Viruses are obligatory intracellular pathogens that infect cells by utilizing cell surface receptors. The innate immune system responds to viruses through induction of type I interferons like IFN-α and IFN-β, which are produced by infected cells and activate natural killer cells. The adaptive immune system mounts both humoral and cell-mediated responses against viruses. However, viruses have evolved multiple mechanisms to evade the host immune response, such as inhibiting interferon activity, blocking antigen presentation, and inhibiting apoptosis of infected cells.

1. IMMUNITY TO VIRUSES
Presented By
Tukendra Verma

2. Viruses
• Obligatory intracellular
pathogens that replicate within
cells.
• Use the nucleic acid and protein
synthetic machineries of the
host cell.
• infect a variety of cell
populations by utilizing normal
cell surface molecules as
receptors to enter cell.

3. Innate Immune Response to Viruses
1. Induction of Type I
interferons
 interferons are antiviral protein
or glycoprotien produced by
several types of cells in response
to viral infection.
 IFN- α by leucocytes
IFN- β by fibroblast
IFN- γ by NK cells
Source -http://www.nature.com/nri/journal/v15/n2/fig_tab/nri3787_F2.html?foxtrotcallback=true

4. Mechanism of activity of interferons I
Source – owen kuby immunology chapter 5, fig. 16

5. 2. Activation of Natural Killer
(NK) cells
The binding of IFN-alfa and IFN-
beta to NK cells induces lytic
activity, making them very effective
in killing virally infected cells.
The activity of NK cells is also
greatly enhanced by IL-12, a
cytokine that is produced very early
in a response to viral infection
Source - http://www.nature.com/nri/journal/v5/n11/fig_tab/nri1711_F2.html

6. Specific Immune Response To Viruses
• Humoral mediated immune
response
• Viral Neutralization by Humoral
Antibody
• i.e. Secretory IgA in mucous secretions
• Viral Neutralization by antibody
sometimes occurs after viral attachment.
• Block viral penetration.

7. Specific Immune Response to Viruses
• Activated Th1 cells produce several
cytokines
 IL-2
 Acts indirectly by assisting the
development CTL precursors
 Activates NK cells
 IFN-γ
 Directly induces an antiviral
state in near by cells
 Activates NK cells
 TNF
• CTL activity
 Arises within 3-4 days after
infections
 Peaks by 7-10 days, and then
declines
 Have viral specificity
 Eliminites specific virus-
infected cells, thus getting rid
of potential new sources of
new virus
• Cell-mediated immune response

8. Cell-mediated immune response
• Antiviral CTL responses occur in 4
phases
• 1. Induction phase
• 2. Activation-induced cell-death
(AICD) phase
• 3. Silencing phase
• 4. Memory phase

10. Viruses Can Evade Host Defence
Mechanism
1. Inhibition of humoral
immune response
 Antigenic variation
• Generation and selecton of
varients with different
antigenic properties
• Genetic variability
• e.g.- influenzae virus & HIV
source -http://www.ppdictionary.com/viruses/flu_a.htm

11.  complement inactivation
• virus encode homologous protein to
regulatory complement compnents
• blocks secertion and activation of
complement system.
• e.g.- HIV, CMV & VV.
 fc receptor encoding
• virus encodes fc receptors
• antibody bound infected cells bound
to virus encoded fc receptors

12. 2. Interfernce with interferons
 type I interferons (α, β)
• virus block JAK-STAT pathway which block interferon activity.
• inhibition of interferon effactor pathway by inhibiting the PKR.
• encoding soluble interferon receptors block functions of
interferons.
 type II interferons (γ)
• block the synthesis of (γ).

13. Some developed the strateries to avade the action of IFN-α &
IFN-β.

14. 3. cytokine inhibition
• block cytokine expression e.g.-
ASFV- encode IkB
• block signal transduction
• mimicry of cytokine and its
receptor for its benefit
• secreting cytokine receptor
homolog

16. 4. Some inhibit the antigen presentation
• i.g.- HSV viruses
• HSV-1 and HSV-2 both express an
immediate-early protein called ICP47.
• Inhibits the human transporter
molecule -TAP.
• Inhibition of TAP blocks antigen
delivery to class I MHC receptors on
HSV-infected cells, thus preventing
presentation of viral antigen to CD8+ T
cells.
• Some down-regulate class I MHC
expression i.g.-adenoviruses and
cytomegalovirus.

17. 5. Inhibition of apoptosis