Module 4

T/F: viruses are very small compared to other pathogens

true

T/F: viruses are obligate intracellular parasites

true, they must infect host cells to replicate, they have no organelles or nuclei, they use host machinery, and energy to synthesize viral particles

How do viruses reproduce?

viruses cannot reproduce by division

viral components must self-assemble in the host cell

nucleocapsid

protein capsid that contains DNA or RNA

How are viruses classified?

strucutre

biochemical properties

disease caused by virus

tropism to host cell

tissue or organ first identified

How do microbiologist prefer to classify viruses?

1. by genome DNA or RNA

2. structure of the virus

DNA nucleocapsid viruses

adenovirus (causes cervical cancer)

DNA enveloped virus

Herpesviridae

HSV1, HSV2, VZV, CMV, EBV

capsid/naked viruses

tolerant in harsh and dry conditions, and can be attacked by the immune system

enveloped viruses

not tolerant in harsh or dry conditions and eaves immune system since its envelop is derived from host cells

consist of a capsid virion surrounded by a membrane envelope

What is the process of capsid assembly?

subunits form protomers

promoters form capsomeres

capsomeres form the capsid

capsids can assemble around the viral genome or empty capsids can be made and later filled with the genome

What is the function of spike proteins?

act as adhesion molecules called viral attachment proteins

What do viral attachment proteins allow for?

allow the virus to target specific host cells, determining tropism for the virion

*antibodies to spikes are an important means of neutralizing virion

What are three important characteristics of capsid/naked virion?

1. capsid virion tolerates hard environments: can tolerate drying, chemicals such as chlorine and acid in gut, extreme temperatures

2. capsid viruses commonly lyse the host cell, spending more time in lytic phase: lysis of host cells can stimulate acute inflammation involving dendritic cells, PMNs to clear the free virus particles and a Type IV hypersensitivity reaction involving CTLs to kill infected cells

3. immune system recognized the capsid fairly easy a being foreign

What does the membrane envelope of a virus consist of?

the membrane is derived from the host cell but has viral proteins and glycoproteins

ex: viral attachment proteins

What are three important characteristics of enveloped viruses?

1. enveloped viruses do not tolerate harsh conditions: must be kept in moderate, damp conditions to survive

2. enveloped viruses are usually released by budding from the cost cell: cell can die from lots of budding, viruses spend a lot of time in the lysogenic phase, becoming with intermittent lytic phases that are clinical, permits the survival of the host cell so that it can continue to produce the virus particles

3. enveloped viruses evade the immune system: the. membrane envelope hides the virus from the immune system because it looks like a host cell. CTLs and NKs are the best defense during the lysogenic phase, don't always cause acute inflammation unless tissue is damage during an episodic lytic phase, may have Fc receptors that can bind Fc portion of antibodies, may have C3b receptors bind C3b backwards so that it is not capable of being recognized by complement

What are the important double-stranded DNA viruses in optometry?

herpes simplex viruses

varicella-zoster virus

Epstein-Barr virus

cytomegalovirus

adenovirus

What are the most important RNA viruses in optometry?

HIV: enveloped retrovirus, positive strand RNA virus

SARS-CoV2

How does a DNA virus replicate?

1. virus targets cell

2. virus enters the target cell and capsize uncoats

3. viral genomic DNA enters the host nucleus

4. host machinery transcribes and translates viral proteins

5. the host replicates viral DNA

6. if it is a capsid virus, the virus will assemble in the cell and be released, if it is a enveloped virus, it will fo through the ER and Golgi and bud off the cell and may lyse the cell

What is the mechanism of RNA viral replication?

1. RNA virus targets & enters host cell

2. the viral RNA is reverse transcribed into cDNA

3. cDNA enters the nucleus and integrates into the host genome. This is dangerous since it may integrate into a gene controlling division and cause cancer

4. the viral cDNA is a template that produces viral proteins and viral RNA

5. the viral cDNA is a template that produces viral protein capsid and some of it is used as mRNA to make viral proteins

6. it buds off the host membrane if it is an enveloped RNA virus that may lyse the cell

T/F: only 1-10% of new viral particles are not infectious

true, because the high mutation rate will produce defective viral particles that are not infectious

recombination

occurs when a single cell gets infected with two different strains of the same virus

What is the mechanism of recombination?

1. One cell gets infected with two strains of the same virus

2. The genetic material recombines (crosses over) in areas of significant sequence homology (similar) to produce a new, third virus genome

* Most commonly occurs in viral genomes that have double stranded DNA

This third virus is now created, produces new progeny, and infects new cells

reassortment

occurs when a virus has a segmented genome (meaning that the genome is in several pieces)

What is the mechanism of reassortment?

1. A single cell is infected with at least two different strains of the same virus with a segmented genome (virus 1 has the red segmented genome, virus 2 and the blue segmented genome).

