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References of the abstract :
Sea cucumbers are curious marine organisms belonging to the echinoderms. Their primitive appearance might suggest that these animals have simple defence mechanisms against pathogens. Instead, sea cucumbers have a complex immune system, capable of neutralising a wide range of invaders. The main actors of this system are circulating cells: the coelomocytes. Several coelomocyte morphotypes are discernible and sea cucumbers appear to have the greatest diversity of cells among the echinoderms. Although their coelomocytes were first described a long time ago, some paradigms concerning them seem incorrect today, and the specific functions of these cells remain poorly understood. This research aims to better understand the diversity and the function of coelomocytes in these particular animals. Initially, a detailed description of coelomocyte morphotypes was performed using different microscopy techniques, including electron microscopy. This allowed to recognise 6 coelomocyte morphotypes, constituting the first description of hemocytes in the Holothuriida clade, and to reconsider the status of vibratile cells – a traditionally recognised coelomocyte type – which are in fact contaminating spermatozoa. Then, to study their immune function, coelomocytes were exposed to lipopolysaccharide (LPS, i.e., molecules characteristic of Gram-negative bacteria). The first investigations consisted of in vivo assays, which allowed us to describe specific cellular behaviours and early events leading to the formation of cellular aggregates. Then, we used a transcriptomic approach to study the differential expression of genes after the LPS challenge. In total, 945 genes were identified as differentially expressed, of which 673 were up-regulated and 272 down-regulated in LPS-treated individuals. Finally, a metabolomic approach was used to identify variations in the abundance of certain metabolites after the LPS challenge. Among the identified metabolites, almost half seemed to be affected by immunostimulation. Overall, the results indicate a multilevel immune response in coelomocytes that illustrates the complexity of the sea cucumber immune system.
