The growing appreciation of immune cell–cell interactions within disease environments has led to extensive efforts to develop immunotherapies. However, characterizing complex cell–cell interfaces in high resolution remains challenging. Thus, technologies leveraging therapeutic-based modalities to profile intercellular environments offer opportunities to study cell–cell interactions with molecular-level insight. We introduce photocatalytic cell tagging (PhoTag) for interrogating cell–cell interactions using single-domain antibodies (VHHs) conjugated to photoactivatable flavin-based cofactors. Following irradiation with visible light, the flavin photocatalyst generates phenoxy radical tags for targeted labeling. Using this technology, we demonstrate selective synaptic labeling across the PD-1/PD-L1 axis in antigen-presenting cell–T cell systems. In combination with multiomics single-cell sequencing, we monitored interactions between peripheral blood mononuclear cells and Raji PD-L1 B cells, revealing differences in transient interactions with specific T cell subtypes. The utility of PhoTag in capturing cell–cell interactions will enable detailed profiling of intercellular communication across different biological systems.

对疾病环境中免疫细胞-细胞相互作用的日益认识导致了开发免疫疗法的广泛努力。然而，以高分辨率表征复杂的细胞-细胞界面仍然具有挑战性。因此，利用基于治疗的模式来分析细胞间环境的技术提供了通过分子水平的洞察力研究细胞间相互作用的机会。我们引入了光催化细胞标记 (PhoTag)，用于使用与可光活化黄素基辅因子偶联的单域抗体 (VHH) 来询问细胞间相互作用。在用可见光照射后，黄素光催化剂会产生苯氧基自由基标签以进行靶向标记。使用这项技术，我们展示了抗原呈递细胞-T 细胞系统中跨 PD-1/PD-L1 轴的选择性突触标记。结合多组学单细胞测序，我们监测了外周血单核细胞和 Raji PD-L1 B 细胞之间的相互作用，揭示了与特定 T 细胞亚型瞬时相互作用的差异。PhoTag 在捕获细胞间相互作用方面的效用将能够详细分析不同生物系统之间的细胞间通讯。