Galectin-9 suppresses B cell receptor signaling and is regulated by I-branching of N-glycans

Abstract

Leukocytes are coated with a layer of heterogeneous carbohydrates (glycans) that modulate immune function, in part by governing specific interactions with glycan-binding proteins (lectins). Although nearly all membrane proteins bear glycans, the identity and function of most of these sugars on leukocytes remain unexplored. Here, we characterize the N-glycan repertoire (N-glycome) of human tonsillar B cells. We observe that naive and memory B cells express an N-glycan repertoire conferring strong binding to the immunoregulatory lectin galectin-9 (Gal-9). Germinal center B cells, by contrast, show sharply diminished binding to Gal-9 due to upregulation of I-branched N-glycans, catalyzed by the β1,6-N-acetylglucosaminyltransferase GCNT2. Functionally, we find that Gal-9 is autologously produced by naive B cells, binds CD45, suppresses calcium signaling via a Lyn-CD22-SHP-1 dependent mechanism, and blunts B cell activation. Thus, our findings suggest Gal-9 intrinsically regulates B cell activation and may differentially modulate BCR signaling at steady state and within germinal centers.

I-branched carbohydrates as emerging effectors of malignant progression

“…In contrast to robust binding on naïve and memory B cells, Gal-9 binding to germinal center B cells is markedly less due, in part, to upregulation of GCNT2/I-branching activity. Gal-9, in the absence of I-branched glycans, imposes a regulatory activity via CD45 binding and downstream suppression of B cell receptor signaling and cell activation (95). Notably, our laboratory also finds that elevated GCNT2/I-branching activity in human B cell lines associates with depressed Gal-3 binding, suggesting that GCNT2 indeed elicits its galectin inhibitory effects across normal and malignant settings.…”

“…This therapeutic notion is dependent, however, on the future discovery and engagement of regulatory factors controlling GCNT2 expression. Coincident with evidence of GCNT2/I-branching antagonizing melanoma galectin ligand activity, intensive glycomic interrogation of human B cell subsets depicts GCNT2 as a major factor controlling Gal-9 binding activity (95). In contrast to robust binding on naïve and memory B cells, Gal-9 binding to germinal center B cells is markedly less due, in part, to upregulation of GCNT2/I-branching activity.…”

“…GCNT2/I-branching activity, however, appears to serve as an end-stage glycosylation event. GCNT2 and B3GNT2, when coexpressed, have a cooperative relationship, in which I-branched poly-LacNAcs are synthesized from linear poly-LacNAcs and the level of I-branched poly-LacNAcs directly correlates with GCNT2 expression (9,94,95). Such endstage glycosylation events, akin to α2,6 sialylation and α1,3 fucosylation, often have profound effects on galectinbinding activities (21,(108)(109)(110).…”

Fc γ receptor compositional heterogeneity: Considerations for immunotherapy development

“…These are predominantly tetraantennary complex-type N-glycans (containing four branches) with four sialic acid residues plus the addition of N-acetyllactosamine (LacNAc) repeats that extend the complex-type branches (Figure 2). LacNAc repeats are potent galectin ligands and are important for T cell and B cell function, though it is not clear how these features impact NK cells and monocytes (47,48). Monocytes expressed glycans at these sites with slightly less Nacetylneuraminic acid and a greater degree of branch fucosylation that may bind to different cross-linking factors.…”

Intracellular Galectin-9 Controls Dendritic Cell Function by Maintaining Plasma Membrane Rigidity

“…Each dot represents apoptosis and differentiation into regulatory T cells (Anderson et al, 2007;Bi et al, 2008;Zhu et al, 2005). Furthermore, extracellular Galectin-9 acts as a suppressor of B cell signaling by binding to the B cell receptor (Cao et al, 2018;Giovannone et al, 2018). Although these studies indicate that Galectin-9 plays an inhibitory role on lymphocytes, its function in myeloid cells remains poorly understood.…”