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The wheezy infant - immunological and molecular considerations
PAEDIATRIC RESPIRATORY REVIEWS (2004) 5(Suppl A), S81–S87
The wheezy infant – immunological and molecular considerations Michela Silvestri, Federica Sabatini, Anna-Carla Defilippi, Giovanni A. Rossi Pulmonary Division, G. Gaslini Institute, 16148 Genoa, Italy
KEYWORDS wheezing; infancy; respiratory syncytial virus; epithelial cells; airway inflammation
Summary Most of the data on the pathogenesis of asthma is based on information obtained through bronchial biopsies and bronchoalveolar lavage in adults and young adults. Ethical considerations linked to the invasive nature of airway endoscopy have limited the studies on the pathophysiology of asthma in infancy and early childhood. Although there is evidence that an asthma-like inflammation, with increased inflammatory cells and thickening of the lung basement membrane, may be present also at a very early age, clinical and epidemiologic studies suggest that asthma manifestations in preschool children may significantly differ from those observed in older subjects. In western countries, the vast majority of infants and young children has episodic (or intermittent) asthma, and the exacerbations generally defined “wheezing episodes” occur more frequently with a seasonal pattern being usually related to acute viral infections. There is strong epidemiological evidence that approximately 2/3 of all children who wheeze because of viral infections in early life (and are not atopic) have a transient condition that tends to disappear during early school years. All respiratory viruses may be implicated in the wheezing episodes, the principal being respiratory syncytial virus (RSV) and, with a lower frequency, adenovirus and parainfluenza viruses during the first 3 years of life, and rhinoviruses after that age. Infants and preschool children have on average 6–8 “colds” per year, but the illness tends to be limited to the upper respiratory tract alone in a considerable proportion of individuals, without causing symptomatic involvement of the lower respiratory tract. The variety of factors determining the different outcomes are only partially known, but complex interactions between the intrinsic pathogenicity of the virus and host factors, including the socio-economic conditions of the family, are central to define the type of manifestations and the severity of the process. © 2004 Elsevier Science Ltd.
ABBREVIATIONS CMI
Cell mediated immunity
* Correspondence to: Giovanni A. Rossi. Tel.: +39-(010)563-6547/8; Fax: +39-(010)-383953; E-mail: [email protected] 1526-0542/$ – see front matter
ICAM-1 sICAM-1 LT PAF RSV TNF-a
Intercellular adhesion molecule-1 Soluble ICAM-1 Leukotriene Platelet activating factor Respiratory syncytial virus Tumor necrosis factor-alpha © 2004 Elsevier Science Ltd. All rights reserved.
S82 URTIs Upper respiratory tract infections VCAM-1 Vascular adhesion molecule-1
INTRODUCTION Asthma is defined as a chronic inflammatory disease of the airways characterised by the local production of mediators that in susceptible individuals may induce long-term pulmonary changes, including bronchial hyperresponsiveness, airway remodelling, and irreversible airflow obstruction.1,2 Most of the data available on the pathogenesis of this disease is based on information obtained through bronchial biopsies and bronchoalveolar lavage in adults and young adults.2 Ethical considerations, created by the invasive nature of endoschoscopic evaluation of the airways, have limited the studies on the pathophysiology of asthma in infancy and early childhood.3 Although there is evidence that an asthmalike inflammation may be present also at a very early age, with increased inflammatory cells and thickening of the lung basement membrane, clinical and epidemiologic studies suggest, that asthma manifestations in preschool children may significantly differ from those observed in older subjects.3,4 In western countries, the vast majority of infants and young children tend to have episodic (or intermittent) asthma, and the exacerbations generally defined “wheezing episodes”,5 occur more frequently with a seasonal pattern and are usually in relation to acute viral infections.6,7,8 There is strong epidemiological evidence that approximately 2/3 of all children who wheeze because of viral infections in early life have a transient condition that disappears during early school years.5 All respiratory viruses may be implicated in the wheezing episodes but the principal ones involved are respiratory syncytial virus (RSV) and, with a lower frequency, adenovirus and parainfluenza viruses during the first 3 years of life, and rhinoviruses after that age.7,9 Infants and preschool children catch on average 6–8 “colds” per year, but in a considerable proportion of individuals the illness tends to be limited to the upper respiratory tract alone, without causing symptomatic involvement of the lower respiratory tract.6 Although the many factors determining the different outcomes are only partially known, complex interactions between intrinsic pathogenicity of the virus and host factors, i.e., anatomic/functional characteristics of the airways, defence mechanisms, socio-economic conditions, are central to define the manifestations and the severity of the process.
