Human Herpesviruses: An evolutionary wonderalthough, not a simple one.

Anything produced by evolution is bound to be a bit of a mess. - Sydney Brenner

Around here, it takes all the running you can do just to stay in the same place.-Lewis Carroll The Red Queen to Alice in Alice in Wonderland

Outline: I. General BackgroundCoexisting with viruses: friends or foe in evolution?Outcomes of InfectionDNA Virus evolutionII. Herpesvirus Background and Human Cytomegalovirus (HCMV)

III. Two examples of evolution: HCMV vs. Human hostInnate Host ResponseAdaptive Host Response

IV. HCMV Latency: paradigms, trends, and work in progress

You will encounter viruses:

Viruses are in everything we touch, eat, breathe: they even become part of our genome!

Every milliliter of sea water has >1 million virus particles

There are 1030 bacteriophage in the worlds water supply, each particle weighing a femtogram. Thus,the planets biomass of bacterial viruses alone exceeds that of elephants by more than 1000-fold.

There are 1016 HIV particles on the planet.

A whale secretes 1013 Calciviruses (whale diarrhea) each day, and they can infect humans!

So, now what?!?!We live and prosper in a literal sea of viruses Viruses dont just impact our biology (disease), they are part of it.

So, you encounter a virus (and you are going to), what now?!We encounter billions of virus particles everyday, the outcome is varied:Nothing

Infection and clearance

Infection and latency

Infection and persistence

Infection and death

Cancer

So, you encounter a virus (and you are going to), what now?!

Viruses are ideal and fascinating models for studying evolution

Every infection results in many new viral genomesDespite a minimal number of genes, viruses exhibit remarkable diversity Phenomenal speed

Virus evolution is defined in terms of population of viruses, not an individual virus particle.No individual particle represents an average for any given population.Every individual virus particle is a potential winner.The most rare genotype in a population can become the most abundant after a single selection event.

A few points unique to DNA viruses:

DNA viruses tend to have narrow host range

Persistent or latent infections common-so replication is less

Replication of DNA virus genomes is less error-prone than that of RNA viruses.

Errors in DNA replication can be corrected, although error rates for viruses are higher than that of cellular DNA replication.

Human HerpesvirusesAncient family of viruses with 9 members

Relatively large (largest known human pathogen), complex viruses

Double-stranded DNA genome

Three subfamilies of herpesviridaeAlpha (variable host range; short replicative cycle) Herpes Simplex Type 1 and 2 Varicella Zoster VirusBeta (restricted host range; long replicative cycle) Cytomegalovirus (human herpes virus type 5) Human herpes virus type 6 and 7Gamma (lymphocyte associated) Epstein-Barr Virus Kaposis sarcoma-associated virus (human herpesvirus type 8)

A hallmark of all herpesviruses is the ability to establish latent infections

Why are herpesviruses (and especially CMV) so fascinating from an evolutionary standpoint?They are ancient

Latency = highly evolved

While many viruses deal with evolution passively (i.e. mutate), herpesviruses actively target mechanisms

Human CMV (HCMV) has evolved with us since the beginning of our time (prior to invertebrate-vertebrate split).

For every defense mechanism we have, HCMV has at least one counter-mechanismtit for tat

Unsuccessful viruses cannot overcome host defenses.Cytomegaloviruses are over 200 million years old...no wonder they are so good at what they do

Human CytomegalovirusA complex -herpesvirusLarge genome (230kb)Slow replicatingRestricted host range

Infects 60-90% of the population worldwide, typically asymptomatic infection

Infection in immunocompromised individuals life threateningStem cell and solid organ transplant recipientsHIV infected individualsCancer patients receiving intensive chemotherapy regimens

Infection in utero: Leading cause of infectious disease related birth defects 1 in 100 infected; 1 in 1000 present symptoms/pathologyMild to severe hearing lossCognitive deficitsPhysical abnormalities

Human CytomegalovirusLatency is a hallmark of all herpesviruses

During LATENCY, viral genomes are maintained in the absence of viral replication = No overt disease

The virus exits latency by REACTIVATION resulting from poorly characterized stimuli = Disease in immunocompromised host

While once considered benign, the latent infection is associated with age-related immune senescence and increased risk of atherosclerosis

tegument200 nmenvelopeglycoproteinscapsidDNA coreHuman CytomegalovirusVirion Structure

INNATE Host DefenseEarly defense that takes care of most infections before we know whats going on--most underrated arm of the immune system

