L7 - acute viral respiratory infections Flashcards
Respiratory tract infections
infection of nasal cavities, pharynx or airways
usually viruses over bacteria
Upper RTIs
affect nasal cavities and pharynx
Lower RTIs
affect airways
upper RTI examples
nasopharyngitis, pharyngitis, sinusitis, laryngitis and influenza
Lower RTI examples
influenza, bronchitis, bronchiolitis, pneumonia and tuberculosis
RTI diagnosis
hemagglutination, blood typing, antibody structure and production, nucleic acid hybridisation, immunoassays, PCR, virus microarrays, qRT-PCR, HTS, VirCapSeq
Respiratory Syncytial Virus
major cause of lower RTIs and hospital visits for severe bronchitis during infancy and childhood
?% of infants infected during their first RSV season
60%
Respiratory syncytial virus in children
nearly all children will have been infected with the virus by 2-3 years old
induces protective immunity which wanes over time therefore people can be infected multiple times
RSV virology
enveloped negative sense ssRNA virus of the Paramyxoviridae family
G-protein
attachment protein, attaches to Annexin II on airway epithelial cells or L-selectin and CXCR1 on immune cells
TLR4
detects fusion protein
TLR3
detects dsRNA replication
TLR7
detects ssRNA
TLR2/6
unknown what it detects
RIG-I
detects ssRNA viral genomes bearing 5’-triphosphate, dsRNA
Nod2
detects ssRNA
NLRSP/ASC
detects small hydrophobic RSV viroporin
RSV prevents an effective host immune response…
- Non-structural bind RIG-I to prevent signalling
- Non-structural proteins bind to IRF3/7 which prevents target binding to the nucleus to produce type I IFNs
- Non-structural proteins bind to STAT1 and cause it to degrade, preventing target binding so ISGs are not produced
G protein binds…
dendritic and T cells to prevent their activation
RSV disease features
cytopathic effect of virus and local inflammatory response
airway epithelial cells highly pervasive to RSV
RSV replication causes…
epithelial damage and therefore necrosis
RSV symptoms
recruitment of neutrophils and lymphocytes
submucosal oedema, mucus secretion, bronchoconstiction
RSV results in…
severe obstruction of airway lumen
Formalin-inactivated vaccine candidate
no protection with higher viral load and 16 fold increase in hospitalisations
2 infant fatalities
Anti-RSV F protein MAB
Palizumab
monthly injections successful in high risk infants
Liv RSV mechanism
dsRNA leads to RSV replication, enters endosome TLR7 to form RSV ssRNA
RIG-I and MDA-5 involved
results in TF activation with antibody affinity maturation
Formalin-inactivated RSV mechanism
No RSV replication, limited TF activation os no antibody affinity maturation
no RIG-I and MDA-5 involvement
RSV in wheezing/asthma conception
severe RSV infection in infancy is a firmly established risk factor for subsequent asthma in later childhood
Underlying biological mechanisms in RSV and asthma
Chronic epithelial and airway reactivity changes to developing infant lung
lung injury altering lung function
immunomodulatory changes
genetic factors that impact patterns of immune response to infectious agents
Infant immune modulation
severe RSV bronchiolitis alters subsequent Th1/2 immune response
associated with Th2 polarisation of the lung immune resposnse
enhances Th2 sensitisation to aeroallergens and induces development of a chronic asthma phenotypes
TLR4 mutations in infants hospitalised for RSV bronchiolitis
ASP299Gly and Thr399Ile
TLR4 mutation mechanism
failure to translocate TLR4 to cell surface resulting in reduced NFKB signalling
RSV
more inducer than trigger
RSV in infants and young children
extensive damage to airways leading to long-lasting bronchial hyperreactivity
HRV
more trigger than inducer
HRV in infants and young children
atopy as facilitator and recurrent infections, leading to pro-inflammatory cell activation and limited damage
Human Rhinovirus
most common cause of upper RTIs
HRV causes…
exacerbations of chronic pulmonary diseases, asthma development, severe bronchiolitis in infants and children, fatal pneumonia in elderly and immunocompromised
HRVs cause a…
self-limiting syndrome with predominantly upper RTI manifestations in healthy adults
airway epithelial cells
respiratory viruses enter and replicate within them, HRV infect nasal epithelial cells but also detected in lower airways
cytopathology of AECs
AECs sloughed but airway lining remains structurally intact, disruption to barrier function by dissociation of zone occludens 1 from tight junction complex
Macrophages and HRV
predominant lymphocyte in airways, release chemicals to result phagocytosis and modulating immune response
HRV prevent macrophages by…
inhibiting antigen presentation, T cell activation and B cell antibody production
Neutrophils in HRV
recruited in response to IL-8/CXCL8
levels in nasal lavage fluid correlate with symptom severity
Neutrophils release….
TNFa and produce elastase
Eosinophils
recruited in response to RANTES/CCL5 with unclear role
T cell responses
recruited in response to RANTES/CCL5 and IP-10/CXCL10
T cells present in nasal lavage and airway epithelium
trigger cytotoxic and AB-mediated immune response - Th1 cytokines
B cell responses
mucosal IgA, then IgM and finally IgG
B cep reformed neutralising IgG
prevents/limits extent of reinfection, elevated serum titers of serotype-specific IgG neutralising antibody correlate with attenuated cold symptoms and reduced viral shedding
rhinovirus infection in those with allergic rhinitis
induces rapid increase in serum IgE without evidence of elevation in antigen-specific IgE
Bradykinin
in nasal lavage
vascular permeability
neutrophil inflex
Histamine
unchanged, as no role for mast cells or basophils
viral-induced acute exacerbations
further increase in airway inflammation
major cause of morbidity, mortality and accelerates disease progression
HRV triggers…
50-85% cause of exacerbations in patients with underlying airway disease
experimental HRV infection studies
viral loads higher in asthma and COPD patients than in healthy controls
interferons
block viral entry, control viral transcription and translation, cleavage of RNA and induction of apoptosis
failure to induce robust IFN response in HRV-induced exacerbations
uncontrolled viral replication and increased inflammatory responses –> possible mechanism
Frequent exacerbators
more susceptible to viral infection and/or more severe clinical disease after infection
Asthma and exacerbations
deficient IFN production, decreased antiviral Th1 cytokines, increased SOCS1 may link Th2 inflammation with IFN deficiency, increased ICAM-1 expression
COPD and exacerbations
deficient IFN production and increased ICAM-1 expression
Acute exacerbation mechanism
inadequate IFN response –> increased viral replication –> cell necrosis, release of virus and pro inflammatory mediators –> airway inflammation –> acute exacerbation
Allergic sensitisation in HRV infection severity
Il-4 and IL-13 production
increased ICAM-1 expression by BEC
Increased BEC infection by HRV
increased HRV infection severity
Th2 bias in HRV infection severity
deficient HRV-induced IFNS and IL-12 production
impaired infected BEC apoptosis and killing
increased virus replication
increased HRV infection severity