M. SILVESTRI ET AL.
INTERACTION BETWEEN VIRUSES AND CELLS IN THE AIRWAYS Almost all respiratory viruses can induce bronchial hyperreactivity in normal subjects or exacerbate this condition in asthmatics for as long as 6 to 7 weeks.10,11 Viruses may be cytotoxic to epithelial cells and disruption of the airway epithelial surface may lead to increased permeability to antigens and allergens, to exposure of bronchial cholinergic sensory nerve fibers to physical or chemical irritants and to loss of epithelial derived relaxant factors, including nitric oxide12,13 (Figure 1). Viral damage to airway epithelium may also induce bronchoconstriction by altering the metabolism of bronchomotor mediators, such as substance P.14 Independently of its activity as bronchoconstrictor, substance P is also a potent proinflammatory mediator, able to increase vascular permeability and to attract and activate leukocytes.14 RSV infection, at least in laboratory animals is indeed causing an increased inflammatory response to substance P, with extravasation of serum and proteins in the airways.15 The specific virus-induced cytotoxic effect to airway epithelium may be considerably enhanced by toxic oxygen metabolites released by the host inflammatory cells, such as neutrophils, eosinophils and, to a lesser extent, alveolar macrophages and interstitial monocytes.1 A considerable contribution to airway narrowing comes from the bronchospastic and inflammatory mediators locally released by inflammatory and parenchymal cells during viral infections4 (Figure 2). Performing bronchoalveolar lavage in infants, undergoing routine surgery, raised eosinophils and eosinophil products have been found in wheezers, compared with non-wheezers.16 Eosinophils may release large numbers of mediators including proinflammatory cytokines, cytotoxic proteases and bronchospastic and proinflammatory substances, such as leukotriene (LT)-C4 and platelet activating factor (PAF).1 Significant higher concentrations of eosinophilic cationic protein was detected in nasopharingeal secretions of infants with RSVinduced wheezing, as compared with those with upper respiratory tract infection alone.7,17 Moreover, acute eosinophilic infiltrates of the bronchial mucosa may be present also after experimental rhinovirus infection.17 The concept of rhinovirus augmenting eosinophil responses is supported by the recent in vitro observation that monocyte-driven T-cell activation after incubation with rhinovirus was productive of factors that enhance eosinophil survival.18 In addition to airway eosinophilia, mast cell hyperplasia and bronchial hyperresponsiveness have been shown in experimental animals with viral
THE WHEEZY INFANT – IMMUNOLOGICAL AND MOLECULAR CONSIDERATIONS
S83
Exposure of bronchial cholinergic sensory nerve fibers to irritants. Increased permeability to antigens and allergens.
RSV
Loss of epithelial-derived relaxant factors (NO). Disruption of epithelial cells
Recruitment and activation of inflammatory cells.
Fig. 1. Injury to airway structures induced by respiratory syncytial virus (RSV).
LTC4, LTD4 and Histamine release Mast cells
Alveolar macrophages
Arachidonic acid metabolism enhancement TNF-α, IL-6, IL-8 release
RSV
TNF-α and IL-10 secretion
RSV RSV
Peripheral blood monocytes
INFECTION
ICAM-1 expression Epithelial cells
Alveolar type II epithelial cell line
ICAM-1, VCAM-1, MHC class I and II antigens expression
Fig. 2. Activation of inflammatory and parenchimal cells leads to synthesis and secretion of proinflammatory mediators and surface molecules expression.
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M. SILVESTRI ET AL.
IFN-γ Cellular antiviral Immunity
NK cell RSV
IFN-γ IL-2 RSV
CD8 + T-cell
A.P.C. IL-12
RSV
APC
IL-10
Th1 CD4 + T-cell Th2
IL-4 IL-5 IL-10 IL-13
Umoral Immunity
Fig. 3. Viral infection stimulates the cellular and umoral immune response.
respiratory infections.4 Mast cells are thought to potentially play a key role in these processes, since they are able to release products, such as histamine, LTC4 and LTD4, found in elevated concentrations in the respiratory secretion of infants with viral wheezing.4 Enhanced arachidonic acid metabolism was also described in alveolar macrophages from wheezy infants, possibly related to a direct effect of viral replication or to cytokines present in the lower respiratory tract.19 Experimental infection of alveolar macrophages by RSV leads to increased secretion of proinflammatory cytokines that include tumor necrosis factor-alpha (TNF-a), interleukin (IL)-6 and IL-8.20 More recently, Barr et al. reported that RSV also significantly enhanced TNF-a and IL-10 production by peripheral blood monocytes.21 IL-6, together with IL-4, IL-5 and IL-10, is classically considered a Th2 cytokine, mediators involved in the pathogenesis of asthma and participating in IgE production and in eosinophil and mast cell activation,1 while antiviral immunity is primarily associated with a Th1 response. Interestingly, one of the major RSV antigen, the “G” surface protein preferentially promotes a Th2-like response that induces some of the “asthmatic features” observed in RSV-associated lower respiratory tract infection4 (Figure 3). Other surface proteins, the adhesion molecules, involved in cell-to-cell or virus-to-cell interaction play a key role in the pathogenesis of inflammatory processes, such as viral infections and asthma.22,23 Evaluating bronchoalveolar lavage fluids obtained from children with a variety of respiratory disorders,
it has been found that asthmatics had significantly higher levels of soluble intercellular adhesion molecule (sICAM)-1, as compared with chronic cough, infantile wheeze, cystic fibrosis and with the control subjects.22,24 ICAM-1 plays a major role in mechanisms that regulate extravasation of eosinophils and neutrophils from the circulation into the tissues, acts as neutrophil and eosinophil receptor on airway epithelial cells and is classically upregulated in the airways of atopic subjects.22,24 Moreover, it has been recently shown that the majority of rhinoviruses attach to the surface of cells via a receptor identified as ICAM-1.25 Experimental infection with RSV enhances ICAM-1 expression on bronchial and nasal epithelial cells25 and, on a type II epithelial cell line, it increases VCAM-1 and class I and II antigen expression, in addition to ICAM-1.26 Infants with viral wheezing appear to have intermediate sICAM-1 levels between normal and asthmatic children, but amongst wheezy infants and asthmatic children a positive correlation between severity of symptoms and sICAM-1 levels was found.24
