Endotoxin levels in Estonian and Swedish house dust and atopy in infancy

Hope for the hygiene hypothesis: when the dirt hits the fan

The hygiene hypothesis was developed in response to data suggesting that the increase in allergic diseases as well as asthma was secondary to a reduced exposure to infectious stimuli. Indeed, the epidemiologic changes, resulting in an increase in atopic disease, have been impressive and intriguing. Furthermore, although there clearly is a genetic component to atopic diseases, genetics cannot account for a marked increase in the incidence and prevalence of allergic manifestations within a few generations. Thus, environmental factors have been suggested as responsible for the changing prevalence. There are two--not mutually exclusive--possibilities, namely, that substances that promote atopy have been added to the environment or that factors that provided protection from allergic disease were lost from the environment. Both outdoor and indoor pollution, along with a long list of other environmental factors, have been proposed. It is of interest that in many developed countries, certain types of pollution have decreased, whereas the prevalence of atopic disease has increased. In this review, we have explored a detailed analysis of a large number of studies that have focused on this issue and suggest that, although the hygiene hypothesis has merit, the stimuli responsible for the new epidemiology remain enigmatic.

Atopic sensitization and atopic dermatitis in Estonian and Swedish infants

BACKGROUND

Early life events seem to have a major impact on the development of tolerance or sensitization.

OBJECTIVE

The aim of the study was to compare the prevalence of sensitization and atopic dermatitis (AD) during the first 2 years of life in Estonia and in Sweden.

METHODS

Two groups comprising 110 Estonian and 123 Swedish infants were followed from birth up to 2 years of age. Data about symptoms of allergy, infections and use of antibiotics were obtained by questionnaires. Clinical examinations, skin prick tests (SPTs) with food and inhalant allergens, and blood sampling for IgE analyses were carried out at 3, 6, 12 and 24 months.

RESULTS

The cumulative incidence of AD and positive SPTs were lower in the Estonian than the Swedish infants (14% vs. 24%; P = 0.06 and 13% vs. 24%; P = 0.03), while circulating IgE antibodies were more common (39% vs. 27%; P = 0.06) and often present without any clinical significance in Estonian children. Estonian infants had respiratory illnesses more often and they had received antibiotics more frequently. Use of antibiotics increased the risk for positive SPT in the Estonian (odds ratio = 1.7, 95% confidence interval = 1.1-2.5), but not in the Swedish infants. This may be explained by the use of broad-spectrum antibiotics in Estonia, while in Sweden mostly penicillin was prescribed.

CONCLUSIONS

The prevalence of AD and positive SPTs was lower in the Estonian than the Swedish infants, while circulating IgE antibodies were more common and often present without any clinical significance. These differences cannot simply be explained by infections, or use of broad-spectrum antibiotics in the two countries, although more the natural lifestyle in Estonia may be contributing factor.

Modest increase in seasonal allergic rhinitis and eczema over 8 years among Estonian schoolchildren

We studied time trends in the prevalence of asthma and allergic diseases in Estonian children born before and after the collapse of the Soviet Union, as this event markedly altered the lifestyle in Estonia. Two identical cross-sectional studies were performed as part of phase I and phase III of the International Study of Asthma and Allergies in Childhood (ISAAC). Children, aged 6-7 yr (n = 3070 in 1993-94 and 2383 in 2001-02) and 13-14 yr (n = 3476 and 3576, respectively), completed ISAAC core-written questionnaires, and 13-14-yr olds (n = 3427 and 3259, respectively) also video questionnaires. The prevalence of respiratory symptoms was mostly similar in the two studies. Despite this, the prevalence of diagnosed asthma increased. This was probably due to modified diagnostic criteria and increased awareness. Furthermore, the prevalence of rhinitis during the pollen season increased, e.g., rhinitis in May from 1.7% to 3.5%; sex-adjusted prevalence odds ratio (POR) 2.09 (95% confidence interval 1.47-2.96) in 6-7-yr olds, and from 2.6% to 5.5%; POR 2.22 (1.72-2.87) in 13-14-yr olds. The prevalence of flexural dermatitis also increased from 12.0% to 13.5%; POR 1.20 (1.02-1.41) in 6-7-yr olds, and from 7.7% to 9.4%; POR 1.26 (1.07-1.50) in 13-14-yr olds. The increase was similar in children born before and after the regaining of Estonian independence, indicating that the influence of factors related to a Western lifestyle and affecting the prevalence of allergic symptoms is not restricted to infancy, but may be operative throughout childhood.

