CD6 is one of many cell surface receptors known to regulate signal transduction upon T cell activation. However, whether CD6 mediates costimulatory or inhibitory signals is controversial. When T cells engage with antigen presenting cells (APCs), CD6 interacts with its ligand CD166 at the cell–cell interface while the cytosolic tail assembles a complex signalosome composed of adaptors and effector enzymes, that may either trigger activating signaling cascades, or instead modulate the intensity of signaling. Except for a few cytosolic adaptors that connect different components of the CD6 signalosome, very little is known about the mechanistic effects of the cytosolic effectors that bind CD6. Jurkat model T cells were transfected to express wild-type (WT) CD6, or a cytoplasmic truncation, signaling-disabled mutant, CD6Δcyt. The two resulting cell lines were directly activated by superantigen (sAg)-loaded Raji cells, used as APCs, to assess the net signaling function of CD6. The Jurkat cell lines were further adapted to express a FRET-based unimolecular HRas biosensor that reported the activity of this crucial GTPase at the immunological synapse. We show that deletion of the cytosolic tail of CD6 enhances T-cell responses, indicating that CD6 restrains T-cell activation. One component of the CD6-associated inhibitory apparatus was found to be the GTPase activating protein of Ras (RasGAP), that we show to associate with CD6 in a phosphorylation-dependent manner. The FRET HRas biosensor that we developed was demonstrated to be functional and reporting the activation of the T cell lines. This allowed to determine that the presence of the cytosolic tail of CD6 results in the down-regulation of HRas activity at the immunological synapse, implicating this fundamental GTPase as one of the targets inhibited by CD6. This study provides the first description of a mechanistic sequence of events underlying the CD6-mediated inhibition of T-cell activation, involving the modulation of the MAPK pathway at several steps, starting with the coupling of RasGAP to the CD6 signalosome, the repression of the activity of Ras, and culminating in the reduction of ERK1/2 phosphorylation and of the expression of the T-cell activation markers CD69 and IL-2R α chain.

中文翻译：

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CD6 介导的通过调节 Ras 抑制 T 细胞活化

CD6 是许多已知在 T 细胞激活后调节信号转导的细胞表面受体之一。然而，CD6 是否介导共刺激或抑制信号是有争议的。当 T 细胞与抗原呈递细胞 (APC) 结合时，CD6 在细胞-细胞界面与其配体 CD166 相互作用，而细胞溶质尾部组装一个由衔接子和效应酶组成的复杂信号体，这可能触发激活信号级联反应，或者转而调节信号的强度。除了一些连接 CD6 信号体不同成分的胞质衔接子外，我们对结合 CD6 的胞质效应子的机制效应知之甚少。转染 Jurkat 模型 T 细胞以表达野生型 (WT) CD6，或细胞质截短、信号禁用突变体 CD6Δcyt。产生的两个细胞系直接被加载超抗原 (sAg) 的 Raji 细胞激活，用作 APC，以评估 CD6 的净信号功能。Jurkat 细胞系进一步适应表达基于 FRET 的单分子 HRas 生物传感器，该传感器报告了免疫突触中这种关键 GTP 酶的活性。我们表明，删除 CD6 的胞质尾部会增强 T 细胞反应，表明 CD6 会抑制 T 细胞活化。CD6 相关抑制装置的一个组成部分被发现是 Ras 的 GTP 酶激活蛋白 (RasGAP)，我们证明它以磷酸化依赖的方式与 CD6 相关联。我们开发的 FRET HRas 生物传感器被证明是功能性的，并报告了 T 细胞系的激活。这允许确定 CD6 胞质尾部的存在导致免疫突触中 HRas 活性的下调，暗示这种基本 GTP 酶是 CD6 抑制的靶标之一。这项研究首次描述了 CD6 介导的 T 细胞活化抑制的事件机制序列，涉及在几个步骤中调节 MAPK 通路，从 RasGAP 与 CD6 信号体的偶联开始，抑制Ras 的活性，最终导致 ERK1/2 磷酸化和 T 细胞活化标记物 CD69 和 IL-2R α 链表达的减少。