PIGMENTED COELOMOCYTES IN HOLOTHUROIDS: NEW PERSPECTIVES ON THE ROLE OF PIGMENTS

Coelomocytes are circulating cells that form the basis of the innate immune system of
echinoderms. They are mainly found in the perivisceral and hydrovascular coelomic
cavities, as well as in association with various internal tissues. Several types of
coelomocytes have been identified, some of which have distinctive red pigmentation. In
sea urchins, red spherule cells are described; and their pigmentation is due to
echinochrome A. This pigment, known for its antimicrobial role, triggers the production
of reactive oxygen species upon immune system stimulation. In sea cucumbers, another
red-pigmented circulating cell type, the haemocytes, is present, with its color traditionally
attributed to a high intracellular concentration of hemoglobin. The presence of
hemoglobin in haemocytes was suggested to play a role in oxygen transport, which could
have been beneficial for burrowing species living in oxygen-poor environments.
However, it has recently been demonstrated that haemocytes are involved in the
encapsulation of foreign particles. In the present study, we investigate the nature of the
red pigmentation of haemocytes found in the hydrovascular fluid of Holothuria forskali.
By comparing perivisceral fluid that does not contain haemocytes and
hydrovascular fluid that contains a high proportion of haemocytes, pigment analyses
using High-Performance Liquid Chromatography (HPLC) revealed a high concentration
of carotenoid pigments, suggesting that they are responsible for haemocyte pigmentation
in this species. The most abundant carotenoid was canthaxanthin. In addition, we took
advantage of the high autofluorescence of the haemocytes to isolate this population by
Fluorescence-Activated Cell Sorting (FACS). This technique allowed us to perform a
targeted analysis of the haemocyte pigments by spectrophotometry which also confirmed
the important quantity of canthaxanthin. Animals generally cannot synthesize carotenoids
on their own and must acquire it from their food. These molecules have a wide range of
functions beyond their role as pigments such as antioxidant defense mechanisms.
Carotenoid supplementation within the echinoderm food sources is known to promote
breeding success and survival of farmed echinoderms. This discovery opens new
perspectives on the function of haemocytes and their pigment in the immune response of
sea cucumbers. Finally, given the presence of these red carotenoids in the pigmented
coelomocytes of H. forskali, we propose to name these cells “carotenocytes”. Our results
strongly suggest that the red pigmentation of haemocytes have been erroneously
attributed to hemoglobin, potentially in all holothuroidea, or at least in some species
inhabiting well-oxygenated environments.