Research Highlights

Dendritic cell development: A sugar symphony

The Glyco-gene Chip array v3 from the Consortium for Functional Glycomics contains probe sets to monitor the expression of approximately 2000 human and mouse transcripts.

Processing and presentation of antigens by dendritic cells (DCs) is required for the immune response to pathogens. Studies on receptors binding antigen glycans such as galectins, siglecs (sialic acid binding immunoglobulin-like lectins) and C-type lectins are rapidly progressing. However, the glycan repertoires of immune cells — while crucial for cell-cell interaction and immune functions — have not yet been extensively examined. Now in the Journal of Immunology, Bax et al. present an exhaustive analysis of glycan expression development during DC maturation.

In immature DCs (iDCs), genes involved in the synthesis of glycan core structures or genes encoding sugar donors and transporters (HKG transcripts) were most strongly expressed, while glycosaminoglyan biosynthesis genes (GAG transcripts) and genes involved in the terminal modification of glycans (Term transcripts) were expressed at lower levels. Mass spectrometric (MS) analysis confirmed that iDCs indeed carry glycans synthesized by Term transcripts, such as terminally 3-fucosylated or poly- and 2,3-sialylated structures. Furthermore, MS analysis also indicated the presence of complex-type, high-mannose and poly-N-acetyllactosamine-elongated N-glycans. Sialylated core-1 structures were the most abundant O-glycans detected. Upon antigen capture-induced DC maturation, HKG transcript levels were mostly unchanged, whereas GAG transcripts were down- and Term transcripts upregulated. The anticipated increase of poly-N-acetyllactosaminylation, 2,3- and 3-fucosylation was again confirmed by MS.

When the authors tested the binding of siglecs and galectins to iDCs and mature DCs (mDCs), they noted that siglecs-1 and -2 and -7 bound strongly to iDCs. Siglec-1 (specific for 2,3-linked sialic acid) and siglec-7 (specific for 2-8 sialylation) binding increased with DC maturation as predicted by gene expression and MS data. Galectins 3 and 8 preferentially bound to mDCs and binding correlated with the enhanced presence of the polylactosamine structures in these cells. Bax et al. also showed that DC activation through the Toll-like receptors (TLRs) 3, 4 and 5 was capable of mediating a similar maturation-induced glycan gene expression profile.

In summary, this study by Bax et al. correlates an analysis of glycan expression during DC maturation with glycan gene expression and lectin binding. The results indicate that gene expression quantification should be complemented with MS analysis to reveal the actual order and stoichiometry in which the glycan building blocks are attached to each other. This important study provides another milestone for studies on the progression and functional significance of cellular glycan expression.

Author: Mirko von Elstermann