Eric Isaac Elliott, MD, PhD

Dr. Eric Isaac Elliott's interest in immunology and cell death research began as an undergraduate at Wheaton College, Wheaton, IL with Dr. Roger Kennett where he studied mouse models of autoimmune neuropathy and generated monoclonal antibodies and attempted to identify small molecules targeting prostate cancer and neuroblastoma cells to promote apoptosis. He continued to pursue his interest in immunology research in the laboratory of Dr. John Colgan at the University of Iowa where he studied the “Justy” mouse that harbors a mutation leading to a dysfunctional Gon4-like (Gon4l) protein and arrest in B lymphopoiesis; he examined the role of Gon4l in T lymphocytes, which are functionally impaired in Justy mice, and investigated how gene expression and cell signaling was altered in these cells.

Dr. Elliott began his study in the area of innate immunity with Dr. Fayyaz Sutterwala at the University of Iowa where he began studying nucleotide binding domain, leucine rich repeat domain-containing receptors (NLR) and focused on mechanisms regulating NLRP3 activation in macrophages. NLRP3 is a cytosolic PRR that perceives diverse pathogenic and sterile stimuli and responds by forming an inflammasome to promote inflammation and pyroptotic cell death. The role of mitochondria in NLRP3 activation is controversial and poorly defined as numerous distinct proteins and molecules are implicated in NLRP3-mitochondria interactions, but mitochondria are postulated to promote NLRP3 activation by providing an activating ligand. Dr. Elliott investigated the role of mitochondria in NLRP3 inflammasome activation in macrophages, the extent of mitochondrial damage required for activation, and features distinguishing inflammasome activation from apoptosis. Some of his initial findings included how perturbations in mitochondrial COX activity and cell ATP and NAD levels were associated with the ability of linezolid to activate the NLRP3 inflammasome, that most NLRP3 agonists cause transient mitochondrial depolarization, and that there is some crossover caspase-1 activation and IL-1b secretion triggered by apoptotic stimuli and some NLRP3 stimuli induce apoptotic caspase activation. These early findings were published in the Immunity (2013) where we also demonstrated that the inner mitochondrial membrane phospholipid cardiolipin is required for NLRP3 mitochondrial translocation and activation through a direct binding interaction.

For his thesis work, Dr. Elliott endeavored to further clarify the roles of mitochondria and cardiolipin in NLRP3 inflammasome assembly and activation. NLRP3 inflammasome activation is tightly regulated, requiring an initial priming signal (e.g. TLR4 activation with LPS) and a secondary activation signal (e.g. the ionophore nigericin). Using differential centrifugation to purify mitochondria, he examined inflammasome localization during step-wise priming and activation and he found that NLRP3 associates with mitochondria upon priming. Independent of NLRP3, the effector caspase-1 also associates with mitochondria at priming. However, mitochondrial localization of the adaptor ASC requires an activation stimulus and is dependent on NLRP3 and calcium flux. Probing the mechanism for priming-induced mitochondrial localization of NLRP3 and caspase-1, he observed that priming induces mitochondrial ROS generation, and pharmacologic inhibition of mitochondrial ROS prevents binding of NLRP3 and caspase-1 to mitochondria. Using mitochondrial flow cytometry, he discovered cardiolipin is externalized to the outer mitochondrial membrane at priming,which is also prevented by mitochondrial ROS blockade. This indicated a mechanism by which NLRP3 could interact with the outer mitochondrial membrane at priming through its direct cardiolipin-binding ability that was previously identified, but did not explain how caspase-1 interacts with mitochondria. Using crosslinking and gel filtration in a cell-free system with cardiolipin liposomes, he found that cardiolipin-containing liposomes induce high molecular weight complexes containing NLRP3 and caspase-1, but not ASC. Complex formation is dependent on membrane density of cardiolipin, acyl chain structure, and cation environment, which suggests that susceptibility to oxidation and phase changes are important properties of cardiolipin promoting NLRP3 and caspase-1 binding. Finally, he prepared nitrocellulose membranes coated with lipids and used liposome sedimentation assays to demonstrate that caspase-1 specifically binds to cardiolipin. Additionally, caspase-11 interacts with cardiolipin to a lesser extent than caspase-1, but other caspases do not interact; he also confirmed previous findings that complexes of caspase-11, but not caspase-1, are induced by LPS and oxPAPC. Collectively, his findings demonstrated NLRP3 and caspase-1 independently bind to cardiolipin, which is exposed on the outer mitochondrial membrane due to oxidative stress at priming; this promotes a localized accumulation of NLRP3 and caspase-1 and initiation of inflammasome assembly at priming, while a subsequent activation signal leads to calcium-driven ASC association with mitochondria to complete NLRP3 inflammasome formation. His findings illustrate that mitochondria serve as innate immune signaling platforms through multiple steps of NLRP3 activation, and further demonstrate a novel capacity of caspase-1 to bind to the lipid cardiolipin, paralleling caspase-11 interactions with the structurally similar lipid A. His results support the “supramolecular organizing center” model of signal transduction, which views innate immune signaling as expanding structural complexes with several mechanisms promoting interactions across multiple interfaces to ensure complex stability and activation of effectors through proximity. Some of these findings were published in the Journal of Immunology (2018).