Role of microbial burden in aetiology of allergy and asthma

Environmental echoes

Allergy, auto immunity and cancer are becoming more prevalent in the developed world. One explanation might be that the immune system required to protect us from such problems is being inadequately trained, perhaps due to our increased separation from the environment which has shaped our mutating genes since we emerged from the primaeval ooze. Those infections which were the essential primers of our immunity are being prevented and action is needed to refocus the immune response without exposing us to the diseases of the past. In this paper we assess our place in relation to the environment and consider ways in which the situation can be redressed. There are considerable similarities between the immune system and human consciousness. Both enter the world in considerable ignorance of the events awaiting them, yet with the genetic ability, endowed by millennia of selection and evolution, to experience the world, to interpret and act on the experiences and to retain memory of the experiences. In both systems, maternal influences and early environmental encounters have profound effects on determining the patterns of subsequent responses. Ideally the 'learned' responses will benefit or protect the individual but inappropriate responses may lead to self damage. As the environment has altered irrevocably, attention must be paid to regulating the balance of immunological responsiveness to that expected of the normal immunological learning process. This should be possible by novel vaccination strategies.

Daycare attendance before the age of two protects against atopy in preschool age children

Early attendance at daycare has been shown to protect against atopy, as defined by a positive skin prick test. One proposed hypothesis for this association is that early exposure to other children protects against atopy by facilitating the spread of infections among children. An alternative hypothesis is that children attending daycare centers have less atopy due to lower levels of exposure to indoor allergens. Our aim was to determine whether attendance at daycare before age 2 years protects against atopy in Australian preschool age children and to test the two alternative hypotheses, as well as a number of potential confounding factors. We conducted a cross-sectional study of children aged 3-5 years living in one humid coastal city (Lismore, n = 286) and one dry, inland city (Wagga Wagga, n = 364) in New South Wales, Australia, in 1995. Atopy was assessed by skin prick tests to six common allergens. Daycare attendance and other possible risk factors for atopy were measured by a parent-completed questionnaire. Children who attended daycare before age 2 years had a reduced risk of atopy at 3-5 years. The greatest effect was seen in children who attended a daycare center (odds ratio (OR), 0.26; 95% CI, 0.14-0.50) rather than family daycare (OR, 0.66; 95% CI, 0.41-1.04). The results of this study do not support either of the proposed hypotheses, nor can the effect be explained by any of the other potential confounders measured. Further work is needed to determine the exposure that is responsible for the protective effect of daycare attendance on atopy.

Development of immunologic memory against tetanus toxoid and pertactin antigens from the diphtheria-tetanus-pertussis vaccine in atopic versus nonatopic children

BACKGROUND

Recent findings suggest that a hallmark of the atopic phenotype is reduced capacity to respond to vaccine antigens, as well as to environmental allergens, during infancy. This deficiency, which is most marked for the cytokine IFN-gamma, appears transient but can result in a long-lasting imbalance within T helper cell (T(H)) memory responses to allergens. Indirect evidence suggests that parallel effects may occur within immunologic memory responses against vaccine antigens in atopic children.

OBJECTIVE

Our purpose was to compare vaccine antigen-specific T(H) memory responses in atopic and nonatopic children.

METHODS

We analyzed specific serum IgG and cytokine responses to pertactin and tetanus antigens as well as to mitogen (PHA) and house dust mite (HDM) allergen in 25 HDM-sensitized atopic and 25 nonatopic 6-year-old children who were vaccinated and boosted with diphtheria-tetanus-pertussis (DTP) vaccine.

RESULTS

PBMCs from the atopic subjects produced higher levels of T(H)1 and T(H)2 cytokines to HDM allergen and PHA. Vaccine antibody titers were normal in the atopic subjects; vaccine-specific T(H)2 responses were rarely detectable, yet T(H)1 (IFN-gamma) responses, in particular against tetanus, were frequent and higher in the atopic subjects (121.5 [SE 64.3] vs 8.0 [3.5] pg/mL culture fluid, P =.04). Corresponding pertactin responses were comparable in both groups.

CONCLUSIONS

At the completion of the full primer-booster DTP vaccination regimen, levels of vaccine-specific immunity in atopic 6-year-old children are at least equivalent to their nonatopic counterparts, indicating that the transient atopy-associated deficiency in T(H)1 function in childhood can be successfully overcome by appropriate vaccination and boosting regimens.

Relation between house-dust endotoxin exposure, type 1 T-cell development, and allergen sensitisation in infants at high risk of asthma

BACKGROUND

Bacterial endotoxin is known to induce interferon gamma and interleukin 12 production, and therefore has the potential to decrease allergen sensitisation. To find out the role of early chronic endotoxin exposure in the development of allergen sensitisation and asthma, we compared concentrations of endotoxin in house dust with allergen sensitisation in infants at high risk for developing asthma.

METHODS

61 infants 9-24 months old with at least three physician-documented episodes of wheezing were studied. Concentrations of house-dust endotoxin and allergens were measured in the infants' homes. Allergen sensitisation was measured by skin-prick testing with a panel of common inhalant and food allergens. In a subset of these infants, proportions of T lymphocytes producing interferon gamma, and interleukins 4, 5, and 13 were calculated by cell-surface and intracellular cytokine staining, with flow cytometry.

FINDINGS

House-dust endotoxin concentrations ranged from 104 to 10,000 endotoxin units (EU) per mL (geometric mean 912 EU/mL). Concentrations did not vary significantly over a 6-month interval. Ten infants (16%) were sensitised to at least one allergen. The homes of allergen-sensitised infants contained significantly lower concentrations of house-dust endotoxin than those of non-sensitised infants (mean 468 vs 1035 EU/mL, respectively; p=0.01). Increased house-dust endotoxin concentrations correlated with increased proportions of interferon-gamma-producing CD4 T cells (p=0.01). Such concentrations did not correlate with proportions of cells that produced interleukins 4, 5, or 13.

INTERPRETATION

This study may provide the first direct in-vivo evidence that indoor endotoxin exposure early in life may protect against allergen sensitisation by enhancing type 1 immunity.

Mimicking microbial 'education' of the immune system: a strategy to revert the epidemic trend of atopy and allergic asthma?

Deficient microbial stimulation of the immune system, caused by hygiene, may underly the atopy and allergic asthma epidemic we are currently experiencing. Consistent with this 'hygiene hypothesis', research on immunotherapy of allergic diseases also centres on bacteria-derived molecules (eg DNA immunostimulatory sequences) as adjuvants for allergen-specific type 1 immune responses. If we understood how certain microbes physiologically 'educate' our immune system to interact safely with environmental nonmicrobial antigens, we might be able to learn to mimic their beneficial actions. Programmed 'immunoeducation' would consist of safe administration, by the correct route, dose and timing, of those microbial stimuli that are necessary to 'train' the developing mucosal immune system and to maintain an appropriate homeostatic equilibrium between its components. Overall, this would result in a prevention of atopy that is not limited to certain specific allergens. Although such a strategy is far beyond our present potential, it may in principle revert the epidemic trend of atopy and allergic asthma without jeopardizing the fight against infectious diseases.

gammadelta T cells provide a breath of fresh air for asthma research

gammadelta T cells downregulate airways responsiveness to allergen challenge, possibly by controlling the 'repair' response of the airway epithelium to alphabeta T cell-mediated damage (pages 1150-1156).