Infectious origins of childhood leukemia

描述（由申请人提供）：多条证据支持感染在引发儿童白血病的继发性病变病因学中的关键作用。这导致了一种假设，即延迟暴露于常见感染的时间更容易导致儿童b淋巴细胞白血病。早期接触传染性病原体，例如通过日托或接种疫苗，可导致轻度和典型的亚临床免疫反应，并与儿童ALL发病风险显著降低相关。相比之下，延迟感染往往更严重，引起更强烈的免疫反应，易患儿童b淋巴细胞白血病前期。本提案的中心目标是通过实验验证“延迟感染”假说，并描述感染背景下人类前b细胞遗传易感性的机制。前b细胞经历rag1 /2依赖性免疫球蛋白V(D)J基因重组，代表儿童ALL的起源细胞。rag1 /2介导的重组酶活性导致DNA双链断裂，并与获得染色体易位的低风险相关。然而，当Rag1/2酶与B细胞特异性突变酶AID同时表达时，这种风险急剧增加。因此，Rag1/2和AID的表达在B细胞发育的早期和晚期是相互排斥和暂时分离的：Rag1和Rag2的表达仅限于免疫球蛋白V(D)J基因在前B细胞和前B细胞中的重组。相反，AID的表达被认为局限于遇到抗原的成熟B细胞中的体细胞超突变和类转换重组。在本次提案的前期工作中，我们发现IL7R？/Stat5/Akt信号对于保持正常的前b细胞处于抗原无应答状态至关重要。在去除IL7或条件删除Stat5后，前B细胞对抗原（如LPS）完全反应，并且在LPS刺激下表达与成熟B细胞相似的AID水平。虽然骨髓中高水平的IL7使前b细胞处于抗原无应答状态，但我们发现了正常前b细胞分化过程中的一个脆弱性窗口：前b细胞受体信号下调IL7R？表达和诱导rag1 /2介导的免疫球蛋白轻链基因重组小静息（部分D）前b细胞。当遇到抗原时，部分D前b细胞除了表达Rag1/2外，还表达高水平的AID，这大大增加了它们的染色体易位倾向。基于这些发现，我们假设D前b细胞在感染的情况下对遗传病变高度敏感。我们提出以下三个目标来验证预测，即分数D前b细胞池的大小和感染免疫反应的频率和强度将决定白血病前（例如TEL-AML1）前b细胞克隆获得关键继发性病变的可能性。

DESCRIPTION (provided by applicant): Multiple lines of evidence support a critical role of infections in the etiology of secondary lesions that initiate childhood leukemia. This led to the hypothesis that delayed exposure to common infections predisposes to childhood pre-B ALL. Early exposure to infectious pathogens, e.g. through daycare attendance or vaccinations, leads to mild and typically subclinical immune responses and is associated with a significantly diminished risk to develop childhood ALL. By contrast, delayed infections are often more serious, cause more vigorous immune responses and predispose to childhood pre-B ALL. The central goal of this proposal is to experimentally test the 'delayed infections' hypothesis and to delineate mechanisms of genetic vulnerability of human pre-B cells in the context of infection. Pre-B cells undergo Rag1/2-dependent immunoglobulin V(D)J gene recombination and represent the cell of origin of childhood ALL. Rag1/2-mediated recombinase activity causes DNA double strand breaks and is associated with a low risk to acquire chromosomal translocations. This risk, however, dramatically increases when Rag1/2 enzymes are expressed concomitantly with the B cell-specific mutator enzyme AID. Hence, expression of Rag1/2 and AID is mutually exclusive and temporally separated in early and late B cell development, respectively: Rag1 and Rag2 expression is limited to immunoglobulin V(D)J gene recombination in pro- and pre-B cells. Conversely, AID expression is thought to be restricted to somatic hypermutation and class-switch recombination in mature B cells that have encountered antigen. In preliminary work for this proposal, we found that IL7R?/Stat5/Akt signaling is critical to keep normal pre-B cells in an antigen- unresponsive state. Upon withdrawal of IL7 or conditional deletion of Stat5, pre-B cells become fully responsive to antigen (e.g. LPS) and express AID at similar levels as mature B cells upon LPS stimulation. While high levels of IL7 in the bone marrow secure pre-B cells in an antigen-unresponsive state, we discovered a window of vulnerability during normal pre-B cell differentiation: Pre-B cell receptor signaling downregulates IL7R? expression and induces Rag1/2-mediated immunoglobulin light chain gene recombination in small resting (Fraction D) pre-B cells. Upon antigen encounter, Fraction D pre-B cells express high levels of AID in addition to Rag1/2, which dramatically increases their propensity to chromosomal translocations. Based on these findings, we hypothesize that Fraction D pre-B cells are highly susceptible to genetic lesions in the context of infection. We propose the following three Aims to test the prediction that the size of the Fraction D pre-B cell pool and the frequency and intensity of immune responses to infections will determine the likelihood of a pre-leukemic (e.g. TEL-AML1) pre-B cell clone to acquire critical secondary lesions.