2. The two viruses replicate

3. In addition to replicating the two original viruses (L and M) genome from the virus particles are mixed and packaged into one virion (R3). This mixed virus is now a new viral strain.

• Unlike recombination, this does not involve "crossing over"

complementation

means that a virus is missing the necessary proteins to replicate so it MUST rely on another virus to provide the missing function

What is the mechanism of complementation?

1. Hepatitis B is a helper virus for Hepatitis D.

A hepatocyte is infected with Hepatitis B Virus

which has everything it needs to reproduce itself

2. When the same hepatocyte gets infected with

the Hepatitis D virus, it can use the gene product

it needs from the Hep B genome to replicate

itself.

What is an example of reassortment?

influenza

What is an example of complementation?

he interaction between Hepatitis B and D viruses (Hepatitis D is not infectious unless there is already an infection with Hepatitis B - Superinfection).

superinfection

an infection on top of another infection

phenotype mixing

the genome of one virus can be coated with the surface proteins (capsid proteins) and envelope of a different virus (Progeny 1 and 2 in top figure)

What is the mechanism of phenotype mixing?

1. The cell is infected by two viruses

2. When the virus replicates itself, it coats

the genome with a different capsid

3. The immune system thinks that it is infected by a specific virus, but the genome carried in that capsid has a different genome = this is a pseudotype virus.

• Kind of like using an orange peel to wrap an apple.

Rare in human disease!

What is the innate immune response against viruses?

Natural barriers (physical, acid, tears, mucus, etc.) inhibit entry of enveloped, and capsid viruses into the body.

Protective interferons alpha and beta decrease viral replication...next slide (see next slide)

Natural Killer cells

What is the acquired immune response specific to the virus?

Antibodies are effective against extracellular viral particles.

- Antibodies opsonize viral particles for phagocytosis, and they can also form immune complexes

that neutralize the virus so that it cannot enter a host cell.

- Antibodies can also be recognized by NK cells as part of a Type II Antibody-Dependent Cellular Cytotoxicity response.

Cell-mediated cytotoxicity kills infected cells preventing further dissemination of

the virus. This type of immunity includes:

- CD8+ cells (Type IV Cell-Mediated Cytotoxicity)

- This relies on activation of TH1 cells

Interferon alpha and beta

Protect uninfected cells from becoming infected.

These interferons will decrease protein synthesis so that fewer viral particles are produced.

These interferons will up-regulate MHC I, enabling CTL attack

How do viruses evade host defenses?

Viruses can stimulate fusion of infected cells to uninfected cells forming syncytia.

• This way the virus never has to leave the cell. Cytomegalovirus loves to do this...

- Viruses decrease MHC I expression to evade CTL recognition.

- Some virion inhibit apoptosis so more viral particles can be

produced.

- Some virion can produce proteins that can bind the Fc portion of antibodies or C3b to evade opsonization and compliment activation.

What is the most common clinical course of viral diseases?

Infection and incubation period.

- This phase is the Prodromal phase in which there are early non-specific symptoms before the appearance of more specific symptoms.

- Non-specific symptoms include fever, and malaise caused by interferon.

Signs and symptoms of the disease specific to the primary location infected,

like the eyes, mouth, or lungs.

- The patient will also exhibit viremia (increased number of viral particles in blood) during this period.

Sometimes signs and symptoms may occur at secondary locations especially if the virus can establish latency (ability of a virus to remain dormant).

Late stage of viral disease:

a) Resolution with associated regeneration, or repair.

b) Alternatively, the virus may persist if it is incurable. Viruses like Herpes will persist in latent form throughout life.

What are the key characteristics of adenovirus?

It is a dsDNA, icosahedral capsid virus

• Does not have an envelope

- It is tolerant to harsh conditions, and

easily spread

Therefore, highly communicable; found in:

- Swimming pools- Classrooms (Ack!)- Military barracks- Hospitals/Nursing homes - Prisons

- It is self-limiting (it goes away on its own)

How does adenovirus establish an infection in the eye?

Fiber proteins (spike proteins) on the surface of the virus target mucoepithelium.

It infects the cell, replicates/assembles as previously described.

Then it lyses the host cells OR can remain latent (live in adenoids - does not do this for eye infections).

It sheds from the infected cells for a long time

How does the adenovirus shed?

irally infected individuals shed viral particles for months after the infection.

It is common for virion to be shed in fluids and excretions, such as:

- Tears - from conjunctiva

- Respiratory droplets(sneezing and coughing) from upper respiratory tract due to drainage from nasolacrimal duct

- Feces from GI tract - if in adenoids

What eye condition can adenovirus cause?

