Absent in melanoma 2 is required for innate immune recognition of Francisella tularensis

Area of Science:

• Immunology
• Cell Biology
• Microbiology

Background:

• Macrophages utilize inflammasomes to detect cytosolic nucleic acids, activating caspase-1 and releasing IL-1beta and IL-18 for innate immunity.
• The sensor for cytosolic double-stranded DNA in inflammasome activation remains incompletely understood.

Purpose of the Study:

• To investigate the role of Absent in melanoma 2 (AIM2) in sensing cytosolic DNA and initiating inflammasome activation.
• To determine AIM2's requirement for caspase-1 activation in response to intracellular bacterial infection.
• To elucidate the interplay between type I interferons (IFN-I) and AIM2 in macrophage immune responses.

Main Methods:

• Utilized macrophages from AIM2-deficient mice to assess DNA sensing and inflammasome assembly.
• Infected macrophages with the intracellular pathogen Francisella tularensis.
• Employed immunofluorescence microscopy to visualize colocalization of bacterial DNA, AIM2, and ASC.
• Measured type I IFN secretion and assessed the impact of IFN-I signaling components (IFN-I receptor, STING) on inflammasome activation.
• Evaluated host susceptibility to F. tularensis infection in AIM2-deficient mice.

Main Results:

• AIM2-deficient macrophages failed to sense cytosolic double-stranded DNA and assemble the inflammasome.
• AIM2 was essential for caspase-1 activation in response to F. tularensis infection.
• Bacterial DNA colocalized with AIM2 and ASC in infected macrophages.
• Type I IFN secretion was AIM2-independent, but IFN-I signaling boosted AIM2-dependent caspase-1 activation.
• AIM2-deficient mice exhibited increased susceptibility and bacterial burden when infected with F. tularensis.

Conclusions:

• AIM2 is a critical sensor for cytosolic DNA, initiating inflammasome assembly and caspase-1 activation.
• AIM2 plays a vital role in the innate immune defense against intracellular bacterial pathogens like F. tularensis.
• IFN-I signaling enhances AIM2-mediated inflammasome activation, highlighting a cooperative immune mechanism.