Earthworm coelomocyte extracellular traps: structural and functional similarities with neutrophil NETs

Invertebrate immunity is associated with natural mechanisms that include cellular and humoral elements, similar to those that play a role in vertebrate innate immune responses. Formation of extracellular traps (ETs) is a newly discovered mechanism to combat pathogens, operating not only in vertebrate leucocytes but also in invertebrate immune cells. The ET components include extracellular DNA (exDNA), antimicrobial proteins and histones. Formation of mammalian ETs depends on enzymes such as neutrophil elastase, myeloperoxidase, the citrullination of histones and protease activity. It was confirmed that coelomocytes—immunocompetent cells of the earthworm Eisenia andrei—are also able to release ETs in a protease-dependent manner, dependent or independent of the formation of reactive oxygen species and rearrangement of the cell cytoskeleton. Similar to vertebrate leukocytes (e.g., neutrophil), coelomocytes are responsible for many immune functions like phagocytosis, cytotoxicity and secretion of humoral factors. ETs formed by coelomocyte analogues to neutrophil ETs consist of exDNA, histone H3 and attached to these structures proteins, e.g., heat shock proteins HSP27. The latter fact confirms that mechanisms of ET release are conserved in evolution. The study on Annelida adds this animal group to the list of invertebrates capable of ET release, but most importantly provides insides into innate mechanisms of ET formation in lower animal taxa.