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Cell Biology and Pathogenesis Molecular and cellular

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					            M229: Cell Biology and Pathogenesis
Molecular and cellular biology of herpesvirus pathogenesis
                              February 2002

                                Ren Sun

                          rsnu@mednet.ucla.edu




     1. Introduction of herpesviruses
     2. Mechanism of herpesviral replication
     3. Herpesviral latency
     4. Transformation by Epstein-Barr virus

     Discussion of viral oncogene.
Take-home messages:
1. The purpose of viral life is to replicate itself. (its associated
        pathogenesis or human diseases are secondary effects)

2. Virus utilizes host machinery to replicate.
         (cellular and organal)

3. The ultimate form of parasitization is to co-exist with the host.
       (at both individual and population levels)

4. Studies of viruses have double meanings.
        (medically important and scientifically interesting)

5. Herpesvirus has two phases of the life cycle:
       latent infection and lytic infection.


        herpein: to creep (Greek)
Herpesvirus particle
Electron cryomicroscopy and 3D reconstruction of HSV-1 B-capsids
                                         Z.Hong Zhou et al. Science, 2000 April
       Virion structure of herpesvirus

1. Herpesvirus particle is composed of an icosahedral
capsid, containing a large linear double-stranded DNA
genome.

2. Viral genome is a double-stranded DNA (150 kb to
250 kb), encoding about 100 genes. Only 40-60% of the
genes are essential for viral replication in tissue culture.

3. The capsid is surrounded by a tegument and wrapped
in a lipid envelope. The tegument, which consists of viral
proteins, is unique to herpesviruses.

4. The particle is about 200 to 300 nm with 100 nm capsid.
              Herpesviruses

• Large genome, complex gene expression and
  regulation
• Ubiquitous infections, primary infections
  usually in-apparent in childhood
• latent/persistent/recurrent infections
• Transition between lytic and latent replication
• Associated with acute and chronic/malignant
  diseases
                 Human herpesviruses
Name Common name associated diseases            subfamily   size

HHV-1 Herpes Simplex Virus 1 (HVS-1)                α       150kb
      Oral, ocular lesions; encephalitis
HHV-2 Herpes Simplex Virus 2 (HSV-2)                α       150kb
      Genital oral lesions; neonatal infections
HHV-3 Varicella Zoster virus (VZV)                  α       130kb
      Chickenpox, shingles
HHV-4 Epstein-Barr virus (EBV)                      γ       170kb
      Infectious Mononucleosis; tumors (BL, NPC, NHL)
HHV-5 Human Cytomegalovirus (HCMV)                  β       230kb
      Congenital infection; systemic infection
HHV-6 Human herpesvirus 6                           β       160kb
      Exanthem subitum; systemic infection
HHV-7 Human herpesvirus 7                           β       160kb
      Exanthem subitum?
HHV-8 Kaposi’s Sarcoma associated herpesvirus       γ       140kb
      Kaposi’s Sarcoma, B cell lymphomas
                 Herpesvirus family
 highly co-evolved with host
 widely disseminated froom human to flog, fish and snake


• Alphaherpesviruses
   – Fast replication, cytolytic; latent in neurons (Herpes
     Simplex, Varicellar Zoster)
• Betaherpesviruses
   – Intermediate replication, cytomegalic; latent in
     glands, kidneys (Cytomegalovirus, HHV-6, -7)
• Gammaherpesviruses
   – Slow replication, latent in lymphocytes,
     lymphoproliferative, (Epstein-Barr virus, HHV-8)
       What is Latency? and why there is latency?
At cellular level, Latency is the reversibly nonproductive
infection of a cell by a replication-competent virus.

(1) They can successfully evade the host immune response.

(2) They enable their genome to persist in the latently
       infected cell, thereby in the host.

(3) They co-exist with the host cells and the host.

(4) The host becomes a active carrier for transmission.

All herpesviruses are capable of establishing latent infection.