Viral conjunctivitis = "pinkeye"

Also known as Epidemic Keratoconjunctivitis or EKC

It causes pink eye in children and adults, equally

It induces a follicular reaction(Type IV cell-mediated cytotoxicity)

Usually accompanied by some corneal involvement as the virus is shed from the conj. (inflammation)

The acute inflammation is stimulated by lyses of host cells (red and burning or painful)

What systemic conditions can adenovirus cause?

Adenovirus can also cause Upper respiratory tract infections:

Infects the upper respiratory tract of children > adults

Causes flu-like symptoms

Dyspnea

This can spread to the eye too and shed into the GI system causing GI problems

What is the immune system respond to adenovirus?

Acute inflammation due caused by cell lysis and free extracellular virion

Cytotoxic T Lymphocytes respond

to the infection (Type IV CMC)

Patients develop follicles and pre- auricular lymphadenopathy (PAL), indicating activation of the cell- mediated immune response.

Natural Killer cells can respond via Type II Antibody-Dependent Cellular Cytotoxicity (ADCC)

What are the treatments for viral conjunctivitis?

Most ODs treat the symptoms with cool compresses and refresh tears since adenovirus is self-limiting (it goes away on its own)

- Some ODs (~30%) have used ocular betadine (see top graph), and of the ODs that use betadine, they typically only use it on the worst cases of EKC.

Betadine can kill the virus butis also a strong irritant

What are the key characteristics of the Herpes virus family?

Herpesvirus is a large family of viruses

Herpes: It's the virus that keeps on giving

It is not curable and never goes away

It can be latent or active

They are dsDNA, icosadeltahedral viruses

- Soccer ball-shaped

- They are large enveloped viruses:

- Not tolerant to dryness, detergent, or acid

• It must be transmitted by fluids!

- The envelope is important for targeting and fusion with target cells

How does Herpes viruses evade immunity?

They have a membrane derived from host cells making it difficult to be recognized as foreign

They have Fc receptors, disabling the humoral response by binding antibodies backwards (shown above) - no opsonization, no complement activation, no Type II ADCC

They establish latency inside cells = live inside a cell hiding from the immune system making it difficult to be eliminated

T/F: The immune system keeps the virus latent so that it does not cause signs or symptoms

true, but it the patient becomes weakened or immunocomprised, the virus will become active which can be devastating and even deadly

What is the pathogenesis of Herpes Infections?

Virus infects host cell

Remember each virus is tropic to a particular host cell

Lytic Infection: Virus lyses the host cell

This is the lytic infection and is typically pretty bad when first infected Vesicular lesions develop

Think of the chicken pox lesions, these are bad

vesicular lesions because they are full of fluid

Establishes latency = Each herpesvirus establishes latency in different locations (see graph) if the patient has a healthy immune system.

Reactivation

6. Primarily relies on cytotoxic T lymphocytes for defense

Usually occurs when the immune system is weakened

- During this period, the virus is replicated and assembles

If latent in neurons, it travels from the cell body backwards to the sensory endings on the surface

- Virus is released

- Infects adjacent epithelium causing lesions to form on body surface

Herpes Keratoconjunctiviits

caused by Herpes Simplex Virus

t is an infection of the cornea and conjunctiva caused by HSV-1 usually, and HSV-2 rarely.

What are the presentations of herpes keratoconjunctiviits?

Early - It is often very difficult to identify (sometimes looks like little dots - not pictured).

- Late - With repeated reactivation, it is characterized by dendritic corneal ulcers (they look like the dendrites of a neuron - see pictures here).

- Repeated inflammation and scarring can eventually become vision-threatening if untreated!

- The treatment goal is to keep the virus from becoming reactivated, and/or to make the virus go back into hiding

How is Herpes Keratoconjunctivitis treated?

Acyclovir is a nucleoside analog, and is only effective against an actively replicating virus not a latent virus!

- Although HSV responds well, there is no cure (it will always remain latent)

- Acyclovir works like AZT, it incorporates into the growing strand of DNA during replication, and stops the Herpes DNA polymerase (see arrow).

What are the key characteristics of Herpes Simplex Virus-1?

Commonly 1st appears in kids and can be reactivated throughout life.

- I differentiate HSV-1 from HSV-2 by the fact that it HSV-1 is first and does not require sexual contact

The virus that infects people earlier in life is first =HSV-1

What disease are caused by lesions of HSV-1?

Cold sores

- Keratoconjunctivitis (corneal ulcer)

More commonly caused by HSV-1, than HSV-2.

What are they key characteristics of HSV-2?

Usually appears during puberty (when sexually active)

If infected, it infects people later in life or second = HSV- 2.

Diseases caused by HSV-2: - Genital herpes

• **Both HSV-1 and HSV-2can cause genital herpes. But it is most commonly HSV-2!

