Responses to cytosolic DNA can protect against both infectious organisms and the mutagenic effect of
DNA integration. Recognition of invading DNA is likely to be fundamental to eukaryotic cellular life, but
has been described only in mammals. Introduction of DNA into chicken macrophages induced type I
interferon mRNA via a pathway conserved with mammals, requiring the receptor cGAS and the signalling
protein STING. A second pathway of cytosolic DNA recognition in mammalian macrophages, initiated by
absent in melanoma 2 (AIM2), results in rapid inflammasome-mediated pyroptotic cell death. AIM2 is
restricted to mammals. Nevertheless, chicken macrophages underwent lytic cell death within 15 min of
DNA transfection. The mouse AIM2-mediated response requires double stranded DNA, but chicken cell
death was maintained with denatured DNA. This appears to be a novel form of rapid necrotic cell death,
which we propose is an ancient response rendered redundant in mammalian macrophages by the
appearance of the AIM2 inflammasome. The retention of these cytosolic DNA responses through evolution,
with both conserved and non-conserved mechanisms, suggests a fundamental importance in
cellular defence.