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Respiratory Medicine (2006) 100, 775–784

KEYWORDAsthma;RespiratorAtypical b

0954-6111/$ - sdoi:10.1016/j.r

�CorrespondiE-mail addr

REVIEW

Respiratory infections and asthma

Girolamo Pelaiaa, Alessandro Vatrellab, Luca Gallellia, Teresa Rendaa,Mario Cazzolac,�, Rosario Masellia, Serafino A. Marsicob

aDepartment of Experimental and Clinical Medicine, University ‘‘Magna Græcia’’ of Catanzaro, ItalybDepartment of Cardiothoracic and Respiratory Sciences, Second University of Naples, ItalycUnit of Pneumology and Allergology, Department of Respiratory Medicine, A. Cardarelli Hospital,Via del Parco Margherita 24, 80121 Napoli, Italy

Received 22 August 2005; accepted 24 August 2005

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y viruses;acteria

ee front matter & 2005med.2005.08.025

ng author. Tel.: +81 404ess: mcazzola@qubisoft

Summary Respiratory tract infections caused by both viruses and/or atypicalbacteria are involved in the pathogenesis of asthma. In particular, several virusessuch as respiratory syncytial virus, rhinovirus and influenza/parainfluenza virusesmay favour the expression of the asthmatic phenotype, being also implicated in theinduction of disease exacerbations. Within this pathological context, a significantrole can also be played by airway bacterial colonizations and infections due toChlamydiae and Mycoplasms. All these microbial agents probably interfere withcomplex immunological pathways, thus contributing to induce and exacerbateasthma in genetically predisposed individuals.& 2005 Elsevier Ltd. All rights reserved.

Contents

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 776Asthma and viral infections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 776

Respiratory syncytial virus (RSV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 776Rhinovirus (RV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 777Influenza and parainfluenza viruses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 778

Asthma and bacterial infections . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 779Chlamydia pneumoniae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 779Mycoplasma pneumoniae . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 780

Hygiene hypothesis and asthma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 780Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 782References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 782

Elsevier Ltd. All rights reserved.

188 813486..it (M. Cazzola).

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Introduction

Asthma is a chronic inflammatory airway diseasecharacterized by bronchial hyper-responsiveness toa wide range of stimuli, recurrent episodes ofwheezing, breathlessness, chest tightness andcoughing, which are associated with reversibleairflow limitation.1 The important medical, socialand economical implications of asthma depend onits very large epidemiological impact, in that over150 million people worldwide are affected by thisdisease, whose prevalence is on the rise especiallyin the industrialized countries.2 Such alarmingnumbers are prompting huge investments of finan-cial and human resources, aimed at furtherelucidating the pathogenic mechanisms underlyingasthma, as well as at developing new and moreeffective anti-asthma treatments.

Current clinical and experimental evidence im-plies that the two essential components of theasthmatic phenotype include bronchial inflamma-tion, mainly involving T lymphocytes, eosinophilsand mast cells, and airway remodelling, character-ized by structural changes spanning throughoutepithelial lining, subepithelial mesenchymal layers,airway smooth muscle, and bronchial vessels.3

These pathophysiologic patterns arise from anintricate network of interactions between geneticand environmental factors, including allergens,pollutants, and infectious agents. With regard tothe latter, their role is indeed very intriguing, inthat it has been hypothesized that respiratoryinfections, in addition to promoting both thedevelopment and the exacerbations of asthma,might also exert a protective effect against thisdisease and atopy.4,5 Therefore, the aim of thisreview is to provide a synthetic update aboutthe complex relationships between asthma andrespiratory infections caused by either viruses orbacteria.

