Immune checkpoints are inhibitory receptors on T cells that tumors exploit to suppress immune responses thereby achieving immune escape. Checkpoint inhibitors, which block these receptors and activate the anti-tumor immune response, have revolutionized cancer treatment—a breakthrough recognized with the 2018 Nobel Prize in Physiology or Medicine. Following PD-1 and CTLA-4, LAG3-targeted drugs receive FDA approval in 2023, the third approved immune checkpoint inhibitors, marking another milestone in cancer immunotherapy. However, like other immune checkpoint therapies, LAG3-targeted treatments benefit only a subset of patients. Identifying those most likely to respond remains a pressing clinical challenge. This requires a deeper understanding of LAG3’s functional mechanisms—a question that has remained unresolved since LAG3’s discovery in 1990. Until now, how ligand binding triggers LAG3 activation has been a major unsolved mystery in the field.
A recent study published in Cell unveils a groundbreaking discovery about the immune checkpoint receptor LAG3. Led by an international team of researchers—including Prof. Xu Chenqi from the Center for Excellence in Molecular Cell Science (Shanghai Institute of Biochemistry and Cell Biology) of the Chinese Academy of Sciences; Prof. Wang Haopeng from ShanghaiTech University; Prof. Dario Vignali from the University of Pittsburgh School of Medicine; Prof. Kong Yan from Peking University Cancer Hospital and Research Institute; and Dr. Shen Zhirong from BeiGene—this study reveals for the first time the molecular switch mechanism underlying LAG3 activation. The team develops a predictive system based on functional biomarkers, providing new strategies for precision immunotherapy.
To investigate how LAG3 is activated, the research team conducts a mass spectrometry-based proteomic analysis of its post-translational modifications (PTMs). Their findings reveal that ligand binding triggers rapid polyubiquitination at the K498 site of the LAG3 receptor. Unlike conventional ubiquitination, which often marks proteins for degradation, this modification plays a pivotal functional role in LAG3 activation. Further studies demonstrate that polyubiquitination acts as a molecular switch to unleash LAG3’s immunosuppressive function. LAG3’s intracellular region contains a Basic residue Rich Sequence (BRS motif), followed by a key signaling domain with the FSALE motif and the polyubiquitination site. In its resting state, LAG3’s critical intracellular signaling motifs are sequestered within the membrane, preventing signal transduction. Upon ligand binding, polyubiquitination facilitates the release of these motifs from the membrane, thereby enabling LAG3’s immune checkpoint function (Figure 1). This “sequestering-unleashing” regulatory mechanism, mediated by polyubiquitination, represents a previously unknown mode of receptor activation.
Building on their discovery of LAG3 activation, the researchers develop a novel biomarker for predicting treatment efficacy. The results show that this functional biomarker (Functionality Biomarker) for characterizing the activated state of LAG3 has significant advantages over traditional biomarkers that merely assess LAG3 expression levels (Expression Biomarker). In clinical studies, the functional biomarker demonstrates superior predictive power: its expression level is 51.7 times higher in responders compared to non-responders (p = 0.0379). In contrast, traditional expression-based biomarkers show only a 6.5-fold difference and lack statistical significance. These findings highlight the functional biomarker’s potential to enhance patient selection for LAG3-based therapies, paving the way for more precise and effective immunotherapy strategies.
Beyond unveiling LAG3’s activation mechanism, this study underscores the broader biological significance of BRS motifs. This work is another significant example showing the biological functions of BRS motifs. Prof. Xu Chenqi’s team has been studying at the forefront of BRS signaling for decades, revealing their crucial roles in diverse immune receptors, including TCR, BCR, CD28, LAG3, PD-L1, and IL7R. Approximately 70% of single-transmembrane proteins have been found to contain BRS motifs in their intracellular juxtamembrane regions. BRS motifs interact with acidic phospholipids and specific protein residues, forming a dynamic electrostatic network that is influenced by factors such as ions and membrane properties. Recent studies from Prof. Xu’s group show that BRS motifs regulate key biological processes, including phosphorylation, ubiquitination, liquid-liquid phase separation, and mechanotransduction. For further details, visit Xu Lab website at https://xulab.sibcb.ac.cn/. Representative publications include Cell (2008), Nature (2013), Cell Research (2017), Nature Structural & Molecular Biology (2017), Cell (2020), and Immunity (2024).
Reference：https://www.cell.com/cell/fulltext/S0092-8674(25)00199-0
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