Research teams based in Germany and Spain have independently discovered that cells transmit inflammation by releasing ASC specks, bacteria-sized clumps of protein key for cytokines’ maturation, according to two papers appearing today (June 22) in Nature Immunology. The protein aggregations are a component of inflammasomes, which sense pathogens and cell damage and set off innate immune inflammation. Researchers previously thought inflammasomes acted only inside single cells, but this latest work has found that the ASC specks can effect extracellular inflammation. The teams also found that macrophages can take up released ASC specks, perpetuating the immune response.
ASC specks are prevalent in the tissues of people with some inflammatory diseases, and could be drug targets for reducing inflammation or diagnostic markers of these diseases, the researchers noted.
Their findings help explain the mystery of how relatively localized contact between a cell and a pathogen or product of cell damage can lead to widespread inflammation, explained George Dubyak, a cell physiologist at Case Western Reserve University in Cleveland, Ohio, who was not involved in the study. “The inflammasome specks can become carriers for intracellular signaling,” he said.
“I think this is much-needed information on how inflammation may actually spread after inflammasome activation and offers a whole host of activities for intervention now that have been unexplored,” said Robert Keane, a professor of physiology and biophysics at the University of Miami Miller School of Medicine who was not involved in the study.
ASC is a soluble protein and, under ordinary circumstances, is distributed throughout the cell. But when a cell encounters a sign of microbial danger or tissue damage, ASC rapidly polymerizes, forming a micron-wide clump that is part of the inflammasome. ASC specks activate the enzyme caspase-1, which cleaves the precursor of the cytokine IL1-β into its mature form.
The German and Spanish teams both encountered ASC specks while working on other projects. Eicke Latz, director of the University of Bonn’s Institute for Innate Immunity in Germany, and his colleagues observed human monocytes and mouse macrophages with fluorescently labeled ASC. They noted that ASC specks accumulated outside the cells at the same time the cells were undergoing pyroptosis, a strategic form of cell death that allows infected cells to kill themselves.
Further, the researchers found that ASC specks—which are made up of a combination of ASC protein and another inflammasome protein—continued to help IL1-β to mature extracellularly. Macrophages ingested ASC specks from the extracellular space, and the ASC specks activated the macrophages’ own inflammasomes, restarting the whole process and multiplying inflammation.
Both outside the cell and when absorbed by macrophages, ASC specks appeared to recruit other free-floating ASC protein, in the manner that resembles how prion proteins recruit others.
When the researchers injected mice with either fluorescent ASC specks or fluorescent beads in the body cavity or ears, they saw that the ASC specks recruited inflammatory immune cells, and injecting mouse footpads with the bacterium Pseudomonas aeruginosa induced release of ASC specks.
Meanwhile, Pablo Pelegrín, an immunologist at the Clinical University Hospital Virgen de la Arrixaca in Murcia, Spain, and his colleagues found that after they activated mouse macrophage inflammasomes, the cells released ASC along with other inflammasome components during pyroptosis. The inflammasome components were active outside the cell, and macrophages that engulfed them soon had their own active inflammasomes as well.
“We think it’s a signaling alert to other cells,” said Pelegrín. “They are releasing these components to activate in other cells the same cascade.”
Pelegrín and colleagues investigated the role of ASC specks in patients with cryopyrin-associated periodic syndromes (CAPS), rare inflammatory disorders caused by gain-of-function mutations in an inflammasome component. They found that CAPS patients have normal levels of ASC specks in their serum during periods of good health but higher levels than healthy controls during disease flare-ups.
Bonn’s Latz and his colleagues investigated ASC specks’ role in inflammatory lung diseases, showing that mice with cigarette smoke-induced chronic obstructive pulmonary disease (COPD) showed greater quantities of extracellular ASC specks compared to controls. Extracellular fluid from humans with COPD and pneumonia also had ASC specks.
The researchers suggested that the specks could serve as diagnostic markers for diseases, or perhaps as drug targets. “If you could find a way to limit or reduce this inflammation cycle to slow down formation or reduce the size of ASC speck, that could potentially be very helpful therapeutically,” said T. Sam Xiao, a structural immunologist at the National Institute of Allergy and Infectious Diseases in Bethesda, Maryland, who was not involved in the study.
Latz warned that some antibodies actually encourage macrophages to take up ASC specks, which may resemble antibody-marked bacteria. But he said that it should also be possible to also make antibodies that can block ASC specks’ action and not stimulate other cells to eat them.
Dubyak suggested that another route would be to block pyroptosis of cells with activated inflammasomes to prevent ASC specks from escaping in the first place. He added that an important next step will be understand how healthy tissues put the brakes on inflammasome-induced inflammation.
“A lot of the inflammasome biology that was not making sense to us is starting to make a lot more sense,” he said.
B.S. Franklin et al., “The adaptor ASC has extracellular and ‘prionoid’ activities that propagate inflammation,” Nature Immunology, doi:10.1038/ni.2913, 2014.
A. Baroja-Mazo et al., “The NLRP3 inflammasome is released as a particulate danger signal that amplifies the inflammatory response,” Nature Immunology, doi:10.1038/ni.2919, 2014.