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Keywords:

  • allergy;
  • asthma;
  • atopy;
  • avoidance;
  • Finland;
  • programme;
  • tolerance

Abstract

  1. Top of page
  2. Abstract
  3. Cellular mechanisms of tolerance
  4. Tolerance vs allergic inflammation of the gastrointestinal tract
  5. Tolerance vs allergic inflammation of the skin
  6. Tolerance vs allergic inflammation of the respiratory tract
  7. Strengthening of tolerance
  8. Psychological tolerance – changes in attitudes
  9. The burden of allergy
  10. Concluding remarks
  11. Acknowledgement
  12. References

 In similarity to many other western countries, the burden of allergic diseases in Finland is high. Studies worldwide have shown that an environment rich in microbes in early life reduces the subsequent risk of developing allergic diseases. Along with urbanization, such exposure has dramatically reduced, both in terms of diversity and quantity. Continuous stimulation of the immune system by environmental saprophytes via the skin, respiratory tract and gut appears to be necessary for activation of the regulatory network including regulatory T-cells and dendritic cells.

 Substantial evidence now shows that the balance between allergy and tolerance is dependent on regulatory T-cells. Tolerance induced by allergen-specific regulatory T-cells appears to be the normal immunological response to allergens in non atopic healthy individuals. Healthy subjects have an intact functional allergen-specific regulatory T-cell response, which in allergic subjects is impaired. Evidence on this exists with respect to atopic dermatitis, contact dermatitis, allergic rhinitis and asthma. Restoration of impaired allergen-specific regulatory T-cell response and tolerance induction has furthermore been demonstrated during allergen-specific subcutaneous and sublingual immunotherapy and is crucial for good therapeutic outcome. However, tolerance can also be strengthened unspecifically by simple means, e.g. by consuming farm milk and spending time in nature.

 Results so far obtained from animal models indicate that it is possible to restore tolerance by administering the allergen in certain circumstances both locally and systemically. It has become increasingly clear that continuous exposure to microbial antigens as well as allergens in foodstuffs and the environment is decisive, and excessive antigen avoidance can be harmful and weaken or even prevent the development of regulatory mechanisms.

 Success in the Finnish Asthma Programme was an encouraging example of how it is possible to reduce both the costs and morbidity of asthma. The time, in the wake of the Asthma Programme, is now opportune for a national allergy programme, particularly as in the past few years, fundamentally more essential data on tolerance and its mechanisms have been published. In this review, the scientific rationale for the Finnish Allergy Programme 2008–2018 is outlined. The focus is on tolerance and how to endorse tolerance at the population level.

Abbreviations
APC

antigen-presenting cell

CD

cluster of differentation

CpG

cytosine guanine (linear dinucleotide)

CTLA

cytotoxic T-lymphocyte antigen

EAE

experimental autoimmune encephalitis

ECRHS

European Community Respiratory Health Survey

FoxP3

forkhead box family transcription factor

GALT

gut-associated lymphoid tissue

GARD

Global Alliance Against Chronic Respiratory Diseases

HDM

house dust mite

ISAAC

International Study of Asthma and Allergies in Childhood

LPS

lipopolysaccharide

MHC

major histocompatibility complex

NALP

NACHT-associated leucine-rich repeat and PYD-containing

NF-κ-B

nuclear factor kappa B

NGO

non governmental organization

NLR NOD

(nucleotide oligomerization domain)-like receptor

PBMC

peripheral blood mononuclear cell

sIgA

secretory IgA

SCIT

subcutaneous immunotherapy

SIT

specific immunotherapy

SLIT

sublingual immunotherapy

TGF

transforming growth factor

TLR

toll-like receptor

TNF

tumour necrosis factor

Tr

Treg T regulatory lymphocyte

WAO

World Allergy Organization

In March 2006, the National Public Health Institute nominated an expert group to evaluate the most recent scientific data and to make a proposal, if justified, for a national allergy programme. The purpose of this programme would be to reduce the burden of allergy in patients and society. In Finland, the trends in occurrence of allergic rhinitis and asthma have been continuously increasing upwards (1), as in many other western countries, and currently more than 40% of the Finnish school children appear to be sensitized to one or more allergens (2). Signs of this problem were discernible already in the 1980s, when the first allergy management guidelines in Finland were published (3), and in 1998, when a consensus report was prepared (4). The time then, however, was not opportune for changes.

The success of the Finnish Asthma Programme 1994–2004 showed that a change for the better could be achieved through an action plan with simple and ambitious but realistic goals (5). A very important factor in this connection is the fact that fundamental new data have been published during the last few years that challenge many of the old dogmas in allergy.

Thus far, little attention in various guidelines and consensus reports, with one exception (6), has been devoted to tolerance. It is, however, becoming increasingly clear that avoidance of allergens is not the right strategy to halt the asthma and allergy epidemic at the population level. Avoidance may be justified in secondary prevention in certain cases, but rather than avoiding allergens, the tolerance in the population must be strengthened.

The expert group recognized that it is now time to act and change the course, and prepared during 2007–2008 the 10-year Finnish Allergy Programme (7), launched in March, 2008, with the support of and in association with the Global Alliance Against Chronic Respiratory Diseases (GARD) (8) programme. The Allergy Programme, more than the Asthma Programme, will focus on children and adolescents, and underscores the importance of strengthening tolerance. The programme comprises six specific goals, each with well-defined tasks, tools and measurements for evaluation. An implementation plan, largely utilizing the network of contact persons (physicians, nurses and pharmacists) created during the Asthma Programme, and an evaluation plan, designed a priori to assess both the efficacy and the process of the programme is also included. The key issues at this stage, indeed, are both implementation and evaluation.

Cellular mechanisms of tolerance

  1. Top of page
  2. Abstract
  3. Cellular mechanisms of tolerance
  4. Tolerance vs allergic inflammation of the gastrointestinal tract
  5. Tolerance vs allergic inflammation of the skin
  6. Tolerance vs allergic inflammation of the respiratory tract
  7. Strengthening of tolerance
  8. Psychological tolerance – changes in attitudes
  9. The burden of allergy
  10. Concluding remarks
  11. Acknowledgement
  12. References

The mammalian immune system protects the host from invading foreign antigens by developing a defence response to those antigens and eliminating them. The function of the defence system is based on its ability to specifically discriminate between ‘danger’ and ‘no-danger’ structures. This function is controlled by the innate immunity and dendritic cells as well as acquired immunity and lymphocytes (9). Thus, the immune system normally not only eliminates deleterious antigens but also develops tolerance against those foreign antigens that are not deleterious but even beneficial. This development of tolerance is crucial in preventing the induction of harmful inflammatory responses. Tolerance prevents self-tissue destruction by own immune system, prevents harmful immune responses against harmless foreign antigens (foods, allergens) and suppresses the appropriate immunological defence responses from proceeding to uncontrolled after the microbes have been destroyed.

The first line of defence system includes the production of antibodies, particulary secretory IgA (sIgA) antibodies by B-lymphocytes. Secretory IgA and other antibodies capture foreign structures, inactivate/neutralize them and prevent the access of such antigens into the tissues beneath the mucosa (10). Secretory IgA is an important factor in the maintenance of tolerance. In animal models, the development of oral tolerance has been found to be associated with high sIgA production in the gut-associated lymphoid tissue (GALT), and in allergic individuals, this sIgA production was shown to be reduced (11).

Central and peripheral T-cell tolerance

T-cell tolerance is classified to central and peripheral tolerance. In central tolerance, the developing T-cells are negatively selected in thymus, and thymocytes with high affinity for self are eliminated already in early stages of differentiation. In peripheral tolerance, the action of autoreactive lymphocytes is eliminated after they have emigrated from the thymus or bone marrow. The essential role of regulatory T-cells in maintenance of peripheral tolerance and the importance of continuous suppression of autoreactive T-cells that escape central tolerance mechanisms has only recently been recognized (12).

The bulk of the data on mechanisms of tolerance has been obtained from studies of oral tolerance. Based on these studies, peripheral tolerance has further been defined as high-dose- and low-dose tolerance (13, 14). In high-dose tolerance, anergy of T-cells and T-cell deletion by programmed cell death appear to be the major mechanisms (14, 15). At lower antigen doses administered repeatedly, tolerance is achieved by active suppression of T effector-cells by the regulatory T-cells (14). The prerequisite for the development of tolerance is repeated exposure to antigens. The development of ‘high-dose’ and ‘low-dose’ exposure in the development of oral tolerance is presented schematically in Fig. 1.

image

Figure 1.  T-cell activation requires the recognition of the presented antigen-MHCII complex and in addition the concomitant costimulatory (B7-CD28) and cytokine signalling. Administration of high doses of antigen leads to antigen presentation to T cell receptor without concomitant costimulatory signalling or soluble signalling by cytokines causing anergy of T-cells. Alternatively in high-dose tolerance antigen presentation with concomitant Fas receptor ligation leads to T cell deletion by programmed cell death (apoptosis). When lower doses of antigen are repeatedly administered low dose tolerance is achieved by active suppression of T effector cells by the cytokines (IL-10, TGF-beta) of regulatory CD4+ T cells.

