Galectin-1: a key effector of regulation mediated by CD4CD25 T cells Short title: Key role of galectin-1 in regulatory T cells

The naturally occurring population of dedicated regulatory T cells that co-express CD4 and CD25 is known to play a key role in the maintenance of peripheral T cell tolerance; however, their mechanism of action has remained obscure. Here we report that a member of the family of β-galactoside-binding proteins, galectin-1, is over-expressed in regulatory T cells, and that expression is increased after activation. Most importantly, blockade of galectin-1 binding significantly reduced the inhibitory effects of human and mouse CD4CD25 T cells. Reduced regulatory activity was observed in CD4CD25 T cells obtained from galectin-1 homozygous null mutant mice. These results suggest that galectin-1 is a key effector of the regulation mediated by these cells.