- RARELY causes keratoconjunctivitis...but it can if genital to ocular transmission

What diseases does Varicella-Zoster virus cause?

chicken pox and shingles

(Varicella = chickenpox)

(Zoster = shingles)

How is chickenpox transmitted?

respiratory droplets or direct touch

chickenpox lesions

dewdrops on rose petal

vesicles are full of virus particles and the lesions crust over after 12 hours

T/F: You cannot catch Shingles since it is reactivation of a virus. But you can catch Chicken Pox from a person with shingles!

true

What are the characteristics of Shingles?

- Dermatomal vesicular rash • Like chicken pox but more

concentrated

- Must be treated within 72 hours or there is a high risk of developing post-herpetic neuralgia:

Post-herpetic neuralgia is severe neuronal pain (this is due to injury of the neuron not the lesions on the surface of the body). It can occur after the lesions heal - not everyone gets it.

This is very serious and can be fatal in immunocompromised patients - dying of pain

What is the pathogenesis of Herpes Zoster Ophthalmicus?

1. Chicken pox (varicella)

2. Virus becomes latent in the ophthalmic division of the trigeminal nerve - especially the nasociliary branch (random).

3. Virus becomes reactivated and causes shingles to the green area which is innervated by the ophthalmic division

4. Notice that the area affected can include the nose, the eye, and the brow on one side of the face.

What are the clinical presentations of Herpes Zoster Ophthalmicus?

Early/Prodromal phase - Before there is any signs of disease (prodromal), the patient can exhibit a fever, malaise, and pain on the skin before lesions ever appear (it can even hurt to comb their hair). So, the patient feels pain but doesn't know why.

Vesicles appear in a dermatomal distribution

The more pigmented the skin, the darker the lesions.

The presence of lesions on the tip of the nose is the Hutchinson sign. The Hutchinson sign indicates a high likelihood for ocular involvement.

Vesicles crust over 2-3 weeks later.

These vesicles can bleed and cause scarring and hyperpigmentation.

Most damage to the eye is caused by the immune system (CTLs killing host cells), not the virus itself...

Treat with steroids, oral antivirals...and pain meds. Again, this can be incredibly painful.

What is the pathogenesis of varicella zoster virus?

Infection (by inhalation or touch)

Causes Chicken pox or Varicella

Virus is transported backward (retrograde) to the nucleus of sensory neurons (ganglia) - where it becomes dormant and establishes latency

When immunity is decreased the virus is reactivated and travels to the skin that is innervated by the sensory neuron, where it infects the epithelium with a dermatomal distribution

Causes zoster or shingles

Notice that the same virus is responsible for both chicken pox and shingles and that shingles is always reactivation of chicken pox - so you can't catch shingles.

What is the largest human herpesvirus?

cytomegalovirus

What type of cell does cytomegalovirus target?

T lymphocytes

How do cytomegalovirus infect cells?

These viruses cause syncytium to form - these are fused T cells

The fusion allows infected cells to "infect" adjacent cells by fusing with them

cytomegalovirus retinitis

This virus does not cause disease unless the patient is severely immunocompromised. When you see CMV you should think = AIDs!!!

• In fact, it is the most common ocular infection and the leading cause of blindness in patients with AIDs.

Necrosis caused by this virus can be extensive

It causes a characteristic "ketchup and cottage cheese" appearance of the retina.

Where the ketchup is hemorrhaging, and the cottage cheese is areas of necrosis.

What cells does Epstein-Barr Virus infect?

B cells

What diseases are caused by Epstein-Barr Virus?

Infectious Mononucleosis or "Mono"

This is typically the initial infection

The virus then establishes latency

Increased risk for autoimmune diseases like Sjogrens, lupus and rheumatoid arthritis

Lymphomas (cancers of B lymphocytes):

1) Burkitt's lymphoma (in Africa)

2) Other B cell lymphomas

B cell lymphoma

EBV is also known to cause different B cell lymphomas which are tumors made of B lymphocytes in lymph nodes

- Ex.EBV can cause Hodgkin's lymphoma and non-Hodgkin's lymphomas

These lymphomas can affect the eye

Burkett's Lymphoma

Burkitt's Lymphoma is a disfiguring cancer that is endemic in Africa. It is thought to be endemic there because malaria and HIV decreases immunity to the virus allowing it to cause B cell cancer. You won't see this in the US.

EBV can transform B lymphocytes; which means that they become "immortal" or cancerous.

- Commonly seen in young boys (6- 12) in Eastern Africa and China who have co-morbidity with malaria

- This was one of the first cancers to be successfully treated with chemotherapy

Human Herpesvirus 8

irus is that it is responsible for Kaposi's Sarcoma in patients with AIDs.

- In 1994, an AIDs patient developed a dark cutaneous lesion containing a previously unknown Herpesvirus, now called HHV-8

- This is a cancerous lesion and is pathognomonic for HIV in the U.S.

Sars-CoV-2 ocular presentation

manifests like pink eye