RESEARCH HIGHLIGHtS

inflammation Taking AIM in the cytoplasm are cytoplasmic The two studies published in Bürckstümmer et al. used immo- multi- complexes that are Nature identified AIM2 as a candidate bilized IFN-stimulatory DNA in a activated rapidly after infection. sensor of cytoplasmic DNA using proteomic screen to purify nucleic- activation results in database screens to search for acid-binding proteins from three the activation of the cysteine protease that contain both a mouse cell lines. Using the same cell 1, which processes the — used to interact with the adaptor lines, a genomic screen was then immature forms of interleukin-1β molecule ASC (apoptosis-associated carried out to look for that (IL-1β) and IL-18 to generate the speck-like protein containing a CARD; are transcriptionally regulated by mature cytokines that induce innate which is necessary for IFNβ. The data from both screens immune responses. It is known that activation) — and an oligonucleotide- were merged and seven proteins that cytoplasmic DNA can trigger an binding domain. Both groups identi- fulfilled both criteria were identified. innate immune response, but the fied four human proteins from the Of these, the authors focused on DNA sensor for this pathway was interferon (IFN)-inducible HIN-200 the three IFN-inducible proteins, undefined. Now, three studies show family. Fernandes-Alnemri et al. as they contain a pyrin domain. that AIM2 (absent in 2) ectopically expressed these proteins in AIM2 was the only protein identified senses cytoplasmic DNA and associ- a 293T cell line that contains caspase 1 that localized to the cytoplasm in ates with the inflammasome, leading and ASC, and only AIM2 could transient transfection assays. AIM2 to caspase 1 activation. activate caspase 1. The pyrin domain showed a preference for binding of AIM2 was crucial for this process, to double-stranded DNA, and as its deletion abrogated caspase 1 immunoprecipitation assays revealed activation. Hornung et al. generated a that tagged forms of AIM2 and fluorescently tagged fusion protein of ASC interacted when co-expressed the AIM2 pyrin domain and showed in human embryonic kidney 293 that it interacted directly with ASC, cells. Knockdown and co-expression leading to the formation of large experiments similar to those carried cytoplasmic aggregates. out in the other two studies yielded Both groups transfected 293T comparable results. In addition, the cells with AIM2 together with other authors identified a point mutation key components and showed that that disrupts the ability of AIM2 AIM2 is necessary and sufficient to to bind to DNA and another that detect cytoplasmic DNA and induce disrupts AIM2 function. ASC oligomerization and caspase 1 Taken together, these studies show activation. Each group also used short that AIM2 is a sensor for double- interfering RNAs in knockdown stranded cytoplasmic DNA that can experiments in macrophages to show activate inflammasomes, leading to that AIM2 is crucial for the activation ASC oligomerization and caspase 1 of caspase 1 by cytoplasmic DNA. Live activation. cell imaging experiments using tagged Elaine Bell

AIM2 and labelled cytoplasmic DNA ORIGINAL RESEARCH PAPERS Fernandes- showed DNA-induced clustering Alnemri, T. et al. AIM2 activates the of AIM2 in the cytoplasm. Finally, inflammasome and cell death in response to cytoplasmic DNA. Nature 21 Jan 2009 experiments by both groups also dem- (doi:10.1038/nature07710) | Hornung, V. et al. onstrated the importance of AIM2 in AIM2 recognizes cytosolic DNA and forms a immune responses against caspase-1-activating inflammasome with ASC. Nature 21 Jan 2009 (doi:10.1038/nature07725) virus and Francisella tularensis. | Bürckstümmer, T. et al. An orthogonal proteomic– The authors of the third study genomic screen identifies AIM2 as a cytoplasmic DNA sensor for the inflammasome. Nature took a slightly different approach Immunol. 21 Jan 2009 (doi:10.1038/ni.1702) to identify DNA sensors. First,

NATURE REvIEwS | Immunology vOLUME 9 | MARCH 2009 © 2009 Macmillan Publishers Limited. All rights reserved