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Retroviruses 反錄病毒 (C65, p657)  Viruses are named because of reverse transcriptase. Family members are listed in Table 1 and Fig. 1.  Human pathogens include HTLV-1, -2 and HIV-1, -2.

Table 65-1. Classification of Retroviruses

Subfamily Characteristics Examples

Oncovirinae Are associated with cancer and neurologic disorders -

B Have eccentric nucleocapsid core in mature virion Mouse mammary tumor virus

C Have centrally located nucleocapsid core in mature virion Human T-lymphotropic virus* (HTLV-1, HTLV-2, HTLV-5), Rous sarcoma virus (chickens)

D Have nucleocapsid core with cylindrical form Mason-Pfizer monkey virus

Lentivirinae Have slow onset of disease: cause neurologic disorders and immunosuppression; are viruses with D-type, cylindrical nucleocapsid core

Human immunodeficiency virus* (HIV-1, HIV-2), visna virus (sheep), caprine arthritis/encephalitis virus (goats)

Spumavirinae Cause no clinical disease but characteristic vacuolated "foamy" cytopathology Human foamy virus*

Endogenous viruses

Have retrovirus sequences that are integrated into human genome Human placental virus

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1. Structure: +ssRNA (about 9 kb for HIV) X2, capsid and envelope) (Fig. 3)

2. viral genome include LTR,

Gag (group specific Ag, capsid, matrix, and nucleic acid proteins),

pol (reverse transcriptase, integrase, polymerase, and protease),

and env (envelope glycoproteins).

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Simple v.s. complex viruses

Table 65-2. Retrovirus Genes and Their Function

Gene Virus Function

gag All Group-specific antigen: core and capsid proteins

int All Integrase

pol All Polymerase: reverse transcriptase, protease, integrase

pro All Protease

env All Envelope: glycoproteins

tax HTLV Transactivation of viral and cellular genes

tat HIV-1 Transactivation of viral and cellular genes

rex HTLV Regulation of RNA splicing and promotion of export to cytoplasm

rev HIV-1 Regulation of RNA splicing and promotion of export to cytoplasm

nef HIV-1 Alteration of cell activation signals; progression to AIDS (essential)

vif HIV-1 Virus infectivity, promotion of assembly, blocks a cellular antiviral protein

vpu HIV-1 Facilitates virion assembly and release, decrease of cell surface CD4

vpr (vpx*) HIV-1 Transport of complementary DNA to nucleus, arresting of cell growth

LTR All Promoter, enhancer elements

﹛ pol

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2. Replication (Fig. 5)-- Virus infects mφ, dendritic cells, microglial cells and activated T cells via CD4 and CCR5 ( M-tropic) and naïve and helper T cells via CD4 and CXCR4 (T-tropic)(Fig. 6 ).

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1. Drug target

2. Provirus

3. Error rate (about

4. 1/2,000 base)

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(provirus)

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4. Human immunodeficiency virus (HIV) -- It is non-oncogenic virus.

-- HIV infects CD4-expressing mφand T cells (M-tropic T-tropic). -- Infection reduces the number (due to cytolysis by virus or cytotoxic T cells) and function of CD4 T cells to cause acquired immunodeficiency syndrome (AIDS).

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Characteristic Function

Infection of lymphocytes and macrophages

Inactivation of key element of immune defense

Inactivation of CD4 helper cells

Loss of activator of the immune system and delayed-type hypersensitivity

Antigenic drift of gp120 Evasion of antibody detection

Heavy glycosylation of gp120

Evasion of antibody detection

Table 65-3. Means of HIV Escape from the Immune System

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Epidemiology: a. Based on the estimation in 2003, there are 5 million new cases each year and 3 million death per year. b. Geographical distribution: HIV-1 is in Africa, Asia, and US. HIV-2 is in Africa. c. Global distribution (Fig. 12)

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d. Transmission: HIV is transmitted by body fluid such as blood, semen, and vaginal secretion (Fig. 11 and Table 4). e. High risk population includes: sexually active people, drug abusers, people receiving blood, blood product, and organs before 1985, and health care workers.

