Endogenous airway mucins carry glycans that bind Siglec-F and induce eosinophil apoptosis

Abstract

Background Siglec-F is a glycan binding protein selectively expressed on mouse eosinophils. Its engagement induces apoptosis, suggesting a pathway for ameliorating eosinophilia in asthma and other eosinophil-associated diseases. Siglec-F recognizes sialylated, sulfated glycans in glycan binding assays, but the identities of endogenous sialoside ligands and their glycoprotein carriers in vivo are unknown. Methods Lungs from normal and mucin-deficient mice, as well as mouse tracheal epithelial cells from mice, were interrogated in vitro and in vivo for the expression of Siglec-F ligands. Western blotting and immunocytochemistry used Siglec-F-Fc as a probe for directed purification, followed by liquid chromatography-tandem mass spectrometric analysis of recognized glycoproteins. Purified components were tested in mouse eosinophil binding assays and flow cytometry-based cell death assays. Results We detected mouse lung glycoproteins that bound to Siglec-F; binding was sialic-acid dependent. Proteomic analysis of Siglec-F binding material identified Muc5b and Muc4. Cross-affinity enrichment and histochemical analysis of lungs from mucin-deficient mice assigned and validated the identity of Muc5b as one glycoprotein ligand for Siglec-F. Purified mucin preparations carried sialylated and sulfated glycans, bound to eosinophils and induced their death in vitro. Mice conditionally deficient in Muc5b displayed exaggerated eosinophilic inflammation in response to intratracheal installation of IL-13. Conclusions These data identify a previously unrecognized endogenous anti-inflammatory property of airway mucins by which their glycans can control lung eosinophilia through engagement of Siglec-F.

Structural basis for sulfation-dependent self-glycan recognition by the human immune-inhibitory receptor Siglec-8

“…x by a distinct mode or, in support of recent findings (44,45), that Gal C6 sulfation is not required for Siglec-F ligands.…”

Abnormalities in MUC5AC and MUC5B Protein in Airway Mucus in Asthma

“…), which is in line with a previous study showing associations for the same allele with lower HHIP expression and lower COPD susceptibility (5). In addition, the minor allele of rs2452785 was associated with lower expression levels of MECR (P value = 6.08 3 10 27 ) and erythrocyte membrane protein band 4.1 (EPB41, P value = 2.44 3 10…”

“…Whereas MUC5B has physiologic functions in the mucus gel that ensure normal mucus clearance (3), MUC5AC has pathologic roles in mechanisms of allergen-induced airway hyperresponsiveness, mucous metaplasia, and airway mucus plugging (4). Changes in the relative proportions of MUC5AC and MUC5B could also affect eosinophil survival in the airway mucus gel because glycoepitopes in the MUC5B glycan coat interact with Siglec-8 on eosinophils to induce apoptosis (5). Thus, a relative deficiency of MUC5B in airway mucus could create an environment that promotes eosinophil survival.…”

Transmembrane Mucin Expression and Function in Embryo Implantation and Placentation

“…Siglecs ( s ialic acid-binding i mmuno g lobulin-like lec tins) are a family of 14 mammalian cell surface proteins primarily expressed in cells of the immune system (Pillai et al 2012 ;Macauley et al 2014 Swanson et al 2007 ;Belisle et al 2010 ;Tanida et al 2013 ;Kiwamoto et al 2015 ). Trophoblast TM binding to inhibitory Siglecs could potentially restrict trophoblast invasion as well as attenuate immune responses to the allogeneic placenta (Redzovic et al 2013 ).…”