Asthma and viral infections

In both children and adults, viral infections of theairways may be associated with the development ofchronic asthma, as well as with acute diseaseexacerbations.6 These associations can now bereliably confirmed by currently available diagnos-tic, molecular tools such as enzymatic amplificationof viral nucleic acids through polymerase chainreaction (PCR).7 Although PCR is not yet widelyused in routine medical practice, this technique isvery useful for identifying respiratory viruses,especially when performed on samples of induced

sputum, thus being much more efficient thantraditional serology and immunofluorescence. Withregard to the pathogenesis of asthma and itsexacerbations, the most commonly involved virusesinclude respiratory syncytial virus (RSV), rhinovirus(RV), influenza and parainfluenza viruses, corona-virus, enterovirus, and adenovirus.8 Wheezing-associated viral infections are characterized by anage-related distribution. In particular, RSV predo-minates in children under 3 years of age, whereasRV represents the most frequent cause of infectiousrespiratory illnesses affecting older children andadults9; influenza and parainfluenza viruses affectall age groups.

Respiratory syncytial virus (RSV)

With regard to RSV, it is noteworthy to point outthat this virus infects about 70% of infants withinthe first year of life, and almost 100% by the age of3 years.10 RSV often produces only non-complicatedinfections of the upper airways. Nevertheless, insome cases RSV causes a severe bronchiolitis whichis frequently associated with the development ofrecurrent wheezing and asthma. However, it is notyet clear whether RSV constitutes a direct cause ofasthma, or if it rather induces airway obstructionexclusively in individuals with a genetic predisposi-tion to asthma. Therefore, it is very difficult toestablish whether the bronchiolitis caused by RSVsimply accelerates the onset of asthma, or if thisvirus instead represents a crucial factor for thesubsequent development of the allergic sensitiza-tion which characterizes many asthmatic patients.In this regard, conflicting data have been indeedprovided by various studies, some of which haveallowed to detect, in contrast with others, a closerelationship between RSV-induced paediatricbronchiolitis and the clinical manifestations ofatopy and asthma. For instance, Stein and collea-gues carried out a wide perspective investigationon more than 1000 children, studied since theirearly infancy until the beginning of adolescence.These authors found that the lower respiratorytract infections caused by RSV within the first 3years of age, though not so severe to requirehospitalization, were associated with an increasedrisk of wheezing by the age of 6.11 However, such arisk subsequently subsided progressively, thus re-sulting to be not significant by the age of 13. On thecontrary, the Scandinavian group led by Sigurshas more recently shown that children whocontracted a severe RSV infection within thefirst year of life experienced, until the age ofabout 7 years, a higher frequency of bronchial

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obstruction and atopic asthma when compared withchildren not previously affected by a seriousbronchiolitis.12 Moreover, by continuing to follow-up their study population, the same authors havefurther confirmed that the risk of developingallergic asthma persisted, until the age of 13, inthose subjects who were hospitalized during theirfirst year of life because of bronchiolitis producedby RSV.13

Several cellular and molecular mechanisms mightcontribute to determine an atopic phenotype inchildren and adolescents with a history of severeforms of bronchiolitis caused by RSV during earlychildhood. Indeed, RSV may be responsible for anincreased risk of allergic sensitization by favouringthe creation of a bronchial and bronchiolar micro-environment characterized by high levels of inter-leukin-4 (IL-4).14 This cytokine represents the mostimportant factor determining the expansion of Th2lymphocytes, the cells primarily implicated in thepathogenesis of allergic processes. Furthermore,IL-4 is directly responsible for the antibody classswitching which promotes the production of IgE byB lymphocytes. The different biological behavioursinduced by RSV might be explained by the tendencyof most children with a common, mild RSV infec-tion, to develop a predominantly Th1-mediatedantiviral immune response, characterized by anintense production of interferon-g (IFN-g). Other-wise, a Th2-dependent immune response asso-ciated with high IL-4 levels can occur in veryyoung patients affected by severe bronchiolitis. Inthis regard, it has been suggested that theindividual susceptibility to the development ofboth allergic sensitization and serious respiratoryviral infections, may be due to genetic factors. Inparticular, a genetically determined delay in thephysiological, early post-natal immune deviationfrom the prevalent Th2 foetal pattern towards thematuration of a Th1 phenotype, could predispose tothe amplification and propagation, within theairways, of inflammatory events triggered by bothrespiratory viruses and inhaled allergens.15 Thishypothesis has been confirmed by the high fre-quency, recently shown in a population of 105Korean children hospitalized for severe RSV infec-tions, of a haplotype detected inside the cytokinegene cluster located on the long arm of chromo-some 5 (-589T IL-4 gene polymorphism), associatedwith an overexpression of IL-4.16 Within such abiological context, a key function is likely exertedby a specific RSV glycoprotein, named protein G,which is able to promote a Th2 immune response.17

