Microbial exposures have been suggested to confer protection from allergic disorders and reduced exposures to gastrointestinal microbiota have been proposed as an explanation for the increase in asthma prevalence. Since the general prevalence of Helicobacter pylori has been decreasing, we hypothesized that H. pylori serostatus would be inversely related to the presence of asthma.
Adults were recruited to participate in the New York University (NYU)/Bellevue Asthma Registry in New York City. Adult asthma cases (N = 318) and controls (N = 208) were identified and serum IgG antibodies to H. pylori whole cell antigens or the immunodominant CagA antigen were measured.
As expected, the asthma cases and controls differed with respect to atopy and lung function. Seropositivity to H. pylori or CagA antigen was present in 47.1% of the total case and control study population. Asthma was inversely associated with CagA seropositivity (OR = 0.57, 95% CI = 0.36–0.89). Median age of onset of asthma (doctor's diagnosis) was older (21 years) among individuals with CagA+ strains than among H. pylori- individuals (11 years) (p = 0.006).
These data are consistent with the hypothesis that colonization with CagA+ H. pylori strains is inversely associated with asthma and is associated with an older age of asthma onset in an urban population. The data suggest H. pylori as a marker for protection.
Citation: Reibman J, Marmor M, Filner J, Fernandez-Beros M-E, Rogers L, Perez-Perez GI, et al. (2008) Asthma Is Inversely Associated with Helicobacter pylori Status in an Urban Population. PLoS ONE 3(12): e4060. https://doi.org/10.1371/journal.pone.0004060
Editor: Dominik Hartl, Yale University School of Medicine, United States of America
Received: September 16, 2008; Accepted: November 28, 2008; Published: December 29, 2008
Copyright: © 2008 Reibman et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: MO 1RR00096, ES 000260 (JR, MM), RO1 GM63270 (MB), Colton Family Foundation, The Diane Belfer Program in Human Microbial Ecology, Senior Scholar Award of the Ellison Medical Foundation (MB). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: Martin J. Blaser, as co-discoverer of cagA at Vanderbilt University can receive royalties from the commercial exploitation of cagA. No diagnostic tests are currently licensed. Dr. Reibman has no conflicts of interest to disclose. Dr. Marmor has no conflicts of interest to disclose. Dr. Filner has no conflicts of interest to disclose. Dr. Fernandez-Beros has no conflicts of interest to disclose. Dr. Rogers has no conflicts of interest to disclose. Dr. Perez Perez has no conflicts of interest to disclose.
The prevalence of atopy and asthma has increased worldwide . The “hygiene hypothesis,” that reduced childhood exposure to microorganisms modifies polarized Th1/Th2 responses leading to more allergic disorders, has been proposed to explain this increase . Relevant microbial exposures may include gastrointestinal biota. Intestinal microbiota differ between healthy infants in countries with low or high allergy prevalence, as well as between allergic and non-allergic infants . Early exposure to orofecal microbes such as Hepatitis A, appears to protect against allergen sensitization –, and in Italian military recruits and Danish adults, HAV, Toxoplasma gondii, and Helicobacter pylori, are inversely associated with atopy , .
H. pylori, gram-negative, microaerophilic gastric bacteria persistently colonize much of the world's population. Whereas nearly all adults are H. pylori-positive in developing countries, with socioeconomic development, prevalence has decreased substantially , . H. pylori is almost exclusively acquired in childhood ,  and antibody responses are present for decades or for life, consistent with the persistent gastric colonization , , . H. pylori virulence is affected by the presence of the 35-40-kb cag pathogenicity island that can be detected by identification of the cagA gene or its product (CagA) . CagA+ strains are more host-interactive , . H. pylori colonization induces continuous gastric inflammation, which is more pronounced with cagA+ strains , and leads toward diminished gastric acidity . Serologic assays to detect antibodies to the CagA protein enhance overall detection of H. pylori, and specifically detection of the more interactive (CagA+) organisms . Antibodies to CagA persist for at least two decades in the absence of antimicrobial treatments that eliminate H. pylori .
We hypothesized that the presence of H. pylori antibodies would be inversely related to asthma and that cagA+ strains of H. pylori would have a more pronounced inverse relationship with asthma. A recent cross-sectional study of adults in Iceland, Estonia and Sweden suggested an inverse association of antibodies to H. pylori and self-reported hay fever or asthma . In the NHANES III population, we demonstrated an inverse association of ever having had asthma with a cagA+ H. pylori strain  and in the NHANES IV population, in which CagA testing was not done, we found inverse associations of H. pylori status with childhood-onset asthma and allergic disorders . We now provide evidence showing an inverse association of CagA serology with asthma in a case control study of an additional well-characterized and separate adult urban population.
