Viral Immune Evasion

Acute viral infection model

Infection Graph

• generally incubation period - couple of days (2-3)

  • virus replication period until high enough conc to cause disease

  • acute illness for a week

  • recovery and long term immunity

• Immunity graph

  • interferon production → begins during incubation = rapid initial immune response

    • dies down as adaptive response starts during peak acute infection period

      • Type 1 interferon (alpha and beta)

      • → cells more resistant to virus replication

      • enhance antigen presentation

  • during end of acute infection → killer T cells [Th0 differention]

    • destruction of viral infected cells

      • before replication cycle completed

      • more virus released into intracellular fluid → targetted by antibodies (IgG = neutralising antibodies)

  • Antibodies from T helper → activates B cells → plasma cells → antibody formation (CAUSES declining acute phase)

    • IgG = virus neuralising antibody

    • IgA of mucosa

    • extracellular fluid neutraliser

    • prevents viral spread (cell→ cell, host→host)

Persistent Infection - viral model

• incubation + protracted clinical disease

Interferons (type 1 - alpha, beta, omega)

• Made by virus infected cells

• Local paracrine → neighbours more resistant

  • Degrade viral mRNA

  • Reduce viral protein synthesis

  • Increased MHC class 1 production → antigen processing and presentation

Interferon and Pestiviruses

1. presence of viral dsDNA or dsRNA in cytoplasm detected by PRR

2. causes cell signalling event → transcription and translation of type 1 interferons (IFNS)

3. IFNS synthesised and secreted

4. acts in paracrine manner → binds to IFN receptor on neighbouring cells

5. upregualation of viral resistance genes

   Pestiviruses (Flaviviridae)

   BVDV, Japanese Encephalitis, Louping ill, West Nile Virus

   • prevent interferon production

   • virulence factors = viral early proteins → facilitate viral replication by downregulating IFN response

     1. bind to PRR (stop signalling to nucelus - within cell cytoplasm)

     2. binding directly to IFN receptor

Avoiding adaptive immunity → hiding in immune priviledged

Brain

Eyes

Gonads

• immunopriviledged sites (protection form inflammatory and destructive immune processes)

• blood brain barrier → physical defence → prevents lymphocyte transmigration and Ab transfer (+ pathogens)

• 2 layers

  • tight junctions capillary endothelium

  • astrocyte tight junctions

• viruses can delib → enter immune privileged sites (Rabies)

Virus neutralising antibody

• virus must bind to host cell receptor before internalised

• B cells RECOGNISE - viral surface antigens

Neutralising antibody prevents attachment of viral attachment protein to host cell receptor

• virus coated in antibody → no cell internalisation → no replication → no cell damage

IgA - mucosal epithelial cells

IgG - protects tissue cells

Avoid VN antibody → Avoid extracellular fluid

• Syncytium formation

• synthesis non-structural fsuing protein

• allows coalescing/fusing of plasma membrane

• forms multinucleated syncytium

• continuous transmission

• gains access to new cellular machinery

Example

Respiratory syncytial virus pneumonia → calves

Avoid VN antibody → Transformation via oncogenic retroviruses

Oncogenic retroviruses integrate DNA (insertional mutagenesis) into host genome

• undergoes maligant transformation

• upstream → transcription facor

• in middle → disrupt DNA

viral genome replicated with each cell division

Virus + cell undergoes uncontrolled proliferation

Example: FeLV

• infects premature T cells in thymus

• Fit 1 gene inserted

Endogenous Retroviruses (mammlian genomes, accumulated over env)

• virus genome integration of germ-line cells that been propagated

• xenotransplacentation → concern → reactivation of virus in human

Avoidance of Antibody via antigen decoys

• fooling immune system into creating abtibodies against an antigen that is not a virulence factor/not important to viral function

1. Ebola virus decoy antigen

   • first secrete similar but soluble structural protein

   • saturate antibody binding sites

   • then ebola decloaks (genome free in cytoplasm) and replicates

   • new generation of virus release, neutralising antibody already complexed

   • viral attachment protein binds to host cell receptor

2. decoy antigens on surface (often enveloped viruses)

   • distracts immune system

   • impacts vaccine development

   • use subunit vaccine (only use viral attachment protein) instead of whole virus that contains the decoy

   • can be positive on ELIZA due to presence of decoy → not protective against virus

