Flashcards in HTLV-1 Deck (156):
How is HTLV-1 transmitted?
breastfeeding; sexual contact; blood transfusion
What type of virus is HTLV-1?
complex human retrovirus
What is the result of HTLV-1 infection for >90%?
Waht are the clinical manifestations of HTLV-1?
adult T-cell leukaemia/lymphoma ; chronic inflammatory disease of the nervous system
What are the symptoms of HTLV-1- associated myelopathy?
spasticity/weakness of the legs; hyperreflexia; bladder dysfunction; lumbar pain; constipation; impotence
What is the median survival of patients with adult t-cell leukaemia?
What is the closest correlate of risk of disease with HTLV-1?
What are the 2 routes of replication?
infectious route via RT; mitotic route via host cell polymerase
How does HTLV-1 get passed around in the infectious route?
not via free cell particles, but directional cell-cell spread via vriological synapse triggered by cell contact
What is the main route by which HTLV-1 persists in the host?
mitotic route ( when cell divides, provirus is replicated)
What suggests that mitotic route is main persistance route for HTLV-1?
get oligoclonal bands, not polyclonal bands detected on southern blot---small number of HTLV-1 clones as mitotic route doesn't use RT
What suggests that HTLV-1 is latent in vivo?
virions; viral mRNA and proteins are usually undetectable in fresh PBMCs
What is the effect of CTLs on HTLV-1?
reduce the proviral load and risk of -HAM-- granzymes and perforin are higher in patients with low proviral load; anti-tax CTL avidity correlates with low proviral laod and expression
What are CTLs mainly directed against in HTLV-1?
How can HTLV-1 escape CTLs?
through spontaneous Tax mutations
What mutations are associated with protection against HTLV-1 in s. Japan? Why?
HLA-A2 and -Cw08--present peptides to CTLs ( further evidence of importance of this cell type)
What is the difference between the CTL responses generated by Tax and HBZ?
Tax- strong, persistently activated CTL response vs HBZ which is a weak CTL immunogen
what is the equivalent of tax in HIV?
What is the function of Tax protein and HBZ?
drive proliferation of hte infected cell
What aspect of hte CTL response determines the host ability to control the proviral load efficiently?
response to HBZ (all hosts respond to Tax- reponse has no correlation with proviral laod)
What is the importance of HLA genotype in protection against HTLV-1?
possession of strong HBZ-binding alleles prevents aroudn 50% of potential cases of HAM in Japan; by looking at the strength of HBZ binding alone you can correctly classify 55% of cases of HAM
What cells does HTLV-1 infect?
CD4 t cells
What determines a T cell clones abundance and pathogenicity?
genomic integration site- as it regulates switch between proviral latency and expression
What defines a clone?
the unique genomic integration site of hte HTLV-1 provirus
How does clonality correspond to proviral load?
proviral load is correlated with total number of clones but not with oligoclonality
How many proviral copies do HTLV-1 infected cells carry?
How many HTLV-1 T cell clones are there in an asymptomatic carrier?
how many HTLV-1 T cell clones are in a patient with HAM?
what is the pathogenic significance of oligoclonal proliferation?
How does HTLV-1 integrate?
targets specific transcription factor binding sites
What is the difference between expression of HBZ and tax?
HBZ is persistently expressed- at low levels but tax is frequently silent - some clones express lots of tax but not all
On what strand is HBZ?
What strand is tax found on?
Why is HBZ more important than tax in terms of immune repsonse?
anti-tax cells will only target some infected cells but antiHBZ targets all infected cells
What is foudn at the 3' end of hte HTLV provirus?
What creates the chromatin loops?
CTCF protein which bind at 2 loci, dimerize and then form a loop
Waht is the function of chromatin loops?
regulate promoter-enhancer contacts
What is the effect of HTLV-1 on CTCF?
CTCF can bind HTLV-1 provirus which can cause dimerisation in the wrong place forming abonromal loops
What can abnormal chromatin looping cause?
increases risk of insertional oncogenesis- proviral CTCF site makes long-range interactions with host chromatin brinding viral enhancer near host genes
What genes are over-expressed in individuals with HAM?
interferon-stimulated genes; p53 signalling
How is HBZ implicated in leukaemia?
persistent clonal proliferation--replicative mutations
What are the 2 mechanisms of insertional mutagenesis with HTLV-1?
local- flanking provirus; distant- via chromatin looping
How is Tax implicated in leukaemia?
acceleration into mitosis; inhibits DNA repair and promotes genome instability
What does HBZ stand for?
