Editorial

Allergy: when does it begin and where will it end?

Although established allergic immune responses havebeen well characterized, relatively little is known abouthow or why these develop. There is also still no definitiveexplanation for the enormous increase in affected indi-viduals. The manifestation of allergic symptoms withinmonths (or even weeks) of life strongly suggests that earlyevents (possibly during gestation) are contributing to this.However, although expanding research in this area hasprovided some important information about earlyimmune development, the most critical questions remainunanswered. These issues are discussed here, in thecontext of our existing knowledge and the new observa-tion by Kihlstrom et al. published in this issue (1) thatsensitization to inhaled allergen (birch pollen) is notrelated to antenatal exposure to this allergen.

When are the immunological events that lead to allergyinitiated?

There is still no clear answer to this, and immunedevelopment is likely to vary between individuals accord-ing to both genetic and environmental influences. Col-lectively, the literature supports intuitive notions thatearly events are important particularly during gestationand early postnatal life when immune response to theenvironment are presumably initiated [as discussedrecently (2)].There has been the assumption that, because allergic

(Type 2) immune responses are directed towards aller-gens, that events during first allergen encounter are themost critical. This has also focused great attention on thedetection of allergen-specific immune responses as early

in life as possible, without a clear understanding ofnormal processes or the significance of these responses.While allergen-specific immune responses have beendetected as early as 22 weeks (3), it is likely that otherearly immunological processes, independent of earlyallergen-specific events, may contribute or predispose tosubsequent allergic disease. Potential environmentalexposures and maternal influences during gestation andperinatal life may exert effects through yet poorly definedimmunological pathways. This emphasises the need toexplore this issue widely, and think beyond the presenceor absence of allergen-specific responses. Many potentialinfluences could operate well before the �22-week� time-point, influencing immune development and susceptibilitybefore specific responses to allergens are evident.

Where do these events occur? How well do systemicimmune response reflect tissue-specific events?

Clearly the expression of disease is regulated and possiblyeven initiated in the organs subsequently affected (andassociated lymphoid tissue). This highlights the limita-tions of using cord blood (CBMC) or peripheral blood(PBMC) mononuclear cell responses to assess compleximmunological processes, many of which are likely to beorgan-specific. Although allergens could enter the fetalcirculation through the placenta, exposure through thefetal gut (and lung) has also been proposed (4). Events atmucosal surface are known to be important for initiatingand informing immune development in the postnatalperiod, and may also be initiated in utero. Allergens havebeen detected in amniotic fluid (5) and cells capable of

S. L. PrescottDepartment of Paediatrics, University of WesternAustralia, Perth, Western Australia, PrincessMargaret Hospital, Perth, Australia

Susan L. PrescottAssociate ProfessorDepartment of PaediatricsUniversity of Western Australia, PerthWestern AustraliaPO Box D184,Princess Margaret Hospital, Perth WA 6001Australia

Accepted for publication 25 March 2003

Allergy 2003: 58: 864–867Printed in UK. All rights reserved

Copyright � Blackwell Munksgaard 2003

ALLERGYISSN 0105-4538

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presenting antigens are present in the fetal gut (4).Proteins involved in the activation of APC and pro-Th1signaling (such as sCD14) have also been detected inamniotic fluid and are reduced in those who go on todevelop atopic dermatis (6). Although studies of cordblood have been of great value, they cannot provideinformation about these events, and it may also bedifficult to extrapolate animal models to the humandilemma.

Do fetal responses to allergens indicate previous exposure(cellular memory)?

