lecture 22: infections in asthma and COPD

Question 1

What are basic aspects of viral structure?

Answer 1

• all viruses exist as a nucleocapsid:
  
  • genome (RNA or DNA, ss or ds) + protein coat
• many have a host-derived lipid envelope
• nucleocapsid (+ envelope) = virion
• viruses must use host cell structures and enzymes to replicate, BUT
• there are a few unique viral enzymes, which may be antiviral drug targets
• e.g. influenza virus virions:
  
  • ~200nm diameter
  • note spikes of haemagglutinin and neuraminidase surface proteins
• respiratory syncytial virus virions
  
  • the viral genome is complexed with the viral structural proteins L, N and P

Question 2

What are general features of virus replication?

Answer 2

• virus may be absorbed to host cell
• virus binds to surface receptor
• virus enters cell and is uncoated
• viral proteins translated and cleaved
• viral genome is replicated
  
  • viral genome may integrate into host
• new virus particles assembled
• new virus particles released

Question 3

What is an introduction to respiratory viruses?

Answer 3

• diverse viruses infect the respiratory tract
  
  • rhinoviruses (RV; common cold, many strains)
  • influenza (animal hosts, antigenically diverse)
  • coronaviruses (SARS; 2003 pandemic)
• no broadly useful vaccines or antivirals available
• at least some pathology is due to host response
  
  • neutrophils release proteases and reactive oxygen species
  • cytotoxic T cells lyse infected host cells

Question 4

How do respiratory viruses cause harm?

Answer 4

• airway macrophages ingest viruses
• many viruses abortively replicate in macrophages
  
  • release of pro-inflammatory cytokines
  • infection curtailed, but some local tissue damage
• influenze and some others are cytolytic
  
  • spread to new cells and hosts
• non-lytic viruses induce cellular and cytokine response
• mucus and dead cells and inflammatory cells clog airways
• damaged airway epithelium becomes more permeable
• bacterial secondary infection is common

Question 5

What is replication of RSV in human bronchial epithelial cells?

Answer 5

• pictures showing replication of RSV in human bronchial epithelial cells
• top panel: mock, cells were not infected with virus, but otherwise treated the same, age, confluent, still adherent to plastic surface, shape normal, not clumping
• bottom panel: 23hr picture of RSV, some cells starting to cluster, adjacent infected cells fuse, cells dying, clumping, detaching from surface at 48 hours, by 3 days monolayer is almost completely gone
• RSV is very common among children

Question 6

What is the development of asthma?

Answer 6

• all that coughs and wheezes is not asthma
• many children outgrow asthma
• asthma has:
  
  • many triggers (allergy, stress, cold, smoke, infection)
  • many genes implicated
  • many presentations (allergic, non allergic, post viral wheeze, steroid resistant, + others)
  • reasonable treatments but no cure
• some develop asthma later in life (occupational exposures, stress)

Question 7

What is asthma prevalence and mortality?

Answer 7

• over 2 million australians have asthma:
  
  • 1 in 8 children, 1 in 10 adults
• 964 deaths in 1989, 378 in 2011 (stable)
• 37,830 hospitalizations in 2010-11

Question 8

What is atopy?

Answer 8

• allergy to innocuous substances

Question 9

What is normal flora?

Answer 9

• microbes, mostly bacteria, present in certain body sites and are normally harmless

Question 10

What are commensals?

Answer 10

• microbes that are normally present and cause no harm, may acquire nutrients from the host

Question 11

What is specific pathogen free/germ free?

Answer 11

• free of disease causing/all microbes

Question 12

What is a URTI?

Answer 12

• upper respiratory tract infection (often, uncomplicated)

Question 13

What is a LRTI?

Answer 13

• lower respiratory tract infection

Question 14

What is eosinophilia?

Answer 14

• a key feature of allergic asthma

Question 15

What is IgE?

Answer 15

• antibody that mediates allergic reactions

Question 16

What is the appropriate Th1 immune response to aeroallergens in the normal airway?

Answer 16

• example of how the immune system should respond to aeroallergens
• CD4 T cells - activated TH1 lymphocyte
• when it encounters an aeroallergen like house dustmite or pet hair → produces TH1 cytokines e.g. IFNgamma, IL-2, TNF-alpha → generally considered beneficial
• activates macrophages
• stimulates production of IgG
• means for airway that things aren’t too bad
• epithelium: normal thickness, no swelling, not a whole lot of mucous
• submucousal glands: not particularly busy

Question 17

What is the Th2 immune deviation to aeroallergens in asthma?

