Lecture 23: HIV (2) Flashcards
Define primary infection in the context of HIV
Refers to the initial phase of the infection that occurs shortly after a person is exposed to the virus. It is the acute phase of CD4 depletion
What is acute HIV syndrome?
Wide dissemination of virus and seeding of lymphoid organs
What is clinical latency of HIV?
Chronic phase of CD4 depletion - Steady state: virus production is equal to virus loss
When do constitutional symptoms of HIV occur?
At about 8 years
What happens in the gut of a HIV+ patient?
Mucosal depletion of CD4+ T cells in the Peyer’s patches instead filled with HIV infected cells
Increased microbial translocation, due to decreased defences
These bacteria activation TLRs, and thus trigger the immune system
What is the clinical hallmark of HIV infection?
CD4+ T cell depletion
Describe the changes in CD4+ T cell number during the different stages of HIV infection
Primary infection: acute depletion
Seroconversion: increases somewhat
Clinical latency: gradual loss
AIDS: less than 200 cells per mm3
What illnesses are seen at the various stages of HIV infection?
Primary infection
* Features of normal viral infection (fever, myalgia)
Clinical latency
* Autoimmune disorders
NB these are not disorders that signal to GPs that the patient may have HIV infection
* Tuberculosis
AIDS:
* Disorders that are only really seen in people who are highly immunosuppressed (PCP, Kaposi sarcoma, non-Hodgkins Lymphoma)
Describe CD4+ T cell homeostasis
Production in bone marrow
Development in thymus
Proliferation of naïve cells
Differentiation into effector cells and memory cells
How does HIV infection lead to decline in CD4+ T cells?
- Increased destruction
a. Direct infection kills the cell
Incomplete reverse transcription in naive T cells
b. Indirect effects
“SAIL”
* Syncitium formation
* Apoptosis
* Immune activation
* Lymph node fibrosis - Impaired production
* in the thymus
* CD34+ progenitor cell suppression / loss
Describe Syncytium formation
Virally infected cell expresses viral proteins and glycoproteins on the cell surface
Many uninfected CD4+ T cells start to gather around the infected cell and fuse membranes
This is called syncytia.
These huge fusion of cells dies, and a single virion has killed many CD4+ T cells by infecting only one cell.
Why is CD4+ T cell depletion variable?
Viral factors
* CXCR4 virus → accelerated T cell loss - quicker than CCR5 because this is only expressed on ACTIVATED T cells
* Nef deleted virus
Host factors
* Genetic: CCR5 del32 heterozygote has a slower disease progression
* Age: thymic function dependent on age
* Immune response: HLA type
*Sex = women have a greater CD4 decline for the same viral load
Which HLA types indicate good HIV prognosis?
- B13, B27, B51, B57
Which HLA types indicate rapid disease progression?
- A23, B37, B49
What is the effect of Nef deleted virus?
Limits T cell loss
Describe the effect of HLA on HIV infection
Certain HLA alleles are associated with good progression, whilst others are associated with poor progression
What happens to CD8+ T cells in HIV infection?
Great number stimulated during acute phase
Later on, decline, because they are exhausted
What happens to NK cells in HIV infection?
Impaired numbers
Describe HIV-induced immunopathology
Depletion / dysfunction of immune cells
* Chronic immune activation
What happens to B-cells during HIV infection?
Increased production of IgG and IgA BUT decreased antibody responses
What is the CMV-specific response?
Expansion of the memory T cells for CMV
This is seen in response to HIV infection
Part of aberrant immune activation in HIV infection
HIV causes C____ I____ A___
Chronic Immune Activation
How does HIV cause chronic immune activation (despite the knocking out of CD4+ T cells)?
Mucosal depletion of CD4 T-cells
* Increased microbial translocation
* Activation of toll-like receptor 4 by bacterial products (LPS)
Activation of innate immune response (pDCs)
* HIV RNA is a TLR7/8 ligand
* Increased plasma IFN-alpha
Cytomegalovirus (CMV)-specific response
* Expansion of CMV-specific activated CD4+ and CD8+ T cells
Loss of T regulatory cells
* Hyperactivation of immune response
What is the difference between elite and viremic controllers?