HOST RISK FACTORS Several endogenous and exogenous factors may increase the risk for an infant to develop viral infections affecting the lower respiratory tract. These include: a) maternal inheritance; b) age (viral wheezing is rare under two months of age); c) gender (RSV and parainfluenza bronchiolitis is
THE WHEEZY INFANT – IMMUNOLOGICAL AND MOLECULAR CONSIDERATIONS more common in males than in females); d) socioeconomic factors (possibly because of crowded living conditions); e) passive smoking (which acts as a non-specific airway irritant); f) baseline pulmonary functions; and g) effectiveness of the immune response to viruses.10 Genetic and epidemiological studies have suggested an important role for maternal inheritance in influencing the development of allergy and asthma in childhood, and more recently in wheezing during infancy.3−5,24 Indeed, cord blood mononuclear cells of newborns from atopic mothers may produce more IL-4 and IL-5 than controls27 and a positive correlation has been found between the occurrence of wheezing in the first year of life and the proportion of cord blood CD4+ T-cells producing IL-13.28 There is no doubt that the relative immaturity of the immune system may, at least in part, explain why a high proportion of wheezy infants, and specifically those with RSV infection, have low gestational age.29 Because of the experimental evidence in animals that immunoprophylaxis with serum-derived antibodies may prevent RSV infection of the lower respiratory tract,30,31 lack of RSV neutralising maternal antibodies has been considered a major factor contributing to severity of RSV infection in preterm infants.29 Although the ability of transplacentally acquired RSV antibodies to fully act against natural infection is still under debate,32,33 there is good epidemiologic evidence that they may be protective against severe RSV-associated respiratory illness.34 Indeed, bronchiolitis is rare in neonates less than 6 weeks old, who have the highest serum concentrations of maternally derived antibodies, and studies on premature infants, but also on healthy newborn, full-term children, showed an inverse relationship between the severity of RSV infection and level of neutralising maternal antibodies.35
CELLULAR IMMUNITY Cell mediated immunity (CMI) is considered to be important in any antiviral defence, including that against RSV, but it may also contribute to enhance the inflammatory-mediated damage to the host structures. As far as the protective effect, there is evidence that children with impaired CMI may show a more prolonged RSV shedding, as compared with normal children,36 and develop RSV pneumonia later in life, i.e., over 3 yrs old, at an age when it is epidemiologically unusual. It is generally accepted that younger children may show a relatively poor CMI response because of immunological immaturity or the inhibition exerted by high levels of pre-existing maternally-derived
S85
antibody. Indeed, RSV-specific CMI was found in over two third of infants aged 6–24 months, but only in 35–38% of those aged <5 months, an age group most severely affected by this infection. An exaggerated CMI may be however a negative prognostic factor in the pathogenesis of RSV. Indeed, previous immunisation with RSV vaccine was associated with more severe pulmonary disease in infants with natural RSV infection.37 In addition, infants with a high degree of RSV-specific CMI 3– 9 weeks after onset of infection were more prone to wheezing over the next six months. Thus, CMI is necessary for the host defence against RSV but, at the same time, may enhance the pathological process.
MUCOSAL IMMUNITY Since most respiratory viruses enter the host via the airways, mucosal immunity seems to play an important role in providing the host’s first line of defence against infections. IgA is the predominant immunoglobulin in respiratory secretions, where it is found in its dimeric form, known as secretory IgA (sIgA). Although it is generally accepted that low concentrations of nasal sIgA may increase the susceptibility to respiratory infection, the ability to mount a greater nasal IgA response to appear to pathogens, rather than the “basal IgA concentration” appears to be protective against viral respiratory infections.38 The finding that nasal IgA response to viral infection in wheezy infants may be similar to their non-wheezing siblings with simple upper respiratory tract infections (URTIs),39 suggest that other factors, such as IgE-mediated immune response, may be involved in airflow limitation in these patients. In agreement with this hypothesis is the demonstration of a positive correlation between the occurrence of wheezing in the first year of life and the proportion of helper T-cells with a Th2 phenotype in cord blood28 and the finding of significantly higher RSV-specific IgE titres in nasopharyngeal secretions in wheezers, as compared with those with nonwheezing RSV infection or parainfluenza infection.40 The interaction between viral antigens and the virusspecific IgEs bound to mast cells and to other cells bearing the IgE receptors results in the release of a variety of vasoactive and inflammatory mediators which cause airway narrowing.1 Indeed, it is possible to demonstrate that bronchoconstrictor mediator, such as LTC4, detected in respiratory secretions of infants with RSV bronchiolitis positively correlate with anti-RSV IgE titres.42 The predisposition to develop a hyperactive IgE response may be related to abnormal T-cell regulatory mechanisms also involving the suppressor/cytotoxic T cells, as partially proven by Welliver who observed
S86 a reduced number of OKT8+ T-lymphocytes during convalescence in children who wheezed with RSV compared with those with URTIs.43 In that study, an inverse correlation between RSV-specific IgE in nasopharyngeal secretions and OKT8+ blood T-cells, suggesting that these cells may include those responsible for suppression of IgE production. Whether this abnormal IgE regulation is virusinduced or constitutionally determined remains unclear.
