The CD28-B7 interaction is required to deliver a second signal necessary for T-cell activation. Additional membrane receptors of the CD28 and B7 families are also involved in immune checkpoints that positively or negatively regulate leukocyte activation, in particular T lymphocytes. BTLA is an inhibitory receptor that belongs to a third receptor family. Fish orthologs exist only for some of these genes, and the potential interactions between the corresponding ligands remain mostly unclear. In this work, we focused on the channel catfish (Ictalurus punctatus), a long-standing model for fish immunology, to analyze these co-stimulatory and co-inhibitory receptors. We identified one copy of cd28, ctla4, cd80/86, b7h1/dc, b7h3, b7h4, b7h5, two btla, and four b7h7 genes. Catfish CD28 contains the highly conserved mammalian cytoplasmic motif for PI3K and GRB2 recruitment, however this motif is absent in cyprinids. Fish CTLA4 share a C-terminal putative GRB2-binding site but lacks the mammalian PI3K/GRB2-binding motif. While critical V-domain residues for human CD80 or CD86 binding to CD28/CTLA4 show low conservation in fish CD80/86, C-domain residues are highly conserved, underscoring their significance. Catfish B7H1/DC had a long intracytoplasmic domain with a P-loop-NTPase domain that is absent in mammalian sequences, while the lack of NLS motif in fish B7H4 suggests this protein may not regulate cell growth when expressed intracellularly. Finally, there is a notable expansion of fish B7H7s, which likely play diverse roles in leukocyte regulation. Overall, our work contributes to a better understanding of fish leukocyte co-stimulatory and co-inhibitory receptors.

CD28-B7 相互作用需要传递 T 细胞激活所需的第二个信号。CD28 和 B7 家族的其他膜受体也参与正向或负向调节白细胞活化（特别是 T 淋巴细胞）的免疫检查点。BTLA 是属于第三受体家族的抑制性受体。鱼类直向同源物仅存在于其中一些基因，并且相应配体之间的潜在相互作用大多仍不清楚。在这项工作中，我们重点关注斑点叉尾鮰（Ictalurus punctatus）（一种长期存在的鱼类免疫学模型），以分析这些共刺激和共抑制受体。我们鉴定了cd28、ctla4、cd80/86、b7h1 / dc 、 b7h3、b7h4、b7h5 、 2 个btla和 4 个b7h7基因的一份拷贝。鲶鱼 CD28 含有高度保守的哺乳动物细胞质基序，用于招募 PI3K 和 GRB2，但鲤鱼中不存在该基序。鱼类 CTLA4 共享 C 端假定的 GRB2 结合位点，但缺乏哺乳动物 PI3K/GRB2 结合基序。虽然人类 CD80 或 CD86 与 CD28/CTLA4 结合的关键 V 结构域残基在鱼类 CD80/86 中显示出低保守性，但 C 结构域残基高度保守，强调了它们的重要性。鲶鱼 B7H1/DC 具有长的胞质内结构域，其中包含哺乳动物序列中不存在的 P 环 NTPase 结构域，而鱼 B​​7H4 中缺乏 NLS 基序表明该蛋白在细胞内表达时可能无法调节细胞生长。最后，鱼类 B7H7 显着扩增，它们可能在白细胞调节中发挥不同的作用。总的来说，我们的工作有助于更好地了解鱼类白细胞共刺激和共抑制受体。