Lecture 17: Retroviruses, HIV & AIDS, Herpesviruses, and Prions

PART 1: RETROVIRUSES (Slides 1-5)

What Are Retroviruses?

• Enveloped virus (surrounded by a lipid membrane)
• Single-stranded, positive-sense RNA genome
• Carry reverse transcriptase, an enzyme that converts their RNA into DNA (hence "retro" - they go backward from the central dogma of molecular biology)
• Their DNA integrates into the host genome, becoming a permanent part of the host cell

The Lysogenic Analogy

Nobel Prize Connection (Slide 4)

How Retroviruses Cause Tumors (Slide 5)

PART 2: HIV & AIDS (Slides 6-18)

Overview (Slide 6)

• HIV (Human Immunodeficiency Virus) causes AIDS (Acquired Immunodeficiency Syndrome)
• HIV is a retrovirus, a member of the Lentivirus genus
• Two types: HIV-1 (more common globally) and HIV-2 (mainly West Africa), distinguished by genome organization and phylogenetic relationships

Transmission (Slides 7 & 10)

• Sexual contact (most common route)
• Parenteral - contaminated blood or blood products (e.g., sharing needles)
• Vertical (mother-to-child):
  • Through sperm or egg
  • Across the placenta
  • In breast milk
• Highest HIV titers are found in blood and semen

Structure (Slide 8)

gag - pol - env

• gag: encodes structural proteins (capsid, matrix)
• pol: encodes enzymes (reverse transcriptase, integrase, protease)
• env: encodes envelope glycoproteins

The HIV Replication Cycle (Slide 9)

1. HIV enters the host cell (binds CD4 receptor)
2. Viral RNA is released into cytoplasm
3. Reverse Transcriptase converts: RNA → DNA
4. Viral DNA enters the nucleus
5. Viral DNA integrates into host genome (becomes a "provirus")
6. Host machinery produces viral RNA and proteins
7. New viruses are assembled and released

Dissemination & Timeline (Slide 11)

1. Primary infection - acute flu-like illness
2. Dissemination of virus to lymphoid organs (lymph nodes, spleen)
3. Clinical latency - asymptomatic but virus still replicating
4. Elevated HIV expression - viral load rises
5. Clinical disease and death

Average time from primary infection to clinical disease: ~10 years (untreated). Death usually occurs within 2 years after onset of symptoms.

Pathogenesis (Slide 12)

1. Inefficient initial transmission (common via sexual contact)
2. Acute phase - intense viral replication; antiretroviral syndrome (flu/mono-like illness)
3. Activation of innate and adaptive immunity, but the immune system cannot contain the rapidly mutating virus
4. Chronic asymptomatic phase (clinical latency) - continued replication
5. Advanced phase - marked depletion of CD4+ T lymphocytes → AIDS and opportunistic infections

Target Cells (Slide 13)

• HIV primarily infects and kills CD4+ helper T cells → suppression of cell-mediated immunity
• Also infects brain monocytes and macrophages → forms multinucleated giant cells → central nervous system (CNS) symptoms

Diagnosis (Slide 17)

Treatment (Slide 18)

• Goals: (1) Restore immune function by raising CD4 count; (2) Reduce viral load to reduce transmission
• HAART (Highly Active Antiretroviral Therapy) - established in 2013 as the standard of care; very effective at prolonging life, improving quality of life, and reducing viral load, but does not cure HIV infection

PART 3: HERPESVIRUSES (Slides 19-60)

General Herpesvirus Properties (Slide 22)

• Icosahedral capsid surrounded by a lipoprotein envelope
• Linear double-stranded DNA genome
• Large: 120-200 nm in diameter
• Replicate inside the nucleus
• Form intranuclear inclusion bodies
• Get their envelope from the nuclear membrane (unusual!)
• Cause latent infections - after the acute disease, they become dormant but can reactivate

Latency Cycle (Slide 24)

