T细胞如何能保持它：在肝脏T细胞的健身？社论

StephenW

T cell fitness in the liver: How can T cells keep it up? EditorialJnl of Hepatology June 2016
Emerging Infectious Diseases (EID) Program, Duke-NUS Medical School, Singapore; Viral Hepatitis Laboratory, Singapore Institute for
Clinical Sciences, Agency of Science Technology and Research (A⁄ STAR), Singapore
See Article, pages 1327–1338



T cells play an important role in the immunological control of pathogens and tumors. CD8+ T cells scan cell surfaces with their T cell receptors for the presence of non-self antigens (originated from viruses, intracellular bacterial or from modified self antigens present in malignancies) bound to MHC-class I molecules. Specific recognition triggers T cell activation and results in the lysis of infected or tumor cells. The efficiency at which this occurs is extremely high such that a functional effector CD8+ T cell can be activated upon recognition of very few MHC-class I/non-self peptides present on the surface of the cells and can kill multiple targets [[url=]1[/url]]. The ability of T cells to access tissues typically requires multiple steps of T cell extravasation/migration that are regulated by the expression of specific adhesion and homing molecules [[url=]2[/url]]. However, such a process is not strictly necessary in the liver where the lack of a continuum basal membrane separating the blood from the hepatic parenchyma allows direct T cell scanning of target cells through the fenestrated liver sinusoidal endothelial cells [[url=]2[/url]]. Since the liver is an organ indispensable for life, the easy accessibility of CD8+ T cells to hepatocytes has been balanced by additional levels of control of intrahepatic CD8+ T cell function to avoid that immunological control of pathogens result in the self-destruction of the organ.
The elegant paper from the group of Bowen and Bertolino published in this issue of the Journal of Hepatology [[url=]3[/url]] directly investigates this problem and analyzes the mechanisms underlying the control of CD8+ T cell mediated liver damage. The authors used the Met-Kb transgenic mouse model in which liver damage is induced by the transfer of transgenic T cells that recognize a self antigen presented by MHC-class I H-2 Kb, expressed only in the liver and in lymph-nodes and they ask whether the most important parameter controlling liver damage is the survival or the cytokine-producing and lytic activity of T cells. To address this question they adoptively transfer in mic,e two types of alloreactive T cells that lack either a gene that regulates T cell survival (BIM-deficient- thus resistant to apoptosis) or one that regulates T cell function (SOCS-1, suppressor of cytokine signaling 1 deficient). As expected, BIM-deficient T cells display increased survival capacity and thus are present in increased numbers in the intrahepatic environment in comparison to normal T cells. However, the increased numbers of alloreactive T cells in the liver have a negligible effect on acute liver damage and do not lead to chronic liver inflammation. In contrast, the transfer of SOCS-1-deficient T cells causes a more severe and slightly prolonged hepatitis but again does not result in the development of chronic liver inflammation. Indeed, wild-type, BIM−/− or SOCS-1−/− T cells became functionally exhausted upon interaction with high levels of their cognate antigen in the liver within 5–7 days after adoptive transfer.
Thus, this work provides a demonstration that an important parameter that determines the extent of liver cell damage during acute hepatitis is the initial functional property of the CD8+ T cells. This finding is not unexpected: it seems logical that CD8+ T cells (SOCS-1 deficient) that possess a higher intrinsic ability to directly kill hepatocytes and produce inflammatory cytokines (such as TNF-α) will cause higher levels of liver damage as compared to equal number of less fit T cells. These findings also confirm the observation that liver damage in acute or chronically virus-infected livers is not directly proportional to the frequency of virus-specific T cells but depends on the functional capacity of these cells [[url=]4[/url]]. However, what is striking and somehow surprising is the similar and rapid kinetics at which intrahepatic T cells with different effector capacities are functionally exhausted and the extent of their functional silencing in a liver that constitutively presents high dose of the T cell cognate antigen. T cells that enter the liver appear to lose their functionality within a few days upon adoptive transfer (5–7 days). The functional exhaustion of the transgenic T cells is observed preferentially in the intrahepatic environment as T cells present in the lymphonodes maintain a degree of function ability.
These findings nicely complement the recent demonstration of the same group that the quantity of antigen expressed by the hepatocytes in the liver is the major determinant of the functional fate of CD8+ T cells [[url=]5[/url]] even though what we are still missing is the exact mechanisms of the intrahepatic T cell functional silencing.
