<i>Staphylococcus aureus</i> δ-Toxin Induces Ca<sup>2+</sup> Influx-Independent Degranulation of Murine Cultured Mast Cells

Cutaneous colonization with Staphylococcus aureus (SA) is frequently observed in patients with atopic dermatitis. SA produces a wide variety of bacterial toxins, among which δ-toxin was found to induce degranulation of mast cells. Degranulation of mast cells could enhance bacterial clearance and protection from future SA infection but lead to exacerbation of atopic dermatitis. Because it remains to be determined how δ-toxin triggers degranulation, we investigated δ-toxin-induced changes in murine bone marrow-derived cultured mast cells in this study. We found that δ-toxin-induced degranulation could be classified into two phases, an early Ca²⁺-independent and a late Ca²⁺-dependent phase. Recent studies suggest that NOD-like receptor family, pyrin domain containing 3 is involved in the degranulation of mast cells, raising a possibility that leakage of K⁺ induced by δ-toxin is involved in the Ca²⁺-independent phase. However, Ca²⁺-independent degranulation remains unchanged although Ca²⁺-influx and degranulation induced by δ-toxin were significantly suppressed in the presence of high concentrations of K⁺. Because actin depolymerization was reported to induce degranulation in the absence of Ca²⁺ in the permeabilized rat peritoneal mast cells, a slow but steady decrease in the amount of filamentous actin observed here may be involved in Ca²⁺-independent degranulation induced by δ-toxin. Although Mas-related G protein-coupled receptor (MRGPR) X2 in humans and Mrgprb2 in mice are regarded as the receptors responsible for immunoglobulin E-independent degranulation, δ-toxin-induced degranulation remained unchanged in Mrgprb2^-/- mast cells. Our findings pave the way for identification of the target receptors of δ-toxin.