17 - HIV - Watson

Question 1

Briefly state the main features of the AIDS timeline

Answer 1

1) CDC collated cases with people affected by Kaposi’s sarcoma and pneumocystis pneumonia (PCP) in NY/LA in gay men
2) retrovirus isolated from lymph system of a patient
3) discovered probable cause of AIDS
4) HIV (human immunodeficiency virus) adopted as the name
5) AZT became available for treatment. blocks RNA transcriptase (key enzyme for retroviral transcription)
6) combination therapies developed
7) protease inhibitors -> death rates from HIV worldwide fell dramatically

Question 2

Briefly describe HIV

Answer 2

lentivirus - slow growing

inserts into the genome as a provirus. enters a state of quiescence until it reemerges

Question 3

What caused for HIV to spread so rapidly within the population?

Answer 3

HIV has always been around. first estimates (through spread and mutation rates) suggest that the first case of transmission between monkey and human was around 1900 (we share 99.9% DNA)

• lifestyle changes eg prostitution, increasing acceptance of gay culture
• better transport hubs linking higher no. people

Question 4

Draw the structure of a HIV virion and label it

Answer 4

311 - 17

Question 5

Describe the genome of HIV

Answer 5

dsRNA with lots of overlapping sequences

Question 6

What is the env protein and why is it important?

Draw a diagram showing its function

Answer 6

env protein is expressed and cleaved by protease -> 2 subunits gp41 and gp120
these form a surface structure that is key to the life cycle of HIV1 and a major vaccine target

Question 7

Give a summary of the infection process

Draw diagrams to aid explanation

Answer 7

1) fusion with membrane. Gp120/41 interact with CD4 and CCR5 (chemokine receptor). all cells that express CD4 and CCR5 are susceptible eg CD4 T cells, macrophages, DCs
2) reverse transcription of genome. RNA -> cDNA. migration of the cDNA to the nucleus and integration into host genome. entering a state of quiescence producing v few viral particles
3) later on, expression of viral proteins, generation of viral genome and encapsulation.
4) expression of env protein and cleavage into gp120/41
5) secretion of mature virions from cell surface

Question 8

What are the effects of infection with HIV?

Answer 8

infection of CD4 T cells

CD8 T cells recognise virally infected cells and they are targeted for apoptosis

Question 9

Describe the pathology of HIV/AIDS

Answer 9

acute phase; CD4 T cells depleted by Cytotoxic T cells (doing their job)
slight recovery of CD4 T cell numbers as HIV-1 evades IS and has integrated into genome
chronic infection; slow depletion of CD4 T cells. because of generalised immune activation and loss of ability to produce new CD4 cells
when CD4 levels fall below certain point, immune system compromised leaving us open to opportunistic infections and rare cancers eg Kaposi’s sarcoma (result of viral infection with human herpesvirus 8 - HHV8)

Question 10

Name AND describe the functions all of the accessory proteins associated with HIV

Answer 10

NEF;
CD4 misdirected and degraded by the endolysosomal compartment. prevents CD4 expression on surface. this assists viral release as the CD4 would tether the virus to the surface via interactions with gp120/41 and prevent its release.
VIF, VPU, VPR;
(all subvert the cellular ubiquitination system to neutralise anti-viral responses)
VIF;
targets apolipoprotein B mRNA editing complex. (this targets ssDNA therefore inhibiting viral replication)
VPU;
dowregulates CD4 therefore assists viral release (like New)
VPR;
targets proteins involved in inhibiting viral replication. subverts cell machinery and allows cells to become viral factory

Question 11

Summarise why HIV is so difficult to eradicate

Answer 11

• HIV targets cells of the immune response
• accessory proteins down regulate the IR
• HIV can be dormant for long periods creating a reservoir of virus. this can re-establish infection later on.
  (even if we were able to treat and remove viral particles etc there would still be a reservoir in the genome that could not be cleared)

Question 12

Why is there a need for a HIV vaccination?

Answer 12

• HIV/AIDS has detrimental impacts on communities

- 1% of world’s population is infected

Question 13

What are the current approaches to treating HIV infections? (2) Describe them

Answer 13

AZT;
blocks RNA transcriptase

Triple Therapy;
inhibitors of integration into genomes
inhibitors of RNA transcriptase
protease inhibitors. prevents maturation of viral proteins (gp120/41) therefore inhibits membrane fusion

Question 14

What evidence is there that suggests a vaccine for HIV/AIDS is possible?

