1. Host Responses to Viral Infection
How do host cells fight back?

2. How do host cells fight back?
Host cells don’t just sit and passively allow the virus to take over their metabolic machinery – they fight back
The immune system defends the host against infection by viral invaders.
There are two types of host response to virus infection
Innate response which involves
Phagocytic cells
Interferon
Natural killer cells
Adaptive response which involves
T lymphocytes (T cells)
B lymphocytes (B cells)
MHC class I and class II molecules

3. How do host cells fight back?
Innate defenses
Phagocytic cells
Include neutrophils (polymorphonuclear leukocytes - PMNs) and macrophages
Phagocytic cells may engulf viruses and take them up into phagosomes.
When the phagosomes fuse with lysosomes, the lysosomal enzymes may or may not destroy the virus.

4. Mechanism of phagocytosis

5. How do host cells fight back?
Innate defenses
Eukaryotic cells produce interferon
Interferon is host specific, not virus specific (reason will be clear later on)
Can be produced by virtually any cell
May be induced by viral infection (RNA viruses are better inducers than DNA viruses), ds RNA molecules, intracellular bacteria like Rickettsia, Gram-negative endotoxin, and metabolic inhibitors of protein synthesis
Interferon is a first line of defense against viruses before a specific immune response is mounted
There are three different classes of interferon (IFN) - , , and ;
 and  are called type I interferon and they function mainly in inducing cells to become resistant to viral infections
 is called type II interferon and it functions to mainly to modulate the immune response

6. How do host cells fight back?
Types I and II IFN cause the following biological effects:
Both types cause an activation of macrophages, natural killer cells, cytotoxic T lymphocytes, and modulation of the synthesis of antibodies
Both types induce increased MHC class I expression.
Both types induce a fever.
Both types cause an inhibition of cell growth and are effective in the treatment of some tumor cells.
Type II induces increased class II antigen expression as well as Fc receptor expression
In type I treated cells, and to a much lesser extent in type II treated cells, there is an inhibition of viral replication.
Interference with penetration and uncoating, or assembly (mechanisms are unknown).
Interference with viral transcription - due to a host protein known as Mx, which is induced by interferon
Interference with viral mRNA translation

7. How do host cells fight back?
The mechanism that activates type I interferon production is as follows:
The regulatory sequence for IFN shows three functional domains
Two domains bind positive regulatory molecules (gene activators)
One domain binds a repressor protein that blocks transcription.
The protein is labile and anything that inhibits protein synthesis results in the protein not being replenished.
When this happens, the two positive regulatory proteins get phosphorylated.
They bind to the two sites upstream from the empty repressor site to activate transcription and interferon is made.
The mRNAs that are made have destabilization sequences and have short half-lives.

8. How do host cells fight back?

9. How do host cells fight back?
What is the action of interferon?
Interferon prepares cells to combat viral infection, if necessary.
Interferon is secreted from an infected cell and must bind to a cellular receptor on the plasma membrane of a neighboring cell ( this accounts for the species specificity) to induce an anti-viral state in the cell.
Both  and  IFN bind to a common receptor
 IFN binds to a different receptor
The species specificity of interferon is due to the specificity of the action of the IFNs with their receptors.

10. How do host cells fight back?

11. IFN activity Binding triggers a signal transduction cascade.
A message is sent through this cascade which involves protein kinases and transcription factors. This leads to the induction of the transcription of several different genes. Cells that have been induced by interferon:
Express new membrane associated surface proteins
Have altered glycosylation patterns
Express at least three proenzymes that, when activated, function to inhibit viral mRNA translation:

12. IFN activity

13. Interferon
The first proenzyme is oligoisoadenylate synthetase (OS). It catalyzes the formation of a 2’-5’ adenylic acid oligo (2’-5’ oligo A) instead of an an oligo with the conventional 3-’5’ phosphodiester linkage.
When the virus comes in dsRNA provides the signal for the proenzyme to be activated to the functional enzyme.
2’-5’ oligo A is made
2’-5’ oligo A activates a second proenzyme, RNAse L which cleaves both both viral and host cell mRNA, thus inhibiting translation for both
This is more effective against RNA viruses. What does the DNA virus need to do to activate this enzyme?

14. The antiviral state

15. Antiviral activity

16. Interferon
The third enzyme is protein kinase RNA dependent (P1 or PKR)
When the virus comes in dsRNA provides the signal to activate the enzyme. The enzyme first catalyzes the addition of a phosphate to itself (self-phosphorylation)
Next the phosphorylated enzyme phosphorylates the  subunit of eIF-2.
This blocks the GDP/GTP exchange necessary for reutilization of the  subunit in translation initiation.
Thus translation of both host and viral mRNAs ceases.
There is some discrimination between viral and host mRNA via localization of the activity to the site of dsRNA. This is also true for the OS system.