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It must, however, be noticed that the route of exposure may play a decisive role in this context. Most recently, tolerance induced by high-dose allergen-exposure via skin in beekeepers was associated with rapid switch and expansion of IL-10-producing regulatory T-cells (16), in agreement with earlier studies including data on high-dose allergen injections during maintenance of subcutaneous immunotherapy (SCIT), high-dose sublingual immunotherapy (SLIT), high-dose indoor exposure to pet allergens, and repeated numerous bee stings in the beekeepers (17–20). In all of these cases, however, another mechanism, the Th2 to Th1 immunodeviation during high-dose exposure must be regarded as another possible existing mechanism (21–24), not necessarily excluding each other.

Regulatory T-cells and cytokines in atopics and healthy

Evidence has accumulated to support the presence of naturally occurring thymus derived regulatory T-cells and inducible regulatory T (Th3, Tr1) cells that are CD4+CD25+ positive. The naturally occurring CD4+CD25+ T regulatory cells likely include subgroups and express variably different cell surface markers (e.g. CD45Rblow, CTLA-4) and transcription factors, especially FoxP3 (forkhead box family transcription factor). FoxP3 is crucial for both the differentiation of regulatory T-cells and the maintenance of their suppressive function (25, 26).

Inducible regulatory T-cells produce IL-10 and TGF-beta, by which the suppressive effects, at least partly, are mediated (27). Interleukin (IL)-10 is a dendritic and T-cell derived anti-inflammatory cytokine that suppresses both Th1- and Th2-type immune responses (28–30). In addition, IL-10 has direct inhibitory effects on mast cells, basophils and T-cells, in which it can induce an anergic state (31–33). Transforming growth factor (TGF)-beta antagonizes both Th1- and Th2-type inflammatory responses and is a crucial cytokine in the immune suppression induced by oral tolerance (34–36). The regulatory cytokine TGF-beta also induces the production of IgA (37). Interleukin (IL)-10 controls the effects of TGF-beta on T-cells by regulating the expression of its receptor (38). Recently found novel suppressive cytokine IL-35, produced by T regulatory cells, is important for the achievement of maximal T-cell suppression (39). Natural CD4+CD25+FoxP3+ T regulatory cells in turn require a cell-cell contact mediated through cell surface bound TGF-beta for maximal suppression (40). Regulatory T-cells and their role in tolerance vs disease have been comprehensively reviewed elsewhere (e.g. in 41–44).

Toll-like receptors and Nod-like receptors.  The regulatory network including, in addition to regulatory T-cells, the dendritic cells and the cytokines produced by these cells, is the major player in the development of tolerance. Toll-like receptors (TLR), the conserved pattern-recognizing molecules expressed by the innate immune cells, are central in capturing microbial components that invade into the body. At least 10 functional TLRs in humans have been identified (45), most of which have a rather limited repertoire of specific ligands. An exception is TLR2 that interact with Gram-positive bacteria and their cell-wall components, lipoteichoic acids, lipoproteins and larger molecules of peptidoglycans, mycobacterial cell-wall components, fungal zymosan and phospholipomannans, and with some parasite cell-wall components (45, 46). All these molecules are abundant in microbe-rich natural environments.

Toll-like receptors (TLRs) are not the only pattern-recognition molecules that capture microbial structures.

Recent research has revealed that besides TLRs located on cell surfaces or endosomes, there are receptors located in the cytosol, called Nod (nucleotide-oligomerization domain)-like receptors (NLR). To date, more than 20 members of the NLRs have been identified, most of which are still poorly characterized (47). Among the best characterized NLRs are NOD1 and NOD2. Both molecules detect peptidoglycan fragments. NOD1 senses those containing diaminopimelic acid typically found in the thin peptidoglycan of Gram-negative bacteria, but also in some Gram-positives (Bacillus sp., Listeria sp.), whereas NOD2 detects muramyl dipeptides released from bacteria of the Gram lineages as degradation products (47–49). In addition, other members of the NLR family, such as NALP receptors, have recently gained considerable attention. NALPs, as a result of interaction with microbial or host danger structures, form protein complexes, called inflammasomes (48, 50), providing a platform for caspase-1 activation. Caspase-1 is necessary for processing of pro-IL-1-beta and pro-IL-18 to their active forms (47, 48).

The decisive role of TLRs in the integrity and maintenance of mucosal tolerance has been demonstrated (51). In addition, NOD2 polymorphism has been strongly linked with Crohn’s disease (52, 53), NOD1 polymorphisms with inflammatory bowel disease (54), atopic eczema (55), allergies (56) and asthma (57), indicating that these receptors are also involved in mucosal integrity and tolerance. Although the exact mechanisms underlying the development of inflammation vs tolerance in these mutations are not yet clear, it is becoming increasingly evident that altered intracellular bacterial recognition is one of the key events in pathogenensis of these diseases (54).

Interaction of microbial components with both TLRs and NODs leads to NF-kappa-B activation, and in the case of NALPs, the formation of inflammasomes and activation of caspase-1, which finally results in the release of proinflammatory cytokines. In response to commensals and saprophytes, inflammatory responses must become silenced. The induction of regulatory network in the absence of true danger signals from the invasive pathogens and/or injured host cells are likely to be involved (Fig. 2).

image

Figure 2.  Development of the two main arms leading to tolerance (saprophytes) and inflammatory responses (pathogens). Different NLRs can sense different microbial structures but also danger signals derived from the host. Additional danger signals from injured host cells and/or from pathogens appear to be required for the induction of inflammatory responses. There is a complex interaction between various signalling pathways and NLRs can co-operate with each other and with TLRs to induce appropriate responses (ref. 47). In addition to NLRs, other cytosolic receptors, such as Rig-1 and Mda5, have been identified but their role is less clear. NLR, Nod-like receptor, TLR, Toll-like receptor.

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Atopics vs healthy.  In allergy, tolerance against environmental allergens does not develop normally but results in prolonged inflammatory responses against them. The crucial observations on the balance between peripheral tolerance and allergy, in mice and men, are associated with regulatory T-cells and cytokines IL-10 and TGF-beta. It is now widely accepted that the question in broken tolerance is largely of an imbalance between different T-cell types. Atopic and healthy individuals appear all to have allergen-specific Th1 and Th2 and regulatory T-cells, but in different ratios. Regulatory T-cells and their immune responses against allergens seem to be a part of healthy immune system. Indeed, Tr1 clones transferred to mice sensitized to ovalbumin inhibited the allergen-specific IgE response (58). This finding gave reason to believe that regulatory T-cells could have a substantial role in the regulation of the balance of tolerance and allergy in man as well. Confirmation was achieved from two parallel studies addressing the allergen-specific responses of CD4+ T-cells in pollen-allergic patients with allergic rhinitis and non atopic healthy controls (59, 60).

These studies showed that the function of regulatory T-cells or the balance between specific regulatory and Th2 cells in atopics is disturbed. It is noteworthy that the prerequisition for tolerance induction is the development of allergen-specific regulatory T-cells activated by a specific antigen. However, activated inducible regulatory T-cells can secrete regulatory cytokines that suppress immune responses even to unrelated antigens in a non specific manner (bystander effect) (61) (Fig. 3).

image

Figure 3.  Tolerance mechanisms may induce bystander effects. Activated regulatory T cells secrete suppressive/regulatory cytokines (TGFβ and/or IL-10) in an antigen-specific manner. These cytokines may then suppress unspecifically ongoing immune responses even to unrelated, co-localized antigens (61).

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Cord blood regulatory T-cell responses and tolerance

Recent findings suggest that the regulatory T-cell responses play an important role in the development of tolerance vs allergies very early in life. Regulatory cell functions are crucial already at birth as cord blood regulatory T-cell responses appear to predict later development of allergy. The cord blood CD4+CD25+ T-cell functions were studied in egg-allergic and healthy infants (62). Although the number of CD4+CD25+FoxP3+ cells did not differ between the groups, the suppressive function of these cells in egg-allergic infants was clearly diminished (62). Concordant results have been obtained when cord blood regulatory T-cells of atopic and non atopic mothers have been compared (63). It seems that the imbalance between tolerance and allergy and the impaired function of regulatory T-cells exist at birth and evidently even during foetal life.

Tolerance vs allergic inflammation of the gastrointestinal tract

  1. Top of page
  2. Abstract
  3. Cellular mechanisms of tolerance
  4. Tolerance vs allergic inflammation of the gastrointestinal tract
  5. Tolerance vs allergic inflammation of the skin
  6. Tolerance vs allergic inflammation of the respiratory tract
  7. Strengthening of tolerance
  8. Psychological tolerance – changes in attitudes
  9. The burden of allergy
  10. Concluding remarks
  11. Acknowledgement
  12. References

The mucosa forms a border between the cavities of the body, e.g. the gastrointestinal tract, and the environment. Thus, intact mucosa prevents the invasion of environmental micro-organisms and macromolecules, such as proteins, into the tissues (64). The mucosa is, however, not completely impermeable, and small amounts of antigens can pass the mucosa to interact with lymphocytes in subepithelial layers. It has been shown both in animal models and in humans that in healthy individuals, antigens that have been administered repeatedly to the mucosa induce the development of tolerance against those antigens, e.g. pollen, pet and food antigens, preventing the development of deleterious inflammatory responses (65). Tolerance of the gut mucosa means that an antigen exposed via ingestion or inhalation results in anergy of the immune response: lymphocytes are first activated but the response is rapidly silenced and the regulatory network induced (65). The prerequisite for the development and maintenance of oral tolerance is the presence of the commensal-rich gut microbiota (66, 67).

