Understanding HIV: The Virus and its Implications

1. HIV (Human Immunodeficiency Virus)

2. Previously known as : • HTLV-3: human T lymphotrophic virus • LAV : Lymphadenopathy associated virus • ARV : AIDS related virus

3. Isolated in 1983 by Frenchman Luc Montagnier and his co-workers. • Robert C Gallo of the NIH USA gave its name HTLV -3 in 1984. • Isolation was from patients with unexplained illness. • Illness was of progressive nature, refractile to treatment and these were homosexuals.

4. Virions were of Retroviridiae family and were the causative agents. • Retroviridae possess reverse transcriptase which transcripts RNA into DNA. • Retroviridae as of now has 3 subfamilies & 7 genera.

5. 3 genera of the 7 contain human retroviruses. • They are enveloped, icosahedral capsid and with 2 +ve ss identical RNA (i.e., +ve polarity)

6. CLASSIFICATION

8. SPUMAVIRUS • Human foamy virus-(HFV) • No human disease is caused • cause problems in viral vaccine production by cell cultures as contaminants. • These cause foamy appearance in culture cells.

9. HIV • Disease-AIDS (Acquired Immuno-Deficiency Syndromes & related infections and conditions) • HIV-1 & HIV-2 cause AIDS; HIV-1 is worldwide & HIV-2 primarily in Africa. • Properties of HIV-1 and HIV-2 are the same with certain differences.

10. Previously , HTLV-3 • Others HTLV-1, HTLV-2, Human T cell leukemia virus, Human T cell lymphotrophic retrovirus. • Prefers and kills Helper (CD4+) T lymphocytes—leads to loss of CMI, high probability of opportunistic infections. • Cells with CD4+protein antigen on cell surface e.g. macrophages and monocytes are also infected.

11. Belong to special sub group lentivirusof retroviruses. • These cause slow infection with long incubation period for disease to develop. • Slow virus diseases—Agents which cause CNS disease, have long incubation periods, gradual onset, invariably fatal course. • It is a disease because rate of replication of viral agent is similar to other viruses- there is no delay.

12. HIV has a bar shaped core called type D. • It is enveloped. Envelope has virus specific glycoprotein: gp 120, gp41. • Genome is diploid because 2 identical molecules of +ve ssRNA. • And is most complex of all retroviruses.

15. 3 genes encode structural proteins—gag, pol, env • 6 other genes found out for regulation of protein synthesis and other properties including species specificity. • gag—internal core protein is encoded by gag. One of them is p24, others are smaller—p17, p9, p7 etc.

16. pol– encodes a) RT-reverse transcriptase  enzyme which synthesizes DNA by using RNA as template. b) Integrase integrates viral DNA into cellular DNA c) Protease  cleaves various precursor proteins. • env-- encodes gp160 a precursor glycoprotein that is cleaved to form the 2 envelope or surface glycoprotein gp120 and gp41.

17. The two HIV (1 & 2) are distinguished on the basis of genomic organization and evolutionary relationship with other primate lentiviruses. • For epidemiologic purpose- env sequences are taken as base. • HIV has 3 distinct groups-> M, N, and O. • Predominant are M contains 11 subtypes or “clades” (A – K). • Similarly there are 6 subtypes for HIV-2 (A-F).

18. The genetic clades do not correspond to neutralization serotypes. • Group A (of HIV-1)  appear world wide • Group B USA—it prefers mononuclear cells, appears to be readily passed through anal sex. • Subtype E  Infects female genital tract and appears to be readily passed through vaginal sex. • Subtype C common in India and China • Subtype H Thailand.

19. HIV-2 was first isolated in India in 1991.

20. Enzymes  3 present in the nucleocapsid of the virion. • Reverse transcriptase (RT) this is a RNA dependent DNA polymerase. • It transcribes RNA genome into proviral DNA. • Other than that it has Ribonuclease H activity and so is a bifunctional enzyme. • Ribonuclease H degrades RNA when it is RNA-DNA hybrid molecules. This is essential for synthesis of proviral ds DNA.

