during this class we talked about the HIV virus and the particular way in which it infects host cells. the structure of the HIV virus was described; a viral "particle" which contains a nucleocapsid core in its center, which contains 3 genes's worth of RNA and 3 proteins. on the surface of the particle is the GP120 receptor, a highly mutagenic receptor which binds to host cells, and the GP40 receptor, which aids in fusion of the viral particle with the host cell.
HIV most commonly binds to the CD4 t cell but can also infect macrophages and dendritic cells. the viral life cycle for HIV infecting a macrophage: GP120 binds to the CCR5 and CD4 receptors on the surface of the macrophage, which stimulates viral fusion with the macrophage. the nucleocapsid core dissolves, releasing the contents into the host cell cytoplasm. the three proteins contained in the core, integrase, protease, and reverse transcriptase, are set into action.
reverse transcriptase produces fragments of cDNA from the viral RNA. integrase caps off the ends of the cDNA and incorporates it into the host cell genome. at this point the host cell is officially a "provirus". the host cell transcribes the viral DNA as its own and produces 3 long viral proteins (hence the 3 genes contained in the nucleocapsid core), which are activated and cleaved by protease into 13 smaller proteins. 10 of these proteins are left in the provirus to aid in replication and 3 are packaged into a new viral bud, which uses for its membrane the membrane of the host cell. one consequence of this is "viral tropism"- a virus that buds off from a macrophage will have the macrophage surface receptors for t cells and thus will be more likely to infect t cells, and vice versa.
the immune response to HIV depends on the pathway of infection and the type of cell it infects. if HIV exists extracellularly, the immune response will be to produce antibodies against it- at first IgM, before t cells are involved. HIV can also reside intracellularly in host cells- either through infection or phagocytosis. in the case of phagocytosis, the APC will present the viral peptides through the MHC class II peptides, and the co-stimulatory molecules will also be expressed (by virtue of the antigen being phagocytosed), thus ultimately provoking the CD8 response against the infected cell. this would occur through activation of CD4 cells that are specific for HIV, which would then secrete IFN-gamma, which would: upregulate MHC's, stimulate the CD8 response, and cause an antibody class switch to IgG.
in the case of infection, the response against HIV gets complicated and potentially dangerous. if HIV infects a macrophage, the GP120 binding to the CCR5 and CD4 jointly will stimulate expression of the co-stimulatory molecules, leading to the CD8 response. however, if it infects a CD4 t cell, there will be no expression of co-stimulatory molecule and therefore no response from the CD8 cells. furthermore, a CD8 response could potentially be triggered by other cells, as described above, which would then begin attacking all of the infected CD4 cells. it is this mechanism which lowers the CD4 count in HIV infected individuals and leads to AIDS.
a few extra notes: a follicular dendritic cell is a special APC which resides in the lymph notes and serves as a "catalogue" for all the antibodies the body has been exposed to- by way of extremely long dendrites that are covered in Fc receptors. unfortunately, this means that in HIV infected individuals, the Fc receptors will bind to antibody which have HIV viral particles bound to it. thus follicular dendritic cells can function as a reservoir for HIV particles, which are then released during the stress response.
the current HIV treatment is a "triple cocktail" of 2 protease inhibitors and 1 reverse transcriptase inhibitor. most single drugs are rendered ineffective because of HIV's high mutagenicity. two HIV tests are the ELISA test and the PCR test. the ELISA test measures for the presence of antibody specific for HIV by using a second, fluorescent-ly labeled antibody specific for the viral antibody. the PCR test tests for the presence of viral DNA by using a primer specific for the infected DNA, which can then be amplified and detected using the PCR technique.
questions
surface receptors...
1. what is GP 120 and what does it do?
2. what is GP 40 and what does it do?
3. what are the two receptors on macrophages that HIV binds to? what are they receptors for?
4. what are the two receptors on t cells that HIV binds to?
5. what is viral tropism?
6. what is the nucleocapsid core and what is inside it?
viral proteins...
7. what does reverse transcriptase do?
8. what does integrase do?
9. what does protease do?
10. what is meant by a "provirus"?
treatments...
