Background: The metabolic syndrome is characterized by a chronic low grade inflammation that increases the risk for cardiovascular events. With its capability to activate IL-1β, the NLRP3-inflammasome has been suggested to impact the inflammatory process. When the inflammasome is primed and activated, cytosolic ASC aggregates into the ASC-speck, which can be detected using fluorescence based methods. 

We aimed to determine the presence of these inflammasome particles in the development of the metabolic syndrome and understand the role of extracellular occurring particles.

Methods and Results: Male C57BL6/J mice received either a chow or high-fat-diet (HFD) for 7, 14 or 21 weeks. Flow cytometry revealed increased priming of the NLRP3-inflammasome in the stromal vascular fraction (SVF) of visceral white adipose tissue (vWAT) when mice are subjected to HFD. Interestingly, HFD did not increase inducibility of ASC-specks in the subcutaneous adipose tissue (scWAT). The frequency of ASC-specks correlated with immune cell infiltration and severity of the metabolic syndrome. Co-staining of Nigericin-induced ASC-specks with CD45.2 and CD11b suggests their derivation from myeloid cells of the SVF. While expression of inflammasome-associated genes in adipocytes reflected increased inflammatory capability of the vWAT compared to the scWAT, adipocytes failed to assemble the NLRP3-inflammasome in vitro and therefore likely do not produce ASC-specks.

Fluorescence microscopy showed that cells carrying cytosolic ASC are equally dispersed in the white adipose tissue of chow-fed mice, but begin to form crown-like-structures (CLS) when a metabolic syndrome is acquired. Inflammasome activation in those CLS is rare, but the resulting ASC-specks appear to accumulate in the extracellular space of the visceral fat.

When incubated with bone marrow derived macrophages, isolated ASC-specks induced a pro-inflammatory polarization.

Discussion: We found that visceral and subcutaneous fat depots of mice differ significantly in their capability to produce inflammasome particles. Our study further suggests that extracellular inflammasome particles accumulate in metabolically altered visceral adipose tissue and increase its inflammatory capabilities. The imbalance between pro- and anti-inflammatory immune response is an important contributor to the development of cardiovascular disease. If our data is found to be applicable to humans, targeting these extracellular inflammasome particles could potentially reduce the cardiovascular risk of abdominal obesity by disrupting a pro-inflammatory positive feedback loop.