Please note: The difference with abortive infection or infection with
defective virus.
        Mechanisms to establish latency
1. HSV infects non-dividing cells such as neurons.

No protein expression.

2. EBV infects dividing or mitotic cells such as B cells.

A set of viral protein(s) and origin of DNA replication initiation is
required for replicating viral genome.

Function of partition is required to retain the viral genome in
daughter cells during the latent infection.

3. Use RNAs to avoid antigenicity.

4. Down-regulation of presentation of viral antigen(s).
                    Life cycle of herpes simplex virus




“Fields Virology”
 HSV-1 infection and
 lytic replication




  HSV-1 latency and
  reactivation




“Fields Virology”
                   A Model of EBV Life Cycles in vivo
    Primary infection                                                  Secondary infection
    of epithelium                                                      of epithelium
                                                   Lytic




        10 infection                                         Reactivation
          of B cell
                                    III                                               III III
                         III                                                                    III
                                                             I/II
                   III               III                                              III III
                              III          Latency switch?          Latency switch?
Resting B cell


                               Kill                                                          Kill




Resting T cell                                             Memory CTL
                         10   CTL response                                            20 CTL response


                                                                    Persistent infection
       Primary infection
                Viral entry of cell
                       (simplex virus)

(1) Virus attachment to the host cell.
         envelope glycoproteins (gC and gB) binds heparin sulfate

(2) Binding of viral surface protein (gD) to one of the cellular
        receptors (herpesvirus entry mediators, Hve)
        HveA (the tumor necrosis factor receptor family)
        HveB and C (nectins, Immunoglobulin superfamily)

(3) Fusion of the virus envelope with cellular membrane.
        gI and gE appear immediately on surface after entry

Multiple interactions during binding, penetration and uncoating.
        (2) Virus attachment to the host cell.

EBV envelope glycoprotein gp350/220 binds to CD21(type 2
complement receptor, CR2) directly, in a fashion similar to that of
C3d of complement.
Activation of tyrosine kinase signal transduction, activate the B cell.
gp42 (BZLF2) binds to HLA class II (MHC class II) as co-receptor.
gp85/gp25/gp42 oomplex mediates membrane fusion.

HHV-7 binds to the CD4.      CD4 is not by itself sufficient for entry.
HHV-7 also can infect certain CD4-negative cells.

Each herpesvirus uses different viral envelope protein to bind
different cellular receptor.
Receptors for other herpesviruses are to be identified.
3) Penetration and uncoating of the virus particle.

For HSV-1, this occurs via a process of fusion of the viral
envelope with the cell plasma membrane. HSV-1
glycoproteins gB, gD, gH and gL involve in the fusion.

Capsid binds to microtubules and is propelled by dynein
(microtubule-dependant motor). Capsid moves to nuclear
pore. Binding to nuclear pore complex causes conformation
change of the capsid and injection of genome into nucleus.

Functions of some virion proteins in the tegument:
      Host shut-off protein (vhs)
      Transcriptional activator (VP16)
      Protein kinases
      Viral mRNAs in virions of HCMV (Bresnahan et al Science 2000)
        Herpesvirus genomes organization

                                           Isomers
e.g.Channel catfish herpesvirus
                                             1

e.g.Human herpesvirus-8
                                             1
e.g.Epstein Barr virus
                                             1

e.g.Pseudorabies virus
                                              2
e.g.Herpes simplex virus
                                              4
 Circularization and replication of viral genome

For herpes simplex virus, the linear genome ends are held
together by viral protein and immediately ligated by cellular
enzymes (via recombination process). The genome remains
nonnucleosomal during lytic replication (nucleosomal during
latency).

For EBV, active DNA synthesis is required over 24 to 36 hours.

The DNA is replicated by viral DNA polymerase during lytic
replication in a mechanism similar to rolling-circle model. Initiated
at specific location(s) by viral protein(s).