REFERENCES 1. Kay AB. Asthma and inflammation. J Allergy Clin Immunol 1991; 87: 893−910. 2. Global initiative for asthma: global strategy for asthma management and prevention. National Institutes of Health, National Heart, Lung, and Blood Institutes. Publication Number 02-3659. Bethesda, MD: February 2002. 3. Martinez FD. Present and future treatment of asthma in infants and young children. J Allergy Clin Immunol 1999; 104: S169−174. 4. Balfour-Lynn IM. Why do viruses make infants wheeze? Arch Dis Child 1996; 74: 251−259. 5. Martinez FD, Wright AL, Taussig LM, Holberg CJ, Halonen M, Morgan WJ. Asthma and wheezing in the first six years of life. N Engl J Med 1995; 332: 133−138. 6. Duff AL, Pomeranz ES, Gelber LE, Price GW, Farris H, Hayden FG, Platts-Mills TA, Heyman PW. Risk factors for acute wheezing in infants and children: viruses, passive smoke, and IgE antibodies to inhalant allergens. Pediatrics 1993; 92: 535−540. 7. Rakes GP, Arruda E, Ingram JM, Hoover GE, Zambrano JC, Hayden FG, Platts-Mills TAE, Heymann PW. Rhinovirus and respiratory syncytial virus in wheezing children requiring emergency care. IgE and eosinophil analysis. Am J Respir Crit Care Med 1999; 159: 785−790. 8. Hall CB. Respiratory syncytial virus: a continuing culprit and conundrum. J Pediatr 1999; 135: 2−7. 9. Pattemore PK, Johnston SL, Bardin PG. Viruses as precipitants of asthma symptoms. I. Epidemiology. I. Clin Exp Allergy 1992; 22: 325−336. 10. Bardin PG, Johnston SL, Pattermore PK. Viruses as precipitants of asthma symptoms. I. Physiology and mechanisms. II. Clin Exp Allergy 1992; 22: 809−822. 11. Noble V, Murray M, Webb MS, Alexander J, Swarbrick AS, Milner AD. Respiratory status and allergy nine to 10 years after acute bronchiolitis. Arch Dis Child 1997; 76: 315−319. 12. Nijkamp FP, Folkerts G. Nitric oxide and bronchial reactivity. Clin Exp Allergy 1994; 24: 905−914. 13. Baraldi E, Dario C, Ongaro R, Scollo M, Azzolin NM, Panza N, Paganini N, Zacchello F. Exhaled nitric oxide concentrations during treatment of wheezing exacerbation in infants and young children. Am J Respir Crit Care Med 1999; 159: 1284−1288. 14. Larsen GL, Colasurdo GN. Neural control mechanisms within airways: disruption by respiratory syncytial virus. J Pediatr 1999; 135: 21−27. 15. King KA, Hu C, Rodriguez MM, Romaguera R, Jiang X, Piedimonte G. Exaggerated neurogenic inflammation and substance P receptor upregulation in RSV-infected weanling rats. Am J Respir Cell Mol Biol 2001; 24: 101−107. 16. Heaney LG, Stevenson EC, Turner G, Cadden IS, Taylor R, Shields MD, Ennis M. Investigating paediatric airways by non-
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bronchoscopic lavage: normal cellular data. Clin Exp Allergy 1996; 26: 799−806. Garofalo JR, Kimpel JL, Welliver RC, Ogra PL. Eosinophil degranulation in the respiratory tract during naturally acquired syncytial virus infection. J Pediatr 1992; 120: 28−32. Gern JE, Vrtis R, Kelly EA, Dick EC, Busse WW. Rhinovirus produces nonspecific activation of lymphocytes through a monocyte-dependent mechanism. J Immunol 1996; 157: 1605−1612. Azevedo I, de Blic J, Scheinmann P, Vargaftig BB, Bachelet M. Enhanced arachidonic acid metabolism in alveolar macrophages from wheezy infants: modulation by dexamethasone. Am J Respir Crit Care Med 1995; 152: 1208−1214. Becker S, Quay J, Soukup J. Cytokine (tumor necrosis factor, IL-6, and IL-8) production by respiratory syncytial virusinfected human alveolar macrophages. J Immunol 1991; 147: 4307−4312. Barr FE, Pedigo H, Johnson TR, Shepherd VL. Surfactant protein-A enhances uptake of respiratory syncytial virus by monocytes and U937 macrophages. Am J Respir Cell Mol Biol 2000; 23: 586−592. Montefort S, Roche WR, Howarth PH, Djukanovic R, Gratziou C, Carroll M, Smith L, Britten KM, Haskard D, Lee TH, et al. Intercellular adhesion molecule (ICAM-1) and endothelial leukocyte adhesion molecule 1 (ELAM-1) expression in the bronchial mucosa of normal and asthmatic subjects. Eur Respir J 1992; 5: 