Asthma cases and non-asthma controls were recruited to participate in the New York University (NYU)/Bellevue Asthma Registry in New York City. The registry was approved by the Institutional Review Board of the New York University School of Medicine. All cases and controls signed informed consent. Letters informing patients of positive H. pylori serology were sent under an IRB-approved protocol. Cases were referred to the registry by the Bellevue Hospital Center Asthma Clinic and local clinics. Controls were referred by asthma cases and by enlisting individuals directly from the community and from other programs within Bellevue Hospital Center. A number of referrals from the cases were unintentionally genetically related and these individuals were accounted for in the statistical methods. Cases and controls were excluded if they were <18 or ≥65 years old; were current smokers; had a history of >10 pack-year tobacco use; or had an unstable cardiac disease, uncontrolled hypertension, lung disease other than asthma, or neuromuscular disease.
Questionnaires and evaluations were completed for 573 persons. Subjects were considered to have a diagnosis of “asthma” based on their response to questions derived from validated questionnaires ,  used for international studies of asthma. All patients were seen by a physician or nurse with extensive experience in asthma diagnoses and management. Because most of these adult patients were using chronic medication for asthma or had longstanding disease, a 12% change in FEV1 was not used as a criterion for diagnosis. We confirmed our diagnosis with the published algorithm of Enright et al. . Twelve persons who could not be classified as either having asthma or being asthma-free and 35 individuals without serum samples were excluded from statistical analyses. Race and ethnicity were self-classified. The final study population included 526 subjects (318 asthma cases and 208 controls).
Serum antibody analysis
Serum anti-H. pylori IgG antibody levels were determined by ELISA using whole cell antigens . CagA status was determined by a separate ELISA, based on the presence of serum IgG antibodies against orv220, a 65 kDa recombinant CagA truncated protein , . Absence of H. pylori was defined as negativity in both the whole cell and CagA assays (H. pylori negative). Individuals were defined as colonized with cagA negative H. pylori strains if they had antibodies to the whole cell antigen but not to CagA (H. pylori+/CagA−). Subjects were defined as being colonized with cagA+ H. pylori strains if they were positive for CagA antibodies, whether or not they were positive for H. pylori antibodies (CagA+). Thirty one persons (5.9%) were CagA+ but had H. pylori serological determinations that did not reach positive values, a finding consistent with previous studies in H. pylori culture-positive subjects .
Measurements of total serum IgE (total IgE) and allergen-specific IgE for allergens considered significant for the Northeastern United States were performed in a commercial laboratory (Pharmacia ImmunoCAP assay; Quest Diagnostics; Teterboro, NJ). Allergen results were available for 525 of the 526 subjects. An allergen-specific IgE level >0.35 kilo-international units (kIU)/L was considered positive.
Pre- and post-bronchodilator spirometry was performed according to American Thoracic Society guidelines ; normal values were obtained from Hankinson et al. . Values were obtained on 516 subjects, but were not available for 10 subjects.
Non-parametric Wilcoxon and Kruskall-Wallis tests were used for crude comparisons of quantitative variables among groups, and the chi-squared test used for comparisons of categorical variables. In multivariable analyses, generalized estimating equations (GEE) were used to confirm the findings because of the presence of matched sets of individuals that occurred when asthma cases referred family members to the study. Data from genetically related cases and controls (n = 104) were entered as “repeated measures” in GEE logistic and linear regression analyses to account for the potentially correlated nature of observations among related individuals . GEE was used with a logit link for logistic regression analyses of risk factors for asthma; risk factors for allergen-specific IgE, and separately, for seropositivity to either cagA− or cagA+ strains of H. pylori. In multivariable analyses, we first included all potential confounders in models and then dropped from the models those potential confounders that did not substantially affect the odds ratios of the variables of interest. GEE analyses were conducted using SAS 9.1 Proc GENMOD (SAS Institute Inc., Cary, NC, USA, 2002). Kaplan-Meier estimation and Cox proportional hazards regression were used to investigate correlates of age of diagnoses of asthma.
