A role for stefin B (cystatin B) in inflammation and endotoxemia.

Stefin B (cystatin B) is an endogenous cysteine cathepsin inhibitor, and the loss-of-function mutations in the stefin B gene were reported in patients with Unverricht-Lundborg disease (EPM1). In this study we demonstrated that stefin B-deficient (StB KO) mice were significantly more sensitive to the lethal LPS-induced sepsis and secreted higher amounts of pro-inflammatory cytokines IL-1β and IL-18 in the serum. We further showed that increased caspase-11 gene expression and better pro-inflammatory caspase-1 and -11 activation determined in StB KO bone marrow-derived macrophages resulted in enhanced IL-1β processing. Pretreatment of macrophages with the cathepsin inhibitor E-64d did not affect secretion of IL-1β, suggesting that the increased cathepsin activity determined in StB KO bone marrow-derived macrophages is not essential for inflammasome activation. Upon LPS stimulation, stefin B was targeted into the mitochondria, and the lack of stefin B resulted in the increased destabilization of mitochondrial membrane potential and mitochondrial superoxide generation. Collectively, our study demonstrates that the LPS-induced sepsis in StB KO mice is dependent on caspase-11 and mitochondrial reactive oxygen species but is not associated with the lysosomal destabilization and increased cathepsin activity in the cytosol.