Oral tolerance and food hypersensitivity

Oral tolerance means the antigen-specific tolerance induced in gastrointestinal immune system against antigens ingested orally (68). The mechanisms of oral tolerance have been difficult to address experimentally and several theories on the immunological basis for oral tolerance have existed. Nonetheless, regulatory T-cells and IL-10/TGF-beta appear to be the major players in natural and acquired tolerance to food antigens.

Data of mechanisms underlying the development of oral tolerance have been largely obtained from studies of sublingual immunotherapy (see Chapter 6.2.) which support the decisive role of regulatory T-cells and the cytokines IL-10 and TGF-beta in this context (69, 70). The original discoveries of both Th3 and Tr1 cells in mice were the first demonstrations of T regulatory cells and IL-10/TGF-beta playing a major role in suppression of inflammation in experimental autoimmune encephalitis (EAE) and autoimmune colitis by oral tolerance (35, 71). Clonal deletion of antigen-reactive T-cells was additionally shown to be one of the mechanisms of oral tolerance (15).

Major players in the development of oral tolerance are the gut epithelial cells, dendritic cells and macrophages. The interplay between all of these cells is central; human gut epithelial cells secrete thymic stromal lymphopoietin, TGF-beta and retinoic acid that steer the development of tolerogenic dendritic cells. These dendritic cells induce in turn the development of naïve T-cells into Foxp3+ regulatory T-cells by mechanisms that are further TGF-beta- and retinoic acid dependent. Macrophages become tolerogenic by stromal cell-derived TGF-beta, and are also able to induce Fox3p+ regulatory T-cells in a manner similar to that of the dendritic cells (reviewed in 72).

The main theories for mechanisms of oral tolerance proposed over 10 years ago are generally still valid: small antigen doses induce primarily an antigen-specific suppression by regulatory T-cells and high antigen doses induce clonal deletion or anergy of T-cells (13–15, 35). Further studies in murine models have shown that regulatory T-cells and cytokines in Peyer’s patches are pivotal in induction of tolerance to food. Indeed, several studies using beta-lactoglobulin as antigen have revealed that oral tolerance is induced by regulatory T-cells and IL-10 and TGF-beta in Peyer’s patches (73, 74).

These mechanisms described in mice are likely to act in men as well. Studies in children allergic to milk showed that T-cells isolated from small intestine mucosa produced Th2-type cytokines IL-5 and IL-13 but not IL-10 or TGF-beta when stimulated with milk protein (75). A prospective study among children with cow’s milk allergy revealed that the number of CD4+CD25+ regulatory T-cells was increased in peripheral blood of children who recovered from IgE-mediated milk allergy as compared with children with persisting allergy (76). Most recently, patients tolerant to heat-denaturated milk showed to have an edge of T regulatory over effector cells. These cells expressed high levels of CD25, FoxP3, and CTLA-4 and were functionally suppressive (77). In adults allergic to hazelnuts, the PBMC cytokine production was studied after sublingual immunotherapy with biologically standardized hazelnut extract. In the actively treated group, the symptom provoking dose in a double-blind, placebo-controlled food challenge increased significantly and was associated with increased IL-10 response in PBMC not found in the placebo group (78). Food allergy vs tolerance has been thoroughly reviewed elsewhere (79, 80).

Most children have a time window in the development of a tolerance response, i.e. repeated exposure to an antigen early, but not too early, leads to tolerance. In a prospective study among infants in the USA, exposure to wheat before the age of 6 months of age reduced occurrence of allergy as compared with children who were given wheat first after 7 months of age (81). It appears that the current recommendations of the 4–6 months’ exclusive breastfeeding are well justified in respect of allergy development (82). Most children outgrow their food allergy quite rapidly, and the mechanisms, i.e. whether it is IgE-mediated or not, behind this condition crucially affects the prognosis. In our prospective study, 10% of the children with IgE-mediated cow’s milk allergy were allergic at the age of 9 years, whereas all children with non IgE-mediated allergy recovered by the age of 4 years. Most children were tolerant to cow’s milk already at the age of 2 years (83). Similar results have been published from a study among egg-allergic children (84).

Maturation of mucosal tolerance

Critical factors in the development of mucosal tolerance, in addition to repeated exposure and the dose of antigen, are the integrity of the mucosa and the maturation stage of mucosal lymphocytes. In infancy, when the mucosa is not fully maturated, the mucosal permeability initially decreases, numbers of mucosa-associated lymphocytes increase and sIgA production starts. In addition, microbiota colonizing the gut mucosa and the food given to the baby play a role, thus many factors concomitantly affect the immune response of a newborn. It is well-known that breast milk contains many factors that affect beneficially the development of gut tolerance, e.g. TGF-beta and polysaccharides that enhance the colonization of gut mucosal flora (85). Thus, exposure to foreign proteins too early in life (<4–6 months) can impair the development of oral tolerance and lead to adverse reactions. On the other side, it is equally clear that the proper development of tolerance against food, pollen and pet allergens requires continuous exposure to these allergens; avoidance can even be deleterious and prevent the development of normal regulatory mechanisms.

Development of gastrointestinal tolerance-clinical perspective

The atopic march.  Food allergy is frequently the first manifestation of atopic disease. Allergies can roughly be categorized as atopic IgE-mediated or non IgE-mediated. For example, of all infants with cow’s milk allergy, the condition in two-thirds of children is IgE-mediated, whereas in one-third of them, no IgE response can be shown at the time of diagnosis (83). The most typical symptoms manifest in skin, gastrointestinal tract, and, more rarely, in respiratory tract. Infant eczema is difficult to define because it can be associated with food hypersensitivity, but it can also be a completely independent condition without any hypersensitivity component. It has, however, been convincingly shown that in children with severe atopic eczema, IgE-mediated food allergy is considerably more common than in children with mild disease, or with no disease at all (86). A part of these allergic children get symptoms immediately, a part with a delay after exposure to the food in question. In both groups, IgE antibodies can be detected, either specifically to that food or to some other food, such as eggs. However, in children with delayed type reaction, the non IgE-mediated mechanism is more common.

Dilemma in allergy prevention among children – intervention or not?  Prevention of food and other allergies in children is desired but difficult. Most immunological measures, such as an elevated cord blood IgE level, egg-specific IgE level, biased cytokine production such as a decreased IFN-gamma/IL-4 ratio in early life, are associated with the development of allergies in later life, and predict to some extent the manifestation of asthma. Nonetheless, none of the indicators is sensitive or specific enough to tell which child really has a high-risk for disease and to whom the intervention should be targeted (87). Family history, e.g. atopy in parents and/or siblings, remains still the best identified risk factor. Another major problem is what would be the proper intervention. Accumulating data now show that dietary interventions during pregnancy or breastfeeding do not work well in the prevention of atopy. A complete avoidance of egg and cow’s milk during the third trimester did not reduce the numbers of children with food allergy or sensitization (88). Elimination of central foods from the diets of pregnant mothers is therefore not recommended (89). There is a large body of data of the effect of allergen avoidance during the breastfeeding period on the occurrence of allergic diseases in the infant. Prevalence of atopic eczema appears to decrease in infants along with the mothers’ dietary interventions according to some studies but contradictory results have also been reported (89, 90). It is possible, to some extent, to reduce or delay the development of specific allergy in the child. However, the effects using different dietary manipulations are mostly only transient, and no long-term effects are to be expected, either in prevention of food allergy or respiratory allergy.

Management of childhood food allergies now and in the future

During the last few years, attention has been devoted rather than to palliative treatment- to treatment that affects mechanisms underlying the development of tolerance leading to recovery. Specific immunotherapy (SIT) of food allergies using subcutaneous injections was earlier tested, but discontinued because of severe adverse effects and inconsistent results. Along with increasing understanding of the mechanisms involved in the development of tolerance, oral tolerance induction is raising growing interest. Evidence of the efficacy in food allergy is, however, still limited. Notably, in many children, who have been partly tolerant to the food, the partial tolerance is broken when the child is put on an elimination diet. A complete elimination diet should thus not be started on the basis of a low IgE response in a preschool or school-aged child. Even after some months’ elimination diet, sensitivity could have paradoxically increased and later contact with that antigen can lead to anaphylaxis (91). An Italian study showed that children with severe cow’s milk allergy (n = 21) who were given milk per os at growing doses for 6 months, 71% (15 children) tolerated milk even 2 dl/day after the treatment (92). This effect was sustained after 4 years (93). Nonetheless, these mechanisms do not apply in food hypersensitivity caused by foods directly releasing histamine or containing biogenic amines.

