Bacterial DNA promoting inflammation via the Sgk1/Nedd4L/Syk pathway in mast cells contributes to antihistamine non-responsive CSU.

Inflammation centered on non-IgE-mediated mast cell activation characterizes chronic spontaneous urticaria (CSU) resistant to non-sedating H1-antihistamines (nsAH). We recently uncovered a strong positive association between inflammation and the fecal Escherichia. To further explore the actions of bacterial DNA (btDNA) derived from Escherichia on mast cells, intestinal permeability of CSU patients with or without nsAH resistance and healthy controls were determined, and LAD2 cells with knockdown of spleen tyrosine kinase (Syk), neuronally expressed developmentally downregulated 4L (Nedd4L), or serum and glucocorticoid-induced protein kinase 1 (Sgk1) or with incubation of inhibitors GS9973, GSK650394, and MG132 were post-treated with btDNA. We found that 1) serum intestinal permeability indices and btDNA markedly increased in CSU patients with nsAH resistance compared with those without (all P<0.001), and btDNA positively correlated with the degree of inflammation; 2) IL-6 and TNF-α levels were time- and dose-dependently up-regulated in btDNA-stimulated LAD2 cells, which relied on unmethylated CpG in btDNA and Toll-like receptor 9 protein in cells; 3) Syk knockdown or inhibition of Syk Tyr525/526 phosphorylation blocked btDNA-initiated cytokine production; 4) Nedd4L interacted with Tyr525/526-phosphorylated-Syk, and inhibition of Nedd4L Ser448 phosphorylation induced by btDNA-activated Sgk1 was mandatory for btDNA's pro-inflammatory property; and 5) Sgk1 suppression showed an inhibitory effect on btDNA-induced inflammation by ensuring Nedd4L-mediated ubiquitination of Tyr525/526-phosphorylated-Syk. Collectively, we identified previously unknown contributory roles of bacterial translocation and serum btDNA on the inflammation phenotype in CSU patients with nsAH resistance and further uncovered a vital negative regulatory role for the Sgk1/Nedd4L/Syk pathway in btDNA-induced inflammation in LAD2 cells.