Firstly, it has not been clear if the fetal T cellsresponding to allergen in vitro have been primed byallergen exposure in vivo. Fetal responses to allergenshave been documented extensively by many groups(reviewed recently by 7) and appear to be of fetal origin(8). This has lead to obvious speculation that allergensencountered by the mother during gestation may bereaching the fetus in some way. While this seems feasiblefor food allergens, which can cross the placenta inanimals (9), transfer of aeroallergens has been moredifficult to conceptualise given the much lower levels ofthese proteins. To address whether allergens can crossthe placenta in humans, Szepfalusi et al. (10) used anovel ex vivo placenta perfusion model to demonstratethat both food and inhaled allergens can pass from thematernal to the fetal circulation. This is not necessarilyphysiological and the transfer of aeroallergens was notas consistent. Thus, the issue of whether the fetus isdirectly exposed to levels of allergens sufficient to induceT cell priming in vivo remains unresolved.Secondly, the significance of fetal responses has been

unclear. Because of controversy about whether the fetusis exposed to allergens, other explanations for theseresponses have been considered. In most cases, CBMCresponses are to allergens to which mother have almostuniversal exposure (such as food allergens and perennialaeroallergens). However, in a number of situationsCBMC responses to specific stimuli have been observedwhen there was no previous documented exposure. Thisincludes responses to tetanus (11) and diphtheria toxoid(12) when there was no apparent exposure to theseantigens in pregnancy. It has been proposed that theseresponses may therefore be nonspecific or because ofcross-reactivity to other antigens (13). There is also nodirect evidence for this. It is not clear what �cross-reacting�antigens could initiate these responses, or why thesewould be generated.A further explanation for this observation has been

that these responses are not true �memory� responses, butsome form of primary response by reactive clones. Inmany ways this also seems unlikely, as the precursorfrequency of un-primed T cells is likely to be extremelylow and generate responses below the threshold of most

assays. Given the relative immaturity of neonatal T cellsand the susceptibility to tolerance, strong allergen-specificprimary responses also seem unlikely. Furthermore, T cellcytokine responses are evident (albeit lower) within asimilar time-frame as adult memory responses. Althoughthe strongest neonatal responses are seen when measuringcytokines [such as interleukin-6 (IL-6) and IL-10] derivedfrom the non-T cell fraction which do not requireprevious exposure, T cell specific cytokines (such as IL-13 and IL-5) are also readily detectable within 24–48 h ofallergen stimulation [(8) and others].Regardless of exposure, there has also been contro-

versy about whether the fetus is capable of generatingclassical �memory� responses as detected in adults usingtraditional lymphoproliferation assays. There are manyclear differences between adult and neonatal T cells,raising concerns that �positive� responses in the fetus maynot have the same meaning. The fact that most (morethan 90%) of fetal T cells are of the CD45RA+ �naıve�phenotype (14) has raised doubts about the likelihood ofa significant repertoire of T cell �memory�. However,one recent study (15) found that 50% of CBMCresponses to grass pollen were mediated by T cells ofthe CD45RO+ �memory� phenotype, which are presumedto have been previously stimulated. More significantly, itis not clear why the remaining 50% of responses weremediated by CD45RA+ cells which are assumed to be�naıve�. As we previously discussed (16), for many reasonsit is probably inappropriate to assume that neonatalCD4 + CD45RO + T cells are equivalent to adultT cells with the same �memory� markers.Thus, further studies are required to determine the

origin and significance of neonatal immune responses toallergens. It is not clear if these responsive clonessubsequently contribute to the memory pool. Whatevertheir origin, these responses are commonly observed andappear to be part of a normal developmental process.Rather than indicating sensitization these early responsesmay be part of the development of normal tolerance.

Is there a relationship between neonatal responsesand allergen exposure in pregnancy?

There are no studies which show a convincing relation-ship between levels of maternal allergen exposure (andpresumed fetal exposure) and the presence, magnitude orcharacter of fetal immune responses to these allergens.Smilie et al. (11) did not find any relationship betweenhouse dust mite (HDM) exposure in pregnancy andCBMC lymphoproliferation. Marks et al. also found norelationship between HDM levels in maternal beds at 36-week gestation and neonatal IL-4, IL-5, IL-10 andinterferon c (IFNc) responses to HDM (17). Others haveeven found negative associations between neonatal HDMallergen-induced proliferation and maternal HDM expo-sure (18).

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Is there a relationship between allergen exposurein gestation and subsequent allergic disease?