21. Integrase mediates the integration of proviral DNA into the host cell DNA hence a very important enzyme. • Protease cleaves the precursor polyproteins into functional viral polypeptides.

22. Regulatory genes a) tat gene most important gene. Means transactivation of transcription gene. Activation of genes distant from the gene region, maybe the genes present on the same proviral DNA or on cellular DNA. • One site of action is the 5’ end of the viral genome.

23. It encodes protein that enhances the viral or cellular gene transcription. • tat protein and another HIV encoded nef (negative factor gene, produces a regulator protein) represses the synthesis of class I MHC proteins .This in turn leads to decreased ability of cytotoxic T-cell to kill HIV infected cell.

24. b) Viral protein u (vpu) in HIV-1 and vpx in HIV-2 enhances maturation and release of progeny virions in cell. • c) Viral infectivity factor gene (vif)-influences the infectivity of viral particles. • d) rev-regulator of expression of viral protein – facilitates export of unspliced viral proteins from nucleus.

25. At both ends are LTR—long terminal repeats (5’ and 3’). These are transcription initiation sites. • Within 5’ LTR is a binding site of tat protein called transactivation response element. tat enhances the initiation and elongation of viral mRNA transcription.

26. Several important antigens of HIV • gp120 and gp41 – type specific envelope proteins • gp 120 protrudes from cell surface and interacts withCD4 receptors along with a chemokine receptor protein • gp41 is embedded in the envelope and mediates the fusion of the viral envelope with cell membrane at the same time of infection

27. The gene that encodes gp120 mutates rapidly- many antigenic variants. • Most immunogenic region of this gp120 is called V3 loop which is a highly variable site. • Antibodies to gp120 neutralizes the infectivity of HIV , but because of the rapid emergence of gp120 variants the production of vaccine is more difficult. • This may be due to the lack of a editing function in the reverse transcriptase

28. b. Group specific antigen p24 located in the core. • It is known to vary like gp120. • Antibody to p24 does not neutralize HIV infectivity but serves as important serological marker of infection

29. Natural host range- humans, but certain primates (chimpanzee) can be infected in the laboratory • It is not a endogenous virus – no HIV sequences are found in normal human cell DNA. The origin of HIV and how it entered human population is uncertain. • Speculations that monkeys or other primates were the source. But the issue is unresolved.

30. Viruses similar to HIV have been isolated- e.g. 1. HIV -2 isolated from AIDS patients in West Africa in 1986, in India 1991. Protein of HIV-2 is 40% homologous to original virus isolates. HIV-2 primarily localized to West Africa and is much less transmissible than HIV-1. 2. SIV- Simian Immunodeficiency Virus isolated from monkeys with AIDS like illness. Antibodies in some African women cross react with SIV. Proteins of SIV resemble closely to the HIV-2 than the original HIV isolates. 3. HTLV – IV infects T cells but does not kill them. It is not associated with any human disease.

31. Replication- many follow the retroviral model.

32. 1. Entry of HIV into cell – binding of virion gp120 envelope protein to CD4 protein on cell surface. Virion gp120 interacts with second protein chemokine receptor on cell surface. • gp 41 then mediates fusion of viral envelope with the cell membrane and virion enters the cell like an injection

33. Chemokine receptors are fusin (CXCR4 and CCR5 proteins). These are required for the virion to enter CD4+ cells. T cell tropic strains need fusin and Macrophage tropic strains need CCR5. • If there is mutation to the gene encoding CCR5 in a person, he will be protected from HIV infection. 1% people of western European ancestry have this homozygous mutation of gene.

34. 2. Uncoating 3. Virion RNA dependant DNA polymerase transcribes the genome RNA into dsDNA. This dsDNA integrates into host cell DNA, at different sites, also multiple copies of viral DNA can integrate and this is mediated by Integrase. 4. Viral mRNA is transcribed from proviral DNA by host cell RNA polymerase and translated into several large polyproteins

35. 5. These proteins are cleaved by virus encoded protease to form viral structural proteins – reverse transcriptase, core proteins and two envelope glycoproteins. 6. Virus assembles in cytoplasm released from cell by budding. But much of virus is cell associated so difficult to neutralize with antibody.

