Hay fever and asthma in relation to markers of infection in the United States

The hygiene hypothesis: does it function worldwide?

PURPOSE OF REVIEW

This article intends to be a systematic review of papers published during 2003-2004 quoted in a Medline search for 'hygiene hypothesis'. The worldwide perspective of the article does not refer just to a geographical concept, but tries also to address the question of whether the consensus on the hypothesis is global or whether it applies to all types of allergic or immunologic disease, to any age sample or infectious agent. Furthermore, the article outlines those clinical and experimental studies which, in the authors' opinion, may represent significant contributions to a better understanding of the hygiene hypothesis and may guide further investigations on the subject.

RECENT FINDINGS

The association between a reduced exposure to infectious agents (as a part of a changed lifestyle) and a higher prevalence of atopy seems now to be confirmed by consistent evidence. Mechanisms underlying this association, however, are not yet completely clear (immune deviation or immune regulation).

SUMMARY

Further experimental and clinical studies are needed, with special reference to the time, duration and intensity of exposure to any specific infectious agent which is related to well-defined allergy outcomes. The background information for using microbial products in allergy prevention and treatment is still limited.

Growth of hepatitis A virus in a mouse liver cell line

Hepatitis A virus (HAV) has been adapted to grow efficiently in primate and some nonprimate cell lines but not in cells of murine origin. To understand the inability of the virus to grow in mouse cells, we studied the replication of HAV in immortalized and nontransformed MMH-D3 mouse liver cells, which require growth factors and collagen to maintain their phenotype. HAV grew in MMH-D3 cells transfected with virion RNA but not in those infected with viral particles, indicating a cell entry block for HAV. However, MMH-D3 cells cultured under suboptimal conditions in the absence of growth factors acquired susceptibility to HAV infection. Serial passages of the virus in MMH-D3 cells under suboptimal growth conditions resulted in the selection of HAV variants that grew efficiently in MMH-D3 cells cultured under both optimal and suboptimal conditions. Nucleotide sequence analysis of the MMH-D3 cell-adapted HAV revealed that N1237D and D2132G substitutions were present in the capsid regions of six viral clones. These two mutations are most likely located on the surface of the virion and may play a role in the entry of HAV into the mouse liver cells. Our results demonstrate that mouse hepatocyte-like cells code for all factors required for the efficient growth of HAV in cell culture.

TIM-4 is the ligand for TIM-1, and the TIM-1-TIM-4 interaction regulates T cell proliferation

The newly identified TIM family of proteins is associated with regulation of T helper type 1 (T(H)1) and T(H)2 immune responses. TIM-1 is genetically linked to asthma and is a receptor for hepatitis A virus, but the endogenous ligand of TIM-1 is not known. Here we show that TIM-4, which is expressed by antigen-presenting cells, is the ligand for TIM-1. In vivo administration of either soluble TIM-1-immunoglobulin (TIM-1-Ig) fusion protein or TIM-4-Ig fusion protein resulted in hyperproliferation of T cells, and TIM-4-Ig costimulated T cell proliferation mediated by CD3 and CD28 in vitro. These data suggest that the TIM-1-TIM-4 interaction is involved in regulating T cell proliferation.

TIM-1 induces T cell activation and inhibits the development of peripheral tolerance

We have examined the function of TIM-1, encoded by a gene identified as an 'atopy susceptibility gene' (Havcr1*), and demonstrate here that TIM-1 is a molecule that costimulates T cell activation. TIM-1 was expressed on CD4(+) T cells after activation and its expression was sustained preferentially in T helper type 2 (T(H)2) but not T(H)1 cells. In vitro stimulation of CD4(+) T cells with a TIM-1-specific monoclonal antibody and T cell receptor ligation enhanced T cell proliferation; in T(H)2 cells, such costimulation greatly enhanced synthesis of interleukin 4 but not interferon-gamma. In vivo, the use of antibody to TIM-1 plus antigen substantially increased production of both interleukin 4 and interferon-gamma in unpolarized T cells, prevented the development of respiratory tolerance, and increased pulmonary inflammation. Our studies suggest that immunotherapies that regulate TIM-1 function may downmodulate allergic inflammatory diseases.

The role of endotoxin and its receptors in allergic disease

OBJECTIVE

To summarize the existing literature on the association of endotoxin with respiratory diseases and allergic sensitization and to review the potentially modifying effects of endotoxin receptor polymorphisms.

DATA SOURCES

English-language articles were identified from the MEDLINE and PubMed databases using combinations of the following search terms: endotoxin, toll-like receptor, polymorphisms, atopy, asthma, and allergy. Other sources included experts in the field and the bibliographies of pertinent articles.

STUDY SELECTION

Relevant articles were selected based on the authors' expert opinion.

