Immune response to viruses Flashcards
What are the main cells in the innate response?
- Neutrophils (phagocytose bacteria and other pathogens)
- Antigen Presenting Cells (APCs) (phagocytose the pathogen)
−Macrophages and dendritic cells - Natural Killer (NK) Cells (non specifically kills virus infected cells)
- Eosinophils (parasites)
What is the function of NK cells?
*Rapidly seek out and destroy
virus-infected cells (not specific)
* Identify markers of stress on
the infected cells
*Has two type of receptor
−Activating receptor
−Inhibiting receptor (prevents NK cells from killing healthy cells)
* no memory or MHC restriction
How are NK cells activated?
Transformed or infected cells sometimes increase expression of molecules that activate NK cell receptors (activating ligands). This activating signal may override the inhibitory MHC class I molecule signal and allow NK cells to attack.
How do NK cells kill?
−Bind to infected cells
*Release perforins
* Induce apoptosis
*Release interferon gamma
What are APCs?
- Involved early in the host
response - Carry immunoglobulin Fc and
C3b receptors (promote
phagocytosis) - Professional antigen presenting
cells
*MHC class II protein binds to antigen fragment and presents antigen - Initiate the adaptive immune
response
What are PAMPs?
Pathogen-Associated Molecular Patterns recognised by innate immune system
* Un-methylated C-G dinucleotides (CpG motifs) on DNA viruses
* high frequencies in viruses
* low frequency in mammalian cells
* Double-stranded RNA (only RNA viruses produce this)
* Uracil-rich, single-stranded RNA
What are toll-like receptors (TLRs)?
- A type of PRR found on the surface of cells (TLR-1,2, 4, 5, 6) and within endosomes of phagocytic
cells (Neutrophils and APCs) (TLR-3, 7, 8, 9) - Detect extracellular viruses
- Sense the presence of viral nucleic acid and other conserved molecular
components of invading pathogens
Examples of TLRs?
TLR-3 binds ds RNA
TLR-7 binds Uracil-rich ss RNA (e.g. HIV)
TLR-8 binds ss RNA
TLR-9 binds CpG motifs within viral DNA
What are cellular PRRs?
- Found in the cytoplasm
- Retinoic acid inducible gene-1-like receptors (RIG-1 and MDA-5)
- RIG-1 recognises ss RNA
- Melanoma differentiation association gene-5 recognises ds RNA
- NOD-like receptors (NOD2)
- Cytosolic DNA sensors
What are interferons?
- Protects adjacent cells from infection
- Inhibition of viral replication
- Helps activates T-cell mediated immunity
−Activation of macrophages
−Up-regulates MHC receptors on virus-infected cells
What are INFalpha and INFbeta (type 1)?
- Produced by most cell types early in infection (Innate response)
- Activate genes that have antiviral activities
−dsRNA dep protein kinase R
−RNase L - Helps stimulate MHC class I (enhance presentation of viral peptides
to T cells) - Activates NK cells
- Induces apoptosis
What is INFgamma (type 2, proinflammatory)?
- Involved in the regulation of nearly all phases of immune and
inflammatory responses (both innate and adaptive) - Produced by NK cells and T lymphocytes
- Enhances MHC expression on APCs
- More important as an immunoregulator than as an antiviral agent
−Enhances the cytotoxic activity of T cells, macrophages and NK
cells
What are the components of the adaptive immune response?
Virus-specific immune response, recognise viral proteins and carbohydrates (antigens); takes days
Humoral Immune response (B lymphocytes)
− Antibody production
* Cellular Immune Response (T lymphocytes)
− Cytotoxic T cells (CTLs) CD8+
* Kill virus-infected cells
* Cytokines that eliminates viral RNA
− T Helper cells (TH) CD4+
* Activate macrophages
* cytokines
What is the humoral immune response?
B lymphocytes
* Resident in the lymphatic tissue
* Respond to antigenic stimulus by
producing and secreting antibodies
* Carry highly specific receptors that
recognise viral epitopes
* Recognise Ag in their native form (no
processing)
How are B cells activated?