• Hallmark of all herpesviruses
• Virus becomes dormant in specific host cells (neurons for HSV/VZV, B cells for EBV)
• Reactivates under stress, illness, sunlight, or immune suppression
• Upon reactivation, virus travels back to skin/mucosa causing recurrent lesions

A. HERPES SIMPLEX VIRUS (HSV) - Slides 25-34

• Gingivostomatitis (primary, mainly in children): fever, irritability, painful vesicular lesions in the mouth; heals spontaneously in 2-3 weeks
• Herpes labialis (cold sores/fever blisters): recurrent crops of vesicles at lips or nose
• Keratoconjunctivitis: corneal ulcers and conjunctival lesions; recurrences can cause scarring and blindness
• Encephalitis: necrotic lesion in one temporal lobe, high mortality, severe neurologic sequelae in survivors

• Genital herpes: painful vesicular lesions of genitals and anal area; primary infections have fever and inguinal lymphadenopathy; asymptomatic infections occur in both sexes
• Neonatal herpes: acquired during birth from vesicular lesions in birth canal; can be prevented by cesarean section when active lesions or positive cultures are present

• Cell culture isolation (cytopathic effect in 1-3 days)
• Tzanck smear (Giemsa stain): shows multinucleated giant cells - presumptive diagnosis
• PCR of spinal fluid for encephalitis diagnosis
• Fluorescent antibody staining or ELISA for confirmation

• Acyclovir (Zovirax) - treatment of choice for encephalitis, systemic disease, neonatal HSV-2
• Also used for genital herpes (shortens duration, reduces shedding, but does not cure latency)
• Viroptic - topical for HSV-1 eye infection
• Prophylactic long-term acyclovir, valacyclovir, or famciclovir can suppress recurrences (but no drug eliminates the latent state)

B. VARICELLA-ZOSTER VIRUS (VZV) - Slides 35-39

• Varicella (chickenpox) = primary disease
• Zoster (shingles) = reactivation/recurrent disease
• Transmitted by respiratory droplets and direct contact with lesions
• Highly contagious childhood disease

• Incubation period: 14-21 days
• Prodrome: fever and malaise, then headache
• Spots appear in crops, develop into itchy blisters anywhere on the body
• Zoster: painful vesicles along the course of a sensory nerve (head or trunk); in immunocompromised patients, can cause life-threatening disseminated pneumonia

• Tzanck smear: multinucleated giant cells (same as HSV)
• Cell culture with specific antiserum
• No antiviral needed in immunocompetent children
• Acyclovir for immunocompromised; foscarnet for acyclovir-resistant strains
• Famciclovir (Famvir) and valacyclovir (Valtrex) also available

C. CYTOMEGALOVIRUS (CMV) - Slides 40-44

• Also called HHV-5, a β-herpesvirus
• Named for its distinctive cytopathic effect: "owl's eye" intranuclear inclusions and cell enlargement (cytomegaly)
• Very common: >60-90% of adults worldwide are infected

• Healthy adults: usually asymptomatic; occasionally mononucleosis or hepatitis
• Immunocompromised (transplant patients, HIV+): serious, life-threatening infections
• Congenital CMV (in newborns): rash, jaundice, microcephaly, IUGR, hepatosplenomegaly, seizures, retinitis; long-term: hearing loss, developmental/motor delay, vision loss, anemia, thrombocytopenia

• Ganciclovir (nucleoside analog, similar to acyclovir) - inhibits CMV replication
• Foscarnet (second-line)
• Cidofovir (nucleotide analog, third option)

D. EPSTEIN-BARR VIRUS (EBV) - Slides 45-54

• HHV-4, a γ-herpesvirus
• ~90-95% of adults worldwide are seropositive
• Transmitted via saliva ("kissing disease")
• Infects mainly B lymphocytes and pharyngeal epithelial cells
• In latently infected cells, EBV DNA is in the nucleus but not integrated into cellular DNA

