Complex immune system of sea cucumbers - into the secrets of coelomocytes

Sea cucumbers (or holothurians), with their primitive and shapeless appearance, could be considered at first sight as organisms lacking strong defences against pathogens. On the contrary, these animals possess a highly efficient immune system, capable of neutralizing a wide spectrum of invaders. This complex system is mediated by enigmatic motile cells found within body fluids of holothurians: the coelomocytes. Although different coelomocyte types were described morphologically a relatively long ago, their respective functions and contribution to holothurian immunity remain poorly understood.
The present research aims to morpho-functionally describe the coelomocytes of the European holothurian species Holothuria forskali Delle Chiaje, 1823 using a multidisciplinary approach. To study the immune functions, coelomocytes were considered after lipopolysaccharide (LPS) challenges mimicking a Gram-negative bacterial infection.
Initially, coelomocytes were described using different microscopy technics including fluorescence microscopy combined with a cytoskeleton staining and scanning electron microscopy. The results allowed the description of 6 coelomocyte types comprising the first observation of hemocytes in a member of the order Holothuriida and a reconsideration of vibratile cells - a historically described type of flagellated coelomocytes that are contaminating spermatozoa. In parallel, in vivo assays, consisting of generating high-resolution videos of coelomocytes, allowed us to describe specific cellular behaviours and early events leading to the formation of cellular aggregates involved in the encapsulation process of foreign bodies, such as microorganisms. Then, coelomocytes were characterised by flow cytometry. Different populations were distinguished according to cell morphology and autofluorescence, most of which varied in quantities and cell viability proportion after immunostimulation. Finally, proton nuclear magnetic resonance spectroscopy (H1-NMR) was used to identify some metabolites specifically produced after immunostimulation. Out of 19 identified metabolites, 9 appeared to be specific to the LPS challenge.
Altogether, results indicate a multi-level immune response of coelomocytes which illustrates the complexity of the immune system in holothurians.