815−823. Johnston SL, Pattemore PK, Sanderson G, Smith S, Lampe F, Josephs L, Symington P, O’Toole S, Myint SH, Tyrrell DA, Holgate ST. Community study of viral infections in exacerbations of asthma in 9–11 year old children. BMJ 1995; 310: 1225−1229. Warner JO, Pohunek P, Marguet C, Roche WR, Clough JB. Issues in understanding childhood asthma. J Allergy Clin Immunol 2000; 105: S473−S476. Tosi M, Stark J, Hamedani A, Smith CW, Gruenert D, Huang YT. Neutrophil adhesion to human airway epithelium: role of epithelial ICAM-1 in cells infected with respiratory viruses or treated with IL-1 or TNF. Pediatr Pulmonol 1991; 6: 301−302. Wang SZ, Hallsworth PG, Dowling KD, Alpers JH, Bowden JJ, Forsyth KD. Adhesion molecule expression on epithelial cells infected with respiratory syncytial virus. Eur Respir J 2000; 15: 358−366. Piccinni MP, Beloni L, Giannarini L, Livi C, Scarselli G, Romagnani S, Maggi E. Abnormal production of T helper type-2 cytokines interleukin-4 and interleukin-5 by T-cells from newborn from atopic parents. Eur J Immunol 1996; 26: 2293−2298. Spinozzi F, Agea E, Russano A, Bistoni O, Minelli L, Bologni D, Bertotto A, de Benedictis FM. CD4°IL13° T lymphocytes at birth and the development of wheezing and/or asthma during the 1st year of life. Int Arch Allergy Immunol 2001; 124: 497−501. Ruuskanen O, Ogra PL. Respiratory syncytial virus. Curr Probl Pediatr 1993; 23: 50−79. Faverio LA, Piazza FM, Johnson SA, Darnell ME, Ottolini MG, Hemming VG, Prince GA. Immunoprophylaxis of group B respiratory syncytial virus infection in cotton rats. J Infect Dis 1997; 175: 932−934. Graham BS, Tang YW, Gruber WC. Topical immunoprophylaxis of respiratory syncytial virus (RSV)-challenged mice with RSV-specific immune globulin. J Infect Dis 1995; 171: 1468−1474. Olgivie MM, Vathenen AS, Radford M, Codd J, Key S. Maternal antibody and respiratory syncytial virus infection in infancy. J Med Virol 1981; 7: 263−271.
THE WHEEZY INFANT – IMMUNOLOGICAL AND MOLECULAR CONSIDERATIONS 33. Glezen WP, Taber LH, Frank AL, Kasel JA. Risk of primary infection and reinfection with respiratory syncytial virus. Am J Dis Child 1986; 140: 543−546. 34. Holberg CJ, Wright AL, Martinez FD, Ray CG, Taussig LM, Lebowitz MD. Risk factors for respiratory syncitial virusassociated lower respiratory illness in the first year of life. Am J Epidemiol 1991; 133: 1135−1151. 35. De Sierra TM, Kumar ML, Wasser TE, Murphy BR, Subbarao EK. Respiratory syncytial virus-specific immunoglobulins in preterm infants. J Pediatr 1993; 122: 787−791. 36. Fishaut M, Tubergen D, McIntosh K. Cellular response to respiratory viruses with particular reference to children with disorders of cell-mediated immunity. J Pediatr 1980; 96: 179−186. 37. Kim HW, Leikin SL, Arrobio J, Brandt CD, Chanock RM, Parrott RH. Cell-mediated immunity to respiratory syncytial virus induced by inactivated vaccine or by infection. Pediatr Res 1976; 10: 75−78. 38. Yodfat Y, Silvian I. A prospective study of acute respiratory tract infections among children in a kibbutz: the role of
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secretory IgA and serum immunoglobulins. J Infect Dis 1977; 136: 26−30. Balfour-Lynn IM, Valman HB, Silverman M, Webster ADB. Nasal IgA response in wheezy infants. Arch Dis Child 1993; 68: 472−476. Stark JM, Busse WW. Respiratory virus infection and airway hyperreactivity in children. Pediatr Allergy Immunol 1991; 2: 95−110. Welliver RC, Duffy L. The relationship of RSV-specific immunoglobulin E antibody responses in infancy, recurrent wheezing, and pulmonary function at age 7–8 years. Pediatr Pulmonol 1993; 15: 19−27. Volovitz B, Welliver RC, DeCastro G, Krystofik DA, Ogra PL. The release of leukotrienes in the respiratory tract during infection with respiratory syncytial virus: role in obstructive airway disease. Pediatr Res 1988; 24: 504−507. Welliver RC, Kaul A, Sun M, Ogra PL. Defective regulation of immune responses in respiratory syncytial virus infection. J Immunol 1984; 133: 1925−1930.
