Immuno 2: HIV Infection (& secondary causes of immunodeficiency) Flashcards
What are the strains of HIV that can cause infection?
- HIV-1
- HIV-2
What is the difference in virulence between HIV-1 and HIV-2?
HIV-2 is less virulent and harder to transmit
Name some retroviruses
- HIV-1
- HIV-2
- HTLV-1 to 4
Explain how HIV replicates
- Genes = RNA molecules
- Replicates inside a cell using Reverse Transcriptase (RT) to convert RNA to DNA to be integrated into the host genome
Describe the structure of HIV
- HIV has an icosahedral (20-faced) structure
- Genome is diploid
- Contains 9 genes (i.e. env, gag, pol) that encode 15 structural, regulatory and auxiliary proteins
- I.E. gp120, gp41 and RT
Which cells does HIV target?
- CD4+ T-helper cells, (esp in the gut)
- CD4+ monocytes / MO,
- CD4+ dendritic cells / DC
Describe how HIV targets cells
- HIV uses host CD4+ cells to replicate and move from cell to cell –> changes the function of these cells
- These are critical to mount a full immune response, hence the critical failure & AIDS
- You will also lose immune memory as the memory T-cells are depleted
- This leads to a selective loss of CD4+ T helper cells
Which cells are necessary for protection from HIV?
- Antibodies (B cells) to prevent infection and neutralise the virus
- Sufficient CD8+ T cells to eliminate latently infected cells
Describe the HIV-1 receptor and co-receptor
- HIV-1 receptor and co-receptor:
- Receptor = CD4 molecule/antigen
- Co-receptor (most strains require) = CCR5 or CXCR4
How is HIV transmitted?
- Sexually – through mucosa (esp. damaged sites / MSM), infects CD4+ cells (inc. CD4+ DC) which carry virus to LN
- Infected blood – transfusion, needle sharing, blood products
- Vertical (mother to child) – ante-/intra-partum, breastmilk
How long does natural immunity to HIV take to be mobilised?
mobilised within hours of infection
What does natural immnity to HIV involve?
- Inflammation
- Non-specific activation of macrophages, NK cells and complement
- Release of cytokines and chemokines (i.e. those made by NK cells can reduce infection of CD4+ T-cells by HIV)
- Stimulation of plasmacytoid dendritic cells (pDC) by toll-like receptors (TLRs)
Which cells are involved in acquired immunity?
Antibody and B cells
Describee acquired immunity to HIV
- Anti-gp120 and anti-gp41 (Nt) antibodies are important in protective immunity
- Non-neutralising anti-p24 gag IgG are also produced
- HIV remains infectious even when coated with antibodies
What is the role of CD4+ T cells / AKA Th-cells in the immune response to HIV?
- Usually recognise processed antigens (especially Gag p24 peptides) in the context of MHC class II molecules
- Important in the coordination of the immune response (i.e. long-term maintenance of CD8+ t-cell response)
What is the role of CD8+ T cells in the immune response to HIV?
- Able to suppress viral replication – can kill HIV-infected cells or other cells that have become malignant
- Secrete soluble molecules (i.e. cytokines/chemokines) to prevent infection by blocking entry to CD4+ T cells
- Recognise processed antigens in the context of MHC Class I
Summarise the effects of HIV on body cells
- Activated infected CD4+ helper T cells die and are lost
- Infected CD4+ T cells are also disabled (ANERGISED) by the virus:
- MO/DC are not activated by the CD4+ T cells and cannot prime naïve CD8+ CTL
- CD8+ T cell and B cell responses are diminished without help
- CD4+ T cell memory is lost
- Infected MO/DC are killed by virus or CTL:
- Defect in antigen presentation
- Failure to activate memory CTL
During the replication of HIV, in which 2 steps can errors occur?
- Reverse Transcriptase (RNA to DNA) – lacks proof-reading mechanisms from cellular DNA polymerases
- Transcription of DNA into RNA copies
What is the clinical significance of errors in HIV replication?
- means that HIV can accumulate a lot of mutations with numerous variants or quasispecies
- can lead to HIV gaining advantageous features
- Escape from neutralising antibodies
- Escape from HIV-1 specific T cells
- Resistance and escape from antiretroviral drugs
Describe the life cycle of HIV
Name some therapy targets within the life cycle of HIV
- Attachment (Attachment Inhibitors / AI)
- Fusion (FI)
- Reverse transcription (RTI)
- Integration of viral DNA into host (INI)
- Transcription of DNA to viral RNA
- Translation of viral RNA to produce viral proteins
- Viral protein cleavage by proteases (PI)
- Assembly and budding of new HIV
Name the suffixes of HIV therapies, and what they inhibit
- -gravir = Integrase Inhibitor
- -avir = protease inhibitor
- -ines = NRTI (nucleotide reverse transcriptase inhibitor)
Describe the clinical course of HIV disease
- Median time from HIV infection to development of AIDS = 8-10 years
- Viral burden predicts disease progression
- Rapid progressors (10%) will take 2-3 years (these are mainly seen in Africa)
- Long-term non-progressors (LTNP < 5%) will have stable CD4+ counts and no symptoms after 10 years
- Exposed-seronegatives (ESN) are people who are repeatedly exposed to HIV but do not seroconvert
- Elite Controllers (EC) can suppress the viral replication
Which therapy can greatly improve the prognosis of HIV infection
HAART (Highly Active Antiretroviral Therapy)
Describe how the numbers of these cells change over the course of HIV infection:
- CD8+ lymphocytes
- CD4+ lymphocyte
- plasma viral load (amount of HIV cells)
Summarise the % of HIV infection that are typical, rapid and long-term non-progressors, and their respective times taken to progress into AIDS
Summarise the host genetic factors that allow long-term-non-progressor infections to occur
- HLA profile (slow progressor)
- Heterozygosity for 32-bp deletion in chemokine-r CCR5
- MBL alleles
- TNF c2 microsatellite alleles
- Gc vitamin D-binding factor alleles
Summarise the host immune response factors that allow long-term-non-progressor infections to occur
- Effective CTL, HTL and humoral responses
- Secretion of:
- CD8 antiviral factor
- IL-16
- Secretion of chemokines that block HIV entry co-receptors CCR5 and CXCR4
- CCR5 chemokines = MIP-1a, MIP-1b, and RANTES
- CXCR4 chemokines = SDF-1
- Maintenance of functional lymphoid tissue architecture
Summarise the virologic factors that allow long-term-non-progressor infections to occur
- Infection with attenuated strains of HIV
Which techniques are used to detect HIV?
- Anti-HIV antibodies (ELISA) – screening test
- Anti-HIV antibodies (Western Blot) – confirmation test
- Viral load (viral RNA detection using PCR) – very sensitive; steps:
- Regent preparation
- Specimen preparation
- Amplification
- Detection
Which factor is a good predictor of the time taken for the disease to appear?
Initial baseline plasma viral load
How are CD4+ T Cell Counts derived/measured?
flow cytometry
Which cell count change correlates with the onset of AIDS
- The onset of AIDS correlates with a decrease in the number of CD4+ T cells
Name the antigens on T cells
- Antigens on T cells:
- CD3, CD4
- CD8
- CD19
- CD56