Molecular And Cellular Mechanisms Of Immunodeficiency Ad

免疫细胞的正常发育和功能需要腺苷脱氨酶（ADA）的活性。缺乏或低水平的ADA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全，T淋巴细胞消耗和自身免疫。细胞内腺苷和脱氧腺苷的积累及其直接淋巴毒性被认为是ADA SCID患者对T细胞有害影响的原因；然而，免疫细胞对ADA缺乏影响的独特高易感性机制尚未完全了解。我们提出ADA SCID患者的T细胞耗竭可以通过胞外腺苷抑制ADA缺乏环境中的TCR信号传导来解释。验证这一假设的实验表明，腺苷，而不是脱氧腺苷，在体外抑制抗原受体（TCR）触发的T细胞活化。重要的是，在体内也观察到tcr触发的基因工程ADA基因缺陷小鼠T细胞活化的强烈抑制。这表明ADA SCID发病过程中T细胞耗竭的新解释是由于腺苷抑制了T细胞活化，而T细胞活化对胸腺细胞成熟、从胸腺迁移和T细胞外周效应的过程至关重要。我们发现体内注射抗cd3单抗能强烈激活表达ada的小鼠的T细胞，但对缺乏ada的小鼠没有作用。在对照实验中，大量体内未激活的ADA缺陷T细胞在体外对TCR交联有激活反应，但即使在没有ADA抑制剂的情况下，细胞外腺苷的加入也会抑制TCR交联。此外，ADA-/- T细胞的实验结果证实，除了ADA之外，T细胞中没有酶能降解细胞外添加的腺苷。这些对ADA+/+和ADA-/-幼崽的研究允许在ADA SCID研究中使用以前无法获得的对照，并提示在体内ADA缺乏环境中，腺苷介导的tcr介导的信号通路受损，这是胸腺细胞和T细胞对腺苷浓度增加的独特敏感性的额外解释。在ADA缺乏环境下，胞外腺苷对tcr触发的胸腺细胞凋亡的“拯救”作用进一步支持了ADA SCID的信号模型与这些条件下腺苷的细胞内淋巴毒性。在ADA缺乏的情况下，细胞外腺苷的A1、A2a和A3受体独立作用的发现表明，有必要研究A2b受体和/或其他尚未确定的受体参与影响ADA缺陷淋巴细胞活化的可能性。这些数据指出细胞外腺苷对分化免疫细胞和成熟T淋巴细胞的重要作用，并强调进一步了解淋巴细胞暴露于腺苷后的分子信号事件的必要性。

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. The finding of A1, A2a and A3 receptor-independent effects of extracellular adenosine in conditions of ADA deficiency suggests the need to investigate the possibility of involvement of A2b receptor and/or other yet to be identified receptor in affecting the activation of ADA deficient lymphocytes. These data point to the important role of effects of extracellular adenosine on the differentiating immune cells as well on mature T lymphocytes and emphasize the further need in understanding of molecular signaling events that follow exposure of lymphocytes to adenosine.