The authors argued that T cell exhaustion is likely mediated by the upregulation of different co-inhibitory molecules like PD-1 and TIM-3. Functionally exhausted T cells present in the liver of these mice overexpressed these two co-inhibithory molecules. However, T cell functional exhaustion was not only mediated by PD-1 engagement since treatment with anti-PD-L1 antibody was not sufficient to restore a hepatic immune response. It is possible that the rapid silencing of T cell functionality in the liver observed few days after adoptive transfer could have be at least partially caused by metabolic changes in the inflamed intrahepatic environment. Arginase which is known to be released by dying hepatocytes [[url=]6[/url]], has been shown to dampen T cell function during acute [[url=]7[/url]] and chronic [[url=]8[/url]] hepatitis through the removal of the amino acid (arginine) essential for T cell function. However, such mechanism which is associated with the presence of liver inflammation, does not explain the prolonged suppression of T cell function observed in these animals after the resolution of liver damage. Despite the absence of any signs of chronic liver inflammation, the T cells that persist in the liver of the treated mice (present in very large numbers when the authors adoptively transfer BIM-deficient T cells) maintain a profound functionally exhausted phenotype as they lack any lytic or cytokine-producing abilities and they are refractory to treatment with anti-PD-L1. It will be interesting to test whether the observed intrahepatic T cell exhaustion is caused by other T cell-intrinsic functional parameters, such as expression of other inhibitory molecules (CTLA4, TGIT) or T cell receptor downregulation, mechanisms which are likely to occur in the presence of the high doses of antigen observed in the liver of these mice. Exhausted T cells in chronically infected livers express more than a single co-inhibitory molecule [[url=][9][/url], [url=][10][/url], [url=][11][/url]]. Nevertheless, a different, although not mutually exclusive possibility is that the maintenance of T cell silencing is mediated by other immune regulatory networks, like regulatory T cells [[url=]12[/url]] or granulocytic myeloid suppressor cells that were recently shown to be present in high numbers in the intrahepatic environment of patients with chronic hepatitis B and low/absence level of liver pathology [[url=]13[/url]]. Evaluation of the contribution of these different factors will be important to understand whether a recovery of intrahepatic T cell function in liver pathologies characterized by persistent and high expression of non-self antigens, (chronic viral hepatitis, hepatocellular carcinoma) would always necessitate a reduction in the number of antigen expressing hepatocytes or whether strategies designed only to alter the liver microenvironment (for example with Toll-like receptors mediated stimulation) might have an effect.
Certainly as the authors pointed out in their discussion, the data presented in their study is relevant for immunological therapies that attempt to restore T cell effector function both in patients chronically infected with hepatotropic viruses or in liver cancer. Despite the success of anti-PD-1 treatment for the restoration of intra-heaptic T cell function in vitro [[url=]14[/url]] and in chimps [[url=]15[/url]], the ineffectiveness of such treatment in this mouse model questions its therapeutic potential in patients with HCC or chronic viral hepatitis, a concept further supported by the modest results of anti-PD-1 clinical trial in HCV-infected subjects [[url=]16[/url]].
The prospect of immunotherapy on these liver pathologies appears even more daunting if we consider that T cell exhaustion was observed here in mice with relatively normal livers, where T cell access and function is not constrained by anatomical barriers [[url=]17[/url]] and by the suppressive immune environment present in livers with chronic inflammation (reviewed in [[url=]18[/url]]). On the other hand, the demonstration that hepatocytes are so well protected against T cell mediated immunological damage could be seen as an incentive to test new approaches of immunotherapy like transfer of gene-modified T cells or that combine check point inhibitors with agents that modify antigen expression in liver cells and T cell induction [[url=]19[/url]]. An approach combining therapeutic vaccination with anti-PD-L1 and antiviral therapies was shown to be effective in woodchucks chronically infected with woodchucks hepatitis virus [[url=]20[/url]]. This study shows that T cells struggle to maintain their fitness in the liver environment and help on multiple fronts may be needed for them to do so.
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StephenW