In fact, by comparatively evaluating the conse-quences of experimentally induced infectionscaused by either normal or G protein-defective

strains of RSV, it has been observed in mice that thisprotein plays an essential role in producing airwayinflammation and functional respiratory impair-ment, both evoked by wild-type RSV.18 Moreover,the RSV-elicited activation of an antiviral proteinkinase may favour, in B cells, isotype switching toIgE.19

Finally, it is also relevant that RSV may con-tribute to airway neurogenic inflammation byinducing the expression of bronchial NK-1 recep-tors,20 stimulated by substance P. This effectcan thus lead to an amplification of the proin-flammatory action of substance P, released by thesensory endings of unmyelinated C-fibres. Theimportance of such a mechanism is experimentallysupported by the demonstrated efficacy of themonoclonal antibody palivizumab, directed againstthe viral protein F, in reducing the excessive neuralinflammation found in the airways of RSV-infectedrats.21

Rhinovirus (RV)

Differently from children under scholar age, duringthe following periods of childhood and adoles-cence, as well as in adults, about 60% of the viral,upper respiratory tract infections involved inasthma exacerbations, are caused by RV.8 There-fore, this virus is not only responsible for thecommon cold, but it also exerts a relevantpathogenic action in acute asthma relapses. Froma clinical and functional point of view, airwayinfection due to some particular RV strains, such asthe RV16 serotype, produces in allergic patients arecrudescence of asthmatic and rhinitic symptoms,a deterioration of respiratory function, and anincrease in bronchial hyper-responsiveness.22 Thelatter may persist for several weeks. Indeed, it isnow strongly evident that RV, in addition toinfecting the upper airways, may also colonize thelower respiratory tract.

The prevalent location of RV-dependent tissuedamage is represented by the bronchial epithelium,whose remarkable fragility typical of asthma makesit particularly susceptible to the viral cytotoxiceffect, which in turn further facilitates thepenetration of allergens and other noxious agentsthus triggering a positive feed-forward circuit. Thecytopathic action exerted by RV on airway epithe-lial cells likely constitutes the initial event inasthma exacerbations induced by this virus. Theairway epithelial receptor for most RV strains isintercellular adhesion molecule-1 (ICAM-1), whichalso significantly contributes to the pathogenesis ofallergic inflammation.23,24 Papi and Johnston25

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have shown that RV is able to up-regulate ICAM-1expression via activation of the transcription factornuclear factor-kappa B (NF-kB), thereby increasingits own pathogenic potential. Moreover, Bianco andcolleagues have found that also IL-4 and other Th2cytokines stimulate the airway epithelial expres-sion of ICAM-1, thus increasing the susceptibility ofasthmatic patients to RV infections.26 On the otherhand, NF-kB is also involved in transcriptionalactivation of many genes encoding proinflamma-tory cytokines and chemokines. In this regard, it isnow clear that intracellular oxidant radicals play akey role in RV-induced stimulation of NF-kB activityoccurring in bronchial epithelium.27 These recentadvances about the molecular mechanisms under-lying the pathogenic action of RV have importanttherapeutic implications, in that both corticoster-oids and reducing agents are able to inhibit theactivation of transcription factors responsible forthe enhanced expression of proinflammatory genesencoding adhesion molecules and various cyto-kines.

