Immune response to virus - an interstesting ppt

immune response to viral infection is important

1. Immune response to virus Ritasman Baisya

2. Points of discussion Properties of a virus Viral replication Innate immune response Adaptive immune response Immune evasion by virus

3. Properties of virus Virus is obligate intracellular parasite It requires host cell for survival Have either  RNA  or  DNA  as their genetic material. The  nucleic acid  may be single- or double-stranded.  Either naked nucleo -capsid or with an envelope The entire infectious virus particle, called a  virion Viral components assemble ,they do not replicate by division Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000. Section 6.3, Viruses: Structure, Function, and Uses .

4. Enveloped vs nonenveloped

5. Classification Gaurab Karki  Classification of virus . Microbiology ,  Virology September 17, 2017  

6. SARS – COV 2

7. Replication

8. 1 2 4 3 5 5 4 6 Golgi Apparatus mRNAs Nucleus Ribosomes Penetration by Endocytosis Viral attachment- TROPISM uncoating – release of viral genome Genomic replication Assembly Transcription & Translation Release

9. Virion release There are two methods of viral release: lysis or budding – Lysis results in the death of an infected host cell, these types of viruses are referred to as cytolytic . example -smallpox. Enveloped viruses, such as Corona virus , are typically released from the host cell by budding . It is this process that results in the acquisition of the viral phospholipid envelope. These types of virus do not usually kill the infected cell and are termed cytopathic viruses .

10. Innate immune response to virus

11. Components of innate immune response Epithelial barrier Early non-specific or innate immune – Interferon (IFN) – Type I IFNs (IFN α and IFN β) Type II IFN (IFN γ) Natural killer cells Macrophages

12. Physical barrier Killing of viral particle by locally produced microbicidal agent – Defensin Cathelicidin Intraepithelial lymphocyte

13. Interferon IFNs are a large family of multifunctional secreted proteins involved in antiviral defense, cell growth regulation, and immune activation . Two dominant types - Type I IFNs ( IFN-α , IFN-β) are produced in direct response to viral infection Type II IFN ( IFN-γ ) - rather than being induced directly by viral infection, is synthesized in response to the recognition of infected cells by activated T lymphocytes & NK cells. IL-29, IL-28A, and IL-28B have been classified as the type III IFN group, designated IFN-λ molecules 1, 2, and 3, respectively

14. Type 1 Interferon - Large family of structurally related cytokines that mediate the early innate immune response to viral infections There are many type I interferons, which are structurally homologous and are encoded by genes in a single cluster on chromosome 9 The most important type I interferons in viral defense are IFN-α (which actually includes 13 different closely related proteins ) and IFN-β , which is a single protein. Plasmacytoid DCs are the major sources of IFN-α, but it also may be produced by mononuclear phagocytes . IFN-β is produced by many cell types in response to viral infection

15. Characteristic The most potent stimuli for type I interferon synthesis are viral nucleic acids The receptor for type I interferons, which binds both IFN-α and IFN-β, is a heterodimer of two structurally related polypeptides , IFNAR1 and IFNAR2, which are expressed on all nucleated cells This receptor signals to activate STAT1, STAT2, and IRF9 transcription factors, which induce expression of several different genes whose protein products contribute to antiviral defense in various ways

16. Pattern recognition receptor The most studied are the Toll-like receptors (TLRs), retinoic acid-inducible gene I (RIG-I; also known as DDX58) and NOD-like receptors (NLRs). Virus infections usually activate the endosomal TLRs (TLR3, TLR7, TLR8 and TLR9) that recognize viral nucleic acids and double-stranded RNA intermediates

17. Anti viral effects of type 1 IFN Induced genes include double-stranded DNA & RNA –activated serine/threonine protein kinase (PKR), which blocks viral transcriptional and translational events , and 2′,5′-oligoadenylate synthetase and RNase L, which promote viral RNA degradation . Primarily , a paracrine action in that a virally infected cell secretes interferon to act on and protect neighboring cells that are not yet infected Interferon secreted by an infected cell may also act in an autocrine fashion to inhibit viral replication in that cell

18. Other actions Sequestration of lymphocytes in lymph nodes , thus maximizing the opportunity for encounter with microbial antigens. The mechanism for this effect of type I interferons is the induction of a molecule on the lymphocytes called CD69, which forms a complex with and reduces surface expression of the sphingosine 1-phosphate (S1P) receptor S1PR1 2 . Increase the cytotoxicity of NK cells and CD8+ CTLs & promote the differentiation of naive T cells to the Th1 subset of helper T cells . These effects of type I interferons enhance both innate and adaptive immunity against intracellular infections, including viruses and some bacteria.

