RETROVIRUS-MICRO {S1}

RETROVIRUS-MICRO {S1} BY RANJEET RAMAN

Retrovirus overview Enveloped, (+)-RNA genome and reverse transcriptase All retroviruses have gag (core proteins), env (viral envelope), pol (enzymatic activities) genes, and may have additional genes Medically important retroviruses: HIV (a Lentivirus) and HTLV-1 (oncogenic)

HIV Life cycle Entry CXCR4 and CCR5 are co-receptor for T-cell and Mf-tropic HIV, respectively sexual/blood/breast milk transmission, transmitted virus usually Mf-tropic gp120 surface glycoprotein, binds CD4, required for neutralizing Ab’s & CTLs gp41 transmembrane protein, mediates cell fusion

p24 core protein, primary component of nucleocapsid Reverse transcriptase RT uses tRNA primer, synthesizes both strands, high mutation rate (10-4) Integration to random site, mediated by integrase, permanent

Latency/reactivation provides reservoir that currently cannot be eliminated Reactivation occurs when infected cell is activated Virus gene/export and virus assembly utilizes RNA splicing and post-trans N L cleavage by proteases

Clinical aspects HIV infects CD4+ cells and other cells including neurons Infection at mucosa spread to local lymph nodes viremia Sx: lymphadenopathy, fever, rash, mm. ache, meningitis, malaise Within few months, CTLs and Ab’s lower blood viral load

Clinical course: Treatment HAART reduces viral load to undetectable levels, but long-lasting latent pool cannot be eliminated Vaccine hard to develop due to high mutation rate/genetically diverse population of viruses, as well as differences between HIV-1 and -2Diagnosis ELISA with confirmatory Western blot PCR for detection of viral load in blood

Retroviruses and HIV Retroviruses are enveloped in host cell membrane including MHC I and II antigens. Surface glycoprotein gp120 mediates the interaction between virus and cellular receptor. It is a target for neutralizing antibodies and killer T cell. Surface glycoprotein gp41 mediates fusion with the host cell.

The genome is positive-strand RNA similar to human mRNA. It has a 5’ cap and 3’ polyA tail. Pol gene encodes reverse transcriptase, protease, and integrase.

Transmission of HIV Usually through genital secretions and blood. HIV is transmitted inside infected macrophages and lymphocytes (not as free virus). Congenital transmission occurs in utero and in breast milk. Viral load in the CSF is very high. Early infections are macrophage tropic; HIV cannot yet attack lymphocytes.

Virus Entry CD4, which is found on lymphocytes, monocytes, and macrophages, is the receptor for HIV. Viral gp120 binds to CD4, which causes a conformation change, exposing gp41 which then injects into cell membrane. Fusion releases viral RNA and reverse transcriptase into cytoplasm.

However, CD4 alone is not sufficient. Macrophage-tropic and lymphocyte-tropic HIV requires a chemokine co-receptor to mediate fusion. For macrophage-tropic strains, the co-receptor is CCR5. For T-cell-tropic strains, the co-receptor is CXCR4. Individuals who don’t express CCR5 are practically immune to HIV (remember that we only get infected with macrophage-tropic strains!)

Upon entry, RTase synthesizes a negative strand of DNA from the RNA. The original RNA strand is then degraded and the double strand DNA completed. The DNA is transported to the nucleus and permanently integrated into host genome, so that future daughter cells inherit it.

RTase is error-prone because it has no editing capacity. Huge diversity both in the population and within a given individual.

Infection frequently has a latent phase, which creates a reservoir of intracellular provirus. As proviruses are produced, full length viral RNAs are packaged into virions, but are also used to make viral proteins.

It is called the Gag polyprotein, and is cleaved by the protease Pro. This gene is what is stopped by protease inhibitors – they prevent cleaving of the polyprotein, which allows formation of immature, non-infectious virions only.

HIV spread and virus load HIV spreads from lymphocytes and macrophages to lymph nodes, and from there to blood. Lymph nodes are the main site of replication in both chronic and acute infection. Acute infection elicits a vigorous immune response (CD8 killer T cells, IgG) and viral load declines.

The latent infection may last years. CD4 levels are almost normal. But the virus is replicating in nodes, spleen, and brain. Germinal center hyperplasia is seen. Over time, anti-viral immune response declines, and the trapping efficiency of follicular dendritic cells declines.

The lymph nodes fail, virus is released, CD4 cells drop, and opportunistic infections set in. The higher the viral load, the faster the progression of the disease. AIDS is when CD4 lymphocytes <200.

Infections of the lungs and brain are common. These organ-specific infections are associated with infection of the many macrophages found within these organs. Interstitial pneumonitis is associated with some control over virus load, so it is a sign of a less progressive disease.

Treatment of HIV HAART therapy reduces viral load, but CD4 cells remain permanently infected with integrated DNA. Patients can never come off the HAART. Diagnosis is first done by ELISA, then followed by Western blot if ELISA is positive.

Why no HIV vaccine HIV is extremely diverse in the population, and generates quasispecies in each individual. An HIV vaccine would have to have sterilizing immunity – no virus can be allowed to infect in the first place. It must be sterilized as soon as it is introduced, or it will set up latency.

SARS SARS SARS is a winter virus, like influenza. It is very stable in colder temperatures, and lasts days in stool. It is killed by normal disinfectants. Animal reservoir is probably

Symptoms include fever, diarrhea, cough. More slow, insidious onset than flu. It can incubate for over a week. Other features that distinguish it from flu include diarrhea and a fever that goes down but then comes back up.

SARS is transmitted by airborne droplets spread from person to person. The smaller the outbreak, the greater the involvement of health care workers. Prevention/control measures must be draconian to prevent spread.

SARS Flu Slower, insidious onset Rapid onset Diarrhea No diarrhea Fever waxes and wanes Fever is constant

RETROVIRUS-MICRO {S1}