1. Viral immune evasion strategies
Frank Momburg
Division of Molecular Immunology
German Cancer Research Center, Heidelberg

2. Viral immune evasion strategies
1. Viral evasion of MHC class I-mediated antigen presentation
2. Viral evasion of MHC class II-mediated antigen presentation
3. Viral evasion of natural killer cell activation
4. Viral interference of chemokine/cytokine action
5. Viral evasion of antibody responses and complement attack
6. Viral interference with apoptosis

3. Life styles and survival strategies of viruses
„hit and run“
„hit and hide“
Small RNA virus
e.g. human influenza virus
Large DNA virus
e.g. CMV
Interference with innate immunity, typically type I interferon
Blockade of host gene transcription
Antigenic hypervariability
Interference with adaptive immunity
Replication at immunoprivileged sites
Molecular latency
In most cases: elimination of the virus
Persistent infection

4. Cytomegalovirus genome (235 kbp, > 200 open reading frames)
Hepatitis B virus
Influenzavirus
Adenovirus
Vaccinia virus
CMV

5. Antigenic drift and antigenic shift of influenza virus

6. Acute infection and latent phase (herpes simplex virus)

7. Viruses involved in immune escape
Herpesviruses
HSV1,2 Herpes simplex virus types 1, 2
VZV Varicella zoster virus
HCMV Human cytomegalovirus
MCMV Mouse cytomegalovirus
EBV Epstein Barr virus
BHV-4 Bovine herpesvirus 4
EHV Equine herpes virus
PRV Pseudorabies virus
HVS Herpesvirus saimiri
HHV-6,7,8 Human herpesvirus types 6, 7, 8
MHV-68 Mouse 2-herpesvirus 68
Poxviruses
Myxoma virus
Cowpox virus
Vaccinia virus
Molluscum contagiosum virus
Variola virus
Papovaviruses
BPV Bovine papilloma virus
HPV Human papilloma virus
SV40 Simian virus 40
Retroviruses
HIV Human immunodeficiency virus
SIV Simian immunodeficiency virus
MuLV Murine leukemia virus
Others
Adenovirus
Influenzavirus
RSV Respiratory syncytial virus
MV Measles virus
HCV Hepatitis C virus
HBV Hepatitis B virus

8. The innate and the adaptive immune system

9. Presentation of antigenic peptides by MHC class I molecules
Cytotoxic T cell Cb Ca
T cell receptor Vb Va a1 a2
CD8 coreceptor
b2m a3
Antigen presenting cell

10. Role of MHC class I-mediated antigen presentation in antiviral immune responses
The antigen-specific T cell receptors of CD8+ cytotoxic T cells (CTL) recognize MHC class I molecules displayed on the surface of virus-infected cells or on professional antigen presenting cells (dendritic cells, macrophages, B cells).
These MHC-I molecules present peptide antigens that are cleaved out of endogenously synthesized viral proteins by proteasomes and other intracellular proteases. Alternatively, viral antigens can be cross-presented by professional APC after uptake of viral particles or of apoptotic, virus-infected cells.

11. Viral subversion of MHC class I-mediated antigen presentation
1. Interference with the biosynthesis of MHC-I molecules
Adenovirus E1A Transcriptional inhibition (via decrease of NFB) of MHC-I heavy chain, TAP1/2, and LMP2/7 expression
2. Interference with antigen processing by proteasomes
HCMV pp65 Kinase (IE phase), inhibits generation of antigenic peptides from a 72 kDa transcription factor (important antigen in immediate early phase)
EBV EBNA-1 Contains a 239 residue Gly-Ala repeat that blocks proteasomal processing
MuLV env-p15E Single amino acid change (K  R) in FMR subtypes  loss of CTL epitope through altered proteasomal cleavage site

12. Viral subversion of MHC class I-mediated antigen presentation
2. Interference with antigen processing by proteasomes
HCMV pp65 Kinase (IE phase), inhibits generation of antigenic peptides of a 72 kDa transcription factor (important antigen in immediate early phase)
EBV EBNA-1 Contains a 239 residue Gly-Ala repeat that blocks proteasomal processing
MuLV env-p15E Single amino acid change (K  R) in FMR subtypes  loss of CTL epitope through altered proteasomal cleavage site

13. Viral subversion of MHC class I-mediated antigen presentation
3. Interference with peptide transport by TAP
HSV1/2 ICP47 Cytosolic protein blocking peptide binding to TAP
HCMV US6 ER protein that inhibits peptide translocation (inducing a conformational change that prevents ATP binding to TAP1)
BHV/EHV/PRV UL Inhibition of peptide translocation (inducing a transport-incompetent arrest and proteasomal degradation of TAP)
HIV ? Inhibition of TAP

