The immune system has various extracellular strategies for clearing infectious agents and recovering homeostasis. A defect in one of these strategies can result in vulnerability to infection, chronic inflammation and/or autoinflammatory disease.
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Cryopyrin-associated periodic syndromes (CAPS) is a spectrum of autoinflammatory syndromes that occur as a result of a rare, heterozygous, gain-of-function mutation in the inflammasome NLRP3. Patients with CAPS have pathological concentrations of C-reactive proteins in their serum, as well as an abundance of extracellular oligomeric ASC+ particles compared with the serum of healthy donors. Although IL-1β appears to drive the clinical phenotype of CAPS, patients don’t consistently have a higher serum concentration of IL-1β than healthy controls.
Traditionally, the NLRP3 inflammasome has been seen as an oligomer with the core sensor NLRP3 and an ASC adaptor. When the two come together, they recruit the protease caspase-1. Caspase-1 controls its own release as well as the release of various leaderless proteins such as the cytokines IL-1β and IL-18. Caspase-1 also induces pyroptosis in macrophages, causing the macrophages to quickly lose plasma membrane integrity and release intracellular proteins, such as the enzyme, lactate dehydrogenase.
A new study expands on this understanding of the NLRP3 inflammasome and indicates that NLRP3 amplifies the inflammatory response through its role as an extracellular oligomeric complex.
Researchers Alberto Baroja-Mazo, PhD, postdoctoral fellow at the Clinical University Hospital Virgen de la Arrixaca in Murcia, Spain, and colleagues describe their model in the August issue of Nature Immunology.1 They write, “In this study, we found that activation of inflammasomes led to the release of functional oligomeric inflammasome particles containing both NLRP3 and ASC that acted as danger signals to amplify inflammation by promoting the activation of caspase-1 extracellularly and in surrounding macrophages following internalization of the particles. Caspase-1 activity was necessary for the release of inflammasome particles from activated macrophages during pyroptosis. The released, extracellular caspase-1 was active and was able to mature pro-IL-1.”
The investigators began their study by determining that activation of the NLRP3, NLRP1 or AIM2 inflammasomes leads to the release of ASC, NLRP3, IL-1β and caspase-1 p10. Activated caspase-1 was able to process pro-IL-1β. After the inflammasomes were activated, NLRP3 continued to be released even after the inhibition of caspase-1. After 10–30 minutes of activation of the NLRP3 inflammasome, extracellular ASC became oligomeric, aggregating into inflammasome oligomeric specks. The ASC specks were able to further increase caspase-1 activity. Thus, an increase in ASC particles over the course of three hours was associated with a continued increase in the release of IL-1β.