Persistent Viral Infections and Human Herpesvirus Latency

Outline: Characteristics of persistent viral infections, types of viral persistent infections, virus-immune responses interactions that favor persistent infections, evasion innate immune response, evasion of CTL responses, Examples of the different types of persistent infections, Human Herpesviruses

Acute Infection

Infection that lasts for a finite period;

• Self-limiting infection (you get sick, then get better and clear the infection)

• Colds inducing viruses - influenza, adenoviruses and rhinoviruses

• Intestinal Viral Infections - norovirus and rotavirus

Persistent Infection

Infection that persists for life

• Periodic stages with symptoms, but the virus remains with you forever

• Herpesvirus

• Human immunodeficient virus (HIV)

• Papillomavirus

Characteristics of Persistent Viral Infections

• Long term infection for the life of the host

• Most persistent infections start as acute infections

• Continuous low-level production of virions, viral proteins, and viral genomes

• Viral genomes may remain after viral proteins could not be detected

• Host defenses are diminished through time

Persistent Viral Infections Reflect Virus and Host Immune Responses Interactions

• Occur when primary infection is not cleared by the host immune responses

• Viruses tend not to produce severe cytopathic effect so host defenses diminshed

• Viruses can evade the host innate-immune response

• Viruses modulate the host adaptive immune response

• Persistence infections are established in: cells with restricted immune surveillance and immune cells

Three Types of Persistant Infections

Latent: Herpes Simplex Virus

Persistent (asymptomatic): Lymphocytic choriomeningitis virus and JC virus

Persistent (pathogenic): HIV, Human T-lympotropic virus, and Measles virus SSPE

Innate Immune Pathways - Establishment of an Antiviral State

Virus Infected Cells produce Type 1 interferons which block non-infected cell receptors

1. Virus-infected cells produce Type 1 IFN

2. Block the IFN receptor of an uninfected cell

3. IFNs induce expression of enzymes that block viral replication (effector function interferon stimulated genes ISGs)

4. Antiviral state is produced

Effector Function Interferon Stimulated Genes (ISGs)

• Inhibition of viral protein synthesis

• Degradation of viral RNA

• Inhibition of viral gene expression and virion assembly

Viral Evasion of Innate Immunity

• Inhibition of Type I IFN Synthesis

• Inhibition of Type I IFN Signaling (attachment occurs, however increased PKR and MxGTPases doesn’t occur)

• Inhibition of ISGs Signaling (prevents phosphorylation of translation initiation factor EIF2a, activation by dsRNA producing Oligo A, and prevents inhibtion of viral gene expression and virion assembly)

Viral Examples: Epstein Barr Virus, Herpes simplex virus type 1, Vaccinia Pox virus, Adenovirus, Human Papillomavirus type 16, Hepatitis C, Hepatitis B, Influenza A, and Reovirus)

Three mechanisms used by viruses to evade the CTL (Cytotoxic T Lymphocytes cells) response

1. Viral proteins antagonize with MHC-1 antigen presentation

2. Emergence of CTLs escape mutants

3. Viral induced apoptosis of CTLs

Viruses that are not cleared by CTLs establish a persistence infection

Viral Proteins Antagonize with MHCI Antigen Presentation

• CD8 T cells survey what’s going on inside the cell to decide, “Should I kill this cell?”

• Does this through MHCI expression

Modulation of MHCI expression by viral infections…

prevents or delays elimination of infected cells by cytotoxic CD8+ T cells (CTLs)

Viral proteins block antigen presentation by interfering with pathways that lead to the assembly and transport of MHC1-peptide complex

Assembly of Transport of MHC1-peptide Complex

1. Viral antigens are processes to individual peptides

2. Viral peptides are load into MHCI molecules in ER

3. Transport of MHCI - peptide loaded molecules through the Golgi apparatus

4. Antigen presentation to CD8 cell

Human Herpesvirus Antagonize with MHCI Antigen Presentation

Viral antigens are not processed to individual peptides, therefore no loading of viral peptides into MHCI molecule in ER, no transport through the Golgi, and no antigen presentation to CD8 cell

Human Immunodeficient Virus 1 (HIV-1) modulations MHC1-peptide - HIV proteins VPU & Tat

Downregulate expression of MHC1 molecules

Human Immunodeficient Virus 1 (HIV-1) modulations MHC1-peptide - HIV Protein Nef

Inhibits the transport to the surface of the MHC-1 complex in lymphocytes

CTL Virus Escape Mutants

Single nucleotide changes in CTL epitopes can be sufficient to evade detection by activated T cells

Antigen with mutations evade T cell receptors in the contact residues (where it makes contact with the recepor) or in the anchor residues (where it attaches to the MHC class 1 receptor)

Why do CTL viral escape mutants arise?