Data obtained from studies of mucosal tolerance have provided an impetus for the development of new treatment strategies to food and respiratory allergies. These treatments are based on repeated administration of increasing doses of the allergen orally or via respiratory mucosa. Adequate doses and routes are sought in ongoing clinical trials, and more conclusive results are still to be awaited. One pitfall of oral sensitization is that at least in some patients, maintenance of tolerance requires continuous exposure to the allergen (94). Another line of treatment, for which only preliminary data are available, is the anti-IgE treatment. A study from the USA showed that in children with severe peanut allergy, the anti-IgE treatment for 16 weeks increased considerably tolerance against peanuts, on the average from a half to nine peanuts (95). Anti-IgE treatment may help in the early stages of tolerance induction by preventing symptoms caused by the allergen in question, and may be withdrawn when some tolerance is obtained. Studies to prove the usefulness of this approach are, however, lacking.

Tolerance vs allergic inflammation of the skin

  1. Top of page
  2. Abstract
  3. Cellular mechanisms of tolerance
  4. Tolerance vs allergic inflammation of the gastrointestinal tract
  5. Tolerance vs allergic inflammation of the skin
  6. Tolerance vs allergic inflammation of the respiratory tract
  7. Strengthening of tolerance
  8. Psychological tolerance – changes in attitudes
  9. The burden of allergy
  10. Concluding remarks
  11. Acknowledgement
  12. References

The tolerance induced by regulatory T-cells appears to have an important role in the regulation of the allergic inflammation of both atopic dermatitis and contact dermatitis. Even though the effector mechanisms in these inflammatory responses in skin differ, the regulatory cells appear to act as suppressors of inflammatory responses in both diseases.

Atopic dermatitis

In atopic dermatitis the acute skin lesions are characterized by the presence of Th2-type cells. This initial Th2-type cellular response is deviated to a Th0/Th1 response by induction of IL-12 in antigen-presenting cells, keratinocytes and eosinophils (96–98). These cutaneous Th1 cells synthetize Fas-ligand and produce pro-inflammatory cytokines IFN-gamma and TNF-alfa, which induce Fas-receptor expression on the keratinocyte surface (98, 99). The ligation of the Fas-receptors by the Fas-ligands incurs a keratinocyte to apoptosis, which leads to chronic inflammation because of pro-inflammatory cytokines released from the dying keratinocytes (99).

A study of the function and cytokine expression of human regulatory T-cells in lesional atopic skin revealed that the inflammatory reaction of atopic skin was associated with defective Tr1 function and the absence of skin infiltrating CD4+CD25+FoxP3+ regulatory T-cells, in other words inadequate induction of tolerance (100).

Contact dermatitis

In contact dermatitis, the early inflammatory response is apparently initiated by elements of innate immunity: NK-cells, dendritic cells and toll-like receptors TLR2 and TLR4 (101). The inflammatory reaction is soon accompanied by cytotoxic CD8+ cells that are able to rapidly recognize the hapten allergen presented along the MHCI complex of the antigen-presenting cells. The activation of the CD8+ T-cells leads to production of IFN-gamma and TNF-alfa and the subsequent Fas-mediated keratinocyte apoptosis (102). These cytokines activate a slower Th1-type inflammatory reaction based on hapten allergen recognition with the MHCII complex by CD4+ T-cells, resulting finally in keratinocyte apoptosis and actuation of chronic inflammation (102).

This described proinflammatory effector T-cell function in contact eczema can in mice be inhibited by IL-10 (103, 104). There is evidence that an experimentally induced dermal tolerance in mice can be transferred by CD4+CD25 cells, but it also requires the presence of IL-10 (105, 106). Nickel-specific, IL-10-producing peripheral blood CD4+CD25+ T-cells of healthy subjects are able to regulate the function of both naive T-cells and Nickel-specific memory cells, whereas the peripheral blood CD4+CD25+ T-cells of nickel-allergic patients lack almost completely regulatory effects (102, 107, 108). Thus, even in the skin the inadequate regulatory T-cell responses and impaired tolerance induction appear to be behind the allergic inflammation.

Skin microbiota

Little is thus far known of the role of skin microbiota in the development and maintenance of tolerance in skin. Modern non culture technologies have provided evidence of the vast diversity and quantity of members in the normal skin (109). For example, a hand surface, on the average, harbours more than 150 different phylotypes (species), mostly members of the genera Propionibacterium, Streptococcus, Staphylococcus, Corynebacterium and Lactobacillus. The diversity of the skin flora is generally higher in women than in men. A core set of bacteria (approximately 13% of all phylotypes) appears to be shared by most individuals, but great intra- and inter-individual variability in skin microbiota also occurs (109) It is becoming increasingly clear that TLRs are expressed on a variety of skin cells, including keratinocytes and Langerhans cells in the epidermis, macrophages, dendritic cells, T, B and mast cells in the dermis, endothelial cells in the skin vasculature, as well as in skin stromal cells, fibroblasts and adipocytes. TLRs have been found to play a pivotal role in skin defence mechanisms and evidently also in inflammatory skin diseases (reviewed in 110), but the role of skin microbiota and TLRs (and NLRs) in tolerance remains to be elucidated.

Tolerance vs allergic inflammation of the respiratory tract

  1. Top of page
  2. Abstract
  3. Cellular mechanisms of tolerance
  4. Tolerance vs allergic inflammation of the gastrointestinal tract
  5. Tolerance vs allergic inflammation of the skin
  6. Tolerance vs allergic inflammation of the respiratory tract
  7. Strengthening of tolerance
  8. Psychological tolerance – changes in attitudes
  9. The burden of allergy
  10. Concluding remarks
  11. Acknowledgement
  12. References

Asthmatic inflammation

Although the family history of asthma or atopic condition is the strongest known risk factor of asthma, most asthmatics have been found to come from families with no history of atopic disease (111); the impact of environmental factors must thus be fundamental. It has also been found that certain gene polymorphisms is associated with the disease only in certain circumstances (e.g. in urban environment) whereas in other circumstances (e.g. in farm environment), the same gene polymorphism exerts no increased risk but rather confers protection (112). It has become clear that along with urbanization, the environmental microbiota –the crucial component in the normal development of tolerance- has in terms of both quantity and diversity dramatically shrunk.

Development of respiratory tolerance

There is evidence that regulatory T-cells control and suppress asthmatic inflammation. In mice sensitized to ovalbumin, an optimal dose of ovalbumin administrated in trachea caused an eosinophilic inflammation, whereas higher doses of ovalbumin induced the attenuation of IL-4 responses in local lymph node T-cells (113). When transferred to mice sensitized to ovalbumin, these lymph node T-cells induced antigen-specific suppression that could be inhibited with neutralizing anti-TGF-beta antibodies, implying that regulatory T-cells suppress mucosal eosinophilic inflammation of the lower airways (113). More recently, antigen-presenting dendritic cells were found to be the key players in differentiation of CD4+ T-cells to Th2 or T regulatory cells in mice sensitized to ovalbumin (114). Both myeloid and plasmocytoid dendritic cells could ingest, process and present inhaled allergens to T-cells. However, mice depleted of plasmocytoid dendritic cells developed eosinophilic asthmatic inflammation, IgE response and production of Th2-type cytokines. The transfer of plasmocytoid dendritic cells into these mice before sensitization suppressed the asthmatic inflammation, IgE response and production of Th2-type cytokines. Myeloid dendritic cells thus induced, by action of Th2 effector cells, an asthmatic inflammatory response, which was inhibited by regulatory T-cells induced by plasmocytoid dendritic cells (114).

Similar observations have also been made in man. The expression of FoxP3, IL-10 and TGF-beta mRNA in CD4+ T-cells isolated from peripheral blood was compared between asthma patients on glucocorticosteroids and healthy controls (115). FoxP3 mRNA expression was found to correlate with IL-10. This cytokine was also produced in higher levels in asthmatic patients receiving either inhaled or systemic steroids. Courses of systemic steroids caused short-term increases of FoxP3 mRNA expression and a long-term increase of CD4+CD25+ T regulatory cells (115). Asthmatic inflammation appears thus to be associated with an inadequate regulatory T-cell response; reduced inflammation achieved by corticosteroids gives way to immunomodulation and induction of tolerance.

Data obtained thus far mainly from animal models suggest that administration of allergen in certain circumstances both locally and systemically can restore tolerance and prevent the development of respiratory allergy in sensitized individuals (116). Even inflammation mimicking allergen-induced asthma in animal models could have been alleviated with repeated allergen-exposure into the respiratory tract, although data from animal models must be interpreted with caution. The dose of allergen and duration of exposure appear to be central to this issue.

Strengthening of tolerance

  1. Top of page
  2. Abstract
  3. Cellular mechanisms of tolerance
  4. Tolerance vs allergic inflammation of the gastrointestinal tract
  5. Tolerance vs allergic inflammation of the skin
  6. Tolerance vs allergic inflammation of the respiratory tract
  7. Strengthening of tolerance
  8. Psychological tolerance – changes in attitudes
  9. The burden of allergy
  10. Concluding remarks
  11. Acknowledgement
  12. References

It was earlier widely believed that avoidance of allergen-exposure can prevent the development of allergy. Although in some situations, e.g. in the case of occupational allergic diseases, there is a clear cause and effect relationship between exposure and disease, an increasing body of evidence from the past decade shows that the disparities in prevalence of allergic diseases between different populations cannot be explained by differences in allergen-exposure.