RESULTS

Cross-sectional studies, particularly those of children raised in rural European communities, suggest that early endotoxin exposure may protect against the development of allergic sensitization and atopic asthma. However, endotoxin exposure may also contribute to other nonatopic respiratory disorders and may exacerbate disease in individuals with preexisting asthma. Paradoxically, among individuals exposed to high levels of endotoxin, carriers of a functional mutation in toll-like receptor 4, which reduces cellular responsiveness to endotoxin, may be at lower risk of developing allergic sensitization.

CONCLUSIONS

The effect of endotoxin exposure on allergic sensitization and asthma appears to be influenced by the timing of exposure, the presence or absence of preexisting disease, and polymorphisms in the genes that encode endotoxin receptors. Further studies are needed to define the window period for this effect, as well as the underlying immunologic mechanism.

Building characteristics affect the risk of allergy development

Damp dwellings increase the risk for house dust mite (HDM) infestation in temperate climate zones and may be associated with an increased risk for allergic disease. The aim of the study was to assess possible relationships between allergen levels in house dust, characteristics of residence buildings and allergic diseases in children. A subsample of 12-yr-old children, having the same address in 1991 and 1996, was selected from a population-based sample of children from the Göteborg area. Health inspectors examined the residences of all the 109 children and several different building characteristics including humidity and indoor temperature were collected. Dust samples for analysis of HDM allergens were collected from the children's beds, and for analysis of cat and dog allergens from the living room. Current health status was assessed by questionnaires, interviews and skin prick tests (SPT). Dog or cat allergens were found in all houses, even in houses without such animals. HDM allergens were found in 60% of the houses, but only six of them had levels exceeding 2 microg/g dust. There was a strong association between HDM-infestation and wheeze, but not with specific sensitization to HDM. The type of building (houses when compared with flats), the ventilation system and the presence of a basement had all major implications on respiratory symptoms, atopy and HDM infestation. We can conclude that dog or cat allergens were found in all houses, and a strong association between HDM infestation and indoor environment. Building construction affected both respiratory morbidity and sensitisation independently, suggesting not only worsening of symptoms but also a causative relationship with disease development.

Th1/Th2 cells in inflammatory disease states: therapeutic implications

Inflammation is initiated as a protective response by the host, but can often result in systemic pathology. Among cells of the immune system, T lymphocytes play a major role in the inflammatory response. T cell inflammation is characterised histologically by an infiltration of mononuclear cells. Key regulators of this response are a subset of T lymphocytes called T helper (Th) cells. These cells secrete soluble mediators called cytokines, which orchestrate the immune response. The appropriate regulation of Th cell immunity is critical in the control and prevention of diverse disease states. This review will focus on the role of Th cells in the inflammatory process involved in allergic disease, diabetes, infectious disease, rheumatoid arthritis, heart disease, multiple sclerosis and cancer. In the area of autoimmunity, in particular, a basic understanding of Th cells and cytokines has contributed to the development of clinically efficacious biological agents. This review also examines current and novel treatment strategies under investigation at present that regulate Th cell immunity, which may result in better treatments for immune-mediated diseases.

Childhood asthma hospitalization risk after cesarean delivery in former term and premature infants

BACKGROUND

Cesarean delivery modifies infant gut bacterial flora composition, which may result in hindered tolerance to allergenic substances, thereby increasing the risk of asthma in accordance with the hygiene hypothesis. Results of previous studies regarding an association between birth route and asthma are conflicting, and these studies have not evaluated some potential confounding effects, including prematurity and maternal asthma.

OBJECTIVE

To determine whether cesarean delivery in full-term and premature infants increases the risk of subsequent childhood asthma hospitalization.

METHODS

We conducted a case-control study using the Washington State Birth Events Record Database linked to statewide hospitalization data. The study included 2,028 children hospitalized for asthma (cases) and 8,292 age-matched controls.

RESULTS

Cesarean delivery was modestly associated with an increased risk of asthma hospitalization (odds ratio [OR], 1.20; 95% confidence interval [CI], 1.04-1.39). However, when analyzed separately, there was an association between cesarean delivery and asthma hospitalization in premature infants (OR, 1.90; 95% CI, 1.09-3.02) but not in full-term infants (OR, 1.15; 95% CI, 0.97-1.34).

CONCLUSIONS

Cesarean delivery was associated with subsequent asthma hospitalization only in premature infants. Because mothers with asthma are reported to have increased rates of cesarean delivery and premature delivery, other factors in addition to the hygiene hypothesis, including genetic and in utero influences associated with maternal asthma, may contribute to the increased risk of asthma in premature infants.

Does atopy protect against enteric infections?

Allergic sensitization to inhaled allergens and foods is common and is linked to the development of asthma, allergic rhinitis and eczema. In view of the morbidity associated with these diseases it is not clear why the genetic predisposition to atopy is so common. Children who are atopic are less likely to develop oral tolerance to ingested foods and as a result are more likely to have manifestations of food allergy including wheezing, skin rashes and gastrointestinal symptoms. Oral tolerance also develops to micro-organisms in the gastrointestinal tract. Children who are atopic may be protected against enteric infections in early childhood because they are less likely to be tolerant of these organisms. This may explain why individuals who are atopic are less likely to have evidence of previous infection with hepatitis A and other enteric organisms although they are just as likely to have been infected with respiratory pathogens.

