Post-Transcriptional Regulation of Interleukin-7 Receptor Expression

GC细胞因子受体在T细胞发育和分化过程中表达的分子机制大多尚不清楚。最近，我们发现了一种新的GC链转录后表达机制，导致了可溶性GC链蛋白的产生。我们在正常人和小鼠血清中检测到大量的可溶性GC(SGC)蛋白，我们发现在免疫激活和炎症条件下，血清SGC的表达增加。为了评估SGC在体内的作用，我们建立了在T细胞中过表达可溶性GC的SGC转基因小鼠。这些SGC转基因小鼠在血清中表达高水平的SGC，并且它们的T细胞激活的记忆表型细胞的百分比增加。然而，转基因SGC不能转导GC细胞因子信号，因为转基因SGC不能恢复GC缺陷小鼠的T细胞发育。因此，SGC诱导了促炎环境，但不是通过促进GC信号来实现的。值得注意的是，我们发现SGC蛋白与专有的细胞因子受体相关，如IL-2Rb和IL-7Ra，即使在没有细胞因子的情况下也是如此。这种配体非依赖的异二聚化进一步导致IL-2和IL-7信号的抑制。另一方面，抑制IL-2信号会促进促炎性IL-17的表达，并增加炎症性自身免疫病的严重程度。由于T细胞活化诱导可溶性SGC的产生，而SGC反过来又增加炎症反应，我们认为SGC的产生是T细胞激活下游加强促炎免疫反应的一种机制。由于SGC是促炎的，因此推测中和血清SGC的表达是否会改善炎症性疾病并抑制破坏性T细胞反应是很有趣的。SGC特异性单抗的产生及其对体内促炎反应的影响是我们计划追求的有趣的问题，我们希望研究利用这种抗体作为治疗药物来抑制炎症和改善疾病的可能性。除可溶性GC受体外，我们还在人和小鼠血清中发现了可溶性IL-7ra蛋白。在人类中，IL-7ra经历了选择性剪接，据报道，选择性剪接诱导产生可溶性IL-7ra链蛋白。这一转录后事件解释了人血清中存在可溶性IL-7ra。相比之下，在小鼠中还没有可溶性IL-7ra蛋白的报道。此外，也没有分子证据表明在小鼠中存在另一种IL-7Ra剪接异构体。因此，小鼠血清中可溶性IL-7ra蛋白的分子基础仍不清楚。我们目前正在使用新产生的ELISA来解决这个问题，以评估小鼠模型中的可溶性IL-7ra。最后，我们希望从转录水平上了解IL-7ra和GC表达的调控机制。目前，还没有关于GC转录如何调控的信息，也没有关于T细胞和胸腺细胞亚群中不同IL-7ra表达的分子解释。我们正在使用T细胞发展作为一个模型来解决这个问题。我们之前曾报道过，锌指蛋白Gfi1在CD8系T细胞中特异性地控制IL-7ra的转录，而在CD4系细胞中不控制。我们现在已经扩大了搜索范围，我们已经确定了其他转录因子，它们以阶段和细胞特异性的方式控制IL-7Ra和GC细胞因子受体的表达。分析它们的作用和调控的分子机制目前正在进行中。

The molecular mechanism of gc cytokine receptor expression during T cell development and differentiation remain mostly unknown. Recently, we discovered a novel post-transcriptional mechanism of gc-chain expression that resulted in the generation of soluble gc-chain proteins. We detected soluble gc (sgc) proteins in significant amounts in both normal human and mouse serum, and we found that serum sgc expression increased upon immune activation and in inflammatory conditions. To assess the role of sgc in vivo, we generated sgc transgenic mice that overexpress soluble gc in T cells. These sgc Tg mice expressed high levels of sgc in serum, and their T cells showed increased percentages of activated memory phenotype cells. However, transgenic sgc was not capable of transducing gc cytokine signals because the sgc transgene failed to restore T cell development in gc-deficient mice. Thus, sgc induces a pro-inflammatory environment but not through promoting gc signaling. Notably, we discovered that sgc proteins associated with proprietary cytokine receptors, such as IL-2Rb and IL-7Ra, even in the absence of cytokines. Such ligand-independent heterodimerization further resulted in inhibition of IL-2 and IL-7 signaling. Inhibition of IL-2 signaling, on the other hand, promoted pro-inflammatory IL-17 expression and increased the severity of inflammatory autoimmune disease. Since T cell activation induces soluble sgc production and because sgc in turn increases inflammation, we propose that sgc production is a mechanism to reinforce pro-inflammatory immune responses downstream of T cell activation. Because sgc is pro-inflammatory, it is interesting to speculate if neutralization of serum sgc expression would ameliorate inflammatory disease and dampen destructive T cell responses. Generation of sgc-specific monoclonal antibodies and assessing their effects on pro-inflammatory responses in vivo are interesting issues that we plan to pursuit, and we wish to examine the potential to utilize such antibodies as therapeutics to dampen inflammation and ameliorate disease. In parallel to soluble gc receptors, we also found soluble IL-7Ra proteins in serum of human and mice. In humans, the IL-7Ra undergoes alternative splicing, and it has been reported that alternative splicing induces the generation of soluble IL-7Ra chain proteins. This post-transcriptional event explains the presence of soluble IL-7Ra in human serum. In contrast, soluble IL-7Ra proteins have not been reported for mice. Also, there is no molecular evidence for an alternative IL-7Ra splice isoform in mice. Thus, the molecular basis of soluble IL-7Ra proteins in mouse serum remains unknown. We are currently addressing this issue using newly generate ELISA for assessing soluble IL-7Ra in mouse models. Finally, we wish to understand the regulatory mechanisms of IL-7Ra and gc expression at transcriptional level. Currently, there is no information available how gc transcription is controlled, and also there is no molecular explanation of distinct IL-7Ra expression in T cell and thymocyte subsets. We are using T cell development as a model to address this question. We had previously reported that the zinc finger protein Gfi1 controls IL-7Ra transcription specifically in CD8 lineage T cells but not in CD4 lineage cells. We have now expanded the search, and we have identified additional transcription factors that control IL-7Ra and gc cytokine receptor expression in a stage and cell-specific manner. Analyzing their roles and the molecular mechanism of regulation is currently under progress.