Possible evolutionary precursors of mast cells: The ‘granular cell’ immunocyte of Botrylloides leachii (Tunicata; Ascidiacea)

Vertebrate mast cells are the first cells to initiate the inflammatory response. The origin of these highly specialised innate immunity cells in chordates is an intriguing unanswered question, and tunicates represent the best candidates to address this question for their close relationship with vertebrates. In the colonial ascidian Botrylloides leachii, a particular cell type circulates in the haemolymph, namely, ‘granular cell’, which is a distinct immunocyte from both phagocytic and cytotoxic lines. Like mast cells and unlike basophils, granular cells were labelled with anti-c-kit antibody on their plasmalemma and exhibited a high content of heparin in their granules, as revealed by various histochemical techniques. Immunohistochemistry revealed the presence of heparin and histamine inside the same granules resembling the granules of mast cells. Histoenzymatic assays revealed the presence of mast cell enzymes such as β-glucuronidase, arylsulphatase, chloroacetyl esterase, and proteases. These cells degranulated after exposure to bacteria, compound 48/80, or heterologous plasma. During exposure to bacteria, they crowd into the perivisceral sinus and then infiltrate the epithelium of the postbranchial gut, where they release the content of their granules, a behaviour remarkably similar to that of the gastric leukopedesis of mast cells.