Association of hepatitis A virus infection with allergic sensitization in a population with high prevalence of hepatitis A virus exposure

BACKGROUND

An inverse association between allergic sensitization and markers of exposure to food-borne and orofecal infections (particularly hepatitis A virus, HAV) has been reported. The prevalence of HAV exposure and allergic sensitization vary widely in different areas, and vary along with age within a given area.

AIM

To investigate the association between HAV exposure and allergic sensitization in adults from a mostly rural area of Spain.

METHODS

An age-stratified random sample of 720 subjects was drawn from the population older than 18 years of A-Estrada, Spain. From 697 eligible subjects, 469 (67.2%, median age 54 years, range: 18-92) participated in the study. Positive skin prick tests to a panel of aeroallergens defined allergic sensitization. Positive serum HAV antibodies (assayed in 465 subjects) defined HAV exposure.

RESULTS

The prevalence of HAV exposure was 83.6% (95% CI: 80.7-86.5). The prevalence of allergic sensitization was lower in subjects with HAV exposure than in patients without it (25.0%vs 40.0%, OR 0.44, 95% CI: 0.25-0.77, P=0.004), but this association became substantially altered after adjusting for age, which was closely linked to both allergic sensitization and HAV exposure (adjusted OR 1.15, 95% CI: 0.60-2.19, P=0.66).

CONCLUSIONS

In a population with high prevalence of HAV exposure, no significant association between HAV exposure and allergic sensitization is observed after controlling for the confounding effect of age.

Farming exposure in childhood, exposure to markers of infections and the development of atopy in rural subjects

BACKGROUND

Within the context of the hygiene hypothesis, we aimed to study the potential association between farming-related risk factors and Toxoplasma gondii (T. gondii) as well as Helicobacter pylori (H. pylori) seropositivity.

METHODS

The study included questionnaire data and serum samples of 321 young adults living in a rural environment. Serum samples were analysed for specific IgE to a common panel of aeroallergens (SX1) as well as IgG against T. gondii and H. pylori.

RESULTS

Regular contact with animal stables before the age of 3 years (odds ratio (OR) (95% confidence interval): 2.0 [1.0; 4.0]) and unpasteurized milk consumption at age 6 years (1.8 [1.0; 3.3]) were the strongest risk factors for T. gondii infection. None of the farming-related factors were significantly associated with H. pylori infection. Current consumption of raw farm milk was not significantly associated with H. pylori infection (2.1 [0.8; 5.3]). Regular contact with animal houses before the age of 7 years was the strongest predictor for atopy (0.49 [0.26-0.96]). The reduction in risk could not be further decreased by any other factor under consideration. After adjustment for animal house contact, the OR for atopy was decreased by raw milk consumption and H. pylori infection in an additive manner.

CONCLUSION

Exposure to farming environments in childhood might predict T. gondii seropositivity in rural subjects. Nevertheless, the strongest predictor for atopy in rural subjects seems to be regular contact with farm animals. Whether T. gondii infection is an intermediate factor in the association between farm contact and atopy needs to be confirmed in larger studies.

Toll-like receptor ligands: hygiene, atopy and therapeutic implications

PURPOSE OF REVIEW

Allergic and certain other inflammatory diseases have become more common in industrialized countries over the past few decades. One potential explanation for such trends is that with a decreased incidence of microbial exposures, as a result of modern public health practices, an important source of immune stimulation has been lost, with a consequent increase in inflammatory responses and their associated diseases. This review will focus on our current understanding of how microbial exposures impact on host immunity and the pathogenesis of allergic diseases.

RECENT FINDINGS

In the past decade, it has become clear that a number of molecular interactions between immunocytes and microbial compounds are mediated by Toll-like receptors on host cells. Moreover, recent investigations have suggested that ligands for different Toll-like receptors have the potential both to inhibit and promote the development of allergic hypersensitivities and diseases.

SUMMARY

On the basis of studies discussed herein, we speculate that physiological exposures to Toll-like receptor ligands have important yet complex effects on immune homeostasis and host susceptibility towards atopic diseases. Moreover, we anticipate that a fuller understanding of how physiological Toll-like receptor ligand exposures impact on immune development will lead to novel therapeutic interventions for the prevention and treatment of atopic diseases.

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.

Infections, medication use, and the prevalence of symptoms of asthma, rhinitis, and eczema in childhood

BACKGROUND

The "hygiene hypothesis" postulates that infections during infancy may protect against asthma and atopy. There is also some evidence that antibiotic and/or paracetamol use may increase the risk of asthma.

METHODS

The study measured the association between infections, and medication use early in life and the risk of asthma at age 6-7 years. It involved 1584 children who had been notified to public health services with serious infections at age 0-4 years, and 2539 children sampled from the general population. For both groups, postal questionnaires were completed by parents.

