Cytosolic LPS sensing triggers the release of a lectin DAMP that promotes inflammation

Abstract

Abstract Activation of inflammatory caspases, such as caspase-11 and caspase-4, by lipopolysaccharide (LPS) from Gram-negative bacteria, is a key mechanism of innate immune defense against infections. This cytosolic LPS sensing pathway results in activation of a pore-forming protein, gasdermin D, which mediates an inflammatory form of cell death called pyroptosis. In addition, caspase-11-mediated cytosolic LPS sensing leads to the activation of caspase-1 and the maturation of IL-1b and IL-18. Importantly, cytosolic LPS-induced pyroptosis is associated with the release of intracellular proteins that act as alarmins or damage-associated molecular patterns (DAMPs) to modulate the inflammatory response and play an important role during infectious diseases and sepsis. However, the identity and function of DAMPs released downstream of caspase-11 activation are poorly characterized. Using a proteomic approach involving ProteomeLab’s PF2D fractionation system followed by mass spectrometry, we have discovered that galectin-1, a b-galactoside-binding lectin, is released as a consequence of cytosolic LPS sensing. By utilizing various mutant cell lines and primary cells we show that galectin-1 secretion is dependent on caspase-11 and gasdermin D, but not NLRP3 or caspase-1. Importantly, in vivo studies using galectin-1-deficient mice showed that galectin-1 acts as a DAMP to promote systemic inflammation and plays a detrimental role during septic shock. Additionally, human septic patients were found to have increased levels of galectin-1 in their serum. Taken together, we have characterized the secretion and function of a novel lectin DAMP released upon lytic cell death and provide new insights into outcomes of inflammasome activation.