Foxp3⁺ T regulatory (Treg) cells promote immunological tumor tolerance, but how their immune-suppressive function is regulated in the tumor microenvironment (TME) remains unknown. Here, we used intravital microscopy to characterize the cellular interactions that provide tumor-infiltrating Treg cells with critical activation signals. We found that the polyclonal Treg cell repertoire is pre-enriched to recognize antigens presented by tumor-associated conventional dendritic cells (cDCs). Unstable cDC contacts sufficed to sustain Treg cell function, whereas T helper cells were activated during stable interactions. Contact instability resulted from CTLA-4-dependent downregulation of co-stimulatory B7-family proteins on cDCs, mediated by Treg cells themselves. CTLA-4-blockade triggered CD28-dependent Treg cell hyper-proliferation in the TME, and concomitant Treg cell inactivation was required to achieve tumor rejection. Therefore, Treg cells self-regulate through a CTLA-4- and CD28-dependent feedback loop that adjusts their population size to the amount of local co-stimulation. Its disruption through CTLA-4-blockade may off-set therapeutic benefits in cancer patients.

Foxp3 ⁺T 调节 (Treg) 细胞促进免疫肿瘤耐受，但它们的免疫抑制功能如何在肿瘤微环境 (TME) 中被调节仍然未知。在这里，我们使用活体显微镜来表征为肿瘤浸润性 Treg 细胞提供关键激活信号的细胞相互作用。我们发现多克隆 Treg 细胞库被预先富集以识别肿瘤相关常规树突状细胞 (cDC) 呈递的抗原。不稳定的 cDC 接触足以维持 Treg 细胞功能，而 T 辅助细胞在稳定的相互作用期间被激活。接触不稳定性是由 CTLA-4 依赖性下调 cDC 上的共刺激 B7 家族蛋白引起的，由 Treg 细胞本身介导。CTLA-4 阻断触发 TME 中 CD28 依赖性 Treg 细胞过度增殖，并且需要伴随的Treg细胞失活来实现肿瘤排斥。因此，Treg 细胞通过依赖于 CTLA-4 和 CD28 的反馈回路进行自我调节，该反馈回路将它们的种群大小调整为局部共刺激的量。通过 CTLA-4 阻断其中断可能会抵消癌症患者的治疗益处。