The wheezy infant – immunological and molecular considerations Michela Silvestri, Federica Sabatini, Anna-Carla Defilippi, Giovanni A. Rossi Pulmonary Division, G. Gaslini Institute, 16148 Genoa, Italy
KEYWORDS wheezing; infancy; respiratory syncytial virus; epithelial cells; airway inflammation
Summary Most of the data on the pathogenesis of asthma is based on information obtained through bronchial biopsies and bronchoalveolar lavage in adults and young adults. Ethical considerations linked to the invasive nature of airway endoscopy have limited the studies on the pathophysiology of asthma in infancy and early childhood. Although there is evidence that an asthma-like inflammation, with increased inflammatory cells and thickening of the lung basement membrane, may be present also at a very early age, clinical and epidemiologic studies suggest that asthma manifestations in preschool children may significantly differ from those observed in older subjects. In western countries, the vast majority of infants and young children has episodic (or intermittent) asthma, and the exacerbations generally defined “wheezing episodes” occur more frequently with a seasonal pattern being usually related to acute viral infections. There is strong epidemiological evidence that approximately 2/3 of all children who wheeze because of viral infections in early life (and are not atopic) have a transient condition that tends to disappear during early school years. All respiratory viruses may be implicated in the wheezing episodes, the principal being respiratory syncytial virus (RSV) and, with a lower frequency, adenovirus and parainfluenza viruses during the first 3 years of life, and rhinoviruses after that age. Infants and preschool children have on average 6–8 “colds” per year, but the illness tends to be limited to the upper respiratory tract alone in a considerable proportion of individuals, without causing symptomatic involvement of the lower respiratory tract. The variety of factors determining the different outcomes are only partially known, but complex interactions between the intrinsic pathogenicity of the virus and host factors, including the socio-economic conditions of the family, are central to define the type of manifestations and the severity of the process. © 2004 Elsevier Science Ltd.
ABBREVIATIONS CMI
Cell mediated immunity
* Correspondence to: Giovanni A. Rossi. Tel.: +39-(010)563-6547/8; Fax: +39-(010)-383953; E-mail: [email protected] 1526-0542/$ – see front matter
ICAM-1 sICAM-1 LT PAF RSV TNF-a
Intercellular adhesion molecule-1 Soluble ICAM-1 Leukotriene Platelet activating factor Respiratory syncytial virus Tumor necrosis factor-alpha © 2004 Elsevier Science Ltd. All rights reserved.
S82 URTIs Upper respiratory tract infections VCAM-1 Vascular adhesion molecule-1
INTRODUCTION Asthma is defined as a chronic inflammatory disease of the airways characterised by the local production of mediators that in susceptible individuals may induce long-term pulmonary changes, including bronchial hyperresponsiveness, airway remodelling, and irreversible airflow obstruction.1,2 Most of the data available on the pathogenesis of this disease is based on information obtained through bronchial biopsies and bronchoalveolar lavage in adults and young adults.2 Ethical considerations, created by the invasive nature of endoschoscopic evaluation of the airways, have limited the studies on the pathophysiology of asthma in infancy and early childhood.3 Although there is evidence that an asthmalike inflammation may be present also at a very early age, with increased inflammatory cells and thickening of the lung basement membrane, clinical and epidemiologic studies suggest, that asthma manifestations in preschool children may significantly differ from those observed in older subjects.3,4 In western countries, the vast majority of infants and young children tend to have episodic (or intermittent) asthma, and the exacerbations generally defined “wheezing episodes”,5 occur more frequently with a seasonal pattern and are usually in relation to acute viral infections.6,7,8 There is strong epidemiological evidence that approximately 2/3 of all children who wheeze because of viral infections in early life have a transient condition that disappears during early school years.5 All respiratory viruses may be implicated in the wheezing episodes but the principal ones involved are respiratory syncytial virus (RSV) and, with a lower frequency, adenovirus and parainfluenza viruses during the first 3 years of life, and rhinoviruses after that age.7,9 Infants and preschool children catch on average 6–8 “colds” per year, but in a considerable proportion of individuals the illness tends to be limited to the upper respiratory tract alone, without causing symptomatic involvement of the lower respiratory tract.6 Although the many factors determining the different outcomes are only partially known, complex interactions between intrinsic pathogenicity of the virus and host factors, i.e., anatomic/functional characteristics of the airways, defence mechanisms, socio-economic conditions, are central to define the manifestations and the severity of the process.