Characteristics of the study groups
Characteristics of the asthma cases and non-asthma controls are shown in Table 1. Cases and controls were similar in age and gender. Cases were more often Hispanic, and income levels were lower in the cases than in the controls. Hispanic ethnicity was not associated with asthma status, once income and race were adjusted for via logistic regression. As expected, total IgE was elevated in the cases compared to the controls, and there was a significant association of asthma with atopy, as defined by the presence of at least one allergen-specific IgE. Lung function parameters including post-bronchodilator forced expiratory volume in one second (FEV1), post-bronchodilator forced vital capacity (FVC), and the ratio of FEV1/FVC were reduced in the cases compared to controls. These characteristics are consistent with expectations for an asthma population compared to a control population.
H. pylori status and asthma
Table 2 shows the crude and adjusted odds ratios (OR) for asthma and H. pylori serostatus. Although there was a suggestion of an association of asthma with CagA+ status, the crude OR for asthma associated with H. pylori+/CagA− or CagA+ status failed reach significance. In contrast, after adjustment for race and income, there was a significant inverse association of asthma and CagA+ status with an OR of 0.63 (95% CI = 0.41–0.98). We also examined the association of atopy and CagA+ status with asthma using GEE logistic regression analysis to adjust for age, race, Hispanic ethnicity, income, and the genetic relatedness among some of the subjects (Table 3). As expected, atopy was associated with asthma. The significant inverse association of CagA+ status with asthma was again demonstrated (OR = 0.57, 95% CI = 0.36–0.89). Analyses that treated all subjects as independent also yielded similar results (data not shown). We also repeated the analyses limiting subjects to those who were neither genetically related nor referred by one another (N = 402). Unconditional logistic regression in this group, adjusted for age, income, education, race/ethnicity, and atopy continued to yield an inverse association of asthma with CagA+ status (OR = 0.49, 95% CI = 0.28–0.85). The inverse OR associated with CagA+ status also was not substantially altered when atopy was excluded from the model. Inclusion of variables representing presence of allergen-specific IgE antibodies or of log10IgE in the multiple logistic regression model for asthma also had no major impact on our primary finding of an inverse association with CagA+ H. pylori strains.
H. pylori as an asthma modifier
We examined the relationship between H.pylori serostatus and IgE. Log10IgE was not associated with either H. pylori+/CagA− status or CagA+ status after adjustment for differences in race, Hispanic ethnicity, and age (GEE, data not shown). No relationship was identified between atopy, and either H. pylori+/CagA− status or CagA+ status. Among atopic subjects (N = 356), 19.7% were H. pylori+/CagA− and 27.3% were CagA+, whereas among non-atopic subjects (N = 169), 19.3% were H. pylori+/CagA− and 27.8% were CagA+ (p = 0.99). Atopy also was not associated with H. pylori status in a multivariable GEE logistic regression model that included adjustments for race, Hispanic ethnicity, education and income. Similarly, atopy was not associated with H. pylori status in the control population (p = 0.8).
In addition, we examined whether H. pylori serostatus was an asthma modifier using post-bronchodilator FEV1 and FEV1/FVC as surrogates of asthma severity. There was a significant difference in FEV1 among the individuals with asthma who were CagA+ (N = 76, median % predicted = 82.0 (interquartile range, IQR = 71–92) compared to those who were H. pylori negative (N = 170, median % predicted = 90.0, IQR = 77–100) (p = 0.008), although FEV1/FVC was not different between the two groups (median = 80.0% predicted, IQR = 74.0–86.5 in CagA+ subjects and 82.0%, IQR = 76–85 in H. pylori− subjects) (p = 0.4).
We next asked whether H. pylori serostatus was associated with the age of onset of asthma. Age of onset of asthma was similar among the H. pylori+/CagA− individuals (N = 64, median age = 19, IQR = 6–30) and the CagA+ individuals (N = 71, median age = 21, IQR = 8–34), but was substantially lower among the H. pylori negative individuals (N = 159, median age = 11 y, IQR = 5–23) (p = 0.006). Similar differences in age were noted when we assessed age at onset of symptoms (data not shown). We performed Kaplan-Meier analysis of the probability of age-related survival until a doctor's diagnosis of asthma (Figure 1) assuming that acquisition of H. pylori occurred close to the time of birth, as suggested in the literature , , , . Cox regression analysis adjusted for age and income (adjustment for race/ethnicity did not affect the results of this analysis) suggested a reduced hazard ratio (HR) in CagA+ individuals compared to H. pylori− individuals (HR = 0.74, 95% CI = 0.54–1.02) whereas the HR for H. pylori+/CagA− individuals compared to H. pylori− individuals was not substantially different from 1 (HR = 0.93, 95% CI = 0.67–1.29). We further modeled ages at doctor's diagnosis of asthma by GEE to take into account potentially correlated times of diagnosis among genetically related family members and to adjust for potential confounders. This analysis showed a significantly greater age at doctor's diagnosis of asthma among CagA+ subjects compared with H. pylori-negative subjects (p = 0.02).
