Molecular/Cellular Mechanisms- Immunodeficiency ADA SCID

免疫细胞的正常发育和功能需要腺苷脱氨酶(ADA)活性。缺乏或低水平的ADA会导致人类疾病、严重的联合免疫缺陷(SCID)，其特征是胸腺发育不良、T淋巴细胞耗尽和自身免疫。细胞内腺苷和脱氧腺苷的积聚及其直接淋巴毒性被认为是ADA SCID患者T细胞受损的原因；然而，免疫细胞对ADA缺乏影响的独特高敏感性的机制尚未完全清楚。我们认为ADA SCID患者的T细胞耗竭可以通过细胞外腺苷抑制ADA缺乏环境中的TCR信号来解释。验证这一假说的实验表明，腺苷，而不是脱氧腺苷，在体外抑制抗原受体(TCR)触发的T细胞激活。重要的是，在体内也观察到了对TCR触发的ADA基因缺陷小鼠T细胞活化的强烈抑制。这提示了ADA SCID发病过程中T细胞耗竭的新解释，即由于腺苷抑制了T细胞的激活，T细胞的激活对胸腺细胞的成熟、胸腺的迁移和外周T细胞效应器的功能至关重要。我们发现，体内注射抗CD3单抗可以强烈激活表达ADA的小鼠的T细胞，但不能激活ADA缺陷的小鼠的T细胞。在对照实验中，大部分在体内未被激活的ADA缺陷T细胞对体外激活的TCR交联物有反应，但即使在没有ADA抑制剂的情况下，细胞外腺苷的加入也能抑制这种反应。此外，对ADA-/-T细胞的实验结果证实，除ADA外，T细胞中没有任何酶能降解细胞外添加的腺苷。这些对ADA+/+和ADA-/-的研究允许在ADA SCID的研究中提供以前无法获得的对照，并建议在体内ADA缺乏的环境中腺苷对TCR介导的信号的损害作为胸腺细胞和T细胞对腺苷浓度增加的影响的独特敏感性的额外解释。在ADA缺乏的环境中，细胞外腺苷对腺苷暴露的胸腺细胞从TCR引发的凋亡中的“拯救”作用的观察，进一步支持了ADA SCID信号模型与细胞内腺苷的淋巴毒性之间的关系。

Normal development and functions of immune cells require adenosine deaminase (ADA) activity. The absence or low levels of ADA results in human disease, severe combined immunodeficiency (SCID), which is characterized by hypoplastic thymus, T lymphocyte depletion, and autoimmunity. Accumulation of intracellular adenosine and deoxyadenosine and their direct lymphotoxicity are believed to be responsible for the detrimental effects on T cells observed in ADA SCID patients; however, mechanisms of uniquely high susceptibility of immune cells to effects of ADA deficiency have not been completely understood. We proposed that T cell depletion in ADA SCID patients could be explained by inhibition of TCR signaling in the ADA-deficient environment by extracellular adenosine. Experiments to test this hypothesis revealed that adenosine, but not the deoxyadenosine, inhibits the antigen receptor (TCR)-triggered activation of T cells in vitro . Importantly, the strong inhibition of TCR-triggered activation of T cells of genetically engineered ADA gene-deficient mice was also observed in vivo. This suggests the novel explanation of T cell depletion during the pathogenesis of ADA SCID as due to inhibition by adenosine of T-cell activation critical for processes of thymocytes maturation, emigration from thymus, and T cell effector functions on periphery. We found that in vivo injection of anti-CD3 mAb strongly activates T cells of ADA-expressing mice but not of their ADA-deficient littermates. In control experiments, the large proportion of ADA-deficient T cells not activated in vivo did respond by activation to TCR crosslinking in vitro, but were inhibited by the addition of extracellular adenosine even in the absence of ADA inhibitor. In addition, results of experiments with ADA-/- T cells confirm that no enzyme in T cells other than ADA degrades extracellularly added adenosine. These studies of ADA+/+ and ADA-/- littermates allowed for previously unavailable controls in investigations of ADA SCID and suggest impairment of TCR-mediated signaling by adenosine in an ADA-deficient environment in vivo as the additional explanation of the unique susceptibility of thymocytes and T cells to the effects of increased concentrations of adenosine. The observations of the "rescue" of adenosine-exposed thymocytes from TCR-triggered apoptosis by extracellular adenosine in ADA deficient environment provided further support of signaling model of ADA SCID vs the intracellular lymphotoxicity of adenosine in these conditions