Answer 14

• presence of long term non-Progressors. people that carry the disease (+30 yrs) and do not progress onto late stage (AIDS). do have some form of natural immunity (Abs?)
• pre-infection with less virulent HIV-2 shown to have protective effect against HIV/HIV-1
• experiments with pre-formed Ab shown to prevent infection (passive immunisation). generally done in primates

Question 15

What are BNABs? Describe them and how they are obtained

Answer 15

broadly neutralising Abs

• have specificity against the gp120 stalk and block infection
• can be used in many strains
• purified from long term non Progressors and expressed in recombinant form
• can be used as a type of passive immunisation

Question 16

Is HIV very infectious and describe its multiple transmission routes

Answer 16

• HIV not infectious. around 200 exposures necessary through sexual transmission to establish infection
  Transmission;
• needle stick, blood transfusion, sexual transmission

Infection normally with one or a v small no. virions (theoretically vaccination could prevent infection and or progression)

Question 17

Describe vaccines that aim to produce a B cell response to HIV infection.
What are the overall aims.

Answer 17

• aim to produce neutralising Abs that prevent infection (by blocking stalk and hence infection)
• want to use recombinant subunit vaccine - dont want to use attenuated HIV strains in case become pathogenic
  Overall aims;
• reduce viraemia - reducing blood-borne levels of virus. acts of exposure therefore reduced
• sterilising immunity -> eradication. vaccinated person will not have the disease nor will they catch it

Question 18

Summarise one early vaccine trial that failed

Answer 18

recombinant subunit vaccines created by expression in viral vectors
gp 120/41
patients could generate Abs against gp120/41
did not prevent HIV infection and transmission

Question 19

Name some features of HIV that hinder immune responses and therefore prevent efficent vaccine production

Answer 19

• genetic variability
• immunodominance and cryptic epitopes
• immunosuppressive factors

Question 20

Describe the genetic variability in HIV

Answer 20

• extremely variable with high levels of variability seen in countries/world. eg phylopgenetic trees show many clades and variants
• around 1 billion new virions produced daily. RT = error prone replication creating many variants.
• therefore high levels of variability seen within one person. co-infection with > 1 strain can exist in people

Question 21

Do we see resistance to anti retroviral drugs in humans?

Answer 21

YES, evolutionary selection occurs within individuals
see AZT resistance developing because HIV is so variable
NATURAL SELECTION

Question 22

Why is it that HIV can escape immune suilveillance?

Answer 22

because HIV is so variable, new populations constantly emerging. so Abs being produced to one variant will not be effective against a separate variant.
- infection with one HIV strain does NOT prevent infection with another HIV strain -> superinfection
-

Question 23

What sites do non-progressors target which allows them to prevent HIV progression?
Draw a diagram showing the natural structure of this site

Answer 23

trimer of membrane bound gp41 in association with gp120
this in complex with CD4 & CCR5.
non-progressors block these sites therefore blocking interaction

Question 24

Why can the binding site between CD4/CCR5 and gp120 not be targeted by Abs?

Answer 24

CRYPTIC EPITOPES;

• CD4 and CCR5 and not exposed to the immune system
• HIV binding to T cell causes slight exposure of these binding sights. transient exposure as a result of conformational change
• immunoglobulins are too large to bind these masked sites

IMMUNODOMINANCE;

• gp120 has no. solvent exposed peptide loops that are highly variable
• preferred targets for B cells responses (decoys!). immunodominant because target B cell responses

Question 25

Is sterilising immunity the only method of reducing rates of infection?

Answer 25

No, if we can live without passing on the virus it will eventually die out.
Reducing the viraemia (viral load) will reduce the rate of transmission.
Making a vaccine that is partially efficent will reduce the spread of the virus and elimination may well be possible.

Question 26

What is the relationship between the probability of infection and viraemia ?

Answer 26

higher the viraemia (higher conc of viral particles in blood) there are higher chances of infection/transmission

Question 27

What is the current focus of vaccines against HIV to date?

Answer 27

production of neutralising Abs (like in non-progressors)
ie making an immunogen that elicits the same Ab response in those that have been vaccinated .
OR cloning Abs of non - Progressors
passive immunisation approach to decrease viral load
reducing viraemia and halting viral progression

Question 28

Briefly describe one relatively successful vaccination trial

Answer 28

1) AIDSVAX
- recombinant purified gp120. developing neutralising Abs against this
- Phase III trials showed no reduction in preventing HIV infection
ALVAC
- expressed viral proteins in harmless canary pox virus.
- both failed in PII trials
- combination of these vaccines AIDSVAX/ALVAC showed a 30% reduction in infection rate in volunteers