Unconventional secretion of a rhamnose-binding lectin by Botryllus schlosseri phagocytes

Botryllus schlosseri is a colonial tunicate diffuse in shallow waters of all seas and oceans. When B. schlosseri haemocytes are challenged with nonself, circulating phagocytes respond to the recognition of foreign molecules with the synthesis and secretion of a rhamnose-binding lectin (previously named BsRBL) with opsonic properties. Here, using a specific antibody raised against BsRBL, we demonstrate that the lectin is present in the supernatants of haemocyte cultures and its synthesis is increased in the presence of nonself. In addition, it is released in the medium via a non-canonical secretory pathway, never described previously in tunicates, involving the formation of extracellular vesicles (ectosomes or microvesicles) deriving from the budding activity of phagocyte plasma membrane.