19. IFN gamma – a few notes IFN-γ is the principal macrophage-activating cytokine IFN-γ is also called immune or type II interferon . Although its name interferon is shared with the antiviral type 1 interferons, it is not a potent antiviral cytokine, and it functions mainly as an activator of effector cells of the immune system . IFN-γ is a homodimeric protein belonging to the type II cytokine family CD4+ Th1 cells ( main source ) , NK cells and CD8+ T cells produce IFN-γ. NK cells secrete IFN-γ in response to activating ligands on the surface of infected or stressed host cells

20. Few notes about IFN gamma IFN-γ activates macrophages to kill phagocytosed microbes IFN-γ promotes the differentiation of CD4+ T cells to the Th1 subset and inhibits the development of Th2 and Th17 cells . IFN-γ stimulates expression of several different proteins that contribute to enhanced antigen presentation and T cell activation IFN-γ acts on B cells to promote switching to certain IgG subclasses, notably IgG2a or IgG2c (in mice), and to inhibit switching to IL-4–dependent isotypes, such as IgE

21. NK cells NK cells kill virus-infected cells and are an important mechanism of immunity against viruses early in the course of infection, before adaptive immune responses have developed. Class I MHC expression is often shut off in virus-infected cells as an escape mechanism from CTLs. This enables NK cells to kill the infected cells because the absence of class I releases NK cells from a normal state of inhibition Viral infection may also stimulate expression of activating NK cell ligands on the infected cells.

22. Macrophages Phagocytosis of virus and virus-infected cells; Killing of virus-infected cells; Production of antiviral molecules - TNFα, nitric oxide, and IFNα. Many viruses that persist trigger innate cells such as dendritic cells (DCs), natural killer (NK) cells and macrophages to produce anti-inflammatory molecules such as interleukin-10 (IL-10) and transforming growth factor- β ( TGF β )

23. Interference with innate immune responses HCV - Blocks RIG-I pathway by degrading IPS1 117 Influenza A virus NS1 protein inhibits RIG-I by direct interaction Paramyxovirus V protein inhibits RIG-I by interacting with MDA5 HIV and human herpesvirus Inhibit IRF3 Hanta virus, CCHFV and Borna disease virus - Viral RNA is undetectable by PRRs owing to removal of 5 ʹ triphosphate

24. Adaptive immune response

25. Antigen presentation and processing

26. Cross presentation If the infected cell is a tissue cell and not an antigen-presenting cell (APC), such as a dendritic cell, the infected cell may be phagocytosed by the dendritic cell, which processes the viral antigens and presents them to naive CD8+ T cells- cross-presentation, or cross-priming

27. Most efficient cross-presenting APCs are the lymphoid tissue dendritic cells that express CD8 or the peripheral tissue subset that expresses the CD103 integrin The corresponding specialized cross-presenting dendritic cells in human tissues express high levels of CD141 , also known as BDCA-3 . Cross presentation

28. Response by CD8 T cell CD8+ T cells undergo massive proliferation during viral infection & become CTL The antiviral effects of CTLs are mainly due to killing of infected cells Other mechanisms include activation of nucleases within infected cells - degrade viral genomes IFN-γ secretion which activates phagocytes and may have some antiviral activity.

29. Role of CD4 T cell CD4+ T cell-derived IL-2: CD8+ T cell growth factor CD4+ T cell-derived chemokines: recruit CD8+ T to site of infection CD4+ T cells secrete IFN γ and TNF α to recruit and activate macrophages

30. Antibody mediated immune response The most effective antibodies are high-affinity antibodies produced in T-dependent germinal center reactions Antibodies are effective against viruses only during the extracellular stage of the lives of these microbes Antiviral antibodies bind to viral envelope or capsid antigens Prevent both initial infection and cell-to-cell spread Secreted antibodies ( IgA isotype ) - important for neutralizing viruses within the respiratory and intestinal tracts

31. http://www.virology.ws/2009/07/24/virus-neutralization-by-antibodies/

32. Latent infection In latent infections, viral DNA persists in host cells, but the virus does not replicate or kill infected cells CTLs generated in response to the virus can control the infection but are unable to eradicate it. As a result, the virus persists in infected cells, sometimes for the life of the individual. Reactivation of the infection is associated with expression of viral genes that are responsible for cytopathic effects and for spread of the virus

33. Tissue injury In some viral infections, tissue injury may be caused by CTLs. Some degree of immunopathology accompanies host responses. The livers of patients with acute and chronic active hepatitis contain large numbers of CD8+ T cells, and hepatitis virus-specific, class I MHC-restricted CTLs - These findings support that the CTL response is the main cause of tissue injury in viral hepatitis . LCMV induced meningitis is basically due to tissue injury