M. SILVESTRI ET AL.
INTERACTION BETWEEN VIRUSES AND CELLS IN THE AIRWAYS Almost all respiratory viruses can induce bronchial hyperreactivity in normal subjects or exacerbate this condition in asthmatics for as long as 6 to 7 weeks.10,11 Viruses may be cytotoxic to epithelial cells and disruption of the airway epithelial surface may lead to increased permeability to antigens and allergens, to exposure of bronchial cholinergic sensory nerve fibers to physical or chemical irritants and to loss of epithelial derived relaxant factors, including nitric oxide12,13 (Figure 1). Viral damage to airway epithelium may also induce bronchoconstriction by altering the metabolism of bronchomotor mediators, such as substance P.14 Independently of its activity as bronchoconstrictor, substance P is also a potent proinflammatory mediator, able to increase vascular permeability and to attract and activate leukocytes.14 RSV infection, at least in laboratory animals is indeed causing an increased inflammatory response to substance P, with extravasation of serum and proteins in the airways.15 The specific virus-induced cytotoxic effect to airway epithelium may be considerably enhanced by toxic oxygen metabolites released by the host inflammatory cells, such as neutrophils, eosinophils and, to a lesser extent, alveolar macrophages and interstitial monocytes.1 A considerable contribution to airway narrowing comes from the bronchospastic and inflammatory mediators locally released by inflammatory and parenchymal cells during viral infections4 (Figure 2). Performing bronchoalveolar lavage in infants, undergoing routine surgery, raised eosinophils and eosinophil products have been found in wheezers, compared with non-wheezers.16 Eosinophils may release large numbers of mediators including proinflammatory cytokines, cytotoxic proteases and bronchospastic and proinflammatory substances, such as leukotriene (LT)-C4 and platelet activating factor (PAF).1 Significant higher concentrations of eosinophilic cationic protein was detected in nasopharingeal secretions of infants with RSVinduced wheezing, as compared with those with upper respiratory tract infection alone.7,17 Moreover, acute eosinophilic infiltrates of the bronchial mucosa may be present also after experimental rhinovirus infection.17 The concept of rhinovirus augmenting eosinophil responses is supported by the recent in vitro observation that monocyte-driven T-cell activation after incubation with rhinovirus was productive of factors that enhance eosinophil survival.18 In addition to airway eosinophilia, mast cell hyperplasia and bronchial hyperresponsiveness have been shown in experimental animals with viral
THE WHEEZY INFANT – IMMUNOLOGICAL AND MOLECULAR CONSIDERATIONS
S83
Exposure of bronchial cholinergic sensory nerve fibers to irritants. Increased permeability to antigens and allergens.
RSV
Loss of epithelial-derived relaxant factors (NO). Disruption of epithelial cells
Recruitment and activation of inflammatory cells.
Fig. 1. Injury to airway structures induced by respiratory syncytial virus (RSV).
LTC4, LTD4 and Histamine release Mast cells
Alveolar macrophages
Arachidonic acid metabolism enhancement TNF-α, IL-6, IL-8 release
RSV
TNF-α and IL-10 secretion
RSV RSV
Peripheral blood monocytes
INFECTION
ICAM-1 expression Epithelial cells
Alveolar type II epithelial cell line
ICAM-1, VCAM-1, MHC class I and II antigens expression
Fig. 2. Activation of inflammatory and parenchimal cells leads to synthesis and secretion of proinflammatory mediators and surface molecules expression.
S84
M. SILVESTRI ET AL.
IFN-γ Cellular antiviral Immunity
NK cell RSV
IFN-γ IL-2 RSV
CD8 + T-cell
A.P.C. IL-12
RSV
APC
IL-10
Th1 CD4 + T-cell Th2
IL-4 IL-5 IL-10 IL-13
Umoral Immunity
Fig. 3. Viral infection stimulates the cellular and umoral immune response.
respiratory infections.4 Mast cells are thought to potentially play a key role in these processes, since they are able to release products, such as histamine, LTC4 and LTD4, found in elevated concentrations in the respiratory secretion of infants with viral wheezing.4 Enhanced arachidonic acid metabolism was also described in alveolar macrophages from wheezy infants, possibly related to a direct effect of viral replication or to cytokines present in the lower respiratory tract.19 Experimental infection of alveolar macrophages by RSV leads to increased secretion of proinflammatory cytokines that include tumor necrosis factor-alpha (TNF-a), interleukin (IL)-6 and IL-8.20 More recently, Barr et al. reported that RSV also significantly enhanced TNF-a and IL-10 production by peripheral blood monocytes.21 IL-6, together with IL-4, IL-5 and IL-10, is classically considered a Th2 cytokine, mediators involved in the pathogenesis of asthma and participating in IgE production and in eosinophil and mast cell activation,1 while antiviral immunity is primarily associated with a Th1 response. Interestingly, one of the major RSV antigen, the “G” surface protein preferentially promotes a Th2-like response that induces some of the “asthmatic features” observed in RSV-associated lower respiratory tract infection4 (Figure 3). Other surface proteins, the adhesion molecules, involved in cell-to-cell or virus-to-cell interaction play a key role in the pathogenesis of inflammatory processes, such as viral infections and asthma.22,23 Evaluating bronchoalveolar lavage fluids obtained from children with a variety of respiratory disorders,
it has been found that asthmatics had significantly higher levels of soluble intercellular adhesion molecule (sICAM)-1, as compared with chronic cough, infantile wheeze, cystic fibrosis and with the control subjects.22,24 ICAM-1 plays a major role in mechanisms that regulate extravasation of eosinophils and neutrophils from the circulation into the tissues, acts as neutrophil and eosinophil receptor on airway epithelial cells and is classically upregulated in the airways of atopic subjects.22,24 Moreover, it has been recently shown that the majority of rhinoviruses attach to the surface of cells via a receptor identified as ICAM-1.25 Experimental infection with RSV enhances ICAM-1 expression on bronchial and nasal epithelial cells25 and, on a type II epithelial cell line, it increases VCAM-1 and class I and II antigen expression, in addition to ICAM-1.26 Infants with viral wheezing appear to have intermediate sICAM-1 levels between normal and asthmatic children, but amongst wheezy infants and asthmatic children a positive correlation between severity of symptoms and sICAM-1 levels was found.24