T细胞如何能保持它：在肝脏T细胞的健身？社论

肝病2016年6月的JNL
新发传染性疾病（EID）计划，杜克 - 新加坡国立大学医学院，新加坡;病毒性肝炎实验室，新加坡研究所
临床科学，局科学技术研究所（A/ STAR），新加坡
见第，1327-1338页


T细胞发挥的病原体和肿瘤的免疫控制中起重要作用。 CD8 + T细胞与T细胞受体的非自身抗原的存在下扫描细胞表面（源自病毒，细胞内细菌或从存在于恶性肿瘤修饰的自身抗原）结合于MHC-I类分子。特异性识别触发感染或肿瘤细胞的裂解T细胞活化和结果。发生这种情况时的效率是非常高的，使得一个的功能效应CD8 + T细胞可以在识别很少的MHC-I类/非自身肽存在于细胞的表面上的被激活并能杀死多个目标[1]。 T细胞的访问组织的能力通常需要T细胞外渗/迁移的多个步骤是由特定接合的表达调控和归巢分子[2]。然而，这样的方法是不严格必要在肝脏中，其中缺乏一个连续基底膜从肝实质分离血液的允许靶细胞的直接的T细胞扫描通过窗孔肝窦内皮细胞的[2]。因为肝脏是器官必不可少的寿命，CD8 + T细胞的容易获得的肝细胞已经通过肝内CD8 + T细胞的功能的控制的附加级别平衡，以避免病原体免疫控制导致器官的自我毁灭。

从组Bowen和Bertolino典雅的纸在中华肝脏病杂志[3]直接调查这一问题，并分析了CD8 + T细胞介导的​​肝损害控制背后的机制这个问题发表。作者使用，其中肝脏损害是由识别由MHC-I类H-2 KB呈现自身抗原的转基因的T细胞的转移诱导的Met-KB转基因小鼠模型中，只有在肝和淋巴结节点表示和他们问控制肝损伤的最重要的参数是存活或T细胞的细胞因子产生和裂解活性。为了解决这个问题，他们在麦克风继转移，例如两种类型的缺乏任一种调节T细胞的存活（BIM-deficient-因而耐凋亡）或一个调节T细胞功能（SOCS-1，抑制一个基因同种异体T细胞的细胞因子信号传导1缺陷）。正如所料，BIM缺​​陷的T细胞显示增加的存活能力，从而存在于肝内环境增加数目相比于正常的T细胞。然而，同种异体T细胞在肝脏中的数量增加对急性肝损伤的影响可以忽略，且不会导致慢性肝脏炎症。与此相反，SOCS-1缺失的T细胞的转移导致更严重的并略微延长肝炎但再次不导致慢性肝炎的发展。事实上，野生型，BIM - / - 或SOCS-1 - / - T细胞相互作用时，在过继转移后的5-7天的肝脏高水平它们的同源抗原的成为功能耗尽。

因此，该工作提供了一种证明，确定急性肝炎中的肝细胞损害的程度的一个重要参数是CD8 + T细胞的最初功能性质。这一发现是不意外：逻辑上似乎比其具有更高的固有能力，以直接杀死肝细胞，并产生炎性细胞因子（如TNF-α）的CD8 + T细胞（SOCS-1缺陷）会导致更高水平的肝损伤的同等数量不太适合T细胞。这些发现也证实观察到急性或慢性病毒感染的肝的肝损伤是不成正比病毒特异性T细胞的频率，但依赖于这些[4]的细胞的功能的能力。然而，什么是惊人的，不知怎么令人惊讶的是类似的，快速的动力学在其中肝内T细胞的不同效应能力在功能上耗尽，其功能沉默在组成呈现T细胞同源抗原的高剂量肝的程度。即进入肝脏的T细胞似乎在内部继转移（5-7天）数天失去其功能。转基因的T细胞的功能衰竭在肝内环境优先观察为存在于淋巴结的T细胞保持了一定程度的功能的能力。