14. Viral subversion of MHC class I-mediated antigen presentation
4. Interference with the heterotrimeric assembly of MHC-I molecules in the ER
HCMV US2, US11 Dislocation of MHC-I through the Sec61p channel to the cytosol  proteasomal degradation
HIV Vpu Dislocation from ER to cytosol  proteasomal degradation
Adenovirus E3/19K Retains MHC-I in the ER, inhibits TAP/tapasin association
HCMV US3 Retains MHC-I in the ER, inhibition of tapasin- dependent peptide loading
MHV-68 mK3 Ubiquitination/degradation of TAP/tapasin associated MHC-I
HPV E5 Retains MHC-I in the Golgi complex

15. Viral factors interfering with the peptide loading complex and assembly of MHC class I molecules

16. Retrotranslocation of MHC class I molecules from the ER to the cytosol for proteasomal degradation

17. Viral subversion of MHC class I-mediated antigen presentation
5. Interference with the intracellular trafficking of MHC-I molecules
MCMV m152/gp40 Retains MHC-I in cis-Golgi network
MCMV m04/gp34 Blocks antigen presentation by MHC-I on the cell surface
MCMV m06/gp48 Targets MHC-I to late endosomes/lysosomes  degradation
HPV E5 Retains MHC-I in the Golgi complex
HIV Nef Enhanced endocytosis of MHC-I  sequestration in trans-Golgi network
HHV-8(KSHV) K3, K5 Enhanced endocytosis of HLA-A, B, (C, E), ubiquitination/degradation
HHV-7 U21 Diversion of MHC-I molecules to lysosomes  degradation

18. Viral factors obstructing the intracellular trafficking of MHC class I molecules

19. Viral inhibition of MHC class I-mediated antigen presentation
6. Mutation of CTL epitopes
Mutations in class I-binding peptides represent an important mechanism to prevent antiviral CTL responses, leading to loss of MHC-I binding, loss of CTL recognition, or antagonism of an existing CTL response.
HIV, EBV, HBV, HCV, Influenza virus

20. Multiple immune evasion mechanisms employed by HIV

21. Role of MHC class II-mediated antigen presentation in antiviral immune responses
Viruses that enter cells through phagocytosis or receptor-mediated endocytosis, or that are enveloped in the trans-Golgi network/early endosome, can undergo degradation into antigenic peptides through the action of endosomal/lysosomal proteases. Such peptides are loaded onto MHC class II (MHC-II) molecules for presentation to CD4+ T cells.
MHC-II molecules are constitutively expressed only by B cells, macrophages, dendritic cells and endothelial cells, however, can be induced on a variety of other cell types by IFN- in the course of inflammtory responses.
Activated CD4+ T cells function as helper T cells assisting the maturation of CD8+ CTL and thus coordinate antiviral responses, or may possess CTL activity themselves (in humans).

22. MHC class II-mediated presentation of exogenous viral antigens

23. MHC class II-mediated presentation of endogenous viral antigens

24. Viral evasion of MHC class II-mediated antigen presentation
1. Interference with the biosynthesis of MHC-II molecules
MCMV M27 IFN-induced transcriptional upregulation of MHC-II expression inhibited (interference with Jak1/STAT pathway  CIITA)
Adenovirus E1A IFN-induced transcriptional upregulation of MHC-II expression inhibited

25. Interferon pathways

26. Viral evasion of MHC class II-mediated antigen presentation
2. Interference with the intracellular trafficking of MHC-II molecules
HCMV US2 Dislocation of MHC-II to cytosol  proteasomal degradation
EBV BZLF2 Association with HLA-DR(DP, DQ)  chains on the cell surface  inhibition of antigen presentation
BPV E6 Interaction with AP-1 adaptor protein  disturbs intracellular transport
BPV/HPV E5 Interaction with 16K subunit of the vacuolar ATPase  endosomal acidification and MHC-II processing 

27. Viral interference with MHC class II-mediated antigen presentation

28. Viral inhibition of MHC class II-mediated antigen presentation
3. Interference with the costimulatory molecule CD4 (T helper cells)
HIV-1,-2, SIV Nef Endocytosis of CD4 via AP-2 adaptor complex (endocytosis motif in Nef), transport to lysosomes via COPI association
HIV-1 Vpu Induction of CD4 ubiquitination in the ER  dislocation to the cytosol  proteasomal degradation
Myxoma virus ? Internalization of CD4  lysosomal degradation
T cell receptor
MHC-II/peptide a1 b1 b2 Vb Va Cb Ca
CD4 coreceptor
Helper T cell
Antigen presenting cell a2

