HIV 1 lecture

HIV/AIDS Lecture Notes

Introduction

• World AIDS Day: December 1st, symbolized by a ribbon.

• HIV: Human Immunodeficiency Virus.

• AIDS: Acquired Immunodeficiency Syndrome.

Learning Objectives

• History, incidence, and clinical progression of AIDS.

• Biology of HIV and immunosuppression mechanisms.

• Immune response to HIV.

• Chemotherapy and vaccine development challenges.

Key Topics and Learning Outcomes

• History of AIDS.

• HIV structure and replication cycle.

• Clinical course of HIV infection.

• Immunology of HIV infection (effects on the immune system and the host's response).

• Chemotherapy of AIDS.

• HIV vaccines and microbicides: frustrations and possibilities.

History of AIDS

• 1981: Cluster of unusual diseases observed.

  • Pneumocystis carinii pneumonia (caused by a yeast). Patients developed severe pneumonia.

  • Kaposi's sarcoma (caused by human herpesvirus eight), which induced tumors. Considered rare in healthy individuals.

• Initial patients: Homosexual men, intravenous drug users, and hemophiliacs.

• Transmissible agent suspected due to the unusual pattern of disease.

• Low CD4 T cell numbers observed, indicating immunosuppression.

• 1982: CDC (Centers for Disease Control) used the term AIDS to describe the disease.

• 1983: HIV isolated by Luc Montagnier from a lymph node of an infected patient, named Human Immunodeficiency Virus (HIV).

• 1986: Second strain identified, called HIV-2. Montagnier's original isolate was named HIV-1.

• HIV-1 and HIV-2 differ in virulence and geographical distribution: HIV-2 is less virulent and primarily found in West Africa.

• Genetic studies: Both HIV-1 and HIV-2 originated from primates (jumped species to infect humans).

  • HIV-1 from chimpanzees.

  • HIV-2 from sooty mangabeys.

• Transmission: Assumed to occur during hunting/killing of monkeys for food.

• HIV-1 and HIV-2 do not cause immunosuppression in chimpanzees or sooty mangabeys; immunosuppression occurs when the virus crosses into humans.

Global Impact

• Since 1981: Over 75 million people have been infected (approaching 90 million).

• Mortality: 43 million deaths.

• Annual deaths: About 1 million people die from HIV/AIDS each year.

• Highest infection rate: Sub-Saharan Africa (20-40% of young adults infected).

• Life expectancy in sub-Saharan Africa has been halved (30s-40s instead of 70).

• Transmission in Africa: Primarily heterosexual contact, affecting men and women equally.

• Transmission in Europe, the USA, and Oceania: Mainly high-risk groups (homosexuals and intravenous drug users), more prevalent in men.

• Changing patterns: Increasing infection rates among women in the USA.

• By the end of 2008, women accounted for 50% of all adults living with HIV around the world.

• Significant numbers of people living with HIV/AIDS in countries like Kenya, Nigeria, and Zimbabwe (percentages reaching 26% or more).

• HIV also infects children (at birth or through living with infected parents).

• AIDS results in orphaned children due to parental mortality.

Strains and Variants

• HIV-1 and HIV-2 exist, with many subtypes of HIV-1.

• Vaccine development challenge: To protect against all the different subtypes.

• High mutation rate of HIV leads to different forms (variants).

• Importance of variants depends on the cell types they infect.

• Despite variations, the structure and replication cycle are collectively described as HIV.

HIV Structure

• Retrovirus: Belongs to the lentivirus family with RNA as genetic material.

• Genome: Two molecules of single-stranded RNA, each bound by a reverse transcriptase molecule.

  • Protease (P10 based on molecular weight).

  • Integrase (P32).

• Nucleocapsid surrounds the genome.

  • Inner layer: Protein subunits called P24.

  • Outer layer: Polyproteins called P17.

  • Both layers form part of the gag complex (group-specific antigen).

• Outer portion: Envelope of lipid derived from host cell membranes.

• Surface viral proteins: GP120 and GP41, critical for attachment to host cells.

Viral Genome Details

• Gag: Precursor that is processed to the gag precursor, then further processed by the protease into P24.

• Pol region: Processed to a pol precursor then processed to:

  • Protease P10.

  • Reverse transcriptase (P66 and P55).

  • Integrase P32.

• Envelope proteins are processed through GP160, then further processed into GP120 and GP40.

• Accessory proteins, such as NF, code for a negative regulator protein associated with viral infectivity.

HIV Gag

• Encodes structural capsid proteins.

• Processed from a precursor by the virus's protease into final products.

• Inner polypeptide in the inner capsid, outer capsid protein P17.

• Membrane-associated gag associates with the host membrane, attracting two copies of viral RNA, along with cellular and viral proteins.

• Triggers budding of the virion from the surface of the cell.

Viral Structure Visual

• Inner nucleocapsid.

• Outer nucleocapsid.

• Lipid membrane surrounding the virus.

• The two molecules of RNA are inside the inner capsid.

• GP120 protein: covered in glycan side chains, believed to help binding of the virus to host cells.

Replication Cycle

• Virus must infect a host cell to replicate.

• HIV infects CD4+ cell type (helper T cells).

• Helper T cells recognize antigens on the surface of virus-infected cells and secrete lymphokines that stimulate B cells and killer T cells.

• HIV infects and kills helper T cells, thereby preventing a normal immune response.

• In addition to CD4 T cells, CD4 markers are also found on monocytes and dendritic cells, although in lower numbers.

