Galectin-1: a key effector of regulation mediated by CD4+CD25+ T cells.

The naturally occurring population of dedicated regulatory T cells that coexpress 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 beta-galactoside-binding proteins, galectin-1, is overexpressed 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 CD4+CD25+ T cells. Reduced regulatory activity was observed in CD4+CD25+ 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.