Gammaherpesvirus (EBV and HHV-8) DNA is replicated by
cellular DNA polymerase during latent infection in a mechanism
similar to plasmid replication in E.coli. A viral protein Initiates the
replication at a specific location and partition to daughter cells.
 Viral DNA replication in the lytic infection

A. Replication origin.
1. Origin of DNA replication during the lytic cycles,
       initially learned from defective particles.
2. A/T rich palindromic sequences,
       located near transcription initiation sites.
3. At least two lytic origin in each wild type virus.
       One is sufficient in vitro.

B. Mechanism of replication
1. Theta form at the early phase.
2. DNA recombination converted to rolling-circle model
      with extensive branches.
3. The final products are head-to-tail concatamers.
    Cleavage and package of viral genome
1. Lytic replication and terminal repeats are essential.
2. pac 1 and pac 2 sequences, located at the each end of the genome,
         are conserved in most herpesviruses.
3. Each herpesvirus has a different terminal sequence arrangement.

              Rolling circle replication




                                                                     Cleavage
                                     Pac 1   Pac 2 Pac 1   Pac 2     & package
         Lytic replication



                   Infection &                              Egress
                   circularization
             Isomerization of HSV genome:
High efficiency and specificity, equal mole after one round of
infection. Defect does not affect in vitro viral replication, but all wild
isolates can isomerize.

   b                 UL                     b’   c US        c’
                                                                      1:1
 Replication and Isomerization
                                                                      1:4
                                                                      1:4
                                                                      1:4
                                                                      1:4
Lytic DNA replication and cleavage of gamma-herpesvirus genome


           Vector        Flag-Rta     Rta                   Rolling-Circle Replication

  I   12    24 36 48 12 24 36 48 12 24 36 48 hr p.t.




                                                       Multimer




                                                                           HindIII


                                                       Monomer




      I : infected BHK cells                                                         Probe
                Maturation of virion:
A. Capsid:
1. Capsid is assembled in the nucleus. Empty capsid can self-
       assemble in vivo and in vitro (from purified proteins).
2. DNA is packed into pre-assembled capsids containing scaffolding
       proteins, proteinase and other viral proteins.
3. Head-full mechanism to measure the length of the DNA.
4. Terminal repeat sequences (pac1 and pac 2) are required. Processing
       massive and inter-connected, non-linear DNA. Capsid proteins
       binding to viral DNA, probably no histones. 70,000 spermidine and
       40,000 spermine per capsid.
B. Envelope:
1. Envelop at nuclear membrane, de-envelop in cytoplasm and re-envelop
       at cell membrane.
2. There are few cellular proteins and a lot viral proteins in envelope..
C. Tegument:
1. 20 to 40 viral proteins (some are well organized & attached to capsid).
2. Some RNAs, eg HCMV.
  Cellular changes during lytic (productive) infection

1. Deposition of materials (tegument proteins) on nuclear
      membranes or into nucleus. Insertion of viral glycoproteins
      into cytoplasmic and plasma membranes.
2. Nucleolus enlarged, displaced toward the nuclear membrane
       and dis-aggregated later.
3. Fusion of infected cells with un-infected cells to form
       polykarycytes (polykarycytosis). Syncytium is common to
       many viruses.
4. Destabilization of all mRNAs by a tegument protein vhs (virion
       host shot-off),which removes preexisting host mRNAs. Vhs
       induce endoribonucleolytic cleavage. Host protein synthesis
       are shot-off at initial infection to ensure the expression of
       viral immediate-early genes.
Cellular changes during lytic (productive) infection

5. Viral immediate-early gene products stimulate
       cellular metabolism (eg. Inactivate p53 & Rb).

6. Host DNA and protein synthesis shot-off during viral
      DNA replication, to ensure the synthesis of viral
      DNA and structure genes.

7. Apoptosis is actively inhibited by viral functions in all
      stages of viral replication (encoding Bcl2, inactivating
      Rb & p53), but the cell is ultimately destructed.

