Regulatory T cells (Treg) limit autoimmunity but can also attenuate the magnitude of anti-pathogen and anti-tumor immunity. Understanding the mechanism of Treg function and therapeutic manipulation of Treg in vivo requires identification of Treg selective receptors. A comparative analysis of gene expression arrays from antigen specific CD4+ T cells differentiating to either an effector/memory or a regulatory phenotype revealed Treg selective expression of LAG-3 (CD223), a CD4-related molecule that binds MHC class II. LAG-3 expression on CD4+ T cells correlates with the cells' in vitro suppressor activity, and ectopic expression of LAG-3 on CD4 T cells confers suppressor activity on the T cells. Antibodies to LAG-3 inhibit suppression both in vitro and in vivo. LAG-3 marks regulatory T cell populations and contributed to their suppressor activity.