Reduced exposure to orofecal organisms has been suggested as an explanation for the increasing prevalence of atopy and asthma, but studies of multiple organisms have had conflicting results . We studied a racially and ethnically diverse urban population in a case control study to examine the relationship between asthma and H. pylori serostatus. H. pylori seroprevalence was common in both cases and controls, in accordance with national estimates . Our analysis identified a trend towards an inverse association between H. pylori and asthma that became significant when we examined individuals who carried cagA+ H. pylori strains. Our data provide evidence that in an urban population, asthma is inversely associated with serologic evidence of the presence of cagA+ H. pylori strains. We used several different analytical approaches, and analytical results were highly consistent. Our findings support and expand upon our recent cross-sectional study of separate populations of individuals with an asthma diagnosis in the NHANES III and NHANES IV populations , , and now include subjects with spirometry and serum IgE determinations. Our analysis of age of onset of disease in an adult population, in which we detect a delay on onset of asthma in CagA+ individuals, is consistent with the age relationships reported in the two NHANES populations.
We did not detect an effect of H. pylori serostatus on total IgE or the presence of atopy in our population. This finding suggests that the ability of H. pylori exposure to modify asthma may be mediated via mechanisms other than those associated with IgE levels. Interestingly, although we detected a delay in the onset of asthma associated with the presence of CagA+ serology, FEV1 was reduced in this group. The finding of reduced lung function in individuals with a later age of onset of asthma is consistent with those recently reported –.
Potential mechanisms by which H. pylori could alter asthma presentation include immune modifications or an effect on gastro-esophageal reflux disease (GERD). Although H. pylori colonization recruits neutrophils, T and B lymphocytes and macrophages to the stomach , H. pylori generally persists for the host's lifetime , reflecting immune evasion and modification of host inflammatory, innate, and adaptive immune responses , , , . H. pylori may alter the polarized Th1/Th2 T cell response through dendritic cell-mediated T-cell expression of IL-12, TNF-α, and IFN-γ –, and cagA translocation induces IL-12 production . H. pylori colonization induces regulatory T cells including CD4+ CD25+ T cells that express the forkhead box P3 transcription factor (Foxp3) ,  and also induces indoleamine 2,3-dioxygenase, mechanisms that suppress T cell function . IL-10 expression is increased in the gastric mucosa of children carrying H. pylori . Such immunosuppressive and immunomodulatory effects have the potential to alter the effector phase of asthma as recently shown in murine models of helminth infection . Helicobacter pylori upregulate Toll-like receptor 4 (TLR4) , and although results are complex, human and murine studies suggest that activation of TLR4 may be protective against allergic asthma .
Associations between GERD and asthma also are well-established –. Longitudinal studies show that asthma is a risk factor for development of GERD, and that GERD can trigger asthma –. H. pylori, especially cagA+ strains, are inversely associated with GERD , . Although we did not specifically assess for GERD in this study, the possibility exists that the inverse association between H. pylori and asthma reflects protection from GERD.
An alternative explanation for our findings is that H. pylori seronegativity is a surrogate for other phenomena such as the presence or absence of other indigenous biota, or merely reflects cumulative early life exposure to antibiotics, identified as a risk factor for asthma. Although possible, the specificity of the relationship to cagA+ strains argues against this point. There are some potential limitations to this study. Although we do not know the age at which H. pylori was acquired in the cases or controls, multiple studies have demonstrated that nearly all acquisition that occurs does so at an early age, usually before the age of five , . H. pylori antibodies reflect the present carriage of H. pylori, and its prior elimination due to antibiotic exposure could lead to seronegativity. Thus, the current serostatus could under-estimate H. pylori acquisition, but not persistence, since with long-term carriage of H. pylori, antibody levels are stable , .
