Foxp3-expressing CD4⁺CD25⁺ regulatory T cells (Tregs) constitutively and highly express the immune checkpoint receptor cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4), whose Treg-specific deficiency causes severe systemic autoimmunity. As a key mechanism of Treg-mediated suppression, Treg-expressed CTLA-4 down-regulates the expression of CD80/CD86 costimulatory molecules on antigen-presenting cells (APCs). Here, we show that Treg-expressed CTLA-4 facilitated Treg-APC conjugation and immune synapse formation. The immune synapses thus formed provided a stable platform whereby Tregs were able to deplete CD80/CD86 molecules on APCs by extracting them via CTLA-4–dependent trogocytosis. The depletion occurred even with Tregs solely expressing a mutant CTLA-4 form lacking the cytoplasmic portion required for its endocytosis. The CTLA-4–dependent trogocytosis of CD80/CD86 also accelerated in vitro and in vivo passive transfer of other membrane proteins and lipid molecules from APCs to Tregs without their significant reduction on the APC surface. Furthermore, CD80 down-regulation or blockade by Treg-expressed membrane CTLA-4 or soluble CTLA-4-immunoglobulin (CTLA-4-Ig), respectively, disrupted cis-CD80/programmed death ligand-1 (PD-L1) heterodimers and increased free PD-L1 on dendritic cells (DCs), expanding a phenotypically distinct population of CD80^lo free PD-L1^hi DCs. Thus, Tregs are able to inhibit the T cell stimulatory activity of APCs by reducing their CD80/CD86 expression via CTLA-4–dependent trogocytosis. This CD80/CD86 reduction on APCs is able to exert dual suppressive effects on T cell immune responses by limiting CD80/CD86 costimulation to naïve T cells and by increasing free PD-L1 available for the inhibition of programmed death-1 (PD-1)–expressing effector T cells. Blockade of CTLA-4 and PD-1/PD-L1 in combination may therefore synergistically hinder Treg-mediated immune suppression, thereby effectively enhancing immune responses, including tumor immunity.

Foxp3 表达 CD4 ⁺ CD25 ⁺调节性 T 细胞 (Tregs) 组成性和高度表达免疫检查点受体细胞毒性 T 淋巴细胞相关抗原 4 (CTLA-4)，其 Treg 特异性缺乏会导致严重的全身性自身免疫。作为 Treg 介导的抑制的关键机制，Treg 表达的 CTLA-4 下调抗原呈递细胞 (APC) 上 CD80/CD86 共刺激分子的表达。在这里，我们表明 Treg 表达的 CTLA-4 促进了 Treg-APC 结合和免疫突触的形成。由此形成的免疫突触提供了一个稳定的平台，Tregs 能够通过依赖 CTLA-4 的吞噬作用将 APC 上的 CD80/CD86 分子提取出来，从而消耗掉 APC 上的 CD80/CD86 分子。即使 Tregs 仅表达缺乏其内吞作用所需的细胞质部分的突变 CTLA-4 形式，也会发生消耗。CD80/CD86 的 CTLA-4 依赖性吞噬作用也在体外和体内加速了其他膜蛋白和脂质分子从 APC 到 Treg 的被动转移，而它们在 APC 表面上没有显着减少。此外，Treg 表达的膜 CTLA-4 或可溶性 CTLA-4-免疫球蛋白 (CTLA-4-Ig) 分别下调或阻断 CD80，破坏 cis-CD80/程序性死亡配体-1 (PD-L1) 异二聚体和树突状细胞 (DC) 上的游离 PD-L1 增加，扩大了表型不同的 CD80 群体^lo free PD-L1 ^hi DCs。因此，Tregs 能够通过 CTLA-4 依赖性吞噬作用降低 APCs 的 CD80/CD86 表达来抑制 APCs 的 T 细胞刺激活性。这种对 APC 的 CD80/CD86 减少能够通过限制 CD80/CD86 对幼稚 T 细胞的共刺激和增加可用于抑制程序性死亡 1 (PD-1) 的游离 PD-L1 对 T 细胞免疫反应产生双重抑制作用- 表达效应 T 细胞。因此，联合阻断 CTLA-4 和 PD-1/PD-L1 可能会协同阻碍 Treg 介导的免疫抑制，从而有效增强免疫反应，包括肿瘤免疫。