Human Immuno Deficiency Virus (HIV VIRUS)

HIV

Viron: spherical, 80-100 nm diameter, cylindric, core (ribonucleoprotein)

Genome: ss RNA, linear, diploid, 9-10kb, +ve sense.

Proteins: RT, Env gp, protease

Envelope: present, acquired from the host cell membrane when budding out of cell.

·        non- oncogenic, may be cytocidal, provirus remain permanently associated with cells.

Genome

-         contain in the same order the genes: gag, pol and env → 5’- gag- pol- env – 3’

-         LTR of both ends of genome contain promoter and enhancer sequences.

-         Gag → group specific Ag ( associated with virion core)

-         env → type- specific / sub group specific (gp of envelope)

-         pol → RT, IN, PR, endonuclease .

-         at least 6 regulatory genes.
- tat (transactivating)
→ general stimulating effect on synthesis of all viral proteins.
- rev(regulatory effect)
→ for expression of viral structural protein.
- nef (negative factor)
→ downregulates expression of CD4 and MHCI.
- vpr → increase transport of pre- integration complex into nucleus.
- vif → promotes virion infectivity
- vp4 (HIV-1)/ vpx (HIV- 2)→ enhances maturation and release of progeny virus.

-         Gag- pol precursors cleaved by viral protease PR.

-         Env is cleaved by cellular PR.

-         Regions of greatest divergence localized to env gene.

-         SU (gp 120) unit determine L- and M- tropisms and carries the major Ag determinant .

-         HIV gp has 5 variable regions (V1-V5).

-         Sequences of TM, more conserved that SU.

-         Due to error prone nature of RT.

-         Infected hosts contain “swarms” of closely related viral genomes, k/as ‘ quasi species’

Classification

-         based on molecular and antigen difference, 2 types of human AIDS viruses,
HIV-1 and HIV-2 → much less virulent; related more to SIV; largely confined to west Africa.

-         Based on env gene sequences, HIV-1 has 3 distinct group: M (major), N(new), and O (outlier).

-         M group contains 9 subtypes / clades (A-K, omitting E and I).

REPLICATION

-         Replicate via an integrated ds DNA stage.

-         RT( RNA directed DNA polymerase) coded by pol gene has 4 activities ( protease, polymerase, RNase H, and integrase).

-         Following entry, synthesis of DNA complementary to viral RNA occurs using RT; primer is a specific t-RNA eg tRNA ^lys

-         Sequences from both ends of viral RNA become duplicated, forming the LTR located at each end of viral DNA.

-         RNase H activity of RT digests RNA from a DNA- RNA hybrid, resulting ssDNA made ds by pol. Activity of RT.

-         Linear ds DNA able to circularize.

-         Integration of circular DNA into host cell DNA → provirus
→ site of insertion non specific
→ provirus flanked by a 4-6 bp direct repeat of host DNA.
→ integrated state is stable and viral DNA replicates along with cellular DNA.

-         Proviral DNA transcribed by host enzyme, RNA pol II.
→ full length transcripts serve as genomic RNA.
→ some transcripts are spliced and the subgenomic mRNAs, translated to produce viral precursors proteins;viral protease involved.

-         Provirus remains integrated within cellular DNA for life of the cell.

HIV PATHOGENESIS

- use as a receptor the CD4 molecule ( macrophages, T- cells)

-         a 2^nd co- receptor is required to gain entry to cells

-         first binds to CD4 and then to the co- receptor  CCRs M- tropic; CXCR4 for T- tropic.

-         After the 2^nd binding step, virus enters by fusion of viral envelope with the cell membrane; requiring exposure of a hydrophobic domain in gp41.

-         Period between HIV- infection and appearance of antibody in serum “ window period” → 2-3 weeks ( Max 6 months).

-         Seronegative person can transmit the virus.

-         Following 10 infection, there is 4-11 day period between mucosal infection and initial viraemia (antigenaemia → free p24 Ag and RNA copy number).

-         Acute illness ( mononucleosis like syndrome) develops in 50-75% patients , 3-6 weeks after 10 infection → significant drop in CD4 cell – count.

-         Seroconversion ( production of detectable level of antibody) occurs 1 week – 3 months after infection. → plasma viremia drops, levels of CD4 – rebound
→ remains negative through out the asymptomatic phase.
→ temporary increase in RNA – level seen during intercurrent infections

-         Immune response unable to clear the infection completely.
→ HIV- infected cells persists in lymph nodes.

-         Period of clinical latency may last for as long as 10 yrs.

-         Increase level of ongoing viral replication ; 10 billion HIV particles produced and destroyed each day.

-         Due to this rapid viral proliferation and inherent error- rate of HIV- RT, every nucleotide of HIV – genome mutates on daily basis.

-         Eventually, patient will develop constitutional symptoms and clinically apparent disease.
→ increase level of virus detectable in plasma ( P24 antigenaemia) during advanced stages of infection.

-         Cardinal sign of HIV infection is depletion of TH- inducer cells
→ result of HIV- replication in these cells as well as death of uninfected T- cells by indirect mechanism.
→ T4:T8 ratio reversed (from 2:1 to 1:2)

-         Early in infection, 1⁰ – HIV – isolates are M- tropic, as the infection progresses, they are replaced by T-tropic strains.