One potential confounding factor could be greater antibiotic use in asthma cases than in controls, which would bias toward consequent elimination of H. pylori. The association of the inverse association with childhood-onset but not later-age onset asthma in this study (Figure 1), and two other recent studies ,  as well as the specificity of the effect with cagA+ positivity argues against that point. However, prospective studies will be needed to clarify this question.
Because adults were studied, the association of H. pylori with delay in asthma onset may be confounded by recall bias or delay in doctor diagnosis. Although these issues suggest the need for future prospective studies, our findings support those of our recent cross-sectional studies of NHANES populations , .
In conclusion, our data suggest that H. pylori, and specifically, CagA positivity is inversely associated with asthma and with a delay in the onset of asthma. That the association was strongest with cagA+ H. pylori strains suggests that the more intensive host-interaction of these organisms may influence disease expression.
We thank Maria Thomas, Nicole Beattie and Wanda Hoerning for their help in conducting this study, Susan Lee for technical support, and Robert Norman for analytic suggestions.
Conceived and designed the experiments: JR LR MJB. Performed the experiments: GPP. Analyzed the data: JR MM JF MEFB LR GPP MJB. Contributed reagents/materials/analysis tools: JR MM MEFB MJB. Wrote the paper: JR MM JF MJB.
- 1. Anderson HR (2005) Prevalence of asthma. Bmj 330: 1037–1038.
- 2. Strachan DP (2000) Family size, infection and atopy: the first decade of the “hygiene hypothesis”. Thorax 55: Suppl 1S2–10.
- 3. Sepp E, Julge K, Vasar M, Naaber P, Bjorksten B, et al. (1997) Intestinal microflora of Estonian and Swedish infants. Acta Paediatr 86: 956–961.
- 4. Bjorksten B (2004) Effects of intestinal microflora and the environment on the development of asthma and allergy. Springer Semin Immunopathol 25: 257–270.
- 5. Ramsey CD, Celedon JC (2005) The hygiene hypothesis and asthma. Curr Opin Pulm Med 11: 14–20.
- 6. Matricardi PM, Rosmini F, Ferrigno L, Nisini R, Rapicetta M, et al. (1997) Cross sectional retrospective study of prevalence of atopy among Italian military students with antibodies against hepatitis A virus. Bmj 314: 999–1003.
- 7. Matricardi PM, Rosmini F, Panetta V, Ferrigno L, Bonini S (2002) Hay fever and asthma in relation to markers of infection in the United States. J Allergy Clin Immunol 110: 381–387.
- 8. Matricardi PM, Rosmini F, Riondino S, Fortini M, Ferrigno L, et al. (2000) Exposure to foodborne and orofecal microbes versus airborne viruses in relation to atopy and allergic asthma: epidemiological study. Bmj 320: 412–417.
- 9. Linneberg A, Ostergaard C, Tvede M, Andersen LP, Nielsen NH, et al. (2003) IgG antibodies against microorganisms and atopic disease in Danish adults: the Copenhagen Allergy Study. J Allergy Clin Immunol 111: 847–853.
- 10. Dooley CP, Cohen H, Fitzgibbons PL, Bauer M, Appleman MD, et al. (1989) Prevalence of Helicobacter pylori infection and histologic gastritis in asymptomatic persons. N Engl J Med 321: 1562–1566.
- 11. Kosunen TU, Aromaa A, Knekt P, Salomaa A, Rautelin H, et al. (1997) Helicobacter antibodies in 1973 and 1994 in the adult population of Vammala, Finland. Epidemiol Infect 119: 29–34.
- 12. Blaser MJ, Atherton JC (2004) Helicobacter pylori persistence: biology and disease. J Clin Invest 113: 321–333.
- 13. Rowland M, Daly L, Vaughan M, Higgins A, Bourke B, et al. (2006) Age-specific incidence of Helicobacter pylori. Gastroenterology 130: 65–72; quiz 211.
- 14. Perez-Perez GI, Salomaa A, Kosunen TU, Daverman B, Rautelin H, et al. (2002) Evidence that cagA(+) Helicobacter pylori strains are disappearing more rapidly than cagA(−) strains. Gut 50: 295–298.
- 15. Blaser MJ (2005) The biology of cag in the Helicobacter pylori-human interaction. Gastroenterology 128: 1512–1515.
- 16. Odenbreit S, Puls J, Sedlmaier B, Gerland E, Fischer W, et al. (2000) Translocation of Helicobacter pylori CagA into gastric epithelial cells by type IV secretion. Science 287: 1497–1500.
