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

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. 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. Additionally, hemocytes exhibit strong autofluorescence. In this study,
we used a combined approach of spectral flow cytometry and FACS to isolate
hemocytes by taking advantage of their autofluorescence. 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.