The current view is that the origin of allergies is indeed in early life. Along with exposure to common environmental allergens, the infant normally develops tolerance, however, in a part of children, hypersensitivity, instead of tolerance, develops. Which course the development will take, depends on complex gene-environment interactions. From the perspective of prevention, a fundamental finding in the past few years has been that exposure to environmental saprophytic micro-organisms is beneficial, even necessary for the normal development of tolerance (117). It was earlier largely thought that particularly intense systemic infections may confer protection. Although the role of childhood infections cannot be wholly ignored (118), the view that has currently gained wide acceptance is that more essential is the overall microbial exposure, both by environmental saprophytes and commensals in the normal flora, particularly in the gut. Not only live cells but even microbial components from dead cells are immunologically active (119, 120).

Disparities in asthma/allergy prevalences even between adjacent areas have been explained by differences in exposure to (i) pathogens, (ii) bacterial components from environmental saprophytes, (iii) gut microbiota, (iv) farm animals and farm milk, and (v) parasites. All these factors are, however, associated in a way or another with microbe-rich environments – farm environments or simple living conditions in non affluent areas. The total exposure via skin, inhalation and gut as well as the diversity of this exposure are apparently the key issues. Sedentary lifestyle and living in urban built environment have greatly shrunk our exposure to environmental saprophytes; modern cleaning practices have further strengthened this phenomenon. Not surprisingly, the very low diversity of bacteria and the near to absence of environmental species characterize urban house dust (121). The parallel trends of two indicators directly associated with environmental saprophytic exposure, the asphalt index and the proportion of households with vacuum cleaners, and asthma are depicted in Fig. 4.

image

Figure 4.  Parallel trends in prevalence of asthma among young men (dotted-dashed line) and the two variables directly associated with environmental microbial exposure, the asphalt index (solid line) and the proportion of households with vacuum cleaners (dotted line) in Finland in 1960–1990. Data on asthma prevalence obtained from Latvala et al (1), data on the asphalt index from von Hertzen et al (117) (both left y-axel), and data on proportions of households with vacuum cleaners from the Association of Electric Companies (right y-axel).

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Another fundamental question relates to timing of exposure. Although it seems that the crucial period for immunomodulation is in early life (122, 123), several studies have provided evidence that immunomodulation may also occur, at least to some extent, even in adulthood. (124, 125).

One of the central issues in the Finnish Allergy Programme is tolerance. How and when tolerance could be strengthened in the population? There are several ways to affect innate and acquired immunity (Table 1).

Table 1.   Endorsement of immunological tolerance
  1. *Efficacy not yet proven in humans

Unspecific and specific ways to affect innate immunity
 Living on a farm
 Adherence to traditional lifestyle
 Adherence to anthroposophic lifestyle
 Marked exposure to allergens
 Use of probiotics
 Use of other bacteria-based products*
 Consumption of farm milk
 Consumption of kefir*
 Breastfeeding for 4–6 months
 Cessation of smoking during pregnancy
Specific ways to affect acquired immunity
 Specific subcutaneous (SCIT) or sublingual (SLIT) immunotherapy

Unspecific and specific ways to affect innate immunity

Farm environment, simple living conditions and anthroposophic lifestyle.  Numerous studies have provided evidence that farming environment and simple living conditions confer protection against atopy and atopic disease, although the precise nature of the factor(s) involved remains to be clarified. One central protective factor emerged in many studies is the child’s contacts with farm animals (reviewed in 126,127). Another factor that has also been identified to strongly protect against allergies is the consumption of farm milk. Most recently two large European studies have corroborated this finding (128, 129).

The Karelia Allergy Study has shed new light on the question of disparities of allergy prevalence between two adjacent areas, and has provided evidence of the importance of environmental microbiota. In our earlier study among adults in 1998, we found that the occurrence of asthma and atopy was substantially lower in Russian Karelia than on the other side of the border, in Finnish Karelia, irrespective of similar geoclimatic and vegetative conditions (130). A further study among schoolchildren and their mothers in 2003 revealed that the risk for atopy was several-fold higher among Finnish children and mothers as compared with their Russian counterparts (2). Interestingly, not only in the occurrence of allergic diseases, but even in that of autoimmune diseases such as type 1 diabetes, the difference between Finnish and Russian Karelia is substantial (131).

In a microbe-rich environment, the continuous stimulation of innate immunity by microbial cell-wall components activates the regulatory network, which in turn appears to be involved in the development of tolerance and prevention/downregulation of deleterious inflammation and Th2 responses characteristic of atopic conditions (132). Much of the research has focussed on endotoxin (LPS), the major cell-wall component of Gram-negative bacteria, bacterial DNA and probiotics as immunomodulatory agents. Besides inhaled air, food and drinking water (133) are major sources of microbial exposure, and data are indeed accumulating that Gram-positive bacteria may predominate in different natural environments (117) and in normal skin, respiratory tract and gut microflora (134–136). Gram-positive bacteria do not contain LPS, but have peptidoglycan and teichoic acids in their cell wall (117).

Surprisingly few studies thus far have examined bacterial components other than endotoxin in house dust in the context of atopy and allergies. Concentrations of muramic acid, particularly abundant in Gram-positive bacteria, have been shown to be inversely associated among children with wheezing (137). In addition, the fungal cell-wall components beta-glucan and extracellular polysaccharides have been reported to have some protective effect against allergic conditions (138). There is evidence that particularly TLR2, the receptor for Gram-positive bacteria, lipoteichoic acid, mycobacterial and fungal components, and TLR9, the receptor for bacterial unmethylated DNA (reviewed in 45), may be of importance in this respect (139–143). Interestingly, farmers’ children were found to express increased levels of TLR2 (and of TLR4 and CD14) (144), and TLR2 polymorphism among farmers’ children emerged as a determinant for occurrence of atopic disease (112). Moreover, we found a striking predominance of Gram-positive bacteria, ligands of TLR2, in Russian house dust (145) (Table 2). Nonetheless, the low atopy risk associated with farming environment and simple living conditions is likely to be the result of an interplay between numerous factors.

Table 2.   Distribution of gram-positive and gram-negative bacteria identified by gene sequencing (A) and concentrations of bacterial components, determined by chemical analyses, and dust mites by microscopic analysis in house dust in Finnish and Russian Karelia. Data based on 10 randomly selected pooled floor dust samples from Finnish (n = 349) and Russian (n = 417) households. Modified from ref (145)
  1. *Species identified: Dermatophagoides sp., Tydeus sp., Glycyphagidae sp., Acarus sp., Euroglyphus sp., Mesostigmata sp., Lepidoglyphus sp.

 FINRUSP-value
(A)
 No of clones231234 
 Gram+%% 
  Actinobacteria20.319.7 
  Firmicutes13.947.0<0.001
 All Gram+34.266.7<0.001
 Gram−
  Proteobacteria33.319.7<0.001
  Bacteroidetes4.30.90.021
  Other Gram−2.03.8 
 All Gram−39.624.4<0.001
 Chloroplasts24.73.0<0.001
 Unclassified bacteria1.36.0 
(B)
 Bacterial components
  Total DNA μg/g 56.571.5 
  Bacterial DNA, μg/g (% of total DNA)2.5 (4.4%)6.5 (9.0%) 
  Muramic acid, ng/mg7.064.5 
  LPS, nmol/mg0.0190.038 
 Mites, no./g<detection limit125* 

In addition to farm environment and simple living conditions, anthroposophic lifestyle has been associated with reduced risk of atopic disease, albeit to a lesser extent as compared with farm environment (146, 147). Antroposophy is characterized by restricted use of antibiotics, antipyretics and vaccination, and frequent consumption of fermented vegetables. In this respect, a recent publication of the typical fermented food, sauerkraut, is interesting. Sauerkraut was found to contain lactic acid bacteria in very high concentrations and their diversity was greater than previously believed (148). Besides anthroposophic families, sauerkraut has traditionally been widely used in many eastern, formerly socialist countries, such as Russia. Both sauerkraut and another microbe-rich product, kefir (149, 150), clearly deserve further studies as possible means to endorse tolerance and prevent allergy.

Exposure to allergens – house dust mite as an example.  The theory that dust mites are closely associated with the development of allergic diseases dates back to 1960s (151) and in the past 45 years numerous studies have shown that both exposure and sensitization to mites are associated particularly with asthma. Mites have been considered to be one of the most important indoor allergens, and it has been widely accepted that there is a linear dose-response relationship between the dose of exposure and sensitization or allergic symptoms (152). Nearly all of the earlier studies are cross-sectional, allowing no cause and effect assessment. In addition, the bulk of these studies performed in the 1980s and 1990s comprised high-risk children, although most of the asthmatic children have been found to come from families without allergic diseases (111). Moreover, hardly any data have been available on dust mite monosensitization (skin prick test positivity to HDM solely), as compared with mite polysensitization (skin prick test positivity to HDM plus one or more other allergens). The relative role of HDM sensitization in allergy/asthma in the context of polysensitization is difficult, if not impossible, to evaluate. Indeed, the most recent data challenge the old doctrine of the close association between mites and atopic diseases. These data have been obtained from:

  • • 
    Prospective studies of exposure among unselected and high-risk children,
  • • 
    Prospective studies of allergen avoidance among asthmatics and high-risk children and
  • • 
    Cross-sectional studies in farming and non western environments.

