Lymphocyte Activation Gene (LAG)-3 is Associated with Mucosal Inflammation and Disease Activity in Ulcerative Colitis.

Background and Aims: Lymphocyte activation gene [LAG]-3 is an immune checkpoint and its expression identifies recently activated lymphocytes that may contribute to inflammation. We investigated the role of LAG-3 by analysing its expression and function in immune cells from blood and tissue of patients with ulcerative colitis [UC]. Methods: The phenotypic properties of LAG-3(+) T cells were determined by flow cytometry, qRT-PCR and single-cell RNA-sequencing. LAG-3(+) cells were quantified and correlated with disease activity.The functional effects of LAG-3(+) cells were tested using a depleting anti-LAG-3 monoclonal antibody [mAb] in a mixed lymphocyte reaction [MLR]. Results: LAG-3(+) cells in the blood were negligible. LAG-3(+) lymphocytes were markedly increased in inflamed mucosal tissue and both frequencies of LAG-3(+) T cells and transcript levels of LAG3 correlated with endoscopic severity. LAG-3 expression was predominantly on effector memory T cells, and single-cell RNA-sequencing revealed LAG3 expression in activated and cytokine-producing T cell subsets. Foxp3(+)CD25(hi)Tregs also expressed LAG-3, although most mucosal Tregs were LAG-3(-). Mucosal LAG-3(+) cells produced mainly interferon gamma [IFN gamma] and interleukin-17A. LAG-3(+) cell numbers decreased in patients who responded to biologics, and remained elevated in nonresponders.Treatment with a depleting anti-LAG-3 mAb led to a reduction in proliferation and IFN gamma production in an MLR. Conclusions: LAG-3(+) cells are increased in the inflamed mucosa, predominantly on effector memory T cells with an activated phenotype and their cell numbers positively correlate with disease activity. Depleting LAG-3 eliminates activated proliferating T cells, and hence LAG-3 could be a therapeutic target in UC.