RESULTS

There was little difference in the prevalence of current wheezing between the childhood infections group (prevalence = 23.5%) and the general population group (prevalence = 24.3%). There was also little difference whether the major site of infection was gastrointestinal (prevalence = 24.1%), invasive (prevalence = 24.6%) or respiratory (prevalence = 21.1%). However, in both groups, there were associations with antibiotic (OR = 1.78, 95% CI 1.49 to 2.14) or paracetamol (OR = 1.38, 95% CI 1.04 to 1.83) use in the first year of life or recent paracetamol use (OR = 2.10, 95% CI 1.78 to 2.49) and current wheezing. There was a weak protective effect of childhood infections in children who had not used antibiotics in the first year of life (OR = 0.78, 95% CI 0.55 to 1.10).

CONCLUSIONS

These findings are consistent with other evidence that antibiotic use early in life may increase the risk of asthma. They are also consistent with some preliminary evidence associating paracetamol use with an increased risk of asthma. Any protective effect of notifiable childhood infections was weak.

No epidemiological evidence for infant vaccinations to cause allergic disease

CONTEXT

The prevalence of allergic diseases has increased considerably over the last decades. The hygiene hypothesis has emerged, linking reduced microbial exposure and infections early in life with the development of allergic diseases. Especially some of currently available non-replicating infant vaccines are unlikely to mimic a natural infection-mediated immune response that protects against the development of allergic diseases. Moreover, several studies suggested infant vaccinations to increase the risk of allergic diseases.

OBJECTIVE

To determine whether infant vaccinations increase the risk of developing allergic disease.

DATA SOURCES

We searched MEDLINE from 1966 to March 2003 and bibliography lists from retrieved articles, and consulted experts in the field to identify all articles relating vaccination to allergy.

STUDY SELECTION AND DATA EXTRACTION

We selected epidemiological studies with original data on the correlation between vaccination with diphtheria, pertussis, tetanus (DPT), measles, mumps, rubella (MMR) and Bacillus Calmette-Guérin (BCG) vaccine in infancy and the development of allergic diseases, and assessed their quality and validity.

DATA SYNTHESIS

Methodological design and quality varied considerably between the studies we reviewed. Many studies did not address possible confounders, such as the presence of lifestyle factors, leaving them prone to bias. The studies that offer the stronger evidence, including the only randomized controlled trial at issue published to date, indicate that the infant vaccinations we investigated do not increase the risk of developing allergic disease. Furthermore, BCG does not seem to reduce the risk of allergies.

CONCLUSIONS

The reviewed epidemiological evidence indicates that, although possibly not contributing to optimal stimulation of the immune system in infancy, current infant vaccines do not cause allergic diseases.

Immunotherapy with mycobacteria

PURPOSE OF REVIEW

To summarize and evaluate critically recent progress with mycobacteria as a potential novel disease modifying treatment strategy in asthma.

RECENT FINDINGS

The link between exposure to pathogenic or saprophytic mycobacteria and protection from allergic diseases is still controversial, and recent epidemiological studies, which addressed only exposure to Mycobacterium tuberculosis or bacillus Calmette-Guérin, did not help to clarify this issue. Moreover, the clear efficacy of mycobacterial treatment seen in animal models has not been reproduced in human asthma, and a recent small study testing the hypothesis that heat-killed Mycobacterium vaccae attenuates asthmatic reactions after allergen challenge did not provide convincing results. However, it has been shown that treatment of mice with M. vaccae induces the generation of allergen-specific T regulatory cells capable of suppressing allergen-mediated eosinophilic lung inflammation, suggesting that a general deficiency of T regulatory cell activity might be responsible for the increased prevalence of asthma. This hypothesis is supported by findings that a lack of T regulatory cells, as found in genetic disorders of man and mouse attributable to a mutation of Foxp3, a transcription factor specifically expressed by T regulatory cells, is associated with manifestations of severe atopy and autoimmunity, precisely the spectrum of diseases linked to the hygiene hypothesis.

SUMMARY

Further studies on the relationship between mycobacteria and atopic disorders are needed, but there is reason to believe that the novel findings and molecular mechanisms associated with mycobacterial infections will further strengthen the currently unproved therapeutic value of immunotherapy with mycobacteria.

