Turka L A, Ledbetter J A, Lee K, June C H, Thompson C B
Department of Medicine, University of Michigan, Ann Arbor 48109.
J Immunol. 1990 Mar 1;144(5):1646-53.
The rearrangement of TCR genes during thymic ontogeny creates a repertoire of T cell specificities that is refined to ensure the deletion of autoreactive clones and the MHC restriction of T cell responses. Signals delivered via the accessory molecules CD2, CD4, and CD8 have a crucial role in this phase of T cell differentiation. Recently, CD28 has been identified as a signal transducing molecule on the surface of most mature T cells. Perturbation of the CD28 molecule stimulates a novel pathway of T cell activation regulating the production of a variety of lymphokines including IL-2. We have studied the expression and function of CD28 during thymic ontogeny, and in resting and activated PBL. A variable percentage of resting thymocytes were CD28+ (3 to 25%, n = 8), but it was found in high density only on mature CD3+(bright) CD4/CD8 cells. Both unseparated thymocytes and isolated CD3-CD28-/dull cells proliferated when stimulated with PMA plus IL-2 or PMA plus ionomycin. PMA treatment also rapidly up-regulated CD28 expression in the CD3- subset as these cells became CD3-CD28+(bright). Despite the ability of PMA to induce high density CD28 expression in CD3- cells, CD3- thymocytes did not proliferate in response to PMA plus anti-CD28 mAb, in contrast to unseparated cells. CD3+ thymocytes stimulated with immobilized anti-CD3 mAb also failed to proliferate in culture. However, the addition of either IL-2 or anti-CD28 mAb supported proliferation, suggesting that only CD3+ cells could respond to CD28 signaling. The comitogenic effect of anti-CD3 and anti-CD28 mAb was IL-2 dependent as it was abrogated by an anti-IL-2R mAb. Interestingly, the expression of CD28 on the cell surface of CD3+ cells was also inducible, as flow cytometric analysis demonstrated a 10-fold increase in cell surface CD28 by 24 to 48 h after anti-CD3 stimulation of both CD3+ thymocytes and peripheral blood T cells. This increase was accounted for by a commensurate increase in CD28 mRNA levels. Together, these results suggest that CD28 is an inducible T cell antigen in both CD3- and CD3+ cells. In addition, stimulation of the CD28 pathway can provide a second signal to support the growth of CD3+ thymocytes stimulated through the TCR/CD3 complex, and may therefore represent a mechanism for positive selection during thymic ontogeny.
在胸腺发育过程中，TCR基因重排产生了一系列T细胞特异性，这些特异性经过优化以确保自身反应性克隆的缺失以及T细胞反应的MHC限制。通过辅助分子CD2、CD4和CD8传递的信号在T细胞分化的这一阶段起着关键作用。最近，CD28已被鉴定为大多数成熟T细胞表面的信号转导分子。CD28分子的扰动刺激了一条新的T细胞激活途径，该途径调节包括IL-2在内的多种淋巴因子的产生。我们研究了CD28在胸腺发育过程中以及静息和活化的外周血淋巴细胞中的表达和功能。静息胸腺细胞中可变比例的细胞为CD28+(3%至25%，n = 8)，但仅在成熟的CD3+(明亮)CD4/CD8细胞上以高密度发现。当用PMA加IL-2或PMA加离子霉素刺激时，未分离的胸腺细胞和分离的CD3-CD28-/暗淡细胞均增殖。PMA处理也迅速上调了CD3-亚群中CD28的表达，因为这些细胞变成了CD3-CD28+(明亮)。尽管PMA能够在CD3-细胞中诱导高密度的CD28表达，但与未分离的细胞相比，CD3-胸腺细胞对PMA加抗CD28单克隆抗体无增殖反应。用固定化抗CD3单克隆抗体刺激的CD3+胸腺细胞在培养中也未能增殖。然而，添加IL-2或抗CD28单克隆抗体可支持增殖，表明只有CD3+细胞能够对CD28信号作出反应。抗CD3和抗CD28单克隆抗体的协同致有丝分裂作用依赖于IL-2，因为它被抗IL-2R单克隆抗体所消除。有趣的是，CD3+细胞表面CD28的表达也是可诱导的，流式细胞术分析表明，在抗CD3刺激CD3+胸腺细胞和外周血T细胞后24至48小时，细胞表面CD28增加了10倍。这种增加是由CD28 mRNA水平的相应增加所导致的。总之，这些结果表明CD28在CD3-和CD3+细胞中都是一种可诱导的T细胞抗原。此外，CD28途径的刺激可以提供第二个信号来支持通过TCR/CD3复合物刺激的CD3+胸腺细胞的生长，因此可能代表胸腺发育过程中阳性选择的一种机制。
