THE HEPARIN-HISTAMINE SYSTEM IN THE PHAGOCYTIC LINE OF A TUNICATE: AN ANCIENT CELL SYSTEM EQUIVALENT TO VERTEBRATE MAST CELLS?

In the compound ascidian Botryllus schlosseri, sentinel-cells were observed in the oral siphon where they play an immunosurvellance role in the opening of the pharynx. Their morphology, histochemical characteristics and ability to engulf test-particles are typical of hyaline amebocytes of the blood phagocytic line. Histochemical and immunohistochemical studies at light and electron microscope reveal that, like in vertebrate mast cells, heparin and histamine co-localize inside the granules of this cell type and exposure to compound 48/80, a specific degranulating agent of vertebrate mast cells, leads to cell degranulation, suggesting that polyfunctional cells separated functions and competences among various specialized cell types of the innate immunity throughout the chordate evolution. Heparin and histamine were found in the temporary “plug” of colloidal matter that closed the oral siphon after 15 min exposure to bacterial spores in seawater, resulting by degranulation of the sentinel-cells. The main physiological functions of these substances is discussed. Heparin might be involved in the releasing activity of proteases, antimicrobial peptides, histamine, cytokines and growth factors. Histamine might be involved in the modulation of the ciliary beat in the pharynx. In short-term branchial cultures, exogenous histamine significantly increases the ciliary beat frequency and the presence of H2 receptors was indirectly demonstrated by means of the specific antagonist ranitidine suggesting a model of clearance similar to the mucociliary transport in the vertebrate respiratory tract.