Population-based study of lymphoma in Germany: rationale, study design and first results

A multi-centre, population-based case-control study of lymphoma among adults was conducted in Germany from 1999-2003. The study comprised 700 incident cases (Hodgkin lymphomas and non Hodgkin's lymphoma, NHL) in the age range 18-80 years and 700 age-, sex- and area-matched controls obtained from population registries. Diagnosis was based on the REAL/WHO classification. Information on demographic characteristics, lifestyle, medical history and occupation was obtained by in-person interviews. Each participant was asked for a 24 ml blood sample. First results are focused on basic demographic characteristics, contact to animals, childhood diseases and vaccinations. Odds ratios (ORs) and 95% confidence intervals (CIs) were estimated using conditional logistic regression. The ORs for lymphoma were decreased for exposure to sheep and goats (OR = 0.7; 95% CI = 0.5-0.9), for rabbits and hare (OR = 0.7; 95% CI = 0.5-0.9), measles infection (OR = 0.6; 95% CI = 0.5-0.9), Bordetella pertussis infection (OR = 0.7; 95% CI = 0.6-0.95), and tetanus vaccination (OR = 0.5; 95% CI = 0.3-0.9). Increased risk of lymphoma was associated with exposure to cattle (OR = 1.3; 95% CI = 1.03-1.7) and immunization for tuberculosis (OR = 1.5; 95% CI = 0.997-2.4). The results of this study are partly consistent with the hygiene hypothesis. The inconsistencies of some of the findings with an explanation by the Th1/Th2 paradigm, however, warrant further research and may indicate that broader explanatory concepts are needed.

Allergy, asthma and markers of infections among Albanian migrants to Southern Italy

BACKGROUND

Studies of immigrants represent an useful tool to determine the relative relevance of environmental vs genetic factors in causing the reported rapid increase of the prevalence of sensitization and allergic diseases.

METHODS

A total of 152 Albanian migrants to Southern Italy responded to a questionnaire based on the European Community Respiratory Health Survey (ECRHS) and 139 of them underwent skin prick test, and 61 serological assays for total IgE and IgG antibodies against Toxoplasma gondii (TG), herpes simplex virus 1 (HSV-1), hepatitis A virus (HAV) and Helicobacter pylori (HP).

RESULTS

Reported asthma was rare (2/152; 1.3%) and reported nasal allergies rather frequent (24/152; 15.8%). Sensitization to common inhalant allergens occurred in 27/139 (19.4%) subjects. The frequency of skin sensitization to pollen (P = 0.003) and that of hay fever (P = 0.004) increased with the time spent in Apulia. All the 61 sera had antibodies against HAV, 59/61 (96.7%) against HSV-1, 48/61 (78.7%) against HP and 34/61 (55.7%) against TG. The prevalence of skin sensitization and hay fever symptoms were correlated to the duration of residence in Southern Italy.

CONCLUSIONS

Data presented indicate that Albanian migrants to Italy, in spite of the low prevalence of allergic diseases and sensitization in their country of origin, manifest with time an increasing prevalence of sensitization to local allergens and nasal symptoms after immigration to Italy. This would suggest a permanent role of allergen exposure and lifestyle factors in influencing the appearance of sensitization and symptoms of allergic diseases.

Influences in allergy: epidemiology and the environment

Many decades after the first descriptions of hay fever in the 19th century, we still struggle with the formation of a cogent conception embracing all the many fragments of insight into the various aspects of allergic and asthmatic diseases. Although in daily practice we can grasp and allocate most of the clinical manifestations, in research the fundamental structures are only in part recognized. We therefore presume that the underlying mechanisms are complex and multifaceted. We have explored in countless directions while branching into ever more detailed analyses of phenomena, all of which are associated with the conditions of interest. But how can we rearrange these pieces to advance toward a broader understanding of asthma and allergies? Epidemiology as a scientific tool to study the occurrence, the natural course, and the determinants of a condition in various sociocultural environments might contribute to the edifice of such a scaffold. This review does not intend to present a comprehensive digest of the relevant literature: the reader can refer to other "state of the art" reviews. The intention is to discuss some conceptual cornerstones and to illustrate them with a few examples.

Clinical phenotypes of asthma

PURPOSE OF REVIEW

Asthma is a phenotypically heterogeneous disorder and, over the years, many different clinical subtypes of asthma have been described. A precise definition of asthma phenotypes is now becoming more and more important, not only for a better understanding of pathophysiologic mechanisms, but in particular to ascertain the specific genes associated with these phenotypes.

RECENT FINDINGS

In children, three asthma phenotypes are now well defined: transient infant wheezing, nonatopic wheezing of the toddler, and IgE-mediated wheezing/asthma. Recently, a fourth phenotype, late-onset childhood asthma has been added to this list. In adults, asthma persisting from childhood into adulthood should be distinguished from asthma starting in adulthood. The phenotypes of adult-onset asthma are still poorly defined. Until now, phenotypic classification has been based primarily on etiologic factors (eg, aspirin sensitivity, persistent respiratory infections, occupational factors, or toxic exposures) or clinical characteristics of the disease (eg, mild, severe, brittle, near fatal, with fixed airflow obstruction, steroid resistant). Novel noninvasive techniques to assess the type and severity of airway inflammation and dysfunction are increasingly used to identify better the different phenotypes.

SUMMARY

The classic phenotype of IgE-mediated asthma starting in childhood is now clearly defined. However, many other phenotypes of asthma in childhood as well in adulthood are being recognized. In particular, asthma starting in adulthood and noneosinophilic asthma constitute an important part of the adult asthma population, and are still poorly defined. A precise definition of these asthma phenotypes is urgently needed because they are likely to be associated with different genotypes, responses to treatment, and prognoses.