In particular, airway epithelial cells react to RVinfection with an inflammatory response mediatedby an increased biosynthesis of interleukins (IL)-6,8, and 16,28 involved in maturation of T and Blymphocytes as well as in fibroblast proliferation(IL-6), recruitment of neutrophils (IL-8), andchemotaxis of eosinophils and monocytes (IL-16),respectively. Bronchial epithelial cells infected byRV also produce high amounts of the chemokineseotaxin and RANTES (Regulated upon Activation,Normal T-cell Expressed and Secreted),28 whichplay a powerful chemotactic role in eosinophilicairway infiltration. Furthermore, by stimulating theepithelial production of fibroproliferative andangiogenic factors such as the fibroblast growthfactor-2 (FGF-2), RV extends its actions to thesubepithelial layers, thus contributing to thestructural changes responsible for airway remodel-ling, another essential component of asthmaphenotype. Therefore, RV affects the functions ofboth resident and immune/inflammatory cells,thereby further addressing the immune responseof atopic, asthmatic patients towards a predomi-nant type 2 pattern, already characterized by anincreased secretion of IL-4. In these subjects, theconcomitant reduced production of type 1 cyto-kines such as IFN-g, may explain at least in part thedefective antiviral defences. Asthma is also char-acterized by an ineffective presentation of viralantigens, that contributes to impair the immuno-logical mechanisms aimed at neutralizing RV. Theresulting persistence of RV infection can thus leadto an exacerbation of asthma symptoms andbronchial hyper-responsiveness.29

Influenza and parainfluenza viruses

The airway immune inflammation occurring in manyasthmatic patients can be further amplified byacute viral infections caused by influenza viruses.The latter often exacerbate respiratory symptomsand bronchial responsiveness to allergic stimuli.Utilizing an animal model of asthma, Dahl andcolleagues have recently shown that the influenza Avirus induces pulmonary changes which last forseveral weeks after infection resolution.30 Inparticular, these authors observed that a previousflu infection was able to enhance the cellularresponse to a subsequent allergen sensitization, asit can be inferred from the increased numbers ofeosinophils, lymphocytes and macrophages, de-tectable in the bronchoalveolar lavage fluid (BALF)obtained from mice infected with influenza A viruswhen compared with control animals undergoingallergen sensitization, but not previously exposedto viral infection. Influenza viruses also contributeto increase neurally mediated bronchial hypertone,in that viral neuraminidases degrade the sialic acidresidues of prejunctional muscarinic M2 recep-tors.31 By inhibiting acetylcholine release frompostganglionic fibres, these cholinergic autorecep-tors exert an inhibitory control on vagal neuro-transmission, Therefore, the influenza virus-dependent M2 receptor dysfunction is responsiblefor an exaggeration of reflex parasympatheticbronchoconstriction.

Endotracheal inoculation of parainfluenza virusinduces, in animal models, biological and histolo-gical changes reminiscent of asthma. In particular,this viral infection elicits an airway influx ofinflammatory cells, bronchial hyper-responsive-ness, epithelial damage and bronchiolar fibrosis.Such two latter alterations are likely due to anincreased production of transforming growth fac-tor-b (TGF-b) from parainfluenza virus-stimulatedalveolar macrophages.32 In fact, it is well knownthat TGF-b has fibrogenic properties and alsoinduces apoptosis of airway epithelial cells.33 Thissuggests that the recurrence of parainfluenza virusinfections may contribute, in patients with asthma,to the airway structural changes typical of thedisease and at least in part mediated by TGF-b.Apoptosis of bronchial epithelial cells may beviewed as a host attempt to limit viral replication,but at the same time cell death is likely a majordeterminant of the epithelial damage occurring inasthma. Furthermore, epithelial loss might be themain cause of the defective bronchial synthesis ofnitric oxide (NO), detected in association with theparainfluenza virus-dependent increase in airwayhyper-responsiveness.