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Routes Specific Transmission

Known Routes of Transmission

Inoculation in blood

Transfusion of blood and blood products

  Needle sharing among intravenous drug abusers

  Needlestick, open wound, and mucous membrane exposure in health care workers

  Tattoo needles

Sexual transmission

Anal and vaginal intercourse

Perinatal transmission

Intrauterine transmission

  Peripartum transmission

  Breast milk

Routes Not Involved in Transmission

Close personal contact

Household members

  Health care workers not exposed to blood

Table 65-4. Transmission of HIV Infection

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Clinical Symptoms: --acute infection: flu like symptoms, mononucleosis, and aseptic meningitis--Stage 1: lymphoadenopathy--Stage 2: weight loss, malaise, opportunist infections, diarrhea, night sweats, fatigue, and wasting (slim) disease in African --Stage 3 (Onset of disease occurs when CD4 T cells are < 200/μl blood) Full blown AIDS with weight loss, diarrhea for > 1 month, and indicator diseases (p. 668, Table 5) .

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Lab. diagnosis: (Table 6)

Table 65-6. Laboratory Analysis for HIV

Test Purpose

Serology

  Enzyme-linked Initial screening

  immunosorbent assay  

  Latex agglutination Initial screening

  Rapid oral antibody test Initial screening

  Western blot analysis Confirmation test

  Immunofluorescence Confirmation test

Virion RNA RT-PCR Detection of virus in blood

Real-time RT-PCR Quantitation of virus in blood

Branched-chain DNA Quantitation of virus in blood

p24 antigen Early marker of infection

Isolation of virus Test not readily available

CD4 : CD8 T-cell ratio Correlate of human immunodeficiency virus disease

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 Prevention, treatment, and control

a. Four types of drugs

b. AZT is for infected pregnant women. c. Antiviral drug cocktail (highly active antiretroviral treatment, HARRT) is used for infected people. d. Education: monogamous and safe sex; use of condoms; promotion of clean needles for drug abusers (needle exchange)

e. Blood, blood product, and organ screening

f. Infection control in the hospitals

c. There is no effective preventive or therapeutic

vaccine because HIV mutates rapidly,

there is no good vaccine strategy, and

animal model is not available. R10

BOX 65-4. Potential Antiviral Therapies for HIV Infection

Nucleoside Analogue Reverse Transcriptase Inhibitors •Azidothymidine (AZT) (Zidovudine/retrovir) •Dideoxycytidine (ddC) (Zalcitabine   ) •Dideoxyinosine (ddI) (Didanosine   ) •d4T (Stavudine   ) •3TC (Lamivudine   ) •Tenofovir disoproxil fumarate    (adenosine    class) (Viread) •ABC (Abacavir) Non-nucleoside Reverse Transcriptase Inhibitors •Nevirapine    (Viramune) •Delavirdine (Rescriptor) •Efavirenz    (Sustiva) Protease Inhibitors •Saquinavir    (Invirase/Fortovase) •Ritonavir    (Norvir) •Indinavir (Crixivan) •Lopinavir (Kaletra) •Nelfinavir (Viracept) •Amprenavir    (Agenerase) •Fosamprevavir (Lexavir) •Atazanavir (Reyataz) Fusion Inhibitor •T-20 (enfuvirtide/Fuzeon) Highly Active Antiretroviral Therapy (HAART) (Combination) •Abacavir/zidovudine/lamivudine (Trizivir) •Indinavir/AZT/3TC •Ritonavir/AZT/3TC •Nelfinavir/AZT/3TC •Nevirapine/AZT/ddI •Nevirapine/indinavir/3T

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2. Oncogenic retroviruses cause leukemia, sarcoma, and lymphoma. They are not cytolytic and belonged to Oncovirinae (also called RNA tumor viruses).  Sarcoma, lymphoma, and acute leukemia (non human) viruses have incorporated cellular genes (protooncogenes) encoding growth-controlling factors. .  Representatives of viral oncogenes are listed in Table 8.