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.

Asthma and atopy - the price of affluence?

Irrespective of improved knowledge of many aspects of atopic diseases, the unfavorable trends in their prevalence particularly among children could not have been reversed. A growing body of evidence suggests that something may lack from our societal affluence that has the capacity to provide protection against the development of atopic diseases. Much attention during the last years has been devoted to the hygiene hypothesis. This review outlines the impact of environment and lifestyle, particularly from the perspective of the East-West gradient, on the development of atopic diseases, with a special emphasis on the hygiene hypothesis in its broadest sense.

TIM-1, a novel allergy and asthma susceptibility gene

Atopic diseases, including asthma, allergic rhinitis, and atopic dermatitis, are caused by environmental factors in genetically predisposed individuals. Although the prevalence of these diseases has risen dramatically over the past two decades, it has been difficult to identify the underlying causes of these diseases due to the complex interplay between the genetic and environmental factors involved. Using a congenic mouse model of asthma, we simplified this complex trait and identified the novel T cell immunoglobulin domain, mucin-like domain (TIM) gene family, that encodes transmembrane proteins expressed by CD4 T cells. Recent studies demonstrate that the TIM family, particularly TIM-1, plays a critical role in immune responses that regulate the development of atopic diseases. In humans, certain polymorphic variants of TIM-1 are strongly associated with protection against atopy, and this association occurs only in individuals who have had past infection with hepatitis A virus (HAV). Since TIM-1 functions as the cellular receptor for HAV, activation of T cells through TIM-1 by HAV or by its natural ligand may affect T cell differentiation and the development of Th2-driven allergic inflammatory responses. Epidemiologically, HAV infection is associated with a reduced risk of developing atopy, and because the incidence of HAV infection has been significantly reduced in industrialized countries over the past 30 years, the discovery of a genetic interaction between HAV and TIM-1 provides the first molecular genetic evidence for the hygiene hypothesis.

Atopic diseases of childhood

PURPOSE OF REVIEW

The incidence of atopic diseases, including atopic dermatitis, allergic rhinitis, and asthma, has increased in developed countries over the past several decades. These diseases comprise a large component of general pediatric practice. This review will highlight some of the recent advances in understanding the pathogenesis and natural history of these diseases, as well as the current approaches to the treatment of children with atopic diseases.

RECENT FINDINGS

Recent studies have identified multiple risk factors for the development and progression of atopic diseases. As a result, much research is focused on identifying therapies that can be initiated at a young age to prevent disease progression. New treatment options have become available in recent years, such as topical immunomodulators for atopic dermatitis, leukotriene antagonists for seasonal allergic rhinitis, and alpha-immunoglobulin E therapy for asthma. The importance of viewing allergic rhinitis and asthma as disorders of a single airway has been emphasized. Finally, an update on the national asthma guidelines was recently released in an effort to promote optimal asthma care.

SUMMARY

This review summarizes many of the recent advances in the diagnosis and treatment of atopic diseases in children. Although not intended to be a comprehensive review of this broad field, it provides a framework for appreciating the complexity of these diseases and for effectively managing them.

T cell activation, from atopy to asthma: more a paradox than a paradigm

During the last 15 years, it was largely shown that allergic inflammation was orchestrated by activated Th2 lymphocytes, leading to IgE production and eosinophil activation. Indeed, Th2 activation was shown to be necessary to induce allergic sensitization in animal models. In humans, a Th2 skewing was shown in atopic children soon after birth. In asthma, descriptive studies showed that Th2 cells were more numerous in patients than in controls. In addition, during specific allergen stimulation, an increase of Th2 cells was described in most cases. According to this Th2 paradigm, it was proposed that early avoidance of microbial exposure could explain the increase of atopic diseases seen in the last 20 years in developed countries, as the "hygiene hypothesis". Recently, it was proposed that early exposure to lipopolysaccharide (LPS) could be protective against atopic diseases. However, it is well established that exposure to LPS can induce asthma symptoms, both in animals and humans, although it induces a Th1 inflammatory response. In addition, most infections induce asthma exacerbations and Th1 responses. Recently, some studies have showed that some Th1 cells were present in asthmatic patients, which could be related to bronchial hyperreactivity. There is therefore an "infectious paradox" in asthma, which contributes to show that the Th2 paradigm is insufficient to explain the whole inflammatory reaction of this disease. We propose that the Th2paradigm is relevant to atopy and inception of asthma albeit a Th1 activation would account at least in part for bronchial hyperreactivity and asthma symptoms.