Endotoxin exposure and atopic sensitization in adult pig farmers

BACKGROUND

Recent studies have reported a low prevalence of atopic sensitization and respiratory allergy in children growing up on farms.

OBJECTIVES

We sought to evaluate the dose-response relationship between endotoxin and atopic sensitization in adult farmers and to assess the effect on respiratory health outcomes.

METHODS

Data on endotoxin exposure and serum IgE levels were available for 162 pig farmers from a cross-sectional case-control study, with case selection on the basis of respiratory symptoms. Exposure to endotoxin was modeled in detail, and respiratory health effects were assessed during a medical examination. Exploratory analysis was done by using nonparametric modeling and was followed by classical parametric regression.

RESULTS

IgE to one or more common allergens was detected in sera from 28 (17%) farmers. The average (geometric mean) total serum IgE levels was 37 IU/mL (geometric SD, 4 IU/mL). A strong inverse relationship was found between endotoxin and sensitization to common allergens for exposures of 75 ng/m 3 or less, with an odds ratio of 0.03 (95% CI, 0.0-0.34) for a 2-fold increase in endotoxin. For endotoxin exposure of greater than 75 ng/m 3 , the association was weak (odds ratio, 1.2 [95% CI, 0.38-3.6]). No association was found between endotoxin exposure and total serum IgE levels. Endotoxin was associated with increased airway hyperresponsiveness to histamine and lower lung function in sensitized farmers, without evidence of a nonlinear relationship.

CONCLUSIONS

The prevalence of atopic sensitization in adult pig farmers is low. Endotoxin or related exposures might protect from sensitization, even in an adult working population exposed to high levels of endotoxin, but is a risk factor for increased airway hyperresponsiveness and low lung function.

The hygiene hypothesis and asthma

PURPOSE OF REVIEW

Reduced exposure to childhood infections may explain the increased prevalence of allergic diseases in industrialized countries (the hygiene hypothesis). This review will examine recent epidemiologic studies of the hygiene hypothesis and asthma.

RECENT FINDINGS

Recent studies have confirmed previous findings of an inverse association between increased exposure to other children during childhood and either allergen sensitization or hay fever. However, there is conflicting evidence regarding the relation between exposure to other children and asthma. Although it has been hypothesized that vaccinations may influence the development of asthma, recent findings do not support this association. Serologic evidence of exposure to certain gastrointestinal pathogens (eg, hepatitis A virus) has been inversely associated with either allergen sensitization or asthma in some, but not all, recent studies. Although heavy infestation with certain parasites (eg, helminths) is protective against allergen sensitization, there is conflicting evidence regarding the relation between parasitic infection and asthma. The results of recent studies suggest that the relation between endotoxin exposure and asthma is complex and likely influenced by factors related to the exposure itself, the host, and other covariates. Although it has been postulated that antibiotic use in early life is a risk factor for asthma, this hypothesis is not supported by recent findings.

SUMMARY

For every exposure studied with regard to the hygiene hypothesis, there are inconsistent findings in relation to asthma. The hygiene hypothesis is not likely to be the sole explanation for the ongoing asthma epidemic in industrialized nations.

TLR2 and TLR4 stimulation differentially induce cytokine secretion in human neonatal, adult, and murine mononuclear cells

Toll-like receptor 2 (TLR2) and TLR4 signaling may induce differential secretion of T helper 1 (Th1) and Th2 cytokines, potentially influencing the development of autoimmune or atopic diseases. To date, the influence of the type of stimulus, timing, and dose of TLR2 and TLR4 ligands on cytokine secretion has not been well established. We tested whether the innate stimuli peptidoglycan (Ppg, TLR2 agonist) and lipid A (LpA, TLR4 agonist) differentially affect the secretion of interleukin-13 (IL-13) (Th2) and interferon-gamma (IFN-gamma) (Th1). Further, we examined the influence of the maturity of the immune system, species, dose, and timing of stimuli in human cord and adult peripheral blood mononuclear cells (PBMC) and murine cells in vitro and in vivo. Stimulation with Ppg induced the secretion of both IL-13 and IFN-gamma, influenced by time and dose in neonates, adults, and mice. In contrast, stimulation with LpA induced primarily time-independent and dose-independent production of IFN-gamma. Pulmonary administration of Ppg in vivo in mice resulted in secretion of IL-13, whereas administration of LpA resulted in secretion of IFN-gamma in bronchoalveolar lavage (BAL). Therefore, TLR2 and TLR4 stimuli differentially influence IL-13 and IFN-gamma secretion in neonates, adults, and mice, supporting a critical role for innate stimuli in the modulation of cytokine responses.