- Activates on binding to virus/Ag; requires a signal from T helper cells
- Clonal expansion of B cells producing a single antibody
- Antibodies early on are low affinity (IgM); B cells evolve by hyper-mutation in the V regions to produce high affinity binding antibodies (IgG) later on
What are IgG, IgM and IgA?
IgG: main circulating
antibody in secondary response; important for long term immunity
IgM: Circulating antibody
produced early in
infection; diagnostic marker
IgA: Secretory antibody; primary defence at mucosal surface
What is the role of an antibody?
- Binds to circulating virus (outside the cell)
−May prevent attachment to susceptible cells (Neutralises virus) - Opsonisation
−Virus-coated with antibody activates complement
−Induces inflammatory response (attracts neutrophils and macrophages) - Plays a major role in recovery following
infection with viruses that cause viraemia
What is the cellular immune response?
- T lymphocytes co-ordinate the activities of the cells involved in generating the immune response
−T helper cells (CD4+ T cells)
−Cytotoxic T cells (CD8+ T cells) - Carry highly specific receptors that recognise specific regions on a pathogen (single T cell recognises a single T cell epitope) in association with MHC proteins
- On activation, T cells will divide and proliferate and secrete cytokines that regulate the immune response
- Some T cells become long-lived memory T cells
What are cytotoxic T cells (CTLs)?
- Recognises viral peptides on the
surface of infected cells in associated
with MHC class I - Kill virus-infected cells
- Granules within the CTL polarise
towards the target cell - Perforin is released and creates
pores in the cell membrane
causing lysis
What are helper T cells?
- Recognises viral peptides on the
surface of APCs in associated with
MHC class II - Plays essential role in initiation of B
cell responses - Secrete cytokines that regulate the
immune response - Main types are Th1 and Th2
What is Th1?
Secretes IL-2
Inflammatory response
Augments immune response by
attracting macrophages to site of
infection
Activates IFN γ and TNF β
Promotes IgG2a production
What is Th2?
Secrete IL-4, IL-5 and IL-6
Provide help for antibody production
Promotes switching of B cells (IgG2a
to IgG1)
What are the targets of the humoral response?
- Surface exposed protein on virus particle
- Envelope proteins
- Capsid proteins (of naked viruses)
- Often spikes or loops that protrude from viral surface
- Epitopes can be linear or “conformational” (critical residues brought together by folding of
the polypeptide chain)
What is western blotting?
- Allows detection of antibodies to some or all the viral proteins of a particular virus
- Can be used to monitor present of antibodies to different antigens at different stages of infection
- Not used routinely in diagnostic setting (not easily standardised or scaled up for automation)
- Generally used as a confirmatory test
How is a western blot performed?
- Run proteins on a SDS polyacrylamide gel
- Transfer separated proteins to a
nitrocellulose/nylon membrane - Incubate membrane with antibody (from sample i.e. diluted serum sample). Wash, incubate with secondary antibody labelled with enzyme/tag that can be visually detected
- Detect/visualise proteins on addition of
substrate
What is a virus neutralisation test?
- Loss of infectivity through reaction of the virus with specific antibody
- Virus is incubated with serum sample (serial dilutions)
− Using constant concentration of virus - Concentration where plaques are visible and beginning to decrease in number
− Virus and antisera are then used to infect cell monolayers in a microtitre plate
− Determine the highest dilution of antibody which when mixed with a standard virus concentration will prevent the formation of plaques
Pros and cons of neutralisation assay
- Used for detection and quantification of virus specific antibodies
- Gold standard in virology
- Neutralising antibody correlates with protective antibody in vivo
- When new virus emerges – such tests can be operational within weeks (take up to a year for an ELISA to be fully validated)
- Cons:
−Slow can takes several days to get a result
−Require infectious virus
−Cost to maintain cell culture facilities
What are uses of serological tests?
Used when it is difficult to detect virus directly
* Widely used when animals are imported and exported from countries
−Governments want certification to ensure animals are not bringing
pathogens with them
* Used to assess vaccine efficacy
−Is vaccination being implemented correctly
−Eradication programs