Antigenic Variation

Drift - spontaneous mutation

• mistake in rep → changes AA chain → chain protein → change in 3D shape - changes EPITOPE (sub-element) of viral antigen

  • bit of the protein that interacts with immune system

Shift - reassorment → radical change in antigen expression

• from a chimeric/hybrid virus

Antigenic variation → impacts on pathogenicity and tissue/species specificity

• FPV mutation → virus can now bind to canine enterocyte transferrin receptors → CPV

• Feline calicivirus (cause of cat flu) → different strains via antigenic drift

  • F9 modified live vaccine → induces neutralising antibody production

    • based on specific surface antigen

  • Variable protection V different strains depending on antigens

CD8 Killer T cells

• detect MHC Class I of virus peptide on surface of infected cells

Killing mechanisms

1. Degranulation

   • Perforin → perforates CSM

   • Granzymes → caspase cascade → apoptosis

2. Fas-Fas L interaction (with ligand) → caspase cascade

3. Binding of TNF cytotoxic cytokines induced cell death → via caspase cascade

   • TNF-α binds to TNF receptor 1 (TNFR1) on the cell surface.

Normal MHC Class 1 pathway

1. INTRACELLULAR antigen degraded by proteasome (enzyme complex) into peptide fragment

2. Fragment transported to ER via TAP transporter (transporter of antigen processing

3. MHC molecules synthesisd by host cell is assembled in ER using chaperone proteins

4. Peptides fragment loades onto empty MHC molecule

5. MHC class 1:peptide complex → Golgi → host cell surface

6. MHC class 1:peptide complex detected by CD8+

Normal MHC Class 1 pathway INHIBITION

• MHC inhibition reduces adaptive immune susceptibility

• BUT INCREASES NK cell response

• decreased MHC class 1 → seen as foreign kill

1. INTRACELLULAR antigen degraded by proteasome (enzyme complex) into peptide fragment

2. Fragment transported to ER via TAP transporter (transporter of antigen processing

   • Viral proteins block TAP transporters → peptide not in ER

3. Peptide fragement and MHC bind in ER

4. MHC class 1:peptide complex → Golgi → host cell surface

   • Viral proteins ‘glue’ MHC1:peptide complex to ER → no movement to Golgi → CSM

   • Viral protein causes complex → cytoplasm instead of CSM → degraded by proteosome

5. MHC class 1:peptide complex detected by CD8+ → immune response

   • retrovirus → superantigen production → nonspecific MHC and TCR binding → immune response to MULTIPLE ANTIGENS → dilutes the effective virus neutralising response

Natural killer Cell avoidance

Virus expresses fake MHCs on CSM (viral glycoproteins) after blocking MHC class 1 → CSM

Blocks NK and CD8 recognition

Avoiding destruction via CD8 killer T cells

Viral SER-PINS (nonstructural)

• bind and inactivate elements of the caspase casade

Subversion of cytokine responses

Virus generates fake cytokine production

• either fake agonist→ binds to biological cell receptor → stimulates anti-inflammatory response/immunosuppressive response

  • E.G. production of IL-10 analogue (usually produced by Tregs) to trigger immunosuppressive response

  • → Epstein Bar and Orf

• or antagonist - fake receptor that normal cytokine binds to to inhibit its action [competitive receptor]

Retrovirus immunosuppresion

• AIDS

• FeLV

• FIV

• AIDS (caused by HIV)→ infect and destroy CD4 T helper and monocytes (macrophages and dendritic cells

  • opportunisitc infection

  • increase cancer → reduced tumour surveillance

• FIV → lentivirus infection (retrovirus)

  • infects CD4 T cells

  • increases susceptibility - opportunistic infection and neoplasia

    • reduced tumour surveillance rather than oncogenes

• FeLV → oncogenic retrovirus → Fit 1 gene insertion

  • neoplasia → lymphoma

  • myelosuppression → damages bone marrow precursors → anaemia

  • immunosuppression (decrease function of lymphocytes)

• FIV Transmission and Pathogenesis

  • enemy - fighting and biting

Transmission

• saliva, blood and other body fluids

• fighting and biting

• outdoor male & feral adult cats most at risk

Pathogenesis

• Acute primary phase

  • malaise, fever, lymphadenopathy (lymph node swelling)

