Lymphocyte activation gene 3 (LAG3) protein is a key immune inhibitory checkpoint receptor that suppresses T cell activation, proliferation and effector functions to maintain immune homeostasis and tolerance. In persistent pathological settings, chronic T cell receptor (TCR) signalling, together with sustained LAG3 signalling and other inhibitory receptor signalling, drives T cell dysfunction, highlighting LAG3 as a promising therapeutic target. However, more than 30 years after its discovery, the fundamental mechanisms governing LAG3 signalling and function remain incompletely defined. Elucidating LAG3 downstream signalling pathways and the mechanisms of action of LAG3-blocking antibodies is essential for identifying optimal clinical indications, developing predictive biomarkers, designing more effective LAG3-targeted therapeutics and maximizing clinical efficacy. Recent advances have revealed key molecular mechanisms, including constitutive tonic signalling, ligand-induced ubiquitination-dependent signalling switches and LAG3–TCR proximity-mediated suppression. In this Review, we comprehensively examine LAG3 signalling mechanisms, synthesize current understanding of its functional effect on T cells, discuss emerging therapeutics and their mechanisms of action, and evaluate biomarker strategies for patient selection. We conclude by highlighting key challenges and outlining future directions for LAG3-targeted immunotherapy in oncology and autoimmune disease.