这些研究结果很好地补充最近该抗原由肝细胞在肝脏中表达的量是CD8 + T细胞的功能命运的主要决定因素的同一组的示范[5]尽管我们仍然缺少是确切的机制肝内T细胞功能的沉默。

作者认为，T细胞耗竭可能是由不同的共抑制分子的像的PD-1和TIM-3的上调介导的。存在于这些小鼠的肝功能耗尽T细胞过表达这两个共inhibithory分子。然而，T细胞功能的衰竭不仅由PD-1卡合，因为用抗PD-L1的抗体治疗介导不足以恢复肝免疫应答。这是可能的T细胞功能在肝脏快速沉默观察几天后过继转移可能至少部分地由在发炎的肝内环境代谢变化引起的。这是已知精氨酸酶由垂死肝[6]被释放，已经显示出通过去除氨基酸（精氨酸）T细胞功能是必需的急性[7]和慢性[8]肝炎中以抑制T细胞功能。然而，这是与肝脏炎症的存在有关这样的机制，并不能解释在这些动物中观察到肝脏损伤的分辨率后T细胞功能的延长抑制。尽管没有慢性肝脏炎症的迹象，即在治疗的小鼠的肝持续的T细胞（展示在非常大的数字，当作者继转移BIM缺陷T细胞）保持一个深刻功能耗尽型，因为它们缺乏任何裂解或细胞因子产生的能力和它们难治用抗PD-L1的治疗。这将是有趣测试是否观察到肝内的T细胞衰竭是由其它T细胞的内在功能参数，如（CTLA4，TGIT）其他抑制分子的表达或T细胞受体的下调，这有可能在发生机制引起在这些小鼠的肝脏中观察到的高剂量的抗原的存在。耗尽T细胞在慢性感染肝表达比单个共抑制分子更[9]，[10]，[11]。然而，一个不同的，尽管不是相互排斥的可能性是，T细胞沉默的维护由其它免疫调控网络介导的，如最近被证明是存在于高数量调节性T细胞[12]或粒细胞的骨髓抑制性细胞慢性乙肝的肝组织病理[13]低/无级肝内环境。这些不同的因素贡献的评估将是重要的，以了解在肝病理学特征在于非自身抗原的持久和高表达，（慢性病毒性肝炎，肝细胞癌）肝内T细胞功能的恢复是否总是必要的减少抗原表达的肝细胞，或者是否只设计改变肝脏微环境（例如用Toll样受体介导的刺激）的策略的数目可能具有的效果。

当然，由于作者在讨论中指出，在他们的研究中提出的数据是相关试图恢复T细胞的效应功能无论是在慢性感染嗜肝病毒或肝癌患者的免疫疗法。尽管抗PD-1治疗的帧内heaptic T细胞功能的体外[14]的恢复和黑猩猩[15]，这种治疗的在该小鼠模型的无效质疑患者其治疗潜力与肝癌或成功慢性病毒性肝炎，由抗PD-1的临床试验中的HCV感染的受试者[16]的适度结果进一步支持的概念。

如果我们考虑到T细胞衰竭在这里观察到在小鼠中相对正常肝，其中T细胞连接和功能不受解剖屏障[17]和由抑制免疫环境的制约就这些肝脏病症免疫治疗的前景看来甚至更艰巨存在于慢性炎症肝脏（综述[18]）。在另一方面，肝细胞对T细胞介导的​​免疫受损，以便很好地保护的示范可以被看作是一种激励具有修改抗原剂测试免疫治疗新方法，如基因修饰的T细胞的转移或结合检验点抑制剂在肝细胞和T细胞诱导[19]的表达。结合抗PD-L1和抗病毒疗法治疗性疫苗接种的方法被证明是有效的慢性感染土拨鼠肝炎病毒[20]土拨鼠。这项研究表明，T细胞很难保持自己的健身在肝脏环境和多方面帮助可能需要他们这样做。