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Asthma and bacterial infections

With regard to the role of non-viral infections inasthma inception and exacerbations, the atypicalbacteria Chlamydia pneumoniae and Mycoplasmapneumoniae are currently believed to be the mostinvolved pathogens.34

Chlamydia pneumoniae

C. pneumoniae is a Gram-negative, obligate in-tracellular bacterium, entirely dependent on en-ergy produced by infected host cells, where itreplicates forming characteristic cytoplasmic in-clusions. Within the respiratory system, the maincellular targets of this pathogen include epithelialand endothelial cells, as well as monocytes/macrophages. C. pneumoniae may cause infectionsof the upper and lower respiratory tracts.35

Furthermore, recurrence and/or persistence ofrespiratory infections caused by C. pneumoniaeare likely implicated in the development andworsening of asthma.36 On the other hand, thismicrobial agent is characterized by an innatetendency to determine chronic infections, whichresult to be particularly insidious in that they arefrequently quiescent and non-symptomatic. Sev-eral different genetic and environmental factors,also including coexistence of other diseases, smok-ing habit, and the therapeutic use of corticoster-oids, contribute to promote the persistence of C.pneumoniae infectious capacity.

A number of epidemiological studies have shownthat many asthmatic patients have high serumlevels of both IgG and IgA anti-C. pneumoniaeantibodies, which can be considered as markers ofpersistent infection. In particular, Hahn and col-leagues were the first to identify a consistentassociation between wheezing and anti-C. pneu-moniae antibody titres in 365 subjects withrespiratory symptoms.37 Moreover, these authorsalso demonstrated that Chlamydial infection was arisk factor for asthma onset in adults. In fact,within the study population, 9 of 19 symptomaticasthmatics exhibited serologic evidence of either acurrent or a recent infection caused by C.pneumoniae. The same authors have furtherextended their observations thus showing, in adultpatients with recently diagnosed asthma, theexistence of serologically positive, chronic respira-tory Chlamydial infections.38 In addition, a recur-rence of dyspnoeic episodes induced by C.pneumoniae may precede the development ofchronic asthma in previously asymptomatic indivi-duals.39 Cunningham et al.40 studied 108 asthmatic

children for 13 months, thus showing that C.pneumoniae-specific secretory IgA antibodies innasal aspirates were much more elevated insubjects with frequent asthma exacerbations. Thelatter subgroup of children also tended to maintaina longer PCR positivity for C. pneumoniae, indica-tive of a chronic infection. A further confirm of thetight relationships linking the infections caused byC. pneumoniae with asthma severity and exacer-bations was provided by Cook and colleagues, whodemonstrated in a cohort of 169 asthmatic patientsthat chronic, severe forms of asthma were corre-lated to high serum concentrations of anti-C.pneumoniae IgG and IgA antibodies.41 More re-cently, Black et al. observed in 619 asthmaticadults that elevated serum titres of anti-C.pneumoniae IgG and IgA were directly associatedwith the severity of asthma symptoms, as well aswith the use of high doses of inhaled corticoster-oids.42 These same authors also detected an inversecorrelation between antibody levels and airwaycalibre, assessed by measuring forced expiratoryvolume in one second (FEV1). Therefore, it can beargued that chronic Chlamydial infections are likelyassociated with markers of asthma severity. Withregard to the relationships between allergen-induced and infection-dependent asthma varieties,von Hertzen and colleagues investigated 332 asth-matic patients thus finding in non-atopic subjectsthat their disease was significantly associated,differently from atopic individuals, with highconcentrations of anti-C. pneumoniae IgG.43 How-ever, it is currently not known whether theassociation between respiratory Chlamydial infec-tions and asthma is causal or coincidental.