• Two stages of viral infection:

  • Binding to the host cell.

  • Fusion with the cell membrane of the host to allow the virus to enter the cell.

• GP120 protein on the virus surface binds to the CD4 receptor on the host cell.

• CD4 receptor is part of the host immune response.

Viral Replication Process

• Virus binds to the CD4 receptor and a co-receptor.

• Once bound, the lipid membrane fuses with the host lipid membrane, and the RNA molecules are released into the cell, where they can then enter the nucleosome and be converted from RNA into DNA.

• They fuse with the host cell's nucleic acid material and, once the virus is transcribed and translated back into RNA and viral proteins, they are then processed and accumulate at the host cell membrane.

• We then get maturation of the virus and ejection from the host.

• The virus binds to the CCR5 receptor (in this particular case) and the CD4 receptor.

• Reverse transcriptase transcribes the RNA into DNA.

• The integrase incorporates viral DNA into the host genome.

• The virus is transcribed and translated.

• Protease cleaves viral precursors into active polypeptides.

Budding Process

• GP120/41 complex assembles on the outside of the host lipid bilayer.

• Viral polypeptides start to accumulate.

• Maturation process is almost complete.

• GP120/41 complex is around the virus portion of the lipid bilayer, just about to go to complete closure.

• Host cell membrane bubbles away when it has the attachment complex, GP120/41, already in place.

• The virus can invade CD4+ helper T cells and can also hijack the immune system by infecting dendritic cells.

• The virus goes through very thin projections called filopodia. These are dependent on actin.

  • Facilitates dendritic cell to T cell interactions.

Co-receptors

• GP120 binds to CD4; originally thought GP41 binds to a second protein, but it is GP120 that binds to the chemokine receptor.

• M-tropic variants: Bind to CCR5 (beta chemokine receptor) on CD4 T cells, monocytes, and dendritic cells.

• T-tropic variants: Bind to CXCR4 (alpha chemokine receptor) on CD4 T cells but not on monocytes or dendritic cells.

• GP160 forms a trimer during the early fusion process.

• GP160 cleaves into GP41 and GP120, remaining in the trimer and non-covalently attached.

• GP41 is thought to be in a high-energy state.

• GP120 binds to CD4, causing a conformational change, allowing it to bind to the chemokine receptor.

• Conformational change releases GP41 from its high-energy state.

• GP120 binds to either CCR5 or CXCR4, not GP41.

• Following chemokine co-receptor binding, GP120 dissociates away.

• GP41 transiently becomes an integral component of the viral membranes.

• For M-tropic variants, GP120 binds to CD4 and then to CCR5; accessory protein MC1 alpha aids binding.

• For T-tropic variants, GP120 binds to CD4 and to CXCR4; accessory factor SDF1 facilitates binding.

Genetic Mutations

• CCR5 delta 32: Non-functional mutant of CCR5 (internal deletion of 32 base pairs) found in high frequency in people in Europe and North America.

• Insertions in the promoter region of MC1 alpha reported in 20% of Indians.

• 3UTF8 to 1: Guanine to adenine point mutation for SDF1 (ligand for CXCR4) found in about 40% of healthy Asian Indians.

• CCR5 gene has two alleles; one mutated in 10% of Europeans, resulting in a non-functional form of the CCR5 protein that delays progression of the disease to AIDS. 1% of Caucasians are homozygous for the known allele, making them almost fully resistant to HIV infection.

• Isolates of HIV in recently infected patients are predominantly M-tropic.

Replication Inside Cell

• GP41 mediates membrane fusion after GP120 binds to CD4 and the relevant chemokine receptor, allowing the viral capsid to enter the cell.

• Nucleocapsid is removed, and the reverse transcriptase copies the RNA to double-stranded DNA.

• The fidelity of reverse transcriptase is very poor, making about ten mistakes per replication round.

• Viral DNA integrates into the host cell DNA (provirus).

• RNase H degrades the original RNA template.

• The virus lies dormant for a long period (latent stage).

• Viral production is initiated by cellular transcription factors, particularly NF-kappa B.

• Viral RNA is transcribed from proviral DNA, and viral proteins are translated.

• Viral proteins and RNA assemble, then bud off.

• The virus takes a lot of the host cell membrane with it.

Clinical Course of HIV Infection

• Three stages: infection, the latent stage, and the development of AIDS.

• Normal CD4 T cell number: approximately 1000 cells per microliter.

• CD4 T cell count drops to about 300-400, followed by a slow recovery during viral infection.

• Seroconversion occurs in 4 to 6 weeks; the latent phase can last from a few months to 17 years.

• Once the CD4 T cell count collapses, we enter AIDS, which can take half a year to two years.

• Early symptoms: Flu-like symptoms (fever, malaise, aching muscles, sore throat, and swollen lymph nodes).

• Seroconversion occurs as antibodies to HIV and antigens are produced.

• The average time for infection to develop into AIDS is about ten years.

• Late-stage symptoms: Weight loss, night sweats, fever, and diarrhea.

• Unusual infections occur, such as oral thrush, herpes simplex, and shingles.

• CD4 T cell count drops below 200 cells per microliter of blood.

• Without treatment, AIDS patients will inevitably die.

• Most common opportunistic infection in Europe and the USA is pneumocystis carinii.

• Common opportunistic infections in Africa and Asia are cryptosporidium and Mycobacterium tuberculosis (TB).

• Typical infections are caused by fungi, viruses, bacteria, or protozoans.