HIV envelope- humoral responses Flashcards
What is the HIV spike composed of?
a trimer of 2xgp120 and gp41
What does viral attachment involve?
interaction of gp120 with CD4 and CCR5/CXCR4
What happens once gp120 binds to CD4?
exposure of coreceptor binding site and the formtion of a triple-strnaded coiled-coil with fusion peptide (gp41) exposure then coreceptor binding
What are the functions of anti-HIV-1 antibody in mucosal tissues?
target cell free infection; ADCC-mediated inhibition; targeting of cellular receptor/coreceptor
What is neutralisation defined as?
ability of natibodies to reduce virus infectivity by interfering at steps in viral life cycle
How do neutralising antibodies work?
prevent cell free infection and spread of infection via cell-to-cell transmission
What is the presence of NAbs correlated to?
most constant correlate of immune protection against viral infections
What is the function of Nabs in HIV vaccination?
essential for successful HIV vaccine but might not be sufficient against HIV
What is the function of the b12 antibody?
binds to gp120 blocking binding to CD4
What is the function of non-NAbs?
don’t bind to an epitope blocking binding but activate complement via Fc and bind to Fc receptors to work with innate effector cells
How can non-Nabs inhibit cell-to-cell transmission?
through their FCyR-mediated inhibitory activity
How may non-NAbs enhance infection?
trapping of antibody and complement opsonised HIV in follicular DCs acts as HIV reservoir
What makes HIV neutralisation difficult?
only one functional spike is needed for entry so complete AB occupancy of spikes is needed; some epitopes are momentarily exposed; size of Ig can sterically hinder Ab binding to some epitopes; conformational masking of epitopes; antigenic variability ; not many spikes
What is the different between the spikes recognised by Nab and non-Nab?
NAbs recognised functional spikes whereas non-Nabs recognse non-functional spikes
what is the function of non-functional env?
molecular decoy
What prevents antibody recognition of spikes?
glycan shield
What is found in the sera of elite controllers of HIV?
broadly neutralising antibodies
What do the serum levels of NAbs correlate with?
protection and lower viral load
What was seen in passive transfer studies of bNAbs ?
in macaques- give n prior to infection: most animals were protected but needed large doses, in those that did become infected, peak viraemia was lower. in humans- infusion can delay viral rebound during treatment holidays
What type of epitope is suitable for a bNAb?
broadly conserved, exposed epitopes
Why do bNAbs have long CDRH3 loops?
to access hidden epitopes
Why do bNAbs have high levels of somatic mutations?
mutating env proteins
What is the structure of gp120?
heavily glycosylated; composed of 5 constant regions interspersed with 5 variable regions
How much of the molecular weight of gp120 is the glycosylation?
40-50%
Where are hte conserved regions of gp120 found?
at the core
Where are the variable regions of gp120 found?
except V5, they form 4 loops which emanates from the surface of the protein
Where do the neutralising epitopres of gp120 cluster?
form a surface named the neutralising face which is exposed and conerved
Where do the non-neutralising epitopes of gp120 cluster?
non-neutralising face which forms the inner domain of the gp120 core and is realtively conserved
What do the silent face and variable loops correlate to in terms of antibody resposne?
a large fraction of the gp120 surface that is protected against antibody responses by a dense array of carbohydrates and by the capacity of variation
What are neutrailsing targets in gp120?
CD4bs; CD4-induced epitopes; 2G12 epitope (binds glycans on the silent face of gp120); V3 loop
Why are there no antibodies made to the extensive glycosylation?
recognised as self
How does the quarternary structure of env help immune evasion?
two-receptor entry mechanism; occlusion of vulenrable epitopes within trimer; surface variable loops cover conserved elemetns- continous mutation acts as immune decoy; extensive glycosylation occludes most ab epitopes
How much affinity maturation is shown by VRC01- a bNAb?
very high degree- v genes differ by 30%, mutated back to germline, there is little/no bidning of gp120
How does reverse vaccinology work?
isolate bNAbs then design antigen using molecular characterisation of antibody-pathogen-antigen interaction and then create vaccine
What is IC50?
concentration needed to stop 50% of infection
How does HIV evade complement mediated lysis?
incorporates completement regulatory proteins into viral membranes as it buds from the host cell, factor H binds directly to gp120/41
What is complement-mediated Ab-dependent enhancement?
infectivity of HIV opsonised with Ab and complement can be increased when complement receptors are expressed on target cells
Which complement receptor is particularly implicated in complement-mediated Ab-dependent enhancement?
