Magor- Topic 8: Viral escape from neutralizing antibodies

Question 1

Monoclonal antibody

Answer 1

• made in lab
• made from fusing a B cell & myeloma cell & then it produces that same antibody forever
• recognizes 1 epitope
• has one specificity

Question 2

Polyclonal antisera

Answer 2

• when you see a virus, you generate polyclonal antisera
• Recognizes many epitopes
• make Abs to different parts of that virus
• has many specificities
• antibodies circulating in your blood are ‘polyclonal’

Question 3

Neutralization assay

Answer 3

• an in vitro test in which antibodies bind to virus and prevent the infectious cycle
• you compare your antisera to the amount of virus that you would get if there was no antisera there
• this is done with a monoclonal antibody or polyclonal antisera for a vaccinated person, or someone who has survived an infection
• the difference in titre tells you whether neutralization has happened

Question 4

Virus neutralization

Answer 4

• free virus particles are susceptible to neutralizing antibodies
• virus particles that have a capsid are easily neutralized
• most of the time, what those Abs are actually doing is blocking the interaction of the virus with its host receptor for attachment
• they can also have other effects -basically changing the structure of that virus so that it doesn’t actually attach properly or preventing the capsid from opening

Question 5

Possible mechanisms of neutralizing antibodies (3)

Answer 5

1. Steric interference with receptor
2. Fix capsid so pH dependent uncoating is blocked
3. Structural changes so that uncoating is blocked

Question 6

__________ viruses survive despite active immunity

Answer 6

Persistent

persistent viruses have virus present all the time

Question 7

How does HIV survive in the face of an active immune system?

Answer 7

Many antibodies are useless, and it can mutate away from good neutralizing antibodies so they stop working

Question 8

Patients who are infected with HIV are making antibody responses all the time. They are making tons of antibodies and most of those antibodies are useless or become useless as that virus ________

Answer 8

evolves

Question 9

Western blot

Answer 9

• detect anti-HIV antibodies in patient serum
• presence of antibodies against HIV is used as diagnostic HIV test
• done to confirm that a patient is HIV positive

Question 10

If HIV positive antibodies make all these antibodies, are any of them effective?

Answer 10

Yes, sometimes.

Case study: 31 yo male with HIV, but no symptoms for 6 years.
B cells expressing antibody were isolated from this patient. Monoclonal antibody B12 from this patient is a broadly neutralizing antibody (neutralizes many strains of HIV)

Isolated B12 antibody prevented infection in macaques

Good neutralizing antibodies do exist & presumably because this patient went for a really ling time w/o symptoms of AIDS, the virus maybe can’t change that spot very easily & so that’s kind of the holy grail-looking for Abs against the region that the virus really can’t change.

Question 11

B12 antibody

Answer 11

• a “neutralizing” antibody that has a projection that allows it to bind snugly to HIV’s gp120 protein
• prevents gp120 from docking to receptors CD4 and CCR5

Question 12

Why do most antibodies fail?

Answer 12

The structure of HIV-1 envelope glycoprotein contributes to evasion from the immune system

Question 13

Variable regions (V) cover the __________________

Answer 13

conserved regions

Question 14

Can you make antibodies against carbohydrates?

Answer 14

Yes, but they’re not particularly good because they don’t have T cell help so they’re not great antibodies generally.

Question 15

What is the only protein that is unique on the HIV viral membrane?

Answer 15

glycoprotein

Question 16

The two receptor mechanism of entry is a ___________

Answer 16

evasion strategy

Important epitopes of the gp120 are completely hidden until fusion. They are only exposed for a moment during the fusion process.
The chemokine receptor was probably the original receptor, and the interaction with CD4 evolved later.

Question 17

Molecular mimicry

Answer 17

• another evasion strategy
• a loop structure on HIV’s gp120 protein has the same shape as part of the chemokine RANTES
• the loop that binds to the chemokine receptor looks like the chemokine that binds to the receptor.
• RANTES is the chemokine receptor that binds to CCR5 chemokine receptor
• V3 region interacts with the CCR5 chemokine receptor
• V3 loop looks identical to the loop of the chemokine that would normally bind this receptor.

Question 18

Quasi-species of HIV recovered

Answer 18

• many different viruses are circulating even within a single patient.
• HIV evolution within a single patient.
• There’s no WT virus for HIV

Question 19

LAV

Answer 19

original sequence is probably closest to this

Question 20

Case Study: Virus neutralization and escape in patient WEAU
(slide 17)

Answer 20

• He makes really good Abs to this virus. That controlled that virus for some time but then the virus mutates away from that pressure & escapes/

Question 21

Changes in ______ glycosylation provide an escape mechanism

Answer 21

gp120

• carbohydrates attaches at NXT or NXS
• Carbohydrates are also shielding a lot of the epitopes that would be good neutralizing epitopes of that virus

Question 22

Antibody responses to HIV infection

Answer 22

• some neutralizing Ab block the fusion process. Block the entry process, prevent that virus form entering
• If you make some Abs that are low affinity but still bind to the virus, some of those Abs may actually help the virus get in through receptor mediated endocytosis using Fc receptors.
• Fc receptors bind to the bottom of the Y of the antibody & then internalize whatever they bound to. HIV can escape from that

Question 23

Antibodies facilitate HIV entry by Fc receptor mediated uptake

Answer 23

• Fc receptor binds to the Ab and internalizes the pathogen
• Most viruses can’t get out of that, can’t escape but HIV infect macrophages. Macrophages have everything they need to make a replication cycle that virus can potentially enter using this pathway.

Question 24

Case studies: Chronically infected patients and understanding HIV escape
(slide 24)

Answer 24

• Basically what you’re looking for is parts of virus that the virus can’t change and yet they’re immunogenic so we can make an Ab response to them. And those are kind of the pieces that will go forward in making the next generation vaccine.

Question 25

Why is it so difficult to make an HIV vaccine?

Answer 25

• Vaccines normally mimic recovered patients but hardly anyone has recovered from HIV.
• The immune system of almost everyone is “blind” to HIV. Effective antibodies to the virus are rare.
• Vaccines protect against disease, not infection, and HIV has a long latent period before disease-AIDS-sets in.
• Most vaccines are killed or weakened viruses: killed HIV is not effective at producing immune response, and any live form of the virus is too dangerous to use.
• Vaccines are usually effective against diseases that are rarely encountered (diphtheria, hepatitis B). People in high-risk groups might be exposed to HIV daily
• A majority of vaccines protect from exposure through the respiratory or gastrointestinal system. HIV enters most often through genital surfaces or blood sharing.
• Whereas most vaccines are tested thoroughly on animal models, there are no really good animal models for HIV/AIDs available.

Question 26

How do vaccines work?

Answer 26

Vaccines work by mimicking disease agents and stimulating the immune system to build up defenses against the,.