High mutation rate of reverse transcriptase (HIV and HBV) and RNA dependent RNA polymerases (IAV and HCV)

Selective pressure of an activated immune response

Programmed Cell Death (apoptosis) Induced by Activated CTLs

Fas/FasL-mediated cell killing (infected cell attaches to CTL cell) where FasL on CTL interacts with Fas on target cell triggering apoptosis of target cell

Why do CTLs express a Fas receptor?

Fas mediated CTL killing removes activated CTLs when no longer needed or when their presence is detrimental to the tissue

How do viral infections induce apoptosis of CTLs?

Viruses induce the rapid expression of Fas ligands on infected cells which interacts with CTL receptors and activate apoptosis pathways to kill activated CTLs before the CTL can kill the infected cell

Viruses that induce the expression of Fas ligand (Fas-L) in the surface of infected cell

HIV

Human Cytomegalovirus (hCMV)

Human T cell leukemia virus (HTLV-1)

Why does HIV cause a persistent-pathogenic infection that results in AIDS many years after the original exposure?

• During non-pathogenic HIV infection of macrophages, dendritic cells, and CD4+ T cells, there is a longer period where the host immune system is still fuctional but compromised

• During pathogenic infection, increased inflammation, depletion of CD4+ T cells in lymphoid nodes, and exhausted CTL response

Human Polyoma Virus

Persistent Aymptomatic Infection

• JCV and BK Polyoma virus infections are life long

• Productive replication where virus particles are produced

• Replication in kidneys, intestine, respiratory tract

• Infections are asymptomatic in immunocompetent individuals

• Mechanism of persistence exactly not known but hypothesis (evading the immune response viruses replicate without causing tissue damage)

• Deadly diseases in immunocompromised patients

Human Polyomaviruses BK and JCV in Immunocompromised Patients

BK: nephropathy (occurs in 18% of kidney transplant recipients; major cause of kidney transplant failure)

JCV: Multifocal Leukoencephalopathy (PML) → Occurs in 1% of patients treated for multiple sclerosis, replicates in the CNS resulting in paralysis and death, PML occurs in 5% of AIDS patients

Herpesvirus Infections

• Persistent infection with long periods of NO production of virus particles

• Cells harboring the silent viral genome are not recognized by the host immune system

• Latency: persistence of the viral genome with very limited viral proteins expression

• Reactivation: productive infection with recurrent symptoms and viral shedding

Subfamily Alpha-herpesvirus

Neurotropic

• Herpes simplex virus types 1 and 2

• Varicella zoster virus

Subfamily Beta-herpesvirus

Lymphotropic

• Human cytomegalovirus

• Human herpesvirus 6 and 7

Subfamily Gamma-herpesvirus

Lymphotropic and Oncogenic

• Epstein-Barr virus

• Kaposi’s sarcoma herpesvirus (HHV-8)

HSV 1 and 2

Trasmitted through contact with lesions, mucosal surface, genital or oral secretions from infected person

• HSV-1 (oral lesions cold sore)

• HSV-2 (genital lesions)

Latent infections: viral genomes are carried in the PNS neurons without symptoms

Reactivation: painful blisters or sores appear at the site of infection, some people can have reactivation without lesions but actively shed virus

HSV-1 Trasmission

3 years old (primary infection gingivostomatitis) → 27 year old

Transmission from adult to child at ~2 years only

Recurrence in both

HSV-1 Site of Active Lesion and Site of Viral Latency

Site of Lesion Innervated by latently infection trigeminal ganglia

• Virus lytic replications in epithelial cells and keratinocytes of the lips, tongue, mouth roof, and gums

HSV-1 Latent Infection 3 Stages

1. Establishment

2. Maintenance

3. Reactivation

Establishment of HSV-1 Latency in Neurons

• Virus capsid and remnant VP16 move through axon (retrogade transport)

• Episome (circular genome that isn’t integrated) persists in ganglia of sensory and/or sympathetic neurons

HSV-1 Contributions to Maintenance of Latency in Neurons

• Latency associated transcripts (LATs) are transcribed from viral episome during latency 

• No proteins are translated from the LATs

• Viral microRNAs helps to silence the viral genome

• Low level expression viral proteins triggers response from resident CD8+ cells 

Host Contributions to Maintenance of Latency in Neurons

Lytic promoters on the viral episome (on the ganglia) are associated with histones repressive factors (methylases and deacetylases) to maintain a repressed chromatin state resulting in silencing of viral gene expression

Only LAT region is associated with histone permissive factors that allows transcription

Activation of Neural Stress Pathways

1. Activation of the JNK pathway (cell signaling pathway response to cell stimulus or stress) induce a switch to displace histones repressive factors for histones permissive factors 

2. Phase I transient expression of IE, E, and L viral genes promote histone permissive factors to restore the complete HSV-1 transcription program (phase II)

