胞外HMGB1协同放大自身DNA诱导自身免疫应答的机制

SLE is a typical lethal autoimmune disease without efficient therapeutic strategies. So far the detailed pathological mechanism is still blurred. We previously found that self-DNA is a critical auto-antigen which initiates potent autoimmune response after being processed by antigen-presenting cells (APC). We and other groups also found that HMGB1 is robustly increased in SLE patients and mice and could potentially promote anti-dsDNA antibody production. More interestingly, only extracellular, but not intracellular HMGB1 possesses such effecs, the underlying mechanism is not clear. Here we are trying to explore the mechanism underlying the synergistic and potentiative effects of extracellular HMGB1 on self-DNA induced autoimmune responses. Our preliminary data showed that accumulation and sustain of self-DNA in endosome is critical for the activation of APC, but DNA was reported to be uptaked by macropinocytosis which is very ineffective in transferring DNA antigen to endosome. HMGB1 has been reported to be accumulated in endosomes by receptor-mediated endocytosis which could very efficiently and specifically transport antigen to endosome. Therefore, we proposed the following hypothesis : the comples of extracellular HMGB1 and self-DNA was uptake by APC via HMGB1 receptor-meidated endocytosis which promote more DNA accumulating and sustaining in endosomes. More than that, HMGB1-self DNA complex could simultaneously activate HMGB1-associated and self-DNA sensing associated signaling pathways, even causing these two pathway crosstalking and the synergize and potentiate elf-DNA induced autoimmune responses. Our study would provide clues for the development of novel SLE therapeutic strategies, and also help understanding the mechanism underlying the initiation and amplification of autoimmune response.

我们前期研究发现自身DNA是系统性红斑狼疮(SLE)自身免疫应答的重要驱动原，其需抗原提呈细胞（APC）加工并呈递给CD4+T细胞，后者辅助B细胞产生抗dsDNA抗体。在SLE和模型鼠内，HMGB1显著升高并促进自身DNA活化APC，有趣的是，仅胞外HMGB1才具备该效应。本项目重点研究胞外HMGB1促进自身DNA活化APC的机制。预实验发现进入并滞留内体(endosome)对自身DNA活化APC十分重要，但游离DNA主要经巨胞饮摄取，进入内体效率低下，而HMGB1可通过受体介导的内吞作用促进抗原内体集聚，因此，我们提出HMGB1可能通过与自身DNA形成复合物并以受体介导内吞方式促进自身DNA在内体集聚和滞留，并同时活化HMGB1相关DNA识别相关两条通路并借助通路间crosstalk协同放大APC的活化。本研究不仅有望为SLE治疗提供新靶点，也有利于理解自身免疫应答的启动和放大机制。

SLE是一类以严重炎症应答为特征的系统性自身免疫病，以持续性高滴度诊断和致病性抗dsDNA抗体产生为主要致病特征，目前其发病机制不明。我们前期原创性工作发现活化活化淋巴细胞来源的自身DNA是诱导机体抗dsDNA抗体产生的重要驱动源，可显著激活抗原提呈细胞巨噬细胞，但机制不明。我们前期发现，自身DNA可诱导高水平HMGB1，后者显著促进巨噬细胞炎症应答，但分子免疫学机制尚待阐明。. 在本研究中，我们发现HMGB1明显改变巨噬细胞摄取自身DNA的内吞途径。游离DNA主要经低效率的巨胞饮作用所摄取，而胞外HMGB1通过与DNA形成复合物，以TLR2/TLR4依赖的方式促进了更高效和更特异的clathrin/caveolin-1依赖性受体介导的内吞途径，引发自身DNA在成熟内体中的快速和大量聚集，放大DNA感受器TLR9介导的炎症非常关键。. 同时我们对于临床标本中持续性高滴度抗dsDNA抗体的产生机制进行了探讨。发现富含HMGB1的DNA复合物经APC加工后可通过TLR2/MyD88通路显著促进抗体产生。该过程依赖于自身DNA诱导的一类新型CD4+Tfh细胞。该细胞分化发育和功能发挥依赖于转录因子RORγt，以分泌IL-17为主要效应分子。当靶向该群IL-17+CD4+Tfh细胞关键转录因子RORγt或效应分子IL-17后，狼疮肾炎病情显著缓解，因此有望成为SLE治疗的新靶点。. 本研究不仅阐明了胞外HMGB1促进自身DNA诱导巨噬细胞炎症的新分子机制，同时也解析了富含HMGB1的DNA复合物诱导新型Tfh细胞促进机体抗体产生的细胞免疫学机制，为SLE发病机理阐明和治疗策略研制提供了新思路。