Immune evasion in viral infections

Question 1

Where should be the point of attacks of viruses

Answer 1

• Point of cell signaling - if cells cannot signal, they can’t mount an immune response
• Antigen processing and presentation
• Co-stimulation of T cells - if they don’t get a second signal they become anergic
• killing of infected cells - virus keeps the host cell alive

Question 2

Viral mechanism of innate immune evasion

Answer 2

Viruses evade the innate IS by blocking different parts of the path from recognition of innate receptor to response

1. Virus block or alter the TLR receptor, the receptor that are responsible to first recognize certain PAMPs specific for viruses
2. Viruses can block the intra cellular cascades that gets activated when the extra- intracellular receptors become activated
3. Viruses blocks end-point of response, the release of interferons by inhibiting the TF etc.

Question 3

Viral mechanism of adaptive immune evasion

Answer 3

• inhibition of T cell activity; virus can produce peptides that look like self antigen so its not recognized, inhibit the proteasome degradation of viral proteins so the peptides are not presented, they can down regulate co-stimulatory signals so the cells go into anergy
• Evading B cells; down regulating BCR, producing viral Fc receptors that can bind to Ab and block, inhibit the complement system, viruses can enter latency, antigen drift and shift

Question 4

What is antigen drift and antigen shift

Answer 4

Antigen drift - virus mutate its proteins so they are no longer recognized by existing Ab

Antigen shift - virus can transfer between species and obtain other species related Ag thereby the same virus now have surface protein from another virus and cannot be recognized by Ab

Question 5

How do viruses escape cytokine/chemokines

Answer 5

• viral chemokines - viruses produce antagonist to normal chemokines, like IL-10 from CMV. Or like in the case of HIV it produces chemokine allowing T cells to go to the LN so it can spread more
• Soluble and decoy receptors - bind the normal cytokine
• False chemokines - so that immune cells go to the wrong place or that Tregs come close

Question 6

How do viruses avoid cell lysis and apoptosis

Answer 6

Viruses produce molecules that inhibit the apoptotic pathway since it want its host to survive

• caspase activity mostly
• Viral Bcl-2 homologues can inhibit apoptosis
• some viruses can produce death-ligands on cells so activated cells die instead of them

Residue in Immune-privileged sites like the brain, testis etc.