In order to settle such a controversy, an usefulcontribute might be provided by studies performedon asthmatic patients infected by C. pneumoniae,with the aim of evaluating the effects of anti-biotics, especially macrolides. In this regard, Blacket al. showed a significant improvement in eveningpeak expiratory flow when asthmatic subjects,serologically positive for Chlamydial infection,were treated for 6 weeks with roxithromycin(150mg b.i.d.)44; subsequently to therapy cessa-tion, there was a decrease in the differencesdetected in comparison with asthmatic individualstreated with placebo. Furthermore, in thosepatients undergoing treatment with roxithromycin,a tendency was found, though statistically notsignificant, towards a reduction of asthma symp-toms. Kraft and colleagues also demonstrated thatchlarithromycin, administered for 6 weeks (500mgb.i.d.), elicited an increase in FEV1 and a decreasein the bronchial expression of IL-5, but only insubjects who were PCR-positive for C. pneumoniae

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or M. pneumoniae.45 Eosinophil levels diminished inthe induced sputum obtained from asthmatic adultstreated for 8 weeks with chlarithromycin (200mgb.i.d.), as reported by Amayasu et al., who alsofound a decreased airway hyper-responsiveness inthe same patients.46 This latter effect was con-firmed by Ekici and colleagues after 8 weeks oftreatment with azithromycin (250mg twice aweek), which however did not induce any FEV1change.47 Moreover, Gotfried et al. have recentlyshown that chlarithromycin (250mg b.i.d.), givenfor 6 weeks to asthmatic individuals treated withoral prednisone, allowed to reduce steroid dosagewithout consequences on asthma symptoms andrespiratory function.48 In order to correctly inter-pret all these data, it is however noteworthy thatmacrolides appear to exert, independently fromtheir antimicrobial properties, a relevant anti-inflammatory action.49

During the last few years, a considerable progresshas been made in our understanding of the cellularand molecular events underlying the potential roleof C. pneumoniae in asthma pathogenesis. Gener-ally, the development of diseases induced byChlamydial infections is dependent on immuno-pathological mechanisms. In this regard, a 57–60-kDa Chlamydial antigen, which is a member of thehsp60 family of heat-shock proteins, seems topossess remarkable immunogenic, proinflamma-tory, and cytopathic properties.50 The immuneresponse to this protein is likely involved in tissuedamage caused by recurrent and persistent Chla-mydial infections, and infected cells produceproinflammatory cytokines such as tumour necrosisfactor-a (TNF-a), IL-1b, IL-6 and IL-8.36,51 In theairways of asthmatic patients, macrophages andbronchial epithelial cells infected by C. pneumo-niae could continuously or repeatedly release thehsp60 antigen, thus contributing to perpetuate andamplify chronic inflammation, as well as to inducethe structural changes typical of asthma. Byactivating NF-kB, in fact, hsp60 is able to stimulatethe transcription of genes encoding proinflamma-tory cytokines and adhesion molecules.52 An im-portant feature of the amino acid composition ofChlamydial hsp60 is to share a high homologysequence with the analogous human protein.Therefore, the bacterial hsp60 might evoke anautoimmune reaction, eventually persisting evenafter infection eradication. Recently, an associa-tion has been identified between adult-onsetasthma and specific antibodies against anotherChlamydial antigen, named hsp10.53 In this regard,it is interesting to point out that a surprisinglyelevated percentage (65.8%) of asthmatic subjects,who are serologically negative for Chlamydial

infections, exhibit an anti-hsp10 antibody re-sponse.