-         At any given time only a small fraction of CD4 cells are productively infected; many infected cells are killed, but a fraction survive and revert to resting memory cells ( reservoir of virus: half life 43 months).

-         When exposed to antigen (foreign Ag and lectin eg phytohaemagglutinin), memory cells become activated and release infectious virus.

-         Monocyte – macrophage major reservoirs for HIV
→ relatively refractory to CPE
→ transports virus to various organs in the body.

-         Throughout the course of untreated infection; HIV is actively replicating in lymphoid tissues.

-         Activation signals required for establishment of productive HIV- infection
→ wide range of invivo antigenic stimuli serve as cellular activators.
→ eg active infection by MTB; other concomitant viral infection; super infection with MTB 2^nd strain in HIV- infected individuals.

-         Destruction of CD4+ cells caused by
- viral replication – syncytium formation – CTL lysis of infected cells.
- CTL lysis of CD4+ cells carrying gp 120 released from infected cells.
- NK- cells. – ADCC.

-         T4 cells don’t release normal amount of IL-2, IFN- γ and other lymphokines
→ damping effect on CMI- response.

-         Humoral mechanisms also are affected.
→ polyclonal activity of B- cells → hypergammaglobulinemia (IgG/ IgA).

-         Chemotaxis , Ag- presentation and intracellular killing by monocytes / macrophages diminished.

CLINICAL FEATURES

-         due not primarily to viral cytopathology but are 20- to failure of immune response; exception in AIDS dementia and other degenerative neurologic lesions due to direct effect of HIV on CNS.

-         Plasma viral load most accurate prognostic marker or the best predictor of long- term clinical outcome.

-         CD4 count assesses the state of patients immune system and the best predictor of short- term risk of developing an opportunistic disease.

1. acute seroconversion illness: in about 50% of cases.
    - resembles glandular fever with adenopathy; after mistaken for infectious mononucleosis.
   - spontaneous resolution occurs within weeks.
   - pathogenesis due to immune complexes as well as to direct effects of viral multiplication.
2. asymptomatic / latent infection: last upto several years.
   - cases are seropositive and infectious
   - period of clinical latency doesn’t mean microbiological latency, virus multiplication goes on throughout.
   - hosts HI and CMI response can only limit the virus load, but not clear it completely; chronic persistent infection is the result.
3. PGL: - in 25-30% patients who are otherwise asymptomatic
   - lymph node enlargement (>1 cm diameter) in two or more non- contiguous extra- inguinal sites that persist for at least 3 months, in the absence of any current illness or medication that may cause lymphadenopathy.
4. ARC
   - patients with constitutional symptoms of fever, weight loss (slim disease), chronic diarrhea and minor opportunistic infections ( oral candidiasis, HZV, salmonellosis, hairycell leukoplakia, TB)

5. AIDS- irreversible breakdown of immune defense mechanism.
   - current case definition of AIDS
   → individual who test +ve for HIV and
   1. have CD4 cell count of <200µl (normal 600-1000/ µl) of whole blood or a CD4+ cell: total lymphocytes % of <14%.
   2. have CD4 cell count of >200 µl and any of following condition
   - parasitic – fungal- bacterial – viral – malignancy
   • commonest presentation → increased dry cough, dyspnoea, fever
   - in west→ Pneumocystis carinii pneumonia and now MTB + MAI complex.
   - in developing countries → MDR TB.
   - recurrent pneumonia indicative of AIDS.

•GI system: oral thrush, herpetic stomatitis, gingivitis, hairy leucoplakia, kaposis sarcoma (Gnwud)

•CNS : toxoplasmosis, cryptococcosis, lymphoma (polyclonal B- cell malignancies).

•Malignancies: kaposis sarcoma; lymphoma (Hodgkin / Non- Hodgkin

6. HIV dementia : - in 25% of patients, due to CPE of virus.

pediatric AIDS; - about 1/3^rd to half the number born to infected mothers
- virus transmission more common perinatally.
- clinical manifestation by 2 yrs of age: death in another 2 yrs.
- lymphoid interstitial pneumonia common in children.

Lab diagnosis

Non specific

1. blood count: leucopenia, thrombocytopenia, lymphocyte count <400µl
2. T- cell subset assay: CD4+ count <200/µl, T4:T8 ratio reversed
3. Hypergamma globulinaemia: ↑ IgG / IgA level
4. CMI : diminished CMI as indicated by skin tests.
5. Lymph node biopsy showing profound abnormalities.

Specific tests:

1. Virus isolation
   - HIV cultured from lymphocytes in peripheral blood.
   - test sample co – cultivated with uninfected phytohaemagglutinin – stimulated donor lymphocytes in presence of IL- 2.
   - virus presence detected by assay for RT and P24 Ag after 7-14 days.
   - time consuming and labourious; PCR amplification technique more commonly used.
2. detection of viral NAs and Ag.

               i.            antigen detection:
- ↓ levels of circulating HIV- 1 p24 Ag detected in plasma by EIA soon after infection.
- Ag becomes undetectable after Ab- develop, may reappear late in course.
- antigenaemia dominant when infection occurs by blood transfusion
- used for screening blood donors in USA.

PCR: gold standard for diagnosis in all stages of infection.
- detection of virus during window period and during extended periods of viral latency.
RNA- PCR:
- RNA sequences found in extracellular virus particles in plasma.
- levels of RNA copy no. indicate the extent of virus replication in patients.