Full Reactivation of HSV-1

Production of Viral Particles

• Small number neurons in ganglion reactivate and produce viral particles that travel by anterograde transport to mucosal tissues innervated by latently infected ganglia

Varicella Zoster Virus

• Virus enters through conjunctiva and upper respiratory track, replicated in lymph nodes, establishes T cell viremia that causes skin rash and viral shedding (chicken pox)

• During Primary infection, the virus invades the nevous sytem and establishes latency in the dorsal root of sensory ganglia (retrograde transport)

• Increased incidence of virus reactivation after 50 years of age

• Anterograde transport of viral particles through the nerve causes rash and painful lesion in the area where that infected ganglia enervates (Shingles)

Human Cytomegalovirus

• High seroprevalence (50-99%) worldwide, seroprevalence raises with age

• Primary infection asymptomatic or induce a feverish mono-like illness that is resolved

• Horizontal transmission through contact with bodily fluids (saliva, urine, cervical, and vaginal secretions, breast milk, semen, blood, and tears) enters through respiratory and upper GI tract 

• HCMV can cross the placenta causing congenital defects and death to the newborn

• HCMV infections are a major problem for organ transplant and immunocompromise patients 

HCMV Infection

1. Initial infection of viral particles is introduced to epithelial cells and Leucocytes

2. Primary dissemination into the liver, lungs and spleen

3. Secondary dissemination into the kidneys, mammary glands and salivary glands

4. Virus shedding through body fluids (due to lytic viral replication within the associated organs)

HCMV Latency

Bone Marrow

Hematopoietic Stem Cell (HSC)

Myeloid cells precursors of monocytes

HCMV Reactivation

Many Tissues

Differentiated Monocytes (Macrophage and Dendritic Cells) and Epithelial Cells

Epstein-Barr Virus

• Exposed in childhood where infection is asymptomatic

• Young adults more susceptible to infection and symptoms of infectious mononucleosis is benign proliferation of B-lymphocytes (glandular fever)

• More tahn 90% of the population in the US is infected by EBV by the age of 30

• Most of the EBV infections are clinically unapperent or very mild

Epstein-Barr Virus Infection

• EBV primary infection occurs in epithelial cells and resting B cells

• Infected resting B cells are transformed to memory B cells most will be cleared out by CTLs and NK cells

• A low percentage of memory B cells will evade immune response and remain as Latently infected resting memory B cells that travel to the bone marrow and lymphoid organs

• Latently infected resting memory B cells undergo reactivation and become infected B cells. Some are eliminated by CTLs while others migrate to oropharyngeal mucosa epithelial cells and shed virus in the saliva

Which virus causes persistent pathogenic infection years after first infection?

HIV

What is not a characteristic of viral persistent infection?

Occurs after primary infection is cleared

What virus causes persistent asymptomatic infection in immunocompetent hosts?

Human Polyomavirus JCV and BK

Epstein Barr Virus establish latency in ______

Resting memory B cells

Herpes simplex virus lytic replication occurs in

epithelial cells and keratinocytes

Herpes simplex virus establish latency in

sensory and sympathetic neurons

Human cytomegaloviruses establish latency in

bone marrow monocyte precursors

Name a virus infection characterized by the emergence of CTL scape mutants

HBV, HIV, HCV

Which are mechanisms utilized by viruses to evade the cytotoxic T lymphocytes responses?

Emergence of CTL escape mutants

Viral proteins antagonize the MHC-1 antigen presentation

Viruses mediate apoptosis of CTLs

Viruses that express Fas ligand in infected cells?

HTLV-1, HCMV, HIV

Why HIV causes a persistent-pathogenic infection that results in AIDS many years after the original exposure?

Infection of CD4, macrophage, and dendritic cells from the start

Late depletion of CD4 and increased inflammation in lymph nodes

Poor (exhausted) CTL response

How do viruses that cause persistent infections evade the innate immune response?

Prevent establishment of an antiviral state

Inhibit ISG pathways

Inhibit Type I IFN signaling

Inhibit Type I IFN synthesis

T/F: Virus infections that are not cleared by CTLs most likely establish persistent infections

True

What viral molecules are synthesized during maintenance of HSV-1 latency?

Viral microRNAs

Latency associated transcript (LAT)

What viral molecules are synthesized during HSV-1 reactivation?

IE proteins, E proteins, Structural proteins, viral genomes

T/F: Histone repressive factors silent the HSV-1 lytic promoters during latency

True

Why HSV-1 and VZV are neurotropic herpesvirus?

Establish latency in neurons

Why HCMV and EBC are lymphotropic?

Establish latency and replicate in lymphocytes

site of HSV latency

trigeminal ganglia

subfamily of neurotropic human HSV

alpha

subfamily of lymphotropic HSV

beta

Herpesvirus that causes infectious mononucleosis a benign proliferation B-lymphocytes

epstein barr virus