34. Immune evasion

35. Immune Evasion by Viruses- antigenic change Viruses can alter their antigens and are thus no longer targets of immune response The principal mechanisms of antigenic variation are point mutations and reassortment of RNA genomes (in RNA viruses) , leading to antigenic drift and antigenic shift. These processes are of great importance in the spread of influenza virus Genomic mutation in virus- antigenic shift Genetic reassortment – antigenic drift

36. Antigenic drift

37. MHC escape phenomenon Some viruses inhibit class I MHC–associated presentation of cytosolic protein antigens. Such viruses cannot be recognized or killed by CD8+ CTLs. NK cells are activated by infected cells, especially in the absence of class I MHC molecules. Some viruses may produce proteins that act as ligands for NK cell inhibitory receptors and thus inhibit NK cell activation

38. Viruses produce molecules that inhibit the immune response Cytokine binding protein is secreted by pox virus and acts as competitive antagonist of cytokines EBV produces a protein homologous to IL-10 i nhibits activation of macrophages and dendritic cells Viruses have acquired genes encoding endogenous inhibitors of immune responses during their passage through human hosts

39. Exhaustion Some chronic viral infections are associated with failure of CTL responses, called exhaustion which allows viral persistence Persistent antigen stimulation may lead to upregulation of T cell inhibitory receptors, such as PD-1 There is evidence for CD8+ T cell exhaustion including HIV and hepatitis virus infection.

40. Inhibitory response to limit tissue damage

41. Inhibitory response to limit tissue damage

42. METHOD OF IMMUNE EVASION with examples Antigenic variation Influenza, HIV Inhibition of antigen processing- Blockade of TAP transporter, Removal of class I molecules from the ER HSV CMV Production of “decoy” MHC molecules to inhibit NK cells Cytomegalovirus (murine Production of cytokine receptor homologues Vaccinia, poxviruses Production of immunosuppressive cytokine Epstein-Barr (IL-10) Infection and death or functional impairment of immune cells HIV Inhibition of complement activation Recruitment of factor H Incorporation of CD59 in viral envelope HIV, vaccinia, human CMV Inhibition of innate immunity - Inhibition of access to RIG-I RNA sensor Inhibition of PKR (signaling by IFN receptor) Vaccinia, HIV HIV, HCV, HSV, polio

43. Li et al., Molecular immune pathogenesis and diagnosis of COVID-19, Journal of Pharmaceutical Analysis, https://doi.org/10.1016/j.jpha.2020.03.001

44. Coronavirus immune evasion Can induce the production of double-membrane vesicles that lack PRRs and then replicate in these vesicles, thereby avoiding the host detection of their dsRNA  IFN-I (IFN-α and IFN-β) has a protective role but the pathway is inhibited in infected mice Gene expression related to antigen presentation is down-regulated Destroying the immune evasion of SARS-CoV-2 is imperative in its treatment and specific drug development.

45. At IFN level At the step of type I IFN induction, SARS- CoV interferes with the signaling downstream of RNA sensors directly or indirectly such as ubiquitination It can cause degradation of RNA sensor adaptor molecules MAVS and TRAF3/6 and inhibiting IRF3 nuclear translocation. Inhibits STAT1 phosphorylation – decrease IFN signaling The viral proteins involved - both structural proteins (such as M, N) and non-structural proteins (ORF proteins).

46. Immune responses in COVID-19 and potential vaccines: Lessons learned from SARS and MERS epidemic Eakachai Prompetchara , Chutitorn Ketloy , Tanapat Palaga.APJAI

47. Take home message Viral cycle involves – entry , replication , assembly , release ( budding / lysis ) Innate response involves Type 1 IFN , NK , macrophages Adaptive response starts when dendritic cell presents MHC1 to naïve CD8 cell Cross-presentation is unique for virus CTL is predominant effector cell causing tissue injury – immunopathology-immunity crosstalk Th1 cell stimulates CTL and cytokine (IFN gamma ) release Humoral immunity acts by neutralization mainly Immune evasion – by multiple mechanism –exhaustion and MHC escape important SARS-COV2 pathogenesis –still at research level

48. References Abbas AK, Lichtman AH, Pillai S, editors. Cellular and molecular immunology. 8th ed. Philadelphia: Elsevier Saunders;2015. Borrow P, Nash AA. Immunity to viruses. In: Male D, Brostoff J, Roth D, Roitt I, editors. Immunology. 8th ed. Philadelph Rouse BT, Sehrawat S. Immunity and immunopathology to viruses: what decides the outcome?.  Nat Rev Immunol . 2010;10(7):514–526. Murray PR, Rosenthal KS, Pfaller MA, editors. Medical microbiology. 8th ed. Philadelphia: Elsevier Saunder ; 2014 Li et al., Molecular immune pathogenesis and diagnosis of COVID-19, Journal of Pharmaceutical Analysis