  • virus replication in lymphoid tissues

• Asymptomatic Phase (variable in length)

  • immune response fails to eradicate virus → persistent infection BUT antibody positive

  • animal healthy - low levels of circulating virus

• Feline AIDS

  • lymphopenia - virus destroys T cells

  • CD4 T cells reach critically low level → evident immunosuppression

FeLV Transmission and Pathogenesis

FeLV = grooming

Transmission

• Saliva → oronasal route → mutual grooming (esp. mother → kitten)

• Kittens most susceptible 6w to 6m

• Infected by carrier animals

Pathogenesis

• Replicates in oropharyngeal lymphoid tissue

• Transient viraemia → lymphoid tissue

• Immunity in 20-30% cats

• Persistently viraemic (70-80%) → high blood antigen

• Latent infection (dormant in bone marrow) is rare

• Most die due to opportunistic infection or cancer

FeLV in house test

antigen ELISA → sandwich

• high antigen numbers in serum

or lateral flow

FIV in house test

antibody ELIZA → non-sandwich

• host immune response but very little antigen present

• if antibody present → FIV positive

or lateral flow

Other Diagnostic labs

1. Virus isolation

2. Western blotting

3. Immunofluorescence assay (IFA)

4. PCR

Palliative Treatment

• Antibiotics for 2ndary infection

• Chemotherapy for tumours

• Interferon

  • Virbac company→ feline omega interferon

  • Reduces viral load to reduce impact on immune system → reduces immunosuppression

  • expensive

Vaccination

FeLV - all work well

• killed whole

• purified subunits

• recombinant subunit

• recombinant canary pox

FIV (USA only because rubbish and not liscenced in UK)

• killed whole virus

• 2 subtypes - A and D

• 70%effective - not advertised or recommended

• only considered for high risk cats
  

BVBV

• Pestivirus (Flaviviridae)

• 2 subtypes

  • Cytopathic - virulent, pathogenic, high cell damage

  • Non-cytopathic - transplacental

    • reproductive disease

    • foetal reabsorption, embryonic loss, abortion → early pregnancy

    • later - persistently infected calf

    • causes hairy shaker - prominent in lambs → cerebellar hypoplasia [Border disease also does this]

Mechanism of tolerance - non-cytopathic transplacental transmission

• Transplacental from dam → foetus

• Viral antigen present in primary lymphoid tissue during early lymphocyte development

• Clonal deletion of antigen reactive lymphocytes since recognised as self antigen

• No lymphocytes recognise BVDV as non self → animal immunotolerant to virus

• Subclinical animals grow more slowly + immunocompromised

Animal diseased if:

1. Virus mutates into pathogenic strain

2. Pathogenic strain invades and is recognised as self

Cytopathic virus destroys mucosal epithelium → mucosal disease

BVDV control

• good biosecurity - prevent entering farm

• test and remove persistently infected calves

• ab pos or neg → cull pos

• vaccinate to avoid infection during pregnancy

Latent virus

Recrudescence - reactivation of latent infection

Trigger

• stress

• mediaction (steroids)

• Pregancy - PPRI (periparturient relaxation of immunity)

• maintenance of viral genome in cells but not replicating

• very little protein synthesis - few antigenic makers → low MHC class 1s

• often cytolytic infection in one cell type and latency in another

  • latency → neurone and lymphocyte

Latent virus examples

• human herpes virus

  • herpes simplex → cold sores (sleeping in trigeminal ganglia in face)

  • chicken pox → shingles

• feline herpes virus-1

  • following pregnancy

  • recrudescence even in vaccinated animal (infected before vac) → vac does not work post exposure

  • kitten acute → latent infection

• equine herpes virus

  • 1 AND 4 most important

    • impossible to test for latent carriers

    • PREVENTION: prevent horse getting stressed or early diagnosis, reduce contact btw pregnant mares and other horses

      • resp, abortion (storms), neurological

  • 2 and 5 → respiratory only

  • 3 genital (STD)

• EEHV = elephant endo-thelio-trophic virus

  • highly haemorrhagic in young asian elephants

  • low virulence (latent carriers and shedders in African Elephants)

  • emerging in UK zoos

EHV Vaccination

• Liscensed against respiratory disease and vaccination

• boosters every 6 months

• vaccinate 5,7,9 months of preganancy