29. Molecular mechanisms employed by the HIV Nef protein

30. Role of NK cells in antiviral immune responses
Natural Killer (NK) cells are important effector components of the innate immune system that function in the initial defense against viruses via direct cellular cytotoxicity and through the production of inflammatory cytokines that promote the influx of CD8+ T cells.
For the control of certain viral infections in mice (RSV, MCMV), an early NK-mediated cytotoxicity and IFN- production plays an important role. In humans, a congenital lack of NK cells is associated with severe herpesvirus infections and low NK cell numbers correlate with more rapid progession towards AIDS in HIV-positive patients.
NK cells are preferentially activated in the presence of low amounts of MHC-I molecules ("missing self") since particular allelic variants of polymorphic MHC-I molecules trigger inhibitory NK receptors.

31. Activation and inhibition of NK cells

32. Viral evasion of NK cell activation
1. MHC-I homologs binding to inhibitory NK receptors
MCMV m144 Peptide-free MHC-I homolog, 2-microglobulin- associated, inhibitory NK receptor unknown
MCMV m157 Distant MHC-I homolog; binding to Ly49I inhibitory receptor in CMV-susceptible mouse strain, to Ly49H activating receptor in CMV-resistant strain
HCMV UL18 MHC-I homolog, 2-microglobulin and peptide- associated, binds to NK-inhibitory LIR-1/ILT-2 receptor

33. Viral evasion of NK cell lysis
2. Selective expression of NK-inhibitory MHC-I molecules
HIV-1 Nef No downregulation of NK-inhibitory HLA-C, -E, -G molecules
HCMV US2, US No downregulation of NK-inhibitory HLA-C, -E, -G molecules
HCMV UL Encodes a leader sequence-derived peptide that is loaded onto inhibitory HLA-E molecules in a TAP-independent fashion
3. Downmodulation of costimulatory molecules
HHV-8 K5 Decreased surface expression and ubiquitination of costimulatory (KSHV) ICAM-1 and B7-2 molecules
HCV E2 Ligation of costimulatory CD81(TAPA-1) tretaspan molecule

34. Viral mechanisms for evading NK cells

35. TAP-independent NK cell inhibition by HLA-E and the HCMV UL40 protein

36. Viral evasion of NK cell lysis
4. Inhibition of NK-activating ligands
HCMV UL16 Intracellular retention of the NK cell activating NKG2D ligands MICB and ULBP1,2 (does not affect MICA and ULBP3)
MCMV m152/gp40 Inhibits surface expression of NK activating ligand H-60

37. Inhibition of NK cell activation by HCMV UL16

38. Role of cytokines and chemokines in antiviral responses
Cytokines and chemokines are secreted polypeptides that coordinate inflammation, cellular activation, proliferation, differentiation, and chemotaxis.
Cytokines and chemokines are immune mediators that are produced early upon virus infection. They induce and maintain innate as well as adaptive immune responses. Cytokines are responsible for flu-like symptoms such as myalgia, fever, headache and drowsiness which are common manifestations of acute virus infections.
Cytokines can be powerful antiviral mediators, allowing clearance of virus infection in the majority of cases. Double-stranded viral RNA  type-I interferons (IFN-/)  protein translation , cell proliferation , cellular RNases for viral RNA .
Pro-inflammatory cytokines are of particular importance and frequently targeted by viruses: IL-1, IL-12, TNF-, IFN-/, IFN-, and several chemokines that activate leukocyte migration.

39. Viral evasion of cytokine action
1. Interruption of cytokine production and maturation
Adenovirus E1A Blocks IRF-3-induced transcription of IFN-/
HPV E Blocks IRF-3-induced transcription of IFN-/
HCMV ? Inhibition of transcription of MCP-1 chemokine
Myxoma virus SERP Inhibition of IL-1 converting enzyme (ICE)  IL-1 
Cowpox virus CrmA/Spi Inhibits several caspases, including ICE
Vaccinia virus B13R Inhibits several caspases, including ICE
Measles virus Hemagglutinin Binding to CD46 (complement receptor/regulator for C3b/C4b) on infected macrophages/DC  IL-12 production   Th1 response (IFN- production by T and NK cells) 