Taken together, the above-mentioned immune-inflammatory pathogenic mechanisms, triggered bychronic infections caused by C. pneumoniae, maythus increase airway susceptibility to other envir-onmental stimuli such as allergens and viruses,thereby accelerating asthma progression. There-fore, within this context antibiotics could play avery important therapeutic role, excellently dis-cussed in recent reviews by Blasi et al.54 andCazzola et al.49

Mycoplasma pneumoniae

Both serological and epidemiological studies sug-gest that infections caused by M. pneumoniae maybe implicated in the pathogenesis of asthma, alsoregarding its exacerbations. For instance, Martin etal. evidenced, in about half of 55 asthmaticpatients, a PCR-documented presence of M. pneu-moniae, which was also associated with a highernumber of tissue mast cell in comparison with PCR-negative subjects.55 Furthermore, administrationof chlarythromycin (500mg b.i.d. for 6 weeks) toPCR-positive asthmatics can significantly improvetheir respiratory function.45 M. pneumoniae mayalso trigger Th2-like cytokine responses, associatedwith elevated serum IgE concentrations. In experi-mental animal models, mice infected with M.pneumoniae may develop airway obstruction andinflammation.56

Airway epithelium represents the most importanttissue target of M. pneumoniae. In vitro, culturedbronchial epithelial cells respond to experimentallyinduced M. pneumoniae infection by enhancing thesynthesis of IL-1b, IL-6, IL-8, TNF-a, RANTES, andTGF-b.57,58 The increase in cell culture supernatantlevels of RANTES and TGF-b can be inhibited, asshown by Dakhama and colleagues, by a treatmentwith either erythromycin or dexamethasone.58 Thisimplies that in the airways both macrolides andglucocorticoids are potentially very useful in pre-venting the inflammatory and structural changescaused by M. pneumoniae, and especially the TGF-b-induced damage of bronchial epithelium.33,58

Hygiene hypothesis and asthma

The remarkable increase in the prevalence ofallergies and asthma, occurred during the lastdecades especially in the industrialized countries,is frequently attributed to a group of factors whichas a whole are referred to the so-called ‘‘hygiene

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hypothesis’’.5,59,60 This theory presumes that thelifestyle of westernized societies characterized byimproved hygienic conditions and lowered risk ofinfections, due to widespread vaccinations anddecreased overcrowding and family size, is respon-sible for a reduced exposure to microbial agentsespecially during infancy and childhood. The resultof such a ‘‘sterilized’’ environment would be areduced activation of the Th1-arm of adaptiveimmunity, which is stimulated by viral and bacterialinfections. Indeed, viruses promote the productionof cytokines such as IFN-g, IL-12 and IL-18, thatorientate lymphocyte differentiation towards a Th1pattern.61 A Th1 response is also favoured by highdoses of bacterial components such as lipopolysac-caride (LPS), which activates the innate immunesystem by interacting with the toll-like receptor 4expressed by dendritic cells.62 Moreover, a delayedhypersensitivity to Mycobacterium tuberculosis canbe associated with a low risk of atopy.63 Therefore,a reduced Th1 activation due to a lack of infectiousstimuli may contribute to polarize the immuneresponse towards a Th2 phenotype, involved in thedevelopment of allergic diseases and asthma.

However, recent epidemiological reports haveraised reasonable doubts about the supposedprotective role of infections on asthma inceptionand development. For instance, a large studycarried out in Europe on more than 18,000 subjectsshowed a direct, rather than inverse correlationbetween the number of siblings and the presence ofasthma symptoms and functional pulmonary im-pairment.64 Another investigation, performed on asample of almost 14,000 British children, did notevidence any influence of pertussis vaccination onallergy and asthma.65 The relationship betweenasthma and bacterial endotoxin is also quiteintriguing. Studying 226 children under the age of5 and with a familial history of atopy, Litonjua etal. have recently demonstrated that exposure tohigh concentrations of endotoxin in house dust wasassociated with an increased risk of wheezing,which however rapidly declined within a fewyears.66 In this regard, a previous study haddetected a direct relationship between inhaledendotoxin levels and the exacerbation of pre-existing asthma symptoms.67 Despite the proposedprotective effect of tubercular infection againstallergy and asthma, no correlation was foundbetween atopy and delayed sensitization to tuber-culin in a population of about 500 African childrenfrom Gambia.68 On the other hand, an intradermalinjection of heat-killed Mycobacterium vaccae,performed by Camporota et al. in allergic asth-matic patients, did not elicit significant changeswith regard to early and late asthmatic reactions to

allergen challenge, bronchial hyper-responsivenessto histamine, serum IgE levels, and IL-5 productionfrom peripheral lymphocytes.69

