Galectins are versatile glycan-binding proteins involved in immunomodulation. Evidence suggests that galectins can control the immunoregulatory function of cytokines and chemokines through direct binding. Here, we report on an inverse mechanism in which chemokines control the immunomodulatory functions of galectins. We show the existence of several specific galectin-chemokine binding pairs, including galectin-1/CXCL4. NMR analyses show that CXCL4 binding induces changes in the galectin-1 carbohydrate binding site. Consequently, CXCL4 alters the glycan-binding affinity and specificity of galectin-1. Regarding immunomodulation, CXCL4 significantly increases the apoptotic activity of galectin-1 on activated CD8+ T cells, while no effect is observed in CD4+ T cells. The opposite is found for another galectin-chemokine pair, i.e., galectin-9/CCL5. This heterodimer significantly reduces the galectin-9 induced apoptosis of CD4+ T cells and not of CD8+ T cells. Collectively, the current study describes an immunomodulatory mechanism in which specific galectin-chemokine interactions control the glycan-binding activity and immunoregulatory function of galectins.

Galectins are versatile glycan-binding proteins involved in immunomodulation. Increasing evidence suggests that galectins can control the immunoregulatory function of cytokines and chemokines through direct binding. Here, we report a new inverse mechanism by which chemokines control the immunomodulatory function of galectins. In a galectin-chemokine interaction screen we identified several specific galectin-chemokine binding pairs, including galectin-1/CXCL4. NMR analyses showed that CXCL4 binds on the surface edge of the galectin-1 ß-sheet causing changes in the galectin-1 carbohydrate binding site. Consequently, the interaction with CXCL4 altered the glycan binding affinity and specificity of galectin-1. With regard to immunomodulation, CXCL4 potentiated the apoptotic activity of galectin-1 on activated peripheral blood mononuclear cells. The potentiation of apoptosis specifically affected CD8+ T cells, while no effect was observed in CD4+ T cells. An opposite regulatory activity was found for another galectin-chemokine pair, i.e., galectin-9/CCL5. While CCL5 reduced the apoptosis induction by galectin-9 in activated PBMCs, this was only statistically significant for CD4+ T cells and not for CD8+ T cells. Collectively, the current study describes a novel immunomodulatory mechanism in which specific galectin-chemokine interactions control the glycan-binding activity and immunoregulatory function of galectins.

Galectins are glycan binding proteins translating the sugar-encoded information of cellular glycoconjugates into many physiological activities including immunity, cell migration, and signaling. During early B lymphocytes (BL) development at the pre-B cell stage, BL express the pre-B cell receptor (pre-BCR) and are supported by mesenchymal stromal cells secreting Galectin-1 (Gal-1). Gal-1 interacts with glycosylated receptors from stromal and pre-B cell surfaces but also with the pre-BCR through a direct carbohydrate-independent contact. How this interaction might interplay with the glycan-decoding function of Gal-1 is unknown. Here, we investigated Gal-1 binding to cell surface ligands using NMR spectroscopy on native membranes. We showed that pre-BCR regulates Gal-1 binding to specifically target α2,3-sialylated receptors on pre-B cells. Upon pre-BCR interaction, dynamic changes resulted in additional contacts with α2,3-sialylated glycans converting Gal-1 from an exo- to an endo-type lectin. Remarkably, this selectivity switch is able to promote pre-B cell survival. Altogether, we shed light on a new mechanism allowing fine-tuning of Galectin specificity at the cell surfaces.