40. Viral interference with chemokine action

41. Viral evasion of chemokine/cytokine action
2. Interference with the receipt of the chemokine/cytokine signal
A. Viral chemokine receptor homologs
HCMV US28 Functional CCR1 chemokine receptor (binds MCP-1, MIP-1, RANTES), co-receptor for HIV entry
HCMV UL33 CCR1 chemokine receptor, expressed in viral coat
MCMV M33 CCR1 homolog, role in salivary gland dissemination and replication
HVS ECRF3 Functional CXCR2 chemokine receptor
HHV-8 ORF74 Constitutively active, agonist-independent CXCR2 receptor
HHV-6/7 ? Downregulation of cellular CXCR4 chemokine receptor in infected CD4+ T cells  response to ligand SDF-1 
B. Viral chemokine/cytokine homologs
HHV-8 vMIP-II Broad spectrum CC-, CXC- and CX3C chemokine antagonist
MCV MC-148P Broad spectrum CC- and CXC chemokine antagonist
HHV-8 vMIP-I MIP-1 homolog, CCR8 agonist, Th2 response 
HHV-6 U83 MIP-1 homolog, CC chemokine agonist
MCMV m131 CC chemokine homolog, promotes virus dissemination
HHV-8 vIL-6 IL-6 homolog, increases angiogenesis and hematopoesis (role in Kaposi sarkoma, IL-6R+)
EBV vIL-10 IL-10 homolog, antagonizes Th1 responses
MCV MC-51L Secreted, binds IL-18  NK activation, Th1 response
C. Virus-encoded secreted cytokine receptors and chemokine-binding proteins
Cowpox virus CrmB, CrmC Secreted TNF-R homologs, sequester TNF- and LT-
Myxoma virus MT Secreted TNF-R homolog, sequesters and inhibits TNF-
HCMV UL TNF-R homolog, retained intracellularly
Vaccinia virus B18-R Secreted type I IFN-R homolog, sequesters IFN-
Vaccinia virus B8-R Secreted type II IFN-R homolog, sequesters IFN-
Myxoma virus M-T Secreted type II IFN-R homolog, sequesters IFN-
Vaccinia virus B15-R Secreted IL-1-R homolog, sequesters IL-1
EBV BARF Secreted protein, sequesters CSF-1

42. Viral evasion of chemokine/cytokine action
3. Interference with interferon-mediated effector functions
A. Inhibition of IFN-induced gene transcription
Adenovirus E1A Depletion of STAT1 or p48  ISGF3  IFN-induced transcriptional activation 
HPV E7 Sequestering of IRF1 IFN-induced gene transcription 
HCMV ? Depletion of Jak1 kinase and p48  ISGF3 
HHV-8 vIRF K9 IRF antagonist, competition with IFN-induced transcription
B. Interference with IFN-induced cellular defence mechanisms
EBV EBNA2 IRF antagonist, competition with IFN-induced transcription
HCV E2 Inhibits phosphorylation of eIF2 by dsRNA-dependent protein kinase (PKR)
HSV  Activates protein phosphorylase 1 to dephosphorylate eIF2
Vaccinia virus K3L eIF2 homolog, inhibits PKR
Adenovirus VAI RNA Viral RNA, inhibits PKR
EBV EBER I Viral RNA, inhibits PKR
Vaccinia virus E3L Sequesters ds-RNA, prevents PKR and 2’5’-OS activation
HSV '-5'-(A) analog Inhibits 2'-5'-oligoadenylate synthase  RNaseL activity 

43. Viral evasion of antiviral cytokine effector functions

44. Role of antibodies and complement in antiviral reponses
Protective antibodies bind to virus surface structures and can block their interaction with cellular receptors.
Antibody-tagged (opsonized) viruses can be cleared from the circulation via IgG-Fc receptors expressed by phagocytes or by germinal center follicular dendritic cells ( viral spread possible).
Fc receptors (CD16) instruct NK cells to lyse antibody-coated virus-infected cells by antibody-dependent cellular cytotoxicity (ADCC).
IgM- and IgG-coated viruses can be neutralized by the classical complement pathway.
Viruses tagged with the complement component C3b (or C4b, C3bi) can be phagocytosed via the CR1 (CD35) complement receptor.
Viruses coated with C3b cleavage products (C3d, C3bi) can activate the CR2 (CD21) complement receptor of B cells. Follicular dendritic cells expressing CR1/CR2 complement receptors trap opsonized viruses and stimulate antibody production by germinal center B cells.
Phagocytosis of opsonized viruses elicits antigen processing, release of pro-inflammatory cytokines and T cell activation. Byproducts of the complement pathway (C3a, C4a, C5a) function as chemotactic peptides and amplify the inflammatory process.

