An innovative strategy for the isolation of circulating pigmented cells : the case of sea cucumber cœlomocytes

The immune system of sea cucumbers, which are marine invertebrates belonging to the
phylum Echinodermata, relies on circulating cells known as coelomocytes. These cells are
found in the fluids of the coelomic cavity and the ambulacral system of these organisms. In
some sea cucumber species, hemocytes are the predominant type of coelomocytes in the
ambulacral fluid and are stored in large numbers in an organ of this system, the Polian vesicle.
Hemocytes have been recently studied for their role in encapsulating foreign particles (Caulier
et al., 2020). Interestingly, they are easily distinguishable from other coelomocytes due to their
intense red color. It has long been assumed that this pigmentation was due to the presence of
hemoglobin. However, our pigment analysis using HPLC revealed a high concentration of
carotenoids in the ambulacral fluid and Polian vesicle, suggesting that these pigments are
responsible for the hemocyte pigmentation. A comparative transcriptomics approach did not
reveal a notable elevated expression of genes coding for globins relative to other coelomocytes,
in contrast to the expression of genes involved in carotenoid metabolism. This finding
corroborates the hypothesis that the observed pigmentation likely does not originate from
hemoglobin. Additionally, hemocytes exhibit strong autofluorescence, and we took advantage
of this property to isolate hemocytes using a combined approach of spectral flow cytometry and
FACS. We successfully obtained a pure and viable cell population that can be processed for
downstream analyses. This strategy could be extended to other pigmented cells and offer new
perspectives for a more targeted study of hemocytes and their function(s) in the immune
response of sea cucumbers.