11. what are some drugs that can aid in preventing HIV infection?
12. what are the disadvantages to these drugs?
13. what is currently the conventional drug treatment plan for HIV?
14. what is the difference between a lytic virus and a budding virus?
FDC's...
15. what is a follicular dendritic cell (FDC)?
16. what is the "job" of an FDC?
17. how can FDC's function as a reservoir for HIV?
18. what is a condition that releases the antibodies from FDC's?
immune response...
19. in general, what are the two pathways of HIV infection?
20. what is the immune response against an extracellular HIV infection?
21. what are the two ways in which HIV can exist intracellularly?
22. describe the immune response when HIV infects a macrophage.
23. describe the immune response when HIV infects a CD4 t-cell.
24. describe the immune response to phagocytose-d HIV in a macrophage.
25. what is the main cytokine that CD4 t cells secrete in response to viral infection and what are its effects?
26. how is the CD4 population depleted by HIV? is this autoimmunity?
testing...
27. what is the ELISA test? how does it work?
28. what is the PCR test? how does it work?
answers
1. GP 120 is a surface protein on the HIV virus particle which facilitates its binding to host cells.
2. GP 40 is the membrane protein on the HIV virus particle which facilitates fusion with the host cells.
3. CD4, CCR5. CD4 is the same receptor on t cells. CCR5 is a chemokine receptor.
4. CD4, CXCR4- another chemokine receptor.
5. the attraction of the viral particle for other host cells depending on what cell the viral particle "buds off" from.
6. the central compartment inside a viral particle that contains 3 genes and 3 proteins. genes: GAG, POL, ENV. proteins: integrase, reverse transcriptase, protease.
7. it transcribes the viral mRNA into cDNA.
8. integrase caps the end of the cDNA, then integrates the DNA fragment into the host genome, where it will be transcribed into proteins.
9. protease cleaves the proteins created by transcription of the cDNA- it cleaves the 3 long proteins into 13 small, active proteins.
10. a provirus is an infected host cell that has had viral cDNA incorporated into its genome.
11. drugs that inhibit or block the action of: reverse transcriptase, integrase, envelope proteins, proteases.
12. HIV mutates at such a fast rate that they are rendered ineffective on the order of months, or even days.
13. a "triple cocktail" of 2 protease inhibitors and 1 reverse transcriptase inhibitor.
14. a lytic virus kills the infected host cell whereas a budding virus simply uses the host cells machinery to replicate itself.
15. a dendritic cell in the lymph nodes with extremely long dendrites which are covered in Fc receptors.
16. to keep a "catalogue" of all the antibodies that the host has been exposed to.
17. the Fc receptors on the dendrites might bind antibodies that have virus particles bound to them.
18. stress.
19. intracellular and extracellular.
20. antibody production- IgM at first, before helper t cells are enlisted.
21. through infection and phagocytosis.
22. HIV infects the macrophage and some of the viral protein is degraded and presented by the MHC class I pathway. CD86 is expressed on the surface due to the joint binding of CCR5 and CD4 with the viral GP120- this stimulates the production of IL-12 and potentially activates a CD-8 t cell response.
23. viral peptide still expressed on MHC class I, but no co-stimulatory molecule is expressed; thus immune response is subverted.
24. the MHC class II pathway is activated, as well as co-stimulatory molecules (which are generally expressed with phagocytosis of antigens). the macrophages can then activate CD4 t cells specific to HIV, which will secrete cytokines to provoke the immune response against it.
25. IFN-gamma, which increases MHC expression, activates CD8 t cells, and causes a class switch from IgM to IgG.
26. this is not autoimmunity. the CD8 t cells that are activated against HIV begin killing the CD4 t cells that have also been infected with HIV (this happens in particular when the virus buds off a macrophage)
27. ELISA is a test for the presence of antibody specific for HIV in the blood. it works by using a secondary antibody which binds to the first- the secondary antibody is labeled with fluorescent material which shows up during spectrophotometry tests.
28. PCR test tests for the presence of infected DNA by using a primer specific for this DNA, which then aids in the "amplification" of this DNA using the PCR (polymerase chain reaction) technique, allowing for its detection.
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