36. Transmission and epidemiology- • Sexual contact, transfer of infected blood, perinatal from infected mother to neonate either at birth or via breast milk. • Infection can be by HIV infected cell or HIV i.e. free – not associated with cell—semen, breast milk.

37. A small amount of virus is found in fluids like saliva, tears but there is no evidence it can play a role in infection. • In general HIV follows same transmission pattern as HBV except that HIV infection is less efficiently transferred. • That means the dose required to cause infection is much higher than HBV.

38. Epidemiology-

39. PATHOGENESIS AND IMMUNITY • Infects helper T cells and kills them, result-suppression of CMI, predisposition to opportunistic infections, certain cancers e.g. Kaposi’s sarcoma and lymphoma. • HIV genes are not found in carcinoma cells-so indirectly involved in causation of tumors. • HIV also infects brain monocytes and macrophages – formation of multinucleated giant cells and significant CNS symptoms.

40. Fusion of HIV infected cells in the brain and other places is mediated by gp 41. It is the main pathological finding – cells ultimately die. • The death of the cells is due to immunologic attack by cyto CD8 lymphocytes or maybe antibodies. • The effectiveness of cytotoxic Tells maybe limited by the ability of viral taq gene which can reduce class I MHC protein synthesis

41. Another mechanism- HIV acts as Super Antigen which indiscriminately activates many Helper T cells and leads to their demise. • (Super Ag- e.g. Mouse mammary tumor virus model) • Persistent non- cytopathic infection of T lymphocyte also occur. HIV is produced and sustenance of infection in vivo occurs

42. A person infected with HIV is considered infected for life. • Reason- integration of viral DNA into the DNA of infected cells. • Macrophage and monocytes are large reservoirs of HIV in AIDS patients. These cells are relatively refractile for viral CPE effects. The virus survives and is transported to lungs and brain. • Lymphoid organs are also a reservoir. Replication goes on for years even during latency of the disease. It is a sequestrated site because of its function and anatomy.

43. If there is mutation in nef gene the person infected with HIV develops no opportunistic infection. So nef gene has a role in pathogenesis. • Antibodies are produced but they neutralize the infectivity of the virus poorly. • Immunity is incomplete because infection and antibodies co-exist. • This is one of the reasons in failure of development of effective vaccines.

44. Abnormalities of B cells also occur. • Polyclonal activation of B cells occurs. • High immunoglobulin levels and also auto immune diseases seen. e.g. thrombocytopenia.

45. Clinical features CDC classification of HIV infection and AIDS –

46. But it can also be divided into 3 stages- • Early – acute stage • Middle- latent stage • Late or immunodeficiency stage

47. Early or acute begins 2-4 weeks of infection. • A mononucleosis like picture- fever, lethargy, sore throat and generalized lymphadenopathy is seen. • Maculo-papular rash is also seen.

48. Leucopenia also occurs. • No. of CD4+ cells are usually normal. • Antibodies to HIV typically appear within 2 months of infection. This delay leads to false negative serological tests based on Antibody detection, so viral Antigen detection assays to be used. (Person is infected but Antibody is not detected).

49. Middle stage a long latent period ensues. • Asymptomatic viremia is low or absent because HIV is produced in lymph node and is sequestrated. • The virus is replicating but disease is latent- clinical latency only but virus does not enter latent stage. • Infected person produces approximately 10 billion new viruses every day.

50. This viral load can be estimated by using an assay for viral RNA in patient plasma. • Amount of viral RNA will be a guide to treatment and prognosis e.g. if drug regimen fails to decrease the viral load it can be changed. • Prognostically if patient has >100,000 copies of viral RNA/ ml plasma, he is more likely to progress to AIDS than the patient having fewer than 100,000 copies.