HTLV-1 basic zipper factor
What genus and subfamily does HTLV-1 belong to?
Which host genes does Tax transactivate?
IL-2 receptor a chain (CD25); IFNy and ICAM1
What mediate the cell-to-cell contact which allows HTLV-1 spread?
ICAM1 on infected cell to LFA-1 on target cell
What is the specialised structure which transmits budding of the virus towards the cell contact area?
What happens when HTLV-1 is expressed in a cell?
the cell becomes activated- large and irregularly shaped with upregaulted ICAM1
When does the HTLV-1 pre-integration complex gain access to the host genome?
when the nuclear envelope dissassembles during mitosis
What are the 2 stages in pathology in HAM?
first- inflammatory phase and then neurodegerative phase
What is seen in the initial inflammatory phase of HAM?
perivascular lymphocytic infiltration
which coutnires have the highest numbers of HTLV-1 infected individuals?
Japan and Brazil
Where are the inflammatory infiltrates of mononuclear cells found most frequently?
upper thoracic spinal cord
What is the makeup of the mononuclear infiltrate in HAM?
CD4 predominate in early active lesions with the proportion of CD8 cells increasing progressively as the lesion progresses
How is HTLV-1 proviral load measured?
number of HTLV-1 DNA copies per peripheral blood mononuclear cell
What happens to the proviral load after the acute phase of infection?
reaches a stable value- set point
What determines the set point proviral load?
quality of that individuals T cell response to the virus
Why is proviral load not the only factor in HAM?
50% of asymptomatic carriers have proviral loads of >1% which is when the exponential increase in prevalence of HAM occurs
What is the most common TLV-1 genotype worldwide?
What does the HLA class I genotype of an individual determine?
the specificity and efficacy of their CD8 T cell repsonse to a virus
When is hte peak incidence of HAM in age?
fifth decade of life
Which gender is more susceptible to HAM?
femailes (4:1 in US)
why is IFNy thought to play a pathogenetic role in HAM?
HTLV-1 Tax protein induces a Th1-like state and IFNy expression in einfected CD4 T cells
What is the effect of type I IFNs a and b on HTLV1?
impair HTLV-1 replication
What is the abundance of HTLV-1 specific CTLs proportional to?
HTLV-1 proviral load
What suggests that HTLV-1 is not latent in vivo but is expressed persistently or at frequent intervals?
CTLs are chronically activated
what suggests that CD4 cells are important in HAM?
HTLV-1 infected CD4 cells secrete IFNy and predominate early in HAM lesions; frequency of IFNy secreting CD4 cells is higher in HAM pts than in asymptomatic carriers with a similar proviral load
What is a possible mechanism for tissue damage in HAM?
IFNy secreted by infected CD4 cells causes astrocytes to secrete CXCL10 which recruits more cells, reuslting a high local conc. of IFNy and neural damage
what other cell type is implicated in HAM?
What causes the high frequency of FOXP3 cells in HAM?
HTLV-1 Tax protein
How does Tax increase FOXP3 cells?
induces Cd4 t cells to produce chemokine CCL22 which prolongs durvival of Tregs by binding to CCR4 which is characteristically expressed on Tregs
Why does HTLV-1 increase Tregs?
Tregs impair the efficiency of the CTL response to HTLV-1
What is the effect of antibodies against HTLV-1?
reduce the efficiency of HTLV-1 transmission by breastfeeding; reduce propagation in mice models
Waht is thought to be the mechanism behind HAM developmnet?
neurons and glial cells are damaged by toxic or inflammatory products released from the HTLV-1 infected T cells and the repsonding CTLs prsent
What are flower cells?
HTLV-1 infected lymphocytes with a characteristic mulilobular nuclei
What were the only clinical correlates of the diagnosis of HAM in a Japanese study?
brisk patellar tendon relfelxes and presence of flow cells
What is hte median time from onset to use of a unilateral walking aid?
What are correlates of a fasster HAM disease progression?
older age at onset (>50 years); high proviral load
What is required for diagnosis of HAM?
detection of HTLV-1 antibodies
how should a psotivie HTLV-1 ELISA be confirmed ?
thourhg western blot analysis or detection of viral nucleic acid
Why is there a high concentration of beta2 microglobulin in patietns with HAM?
reflects high freq of Cd8 cells (beta2 microglobulin is bound MHC-I to the cell
What drugs form the mainstay of HAM treatment?