45. Fc receptors

46. Viral evasion of antibody responses
Viral Fc receptor homologs
HSV-1 gE-gI FcR homolog for monomeric or aggregated IgG, expressed on virus particles or infected cells, gE contains YXXL internalization motif for endocytosis of immune complexes inhibition of ADCC
HSV-2 gE FcR homolog
MCMV Fcr1 FcR homolog
PRV gE-gI FcR homolog
VZV gE-gI FcR homolog
MV NP Ligates cellular inhibitory FcRII receptor (CD32)
downregulation of antibody production by B cells

47. Evasion of antibody responses by viral Fc receptors

48. The complement pathways and their regulation

49. Viral subversion of complement responses
1. Enhanced factor I-mediated inactivation of C3b, C4b, or C3 convertases by viral proteins mimicking regulators of complement activation (RCA)
HSV1/2 gC-1/2 CR1(CD35) homolog
HVS ORF-4 C4b-BP, CD46(MCP), CD55(DAF) homolog
HHV-8 CCPH C4b-BP, CD46(MCP), CD55(DAF) homolog
Cowpox virus IMP C4b-BP homolog
Vaccinia virus VCP C4b-BP homolog
Variola virus SPICE C4b-BP homolog
HCMV ? upregulation of host cell CD46 and CD55
HIV ? downregulation of host cell CR1/CR2, recruitment of factor H by gp41 and gp120
2. Inhibition of C9 polymerisation
HVS HVS-CD59 CD59(MIRL) homolog

50. Viral evasion of complement activation

51. Viral interference with apoptosis (programmed cell death)
Replicating viruses may stimulate suicide of the host cell directly or provoke recognition by cytolytic T cells and NK cells.
These immune effector cells induce apoptosis by secretion of cytotoxic cytokines (such as TNF) and by processes requiring direct cell-cell contact (perforin/granzyme system or CD95/CD95-L system).
Premature cell death would limit the time available for the production of new virions and interrupt cycles of latency and reactivation used by persistent viruses.

52. Molecular pathways of apoptosis

53. Viral evasion of the extrinsic apoptosis pathway
1. Inhibition of TNF/Fas-mediated apoptosis
Myxoma virus MT-2 TNF-R homolog, secreted form blocks TNF- induced apoptosis, intracellular form protects T cells from apoptosis
Cowpox virus CrmB, C, D Secreted TNF-R homologs, neutralize TNF- and LT-
Adenovirus E3-10.4/ Multimeric complex (RID) that forces internalization and K lysosomal degradation of CD95 (Fas, APO-1)
HCMV UL TNF-R homolog, retained intracellularly, function unclear
2. Death effector domain (DED)-containing viral proteins (vFLIPs)
HHV-8 K Viral FLICE inhibitory proteins (vFLIPs) with 2 DEDs, bind
HVS ORF FADD/TRADD adaptor proteins, prevent activation of
MCV MC159, FLICE (Caspase-8)/Caspase-10 following ligation of CD95,
BHV-4 BORFE TNFR1, TRAMP, TRAIL-R1/R2 death receptors

54. Viral regulation of the extrinsic apoptosis pathway

55. Viral evasion of the intrinsic apoptosis pathway
3. Viral caspase inhibitors
Cowpox virus CrmA/SPI Serpin, inhibits caspase-8, caspase-1 (ICE), granzyme B
Vaccinia virus SPI-2/B13R2 Serpin, inhibits caspase-8, caspase-1 (ICE), granzyme B
Baculoviruses p35 Inhibits multiple caspases
vIAP Inhibits initiator caspase-9 and effector caspases-3 and -7
Adenovirus 14.7K Inhibits caspase-8
4. Viral antiapoptotic Bcl-2 homologs
EBV BHRF1 Bcl-2 homologs, prevent cytochrom c release from BALF1 mitochondria  activation of caspase-9 
HHV-8, HVS ORF16 dito
Adenovirus E1B/19K dito
EBV LMP-1 Bcl-2 transcription mimics anti-apoptotic TNF-R signals

56. Viral regulation of the intrinsic apoptosis pathway

57. Viral immune evasion strategies
Co-evolution of viruses with their hosts for millions of years has led to a host immune system of high complexicity and, likewise, sophisticated viral mechanisms to antagonize immunity.
Viral antagonists interfere with all relevant effector pathways of the innate and the adaptive immune system including
the MHC class I-mediated antigen presentation activating cytotoxic T cells
the MHC class II-mediated antigen presentation activating helper T cells
the activation of natural killer cells
the function chemokines and cytokines orchestrating cellular antiviral responses
the function of antiviral antibodies
the function of the complement system
the induction of the extrinsic and intrinsic pathways of programmed cell death