Answer 17

• TH2 responses are not all bad, times when you want them, but in asthma bad
• activated TH2 lymphocyte produces a range of TH2 cytokines → IL-4, IL-5, → recruit and activate a lot of cells such as eosinophils (MBP, ECP, leukotrienes, cytokines) , Mast cells (histamine, leukotrienes, cytokines), Plasma cell (IgE)
• these cells have a normal job and function (e.g. helminth infections)
• → airway oedema, goblet cell hyperplasia, subepithelial fibrosis, smooth muscle hyperplasia and/or hypertrophy
• airway becomes remodelled
• airways can become chronically inflamed
• smooth muscle thickening means less responsive to bronchodilators

Question 18

What is virus infection and TLR activation?

Answer 18

• pathogen-associated molecular patterns (PAMPs) are conserved microbial patterns
• PAMPs are recognised by pattern recognition receptors (PRRs) on phagocytic cells
• Toll-like receptors (TLRs) are the first responders to microbial infection
• TLR ligands may be:
  
  • extracellular (LPS on gram neg. bacteria - TLR4) or,
  • intracellular (viral dsRNA - TLR3)
  • activation of phagocytes, release of inflammatory mediators

Question 19

What is the hygiene hypothesis?

Answer 19

• a lack of early childhood exposure to infectious agents increases susceptibility to allergic diseases, as the immune system does not develop properly
• the risk of some autoimmune diseases is also increased
• 17,414 children born in march 1958, followed to 23
• incidence of hay fever recorded at age 11 and 23, and eczema in first year of life
• both disorders less common in larger families and with more older siblings
  
  • adjusted for several confounders incl. breastfeeding
• does catching infections from siblings reduce atopy?
  
  • Strachan, D.P. (1989( Br Med
  • 3075 (sept 2014; 369 in last year!)

Question 20

What are mechanisms of the hygiene hypothesis?

Answer 20

• Th2 immune responses predominate in babies, which promotes antibody production
• cytokine response to early infections leads to overall Th1 response
• Th1 response → efficient pathogen clearance
• Th2 response → more mast cells, eosinophils, IgE
• the infant’s immune system needs stimuli to develop regulatory T cells, which mediate tolerance and dampen immune responses

Question 21

What is more recent work on the HH?

Answer 21

• benefit of early infection depends on the type and timing of infection
• more antibiotics now given in early life
  
  • affects colonisation of bowel by “good” bacteria
  • exposure to non-pathogenic microbes may give the greatest protection
    
    • favours Th1 over Th2 responses
    • stimulates regulatory T cell development

Question 22

What is evidence for the HH?

Answer 22

• allergic and autoimmune disorders less common in developing countries
• migration from developing to developed world → greater incidence of these conditions
• early life exposure of mice to infections → a reduced incidence of autoimmune disorders
• children raised on farms are less likely to have allergies and asthma
• pet ownership linked to decreased atopy and asthma

Question 23

What is evidence against the HH?

Answer 23

• association does not prove causation
• microbial diversity (host microbiome) may be more important than the number of infections
• asthma rates are decreased in some developed nations while food allergy continues to rise
• many factors linked to increased asthma and allergy:
  
  • delayed introduction to solids
  • air pollution
  • household damp and airborne moulds family history
  • family history

Question 24

What is the relationship between early respiratory infections and asthma?

Answer 24

• most asthmatics are atopic, but only some atopic children progress to asthma
• viral infections particularly relevant, as
  
  • weak specific immunity (RSV) and large numbre of strains (RVs, flu) → re-infections common
  • viral infections are not treatable with antibioitcs
  • early virus infection may affect lung development
• long term birth cohort studies: wheeze is common, but LRTI + aeroallergen sensitization + family Hx of asthma = → greatest risk of persistent asthma

Question 25

What are likely mechanisms related to respiratory infections and their relationship with asthma onset?

Answer 25

• viral infection alters normal maturation and turnover of dendritic cells → altered tolerance
• more severe LRTIs (e.g. RSV) more strongly linked to asthma than uncomplicated URTIs
• Th2 cytokines stimulate IgE production
• damaged tissue → inflammatory cell recruitment
  
  • early respiratory infection may be a sign of predisposition to asthma, and not a cause

1. symptoms are initially intermittent; airway oedema and constriction
   
   • infection worsens inflammation
   • antiviral and allergic cytokines amplify inflammation
2. persistent inflammation → repeated cycles of tissue repair and remodelling
3. lung structures may be irreversibly damaged

Question 26

What are some of the many risk factors for asthma?