Elite controllers: defined as viral load <50 copies/ml in the ABSENCE of ART
Viremic controllers: defined as viral load <400 or <2000 copies/ml in the absence of ART
What classes of antiviral drugs are used for HIV? Name some drug examples
- Nucleoside/nucleotide reverse transcriptase inhibitors (NRTIs): These drugs inhibit the reverse transcriptase enzyme, which is necessary for HIV replication. Examples of NRTIs include zidovudine, lamivudine, and tenofovir.
- Non-nucleoside reverse transcriptase inhibitors (NNRTIs): These drugs also inhibit reverse transcriptase, but by binding directly to the enzyme rather than being incorporated into the viral DNA chain. Examples of NNRTIs include efavirenz and nevirapine.
- Protease inhibitors (PIs): These drugs inhibit the HIV protease enzyme, which is necessary for viral maturation. Examples of PIs include lopinavir, ritonavir, and darunavir.
- Integrase inhibitors (INSTIs): These drugs inhibit the HIV integrase enzyme, which is necessary for viral integration into the host genome. Examples of INSTIs include raltegravir and dolutegravir.
- Fusion/Entry inhibitors: These drugs block the entry of HIV into host cells. There are two types of entry inhibitors: fusion inhibitors and CCR5 antagonists. Fusion inhibitors, such as enfuvirtide, block the fusion of the viral and host cell membranes. CCR5 antagonists, such as maraviroc, block the interaction of the viral envelope protein with the CCR5 co-receptor on host cells, preventing viral entry.
What is the relationship between viral load and anti-retroviral therapy?
- Viral load becomes undetectable on ART
- Viral load rapidly rebounds if you stop ART, because the virus persists in the latent reservoir
What was the breakthrough idea concerning antiviral HIV drugs?
The breakthrough idea concerning antiviral HIV drugs was the use of combination therapy, also known as Highly Active Antiretroviral Therapy (HAART). The combination of drugs targets multiple steps in the viral life cycle, making it more difficult for the virus to develop resistance to all of the drugs at once.
Describe HAART
What are some of the classes of anti-retroviral drug?
Combination of several (3) different classes of retroviral drugs
Classes:
* Cellular chemokine receptor antagonists
* Fusion inhibitors
* RT inhibitors
* Integrase inhibitors
* Protease inhibitors
It is very important that several classes are used in combination.
This ensures that the virus does not rapidly evolve resistance to the treatment
What are the challenges of antiretroviral therapies?
- Life expectancy is almost normal but is still reduced if the treatment is started too late
- Lifelong adherence: new advances in long-acting injected ART
- Toxicities of medications: hyperlipidaemia, renal toxicity, bone disease
- Drug resistance: very rare if adherent
- Immune activation: multiple drivers and consequences
What is the START study for HIV?
The Strategic Timing of AntiRetroviral Treatment (START) study was a clinical trial designed to evaluate the optimal time to start antiretroviral therapy (ART) in HIV-positive individuals with high CD4+ T-cell counts.
What are the goals of ART and defining virological failure ?
- > 1000 copies/ml virus sequencing is possible
- > 200 copies/ml is the definition of virological failure
- <200 copies/ml means no sexual transmission
- <20 copies/ml = treatment success
What are the ageing risks associated with HIV?
- Stroke
- Malignancy (non-AIDS)
- Renal disease
- Liver disease
- Fracture/osteoporosis
- Dementia
- Type 2 DM
- COPD Frailty
T-cell activation on ART is still_____
Persistent HOWEVER it never normalises/stabilises
What mechanisms drive Immune activation on ART?
Microbial translocation, persistent HIV, co-infections: CMV, HCV, Activation of pDC and altered Treg/Th17
What mechanisms drive disease on ART?
Lymph node fibrosis, T-cell exhaustion and local inflammation
WHICH LEADS TO
Impaired CD4 T-cell recovery –> end-organ disease e.g., CVD, liver, dementia
Explain the concept of the Combination Prevention Toolbox in treating HIV
- Blood supply screening
- Condoms
- Education/behaviour modification
- Clean syringes
- Treatment/Prevention of Drug/Alcohol Abuse
- STI Treatment
What are the biomedical approaches added to the combination prevention toolbox?