CR2
How does complement-mediated Ab-dependent enhancement work?
increasing adhesion of virion to target cell allowing more efficient engagement with CD4
What is Ab-dependent cellular cytotoxicity?
a complex bewteen the IgG Fab portion of antibody bound to envelope protein on the cell surface and the Fc portion to Fc receptors on effetor cells leads to lysis of the infected cell
What is Ab-dependent cell-mediated viral inhibition?
measure the effects of ADCC-mediated cell killing, which leads to reduced virus production as well as virus inhibition by antiviral cyokines and other secondary effects of FcR-vius interactions eg phagocytosis
What IgA response is seen in plasma and mucosal fluids within weeks of transmission?
anti-gp41 but not anti-gp120
What happens to the concentrations of IgA anti-HIV response?
intiial mucosal response rapidly declines
How may IgA stop viral transfer across mucosal surfaces?
dimeric IgA can aggregate virus impeding viral transfer
What are the potential roles of IgA in HIV infection?
deleterious effect by competing with potential IgG protective Abs or significant protective effect by limiting HIV transmission at the mucosal site
What did the first partially efficacious vaccine trail-RV144 demonstrate about the role of IgA?
levels of vaccine-induced IgA in serum were associated with a lack of protection against HIV acquisition and that IgA competed with IgG for ADCC avidity
Why do the Ab responses to HIV-1 infection change over time?
B cell epitopes change through mutation; change in CD4 help- changes in CD4 epitopes or elimination of CD4 cells
What do non-neutralising antibodies typically bind to?
env epitopes that are not presented on the functional spike
Why may non-neutralising antibodes become more useful over time?
functional spikes are relatively unstable and over time the spike structure decays to reveal epitopes that can be recgonised by this class of Abs
What are gp41 stumps?
when gp120 is shed from the trimer, gp41 is left in the viral membrane as 6-helical bundles
What do antibodies that neutralise virus in a highly strain-specific manner bind to?
target regions on the virable loops or other regions of gp120 with relatively high sequence variation
What is the main mechanism by which HIV-1 continually avoids Ab control?
highly strain specific Abs emerge early in infection and tend to be immunodominant and HIV-1 mutates these variable epitopes leading to neutralisation escape
When do broadly neutralising antibodies typically arise?
take years to evolve
How do gp120 and gp41 associate?
non-covalently
Antibodies to what are detected first in the HIV infection?
to gp41
Why may the early Ab responses be ineffective?
directed to dissociated gp120 and gp41 proteins which do not react with native env trimers
What happens when galactose or sialic acid residues are added to IgG glycans?
promotes anti-inflammatory activity by enhancing the inetraction wit hthe inhibitory FcyRIIb
What demonstrated the importance of non-neutralising antibodies in controlling HIV?
RV144 vaccine study showed a 31% reduction in infection rate despite absence of NAbs
Aside from aggregating virions to prevetn transfer over mucosal surface, how may IgA work?
interact with mucosal secretions eg mucins to trap and clear virions away from the mucosal surface
What is thought to drive development in some individuals of cross-reactive neutralising abs?
ongoing process of viral escpae and Ab evolution
What is one hypothesis of serological neutralisation breadth?
accumulation of large number of strain-specific neutralising Abs results in a polyclonal response that can neutrailse many strains however, studies suggest the acivity is from a small number of neutralising Ab specificities
What were the three major env entigenic sites initially defined as targets of bNAbs?
membrane-proximal external region of gp41; CD4bs; glycan epitope on the outer domain of gp120
Why should bNAbs be given in combination?
to prevent viral escape from neutralisation
What features of HIV-specific CD4 cells are impaired with high VL?
proliferative capacity; surface phenotype; secretion of multiple cytokines
What suggests that the CD8 T cell response of elite controllers is repsonsible for restricting HIV-1 replication?
high percentage of elite controllers have HLA-B*5701; depletion og CD8 T cells reuslts in a 1-2x log increase in VL
What suggests that differences between elite controllers and progressors in CD8 cells is due to qualitative rather then quantitave factors?
both groups maintain a high-frequency response to HIV-1
What suggests that elite control is not due to a predominance of a particular CD8 phenotype?
although differences in markers are foudn between controller and progressors, once on ART, the markers become very similar to that of ECs, despite CD8 functionality not being restored
What is the evidence to suggest that there is greater antigen sensitivity in CD8 cells of controllers?
there have been some studies to suggest that B57 restricted responses have higher T cell avidity than other alleles; and other studies found higher functional avidity and broad variant reactivity more prevalent in ECs, however other studies have foudn no difference, and even when observed tended to be modest
What is polyfunctionality?
ability to simultaneously produce a greater number of cytokine and chemokes and degranulate
On what basis have the cells of controller been found more functional?
greater polyfunctionality; ability to suppress HIV-1 in vitro and viv; proliferate; produce perforin and kill infected targets
What have been the greatest differences found between the CD8 cells of controllers vs progressors?
ability to proliferate; produce perforin; kill infected targets after prolonged stimulation
What in macaque models confirms the role of Cd8 cells in controlling Hiv?
infection of macaques carrying protective MHC alleles with a highly pathogenic strain of SIV often results in nonprogressive infection and control of SIV is largely eliminated by passive transfer of anti-CD8 antibodies
What might be the role of CD8 responses in vaccination ?
complementary role to HIV-specific antibodies that involves eliminating cells infected by neutralisation resistant viral escape mutants
What is the viral set point established in HIV?