HOST RISK FACTORS Several endogenous and exogenous factors may increase the risk for an infant to develop viral infections affecting the lower respiratory tract. These include: a) maternal inheritance; b) age (viral wheezing is rare under two months of age); c) gender (RSV and parainfluenza bronchiolitis is
THE WHEEZY INFANT – IMMUNOLOGICAL AND MOLECULAR CONSIDERATIONS more common in males than in females); d) socioeconomic factors (possibly because of crowded living conditions); e) passive smoking (which acts as a non-specific airway irritant); f) baseline pulmonary functions; and g) effectiveness of the immune response to viruses.10 Genetic and epidemiological studies have suggested an important role for maternal inheritance in influencing the development of allergy and asthma in childhood, and more recently in wheezing during infancy.3−5,24 Indeed, cord blood mononuclear cells of newborns from atopic mothers may produce more IL-4 and IL-5 than controls27 and a positive correlation has been found between the occurrence of wheezing in the first year of life and the proportion of cord blood CD4+ T-cells producing IL-13.28 There is no doubt that the relative immaturity of the immune system may, at least in part, explain why a high proportion of wheezy infants, and specifically those with RSV infection, have low gestational age.29 Because of the experimental evidence in animals that immunoprophylaxis with serum-derived antibodies may prevent RSV infection of the lower respiratory tract,30,31 lack of RSV neutralising maternal antibodies has been considered a major factor contributing to severity of RSV infection in preterm infants.29 Although the ability of transplacentally acquired RSV antibodies to fully act against natural infection is still under debate,32,33 there is good epidemiologic evidence that they may be protective against severe RSV-associated respiratory illness.34 Indeed, bronchiolitis is rare in neonates less than 6 weeks old, who have the highest serum concentrations of maternally derived antibodies, and studies on premature infants, but also on healthy newborn, full-term children, showed an inverse relationship between the severity of RSV infection and level of neutralising maternal antibodies.35
CELLULAR IMMUNITY Cell mediated immunity (CMI) is considered to be important in any antiviral defence, including that against RSV, but it may also contribute to enhance the inflammatory-mediated damage to the host structures. As far as the protective effect, there is evidence that children with impaired CMI may show a more prolonged RSV shedding, as compared with normal children,36 and develop RSV pneumonia later in life, i.e., over 3 yrs old, at an age when it is epidemiologically unusual. It is generally accepted that younger children may show a relatively poor CMI response because of immunological immaturity or the inhibition exerted by high levels of pre-existing maternally-derived
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antibody. Indeed, RSV-specific CMI was found in over two third of infants aged 6–24 months, but only in 35–38% of those aged <5 months, an age group most severely affected by this infection. An exaggerated CMI may be however a negative prognostic factor in the pathogenesis of RSV. Indeed, previous immunisation with RSV vaccine was associated with more severe pulmonary disease in infants with natural RSV infection.37 In addition, infants with a high degree of RSV-specific CMI 3– 9 weeks after onset of infection were more prone to wheezing over the next six months. Thus, CMI is necessary for the host defence against RSV but, at the same time, may enhance the pathological process.
MUCOSAL IMMUNITY Since most respiratory viruses enter the host via the airways, mucosal immunity seems to play an important role in providing the host’s first line of defence against infections. IgA is the predominant immunoglobulin in respiratory secretions, where it is found in its dimeric form, known as secretory IgA (sIgA). Although it is generally accepted that low concentrations of nasal sIgA may increase the susceptibility to respiratory infection, the ability to mount a greater nasal IgA response to appear to pathogens, rather than the “basal IgA concentration” appears to be protective against viral respiratory infections.38 The finding that nasal IgA response to viral infection in wheezy infants may be similar to their non-wheezing siblings with simple upper respiratory tract infections (URTIs),39 suggest that other factors, such as IgE-mediated immune response, may be involved in airflow limitation in these patients. In agreement with this hypothesis is the demonstration of a positive correlation between the occurrence of wheezing in the first year of life and the proportion of helper T-cells with a Th2 phenotype in cord blood28 and the finding of significantly higher RSV-specific IgE titres in nasopharyngeal secretions in wheezers, as compared with those with nonwheezing RSV infection or parainfluenza infection.40 The interaction between viral antigens and the virusspecific IgEs bound to mast cells and to other cells bearing the IgE receptors results in the release of a variety of vasoactive and inflammatory mediators which cause airway narrowing.1 Indeed, it is possible to demonstrate that bronchoconstrictor mediator, such as LTC4, detected in respiratory secretions of infants with RSV bronchiolitis positively correlate with anti-RSV IgE titres.42 The predisposition to develop a hyperactive IgE response may be related to abnormal T-cell regulatory mechanisms also involving the suppressor/cytotoxic T cells, as partially proven by Welliver who observed
S86 a reduced number of OKT8+ T-lymphocytes during convalescence in children who wheezed with RSV compared with those with URTIs.43 In that study, an inverse correlation between RSV-specific IgE in nasopharyngeal secretions and OKT8+ blood T-cells, suggesting that these cells may include those responsible for suppression of IgE production. Whether this abnormal IgE regulation is virusinduced or constitutionally determined remains unclear.
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