Abstract
The galectin family of secreted lectins have emerged as important regulators of immune cell function; however, their role in B-cell responses is poorly understood. Here we identify IgM-BCR as a ligand for galectin-9. Furthermore, we show enhanced BCR microcluster formation and signaling in galectin-9-deficient B cells. Notably, treatment with exogenous recombinant galectin-9 nearly completely abolishes BCR signaling. We investigated the molecular mechanism for galectin-9-mediated inhibition of BCR signaling using super-resolution imaging and single-particle tracking. We show that galectin-9 merges pre-existing nanoclusters of IgM-BCR, immobilizes IgM-BCR, and relocalizes IgM-BCR together with the inhibitory molecules CD45 and CD22. In resting naive cells, we use dual-color super-resolution imaging to demonstrate that galectin-9 mediates the close association of IgM and CD22, and propose that the loss of this association provides a mechanism for enhanced activation of galectin-9-deficient B cells.
Highlights
The galectin family of secreted lectins have emerged as important regulators of immune cell function; their role in B-cell responses is poorly understood
We found no difference in the H function curve in Gal9-KO B cells compared to WT B cells. These findings suggest that galectin-9 does not mediate formation of IgM-B-cell receptor (BCR) nanoclusters; galectin-9 is sparsely distributed at the cell surface of naive B cells (Fig. 1b), and direct stochastic optical reconstruction microscopy (dSTORM) analysis based on randomly selected regions may underestimate an effect within the galectin-9 lattice
We found that BCR microcluster formation and signaling are enhanced in galectin-9-deficient B cells
Summary
The galectin family of secreted lectins have emerged as important regulators of immune cell function; their role in B-cell responses is poorly understood. Two key parameters influencing the assembly of signaling clusters and regulation of membrane receptor activation are the constitutive nanoscale clustering of membrane proteins referred to as nanoclusters or protein islands[8,9,10], and the cell surface mobility of membrane proteins (or nanoclusters of proteins) 7,11,12. These parameters have important implications for receptor triggering and the assembly of signaling complexes as they influence the interaction between protein partners. Several mechanisms have been identified that impact on the organization and mobility of membrane proteins, including the actin cytoskeleton[11,12,13], protein–protein interactions[9,14,15,16], and membrane microdomains defined by lipid composition[8,17]
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