IgG sialylation occurs via the FcRn-mediated recycling pathway in endothelial cells.

IgG is a key mediator of immune responses throughout the human body, and the structure of the conserved glycan on the Fc region has been identified as a key inflammatory switch regulating its downstream effects. In particular, the absence of terminal sialic acid has been shown to increase the affinity of IgG for activating Fc receptors, cascading the inflammatory response in a variety of diseases and conditions. Previously, we have shown that IgG sialylation is mediated by B cell-extrinsic processes. Here, we show that the FcRn-mediated recycling pathway within endothelial cells is a critical modulator of IgG sialylation. Building a deeper understanding of how IgG sialylation is regulated will drive the development of novel therapeutics which dynamically tune IgG functionality in vivo.

Acute and chronic lung inflammation drives changes in epithelial glycans.

Introduction: Asthma is the most common chronic inflammatory disease and it is characterized by leukocyte infiltration and tissue remodeling, with the latter generally referring to collagen deposition and epithelial hyperplasia. Changes in hyaluronin production have also been demonstrated, while mutations in fucosyltransferases reportedly limit asthmatic inflammation. Methods: Given the importance of glycans in cellular communication and to better characterize tissue glycosylation changes associated with asthma, we performed a comparative glycan analysis of normal and inflamed lungs from a selection of murine asthma models. Results: We found that among other changes, the most consistent was an increase in fucose-α1,3-N-acetylglucosamine (Fuc-α1,3-GlcNAc) and fucose-α1,2-galactose (Fuc-α1,2-Gal) motifs. Increases in terminal galactose and N-glycan branching were also seen in some cases, whereas no overall change in O-GalNAc glycans was observed. Increased Muc5AC was found in acute but not chronic models, and only the more human-like triple antigen model yielded increased sulfated galactose motifs. We also found that human A549 airway epithelial cells stimulated in culture showed similar increases in Fuc-α1,2-Gal, terminal galactose (Gal), and sulfated Gal, and this matched transcriptional upregulation of the α1,2-fucosyltransferase Fut2 and the α1,3-fucosyltransferases Fut4 and Fut7. Conclusions: These data suggest that airway epithelial cells directly respond to allergens by increasing glycan fucosylation, a known modification important for the recruitment of eosinophils and neutrophils.

Platelet-localized ST6Gal1 does not impact IgG sialylation.

The IgG antibody class forms an important basis of the humoral immune response, conferring reciprocal protection from both pathogens and autoimmunity. IgG function is determined by the IgG subclass, as defined by the heavy chain, as well as the glycan composition at N297, the conserved site of N-glycosylation within the Fc domain. For example, lack of core fucose promotes increased antibody-dependent cellular cytotoxicity, whereas α2,6-linked sialylation by the enzyme ST6Gal1 helps to drive immune quiescence. Despite the immunological significance of these carbohydrates, little is known about how IgG glycan composition is regulated. We previously reported that mice with ST6Gal1-deficient B cells have unaltered IgG sialylation. Likewise, ST6Gal1 released into the plasma by hepatocytes does not significantly impact overall IgG sialylation. Since IgG and ST6Gal1 have independently been shown to exist in platelet granules, it was possible that platelet granules could serve as a B cell-extrinsic site for IgG sialylation. To address this hypothesis, we used a platelet factor 4 (Pf4)-Cre mouse to delete ST6Gal1 in megakaryocytes and platelets alone or in combination with an albumin-Cre mouse to also remove it from hepatocytes and the plasma. The resulting mouse strains were viable and had no overt pathological phenotype. We also found that despite targeted ablation of ST6Gal1, no change in IgG sialylation was apparent. Together with our prior findings, we can conclude that in mice, neither B cells, the plasma, nor platelets have a substantial role in homeostatic IgG sialylation.