Furthermore, Holtzman and colleagues haverecently proposed a new theory, decisively alter-native to the hygiene hypothesis, according towhich both Th1 and Th2 branches of the adaptiveimmune response would be implicated in asthmapathogenesis.70,71 In particular, within a geneticallypredisposed individual context, the asthmaticphenotype would develop as a consequence of aconcomitant activation of allergic and antiviralmechanisms. Indeed, respiratory viruses mightinduce bronchial epithelial cells to produce IFN-gand IL-12, thus favouring the creation within theairways of a local environment characterized by aTh1 immune pattern. At the same time, inhaledallergens would promote a deviation of the immuneresponse towards a Th2 secretory profile, mainlyincluding IL-4, 5, 9, and 13. Therefore, the Th1 andTh2 arms of the adaptive immune system, ratherthan antagonizing and reciprocally inhibiting eachother, may contribute together to unveil andamplify the asthmatic phenotype, featured bychronic inflammation, bronchial hyper-responsive-ness, and airway remodelling. This pathogenetictheory appears to be also supported by the above-mentioned data obtained by Dahl and colleagues30

These authors inoculated intranasally the influenzaA virus into some mice which subsequently under-went, several weeks after viral infection, anallergen sensitization. Under such experimentalconditions, the influenza A virus dramaticallyincreased the secretion of IFN-g from pulmonarydendritic cells, thus inducing a strong Th1 re-sponse. The latter did potentiate, rather thaninhibiting, the expansion of the Th2 lineagetriggered by the following allergic challenge.Within genetically mediated host susceptibility,the dual involvement of Th1 and Th2 cells activatedby viral agents and inhaled allergens, respectively,may thus determine the immune-inflammatorychanges underlying asthma.

Taking together all these considerations, andkeeping in mind the complexity of the asthmaticphenotype, it seems unlikely that the increasedprevalence of asthma in the industrialized worldmay be uniquely due to a reduced exposure toinfections during childhood. However, the contro-versial hygiene hypothesis could not be completelyinconsistent, but it probably needs to be revisedand referred only to a limited group of infectiousagents, such as Toxoplasma gondii, Helicobacterpylori and Hepatitis A virus.72–74 The latter appearsin fact to play a relevant protective role against thedevelopment of atopic disorders.74 In any case, a

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growing body of both in vitro and in vivo evidencedoes not corroborate the potential utility forasthma prevention and treatment of the immunos-timulatory approach, also based on the use ofunmethylated cytosine–guanosine (CpG) dinucleo-tides derived from bacterial genomes.75 CpG DNAsequences are potent inducers of the Th1 immuneresponse, which however could result to be notcompletely effective in preventing or curing atopyand asthma, in addition to representing an eventualrisk factor for the development of Th1-mediateddisorders like autoimmune diseases and diabetes.

Conclusions

Respiratory infections may significantly affect, viacomplex cellular and immunological mechanisms,the development and progression of asthma, alsoincluding its exacerbations. On the other hand,routine clinical practice clearly indicates that suchinfections often cause, in asthmatic patients, aworsening of symptoms and a deterioration ofpulmonary function. Therefore, anti-bacterialagents such as macrolides and fluoroquinolonesmay represent a useful therapeutic tool in limitingboth the duration and severity of asthma exacer-bations induced by atypical bacteria like Chlamy-diae and Mycoplasms. Moreover, further progressesmay be achieved in asthma therapy if pharmacolo-gical and immunological research efforts willprovide new molecules and vaccines, able to bereally effective against those viruses most com-monly involved in inducing and exacerbating such awidespread disease.

References

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