3-6 months after infection
What can slow diffusion of HIV-1 across the vaginal mucosa?
cervicovaginal mucosa
How may virus cross the mucosal epithelium?
by transcytosis or by making direct contact with dendrites of intraepithelial DCs
What is the eclipse phase?
after transmission of the virus before viral RNA becomes detectable in the plasma
How long is the eclipse phase?
around 10 days
What is the effect of APOBEC?
apolipoprotein B mRNA-editing enzyme, catalytic polypeptide-like cytidine deaminases which causes many viruses produced in infected CD4 cells to be defective
How are DCs inovlved in augmented the infection of CD4 cells by virus?
bind and internalise virus through CD-speicfic ICAM3-grabbing non-integrin and carry the virus to activated T cells
Why does the virus particualrly like GALT?
activated CD4 CCR5 memory T cells are present in high numbers
How many of the cd4 T cells in the GALT are infected in HIV?
20%
What happens to the uninfected CD4 cells in the GALT?
upto 60% of uninfected CD4 T cells become activated and die by apoptosis
What happens to CD4 counts in acute infection?
low at time of peak viraemia but later return to near normal levels in the blood but not in the GALT
What happens to the germinal centres in the gut and why?
upto 50% of the GCs are lost within the first 80days of infection as there is destruction of the cell types improtant for their development/maintenance
What happens after the peak viraemia as the viral load is decreasing?
virus diversification occurs and multiple escape mutants are selected under the pressure of hte adaptive immune repsonses that are first detectable just before peak viraemia
What suggests a role for immune activation in AIDS development?
chronic immune activation is not observed in naturally SIV-infected sooty mangabeys in which the infections rarely prgress to AIDs despite high elvels of viral replication and acute CD4 depletion;; positive correlation between markers of CD8 activation and HIV disease progression
What is the extensive immune activation of T and B cells assocaited with?
release of apoptotic microparticles into the blood and increased expression of TRAIL and FAS ligand –kill bystander cells and are immunosuppressive
What are some of the causes of hte immune activation seen with HIV infection
?
direct viral infection of immune cells; proinflammatory cytokine production; translocation of microbial products into the blood through damaged intestinal epithalium; loss of virally infected Tregs
What is the most dramatic genetic influence on HIV ontrol?
homozygosity for CCR5
What is thought to be the reason for some HLA-B types being associated with good control of the virus and a slower progression to AIDS?
partly because the epitopes recognised by the T cells are focused on conserved regions of the viral Gag protein
What is the first detectable innate immune response after HIV infection?
increase in acute phase proteins e.g serum amyloid A
Why is LPS detectable in the plasam during chronic infection with HIV-1 or SIV?
may be derived from commensal bacteria that translocate from the gut lumen following depletion of HIV infected Th17 cells
What does a further wave of acute phase protein production coincide with?
a cytokine response and increase in plasma viraemia
What can trigger the production of acute- phase proteins?
proinflam cytokines e.g IL-1 and also LPS
What suggests that a systemic cytokine response of the magnitude seen with HIV is not a prerequisite for viral clearance?
cytokine storm in HIV is much greater than seen with acute hep B and C
What are hte possible reasons for a decrease in circulating DCs seen in acute HIV?
may be activation-induced cell death or migration of activated DCsinto lymph nodes
What do pDCs produce when exposed to HIV?
IFNa which enhances adaptive responses but also IDO which induces Treg differentaiton and may suppress immune responses
What happens to HIV exposed conventional DCs?
do not become fully activated and dhow defective IL-12 production
What suggests taht NK cells are important in HIV infection?
HIV has evolved a strategy to reduce expression of ligands for NK ell receptors by infected cells and the role of KIR3D molecules in determining viral set point
Give an example of innate immune repsonses being harmful in HIV infection?
induction of mucosal inflamamtory repsonses by some microbicides has lead to increased acquisition of HIV-1 infection
When do the first T cell responses to HIV infection arise?
as viraemia approaches its peak
Which cells are involved in the selection of initial escape mutants?
CD8 cells
What are the earliest T cell responses often speciigc for?
env and nef
what type of T cell response would result in a lower level of viraemia at the set point?
immunodominant responses to more highly conserved epitopres and which escpe occurs at a cost to the fitness of the virus
What is the level of set point viraemia influenced by?
the nature of hte transmitted virus and the specificity of the early CD8 T cell responses
Why is there a rapid decline of CD8 T cells after a reduction in some viraemia?
after the founder epitope is eliminated due to escape mutations
What happens to CD8 T cells as HIV infection progresses?
devleop abnormally and become dysfunctional
what is the first free natibody detected after HIV infection?
gp41
What indicates that the early arising antibodies are ineffective against HIV?
they do not select escpe mutations
When are the first antibodies to induce escpe mutants developed?
around 12 or more weeks after transmission
Give a reason why CD8 T ell responses are hard to vaccinate for?
CD8 T cells are highly sensitive to singl aa variation in epitope peptides so even minor mismathces between vaccine-encoded epitoptes and the virus could be a serious problem
What is good bout the transmission of HIV being mediated by only one virion?
indiactes the vulnerability of hte virus to immune attack during the eclipse phase