Prospective studies of exposure among unselected and high-risk children.  No associations have been found between the level of mite exposure at home in early life and occurrence of asthma in later life both in general population and high-risk children (153–158). Exposure to HDM in many studies seems to be associated with sensitization, but this sensitization in the light of recent prospective studies may not have any clinical significance, rather, the association between exposure/sensitization to mites and asthma may be because of reversal causation, i.e. asthmatics and individuals genetically prone to develop asthma are sensitized to mites and other common allergens in their environment more readily than normal population. Asthma and allergies appear to have their origin very early in life, even in utero (122, 123, 159).

Prospective studies of allergen avoidance among asthmatics and high-risk children.  A recent updated Cochrane-meta-analysis of reducing exposure to mites in homes of asthmatics (3002 subjects with asthma in 54 studies) came to the conclusion that reducing mite allergens with chemical (acarocides) or physical (heating, freezing, repeated washing of bedding, cover textiles, indoor airfilters, ionisators) methods or with the combination of these, no clinical benefits are achieved, irrespective of reduction in allergen concentrations in several studies (160). As to other atopic conditions, a recent meta-analysis of mite allergen avoidance among patients with allergic rhinitis (seven studies) found that mite-impermeable beddings had no effect (two high-quality studies) or the trials were small and of poor quality (five studies) (161).

Cross-sectional studies in farming and non western environments.  Exposure to dust mites appears to be considerably higher in farm houses as compared with other dwellings (162, 163), and sensitization rates to HDM have been found to be similar or higher among farmers’ children as compared with other children (162, 164). In spite of this, occurrence of atopic diseases has, in numerous studies, been convincingly shown to be lower than that in non farmers’ children (117). We found recently that in the Karelia Allergy Study, 43% of all school children in Finland were sensitized to one or more allergens, whereas a positive skin prick test result was rare in Russian children. An exception was sensitization to HDM, to which sensitization rates were similar (9%) on both sides of the border (2). Russian children were mostly sensitized to mite only (monosensitization), whereas the majority of Finnish children were sensitized to one or more allergens besides HDM (polysensitization). Microscopic analysis of house dusts revealed that Russian dusts were rich in mites, both house dust and storage mites, whereas no mites were found in Finnish dusts (165) (Table 2). Irrespective of this, atopic diseases were rare among Russian children and adults (2).

Farm environment and simple living conditions appear to favour the growth of dust mites, but confer protection against atopic diseases. It is evident that in such environments, tolerance against dust mites develops in a non linear dose-dependent manner (22, 153, 166), a phenomenon that has been described already for many allergens and bioparticles, such as bee venom allergen (167), LPS (163), cat, mouse and rat allergens (168–171). As an example, the non linear dose-response relationship for cat allergen in different studies is illustrated in Fig. 5.

image

Figure 5.  The sensitization to cat as a function of Fel d 1 cat main allergen concentration in indoor living environment and the amount of Fel d 1 cat main allergen in living rooms of homes with or without a cat. A combined presentation with adjusted estimates from data presented by Custovic et al (173), Platts-Mills et al (20), Custovic et al (168), Cullinan et al (153) and Gore et al (172).

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Avoidance of inhalant allergens is difficult, if not impossible. As referred above, the Cochrane meta-analysis (160), similar to the earlier one (172), showed clearly that avoidance of mite allergens using various methods among asthmatics sensitized to HDM provided virtually no clinical benefits, although reduction of mite allergen levels was demonstrated in several studies. Multifaceted interventions have shown some benefits (reviewed in 173, 174), but the role of allergen avoidance vs cessation of indoor smoking, for example, cannot in such settings be reliably separately assessed. Moreover, complicated multifaceted interventions are not the solution for this kind of a public health problem.

Probiotics and other bacteria-based products

Probiotics.  A number of trials suggest that the mucosal microbiota is a central player in the development of mucosal tolerance, and based on these findings, the possibility that mucosal defence mechanisms are strengthened or balanced by probiotics, has been examined (175). The potential of probiotics to improve mucosal health is based on their dual effects; on one side, their ability to strengthen tolerance against e.g. allergens, and on the other side, improve mucosal defence system against harmful pathogens. Probiotics appear to be beneficial particularly in young children with IgE-mediated atopic eczema (175, 176).

Use of probiotics, mainly lactobacilli, during pregnancy, and their administration to the newborn or to the breastfeeding mother appears to reduce prevalence of atopic eczema in high-risk children or children delivered by cesarean section (177–179). Even this treatment could not reduce specific IgE-mediated responses to food. Whether the large-scale use of lactobacilli in pregnant women to prevent atopic eczema in the offspring is beneficial is an issue that still needs confirmation. One problem shared by many probiotic studies is the too short intervention period (commonly 6 months in infants), consumption of probioitics may apparently be continuous to exert maximal effect.

Data from animal models have shown that the colonization of the normal flora on the gut lumen is a prerequisite for the development of tolerance against food and other allergens (180). Probiotic lactobacilli have been shown in animal models to enhance the development of tolerance and have therefore been examined as tolerance-strengthening agents, e.g. in food and pollen allergy in humans, but their role in these conditions still remains unclear. One study found that the use of lactobacilli ameliorated skin and eye symptoms in house dust allergy (181). In many other studies among patients with e.g. birch pollen allergy, the efficacy of probiotics on respiratory allergy could not have been demonstrated (182). Accumulating data now show that there are disparities between different bacterial strains in this respect, and therefore the comparison of different studies is difficult. Probiotics appear at best to affect only IgE-mediated atopic eczema in young children, frequently associated with food allergies, whereas no beneficial effects have convincingly been reported for non IgE-mediated conditions. More data of probiotics in strengthening tolerance are undoubtedly needed before their role in prevention/treatment of allergy can be evaluated.

The effects of probiotics in mucosal defence appear to be multifaceted. Probiotics have been found to enhance the function of mucosal epithelial cells shown as decreased permeability of gut mucosa in atopic children (183). Probiotics appear to increase mucosal IgA production (184), which in turn can decrease mucosal permeability, and further strengthen tolerance even against environmental allergens. Our recent studies have revealed that administration of probiotics to infants elicits a mild systemic inflammatory response, which can strengthen tolerance against environmental allergens via mechanisms similar to infections, such as some helminth infections, that appear to augment the function of mucosal T reg cells and confer protection against allergies via the function of regulatory network (185, 186). These studies and other data from population surveys and animal models (175) lend support to the concept that exposure to micro-organisms fundamentally affects the development and function of our defence system and regulates immune responses, including tolerance responses, to environmental antigens.

Other bacteria-based products.  Allergen-specific immunotherapy is thus far the only curative therapy for allergic diseases. It requires, however, good compliance and adherence because of long duration of treatment, and may sometimes cause anaphylactic reactions. Alternative treatment and prevention approaches are therefore needed. Several microbes -based strategies, in addition to probiotics, have been or are currently being tested:

  • • 
    bacterial extracts and cell-wall components.
  • • 
    mycobacteria.
  • • 
    CpG oligonucleotide motifs.
  • • 
    helminths.

The idea of using bacterial products, alone or as adjuvants, to treat or prevent atopic disease is not new. Double-blind and longitudinal studies of bacteria-based vaccines in asthma had been performed already more than 40 years ago (187), but their effectiveness could not be proven (188).

The concept of bacterial products in prevention of allergies is nonetheless still tempting. It is noteworthy that bacteria need not have to be viable to exert their immunomodulatory effects. Accumulating evidence shows that different bacterial components are strong immunomodulators (189, 190). Moreover, acellular vaccines based on bacterial cell-wall components have successfully been used for years to elicit protective immune responses. Products containing components of environmental saprophytes could provide a method to endorse tolerance in populations.

Several studies using murine models of asthma revealed that mycobacteria are able to exert immunomodulatory potential and alleviate inflammation via induction of regulatory T-cells (191, 192). However, results from human trials among asthmatics have been less encouraging (193, 194). Killed Mycobacterium vaccae showed no efficacy in atopic eczema either, (195), contrary to preliminary results from an earlier smaller study (196).

CpG oligonucleotides, similar to the case of mycobacteria, have proved to be highly effective in mice but success in human trials is still awaited. Nonetheless, recent studies have shown that CpG motifs, in addition to their well-known propensity to drive the response to the Th1 arm, elicit also the regulatory network (197, 198). Large human trials using CpG motifs alone or conjugated with an allergen have shown promising results (199).

Certain helminth infections have been associated with reduced risk of asthma and allergy. It has been emphasised that the species, chronicity, and intensity of infection are the decisive factors pertaining to this issue (200, 201). A whipworm infecting pigs, Trichuris suis, has been used in patients with colitis (202), but thus far the role of helminths in prevention or treatment of asthma and allergies, including food allergies (203) remains to be clarified. Helminth-derived molecules, possibly coupled to allergens, could be suitable candidates for trials (204).

