Human autoinflammatory syndromes represent a diverse set of diseases, including hereditary periodic fevers (HPF), Type 1 diabetes and gout, which are characterized by overexpression of IL-1β in the absence of recognized inflammatory stimuli. IL-1β is made by myeloid cells in a two-step process involving TLR induced production of pro-IL-1β followed by cleavage into mature IL-β by caspase-1 in a cytoplasmic multi-protein complex called the inflammasome. In addition to caspase-1 and the adaptor protein ASC, the inflammasome contains an NLR protein that acts as a cytoplasmic sensor of pathogens or host cell damage. While all known inflammasome associated NLRs are thought to promote IL-1β secretion, our published and preliminary studies suggest that another NLR protein, NLRP12, is a novel member of the inflammasome complex and functions to inhibit IL-1β production. Our studies suggest a mechanism by which loss-of-function mutations in NLRP12, which are associated with the autoinflammatory syndrome HPF, result in inflammasome hyperactivation and uncontrolled IL- 1β secretion. Nlrp12-/- mice housed in standard facilities, but not Nlrp12-/- mice housed in a reduced pathogen environment, displayed significantly increased levels of IL-1β relative to wild type mice at baseline, suggesting that IL-1β expression in Nlrp12-/- mice is triggered by minimal stimuli. In a murine model of asthma, Nlrp12-/- mice display increased lung inflammation characterized by recruitment of inflammatory cells to the lungs and lymph nodes, increased cytokine expression including IFNg and IL-17 which is suggestive of Th1 and Th17 T cell skewing, and increased airway hyperactivity. In a pulmonary Cryptococcus fungal infection model, Nlrp12-/- mice had significantly elevated levels of IL-1β in the lung, increased lung inflammation, lower fungal burden, and increased dendritic cell numbers in the lymph nodes. In biochemical studies, we made the novel observation that NLRP12 associates with inflammasome proteins ASC, NLRP3, and caspase-1. In this proposal, we will test our hypothesis that NLRP12 normally inhibits IL-1β and IL-18 production by myeloid cells by competing with or inhibiting the activity of stimulatory NLRs in the inflammasome, and that an increase in IL-1β and/or IL-18 production by monocytes, macrophages, and dendritic cells in NLRP12-/- mice leads to increased inflammation and increased Th1 and Th17 polarized T cell responses.
In Aim 1 we will determine the mechanism by which NLRP12 inhibits IL-1β by examining the protein associations of NLRP12 within the inflammasome.
In Aim 2 we will determine the mechanisms by which NLRP12 deficiency leads to increases in complex immune responses.
When completed our studies will determine the mechanism by which NLRP12 inhibits IL-1b production and generally contribute to our understanding of the importance of negative regulators of autoinflammation.
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