Innate immunity: the bacterial connection.

Bacteria encode a complex repertoire of innate immune systems to fend off viruses. Over 150 defense systems disseminated throughout bacterial genomes have now been shown to block viral infection via diverse molecular mechanisms. Multiple components of eukaryotic innate immunity were shown to have bacterial homologs that function in antiviral defense, such as gasdermins, cGAS-STING, and argonautes. These immune pathways likely emerged in bacteria, transmitted to eukaryotes over the course of evolution. New findings on innate immune pathways in one domain of life can provide novel hypotheses in other domains of life. Bacteria might be the source of novel immune proteins across the tree of life. ATP nucleosidases have been reported in diverse prokaryotic and eukaryotic organisms, and homologs of the Eleos and Lamassu bacterial defense systems might be encoded in the human genome. Living cells encode diverse innate immune mechanisms to block pathogens. Bacteria encode a large repertoire of immune strategies, some of which may have been transmitted all the way to humans over the course of evolution. Dissecting the immune mechanisms of bacteria can expand our understanding of innate immunity in all domains of life. Pathogens have fueled the diversification of intracellular defense strategies that collectively define cell-autonomous innate immunity. In bacteria, innate immunity is manifested by a broad arsenal of defense systems that provide protection against bacterial viruses, called phages. The complexity of the bacterial immune repertoire has only been realized recently and is now suggesting that innate immunity has commonalities across the tree of life: many components of eukaryotic innate immunity are found in bacteria where they protect against phages, including the cGAS-STING pathway, gasdermins, and viperins. Here, I summarize recent findings on the conservation of innate immune pathways between prokaryotes and eukaryotes and hypothesize that bacterial defense mechanisms can catalyze the discovery of novel molecular players of eukaryotic innate immunity. [ABSTRACT FROM AUTHOR]