- ARVs for PMTCT, PEP, PrEP
- Treatment as Prevention
- HIV Testing/Counselling
- Microbicides
- Medical male circumcision
List the prevention modalities built on antiviral therapy
- Prevention of mother-to-child transmission
- Pre-exposure prophylaxis
- Post-exposure prophylaxis
- Treatment as prevention
Explain Treatment as Prevention (TASP) for HIV
Refers to the use of antiretroviral therapy (ART) for HIV-positive individuals to reduce the risk of transmission of the virus to HIV-negative individuals. The idea is that by suppressing the virus in the bloodstream of HIV-positive individuals, the risk of transmission to others can be reduced or eliminated. TASP is based on the fact that HIV is most infectious when it is present in high levels in the blood (known as the viral load). When HIV-positive individuals are treated with ART, their viral load can be suppressed to very low levels, which greatly reduces the risk of transmission.
Explain Pre-exposure prophylaxis (PREP)
- PREP is highly efficacious in MSM
o Taken daily
o Taken intermittently
o Reduces acquisition by 95% - PREP activity is lower in women
- Main concerns with implementation:
o Adherence
o Effects on condom use
o STIs effect
How does a Dapivarine Vaginal Ring work?
The dapivirine vaginal ring is a type of microbicide that is designed to prevent the transmission of HIV during vaginal sex. It works by releasing a small amount of the antiretroviral drug dapivirine, which inhibits the replication of the virus, into the vaginal tissue over a period of 28 days.
Explain circumcision for HIV prevention
Circumcision reduces the risk of HIV acquisition by 70%:
The foreskin of the penis contains a high concentration of Langerhans cells, which are target cells for HIV. Circumcision involves surgical removal of the foreskin, reducing the surface area available for HIV to enter the body.
What 2 components are needed for an effective vaccine response to HIV?
Neutralizing antibodies: These are antibodies that can bind to and inactivate the virus by preventing it from infecting healthy cells. Neutralizing antibodies are critical for preventing the initial infection of HIV and reducing the viral load during established infections. The development of neutralizing antibodies against HIV is challenging due to the high variability of the virus.
Cell-mediated immunity: This refers to the activation of T-cells, which are specialized immune cells that can recognize and destroy virus-infected cells. Cell-mediated immunity is important for controlling viral replication during established infections and for providing long-term protection against the virus. An effective HIV vaccine should elicit a strong and sustained T-cell response against the virus.
What are the 4 different HIV Vaccine approaches:
- Recombinant proteins
o Good antibody but poor T-cell responses
o No protection - Live vectors:
o Good T-cells responses but poor antibody responses, includes adenoviruses (Ad, chimp or human) modified vaccinia Ankara (MVA), pox viruses - Nucleic acid vaccines
o DNA: Good T-cell responses but poor antibody responses
o mRNA: broad T cell responses - Live attenuated virus
o Potentially unsafe (need to make sure that all of the HIV is inactivated) and so has not been pursued further
What are the prime boost HIV vaccine approaches?
- DNA + protein e.g., DNA + protein or vector
- DNA + live vector
o Non-replicating poxviruses expressing HIV-1 genes good T cell responses Ad5 virus
DNA vaccine + Ad5 boost
Poor approach due to a possible increased risk of infection - Live vector + protein e.g., Ad26 vector + gp140 protein (mosaic immunogen)
o Phase 2b Imbokodo trial and phase 3 Mosaico showed no benefit
What are the most promising HIV vaccine approaches?
- Broadly neutralising antibodies
o Passive administration (phase 3)
o Active vaccination much more challenging (phase 1) - CMV vectors
o Persistent antigen presentation - mRNA vaccines
o Unclear if it will be a major advance, but immunogen can be changed easily
o Phase 1 human studies underway with 1) BG505 MD39.3 mRNA 2)BG505 MD39.3 gp151 mRNA and 3) BG505 MD39.3 gp151 CD4KO mRNA
Outline the escape mechanisms of HIV
HIV is highly mutable
* Due to error prone reverse transcriptase
* Immune system can not keep up with the escape variants
* Env can withstand much antigenic variation
Unstable association between gp120 and gp41
* gp120 release to expose irrelevant Ag
N-glycan carbohydrate shielding
* Immune system not good at making high affinity Ab against carbohydrates
* Antigenic sites of gp120 hidden
Hidden CoR binging site on gp120
Describe the unstable association between gp120 and gp41 and the implication
gp120 head can dissociate
2. Exposes irrelevant epitopes which can dominate the immune response
Why is HIV highly mutable?
What does this mean for our immune system?
Error prone reverse transcriptase
The immune system may make antibodies against an antigen of the HIV, only for mutation in this antigen to render the antibodies useless.
