LAG-3 expression on Foxp3+ regulatory T cells is critical for controlling homeostasis and autoimmune neuroinflammation

Abstract

Abstract Lymphocyte activation gene-3 (LAG-3) is increasingly recognized as an important immune checkpoint molecule that controls T cell activation. However, the role of LAG-3 on Foxp3+ regulatory T cell (Treg) function remains largely unknown. To test if LAG-3 expression is critical for Treg function in vivo, we generated a novel Treg-specific LAG-3 knock out mouse model (TregΔLag3). In steady state conditions, the lack of LAG-3 in Tregs significantly elevated activated memory-like phenotypes (CD62LlowCD44hi) and higher cytokine production of peripheral T cells. Following induction of experimental autoimmune encephalomyelitis (EAE), a murine model of autoimmune inflammation in the central nervous system, TregΔLag3 mice developed severe EAE compared to that of littermate controls. Flow cytometric analyses showed that TregΔLag3 mice had increased Foxp3+ Tregs in the CNS but that these Tregs expressed lower level of CD44, CD25, ICOS, and GITR, and higher inflammatory cytokines, suggesting their functions may be defective. Unlike WT Tregs, LAG-3-deficient Tregs adoptively transferred into Treg-depleted recipients were unable to rescue the mice from lethal disease, further corroborating the findings. Importantly, Tregs expressing mutant LAG-3 deficient in the cytoplasmic domain of the LAG-3 also failed to control EAE development in vivo, suggesting the key role of LAG-3 intracellular signaling on Treg function. Taken together, our study uncovers a novel Treg-intrinsic role of LAG-3 in regulating homeostasis and inflammation.