8. A late protein γ34.5 is required in neuron cells to prevent
       triggering of cellular stress responses that result in a
       premature total shut off of protein synthesis.
     Gene expression Kinetics of HSV
                         Hours post-infection


0   2        4    5       7        10             15              20


Peak gene Peak gene                       Late ( gene
expression expression                     expression
Immediate-early       Early                     Late



                              DNA replication

                                                Virion assembly
            DNA viruses inactivate p53
After viral infection, p53 becomes activated and induces the
apoptosis pathway. Most DNA viruses have evolved mechanisms to
inhibit p53.
SV40 Large T-antigen binds to the p53 DNA-binding domain,
preventing p53 from binding to its control elements in DNA.
Ad E1B 55K stimulates p53 DNA-binding, but it also contains a
strong repression domain, so that it turns p53 from an activator into
a repressor of genes regulated by p53.
HPV E6 protein binds to p53 and induce its degradation via
ubiqutination and proteosome degradation.

EBV Zebra inactivates p53 by direct binding. EBV Rta binds to Rb.
HHV-8 LANA inhibits the trans-activation function of p53.
                                      (Friborg et al, Nature, 1999)
     Cellular targets of DNA tumor viruses
                            Ad, SV40, HPV, EBV, HHV-8
        M                   E1A, T,    E7, Rta, Rta?


G2
                  G1                  Rb

                                     E2F

                                             EBV induces
                       G1 Cdk-Cyclins        HHV-8 encodes
       S
                          Cki1                   Ad      E1B
        Ad    E1B                     Bcl2       EBV     bcl2
        SV 40 T                                  HHV-8   bcl2
        HPV   E6
        EBV   Zebra
                          p53       Apoptosis
        HHV-8 LANA
Expression and Functions of Herpesviral Genes

  Latent genes Suppressers of immune recognition,
                  Activators of transcription & proliferation
    +      ?
      genes        Transcription activators,
                   Suppressers of immune recognition
Immediate-early
     +     _
                   Non-structural regulatory proteins,
        genes      Enzymes for viral DNA replication,
       Early       Inhibitors of apoptosis
   +      _
       genes       Major structural proteins,
       Late        Regulatory factors in virion (VP16, vhs)
           DNA replication enzymes

• Defining characteristic of herpesviruses (reactivation
  from resting cells).
• Early ( ) genes encode 2 groups proteins which
  increase DNA synthesis
   – Increase DNA replication (viral DNA polymerase,
     single-stranded DNA binding protein,
     helicase/primase, etc)
   – Increase nucleic acid metabolism (thymidine
     kinase, ribonucleotide reductase, dUTPase)
       Thymidine

           Thymidine       Kinase


       d TMP                dTDP            dTTP        DNA
                            Dihydrofolate
                                                   Dihydrofolate
               Thymidylate      Synthase              Reductase
        dUMP        methylene       THF    THF

           dUTPase
dUDP    dUTP        dCTP
   Ribonucleotide
      Reductase
UDP

        UMP
Human herpesvirus-8 encoded cellular homologues
ssDNA binding protein                Complement Binding Protein
DNA Polymerase                       IL-6
DHFR                                 vMIP1 (macrophage Inflammatory Protein)
TS                                   vMIP2
TK                                   vMIP3
Alkaline Exonuclease
Helicase-primase                     Bcl-2
Uracil DNA glucosidase               Interferon Response Factor-1
dUTPase                              Cyclin D
Ribonucleotide Reductase, small      FLIP
Ribonucleotide Reductase, large      OX-2
                                     IL-8 receptor

                                     AP-1 like transcriptional activator
                                     myb like transcriptional activator


 In simplex virus, nearly 50 % of viral genes are not required for replication
 in fibroblast cells in culture.
Gene expression cascade during lytic replication

                              Virion
                        $
                       VP16


              +

     _             +            +
+         -genes       -genes          -genes
                   _            _
                        +
                        _
The balance between latency and lytic replication


                                      $        Virion
                                   VP16, vhs
              latent-genes
Latency
                               +
Lytic replication
                               +                +
          +      _    -genes        -genes              -genes
                               _                _
                                       +
                                       _
        Latency of herpes simplex virus
1. Viral DNA exists as a circular double stranded DNA in the
       nucleus of neuron cells for life time until reactivation.