Provides early warning to activate the adaptive defenses

MAJOR PLAYERS:Toll receptors (recognize microbial macromolecular patterns)cytokines/interferons (soluble messengers)natural killer cells (eliminate infected cells by direct lysis)complement (antibody-activated)

Viral infection stimulates the release of interferons and interleukins (cytokines) that establish an antiviral state in the infected cell and neighboring cellsTo contain and alert

Type-I IFN ResponseNFBIRF3AP1IFN/ ProductionTLR?Tyk2Jak1STAT1STAT2p48PPPKROAsMxAIRF-7HCMV1. Induction2. Receptor Signal Transduction3. IFN-induced Gene TranscriptionIFNRIFN/CytoplasmNucleusISREAntiviralResponseRIG-1MAVS

INNATE Host DefenseInterferons induce death of the infected cell and also ensure that surrounding cells will die if they become infected

INNATE Host DefenseViruses have evolved complex strategies to by-pass the innate immune response (antiviral state)

-Block the trigger: make proteins that bind dsRNA-Cytokine homologues (decoys)-Cytokine receptors (bind up/inactivate IFN)-Inhibition by viral proteins-Inhibition by viral miRNAsmaybe

1. InductionType-I IFN ResponseNFBIRF3AP1IFN ProductionTLR?Tyk2Jak1STAT1STAT2p48PPPKROAsMxAIRF-7HCMV2. Receptor Signal Transduction3. IFN-induced Gene TranscriptionIFNRIFNCytoplasmNucleusISREAntiviralResponseIE2IE1XRIG-1MAVS

ADAPTIVE Host ResponseT Cells (Cellular Immunity)cytolytic T-cells (CTL)helper T-cells TH-1: activate CTL TH-2: activate antibody productionB Cells (Humoral Immunity-Antibodies)viremic infections

Cell-mediated defense is essential for clearing most viruses--downside is a lot of immunopathology

ADAPTIVE Host ResponseViruses have evolved complex strategies to by-pass the adaptive immune response:

-Block MHC class I or II presentation-Bind MHC in secretory pathway-Degrade MHC-Compete with MHC-Prevent antigen loading onto MHC-Block TH1 (B cell) activation with viral IL-10 homologue

When T cells attack CD8 T cells kill virus-infected cells and tumor cellsfrom Zweifach, et al. 2001. Immunity. 15:847 T cell = blueInfected cell = greenRed = toxic granules inT cell delivered to infected cellVideo microscopyElapsed time = 400 seconds

calreticulinERLumencalnexintapasinTransportclass Ihetero-trimerclass Ihetero-dimerTAP1TAP2Peptidesclass IH-chainb2mProteasomeProteinGolgiCytosolExtracellularMatrixERp57MHC Class I Assembly/Antigen Presentation

calreticulinERLumencalnexintapasinTransportclass Ihetero-trimerclass Ihetero-dimerTAP1TAP2Peptidesclass IH-chainb2mProteasomeProteinGolgiCytosolExtracellularMatrixERp57MHC Class I Assembly/Antigen PresentationUS10US6US2US11US3US8UL18

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TAPTAPProteasomePeptideDislocationHCMV (US2, US11)ERGOLGIHSV (ICP47)cmtLysosomeHCMV (US6)ERGICMCMV (gp40)TAPTAPcSurfaceKSHV (kK5/3)Adv (E3-19K)FoldedProteinVirus Evasion of CD8+ T Cell Responses

Viral Latency and PersistenceThe reward for subverting detection and elimination

Herpesvirus evolution is a complex and elegant example of molecular biological warfare

Its just not always clear whos winning!In healthy individuals, herpetic pathologies are rarely fatal.

Herpes is foreverbig bonus for the virus!

The better the virus, the less disease it causes, the more it is tolerated by the host

Herpes B Virus80% of untreated cases are fatal in humans

Why?

Herpes B Virus80% of untreated cases are fatal in humans

22 reported cases, 20 developed encephalitis, 15 were fatal hmmmm, very rare

Herpes B Virus80% of untreated cases are fatal in humans

22 reported cases, 20 developed encephalitis, 15 were fatal

Herpes B virus infection occurs naturally in Macaque monkeys, most with NO obvious signs of infection. Those with signs of infection have small blisters or ulcers on the mouth, face, lips, genitalia. Reactivation/relapse can occur in stressed monkeys

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