XCL2  (趋化因子 C-基元配体 2)

XCL2 (也称为 SCM1-β 和 SCYC-2) 是与 XCL1 高度相关的 C 趋化因子亚家族的成员。XCL2 结合并激活 G 蛋白偶联受体 XCR1。XCL2 由未受刺激的自然杀伤 (NK) 细胞组成型表达，在较小程度上由非活性 CD8⁺ T 细胞表达^[1]。
在结构上，XCL1 和 XCL2 非常相似，在生理 NaCl 和温度条件下在单体趋化因子折叠和不相关的二聚体状态之间进行交换。XCL1 和 XCL2 映射到染色体 1，RT-PCR 分析显示有丝分裂原刺激和非刺激细胞中的 XCL1 和 XCL2 转录物。Ca²⁺ 通量、趋化性和肝素结合试验表明，XCL2 的单体形式负责 G 蛋白偶联受体的激活，而二聚体形式对 GAG 结合很重要。此外，XCL2 对肝素的亲和力略高于 XCL1。T 细胞的激活导致 CD8⁺ T 细胞中 XCL1 和 XCL2 表达的显着增加。活化的 CD4⁺ T 细胞表现出增加的 XCL2 量，但不是 XCL1^[1]。
XCL2 与 XCR1 的结合启动了树突状细胞以及 T 和 B 淋巴细胞中的趋化细胞迁移。 XCL2 也由癌症和细菌病原体诱导。在上皮性卵巢癌和肝细胞癌等癌症中，增加的 XCL2 表达与癌症进展相关。在另一项研究中，加入结核病抗原后，Wag31 可诱导 XCL2 而不是 XCL1 在巨噬细胞中的表达^[1]。

XCL2 (also knowns as SCM1-β and SCYC-2) is a member of C chemokine sub-family that is highly related to XCL1. XCL2 binds and activates the G protein-coupled receptor, XCR1. XCL2 is constitutively expressed by unstimulated natural killer (NK) cells and to a lesser extent by inactive CD8⁺ T cells^[1].
Structurally, XCL1 and XCL2 are very similar, exchanging between the monomeric chemokine fold and an unrelated dimeric state under physiological NaCl and temperature conditions. XCL1 and XCL2 mapped to chromosome 1, and RT-PCR analysis revealed XCL1 and XCL2 transcripts in both mitogen stimulated and non-stimulated cells. Ca²⁺ flux, chemotaxis, and heparin binding assays showed that the monomer form of XCL2 is responsible for G protein-coupled receptor activation while the dimeric form is important for GAG binding. Furthermore, XCL2 displays a slightly higher affinity for heparin than XCL1. Activation of T cells resulted in a dramatic increase in XCL1 and XCL2 expression in CD8⁺ T cells. Activated CD4⁺ T cells demonstrated increased amounts of XCL2, but not XCL1^[1].
Binding of XCL2 to XCR1 initiates chemotaxic cell migration in dendritic cells as well as in T and B lymphocytes. XCL2 is also induced by cancer and bacterial pathogens. In cancers such as epithelial ovarian and hepatocellular carcinoma, increased XCL2 expression correlates with cancer progression. In a separate study, addition of a tuberculosis antigen, Wag31 induced the expression of XCL2 but not XCL1 in macrophages^[1].

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