Early Life Exposure to Microbial Products and Asthma

描述(申请人提供)：在过去的二十年里，哮喘在工业化世界中的患病率和严重性有所增加；它是最常见的儿童慢性病，死亡率不断上升。“卫生假说”提出，早期接触微生物和微生物产品的减少是造成这种流行病的原因。根据这一范式，微生物暴露抑制环境过敏原Th2偏斜反应的发展，推测是通过诱导Th1和/或调节型反应来实现的。支持这一假说的流行病学数据是令人信服的，如果相关的话：在农场环境中长大的儿童(尤其是大量接触牲畜的儿童)、较年长的兄弟姐妹或早期托儿所的数量增加(因此在较小年龄时暴露于儿童感染)，或早期与宠物接触的儿童患哮喘和特应性疾病的风险较低。卫生学假说很难在人类身上直接测试；确定促进接触微生物的儿童免疫耐受的途径仍然很重要。了解这种耐受的机制将有助于开发新的诱导耐受的治疗方法，从而减轻哮喘和特应性疾病的负担。诱导耐受性的一个候选因素是内毒素；在一些研究中，环境粉尘中的内毒素水平与特应性疾病风险的降低相关。然而，吸入内毒素并未被证明能预防人类的特应性疾病；事实上，许多证据表明内毒素在哮喘中具有有害作用。第二个候选是细菌(富含CpG)DNA。我们以前已经用小鼠模型证明了含有CpG中心序列基序的寡核苷酸(CpG ODN)在预防和逆转特应性呼吸道疾病方面都非常有效。这些化合物可以抑制过敏原特异性免疫(Th2型)反应，并诱导Th1型和调节性(IL-10)细胞因子反应。CpG-ODN模拟天然细菌DNA的免疫刺激作用。像内毒素一样，细菌DNA由Toll样受体(TLR-9)识别，并有效地激活天然免疫系统。我们认为，接触细菌DNA可能介导了对特应性疾病的保护。因此，我们建议检验卫生假说，并比较内毒素和富含CpG的DNA的调节作用。推动这些研究的首要假设是，早期接触微生物产品可以促进耐受性，并降低后来发展为哮喘等特应性疾病的易感性。吸入内毒素和细菌DNA可能通过不同的介质和途径影响对吸入性变应原的反应，这些作用可以是相加的，也可以是协同的，甚至是拮抗的。我们建议在以下目标中解决这些假设：目标1：检验早期接触微生物产品(细菌DNA或内毒素)可降低日后发生特应性疾病的易感性的假设。问题1A：生命早期接触微生物产品是否会改变肺部炎症环境？问题1B：早期接触微生物产品是否会改变日后发展为特应性哮喘的易感性？问题1C：早期接触抗原和微生物产品是否需要抗原特异性耐受？目的2：验证早期接触微生物产品通过诱导调节细胞群促进免疫耐受的假设。问题2A：哪些介体促进了生命早期接触微生物产品诱导的免疫耐受？问题2B：早期接触微生物产品诱导免疫耐受的细胞是什么？

DESCRIPTION (provided by applicant): Asthma has increased in prevalence and severity in the industrialized world for the past two decades; it is the most common chronic illness of children and has had a rising mortality rate. The "hygiene hypothesis" proposes that reduced early-life exposures to microbes and microbial products are responsible for this epidemic. According to this paradigm, microbial exposures inhibit the development of Th2-skewed responses to environmental allergens, putatively through the induction of Th1 and/or regulatory-type responses. The epidemiological data supporting this hypothesis are compelling, if correlative: children raised in a farm environment (especially with significant exposure to livestock), with increased numbers of older siblings or early daycare attendance (and thus exposed to childhood infections at an earlier age), or with early life exposure to pets have a reduced risk of developing asthma and atopic disorders. The hygiene hypothesis has been difficult to directly test in humans; it remains important to identify the pathways that promote immune tolerance among microbe-exposed children. Understanding the mechanisms of this tolerance will aid development of novel tolerance-inducing therapeutic approaches, and thus reduce the burden, of asthma and atopy. One candidate for inducing tolerance is endotoxin; endotoxin levels in environmental dust correlate with reduced risk of atopy in some studies. Nevertheless, endotoxin inhalation has not been shown to protect against atopic disease in humans; indeed, much evidence points to a deleterious effect of endotoxin in asthma. A second candidate is bacterial (CpG-rich) DNA. We have previously demonstrated, using murine models, that oligonucleotides containing CpG-centered sequence motifs (CpG ODN) are highly effective in both preventing and reversing atopic airway disease. These compounds can suppress allergen-specific immune (Th2-type) responses and induce Th1-type as well as regulatory (IL-10) cytokine responses. CpG-ODN mimics the immunostimulatory effects of native bacterial DNA. Like endotoxin, bacterial DNA is recognized by a toll-like receptor (TLR-9) and potently activates the innate immune system. We suggest that exposure to bacterial DNA may mediate protection against atopy. Thus, we propose to test the hygiene hypothesis, and to compare the modulatory effects of endotoxin and CpG-rich DNA. The overarching hypothesis driving these studies is that early-life exposure to microbial products promotes tolerance and reduces later susceptibility to developing atopic disorders such as asthma. Inhalation of endotoxin and bacterial DNA may influence response to inhaled allergens through different mediators and pathways; these effects may be additive, synergistic, or even antagonistic. We propose to address these hypotheses in the following Aims: Aim 1: To test the hypothesis that early-life exposure to microbial products (bacterial DNA or endotoxin) reduces later susceptibility to developing atopic disorders. Question 1A: Does early life exposure to microbial products alter the pulmonary inflammatory milieu? Question 1B: Does early life exposure to microbial products alter later susceptibility to developing atopic asthma? Question 1C: Is early-life exposure to antigen as well as microbial products required for antigen-specific tolerance? Aim 2: To test the hypothesis that early-life exposure to microbial products promotes immune tolerance by inducing a regulatory cell population. Question 2A: What mediators promote immune tolerance induced by early life exposure to microbial products? Question 2B: What cells are responsible for immune tolerance induced by early life exposure to microbial products?