Alteration of hepatitis A virus (HAV) particles by a soluble form of HAV cellular receptor 1 containing the immunoglobin-and mucin-like regions

Hepatitis A virus (HAV) infects African green monkey kidney cells via HAV cellular receptor 1 (havcr-1). The ectodomain of havcr-1 contains an N-terminal cysteine-rich immunoglobin-like region (D1), followed by a mucin-like region that extends D1 well above the cell surface. D1 is required for binding of HAV, and a soluble construct containing D1 fused to the hinge and Fc portions of human immunoglobin G1 (IgG1), D1-Fc, bound and neutralized HAV inefficiently. However, D1-Fc did not alter the virions. To determine whether additional regions of havcr-1 are required to trigger uncoating of HAV, we constructed D1muc-Fc containing D1 and two-thirds of the mucin-like region fused to the Fc and hinge portions of human IgG1. D1muc-Fc neutralized 10 times more HAV than did D1-Fc. Sedimentation analysis in sucrose gradients showed that treatment of HAV with 20 to 200 nM D1muc-Fc disrupted the majority of the virions, whereas treatment with 2 nM D1muc-Fc had no effect on the sedimentation of the particles. Treatment of HAV with 100 nM D1muc-Fc resulted in low-level accumulation of 100- to 125S particles. Negative-stain electron microscopy analysis revealed that the 100- to 125S particles had the characteristics of disrupted virions, such as internal staining and diffuse edges. Quantitative PCR analysis showed that the 100- to 125S particles contained viral RNA. These results indicate that D1 and the mucin-like region of havcr-1 are required to induce conformational changes leading to HAV uncoating.

The TIM gene family: emerging roles in immunity and disease

The search for cell-surface markers that can distinguish T helper 1 (T(H)1) cells from T(H)2 cells has led to the identification of a new gene family, encoding the T-cell immunoglobulin mucin (TIM) proteins, some of which are differentially expressed by T(H)1 and T(H)2 cells. The role of the TIM-family proteins in immune regulation is just beginning to emerge. Here, we describe the various TIM-family members in mice and humans, and discuss the genetic and functional evidence for their role in regulating autoimmune and allergic diseases.

IgG antibodies against microorganisms and atopic disease in Danish adults: the Copenhagen Allergy Study

BACKGROUND

Seropositivity to food-borne and orofecal microorganisms (hepatitis A virus, Helicobacter pylori, and Toxoplasma gondii ), which are considered to be markers of poor hygiene, has been reported to be associated with a lower prevalence of atopy. In contrast, colonization of the gut with Clostridium difficile, a potential intestinal bacterial pathogen, in early childhood may be associated with a higher prevalence of atopy.

OBJECTIVE

The objective of this study was to investigate the association between atopy and exposure to 2 groups of food-borne and orofecal microorganisms: (1) markers of a poor hygiene and (2) intestinal bacterial pathogens.

METHODS

A cross-sectional population-based study of 15- to 69-year-olds living in Copenhagen, Denmark, was carried out in 1990 to 1991. Atopy was defined as a positive test result for specific IgE to at least 1 of 6 inhalant allergens. Exposure to microorganisms was assessed as IgG seropositivity to microorganisms.

RESULTS

Seropositivity to 2 or 3 markers of poor hygiene (hepatitis A virus, H pylori, and T gondii ) was associated with a lower prevalence of atopy (adjusted odds ratio, 0.5; 95% CI, 0.3 to 0.8). In contrast, seropositivity to 2 or 3 intestinal bacterial pathogens (C difficile, Campylobacter jejuni, and Yersinia enterocolitica ) was associated with a higher prevalence of atopy (adjusted odds ratio, 1.7; 95% CI, 1.2 to 2.6).

CONCLUSION

Exposure to markers of poor hygiene was associated with a lower prevalence of atopy, whereas exposure to intestinal bacterial pathogens was associated with a higher prevalence of atopy. These findings raise the hypothesis that different groups of food-borne and orofecal microorganisms may have different effects on the risk of atopy.

Hygiene hypothesis: fact or fiction?

The hygiene hypothesis of asthma and allergy has recently received a swell of popularity and published supporting evidence, and has been extended to autoimmune conditions of childhood. Broadly stated, naturally occurring infections and microbial exposures might essentially immunize against the development of asthma and allergic and autoimmune diseases. If true, then reductions in nature's immunotherapy over the past century might be a major factor in the global increase of these conditions (eg, the higher prevalence of asthma and allergies in urban metropolitan areas compared with rural and farm communities) and might lead to new therapies for these conditions. Although such a unifying hypothesis has great appeal, currently it is only speculation about what might be at the end of the investigative road. How close are the current studies to establishing a causal relationship between microbial exposures and a reduction in allergic, asthmatic, and autoimmune disease prevalence? A systematic epidemiologic appraisal of the current hygiene hypothesis evidence can provide a critical analysis of what is currently known and an investigative blueprint for future studies that can ultimately prove causation and improve recommendations, interventions, and therapies.