2. LATs (latency-associated transcripts, discovered by Jack
      Stevens at UCLA, are the only RNAs expressed during
      latency. 2 kb LATs are circular RNAs, resulted from
      splicing of a 8.3 kb primary transcript. Function is not
      clear.

3. LATs are non-coding nuclear RNAs. No viral gene proteins
      are expressed or required to establishing latency!!??

4. The molecular mechanism of HSV latency and reactivation
      is not clear.
   Epstein-Barr virus (gammaherpesvirus)
• Lytic infection in epithelial cells of
  nasopharynx and salivary gland
• Latent in B lymphocytes
• Multiple copies of viral episome in nucleus
  are replicated synchronously with cellular
  chromosomal replication.
• Sets of latent gene products (proteins and
  RNAs) contribute to viral episome replication
  and cell proliferation.
        Epstein-Barr virus and cancer
•   Nasopharyngeal carcinoma
    – One of the most prevalent malignancies in East Asian
    – Antibody responses to EBV antigens
    – EBV genome always found in tumor cells
•   Gastric carcinoma (NCNT)
    – 1000 % in NCNT 1
    – 10 % in general gastric carcinoma
    – EBV genome always found in tumor cells, high a-EBV IgA
•   Hodgkin’s lymphoma
    – Widespread in W Europe and US
    – EBV DNA in Reed-Sternberg cells (~50% cases)
•   Burkitt’s lymphoma
    – Common in Africa, EBV always found in endemic BL tumor cells
    – EBV genomes (parts) found in some sporadic BL
•   Immunoblastic B-cell lymphomas
    – Common in the immunosuppressed patients
    – EBV genome always found in tumor cells
    Evidence for EBV is a tumor virus
     (First human tumor virus, 1964)

• Sero-epidemiology, elevated antibody titers:
   VCA (IgA), EA (IgG)
•   Viral genome and expression in tumor cells
•   Monoclonal viral genome in tumor cells
•   Transform primary B cells in vitro
•   Cause tumor in new world monkey
•   Prevent and cure of immunoblastic
    lymphoma by CTL specific for EBV antigens
    Cleavage and package of viral genome
1. Lytic replication origin and terminal repeats are essential.
2. pac 1 and pac 2 sequences, located at the each end of the genome,
         are conserved in most herpesviruses.
3. Each herpesvirus has a different terminal sequence arrangement.

              Rolling circle replication




                                                                     Cleavage
                                     Pac 1   Pac 2 Pac 1   Pac 2     & package
         Lytic replication



                   Infection &                              Egress
                   circularization
Lytic DNA replication, cleavage package and circularization of
              gamma-herpesvirus genome




Multimer
                                                       Transformation &
                                                       clonal tumor



                HindIII
Monomer
Establishment of latency in primary B cells (in vitro):
Virology
1. Activation of B cells by binding of virions to the receptor CD21.
2. Circularization of genome requires (repair) DNA synthesis.
3. The copy number per cell increases in the first week to
about 10 to 50 per cell without lytic replication, and remains
stable thereafter. Viral genome is replicated, once and only once,
by cellular DNA polymerase in early S phase. The only viral
protein required for maintaining latent replication is EBNA1
(Epstein-Barr viral nuclear antigen 1).
4. The viral genome undergoes progressive methylation,
except the OriP region, which contains multiple binding sites for
EBNA1 and is the nuclear matrix attachment
site and transcription regulation sequences for latent RNAs.
      Transformation of primary B cells (in vitro)
1. Six viral nuclear antigens (EBNAs) are expressed. Many
of them have transforming activities.
2. Three cytoplamic membrane protein (Latent membrane
protein, LMPs) have transforming activities.
3. Two novel RNAs, Epstein-Barr virus encoded RNAs
(EBER1, EBER2) are expressed to over one million copies
per cell. Good diagnosis marker for EBV related diseases.
Transforming activity?
4. The latency can be disrupted and switch to lytic replication.
The frequency is dependent on the host and can be
enhanced by TPA, sodium butyrate or α-IgM antibody
crosslinking. The switch is controlled by transcription factors
Zebra and Rta. The lytic genes are expressed in a cascade.
Establishment of latency and transformation of
primary B cells (in vitro, at least three steps)
Cell biology:
1. Activation of primary B cells by binding of the receptor.
2. Induce B cell gene expression and proliferation in the
way similar that in responses to antigens, mitogens or IL-4 or
α-CD40. The cells are dependent of cytokines and high cell
density. This is the results of the expression EBNA2 and
LMP1 (type II latency).

