Causes and consequences of declining B cell-mediated central T cell tolerance throughout the lifespan

总结 胸腺B细胞最近被证明表达转录调节因子Aire，一种TRA的关键介质 表达和T细胞耐受性。以前唯一已知的胸腺内Aire来源 表达是髓上皮细胞（mTEC）的子集。令人惊讶的是，在转录水平上， 在B和mTEC中，由Aire激活的细胞几乎完全不同，表明这些群体耐受T 细胞与自身抗原的独特群体。在新自身抗原/TCR转基因小鼠模型中，胸腺B细胞被 胸腺B细胞介导的免疫耐受性也被证明是对Aire依赖性自身抗原的耐受性介导的，然而， 在生理学多克隆系统中还没有证明中心T细胞耐受性。这部分是由于 与特定自身抗原特异性T细胞的低频率相关的技术困难， 多克隆T细胞受体（TCR）库，以及胸腺B细胞和mTEC的缺乏。克服这些 障碍是必要的，以了解机制强加的耐受性，以减轻自身免疫，无论是在 稳定状态，尤其是在老化过程中。衰老与许多自身免疫性疾病的发病率增加有关。 长期以来，胸腺功能下降一直被认为是与年龄相关的疾病的重要贡献者。 免疫失调我们最近的工作揭示了小鼠胸腺B细胞中Aire表达随年龄增长而下降 和人，这将被预测为减少对B细胞特异性Aire依赖性自身抗原的耐受性。 然而，严格测试胸腺功能障碍和衰老中T细胞自身免疫之间的机制联系， 已经受到阻碍，因为上述技术障碍在老年人、萎缩的 胸腺我们建议全面检验以下假设： 胸腺B细胞利用互补方法促进自身免疫：基于四聚体的T细胞富集 识别内源性TRA和对neo自身抗原的耐受性诱导的转基因（Tg）TCR模型。我们 模型内源性自身抗原Titin和载脂蛋白B与晚发型重症肌无力相关 和动脉粥样硬化，并且我们还将采用糖尿病的Tg TCR模型（BDC2.5 TCR Tg）。我们 预测老化的胸腺B细胞将足以减少T细胞耐受， 将足以挽救它们的耐受化潜力。我们也会全面评估年龄- 人类胸腺TEC和tB细胞的相关变化，以比较啮齿类动物中年龄相关的变化， 人类总之，这些数据将为合理设计预防年龄相关疾病的战略提供信息。 自身免疫，可能揭示新的治疗靶点。

Summary Thymic B cells have recently been shown to express the transcriptional regulator Aire, a critical mediator of TRA expression and T cell tolerance in mice and humans. The only previously known source of intrathymic Aire expression was a subset of medullary epithelial cells (mTECs). Surprisingly, the cohort of genes transcriptionally activated by Aire are almost entirely distinct among B and mTEC, indicating that these populations tolerize T cells to unique cohorts of self-antigens. In a neo-self-antigen/TCR transgenic mouse model, thymic B cells were also shown to mediate tolerance to Aire-dependent self-antigens, however, the role of thymic B cell-mediated central T cell tolerance has not been demonstrated in a physiologically polyclonal system. This is in part due to the technical difficulties associated with the low frequency of T cells specific for a given self-antigen in a polyclonal T cell receptor (TCR) repertoire, and the paucity of thymic B cells and mTECs. Overcoming these obstacles is necessary to understand the mechanisms imposing tolerance to mitigate autoimmunity both in the steady state, and especially during aging. Aging is associated with increased incidence of many autoimmune diseases, and declining thymus function has long been considered an important contributor to age-associated immune dysregulation. Our recent work revealed that Aire expression declines with age in thymic B cells in mice and humans, which would be predicted to diminish tolerance to B cell-specific Aire-dependent self-antigens. However, rigorously testing the mechanistic link between thymic dysfunction and T cell autoimmunity in aging has been hindered because the technical barriers described above are exacerbated in the aged, atrophied thymus. We propose comprehensively testing the hypothesis that age-associated loss of TRA expression in thymic B cells promotes autoimmunity using complementary approaches: tetramer-based enrichment of T cells recognizing endogenous TRAs, and transgenic (Tg) TCR models of tolerance induction to neo self-antigens. Our model endogenous autoantigens, Titin, and Apolipoprotein B, are associated with late-onset Myasthenia Gravis and atherosclerosis, respectively, and we will also employ a Tg TCR model of diabetes (BDC2.5 TCR Tg). We predict that aged thymic B cells will be sufficient to diminish T cell tolerance, and that rescued TRA expression in aged cells will be sufficient to rescue their tolerization potential. We will also comprehensively assess age- associated changes in TEC and tB cells from human thymi to compare age-associated changes in rodents and humans. Together, these data will inform the rational design of strategies to prevent age-associated autoimmunity, potentially revealing novel therapeutic targets.