While there are many studies linking allergic sensitiza-tion with early postnatal allergen exposure (with therecent exception of pets), there is still considerablecontroversy about associations with antenatal allergenexposure. In this issue, a retrospective study byKihlstrom et al. (1) addressed this question by com-paring clinical outcomes (allergic disease and sensitiza-tion) in two small cohorts of children with differentlevels of maternal birch pollen in pregnancy. Takingadvantage of an unusually high pollen count inStockholm during the spring of 1993, Kihlstrom et al.compared outcomes in infants of mothers exposed tohigh levels of birch pollen while pregnant during thisseason (born autumn 1993) to those with significantlylower maternal exposure the following season (bornautumn 1994). They did not observe any significantdifferences in allergic symptoms and sensitisation at4 years of age. Although there were some trends formore frequent sensitisation to birch with �high� mater-nal exposure, this did not approach statistical signifi-cance. There is no way of determining that theseseasonal differences resulted in major differences in fetalexposure. Even if birch pollen allergens reach the fetus,these would presumably be at extremely low levels, andit is unclear if seasonal fluctuations could change thissubstantially. This situation cannot be extrapolated toanimal models of �high dose� or �low dose� antigenexposure in which tolerance in which high antigenexposure induces tolerance. It is also not clear whetherthere are �more critical periods� during gestation whenexposures are potentially more significant. It is alsoimportant to recognize that numerous potential con-founding lifestyle factors and other environmentalexposures, can make interpretation difficult in popula-tion-based studies of this nature. This is likely to becompounded by retrospective data collection. Howeverthese limitations are extremely difficult to overcome inhuman studies, which can still provide some valuableinformation.These issues aside, this study supports a growing body

of evidence that antenatal allergen exposure is not a keyfactor in the development of atopic sensitization [(18) andothers]. In keeping with earlier studies, Kihlstrom et al.also observed that postnatal allergen exposure was morestrongly related to clinical outcomes than antenatalexposure (1). This reinforces the notion that antenatalallergen exposure is not a detrimental process, andallergen avoidance during pregnancy may have littlebenefit in preventing allergy.

Other unanswered questions

It is not clear when Type 2 responses, which are normalin the neonate, become pathological and lead tosensitization and disease. It is also not clear when (orif) these immune responses become �committed� to aparticular pattern of reactivity. Furthermore, although anumber of subtle differences have been observed whencomparing high risk neonates (of atopic parents) andlow risk neonates (of nonatopic parents), these havebeen conflicting, and are not of predictive value. Morerecent studies suggest that atopic risk (maternal atopy)is associated with higher numbers of Type 2 helper cellCD4+IL13+ cells in cord blood (19) and strongerneonatal IL-13 responses to milk allergen (b lactoglob-ulin) (20). Atopic disease (at 1 year) has also beenassociated with higher neonatal polyclonal IL-13responses (21) or increased numbers of CD4+IL13+cells in cord blood (19). Reduced capacity for Th1 IFNcresponses is another key functional difference in highrisk neonates (22–24 and others), but the significance ofthis is still unclear. Although, this may be because ofAPC immaturity resulting in reduced pro-Th1 signaling,there is no direct proof for this. There may also befunction differences in the capacity of T cells of high riskinfants to respond to IL-12 and other pro-Type 1 signalsfrom APC, as observed recently by Janefjord et al. (25).A great deal more information is needed to understandhow these early processes contribute to establishedallergic responses.

Conclusion

Although allergen exposure in pregnancy does notappear to be a major factor determining the develop-ment of subsequent allergic disease, it is likely thatother early environmental influences have importanteffects on immune development. This is in keeping withthe growing sense that allergens may not play a majorcausal role in the recent increase in allergic disease, butthat other environmental changes are contributing tothis process. Many potential exposures are likely tohave greater effects during formative periods of devel-opment when response patterns are being established.Thus, although allergen exposure in utero may not be amajor issue, immune interactions in this early periodremain fundamentally important in defining allergypathogenesis.

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References

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PA, Bishop GA, Britton WJ. Cordblood mononuclear cell cytokineresponses in relation to maternal housedust mite allergen exposure. Clin ExpAllergy 2002;32:355–360.

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