• VCA (Viral Capsid Antigen) - most important; used most often diagnostically
• EA (Early Antigens) - produced before viral DNA synthesis
• EBNA (Nuclear Antigen) - diagnostically helpful

1. Infectious Mononucleosis (IM) - fever, malaise, pharyngitis, tender lymphadenopathy, splenomegaly; lasts days to weeks; 1-5% develop complications (splenic rupture, encephalitis, hemolytic anemia)
2. Burkitt Lymphoma (BL) - most common malignancy in young children in sub-Saharan Africa; rare, aggressive non-Hodgkin's lymphoma; linked to EBV + chronic malaria; symptoms include abdominal swelling, facial bone distortion, night sweats
3. Nasopharyngeal Carcinoma (NPC)
4. EBV can immortalize B-lymphocytes in vitro and in vivo

E. HHV-6 AND HHV-7 (Slides 55-57)

• β-herpesviruses; transmitted via saliva and breastfeeding
• Main target: T-lymphocytes (and B-lymphocytes)
• Ubiquitous worldwide
• HHV-6 and HHV-7 are related to each other similarly to HSV-1 and HSV-2

• Classical childhood disease; ages 4 months to 2 years
• Spiking high fever for 2 days → mild rash
• Fever high enough to cause febrile convulsions; may be complicated by encephalitis

F. HHV-8 (KSHV) (Slide 58)

• Human Herpesvirus 8, also called Kaposi's Sarcoma-Associated Herpesvirus (KSHV)
• Identified in 1994 from AIDS-associated Kaposi's sarcoma
• Belongs to the γ-herpesvirus subfamily
• Strongly associated with Kaposi's sarcoma in AIDS patients

PART 4: PRION DISEASES (Slides 61-68)

What Are Prions? (Slide 61)

• Composed only of protein - no nucleic acids (no DNA or RNA genes)
• Impervious to standard microbial disinfection
• Diseases have very long incubation periods (up to 30 years)
• Can cause inherited, communicable, or sporadic diseases
• Affect both animals and humans

Mechanism (Slide 63)

• A normal cellular gene PrP (on chromosome 20) encodes a normal protein designated PrPc
• This normal protein can be converted by a conformational change into a disease-causing form: PrPsc
• PrPsc is the actual prion - it can template the conversion of more normal PrPc proteins into PrPsc, causing a chain reaction
• PrPsc accumulates in the brain and causes disease

Pathology (Slide 65)

• These diseases are called Subacute Spongiform Encephalopathies (SSE) or Transmissible Spongiform Encephalopathies (TSE)
• Named "spongiform" because of vacuolar (sponge-like) changes in the cortex and cerebellum
• Pathological features: neuronal loss and astrocyte proliferation
• All are progressive and fatal

Specific Prion Diseases:

• Affected the Fore people of Eastern Highlands, New Guinea
• Mainly adult women and children of either sex
• Symptoms: ataxia, hyperreflexia, spasticity → progressive dementia → death
• Cause: Ritual cannibalism (eating brain soup of dead relatives)
• Incubation: 4-20 years after exposure
• Has disappeared since the elimination of cannibalism from the Fore culture

• Progressive, fatal CNS illness; most common in the 6th and 7th decades of life
• Initial symptoms: change in cerebral function, often diagnosed initially as a psychiatric disorder
• Sporadic (85%) or familial (15%)
• Also transmitted by: dura mater grafts, corneal transplants, contaminated neurosurgical instruments, pituitary-derived growth hormone
• No effective therapy - all cases fatal
• Nosocomial infections preventable by careful sterilization

• Identified in 1986 in the United Kingdom
• Cows became uncoordinated and unusually apprehensive
• Source: food supplement containing meat and bone meal from dead sheep
• The BSE prion survived cooking and was transmitted to humans who consumed infected bovine neural tissue or bone marrow (found in processed meats)
• Led to variant CJD (vCJD) in humans
• Destruction of diseased cattle and changes in livestock feeding have prevented further cases

Summary Table: Herpesviruses at a Glance