Response:
-
Recognition and Immediate Defense:
- The innate immune response is the first line of defense and acts rapidly upon encountering the herpesvirus.
- Pattern recognition receptors (PRRs) on various cells, such as macrophages and dendritic cells, recognize pathogen-associated molecular patterns (PAMPs) on the virus, triggering an immediate response.
- Natural killer (NK) cells play a crucial role in detecting and eliminating virus-infected cells.
-
Interferon Production:
- Infected cells release signaling molecules, including interferons, to alert neighboring cells about the presence of the virus.
- Interferons induce an antiviral state in nearby cells, inhibiting viral replication and spread.
-
Inflammatory Response:
- Inflammation is triggered to recruit immune cells to the site of infection.
- Neutrophils and other immune cells help contain and control the virus.
-
Recognition and Immediate Defense:
-
Adaptive Immune Response:
-
Antigen Presentation:
- Antigen-presenting cells, such as dendritic cells, engulf and process viral antigens.
- Viral peptides are presented on the cell surface by major histocompatibility complex (MHC) molecules.
-
T Cell Activation:
- CD4+ T helper cells recognize viral peptides presented on MHC class II molecules, stimulating the production of cytokines and activating other immune cells.
- CD8+ cytotoxic T cells recognize and kill virus-infected cells by directly destroying them.
-
B Cell Activation and Antibody Production:
- B cells recognize viral antigens and get activated with the help of T cells.
- Activated B cells differentiate into plasma cells, producing antibodies specific to the virus.
- Antibodies neutralize the virus, mark it for destruction, and enhance other immune responses.
-
Antigen Presentation:
-
Memory Response:
-
Establishment of Immunological Memory:
- Some activated T and B cells transform into long-lived memory cells.
- Memory cells “remember” the virus and can respond more rapidly and effectively upon re-exposure.
-
Long-Term Protection:
- Immunological memory provides long-term protection against herpesvirus.
- If the individual encounters the same virus again, memory B cells quickly produce antibodies, and memory T cells rapidly initiate a targeted immune response.
-
Establishment of Immunological Memory:
13
Q
Discuss drug-inhibitors for herpes viruses(a,b,c)?
A
- Pharmacological agents target differences between the host and the pathogen.
- Most current drugs target viral DNA Polymerase.
- Acyclovir (ACV) & valacyclovir – guanosine analogues: HSV-1>HSV-2>VZV. Must be converted to active form by viral thymidine kinase (TK). ‘Safety Gate’ concept!
- Ganciclovir & valganciclovir (Cidofovir, second-line) – Primarily CMV disease. Converted to final active form by UL97 and then cellular kinases. ‘Safety Gate’ concept, again!
- Antisense to mRNA:
- Fomivirsen – 21 nucleotide antisense inhibitor of IE region: Primarily for intraocular CMV.
- DNA Packaging (Terminase) inhibitors:
- Letermovir – Recently (2017) FDA-approved for CMV prophylaxis in HSCT recipients. Prevents DNA genome cleavage resulting in non-infectious, long DNA. Specific for CMV.
14
Q
Discuss the importance of a safety gate in inhibitory drugs.
A
The “safety gate” concept is crucial in the context of certain antiviral drugs, particularly those that rely on activation by viral kinases. The concept ensures that the drugs are selectively activated within infected cells by specific viral enzymes, minimizing their impact on uninfected cells. This selectivity is important for therapeutic efficacy and minimizing potential side effects.
Several antiviral drugs, such as acyclovir, ganciclovir, and valganciclovir, rely on a viral kinase for their activation. Here’s why the safety gate concept is significant:
-
Selective Activation:
- These drugs are typically administered in an inactive form and need to be converted to their active form to exert their antiviral effects.
- The viral kinase (e.g., thymidine kinase for acyclovir or UL97 for ganciclovir) is present only in infected cells, ensuring that the drug is activated selectively within those cells.
-
Reduced Impact on Uninfected Cells:
- The absence of the specific viral kinase in uninfected cells reduces the likelihood of drug activation in healthy tissues.
- This selectivity is crucial for minimizing potential toxicity and side effects associated with the drugs, as they primarily act on cells actively undergoing viral replication.
-
Increased Therapeutic Window:
- The safety gate concept contributes to a wider therapeutic window, allowing for effective inhibition of viral replication with lower toxicity to host cells.
- A broader therapeutic window provides clinicians with more flexibility in drug dosing and reduces the risk of adverse effects in patients.
-
Prevention of Resistance Development:
- Viral kinases targeted by these drugs are essential for the virus’s replication cycle.
- By relying on viral kinases for activation, the drugs reduce the likelihood of resistance development in the virus, as mutations that affect the activation process may also impair viral fitness.
15
Q
Discuss herpes a,b,c…
A
A
- Following primary infection of sensory neurons, HSV and VZV (chickenpox) establish latency in dorsal root ganglia.
- In VZV infection, oropharyngeal portal of entry is silent, but transported by tonsillar T cell to skin. HSV infection of skin is direct.
- Latency very different. VZV latency characterized by expression of restricted set of IE and E proteins. HSV only makes ‘latency-associate transcripts’ (LATs) and no proteins.
- VZV reactivation very rare (herpes zoster ‘shingles’). HSV reactivation very common (3-5 times/yr).
- VZV reactivation results in productive infection, spreads within ganglion and kills multiple neurons affects entire dermatome. HSV limited to sensory field of single neuron without neuronal death.
B
- Infection common in daycare and from mother during nursing.
- Except for mild mononucleosis during primary infection (10% of cases) does not cause disease in immunocompetent host.
- Latency in hematopoietic cells (CD34+), but also chronic, persistent low level replication.
- Can cause severe disease in immunosuppressed:
– Leading cause of infectious deafness from congenital infection (1 in 750 symptomatic).
– Primary infection of CMV sero-negative women during pregnancy.
– Prior to HAART major cause of CMV retinitis in AIDS patients.
– Remains major source of disease in transplant patients.
Y
- 25-50% primary EBV infections result in infectious mononucleosis, with 10-50% circulating B cells EBV infected. Otherwise chronic infection asymptomatic in immune-normal individuals.
- In EBV infection, oropharyngeal portal of entry, followed by colonization and expansion in epithelial cells and B lymphocytes. Initial expression of lytic antigens (VCA) followed by latent antigens (EBNA).
- High levels of EBV excreted in saliva during acute infection, but then low, but persistent levels for life. Low level of latently infected B cells persist for life (1-50 cells / million).
- Disease in immunosuppressed (transplant, HIV, cancer): oral hairy leukoplakia, post-transplant lymphoproliferative disease (PTLD), B cell lymphomas, Endemic Burkitt’s Lymphoma (most common cancer of children in Equatorial Africa), Hodgkin’s Lymphoma.
