For the activation of T cells, it is necessary the specific recognition of the peptide by the T cell receptors (TCR) in the surface of antigen-presenting cells (APCs) and additional signals delivered by costimulatory receptors. In fish, knowledge about the presence of these costimulatory signals is limited and functional evidence almost absent. Thus, in this study, we have identified the stimulatory CD28 and the inhibitory cytotoxic T-lymphocyte-associated protein 4 (CTLA4) coreceptors in the European sea bass (Dicentrarchus labrax), and evaluated their transcription. In parallel, the transcription encoding for the T cell markers CD8α and CD4 was also evaluated. Both coreceptors showed the canonical architecture including a signal peptide, an immunoglobulin domain, a transmembrane region and a cytosolic tail. Protein predictions and phylogenetic tree identify them as true mammalian orthologues of CD28 and CTLA4. We found these genes constitutively expressed in all studied organs of European sea bass with high expression in lymphoid organs (thymus, spleen and head-kidney) and liver. The molecular expression pattern of these genes was up-regulated in head-kidney leucocytes stimulated with T mitogens as concanavalin A and phytohemagglutinin (PHA), but not with the B cell mitogen lipopolysaccharide (LPS). Fish challenged with nodavirus (NNV) evidenced a differential and opposing regulation of the cd28 and ctla4 transcription levels in the brain, the target organ for viral replication, and head-kidney. While cd28 transcription tends to decrease over the infection time in both organs the expression of the ctla4 gene tends to increase. Interestingly, the coreceptor expression is highly and significantly correlated to the transcription of the T cell markers. Our results highlight the important role of CD28 and CTLA4 as costimulatory receptors of T cells in European sea bass but further studies are deserved.

为了激活T细胞，有必要通过抗原呈递细胞（APC）表面的T细胞受体（TCR）特异性识别肽，并通过共刺激受体传递额外的信号。在鱼类中，关于这些共刺激信号的存在的知识是有限的，几乎没有功能证据。因此，在本研究中，我们确定了欧洲鲈鱼（Dicentrarchus labrax）中的刺激性CD28和抑制性细胞毒性T淋巴细胞相关蛋白4（CTLA4）受体。），并评估其转录。同时，还评估了编码T细胞标记CD8α和CD4的转录。两个共受体都显示出规范的结构，包括信号肽，免疫球蛋白结构域，跨膜区和胞质尾巴。蛋白质预测和系统发育树将它们识别为CD28和CTLA4的真正哺乳动物直系同源物。我们发现这些基因在欧洲鲈鱼的所有研究器官中组成性表达，并在淋巴器官（胸腺，脾脏和头部肾脏）和肝脏中高表达。这些基因的分子表达模式在以T促细胞分裂剂如伴刀豆球蛋白A和植物血凝素（PHA）刺激的头肾白细胞中被上调，而在B细胞促细胞分裂剂脂多糖（LPS）刺激下却没有。大脑，病毒复制的靶器官和头肾中的cd28和ctla4转录水平。尽管在两个器官中，随着感染时间的延长，cd28的转录趋于减少，但ctla4基因的表达趋于增加。有趣的是，共受体表达与T细胞标志物的转录高度且显着相关。我们的结果强调了CD28和CTLA4作为欧洲鲈鱼T细胞的共刺激受体的重要作用，但值得进一步研究。