- 17. Suerbaum S, Michetti P (2002) Helicobacter pylori infection. N Engl J Med 347: 1175–1186.
- 18. Kuipers EJ, Perez-Perez GI, Meuwissen SG, Blaser MJ (1995) Helicobacter pylori and atrophic gastritis: importance of the cagA status. J Natl Cancer Inst 87: 1777–1780.
- 19. Everhart JE, Kruszon-Moran D, Perez-Perez G (2002) Reliability of Helicobacter pylori and CagA serological assays. Clin Diagn Lab Immunol 9: 412–416.
- 20. Thjodleifsson B, Asbjornsdottir H, Sigurjonsdottir RB, Gislason D, Olafsson I, et al. (2007) Seroprevalence of Helicobacter pylori and cagA antibodies in Iceland, Estonia and Sweden. Scand J Infect Dis 39: 683–689.
- 21. Chen Y, Blaser MJ (2007) Inverse associations of Helicobacter pylori with asthma and allergy. Arch Intern Med 167: 821–827.
- 22. Chen Y, Blaser MJ (2008) Helicobacter pylori colonization is inversely associated with childhood asthma. J Infect Dis 198: 553–560.
- 23. Burney PG, Laitinen LA, Perdrizet S, Huckauf H, Tattersfield AE, et al. (1989) Validity and repeatability of the IUATLD (1984) Bronchial Symptoms Questionnaire: an international comparison. Eur Respir J 2: 940–945.
- 24. Asher MI, Keil U, Anderson HR, Beasley R, Crane J, et al. (1995) International Study of Asthma and Allergies in Childhood (ISAAC): rationale and methods. Eur Respir J 8: 483–491.
- 25. Enright PL, McClelland RL, Newman AB, Gottlieb DJ, Lebowitz MD (1999) Underdiagnosis and undertreatment of asthma in the elderly. Cardiovascular Health Study Research Group. Chest 116: 603–613.
- 26. Blaser MJ, Perez-Perez GI, Kleanthous H, Cover TL, Peek RM, et al. (1995) Infection with Helicobacter pylori strains possessing cagA is associated with an increased risk of developing adenocarcinoma of the stomach. Cancer Res 55: 2111–2115.
- 27. Romero-Gallo J, Perez-Perez GI, Novick RP, Kamath P, Norbu T, et al. (2002) Responses of endoscopy patients in Ladakh, India, to Helicobacter pylori whole-cell and Cag A antigens. Clin Diagn Lab Immunol 9: 1313–1317.
- 28. ATS (1995) Standardization of Spirometry, 1994 Update. American Thoracic Society. Am J Respir Crit Care Med 152: 1107–1136.
- 29. Hankinson JL, Odencrantz JR, Fedan KB (1999) Spirometric reference values from a sample of the general U.S. population. Am J Respir Crit Care Med 159: 179–187.
- 30. Hanley JA, Negassa A, Edwardes MD, Forrester JE (2003) Statistical analysis of correlated data using generalized estimating equations: an orientation. Am J Epidemiol 157: 364–375.
- 31. Banatvala N, Mayo K, Megraud F, Jennings R, Deeks JJ, et al. (1993) The cohort effect and Helicobacter pylori. J Infect Dis 168: 219–221.
- 32. Malaty HM, El-Kasabany A, Graham DY, Miller CC, Reddy SG, et al. (2002) Age at acquisition of Helicobacter pylori infection: a follow-up study from infancy to adulthood. Lancet 359: 931–935.
- 33. Everhart JE, Kruszon-Moran D, Perez-Perez GI, Tralka TS, McQuillan G (2000) Seroprevalence and ethnic differences in Helicobacter pylori infection among adults in the United States. J Infect Dis 181: 1359–1363.
- 34. Cassino C, Berger KI, Goldring RM, Norman RG, Kammerman S, et al. (2000) Duration of asthma and physiologic outcomes in elderly nonsmokers. Am J Respir Crit Care Med 162: 1423–1428.
- 35. Sorkness RL, Bleecker ER, Busse WW, Calhoun WJ, Castro M, et al. (2008) Lung function in adults with stable but severe asthma: air trapping and incomplete reversal of obstruction with bronchodilation. J Appl Physiol 104: 394–403.
