19. Patterns of Viral Infection

Question 1

What are viruses?

Answer 1

obligate intracellular pathogens
packets of genetic information that can reprogram the host cells

Question 2

what do the different viral genomes infer?

Answer 2

different intracellular lifestyles

Question 3

what is the problem with different viral lifestyles?

Answer 3

makes it hard to develop broad spectrum treatments

Question 4

how do viruses trigger the immune response?

Answer 4

viral particles being detected
viral proteins on the cell surface being detected

Question 5

What is the progress of viral infection?

Answer 5

establish infection
regulation of gene expression
antigenic variation
immune invasion
immune subversion

Question 6

What are the basic requirements for viral infection?

Answer 6

enough virus present to initiate infection
cells at the site of infection must be susceptible to the virus
local antiviral defences must be absent or ineffective

Question 7

what causes viral tissue tropism?

Answer 7

Susceptibility of cells at site of infection

Question 8

what are innate defences to viruses?

Answer 8

type 1 interferons inducing the transcription of antiviral genes
lysis of infected cells by NK cells triggered by changes in the cell surface

Question 9

what antibodies are useful in viral infection?

Answer 9

IgG
IgM
IgA

Question 10

What antibody function is the most effective against viruses?

Answer 10

neutralisation

Question 11

What is neutralisation?

Answer 11

antibodies binding to viral particles and blocking their receptors

Question 12

Does opsonisation occur in the antiviral response?

Answer 12

yes but less likely to occur then neutralisation

Question 13

What is antibody dependant cell-mediated cytotoxicity?

Answer 13

Antibodies bind to viral receptors and Fc receptors on NK cells.
NK cells release cytotoxic granules to kill the virus

Question 14

What kind of adaptive defences are more important in viral defence?

Answer 14

cell-mediated immunity

Question 15

what are the 2 cells involved in viral cell mediated defences?

Answer 15

CD8+ T cells
NK cells

Question 16

What is the role of CD8+ T cells in viral cell mediated defence?

Answer 16

MHC restricted killing
Recognise infected cells
HLA process and present the antigenic peptide
kill the cells that they recognise
provide long term response

Question 17

What is the role of NK cells in viral cell mediated defence?

Answer 17

Non- restricted killing
kill any cells that trigger PRR

Question 18

What is the role of ADCC in viral cell mediated defence?

Answer 18

antibody binds to a viral protein and helps cytotoxic cells recognise them by binding to Fc receptors

Question 19

How can viruses evade host defences?

Answer 19

genetic reassortent to change the structure of surface receptors (dramatic changs can make new strains)
stealth - low levels of infection so don’t generate an immune response
disruptions - killing immune cells like HIV
non-cytopathic infections

Question 20

what are the 4 different type of virus infection?

Answer 20

Acute - eg smallpox
persistent - eg hep B
latent - eg herpes
slow - eg HIV

Question 21

what is the difference between a persistent and latent infection?

Answer 21

persistent - can be asymptomatic but still transmissible
latent - only transmissible when infections are active

Question 22

what is the course as a typical acute infection?

Answer 22

initially innate defence while the virus is at lower levels
stimulate the adaptive immunity as the infect reaches its peak
adaptive immunity is responsible for clearing infection

Question 23

what are the main defences against acute viral infection?

Answer 23

innate - interferon response and NK cells
Adaptive - final clearance of virus, provides memory

Question 24

what may happen in an immunocompromised host?

Answer 24

the infection may not stay local to the primary infection site

Question 25

what are the general symptoms of viral infection resulting from immune response?

Answer 25

fever malaise aches nausea

Question 26

how can viruses cause direct cell damage?

Answer 26

cell lysis toxic viral proteins cell killing by the immune system

Question 27

what are the symptoms of viral infection resulting from cellular damage?

Answer 27

depends of virus: diarrhoea poor lung function poor liver funciton breakdown of capillary beds

Question 28

what are clinically inapparent acute infections?

Answer 28

successful infections with no symptoms still active virus infection cycle infection rapidly controlled by immune system usually well adapted pathogens

Question 29

what are multiple acute infections?

Answer 29

initial infection followed by 2nd or 3rd round of infection could be 2nd acute infection

Question 30

Acute infection example = human poliomyelitis

Answer 30

typically mild intestinal infection with no symptoms can enter the bloodstream (usually in immunocompromised) can enter multiple different tissues enter CNS = paralysis and death

Question 31

why are acute viral infection important public health matters?

Answer 31

often associated with serious epidemics difficult to treat or detect before transmission preventative measures often too late

Question 32

What are the characteristics of persistent viral infections?

Answer 32

virus particles/products are produced for long periods virus could be produced intermittently or after long periods of latency persistent infections can eventually be cleared latent and slow infections will last the life of the host

Question 33

What is integral to persistent infections?

Answer 33

virus adjusting its replication and pathogenicity evastion of host immunity

Question 34

How do persistent viruses AVOID the host immune system?

Answer 34

Infection of immune privileged sites (no active immune Surveillance) eg skin, eye, brain, CNS down regulation of viral gene expression viral protein structure limits antigenicity high frequency variation of antigenic epitopes

Question 35

How do persistent viruses INHIBIT the host immune system?

Answer 35

subvert the immune system - inhibit MHC1 and MHC2 - inhibit NK cell lysis - interfere with apoptosis - inhibit cytokines like interferons produce host mimicking compounds direct infection of immune cells like HIV

Question 36

what are the 4 characteristic properties of latent infections?

Answer 36

1. infect a non replicating cell or the viral and host DNA is replicated together as not to disrupt the cell 2. immune detection of infected cell is reduced/eliminated 3. expression of viral genes for making virions is absent 4. the viral genome persist and is capable to starting an acute infection later

Question 37

an example of a latent infection

Answer 37

Herpes simplex 1 primary infection = cold sores, fever latent infection = asymptomatic with no virus production, viral DNA resides in trigeminal nerve ganglion reactivation = virus replicated and travels down sensory nerse to infect epithelial cells

Question 38

what is the main feature of slow viral infections?

Answer 38

many years pass between the initial contact with infectious agent and fatal disease difficult to study due to long duration

Question 39

what are some unconventional slow diseases?

Answer 39

prions - transmissable spongiform encephalopathies

Question 40

an example of a slow infection

Answer 40

measles virus - normally acute but can switch to persistent subacute sclerosing panencephalitis appears 7-10 years after infection and effects 1 in 1 million cases causes progressive neurological deterioration and death defective measles virus can be detected