3. The expression of other EBNAs and LMP2s leads to
full transformation. EBV in these cells induces express of
bcl-2, activates the signal transduction pathways of TRAFs,
Notch, NFκB and inactivates Rb.
    Latent genes of Epstein Barr virus
• 6 Epstein-Barr virus Nuclear Antigens (EBNA’s)
  – EBNA 1 maintains viral episome via OriP.
  – EBNA 2, 3A, 3B, 3C and LP involve in proliferation
    via Notch signaling pathway. (Hsieh, Science 1995, PNAS 1999)
• 3 latent membrane proteins (LMP’s)
  – LMP1 activates membrane signaling (TRAFs)
    activate proliferation and block apoptosis.
  – LMP2 A&B sequester B cell receptor-associated
    tyrosine kinase (α-IgM reactivates EBV), inhibit
    reactivation from latency. Not directly contribute to
    transformation, but necessary.
• 2 small RNAs (EBERs): transformation?
Activation of TRAF signal transduction pathway by EBV LMP1



TNFR
                        TNF                    TNFR



                              TRAFs
        TRAFs                                NFkB
                                                 Growth
                                   LMP1

EBV infection




                                  TRAFs       NFkB
                                                 Growth
        Notch signal transduction pathway


Notch                                                        Delta
                                              Notch


                                                             Protease cleavage
           Notch IC                                        Notch IC

                       HDAC
                      SAPCIR                                   Notch IC
                               X
                      CBF/Su                                         CBF/Su




                                          After ligand binding and cleavage, Notch IC with an
Without ligand binding, CIR recruit SAP   activation domain, enters nucleus, replaces CIR and
and HDAC which represses transcription.   activates transcription.
         Activation of Notch pathway by EBV



Notch                                          Notch




            Notch IC                                       Notch IC


                                                                      EBNA3
                 EBNA2
                                                                   CBF/Su
                       CBF/Su




 EBNA2 with an activation domain, enters nucleus,      EBNA3A&C binds CBF, prevents DNA
 replaces CIR, release the suppression and activates   binding and releases the suppression.
 transcription.
Gene expression regulation during latency
EBERs are always expressed.
Type I === use of Qp.
Expression of EBNA1 only, no CTL recognition. EBNA1 is essential forgenomic
Maintenance.
Type II === use of Qp and LMP2p
Expression of EBNA1 and low levels of LMP2s, weak CTL recognition.
Type III=== use of Cp, Wp and LMP1&2p.
Expression of all six EBNAs, and higher levels of LMPs. Presenting
strong transforming activities and strong CTL epitopes.


