Reinhold M I, Lindberg F P, Kersh G J, Allen P M, Brown E J
Department of Medicine, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
J Exp Med. 1997 Jan 6;185(1):1-11. doi: 10.1084/jem.185.1.1.
The integrin-associated protein (IAP, CD47) is a 50-kD plasma membrane protein with a single extracellular immunoglobulin variable (IgV)-like domain, a multiply membrane-spanning segment, and alternatively spliced short cytoplasmic tails. On neutrophils, IAP has been shown to function in a signaling complex with beta 3 integrins. However, the function of IAP on T cells, which express little or no beta 3 integrin, is not yet defined. Here, we show that mAbs recognizing IAP can enhance proliferation of primary human T cells in the presence of low levels of anti-CD3, but have no effect on T cell proliferation on their own. Together with suboptimal concentrations of anti-CD3, engagement of IAP also enhances IL-2 production in Jurkat cells, an apparently integrin-independent function of IAP. Nonetheless, costimulation by IAP ligation requires cell adhesion. IAP costimulation does not require CD28. Furthermore, anti-IAP, but not anti-CD28, synergizes with suboptimal anti-CD3 to enhance tyrosine phosphorylation of the CD3 zeta chain and the T cell-specific tyrosine kinase Zap70. Ligation of human IAP transfected into the hemoglobin-specific 3.L2 murine T cell hybridoma costimulates activation for IL-2 secretion both with anti-CD3 and with antigenic peptides on antigen-presenting cells (APCs). Moreover, ligation of IAP but not CD28 can convert antagonist peptides into agonists in 3.L2 cells. Using costimulation by IAP ligation as an assay to analyze the structure-function relationships in IAP signaling, we find that both the extracellular and multiply membrane-spanning domains of IAP are necessary for synergy with the antigen receptor, but the alternatively spliced cytoplasmic tails are not. These data demonstrate that IAP ligation initiates an adhesion-dependent costimulatory pathway distinct from CD28. We hypothesize that anti-IAP generates the costimulatory signal because it modulates interactions of the IgV domain with other plasma membrane molecules; this in turn activates effector functions of the multiply membrane-spanning domain of IAP. This model may have general significance for how IAP functions in cell activation.
整合素相关蛋白（IAP，CD47）是一种50kD的质膜蛋白，具有单个细胞外免疫球蛋白可变（IgV）样结构域、多个跨膜区段以及可变剪接的短细胞质尾巴。在中性粒细胞上，IAP已被证明在与β3整合素的信号复合物中发挥作用。然而，IAP在几乎不表达或不表达β3整合素的T细胞上的功能尚未明确。在此，我们表明，识别IAP的单克隆抗体在低水平抗CD3存在的情况下可增强原代人T细胞的增殖，但自身对T细胞增殖无影响。与次优浓度的抗CD3一起，IAP的结合还可增强Jurkat细胞中IL-2的产生，这是IAP一种明显不依赖整合素的功能。尽管如此，IAP连接介导的共刺激需要细胞黏附。IAP共刺激不需要CD28。此外，抗IAP而非抗CD28与次优抗CD3协同作用，增强CD3ζ链和T细胞特异性酪氨酸激酶Zap70的酪氨酸磷酸化。转染到血红蛋白特异性3.L2小鼠T细胞杂交瘤中的人IAP的连接，在抗CD3以及抗原呈递细胞（APC）上的抗原肽作用下，共刺激IL-2分泌的激活。此外，在3.L2细胞中，IAP而非CD28的连接可将拮抗剂肽转化为激动剂。利用IAP连接介导的共刺激作为分析IAP信号传导中结构-功能关系的检测方法，我们发现IAP的细胞外和多个跨膜结构域对于与抗原受体的协同作用都是必需的，但可变剪接的细胞质尾巴则不是。这些数据表明，IAP连接启动了一条与CD28不同的依赖黏附的共刺激途径。我们推测抗IAP产生共刺激信号是因为它调节了IgV结构域与其他质膜分子的相互作用；这反过来又激活了IAP多个跨膜结构域的效应器功能。该模型可能对IAP在细胞激活中的功能具有普遍意义。
