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Flashcards in *M39: Persistence and Latency Deck (25)

Persistence vs. Latency:

1. Persistence: ability of a pathogen to remain long-term within the host.

2. Latency: form of persistent infection in which the pathogen is quiescent (NO overt disease).

3. Reactivation: persistent/latent pathogen re-enters PRODUCTIVE replication.

4. Carrier state: infected individual that displays no overt disease (serves as a reservoir)



Pathogens that Establish a Persistent or Latent Infection:

1. Bacteria: Mycobacteria, Treponema.

2. Protozoa: Plasmodium, Toxoplasma, Trypanosomes.

3. Viral: EBV, HSV-1, HSV-2, CMV, VZV, KSHV, HIV, HBV, HCV, HPV.



How Pathogens Modulate the Host Response to Establish Persistence:

1. Infection and destruction of host immune cells.
a. Human immunodeficiency Virus (HIV): T-cells.
b. Mycobacteria: macrophages.
c. Epstein-Barr Virus (EBV): B-cells.
d. Trypanosomes: macrophages.
e. Human cytomegalovirus (CMV): T-cells and macrophages.

2. Establish latency.
a. EBV: B-cells.
b. Herpes simplex virus type-1 (HSV-1): trigeminal ganglion neurons.
c. Herpes simplex virus type-2 (HSV-2): dorsal root ganglion neurons.
d. Varicella zoster virus (VZV): neurons and glia.
e. CMV: monocytes.
f. Human papilloma virus (HPV): keratinocytes.
g. HIV: resting T-cells.



How Pathogens Modulate the Host Response to Establish Persistence:

3. Induction of autoimmune disease.
a. Streptococci: M-protein.

4. Antigenic variation of major surface antigens.
a. Trypanosomes: switch in major surface antigen.
b. Influenza virus: antigenic shift/drift in HA and NA envelope glycoproteins.
c. HIV: antigenic variation in gp120/gp41 envelope glycoproteins.
d. Neisseria: pillin switching.

5. Molecular mimicry of host molecules.
a. Treponema: outer sheath antigens block ADCC.
b. Neisseria: group B capsular polysaccharide.



A Case History: A 60 year-old patient is admitted for abdominal pain and fever. A CT scan shows acute cholecystitis and he undergoes an emergent cholecystectomy. Post-operatively he develops multi-organ failure and requires mechanical ventilation. Two weeks into his illness, the ICU physicians note hemorrhagic lesions on his lips and in the mouth-around the intubation tube. A culture swab is used to collect fluid from the lesions in the mouth, and sent for a Tzanck smear and viral culture. The Tzanck smear shows multi-nucleated giant cells, suggestive of an infection/reactivation with one of the human herpesviruses. Empirically, the patient is started on IV acyclovir. _ grows on tissue culture, confirming the diagnosis of orolabial herpes infection. Acyclovir was continued and the lesions healed. The patient was eventually discharged to a nursing home.

Cause: _


Herpes simplex virus (HSV)


Herpes simplex virus (HSV):

a. Member of herpesvirus family.
i) 8 known human viruses (HSV-1, HSV-2, VZV, CMV, EBV, HHV6, HHV7, KSHV).

b. Serotypes.
i) HSV-1: 60% orofacial lesions/40% genital lesions.
ii) HSV-2: 40% orofacial lesions/60% genital lesions.

c. Infects a variety of cell types; dependent on presence of virus receptors.

d. Not species-specific as virus can infect cells from various species in culture, BUT humans are the sole reservoir for virus transmission.



Herpes simplex virus (HSV):

e. Virus particle.
i) Envelope: acquired from host, contains 13 viral glycoproteins.
ii) Tegument: surrounds capsid, 23 viral + 49 cell proteins.
iii) Capsid: icosahedral-shape, 162-capsomeres, 4 major proteins.
iv) DNA genome: double-stranded linear DNA genome of 152 kb.

g. Latent infection of neurons.
i) Spread: retrograde axonal transport to PNS.
ii) DNA persists as an extrachromosomal circular episome.
iii) Absence of lytic gene proteins.
iv) Expression of Latency-Associated Transcript (LAT).
v) No overt disease.



Herpes simplex virus (HSV):

f. Lytic [cell death] life cycle (Production of virus particles).

i) Attachment: two receptors
1. Heparan Sulfate (HS): present on most cells, involves gC, gB.
2. HveA (HVEM/TNFR14) and HveC (nectin-1/CD115), involves gD.

ii) Entry: fusion usually occurs at the cell surface, may be receptor-dependent, involves gD, gB and gH.
iii) Transcriptional cascade: Over 84 gene products (very complex).
iv) DNA replication: Nuclear, see intra-nuclear inclusions.
v) Assembly: Nuclear.
vi) Egress: usually eventually results in cell death



Herpes simplex virus (HSV):

h. Recurrent Infection / Reactivation.
i) Stimuli: UV, surgery, stress, trauma, hormones, immune suppression.
ii) Virus re-enters lytic cycle.
iii) Anterograde axonal spread from PNS to site of primary infection.
iv) Frequency: high -> occasional recurrences -> never.
v) Asymptomatic virus shedding (60-90%) affects spread.
vi) No apparent lysis of virus-infected neurons [WHY??].
vii) Role of host immune system.