Nearly a decade ago, the potential of bacteria-based products in educating the developing immune system and maintaining the mucosal integrity was recognized (205). A critical appraisal of the data on the potential use of microbial products in allergy prevention and therapy was published in 2003 by the European Association of Allergology and Clinical Immunology Task Force Working Group (206). The progress in this matter has however been slow. None of the agents above has thus far got a validated position in the treatment or prevention of allergies.

Specific ways to affect acquired immunity

Specific immunotherapy.  Specific immunotherapy has been used in the treatment of IgE-mediated allergic diseases for nearly 100 years. Data on efficacy and safety of specific immunotherapy obtained from randomized placebo-controlled double-blind studies were published already in the 1960s (207, 208), and since the 1990s, data on long-term efficacy of immunotherapy in prevention of asthma among patients with allergic rhinitis and of sensitization among high-risk children have been available (209, 210). Along with unravelling the mechanisms behind immunotherapy, it is now widely accepted that the therapy should be started early in the manifestation of an IgE-mediated condition together with guided self-management, symptom-based therapy and possibly with avoidance of allergens that clearly worsen symptoms. According to WHO guidelines from 1998, specific immunotherapy acts globally on IgE-mediated inflammation in various target organs (211).

The guidelines of specific immunotherapy by the European Association of Allergology and Clinical Immunology have been followed in Finland from 1993. These management guidelines have been updated and published in Allergy in 2006 (212). Both subcutaneous injection and sublingual therapy are effective in allergic rhinitis and asthma both among adults and children. A working group set by the Finnish Association of Allergists and Immunologists published management guidelines in 2002 for specific immunotherapy. This therapy is currently given to 7000–8000 Finns, however, based on epidemiological surveys, even 200 000 individuals could benefit from this therapy (considering a population of 5.2 million inhabitants).

There are no cost-effectiveness studies of specific immunotherapy from Finland, and overall, very few such studies have been performed in other countries, either. The need is however obvious, particularly when possible preventive effects of immunotherapy are considered, such as the development of new sensitizations and asthma (213). Encouraging results were obtained in a recent cost-effectiveness study from Denmark (214). Issues concerning the quality of life have recently been included, and the thus far limited data available show that the quality of life will improve along with immunotherapy. Efficacy and strength of evidence of injection therapy have been reviewed in detail previously (212).

Effector mechanisms of specific immunotherapy.  Immunotherapy blocks the Th2 effector functions and leads to reduced production of IgE and reduced maturation and activation of basophils and eosinophils, resulting in silenced local mucosal inflammation. The tolerance induced by the regulatory T-cells appears to be a main event behind this effect.

Venom immunotherapy.  Increased levels of allergen-induced IL-10 in human PBMCs were first observed in venom immunotherapy (215, 216). The increased IL-10 levels were associated with attenuated allergen-specific lymphoproliferative responses that could be re-established with neutralizing anti-IL-10 antibodies (215, 216). A dose-dependent allergen-induced IL-10 response in PBMC was also seen in bee-keepers that were naturally tolerized to bee venom by repeated bee stings (216).

Subcutaneous immunotherapy with aeroallergens (SCIT).  In subcutaneous immunotherapy (SCIT) with aeroallergens, the regulatory T-cell responses were documented a couple of years after the bee venom studies. A Swiss study showed that SCIT with house-dust mite induces house-dust-mite-specific T-cells producing IL-10 and TGF-beta in peripheral blood. These cells also suppressed Th1 and Th2 cell-responses in vitro (17). A study from the UK demonstrated that after timothy SCIT, there was an increased proportion of CD4+CD25+ cells in PBMC as well as increased production of IL-10 in the PBMC after stimulation with allergen in vitro (18). In both the Swiss and UK studies, the increased proportion of CD4+ cells in PBMC was associated with suppression of T-cell responses. In a third, Finnish study, elevated allergen-stimulated IL-10 mRNA responses in PBMC were observed after SCIT with birch or timothy. The increased IL-10 mRNA responses immediately after the up-dosing phase predicted good therapeutic outcome after on year of therapy (19). It was recently shown that in addition to inducing a peripheral regulatory T-cell response, grass SCIT increases the number of Foxp3+CD4+CD25+ cells in nasal mucosa during pollen season (217). There is substantial evidence to support the concept that tolerance to aeroallergens achieved after SCIT is induced and maintained by regulatory T-cells and cytokines.

Sublingual immunotherapy.  The mechanisms of sublingual immunotherapy (SLIT) differ somewhat, but not fundamentally, from those of venom immunotherapy or aeroallergen SCIT. Examination of birch-allergen-stimulated PBMC from tree-pollen-allergic children receiving high-dose-, low-dose- or placebo SLIT revealed increased expression of IL-10 mRNA and decreased expression of IL-5 mRNA in PBMC after 1 year’s therapy only in children receiving the high-dose SLIT (69).

In a small study population, the CD4+CD25+ cell, FoxP3+ and IL-10 responses in PBMC stimulated with different antigens were studied after 4 and 52 weeks of SLIT (70). After 4 weeks, increased number of CD4+25+ T-cells and expression of IL-10 and FoxP3 mRNA but decreased expression of IL-4 and IFN-gamma mRNA were observed. The lymphoproliferative responses to birch and cross-reacting main apple allergens (Bet v 1 and Mal d 1) and tetanus toxoid after 4 weeks of SLIT were diminished but could be re-established with depletion of CD25+ cells or neutralizing anti-IL-10 antibodies. After 1 year of SLIT, the lymphoproliferative responses to tetanus toxoid and apple main allergens were restored to pretreatment levels whereas Bet v 1-specific lymphoproliferation remained diminished and could not be restored either with depletion of CD25+ cells or by neutralizing anti-IL-10 antibodies.

It appears that during the early course of SLIT, an IL-10-mediated specific and non specific tolerance is induced, which gives way and is replaced by an allergen-specific and a more abiding tolerance.

Future.  Improvements concerning dosing and composition of allergen products are currently in progress. Sublingual immunotherapy has become popular in Middle Europe and for example, 70% of all immunotherapy in France is sublingual. The next few years will show which course the Nordic practice in sublingual immunotherapy will take.

Sublingual immunotherapy is considered to be virtually free of adverse reactions and the treatment can be administered at home. There are however also problems, as 3 years of treatment daily all through the year just to treat a 3-month period with symptoms needs treatment adherence. Sublingual therapy with non standardized preparations is additionally associated with the small risk of anaphylaxis. A recent placebo-controlled double-blind study from Finland showed that in children with pollen allergy, related symptoms and use of medication reduced significantly in the active treatment group (218). More well-conducted studies are undoubtedly needed, particularly comparative cost-effectiveness studies of injection and sublingual immunotherapy. More data are also needed of immunotherapy in prophylaxis. Patient selection and early start of the treatment, i.e. whether sublingual therapy should be started even in children younger than 5 years, are similarly still open questions. Sublingual immunotherapy is becoming increasingly popular and will evidently be the most widely used immunotherapy in the future. Currently there is only one registered product for sublingual use in Finland.

Tolerance and preventive measures

The focus of this communication is in endorsing tolerance in the population before any signs of sensitization are discernible. Primary prevention, closely associated with tolerance, is considered comprehensively in recommendations published in 2004 by WHO/WAO (World Allergy Organization). These recommendations are based on evidence and on the quality of the evidence using WHO categorization criteria (219).

According to WHO/WAO, evidence of the category A (evidence based on ≥1 randomized controlled study) or B (evidence based on ≥1 controlled or other non randomized experimental study) is available only for the three following measures:

  • • 
    Avoidance of smoking (also passive smoking) during pregnancy and in the infant’s first years of life (B).
  • • 
    Exclusive breastfeeding during the first 4–6 months (B). The diet of the breastfeeding mother has no role in the development of allergy in the baby.
  • • 
    Avoidance of animal allergens in children with family history of allergies (B). The data concerning this point are, however, somewhat conflicting. Recent data suggest that exposure to pets in the early life decreases the risk of atopic disease in later life (even among those with a family history of atopic disease (220), provided that the exposure is strong enough. The dose of exposure appears thus to have a major impact on the development of allergy (the non linear relationship between exposure and allergy; see above).

As to secondary prevention, there is evidence to show that allergic rhinitis is a risk factor for the development of asthma in later life (213, 221, 222). Specific immunotherapy of allergic rhinitis could thus prevent the development of asthma in some cases (223).

Psychological tolerance – changes in attitudes

  1. Top of page
  2. Abstract
  3. Cellular mechanisms of tolerance
  4. Tolerance vs allergic inflammation of the gastrointestinal tract
  5. Tolerance vs allergic inflammation of the skin
  6. Tolerance vs allergic inflammation of the respiratory tract
  7. Strengthening of tolerance
  8. Psychological tolerance – changes in attitudes
  9. The burden of allergy
  10. Concluding remarks
  11. Acknowledgement
  12. References

A recent survey among members of allergic patient organizations in 11 European countries revealed that the burden of allergic rhinitis, e.g. is often much greater to the patient than generally acknowledged, and underscored the importance of listening to the patient’s voice in allergic rhinitis, as well as in other seemingly ‘mild’ chronic diseases (224). On the other side, imagined allergy (or pseudoallergy) is also common in western societies. The Finnish Allergy Programme aims to strengthen even the psychological tolerance among people. Mild allergy may be considered as a personal trait or characteristic rather than a disease that needs specific measures. People with mild allergy can in most cases live normal life in spite of their disorder. This concept of ‘allergy health’ was introduced in the Allergy Programme (7). Most childhood allergies also disappear in time when tolerance is naturally developed. In allergy, there is no ‘law’ of symptom escalation, i.e. symptoms get worse if nothing is done.