                            Circular genome

        Cp   Wp                         Qp            LMP1p LMP2p




                                                                TR
EBERs
 OriP




                  EBNA2-6                     EBNA1   LMP1           LMP2
 Switch among different latent forms and lytic cycle
                   Latency I
                     EBNA1
                     EBERs


Latency II                         Latency III
  EBNA1                                EBNA1, 2,
  EBERs                               3A,3B,3C,LP
                                       EBERs
  LMP1,2s
                                        LMP1,2s



                   Lytic cycle
             p53
                    Zebra
             Rb     Rta
                                      $     Virion
                   A Model of EBV Life Cycles in vivo
    Primary infection                                                  Secondary infection
    of epithelium                                                      of epithelium
                                                   Lytic




        10 infection                                         Reactivation
          of B cell
                                    III                                               III III
                         III                                                                    III
                                                             I/II
                   III               III                                              III III
                              III          Latency switch?          Latency switch?
Resting B cell


                               Kill                                                          Kill




Resting T cell                                             Memory CTL
                         10   CTL response                                            20 CTL response


                                                                    Persistent infection
       Primary infection
Similarity of Gammaherpesvirus Genome Organization



MHV-68      4-11           17-50        *        52-69         72-75




HHV-8    4-11               17-50                    80
                                                     52-69                   72-75
                                            *




EBV       4-11        17-50             *                52-69                               75




                 20   40           60           80       100           120       140   160        Kb
                        Genomic difference among
                         gammaherepesviruses
                                                                                 ORF72 cyclin D
                                                        ORF50 Rta
                                                    *                            ORF73 LANA
                                                                                 ORF74 IL-8R(GPCR)

        tRNA 1-8                                        M8              bcl-2

         M1-4        M5,K3,M6                       M7                 M10a,b,c              M12-14
MHV-68             4-11                17-50        *          52-69           72-75
                            PAN

                   IL-6,DHFR,K3,TS,
                                                             vIRF,K10-11               OX-2
                   MIP-II,K5,MIP-I
         K1        bcl-2                                Zebra                     K12,FLIP

HHV-8         4-11                      17-50                       80
                                                                    52-69                     72-75
                                                        *
                                                    Zebra,BZLF2
        LMP1,2                                      EBNA3a-c                                                EBER1-2
        BARF1        BILF1                          gp350/220
        BALF2        BILF2               EBNA1      BLLF2                          BHRF1,EBNA2,EBNA4,vIL-10,LMP2

EBV             4-11              17-50             *                  52-69                                     75


                       20         40           60             80         100           120            140      160    Kb
         KSHV gene expression program
                     +
     Latency                     Lytic replication



   Latent      Immediate-early     Early        Late
                           PAN RNA            sVCA
                             vIL-6
Cyclin D                      vMIP-I
 FLIP        +            +     vMIP-II
                    Rta
 LNA     n-butyrate               vMIP-III
            TPA                    vIL-8 receptor
                                    Bcl-2
                                      DHFR
                                        TS
                                           TK
Mechanism controlling the balance between latency and lytic replication of KSHV



       Latency                                Lytic replication


                          NF B
                           p65
          +
                                                      PAN RNA, vIL-6
Latent                                   +                    &
                            Rta
genes                                                 Other lytic genes
               +             +
              PKC

                        +
      The life cycle of gammaherpesvirus

                               $         Virion
Transformation              Tegument
                                         cell death




     Latency               Lytic cycle
                  Rta
                  -genes      -genes         -genes

                   +
       Biological implication of complicated gene
             expression regulation of EBV
Multiple level of regulation:
       The switch between type I, II, and III latencies;
       The switch between latency and lytic replication.

Purposes:
       Evade the immune system, maximize production of
virions or proliferation of infected cells and minimize damage
to the host.

Conclusion:
        The viral gene expression regulation plays a critical
role in determining the biology and pathogenesis of EBV
Infection.
Take-home messages:
1. The purpose of viral life is to replicate itself. (its associated
        pathogenesis or human diseases are secondary effects)

2. Virus utilizes host machinery to replicate.
         (cellular and organal)

3. The ultimate form of parasitization is to co-exist with the host.
       (at both individual and population levels)

4. Studies of viruses have double meanings.
        (medically important and scientifically interesting)

5. Herpesvirus has two phases of the life cycle:
       latent infection and lytic infection.


        herpein: to creep (Greek)

				
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