Lecture 27 Retroviruses: Classification, Structure, & Replication

What group of viruses has been studied more than any other?

Retroviruses

What type of viruses are retroviruses?

Spike-enveloped (+)sense RNA viruses with polyhedral capsids

What molecular biology tenet do retroviruses contradict?

The transfer of genetic information from DNA to RNA to protein

What enzyme do retroviruses use to transpose their RNA genome into DNA?

Reverse transcriptase

Why is reverse transcriptase essential for retroviruses?

It is essential for retroviral replication in host cells

How is reverse transcriptase used outside of retroviral replication?

As a tool in molecular biology research and therapeutic developments

Reverse Transcriptase

An RNA-dependent DNA polymerase that replicates a DNA intermediate from a single-stranded (+) sense RNA molecule

Three human retrovirus subfamilies

-Lentivirinae

-Oncovirinae

-Spumavirinae

Lentivirinae

-Primarily immunosuppressive

-HIV-1, HIV-2

-Slow viruses associated with neurologic and immune system diseases

Oncovirinae

-Primarily oncogenic

-HTLV-1, HTLV-2, HTLV-5

-Only retrovirus that can immortalize or transform targeted

Spumavirinae

-Not associated with human disease

-Human foamy virus, HERVs

-Endogenous retroviruses - inert viruses that take up ~8% of the human genome

Retrovirus classification

-Pathogenicity

-Tissue tropism (preference)

-Host range

- Genetic complexity

-Virion morphology (main category)

Retrovirus structure

Roughly spherical, enveloped RNA virus (Diameter: 80-120 nm)

-envelope acquired by budding from host cell plasma membrane

-Studded with viral glycoproteins (HIV= gp120 & gp41

Retrovirus features

-Contains two identical single-stranded (+)sense RNA genomes

-Contains 10-50 reverse transcriptase and integrase enzymes

-Contains two cellular transfer RNAs (tRNAs)

Simple retrovirus genomes contain three primary genes:

-Gag = Group-specific antigen (capsid, matrix, & nucleic acid binding proteins)

-Pol = Reverse transcriptase (polymerase, protease, & integrase)

-Env = Envelope glycoproteins (HIV: gp120, gp41; HTLV: gp46, p21)

-Long terminal repeat (LTR) sequences = Promoters, enhancers, transcription factor binders

(Retrovirus replication) infection

Viral glycoprotein spikes (HIV: gp120, gp41) bind with host cell

-Primary receptor - CD4 protein

-Co-receptor - Chemokine receptor

Major retroviral determinants of tissue tropism and host range.

CCR5

-Initial HIV infection

-CD4 T-cell subsets

-(Memory, intestinal, peripheral)

-Macrophage

CXCR4

-Chronic HIV infection

-CD4 T-cell subsets

-(Memory, intestinal, peripheral)

(Retrovirus replication) early phase

Reverse transcriptase synthesizes a complementary (-)sense DNA strand (cDNA) and degrades the (+) sense viral RNA strand

-Reverse transcriptase synthesizes a (+)sense DNA strand for the (-)sense DNA strand

Reverse transcriptase is very error-prone

-One error per 2,000 bases

-Five errors per genome (HIV)

-Responsible for promoting new viral strains; promotes antiviral resistance and immune escape.

(Retrovirus replication) Mid/late phase

-Integrase inserts viral DNA (provirus) into the host genome

-Provirus transcribed to manufacture viral proteins and RNA

Human Immunodeficiency Virus

Two known variants:

-HIV-1 = Predominate strain; worldwide; similar to chimpanzee strain

-HIV-2 = Prevalent in West Africa; Similar to simian strain

Four genotypes of HIV-1

M, N, O, P

-Majority is M subtype (divided into 11 subtypes/clades (A-K))

-Designations are based on differences in their env and gag genes

Viral tropism for CD4-expressing T-cells and myeloid cells

-Major determinant of HIV pathogenesis

-AIDS results from CD4 T-cell reduction

How does HIV infect the body?

HIV infects mucosal surfaces and enters the mucosa-associated lymphoid tissue (MALT).

What receptors does HIV use to enter cells?

HIV uses CD4 and chemokine receptors (CCR5/CXCR4) to enter cells.

What is the CCR5-Delta 32 mutation?

The CCR5-Delta 32 mutation prevents surface expression of the CCR5 receptor, providing resistance to HIV.

What are the consequences of the CCR5-Delta 32 mutation?

Individuals with the CCR5-Delta 32 mutation are more vulnerable to influenza and West Nile virus.

What are the major reservoirs and means of distribution of HIV?

Macrophages, dendritic cells, memory T-cells, and hematopoietic stem cells are the major reservoirs and means of distribution of HIV.

How does the course of HIV disease relate to CD4 T-cell count?

The course of HIV disease parallels the reduction in CD4 T-cell count and the amount of virus in the blood.