Exposure to pets, and the association with hay fever, asthma, and atopic sensitization in rural children

BACKGROUND

An increasing number of studies report pet exposure to be associated with lower risk of asthma and allergies. This 'protective pet effect' has been suggested to result from a modified T-helper (Th)2-cell response, or because of increased microbial load in homes where pets are kept. We examined the associations between pet contact and the occurrence of asthma and allergies in children of the rural Allergy and Endotoxin (ALEX) population, taking farm animal contact, endotoxin and cat allergen levels in mattress dust into account.

METHODS

Information about contact with pets and farm animals, asthma and allergy were collected for 812 children by a standardized parents' questionnaire and an interview. Mattress dust endotoxin and cat allergen levels as well as specific IgE and IgG4 antibodies to Fel d1 were determined.

RESULTS

Current contact with dogs was inversely associated with diagnosed hay fever (OR 0.26, 95% CI 0.11-0.57), diagnosed asthma (OR 0.29, 95% CI 0.12-0.71), sensitization to cat allergen (OR 0.48, 95% CI 0.23-0.99) and to grass pollen (OR 0.55, 95% CI 0.33-0.94), but not with increased IgG4 levels. Early and current contact with cats were associated with reduced risk of wheezing (OR 0.48, 95% CI 0.23-1.00, and OR 0.49, 95% CI 0.26-0.92, respectively) and grass pollen sensitization. Adjustment for farm animal contact but not for endotoxin and cat allergen exposure attenuated these associations and the effect of pet was stronger among farmers' children.

CONCLUSION

Although pet exposure was very frequent in this rural population, the inverse relation between current dog contact, asthma and allergy was mostly explained by simultaneously occurring exposure to stable animals or was restricted to farm children. In addition, a subtle form of pet avoidance may contribute to the protective effect of pet.

Hygiene hypothesis and endotoxin: what is the evidence?

PURPOSE OF REVIEW

The hygiene hypothesis has gained strong support over the past few years. Exposure to microbial products in early life could be an underlying factor in this hypothesis, but the mechanisms that lead from a less clean and more crowded environment to a lower prevalence of asthma and allergies are not known. Among the variety of potential microbial molecules that may confer protection against the development of asthma and allergies, endotoxin, a component of Gram-negative bacteria, has incited lively as well as controversial discussions. This review focuses on recent studies on endotoxin and its role in the context of the hygiene hypothesis.

RECENT FINDINGS

Results from cross-sectional surveys, prospective cohorts, and experimental studies in vitro and in rodents suggest that exposure to house dust endotoxin in early life protects from atopic sensitization and IgE-mediated diseases, but is a risk factor for wheezing in infancy.

SUMMARY

Numerous studies have supported the hygiene hypothesis, but whether endotoxin by itself confers the protection or whether it acts as a marker for another environmental exposure is still unclear. The challenge for the future will be to identify those factors that confer the protection proposed by the hygiene hypothesis, and to find strategies to modify the environment without causing harm to susceptible individuals.

Reduced levels of soluble CD14 in atopic children

BACKGROUND

A reduced microbial stimulation has been reported as a reason for the increasing prevalence of atopic diseases in industrialized countries. Antigen-presenting cells (APC), responding to microbial signals by pattern recognition receptors such as CD14, have an important role in the development of the Th1/Th2 balance.

OBJECTIVE

We hypothesized that atopic children have a lower expression of CD14 on monocytes and lower soluble CD14 levels (sCD14).

METHODS

Seventy-six children were followed prospectively from birth and signs of atopic disease were evaluated. The expression of CD14 on monocytes was analysed with flow cytometry at 0, 3, 6, 12 and 18 months. Circulating levels of sCD14 were analysed by ELISA and total IgE was analysed by fluoroenzymo immunoassay at these ages, and in a subgroup, followed up at 7 years.

RESULTS

Levels of sCD14 were reduced at 7 years both in children with a current or a cumulative history of atopy compared to non-atopic children with P=0.002 and 0.001, respectively. Sensitized children with atopic symptoms had lower sCD14 at 3 and 18 months and at 7 years of age than non-atopic non-sensitized children with P=0.023, 0.039 and 0.008, respectively.

CONCLUSION

The lower levels of sCD14 observed in atopic children may be a consequence of an atopic family heredity and/or atopic disease, but it may also reflect a reduced capacity to respond to microbial signals.