Describe the unstable association between gp120 and gp41 and the implication
gp120 head can dissociate
2. Exposes irrelevant epitopes which can dominate the immune response
Describe the function of the hidden CoR binding site on gp120
This binding site is only revealed once CD4 binds to gp120
The site is not available for neutralising antibodies
What features of the HIV Env protein that render it difficult for the immune system to control?
Env can withstand many mutations and still be functional, thus ESCAPING from the immune system
What are broadly neutralising Abs?
Broadly neutralizing antibodies (bNAbs) are a class of antibodies that are capable of neutralizing multiple strains of a virus, including HIV. HIV is a virus that rapidly mutates, and as a result, the virus has a high degree of genetic diversity. This makes it difficult for the immune system to produce antibodies that can effectively neutralize all the different strains of the virus
Describe the various methods of enrichment of bnAbs from elite controllers
Single-cell sorting: Isolation of individual B cells from the blood of elite controllers and the screening of these cells for the production of bnAbs.
Memory B cell sorting
Phage display: Phage display is a technique that allows for the screening of large libraries of antibodies for specific binding properties. Elite controller-derived B cells or antibody fragments are cloned into bacteriophages, which are then screened for the production of bnAbs.
Serum screening: Elite controller serum samples can be screened for the presence of bnAbs.
High-throughput sequencing: used to identify the genetic material of bnAbs in elite controllers.
Future directions and challenges for broadly neutralising antibodies as vaccines
- Monovalent antibodies in combination, bi- or tri-specific bNAbs to enhance efficacy and reduce the chance of resistance
- Modification of the Fc tail of a bNAb to increase half life or activate Fc receptors
- The biggest challenge is to generate bNAbs in vivo
What is the schematic of lineage-based vaccine design?
- Immunogen for priming
- Immunogen for 1st boost
- Immunogen for 2nd boost
- Immunogen for 3rd boost
- bNAb
Describe the VRC01 story
Participants were randomised to receive infusion of bNAb (VRC01) every 8 weeks at either 10 or 30 mg per kg (low dose or high dose) or placebo for 10 infusions in total
The VRC01 antibody targets a specific region on the surface of the HIV virus, known as the CD4 binding site, which is essential for the virus to infect human cells. This makes it a particularly potent and broadly neutralizing antibody (bnAb)
Scientists used the genetic information of VRC01 to create synthetic versions of the antibody, which could potentially be used as a therapy for HIV-infected individuals or as a prophylactic vaccine to prevent HIV infection
How did these germline Abs ever become specific for HIV antigen, if they don’t react at all?
The process by which germline antibodies eventually become specific for HIV antigens is called affinity maturation. Germline antibodies are typically low-affinity and are not capable of binding strongly to specific antigens like those found on the surface of the HIV virus.
However, when the body is exposed to HIV, certain B cells that produce antibodies against the virus are selected and undergo a process of mutation and selection in the germinal centers of lymph nodes, spleen, and other lymphoid tissues. This process is called somatic hypermutation and it introduces random mutations in the genes that encode the antibody variable regions
What are CMV vectors?
- Induced unusually broad HIV-specific T-cells
- Peptides presented by HLA-E (instead of HLA-A and B)
- Alternative antigen presentation triggered by CMV itself
Does Antiretroviral Therapy only prevent HIV infecting activated CD4+ T lymphocytes? Is that why there can still be residual replication in resting CD4+ T cells?
ART can only prevent NEW rounds of infection. Residual replication is likely either coming from the release of virus from cells that were infected prior to initiation of ART (ie latently infected cells); OR perhaps replication is occurring in tissues where there is poor drug penetrance.
With regard to the reverse transcriptase inhibitors, how do the nucleoside RT inhibitors compare to the non-nucleoside ones? Are they equally effective or do they need to be used in combination?
Nucleoside RT inhibitors are analogues of naturally occurring deoxynucleotides, they compete with the natural deoxynucleotides for incorporation into the growing viral DNA chain. They typically lack a hydroxyl group on the 3’ end which means the next deoxynucleotide can not bond to it leading to early termination of the DNA chain. Non-nucleoside RT inhibitors typically bind allosterically to the RT enzyme and prevent its function. All antiretroviral drugs need to be used in combination, due to the rapid emergence of resistant strains when you only use one drug.