Herpes simplex virus (HSV):

a. One of the most widespread infectious diseases (776,000 cases/yr USA)
i) 65% adults HSV-1 sropositive; 25% adults HSV-2 seropositive.
ii) 16% of individuals age 14-19 have genital HSV-2 infections.

b. Humans are the sole reservoir.

c. Not seasonal.

d. Spread by intimate (STD) or direct contact.
i) Spread in secretions and respiratory droplets.
ii) Spread to mucosal surfaces or via skin abrasions.



Herpes simplex virus (HSV):
Virulence Factors:

a. Viral DNA synthesis: maintain nucleotide pools in quiescent cells (neurons).
i) thymidine kinase (tk) and ribonucleotide reductase (RR).

b. Control host cell function: shut-off protein synthesis and block apoptosis.
i) UL41 (vhs) and g34.5.

c. Control host cell function: toxicity/apoptosis.
i) ICP0 protein processing and stability altered in neurons.

d. Immune regulation: block complement, antibody and Ag recognition.
i) gE/gI, gC and ICP47.



Herpes simplex virus (HSV):

a. Vesicular lesion formation.
i) Cytopathic effects (CPE).
ii) Inflammation: host recognition of virus-infected cells.

b. Virus shedding occurs in presence of high levels of neutralizing antibody.

c. CMI response is important in resolving lesions (usually takes 1-2 weeks).

d. Severity of primary infection.
i) Correlates with increased frequency of reactivation.
ii) Correlates with increased duration of reactivation.
iii) Correlates with increased viral load.
iv) Usually lesions are more severe with longer duration (10-14d).



Herpes simplex virus (HSV):

e. Severity of recurrent infection (reactivation).
i) Varies with serotype (HSV-1 vs. HSV-2).
ii) Varies with between herpesviruses (HSV vs. VZV).
iii) Spread to CNS: encephalitis.
iv) Usually lesions are less severe with decreased duration.

f. Complications Detected in Immunocompromised Patients.
i) Neonates: 1/10,000 live births -> Cesarean births (720,000/yr).
1. Disseminated.
2. CNS.
3. SEM: Skin, Eye, Mouth
ii) AIDS.
1. Increase in HSV-2/HIV-1 seropositive individuals



Herpes simplex virus (HSV):

a. Important for pregnant women: asymptomatic shedding (5-28%).
i) Infants with disseminated disease born to women shedding virus.

b. Clinical examination: problem of asymptomatic shedding.

c. Serologic tests: sensitive, type specific.

d. Tzanck smear stain: multi-nucleated giant cells with inclusion bodies.
i) Rapid and inexpensive.

e. Direct virus isolation from infected tissues.

f. Fluorescent In Situ Hybridization (FISH).

g. PCR.



Herpes simplex virus (HSV):

a. Incurable disease that persists for life.

b. Prevention by barrier method: STD.

c. Nucleoside antiviral therapy: decreases severity and duration.
i) Acyclovir (ACV): oral, i.v., topical, also valcyclovir.
ii) Penciclovir: topical cream.

d. Drugs to block viral entry: Abreva (n-docosonal).

e. Cesarean delivery: prior exposure to HSV.

f. Vaccines: target HSV glycoproteins, not yet effective.

g. Use as gene delivery vehicle in gene therapy applications (T-Vec and MM).



Neoplasia: growing mass of cells that have lost growth control:

1. Benign tumor: non-metastatic.

2. Malignant tumor: metastatic, spread to other tissues/sites within the host.



Mechanisms of tumor formation:

1. Indirect: disruption of cellular processes usually involving DNA replication.

2. Direct: mediated by outside source, bacterial or viral pathogen.

a. Direct expression of an oncogene encoded by a bacteria or virus capable of transformation.

b. Definitions:

i) Oncogene: gene product capable of cell transformation.

ii) Proto-oncogene: normal cellular gene that when improperly regulated is capable of cell transformation.

iii) Tumor suppressor gene: normal cellular gene capable of suppressing oncogene products.



Mechanisms of tumor formation:

c. Examples:
i) Avian oncoretroviruses: express altered proto-oncogenes.
1. Growth factors.
2. Receptors.
3. Receptor-kinases.
ii) HPV.
1. E6 binds and degrades p53 tumor suppressor protein.
2. E7 binds and sequesters Rb tumor suppressor protein.

d. Mutation of a crucial cellular gene such as a tumor suppressor involved in the regulation of cell DNA synthesis/repair: may be by insertional inactivation.
i) Retroviruses: Freund leukemia virus (FLV).
ii) HPV.