17. The antiviral state

18. Antiviral activity

19. How Does PKR Really Work?
The host cell enzyme that phosphorylates eIF2 is called protein kinase RNA dependent (PKR).
PKR is synthesized as an inactive proenzyme that must be phosphorylated in order to become active.
The trigger for phosphorylation is ds RNA which is produced not only when RNA viruses express their genomes, but also when DNA viruses transcribe from both strands in the same location in their genome, producing complementary RNA molecules that can base-pair with each other.

20. How does PKR Work?
When two PKR proenzyme molecules bind to dsRNA, each undergoes a conformational change that cause it to phosphorylate the other PKR molecule.
The activated PKR molecules than attach phosphates to eIF-2 proteins.

21. How does PKR Work?

22. Inhibition of Eukaryotic Protein Synthesis

23. Interferon
What countermeasures do viruses have to combat the host interferon system defenses?
Reovirus has a core protein that binds to dsRNA to prevent its activation of the PKR system
Adenovirus VA1 protein also prevents dsRNA from binding to PKR
Poxvirus encodes a protein that binds to dsRNA and blocks activation of both the PKR and OS proenzymes

24. How do host cells fight back?
Innate defenses
Natural killer (NK) cells – NK cells are large, granular lymphocytes that are activated by three types of targets:
Antibody coated cells
Cells infected by viruses and some intracellular bacteria
Cells lacking class I MHC molecules. NK cells express inhibitory receptors that recognize class I MHC molecules, and therefore NK cells are inhibited by class I expressing cells and activated by target cells lacking class I molecules. Some viruses are known to down regulate expression of class I molecules.
Activated NK cells lyse the target cells by releasing granules that contain perforin that creates pores in the target cells and granzymes that enter the target cells through the pores and induce apoptosis of the target cell.

25. NK activity

26. NK activity with normal cell

27. NK activity with cell lacking MHC class I molecules

28. How do host cells fight back?
Adaptive responses
B cells have structures on their surface called antibodies or the B cell receptor (BCR) that recognize and bind to foreign antigens (more specifically antigenic determinants or epitopes).
Similarly, T cells have structures on their surface called T cell receptors (TCR) that recognize and bind to foreign epitopes.
Each B or T cell has many identical BCRs or TCRs on its surface and they all recognize the same epitope.
Different B or T cells have different BCRs or TCRs and each recognizes its own unique epitope.
The epitopes recognized by BCRs are peptides, nucleic acids, or polysaccharides and they can be recognized in solution.
The epitopes recognized by TCRs are peptides and they can only be recognized in the context of a self antigen (More on this later).

29. How do host cells fight back?
Binding of a BCR to its epitope will cause that B cell to proliferate and differentiate into a plasma cell that will secrete antibodies of the same specificity that was on the surface of the original B cell.
Binding of a TCR to its epitope will cause that T cell to proliferate and differentiate into an effector cell that will be responsible for a type of immunity that can kill infected cells.
Some effector cells have CD4 ( T helper cells) on their surface and some have CD8 (cytotoxic T cells) on their surface.

30. How do host cells fight back?
The TCR can only recognize and bind to its specific epitope if that epitope is associated with a self MHC (major histocompatability) molecule.
There are two basic types of MHC molecules
Class I MHC molecules are found on the surface of all nucleated cells.
Class I molecules present peptides made inside the cell to
CD8 +cytotoxic T cells.
The class I molecule with its associated peptide is recognized and bound by both the TCR (recognizes the peptide in conjunction with the MHC class I molecule) and the CD8 molecule.
The interaction triggers the CD8+ cytotoxic T cell to cause the infected cell to undergo apoptosis

31. MHC Class I Interaction with CD8+ T Cell

32. Presentation of Foreign Antigen Made Within the cell to CD8+ T Cells

33. Killing by cytotoxic T cell

34. How do host cells fight back?
Class II MHC molecules are found on the surfaces of cells of the immune system, including B cells and macrophages.
Class II molecules present peptides made outside the cell to CD4 + helper T cells.
The class II molecule with its associated peptide is recognized and bound by both the TCR (recognizes the peptide in conjunction with the MHC class II molecule) and the CD4 molecule.
The interaction triggers the CD4+ helper T cell to release cytokines that help B cells to proliferate and differentiate into plasma cells that secrete antibody of the same specificity that was on the surface of the original B cell.

35. MHC Class II Interaction with CD4+ T Cells

36. Presentation of Foreign Antigens Made Outside the Cell to CD4+ T Cells

37. MHC Class II Interaction with CD4+ T Cells

38. How do host cells fight back?
How do antibodies effect viruses?
Antibodies may directly neutralize viruses by preventing their binding to their host cell receptor.
They can indirectly inactivate or neutralize the virus by signaling other mediators (phagocytic cells and complement) that inactivate the virus.