TCR Cis-Proximity-Guided LAG-3 Suppression of T Cell Activation and Autoimmunity [bulk RNA-seq]

Therapeutically targeting pathogenic T cells in autoimmune diseases has been challenging. LAG-3, an inhibitory receptor specifically expressed on activated T cells, binds to MHC-II as its canonical ligand. Using artificial antigen-presenting-cells reconstituting cognate or non-cognate peptide-MHC-II, we discovered that MHC-II interaction alone is insufficient for LAG-3's function. Instead, LAG-3's proximity to TCR but not CD4 coreceptor, facilitated by cognate peptide-MHC-II, is crucial in mediating T cell suppression. Mechanistically, LAG-3 forms condensate with TCR signaling component CD3ε through its intracellular FSAL motif and disrupts CD3ε/Lck association. To enhance LAG-3's proximity to TCR complex, an Fc-attenuated LAG-3/TCR bispecific antibody was generated, transforming LAG-3 antagonists into LAG-3-dependent potent suppressors of both CD4 and CD8 T cells, and alleviating autoimmune symptoms in mouse models. Our findings reveal a unique checkpoint cis-modulatory mechanism and provide potential strategies, distinct from PD-1 agonists and others, for T cell-driven autoimmune diseases that lack effective and well-tolerated immunotherapies.