-HIV infection promotes a mononucleosis-like syndrome.

HIV Time course and stages

1 = Primary infection: M-tropic virus (CD4 & CCR5)

2 = Large HIV Viremia: Surge in viral load & decreased CD4 cell count

3 = Clinical Latency: Virus levels in the blood decrease, and host appears asymptomatic; CD4 T-cell count declines

4 = Immunodeficiency: CD4 T-cell count < 200/mm

5 = Opportunistic infection: Secondary infection is severe/ life-threatening

Acquired Immunodeficiency Syndrome

is not a disease but a syndrome

-Opportunistic or rare infections are the common clinical presentation associated with this condition

Occurs in the presence of antibodies against the human immunodeficiency virus (HIV) and a CD$ count <200/mm (normal is 500-1,000/mm)

HIV

-Likely arose from mutation of the simian immunodeficiency virus (SIV), found in African primates, about 1920

-HIV only replicates in humans and destroys the human immune system (Helper T-cells)

Opportunistic infections

-TB accounts for 13% of AIDS

-If an HIV patient contracts TB, he or she has a 90-95% chance of dying in a few months without treatment

-CMV causes symptomatic primary or recurrent infections

--Pneumonia/pneumonitis

--Retinitis

--Colitis

--Esophagitis

HIV transmission

occurs via contact with select bodily secretions with sufficient concentration of the virus

-Blood, semen, vaginal secretions, and breast milk

-Primarily transmitted via sexual contact and intravenous drug use

-Also transmitted from mother to baby across the placenta and in breast milk

-Infected fluid must encounter a tear or lesion in the skin or mucous membranes or be injected into the body

AIDS epidemiology

First recognized in young male homosexuals in the U.S.

-Now found worldwide

HIV Diagnosis

-Serological diagnosis involves detecting antibodies against HIV

--A positive test indicates infection with HIV but not the presence of AIDS

-Signs and symptoms of AIDS vary according to the diseases present

-Long-term non-progressors appear not to develop AIDS

--Most probable that these individuals lack effective coreceptors (CXR4 or CCR5) for the virus

-Presence of Thrush or Kaposi Sarcoma

HIV treatment

-A "cocktail" of several antiviral drugs is currently used (HAART - Highly active antiretroviral treatment)

-Cocktails reduce viral replication but do not cure the infection

-Vaccine development is difficult because various problems must be overcome

HIV prevention

-Behavioral changes can help slow the progression of AIDS epidemics

--Includes abstinence and safe sex, use of clean needles, screening of blood products, and administering AZT to infected pregnant women

Oncogenic retroviruses

-Originally called RNA tumor viruses

-35 have been identified to date

-Not cytolytic; immortalization of the host cell line

Sarcoma and acute leukemia viruses

-Can cause rapid transformation of cells and are highly oncogenic

-No human viruses of this type has been identified

Human oncoviruses

-HTLV-1 = First identified human oncovirus (Adult T-cell leukemia ATLL)

-HTLV-2 = Isolated from atypical hairy cell leukemia forms

-50% homology between HTLV-1 and HTLV-2

HTLV-1

-Spread in cells after blood transfussion, sexual intercourse, or breastfeeding

-Long latency period (30+ years) before onset of leukemia

-Transmitted and acquired by the same routes as HIV

--Endemic in southern Japan, Australia, the Caribbean, Central Africa, and among African Americans in the southeastern US

-Usually asymptomatic, but progresses to ATLL in approximately 1 in 20 persons over a 30-50 year period

-ATLL is fatal within a year of diagnosis, regardless of treatment

Mature T-cell neoplasms

Peripheral T-cell lymphomas have varied presentations

-Retain some of the functions of normal mature T-cells

-Have more aggressive course than B-cell lymphomas

Cutaneous T-cell Lymphoma

-Skin disease due to the infiltration of malignant CD4+ T-cells

-Homing receptors in the T-cells are believed to be the reason they travel to the epidermis

-Historical name: mycosis fungoides

--Thought a chronic fungal infection caused it

Adult T-cell Leukemia/Lymphoma

-Abbreviated ATLL

-Neoplasm of mature CD4+ T-cells

--Caused by retrovirus Human T Lymphotropic Virus 1 (HTLV-1) (Recognized before AIDS and HIV)

-Constantly express/high affinity for IL-2

--Without antigen presence -> autocrine stimulation

--Leads to uncontrolled growth and proliferation

-Patients mainly infected during infancy

--Incubation period for HTLV-1 is 20-40 years

Acute leukemia or sarcoma (Mechanisms of retrovirus oncogenesis)

Fast: oncogene

Direct effect: Provision of growth-enhancing proteins

Leukemia (Mechanisms of retrovirus oncogenesis)

Slow: transactivation

Indirect effect: Transactivation protein (Tax) or long terminal repeat promoter sequences that enhance expression of cellular growth genes