Mechanisms of tumor formation:

e. Overexpression or disregulation of normal cellular proto-oncogene by cis-activation:
integration of promoter directly upstream of proto-oncogene.
i) Retroviruses: LTR

f. Overexpression or disregulation of normal cellular proto-oncogene by trans-activation: bacterial/viral protein activates transcription of proto-oncogene.
i) HTLV-1: Tax protein.



A Case History: A 65-year-old woman with stage IIb cervical cancer underwent transabdominal hysterectomy and bilateral salpingoophorectomy. Four years later she presented with a vaginal tumor that was excised along with the pelvic lymph nodes. Tissue samples from the vaginal tumor and four lymph nodes were studied histologically and virologically. The present vaginal tumor was an adenosquamous carcinoma as was one of the lymph nodes. The other nodes contained metastases of a highly differentiated squamous cell carcinoma. Samples were analyzed by Southern blot hybridization and by PCR. PCR analysis demonstrated the presence of _ DNA in all the tissues. Analysis of the tumor tissue DNA by Southern blot showed that the vaginal tumor and three of the nodes contained integrated _ DNA, whereas one of the nodes contained episomal _ DNA.

Cause: _


Human papillomavirus (HPV)


Human papillomavirus (HPV):

a. Member of papillomaviruses, subfamily of papovaviruses.

b. Over 110 human isolates (70 benign/40 malignant lesions).

d. HPV genome: Circular dsDNA genome of 8 Kilobases encodes 7-9 genes.
i) 5-7 Early genes: E1-E7 transcription and replication.
ii) 2 Late genes: L1 and L2 capsid proteins.



Human papillomavirus (HPV):

c. Associated tumors.
i) Skin warts: benign (HPV1, 2, 3, 4, 7, 10, 63).
ii) Anogenital warts: benign (90% HPV6 and 11; also HPV42, 44).
iii) Cervical cancer: High risk-malignant (70-90% HPV16 and 18). Other (HPV 33, 35, 39, 51, 52, 56, 58, 59).
iv) Laryngeal warts: benign (HPV6 and 11).
v) Oral papillomatosis: benign (HPV6 and 11; also HPV7, 16, 32).
vi) Oropharyngeal cancer: malignant (HPV16) [major cause H&NC].

e. Replication- species and tissue specific.
i) No cell culture system to grow and study the virus: New Raft system.
ii) Maintained in basal cells as episome: no replication/transcription.
iii) As basal progress through proliferation: E gene -> L gene -> virus.



Human papillomavirus (HPV):


a. Humans are the sole reservoir (most common STI, 20 million in US HPV+).

b. Spread by intimate (STD) or direct contact, vertical transmission to fetus.

Virulence Factors.

a. Ability of virus to enter and remain in latency in keratinocytes.

b. May integrate into host DNA: usually results in transformation.
i) Result of inactivation of E1 or E2 genes by integration (E1 and E2 normally down-regulate E6 and E7).
ii) Results in up-regulation of E6 and E7 oncogenes.



Human papillomavirus (HPV):


a. Integration usually leads to transformation that results in hypertrophy.
i) Reversible by E2 expression.
ii) Depends on serotype and cellular factors.
iii) Role of other viral pathogens: CMV, HSV, HIV.

b. Benign skin or mucosal lesions: 90% of genital caused by HPV6 and HPV11.

c. Metastatic disease: cervical carcinoma: 80% HPV16 and HPV18.
i) Slow developing: several decades, 70% integrated / 30% episomal (E1).
ii) Cervical intraepithelial neoplasia (CIN1,2,3) -> Carcinoma In Situ (CIS).
1. 40% of CIN3 -> invasive carcinoma in 15 years.


a. Clinical examination with biopsy: subclinical disease.
b. PAP smear.
c. Fluorescent In Situ Hybridization (FISH).
d. PCR.



Human papillomavirus (HPV):

a. Barrier method helps to prevent spread: STD.

b. Benign skin (condyloma acuminatum) and anogenital warts.
i) Spontaneous regression; surgical removal if painful.

c. Cervical cancer.

i) Topical agents: Podophyllin or TCA- painful, not effective (35-70%).

ii) Cryotherapy (65-70%). also effective against genital warts

iii) Surgery: effective therapy (60-90%).

iv) Interferons: painful, not effective (40-50%) better with surgery.

v) Vaccination: very successful (99 vs 97% efficacy in blocking CIN2/3).
1. quadrivalent (HPV16, 18, 6, and 11) to Lp1 VLP (Gardasil:Merck).
also effective against anal, cervical, vulvar and genital warts.
2. bivalent (HPV 16 & 18) to Lp1 VLP (Cervarix:GSK).
3. 56% decrease in HPV infection rate in female teens (~18K CA/yr).