Crucial elements in bringing about changes in attitudes are awareness and dissemination (8). All parts involved in the Allergy Programme, the government, the non governmental organizations (NGOs), media, the population, patients and health-care professionals, must become aware of the key messages of the programme and must be informed what can be done to strengthen tolerance in the population.

Important in dissemination of the message is indeed to reach the critical mass, i.e. the point where the message diffuses in a self-sustaining way. It has been estimated that the dissemination begins rather slowly, then, the time 15–25% of the target audience adopts the message, the adoption rate grows quickly (225).

The North Karelia Project, considered often as a model programme for chronic disease prevention, was launched in 1972 to prevent and control cardiovascular disease in Finland (226). This project is another example to show that a programme which has a strong basis of theory employing comprehensive strategies with community organizations and large participation of people can change a poor health situation prevailing persistently for the better. The key issues were strong community participation and high motivation of health personnel and citizens involved.

The burden of allergy

  1. Top of page
  2. Abstract
  3. Cellular mechanisms of tolerance
  4. Tolerance vs allergic inflammation of the gastrointestinal tract
  5. Tolerance vs allergic inflammation of the skin
  6. Tolerance vs allergic inflammation of the respiratory tract
  7. Strengthening of tolerance
  8. Psychological tolerance – changes in attitudes
  9. The burden of allergy
  10. Concluding remarks
  11. Acknowledgement
  12. References

The prevalence of atopic diseases (Fig. 6) and increasing trends in Finland have been discussed in detail earlier (7), and are not reiterated further.

image

Figure 6.  Prevalence of asthma, allergic rhinitis and atopic eczema in Finnish military recruits during 1966–2003. Prevalence of disabling asthma at call-up and during service also presented. Adopted from ref (1).

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Prevalences in the light of international comparisons

Two large international studies, the ISAAC (the International Study of Asthma and Allergies in Childhood) among children (227) and the ECRHS (the European Respiratory Health Survey) among adults (228) allow a reliable comparison of prevalences of allergic diseases in Finland with other countries. Occurrence levels of asthma (4–7%) among school children at the age of 13–14 years in the ISAAC phase I in Finland was lower than that prevailing in many English-speaking countries, higher than the same in East-European countries, and at the same level as in other West-European countries. The prevalences of rhinitis were relatively high; 15–23% of the participants in the ISAAC and 28–34% in the ECRHS reported symptoms of allergic rhinitis during the past 12 months. In similarity to the case of atopic eczema, rather high prevalence rates for rhinitis in Finland, as in Sweden, were found. Overall, high rates for allergic rhinitis and atopic eczema but intermediate rates for asthma were reported. The ISAAC was repeated in a timeframe 10 years apart, albeit only in one area in Finland. Self- or parental-reported symptoms of asthma increased markedly, whereas the increase in atopic eczema was only moderate and hay fever showed no increase at all during the 10-year period (229). The most recent prevalence rate for physician-diagnosed asthma both among school children (2) and adults (the FinEsS study, unpublished data) is 9%.

Costs attributable to allergic diseases in Finland in 2004–2005

Allergic diseases are a problematic disease entity from the perspective of cost evaluation. Allergies can manifest in skin, nasal mucosa, eyes, respiratory tract or in the gut, but symptoms of these conditions are often difficult to differentiate from those of other conditions. Therefore, allergies are recorded in statistics and registers inaccurately.

Mortality of allergic diseases is relatively low, and they seldom cause long-term working inability. The impact of allergies on the quality of life is considerable, when taking into account their frequency and recurrent as well as chronic nature. Most of asthma and allergy patients are treated in outpatient clinics, and a considerable part of treatment consists of medication. In the following, the direct costs of asthma and allergies are considered, i.e. costs because of diagnostics, treatment and rehabilitation. Asthma has been included as one entity, although a proportion of asthma is not associated with allergy. Cost evaluation is based on registers of the Social Insurance Institute, the National Research and Development Centre for Welfare and Health, and the Finnish Centre for Pensions.

Direct costs comprise hospitalization days, outpatient visits to physicians and nurses, medications, laboratory examinations associated with diagnostics and follow-up, travel costs and rehabilitation. The numbers of patients and treatment-days in specialist care attributable to allergic symptoms can be accurately registered, but not so for outpatient visits. It was estimated in 1996 (Primary Care Population Survey, Social Insurance Institute) (230), that the proportion of visits because of asthma and allergies comprised approximately 5% of all visits to primary care. When the increase in prevalence of allergic diseases is taken into consideration, the total sums of 80 million euros for physician and 40 million euros for nurse visits are obtained. The costs for outpatient visits in specialist care are estimated to be at least comparable, totalling in 240 million euros for all outpatient visits (Table 3).

Table 3.   Annual costs attributable to allergic diseases in Finland. Loss of productivity not included (data from years 2004 and 2005)
 Million euros (%)
  1. *Of which medication for asthma comprises 70.3%, for allergic rhinoconjunctivitis 10.7%, and for atopic eczema 6.8%. The rest comes from systemic antihistamines 9.3% and glucocorticoids 2.9%.

Direct costs
 Hospital days11 (2.1)
 Outpatient visits240 (46.2)
 Medication*192 (36.9)
 Travelling12 (2.3)
 Private sector examinations and treatm.10 (1.9)
 Rehabilitation3 (0.6)
Indirect costs
 Disability pensions36 (6.9)
 Sickness allowances10 (1.9)
 Per diem for patients5.7 (1.1)
Total519.7 (99.9)

Costs attributable to medication can be accurately evaluated according to the ATC classification. For this purpose, the costs of medication for allergic symptoms are based on retail prices in 2005. Direct costs attributable to allergic diseases are summarized in Table 3. The sum of direct costs, 468 million euros, is approximately 4% of the total costs of health care in Finland, which in 2004 were 11.2 billion euros. In addition to direct costs, there are several indirect costs, primarily caused by pensions for working inability (36 million euros), sickness allowances (10 million euros) and per diems for patients (5.7 million euros), totalling in 51.7 million euros in 2005. Loss of productivity is not included in the present calculations.

Concluding remarks

  1. Top of page
  2. Abstract
  3. Cellular mechanisms of tolerance
  4. Tolerance vs allergic inflammation of the gastrointestinal tract
  5. Tolerance vs allergic inflammation of the skin
  6. Tolerance vs allergic inflammation of the respiratory tract
  7. Strengthening of tolerance
  8. Psychological tolerance – changes in attitudes
  9. The burden of allergy
  10. Concluding remarks
  11. Acknowledgement
  12. References

A comprehensive plan to prevent and treat allergic disorders, the Finnish Allergy Programme, was created in 2008 for a counter-action in a situation where nearly half of the school-aged children are sensitized to one or more common allergens; asthma and allergy prevalences are high and trending still upwards. At the population level, actions undertaken and treatment strategies used had not been able to reverse these unfavourable trends. New data have accumulated to show that many of the old dogmas concerning allergy must give way and a re-orientation of attitude towards allergy is called for.

One of the key issues in the Allergy Programme is tolerance; how to endorse tolerance and health instead of avoiding allergens and treating allergies. Unravelling the basic mechanisms of tolerance including the regulatory network and the receptor repertoire in innate immune cells has greatly improved our understanding of the importance of environmental exposure in the development of tolerance. In allergy, there is no law of symptom escalation, and many children also outgrow their allergies. Adopting a new attitude to allergy – from avoidance to tolerance – is necessary; allergen avoidance is important in secondary prevention, but only on justified and well-defined grounds. In addition, patients with severe disease must be treated in a better manner than what is being done at present. The Allergy Programme emphasizes the importance of early recognition and treatment of patients with severe allergies. Further, an important, albeit often neglected issue in allergy is psychological tolerance. Imagined (pseudo-) allergy is common among people; the Allergy Programme aims at reducing even this by strengthening psychological tolerance. Indeed, the concept of ‘Allergy Health’ was introduced in this programme, and means that mild allergy is considered as a personal characteristic rather than a disease that needs specific measures, and that people with mild allergy can in most cases live normal life in spite of their disorder.

References

  1. Top of page
  2. Abstract
  3. Cellular mechanisms of tolerance
  4. Tolerance vs allergic inflammation of the gastrointestinal tract
  5. Tolerance vs allergic inflammation of the skin
  6. Tolerance vs allergic inflammation of the respiratory tract
  7. Strengthening of tolerance
  8. Psychological tolerance – changes in attitudes
  9. The burden of allergy
  10. Concluding remarks
  11. Acknowledgement
  12. References