Inhibition of FcϵRI-dependent mediator release and calcium flux from human mast cells by sialic acid-binding immunoglobulin-like lectin 8 engagement

Background: Sialic acid-binding immunoglobulin-like lectins (Siglecs) are a family of glycan-binding inhibitory receptors, and among them, Siglec-8 is selectively expressed on human eosinophils, basophils, and mast cells. On eosinophils, Siglec-8 engagement induces apoptosis, but its function on mast cells is unknown. Objective: We sought to study the effect of Siglec-8 engagement on human mast cell survival and mediator release responses. Methods: Human mast cells were generated from CD34⁺ precursors. Apoptosis was studied by using flow cytometry. Mast cell mediator release or human lung airway smooth muscle contraction was initiated by Fcε{lunate}RI cross-linking with or without preincubation with Siglec-8 or control antibodies, and release of mediators was analyzed along with Ca⁺⁺ flux. RBL-2H3 cells transfected with normal and mutated forms of Siglec-8 were used to study how Siglec-8 engagement alters mediator release. Results: Siglec-8 engagement failed to induce human mast cell apoptosis. However, preincubation with Siglec-8 mAbs significantly (P < .05) inhibited Fcε{lunate}RI-dependent histamine and prostaglandin D₂ release, Ca⁺⁺ flux, and anti-IgE-evoked contractions of human bronchial rings. In contrast, release of IL-8 was not inhibited. Siglec-8 ligation was also shown to inhibit β-hexosaminidase release and Ca⁺⁺ flux triggered through Fcε{lunate}RI in RBL-2H3 cells transfected with full-length human Siglec-8 but not in cells transfected with Siglec-8 containing a tyrosine to phenylalanine point mutation in the membrane-proximal immunoreceptor tyrosine-based inhibitory motif domain. Conclusion: These data represent the first reported inhibitory effects of Siglec engagement on human mast cells.