Histamine regulates the inflammatory response of the tunicate Styela plicata

Histamine is stored inside hemocytes of the tunicate Styela plicata (Chordata, Tunicata, Ascidiacea), but no evidence on its role in the regulation of the immune response of this species has been reported. We examined whether histamine participated in the regulation of inflammation and host defense in S. plicata. The presence of histamine inside S. plicata hemocytes was confirmed by flow cytometry, and histamine release was detected by ELISA, after in vitro hemocyte stimulation with different PAMPs. In vitro hemocyte treatment with histamine, or specific histamine-receptor agonists, reduced their phagocytic ability. Injection of histamine into the tunic recruited hemocytes to the site of injection. Systemic injection of histamine, or the histamine-releasing agent compound 48/80, decreased the phagocytic ability of hemocytes. Histamine promoted the constriction of tunic hemolymph vessels in vivo, having a direct effect on vasoconstriction in tunic explants. These results provide for the first time clear evidence for the involvement of histamine in the regulation of inflammation and host defense in tunicates.