Galectin-3 negatively regulates TCR-mediated CD4⁺ T-cell activation at the immunological synapse

We have investigated the function of endogenous galectin-3 in T cells. Galectin-3-deficient (gal3^-/-) CD4⁺ T cells secreted more IFN-γ and IL-4 than gal3^+/+CD4⁺ T cells after T-cell receptor (TCR) engagement. Galectin-3 was recruited to the cytoplasmic side of the immunological synapse (IS) in activated T cells. In T cells stimulated on supported lipid bilayers, galectin-3 was primarily located at the peripheral supramolecular activation cluster (pSMAC). Gal3^+/+ T cells formed central SMAC on lipid bilayers less effectively and adhered to antigen-presenting cells less firmly than gal3^-/- T cells, suggesting that galectin-3 destabilizes the IS. Galectin-3 expression was associated with lower levels of early signaling events and phosphotyrosine signals at the pSMAC. Additional data suggest that galectin-3 potentiates down-regulation of TCR in T cells. By yeast two-hybrid screening, we identified as a galectin-3-binding partner, Alix, which is known to be involved in protein transport and regulation of cell surface expression of certain receptors. Co-immunoprecipitation confirmed galectin-3-Alix association and immunofluorescence analysis demonstrated the translocation of Alix to the IS in activated T cells. We conclude that galectin-3 is an inhibitory regulator of T-cell activation and functions intracellularly by promoting TCR down-regulation, possibly through modulating Alix's function at the IS.