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Function Oncogene Virus

Tyrosine kinase Src Rous sarcoma virus

  Abl Abelson murine leukemia virus

  Fes ST feline sarcoma virus

Growth factor receptors erb-B (EGF receptor) Avian erythroblastosis virus

  erb-A (thyroid hormone receptor) Avian erythroblastosis virus

Guanosine triphosphate-binding proteins Ha-ras Harvey murine sarcoma virus

  Ki-ras Kirsten murine sarcoma virus

Nuclear proteins Myc Avian myelocytomatosis virus

  Myb Avian myeloblastosis virus

  Fos Murine osteosarcoma virus FBJ

  Jun Avian sarcoma virus

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 Human pathogens include HTLV-1, 2, and 5

a. HTLV-1 causes adult acute T-cell lymphocytic leukemia (ATLL) and HTLV-1-associated myelopathy (an neurological disorder) HTLV-2 is isolated from atypical form hairy cell leukemia, and HTLV-5 is isolated from malignant cutaneous lymphoama.

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b. They can not transform cells so they induce disease by different mechanisms.

Disease Speed Effect

Acute leukemia or sarcoma

Fast:  Oncogene

Direct effectProvision of growth-enhancing proteins

Leukemia Slow: Indirect effect

    Transactivation

Transactivation protein (tax) or long-terminal repeat promoter sequences that enhance expression of cellular growth genes

Table 65-7. Mechanisms of Retrovirus Oncogenesis

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b.  Pathogenesis and immunity Virus is transmitted by sexual intercourse and body fluid like blood and breast milk. Virus infects CD4 T cells (which reside in skin) and neurons.  Viral gene, tax, induces IL-2 and its receptor to cause clonal outgrowth of particular T-cells in adult ATLL in about 30 years.  Infected hosts elicit Ab against virus glycoprotein gp46. Virus infection causes immunosuppression.

c.  Epidemiology.  HTLV-1 is transmitted by the same route as HIV.  It is endemic in some areas. (35% people are infected in Okinawa). In those areas, children acquire virus in breast milk and adults are infected sexually. Intravenous drug abuse and blood transfusion also transmit virus in US.

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d.  Clinical syndromes  ATLL develops in 5% infected patients over a 30- to 50-year period. Malignant CD4 helper cells are pleomorphic and have lobulated nuclei (like flowers).  Skin lesions develop in ATLL patients.  ATTL patients die within a year of diagnosis.

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e.  Lab. Diagnosis ELISA is used to detect viral antigens and specific Ab, and RT-PCR is used to detect viral genome.  f.  Treatment, prevention and control.  AZT plus interferon alpha are used to treat ATTL. No treatment and vaccine are available for virus infection. Control measures are similar to HIV.

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A 28-year-old man had several complaints. He had a bad case of thrush (oral candidiasis) and low-grade fever, had serious bouts of diarrhea, had lost 20 pounds in the past year without dieting, and, most seriously, complained of difficulty breathing. His lungs showed a bilateral infiltrate on radiographic examination, characteristic of P. carinii pneumonia. A stool sample was positive for Giardia organisms. He was a heroin addict and admitted to sharing needles at a "shooting gallery."

1. What laboratory tests should have been done to support and confirm a diagnosis of HIV infection and AIDS?

2. How did this man acquire the HIV infection? What are other high-risk behaviors for HIV infection?

3. What was the immunologic basis for the increased susceptibility of this patient to opportunistic infections?

4. What precautions should have been taken in handling samples from this patient?

5. Several forms of HIV vaccines are being developed. What are possible components of an HIV vaccine? Who would be appropriate recipients of an HIV vaccine?