Why might there be a lack of goof results with pharmacological studies of HAM treatment?
tx is started in the late phase of hte disease when infalammation has subsided-- clinical trials should take into account the pahse of the illness
What receptor, not HLA enhances HLA class I mediated protection in HTLV1?
why are biomarkers for HAM important to develop?
can lead to more accurate diagnosis and prognostication; as well as allowing clinical trials to assess treatment effects faster as HAM is very slow progressing
What chemokine receptor is expressed on a majority of HTLV infected T cells?
What are teh cellular receptors for HTLV-1?
neuropilin-1 and GLUT1
What are the 4 factors which govern HTLV-1 integration?
integration is mainly in open, transcriptionally-active chromatin; places where certain proteins eg STAT1 are bound; protien phosphatase 2A binds the integration complex; targetd to a primary DNA sequencce motif
Why does increasing the number of infected cells in the host augment transmission of HTLV1?
transmits through cell-to-cell contact, therefore transmission needs living infected cells
Why is breast-milk and semen infectious with HTLV1?
HTLV1 increases the migratory capacity of infected T ells into the breast milk and semen for transmission
What are the 2 broad viral strategies for HTLV1?
host cell prolioferation and cellular motility
What 2 regulatory genes does HTLV1 have?
tax and rex
What is the only gene found on the minus strand of the provirus?
HBZ - transcribed frmo the 3' LTR
Which type of transcription is Tax able to heavily influence?
from the 5' LTR (sense transcripts)
What protein is critical for the constant expression of HBZ?
What other protein enhances the trnascription of HBZ by cooperating with SP1?
What suggests that CTLs suppress Tax expression?
when CD8 cells are removed in vitro Tax expression increased
Why does mitotic replication generate much less sequence diversity than infectious replication?
mitotic route uses DNA Pol2 rather than RT
What is the relationship between clonality and proviral load?
it is the clonal diversity not that degree of oligoclonal proliferation that correlates with proviral load
Which cell type are infected with 95% of the HTLV1 proviral load?
CD4 T cells
What type of cell does HTLV2 mainly infect?
What is the equivalent of HBZ in HTLV2?
Waht is the equivalent of Tax in HTLV2
What is the function of SAMHD1?
limits the supply of nucleoside triphosphates for virion replication, inhibits HTLV1 infection of macropahges
What is the function of APOBEC3G in HTLV1 infection?
generates nonsense mutations but targets the plus strand genes so spares HBZ, this sustained expression of HBZ may favour viral persistence
How does HTLV1 reduce type I IFN production?
induces the cytokine suppressor SOCS1 and Tax-mediated inhibition of RIG1 and MDA5
which IFN does Tax induce?
What demonstrates that HTLV1 is persistenyl expressed in vivo?
HTLV1 infected pts have high freq of persistently activated HTLV1 specific CTLs- indicates recent exposure to newly synthesised viral antigen
What enhances the protective and pathogenic effects of HLA class I alleles in HTLV1?
what type of cell is much higher in patients with HAM than in asymptomatic carriers?
HTLV-1 specific CD4 cels
Which cytokine does Tax induce CD4 cells to secrete?
How does Tax maintain a high freq of CD4 FOXP3 cells?
induces secretion of CCL22, CCR4 which is the receptor for CCL22 is found on FOXP3 cells
How does HTLV1 mainly increase the proviral load?
driving proliferation of infected cells- mitotic division
What indicates the de novo infection does not contribute significantly to the proviral load in the chonic phase?
RT inhibitors and integrase inhibitors don't change the proviral load in HTLV-1 infected individuals
How does HBZ inhibit apoptosis of hte infected cells?
perturbs the location and function of FoxO3a which is a transcriptional activatro for genes encoding Bim and Fas ligand; interacts with the Rb/E2F-1 complex; its RNA which stays in the nucleus inhibits apoptosis
How does HBZ change the immunophenotype of infected cells?
induces cell surface expression of TIGIT and CCR4
How does Tax inhibit apoptosis?
activates NFkB which leads to expression of anti-apoptotic gene c-FLIP
How does Tax promote vell proliferation?
What protein is thought to play a role in the differential expression of plus and minus strands ?