Advances in childhood asthma: hygiene hypothesis, natural history, and management

There is significant interest in early identification and intervention in childhood asthma. Current asthma guidelines identify inhaled corticosteroids (ICS) as the preferred initial long-term control therapy even in young children. ICS clearly improve asthma control in children with mild to moderate persistent asthma, but it is not clear that they can alter the natu-ral history and progression of asthma. New insights regarding the origins of asthma and allergy and their natural history will continue to stimulate questions regarding the appropriate time for intervention and will stimulate the design of new treatment strategies and the discovery of new medications.

Role of intestinal flora in the development of allergy

PURPOSE OF REVIEW

The frequency of allergic diseases is increasing worldwide. Experimental and clinical studies have linked a reduced number of early infections to this trend. The gastrointestinal system, which comprises the largest lymphoid tissue and microbial reservoir of the body, has received more attention during the last few years as a potential determiner in the development of atopic disease.

RECENT FINDINGS

Alterations in intestinal microbiota have been detected both in infants suffering from allergic disease and in those later developing the disorder. Delay in the compositional development of and in gut microflora was a general finding in allergic children. In a subsequent study, perinatal administration of lactobacilli halved the later development of atopic eczema during the first 2 years of life. Specific strains of the healthy gut microbiota have been shown to induce the production of IL-10 and transforming growth factor-beta, which possess an important regulative role in the development of allergic type immune response. Probiotics also strengthen gut defence barrier mechanisms and reduce antigen load in the gut. Pattern recognition receptors in intestinal epithelial and antigen-presenting cells have been demonstrated to mediate a continuing dialogue between host and gut microbiota.

SUMMARY

Despite several promising findings, the exact role of gut normal microbiota in the development of allergy remains to be elucidated. For successful interventions, more data concerning a communication between host and specific microbial species are needed.

The role of respiratory viruses in the origin and exacerbations of asthma

PURPOSE OF REVIEW

The present review focuses and comments on the increasing body of evidence correlating respiratory viral infections with asthma onset and exacerbations.

RECENT FINDINGS

Recent data suggest multiple and some time contrasting roles for viral infection in the origin of asthma. These data also indicate that the immune status of the host, including atopy, may interactively contribute to this process, conferring susceptibility or even resistance to the development of asthma in virus-infected individuals. In the presence of asthma, the role of viral infection in triggering exacerbations is clearly established. Chemokine and cytokine responses of the respiratory epithelium, a biased type 1/type 2 cytokine balance, defective costimulation, as well as abnormal neural control have been suggested as possible mechanisms. The importance of concurrent or synergistic effects of allergen exposure is currently under scrutiny.

SUMMARY

Viruses may initiate and certainly exacerbate asthma. Mild repeated infections early in life could also stimulate type 1 immune responses conferring protection from atopy and asthma. The host's immune status, the type of viral infection and the timing of exposure to various environmental stimuli are probably the key factors in this process. Mechanistic insights deduced from recent work should allow for the development of intervening strategies in the near future.

Asthma: environmental and occupational factors

Asthma is in several ways a difficult disease to study. Generally arising in childhood, its pattern is often one of remission and relapse; at any point there are difficulties in translating its characteristic, clinical features into an operational definition. Geographical and temporal patterns in its distribution - whereby the disease appears to have increased in frequency in more 'westernised' countries -suggest strong environmental determinants in its causation although there are, too, undoubted and important genetic influences on both its incidence and presentation. Recent aetiological research has concentrated on the function of allergen exposure or on the role of early-life microbial contact that may regulate the development of a range of childhood allergies, including asthma. To date the 'hygiene hypothesis' offers the most efficient explanation for the distribution of the disease in time and place although convincing evidence for it remains elusive.

Inner-city asthma and the hygiene hypothesis

OBJECTIVE

Our goals were to analyze some of the similarities and differences in the increase in asthma, hay fever, and atopic sensitization between Europe and the United States and attempt to explain "inner-city asthma" within the framework of the hygiene hypothesis.

DATA SOURCES

We reviewed historical descriptions of hay fever and asthma as well as the currently available related literature.

STUDY SELECTION

The authors' judgment was used in the selection of historical and epidemiologic evidence.

RESULTS

Analyses of patterns of risk factors for allergic rhinoconjunctivitis and asthma in Europe led to a causal theory of the epidemic: the hygiene hypothesis. This theory claims that hygiene removed a protective influence against atopy and asthma that was once provided by exposure to infections in early life. This hypothesis has been questioned in the United States, where allergic asthma since the 1970s has increased among minorities living in poverty and with suboptimal hygienic conditions (inner-city asthma). When seen from a historical perspective, the recent increasing trend in respiratory allergies among the less advantaged in the United States may be explained as the consequence of several epiphenomena linked to westernization (including declining exposure to foodborne and orofecal infections) that first affected the richest socioeconomic strata during the 19th century, expanded among the middle classes during the first half of the 20th century, and eventually cascaded down to affect the least-advantaged Americans.

CONCLUSION

Inner-city asthma may be the final stage of a class-driven urbanization and westernization that began 2 centuries ago in the United States and that is now coming full circle.