- 36. Moore WC, Bleecker ER, Curran-Everett D, Erzurum SC, Ameredes BT, et al. (2007) Characterization of the severe asthma phenotype by the National Heart, Lung, and Blood Institute's Severe Asthma Research Program. J Allergy Clin Immunol 119: 405–413.
- 37. Everhart JE (2000) Recent developments in the epidemiology of Helicobacter pylori. Gastroenterol Clin North Am 29: 559–578.
- 38. Mai UE, Perez-Perez GI, Allen JB, Wahl SM, Blaser MJ, et al. (1992) Surface proteins from Helicobacter pylori exhibit chemotactic activity for human leukocytes and are present in gastric mucosa. J Exp Med 175: 517–525.
- 39. O'Keeffe J, Moran AP (2008) Conventional, regulatory, and unconventional T cells in the immunologic response to Helicobacter pylori. Helicobacter 13: 1–19.
- 40. Guiney DG, Hasegawa P, Cole SP (2003) Helicobacter pylori preferentially induces interleukin 12 (IL-12) rather than IL-6 or IL-10 in human dendritic cells. Infect Immun 71: 4163–4166.
- 41. Bergman MP, Engering A, Smits HH, van Vliet SJ, van Bodegraven AA, et al. (2004) Helicobacter pylori modulates the T helper cell 1/T helper cell 2 balance through phase-variable interaction between lipopolysaccharide and DC-SIGN. J Exp Med 200: 979–990.
- 42. Hafsi N, Voland P, Schwendy S, Rad R, Reindl W, et al. (2004) Human dendritic cells respond to Helicobacter pylori, promoting NK cell and Th1-effector responses in vitro. J Immunol 173: 1249–1257.
- 43. Segal ED, Cha J, Lo J, Falkow S, Tompkins LS (1999) Altered states: involvement of phosphorylated CagA in the induction of host cellular growth changes by Helicobacter pylori. Proc Natl Acad Sci U S A 96: 14559–14564.
- 44. Lundgren A, Stromberg E, Sjoling A, Lindholm C, Enarsson K, et al. (2005) Mucosal FOXP3-expressing CD4+ CD25high regulatory T cells in Helicobacter pylori-infected patients. Infect Immun 73: 523–531.
- 45. Beswick EJ, Pinchuk IV, Das S, Powell DW, Reyes VE (2007) Expression of the programmed death ligand 1, B7-H1, on gastric epithelial cells after Helicobacter pylori exposure promotes development of CD4+ CD25+ FoxP3+ regulatory T cells. Infect Immun 75: 4334–4341.
- 46. Raitala A, Karjalainen J, Oja SS, Kosunen TU, Hurme M (2005) Indoleamine 2,3-dioxygenase (IDO) activity is lower in atopic than in non-atopic individuals and is enhanced by environmental factors protecting from atopy. Mol Immunol.
- 47. Oderda G, Vivenza D, Rapa A, Boldorini R, Bonsignori I, et al. (2007) Increased interleukin-10 in Helicobacter pylori infection could be involved in the mechanism protecting from allergy. J Pediatr Gastroenterol Nutr 45: 301–305.
- 48. Wilson MS, Taylor MD, Balic A, Finney CA, Lamb JR, et al. (2005) Suppression of allergic airway inflammation by helminth-induced regulatory T cells. J Exp Med 202: 1199–1212.
- 49. Ishihara S, Rumi MA, Kadowaki Y, Ortega-Cava CF, Yuki T, et al. (2004) Essential role of MD-2 in TLR4-dependent signaling during Helicobacter pylori-associated gastritis. J Immunol 173: 1406–1416.
- 50. Garantziotis S, Hollingsworth JW, Zaas AK, Schwartz DA (2008) The effect of toll-like receptors and toll-like receptor genetics in human disease. Annu Rev Med 59: 343–359.
- 51. Harding SM, Richter JE (1997) The role of gastroesophageal reflux in chronic cough and asthma. Chest 111: 1389–1402.
- 52. Kiljander TO, Laitinen JO (2004) The prevalence of gastroesophageal reflux disease in adult asthmatics. Chest 126: 1490–1494.
- 53. Harding SM (2005) Gastroesophageal reflux: a potential asthma trigger. Immunol Allergy Clin North Am 25: 131–148.
- 54. Peek RM Jr, Blaser MJ (2002) Helicobacter pylori and gastrointestinal tract adenocarcinomas. Nat Rev Cancer 2: 28–37.