Answer 26

• inactivity/too much time indoors
  
  • too little vitamin D → impaired immunity obesity → dyspnoea and inflammation
• C-section delivery → slight increase in asthma (early colonization by mother’s flora)
• cigarette smoke exposure irritates airways and reduces protective immune responses
• some risk factors overlap with the HH

Question 27

What are important public health messages?

Answer 27

• “dirt is good”… to some extent!
• not all micro-organisms are harmful, BUT
• little evidence for benefits of probiotics, and
• it’s important to avoid serious infections
  
  • ear and throat infections, pneumonia
• vaccination significantly decreases occurence of infections that can kill or have permanent, serious consequences
  
  • whooping cough, measles, mumps, varicella
• asthma is fairly treatable - for most patients

Question 28

What are asthma exacerbations?

Answer 28

• cause illness, hospitalisation and death
• initial treatment: inhaled beta2-agonists and steroids
• moderate cases: antibiotics, nebulized beta2-agonists and anti-cholinergic bronchodilators, plus oral steroids
• severe cases: i.v. bronchodilators and steroids, possibly adrenaline
• most severe cases: intubation and mechanical ventilation
• hard to detect viruses in AE by cell culture
• PCR indicated that ~80% of AE are caused by viruses, mostly rhinovirus (RV)
• 76 cohabitating couples (1 with atopic asthma)
  
  • daily respiratory diary, 2x daily peak flow
  • nasal aspirates taken fortnightly for RV culture
  • asthmatics don’t more colds, but longer lasting and worse colds

Question 29

What are symptoms of asthma exacerbations?

Answer 29

• reduced peak flow, dyspnoea
• increased mucus and constricted airways
• persistent coughing affects eating and sleeping
  
  • reflects…
• excessive influx of inflammatory cells
• dysregulated leukocyte activation and resolution
• ? persistent viral replication/presence in LRT
• concurrent or sequential bacterial infection

Question 30

What does viral infection induce?

Answer 30

• dsRNA is present either in viral genome or as a replicative intermediate → induces IFNs
• IFN-alpha/beta produced early by infected epithelial cells and some leukocutes
• IFN-gamma produced later by lymphocytes
• IFN binding to receptor → Janus-family tyrosine kinase → phosphorylation of STAT proteins

Question 31

What are the diverse antiviral effects of IFN?

Answer 31

• direct interference with viral replication
  
  • degrades viral RNA
  • blocks initiation of translation
• indirect antiviral effect
  
  • activation of NK cells and macrophages
  • increases antigen presentation and MHC class I
• IFNs used clinically to treat hepatitis
  
  • IFNs cause fever and other flu-like symptoms

Question 32

Why do asthmatics get worse and longer colds?

Answer 32

• primary bronchial epithelial cells grown from those +/- asthma, infected with rhinovirus (RV)
• both types have similar basal ICAM-1 levels
• asthmatic cells have higher RV titres and delayed lysis, due to impaired apoptosis
• asthmatic cells produced less IFN-beta
• adding IFN-beta to infected asthmatic cells induced apoptosis and reduced viral replication

Question 33

What research has been done on IFN-beta to treat asthma exacerbations?

Answer 33

• 147 steroid-using asthmatics (18-65 years) with a history of viral exacerbations
• within 24 hr of cold onset, randomized to 14 d of inhaled IFN-beta, or placebo
• IFN-beta did not reduce asthma symptoms, but treated subjects had:
  
  • better morning peak flow
  • less need for more intense treatment
  • increased innate immunity markers in blood and sputum

Question 34

summary

Answer 34

• the hygiene hypothesis is well supported but controversial
• types and timing of infection and microbial diversity important
• asthma is a variable disease with multiple, overlapping causes
• respiratory virus infections are involved in onset and exacerbation of asthma
• mechanisms are complex but involve:
  
  • altered signalling by infected cells
  • altered recruitment and activation of immune cells
• dirt can be good… sometimes

Question 35

What are prospects for asthma treatment, and prevention?

Answer 35

• Th2/IgE blockade is of limited benefit in established asthma
• can early anti-Th2 treatment stop asthma in high risk children?
• promotion of IFN responses in early infection
• a vaccine for RSC - one day?
• deliberate infection with helminths (helminth infection inversely correlates with allergies)

Question 36

What are the signalling events once TLRs are activated?

Answer 36

• MyD88: myeloid differentiation primary response protein →
• IRAKs: IL-1 receptor associated kinases +
• TRAF6: tumour necrosis factor receptor associated factor
• NFkB is uncoupled from its inhibitors, translocates to the nucleus and initiates transcription of proinflammatory cytokines