Retroviruses

RNA viruses Distinguished by presence of an unusual enzyme, reverse transcriptase. Retro = reversal Genus: -Lentivirus Human immunodeficiency viruses, Types HIV-1 & -2 Human T-cell leukemia viruses, Types HTLV-1 & -2

Retroviridae

HIV-1, HIV-2 and SIV HIV-1 is devided into 3 groups (based on env gene): O, N, M Group M is devided into at least 10 sub groups (A-J) HIV-2 includes 5 subtypes (E-A) HIV-2 and SIV (in Macaque & Chimpanzee) are much more similar.

Structure Envelope contains 72 spiked knobs: a transmembrane protein (gp 41), surface protein (gp 120). The virion has cone-shaped, icosahedral core, containing the major capsid protein, CA (p24). Outer matrix protein, MA (p17): Between capsid and envelope.

Structure Two identical copies of positive sense ssRNA genome (retroviruses are diploid). Enzymes: reverse transcriptase, integrase and protease. Acquired immunodeficiency syndrome (AIDS) was first reported in US in By 1984, AIDS was recognized as an infectious disease caused by a retrovirus.

HIV particles

HIV Genome Three major genes: Gag gene codes for CA (major capsid protein: p24), MA (outer matrix protein: p17) and NC (nucleocapsid proteins). Pol gene codes for reverse transcriptase, protease, integrase and ribonuclease. Env gene codes for TM (transmembrane protein: GP41) and SU (surface protein: GP 120)

HIV Replication Attachment to specific cell surface receptor: gp120 binds CD4 molecule on the helper T cells, monocytes and dendritic cells Viral entery. Reverse transcription of viral RNA into DNA. Integration of DNA (provirus) into host cell DNA. Transcription. The provirus is transcribed into a full length mRNA by the cell RNA polymerase II. Translation of viral RNA. Assembly and maturation of progeny virus.

Transmission of HIV Sexual contact: HIV is present in semen and vaginal secretions; either homoxesual or heterosexual contact Transfusions: whole blood, plasma, clotting factors or cellular fractions of blood. Contaminated needles: accidentally or sharing needles by drug users. Perinatal: Transplacental, during delivery or via breast milk.

Pathogenesis and clinical significance (1) Initial infection:  genital tract macrophages  HIV disseminates via blood  Dendritic cells in lymphoid tissue  CD4+ lymphocytes

Acute phase viremia several (2-4) weeks after the initial infection, 1/3 – 2/3 of individuals experience an acute disease syndrome (similar to infectious mononucleosis). Circulating antibody appears in 2 – 10 weeks after the initial infection

Latent period Latent period lasts from months to many years (average 10 years). 90% of HIV proviruses are transcriptionally silent. Continous loss of CD4+ cells in which HIV is replicating Active replacement through stem cell multiplication The infection remains clinically asymptomatic

During latent infection 10,000,000 virion particles is degraded every day. The half life time for HIV is 6 hours in plasma. The cycle life of virus (from infecting one cell until producing new virus infecting another cell) is 2.6 days. The half life of each infected T CD4 cell is 1.6 days on average.

Pathogenesis and clinical significance (2) Clinical complications during the latent period Multiple non-specific conditions:  persistent generalized lymadenopathy  Diarrhea  Chronic fevers  Night sweats  Weight loss  Opportunistic infections

Clinical complications during the latent period Coinfection with other pathogens, such as HHV-6 can transactivate transcription from the silent HIV provirus. Appearance of HIV mutants with altered antigenic specificity also contributes to progression with CD4+ count falling below 200 / ul.

Opportunistic Infections (1) Bacteria: Mycobacterium avium complex Disseminated miliary disease Chronic bronchitis, pneumonia Chronic osteomyelitis, renal infection Mycobacterium tuberculosis Chronic pneumonitis, osteomyelitis Meningitis, miliary disease Haemophilus influenza (pneumonia) Campylobacter spp. (diarrhea) Shigella spp. (diarrhea)

Opportunistic Infections (2) Fungi: Candida spp. Oral, vaginal or systemic candidiasis Histoplasma capsulatum (disseminated disease) Cryptococcus neoformans (meningitis) Pneumocystis carinii: Unicellular eukaryote Taxonomic status is uncertain Most common opportunistic pathogen in AIDS patients Fatal pneumonia

Opportunistic Infections (3) Parasites: Toxoplasma gondii (focal encephalitits) Viruses: HHV-8 (Kaposi’s sarcoma-associated herpesvirus) EBV (Lymphomas) HSV (oral, genital ulcers) JCV (progressive multifocal leukoencephalopathy, affect the white matter of the brain) CMV (Chorioretinitis, encephalitis, enterocolitis, gastritis)

Malignancies associated with AIDS Kaposi’s sarcomaHHV-8 LymphomasEBV

Kaposi’s Sarcoma

Laboratory diagnosis Screening of blood donors using ELISA (Enzyme-linked immunosorbant assay) for detection Antibodies. Confirmation antigenes by western blotting Confirmation genome by PCR

ELISA for HIV antibody Microplate ELISA for HIV antibody: coloured wells indicate reactivity

Western blot for HIV antigens There are different criteria for the interpretation of HIV Western blot results e.g. CDC, WHO, American Red Cross. The most important antibodies are those against the envelope glycoproteins gp120, and gp41 p24 antibody is usually present but may be absent in the later stages of HIV infection

Treatment Anti-retroviral drugs Reverse transcriptase inhibitors Protease inhibitors Nucleoside analog reverse transcriptase inhibitors Multidrug therapy RT has no proofreading activity, resulting in production of many errors during viral DNA synthesis which leads to mutations in all HIV genes and accumulation of mutant viral strains.

Highly active antiretroviral therapy (HAART) Nucleoside analog reverse transcriptase inhibitors Act by serving as a chain terminator Zidovudine (AZT) Zidovudine (AZT) Didanosine (ddi) Didanosine (ddi) Lamivudine (3TC) Lamivudine (3TC)

Prevention Zidovudine Zidovudine (AZT) had been shown to be effective in preventing transmission of HIV from the mother to the fetus. The incidence of HIV infection in the baby was reduced by two-thirds.