What is the normal function of CTCF?
form chromatin loops- organise the chromatin structure and to bring enhancers near their cognate promoters
Why will each infected T cell clone have its own characteristics of proviral expression?
the genomic integration site influences the expression of the provirus - whichi s unqie to each clone
What happens when a host promoter is in the same trancriptional sense upstream of hte 5'LTR?
transcriptional silencing of hte HTLV1 plus strand
What happens when a host promoter is upstream of the 3'LTR (same sense as the proviral negativ strand)?
activation of plus strand expression
The immune system puts a constant pressure on the virus selecting for clones withan optimal pattern of proviral expression, what is this expression?
minimal but persistent HBZ expression, and bursts of plus-strand expression that may be driven by cellular stress
What is the function of the plus strand being expressed during cellular stress?
cellular stress eg when lymphocytes are transferred between individuals, so expression would promote viral transmission
Why was Tax thought to be necessary and suggicient for malignant transformation of HTLV1 infected cells?
transduction of Tax-expressing vectors can immortalise T cells in vitro; transgenic expression of Tax induced cancers in vivo- different cancers based on the promoter causing Tax expression
What suggests that Tax is not as important in oncogenesis as previously thought?
half of ATL cells dont express Tax whereas all ATL cells express HBZ
What are the 3 mechanisms to inactivate Tax expression?
nonsense mutations of hte tax gene; DNA methylation of hte 5'LTR; deletion of hte 5'LTR
Why is transcription of the sense strand finally silenced during the natural course of infection?
DNA methylation of hte 5'LTR naturally accumulates
What role may CTCF play in preventing methylation of the 3'LTR?
CTCF binding region in pX region which may account for the arrest of DNA methylation before pX and 3'LTR
What suggests taht multiple genetic and epigenetic alterations are needed for ATL?
long latent period is necessary before onset of ATL
Where are the genetic alterations foudn in ATL cells generally foudn?
in the genes assocaited with pathways that Tax and HBZ infect-- clones persistent and proliferate by actions of Tax and HBZ during carriage thereafter genetic and epigenitc alterations fix or potentiate these changes
Give an example of a mutation found in 20% of ATL cells in a HBZ targeted pathway?
gain of function mutation in CCR4 (HBZ induces expression)-increased migration and proliferation of infected cells
What genetic mutation is involved in the Tax pathway?
expression of miR31 enhances NFkB in ATL cells even in the absenec of Tax
What are the 4 subtypes of ATL?
acute; lymphoma-type; chronic and smouldering
How does HBZ induce CCR4 expression?
enhanced GATA3 trancription
What receptor promotes viral entry but isn't bound by the env proteins?
heparan sulphate proteoglycans
What is the function of Rex?
regulation of nuclear RNA export
How did most patients with ATL get the virus?
How does the genotype of HTLV-1 differ between pts with ATL or HAM and asympatomatic carriers?
How does the NK response to HTLV1 differ between pts with HAM and asymptomatic carriers?
frequency of and lytic activity of NK cells was significantly lower in patietns with HAM
What is thought to be the mechanism behind using AZT and IFNa to treat chronic ATL?
antiproliferative rather than antiviral
What mainly determines whether a host will have a high or low quality CTL response?
host genotypes at the HLA-I and KIR loci and therefore the ability to present epitopes from the HBZ antigen to CTLS
what proteins suppress Tax activity?
Wnt transcription factors TCF1 and LEF1
What can stop the suppression of Tax activity by TCF1 and LEF1?
T cell activation
What genes are overexpressed in models with trangenic expression of Tax?
NFkB pathways genes
How does Tax affect NFkB?
directly binds to IKKy and p100, alters the ubiquitination of TRAF6 etc (regulatory proteins)
How does Tax induce genetic instability?
thourhg the generation of reactive oxygen species
How does HbZ minimise the effect of repeated mitosis on cellular aging ?
enhances expression of the host genes telomerase reverse transcriptase and JunD
What infection increases the number of TLV1 clones in a host?
Is there an integration spot associated with ATL?
no, but there is evidence that integration close to host genes that frequently dysregulated in otehr leukaemias occurs more frequently than by chance
What determines the rate of spontaneous Tax express ion in a given clone?
influenced by the genomic integration site of hte provirus- distance and transcriptional orientation of hte provirus relatige to the nearest host gene
Which clones express Tax more frequently?
What does CTCF stand for?