Environmental exposure to endotoxin and other microbial products and the decreased risk of childhood atopy: evaluating developments since April 2002

PURPOSE OF REVIEW

The hygiene hypothesis proposes an association between the change in exposure to microbes and the increased incidence of atopic diseases in recent decades. Exposure to microbes and childhood infections and their association with atopy has thus attracted much scientific interest. This review focuses on new developments in the field of epidemiology.

RECENT FINDINGS

Studies in adults confirm that exposure to orofaecal pathogens are associated with less asthma and allergies. In seropositive individuals, no increase in allergy prevalence over time was noted. Similarly, the generational increase in atopy and allergic rhinitis was not observed in individuals who were exposed to a farming environment in childhood. More than 20 studies have been published examining the effect of exposure to a farm environment in children and adults. Most consistently, the 'protective' farm effect was related to livestock farming and thus to microbial exposure. A dose-dependent inverse relationship between exposure to endotoxin in the mattress dust of children and the occurrence of atopic diseases was shown in rural environments in Europe. In addition, the blood cells of farmers' children were shown to express higher amounts of innate immunity receptors. Only a few farm studies have so far included an objective measure of the microbial environment. The examined exposure to endotoxin might partly be a surrogate measure of a much broader spectrum of immunomodulatory microbial compounds present in a rural environment.

SUMMARY

The 'hygiene hypothesis' has gained much credibility, but the results should be balanced against the benefits of established hygiene standards.

Effects of intestinal microflora and the environment on the development of asthma and allergy

The aim of previous research into the causes of allergic diseases, including asthma was mostly to identify potential risk factors in the environment. No major risk factors have been identified, however. Over the past 10 years, focus has, therefore, more been directed towards protective factors that could enhance the development of tolerance to allergens which were previously encountered early in life, but are now lost in modern affluent societies. In particular, the role of childhood infections has been discussed, but so far these studies have not been conclusive. Recent epidemiological studies and experimental research suggest that the microbial environment and exposure to microbial products in infancy modifies immune responses and enhances the development of tolerance to ubiquitous allergens. The intestinal microflora may play a particular role in this respect, as it is the major external driving force in the maturation of the immune system after birth, and animal experiments have shown it to be a prerequisite for normal development of oral tolerance. Recent studies have shown differences in the composition of the microflora between healthy and allergic infants in countries with a high and low prevalence of allergies and between healthy and allergic infants within such countries. These differences are apparent within the first week of life and thus precede clinical symptoms. The use of live microorganisms that might be beneficial to health has a long tradition and the safety is well documented. Very recently, several prospective intervention studies, modifying the gut flora from birth have yielded encouraging results and may suggest a new mode of primary prevention of allergy in the future.

Epigenetic inheritance of fetal genes in allergic asthma

Asthma has been associated with an exaggerated T-helper type 2 (Th2) over Th1 responses to allergic and nonallergic stimuli, which leads to chronic airway inflammation and airway remodeling. In the present article, we propose that many of the genes involved in IgE synthesis and airways (re)modeling in asthma are persistent or reminiscent fetal genes which may not be silenced during early infancy (or late pregnancy). Genes of the embryologic differentiation of ectodermic and endodermic tissues may explain some of the patterns of airway remodeling in asthma. In utero programming leads to gene expression, the persistence of which may be associated with epigenetic inheritance phenomena induced by nonspecific environmental factors. Clear delineation of these issues may yield new information on the mechanisms of asthma and new targets for therapeutic intervention and primary prevention.

The genetics of CD14 in allergic disease

PURPOSE OF REVIEW

The 'hygiene hypothesis' implies that infections in early childhood prevent the development of atopy. Atopy is caused by the interaction of the environment with genetic factors. Therefore, both contacts with microbial products such as endotoxin as well as CD14, a gene involved in the immunological response to endotoxin, are discussed in this review.

RECENT FINDINGS

CD14 is a multifunctional receptor for endotoxin and other bacterial wall components. Membrane-bound CD14 is expressed on monocytes and macrophages. It forms a complex with Toll-like receptor 4. Genetic studies have shown an association of variants in the CD14 gene with the prevention of (and severity of) atopy. However, phenotype definitions and the allele that showed association differed between studies. In addition, the presence of endotoxin in house dust appears to be inversely related to atopy in different studies.

SUMMARY

It can be hypothesized that different levels of endotoxin exposure could interact with specific CD14 variants in the prevention of atopy. If confirmed in prospective cohort studies, this might represent an important gene by environmental interaction in the development of atopy. This could possibly open ways for the primary prevention of atopy.