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Abstract
Sialoadhesin (Sn) is a sialic acid-binding Ig-like lectin expressed selectively on macrophage subsets. In a model of experimental autoimmune encephalomyelitis, Sn interacted with sialylated ligands expressed selectively on CD4(+)Foxp3(+) regulatory T cells (Tregs) and inhibited their proliferation. In this study, we examined the induction of Sn ligands (SnL) on all splenic CD4(+) T cells following in vitro activation. Most CD4(+) Tregs strongly upregulated SnL, whereas only a small subset of ~20% CD4(+)Foxp3(-) T cells (effector T cells [Teffs]) upregulated SnL. SnL(+) Teffs displayed higher levels of activation markers CD25 and CD69, exhibited increased proliferation, and produced higher amounts of IL-2 and IFN-γ than corresponding SnL(-) Teffs. Coculture of activated Teffs with Sn(+) macrophages or Sn(+) Chinese hamster ovary cells resulted in increased cell death, suggesting a regulatory role for Sn-SnL interactions. The key importance of α2,3-sialylation in SnL expression was demonstrated by increased binding of α2,3-linkage-specific Maackia amurensis lectin, increased expression of α2,3-sialyltransferase ST3GalVI, and loss of SnL following treatment with an α2,3-linkage-specific sialidase. The induction of SnL on activated CD4(+) T cells was dependent on N-glycan rather than O-glycan biosynthesis and independent of the mucin-like molecules CD43 and P-selectin glycoprotein ligand-1, previously implicated in Sn interactions. Induction of ligands on CD4(+)Foxp3(-) Teffs was also observed in vivo using the New Zealand Black × New Zealand White F1 murine model of spontaneous lupus and SnL levels on Teffs correlated strongly with the degree of proteinuria. Collectively, these data indicate that SnL is a novel marker of activated CD4(+) Teffs that are implicated in the pathogenesis of autoimmune diseases.
Highlights
We show that exposure of activated CD4+ T cells to Sn expressed by bone marrow–derived macrophages (BMDM) leads to increased cell death and that Sn ligands (SnL) induced on effector T cell (Teff) is associated with a2-3-linked sialic acid (Sia) on N-glycans rather than O-glycans
Following activation with plate-bound anti-CD3 mAb for 48 h, strong upregulation of SnL was seen for most CD4+ Foxp3+ T cells (Tregs), consistent with the previous observation of SnL expression induced on a subset of these cells in EAE [9]
These findings suggested that SnL induction is an event that follows TCR ligation and that the expression was biased toward TCRtriggered regulatory T cell (Treg) compared with Teffs
Summary
We show that exposure of activated CD4+ T cells to Sn expressed by bone marrow–derived macrophages (BMDM) leads to increased cell death and that SnL induced on Teffs is associated with a2-3-linked Sias on N-glycans rather than O-glycans. Following activation with plate-bound anti-CD3 mAb for 48 h, strong upregulation of SnL was seen for most CD4+ Foxp3+ T cells (Tregs), consistent with the previous observation of SnL expression induced on a subset of these cells in EAE [9].
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