Sepsis is a highly lethal syndrome that affects more than 1.2 million people in the Unites States and 18 million globally each year. A better understanding of the underlying immune mechanisms is greatly needed to develop specific and effective drugs. Innate immune system is central to the sensing of invading pathogens and the activation of the host immune response. A diverse set of germ-line encoded innate immune receptors survey nearly all-cellular compartments for the presence of pathogens and their products. Inflammasomes are multi- protein scaffolds in the cytosol containing a NLR receptor, an adapter ASC, and an effector, caspase-1. Inflammasome is an integral part of the immunosurveillance of the cytosol. Inflammasomes directly detect various signature microbial products or indirectly sense signs associated with an infection. Although lipopolysaccharide (LPS) of Gram-negative bacteria was believed to be exclusively detected at the cell surface by Toll-like receptor-4 (TLR4), it has very recently been described that the LPS is sensed in the cytosol in a TLR4-independent manner by caspase-11, an inflammatory caspase. Activation of caspase-11 by intracellular LPS leads to the proteolytic activation of caspase-1, which then executes the activation of IL-1? and IL-18. Importantly, active caspase-11 triggers an inflammatory form of cell death (pyroptosis) and the release of endogenous alarmin or danger molecules that perpetuate the inflammatory reactions. Cytosolic sensing of LPS and the ensuing caspase-11 activation is the central mediator of sepsis. Despite its profound clinical implications the mechanistic details of this pathway regarding the molecular basis of cytosolic entry of LPS and the regulation of the downstream signaling cascade remains largely unknown. This study seeks to comprehensively address these critical knowledge gaps in three specific aims.
Aim 1 and 2 will identify how LPS enters the cytosol and activates caspase-11 and Aim 3 will characterize a regulatory mechanism that keeps inflammatory caspases-mediated immune responses in check in mice and humans. By uncovering the molecular details of cytosolic LPS sensing-driven responses in humans, the findings from this study could offer new immunomodulatory strategies and targets to bolster protective immunity as well as block detrimental inflammation as desired in infectious diseases.
Sepsis affects 1.2 million people in the US with approximately 30% death rate. The annual cost associated with sepsis care in the US is estimated to exceed 17 billion. Excessive inflammatory responses to the immune recognition of LPS in the cytosol is a key mediator of sepsis. The findings and knowledge derived from this study on cytosolic LPS sensing-driven inflammatory responses in mice and human cells could offer new strategies and targets to treat sepsis.
|Vanaja, Sivapriya Kailasan; Russo, Ashley J; Behl, Bharat et al. (2016) Bacterial Outer Membrane Vesicles Mediate Cytosolic Localization of LPS and Caspase-11 Activation. Cell 165:1106-1119|