When viruses or bacteria invade cells, aberrant cytosolic DNA serves as a critical danger signal. How the immune system efficiently recognizes such DNA and mounts rapid defense responses has long been a central question in immunology. Now, a study published in Protein & Cell by researchers from the Institute of Biophysics, Chinese Academy of Sciences, in collaboration with Beijing Institute of Technology and other institutions, uncovers a previously unrecognized mechanism: the DNA sensor AIM2 activates immune signaling through liquid-liquid phase separation (LLPS).
The team demonstrated that upon binding double-stranded DNA, AIM2 rapidly forms dynamic liquid-like condensates both in vitro and in cells. These condensates are not random aggregates but highly organized signaling platforms assembled through multivalent interactions. Within these compartments, AIM2 efficiently recruits the downstream adaptor ASC and protease caspase-1 to assemble the inflammasome. Remarkably, these condensates also integrate multiple PANoptosome components—including ZBP1, RIPK1, RIPK3, FADD, and caspase-8—simultaneously engaging pyroptosis, apoptosis, and necroptosis pathways, thereby orchestrating a broad antimicrobial and antiviral immune response.
The study further dissected the molecular basis of AIM2-DNA condensation. All three domains of AIM2—PYD, HIN, and the intrinsically disordered region (IDR)—are indispensable for condensate formation. Intriguingly, the IDR, previously considered merely a flexible linker, is enriched with positively charged residues that critically drive phase separation. When the researchers specifically disrupted charge interactions within the IDR or HIN domain, AIM2 retained DNA-binding ability but failed to form condensates—and consequently, downstream immune activation was severely impaired.
In vivo experiments using knock-in mice carrying condensation-deficient AIM2 mutants revealed dramatically increased mortality and bacterial loads following Francisella novicida infection, alongside exacerbated susceptibility to DSS-induced colitis. These findings underscore the physiological indispensability of AIM2 condensation in antimicrobial host defense and intestinal homeostasis.
Beyond its role in immune activation, AIM2-DNA condensates also serve as regulatory hubs targeted by host and pathogen factors. The host inhibitory protein p202 disrupts AIM2-DNA phase separation in a DNA-binding-dependent manner, while the herpes simplex virus protein VP22 hijacks the condensation process to evade immune detection—revealing phase separation as a key battleground in host-pathogen interplay.
By establishing LLPS as a fundamental mechanism of AIM2 activation, this work not only reshapes our understanding of DNA-sensing innate immunity but also opens new avenues for therapeutic intervention in autoimmune diseases, infections, and cancers by targeting the condensation process itself.
DOI：10.1093/procel/pwag024