Morula cells as key hemocytes of the lectin pathway of complement activation in the colonial tunicate Botryllus schlosseri

The complement system is deeply rooted in the evolution of humoral mechanism of innate immunity. In addition to the alternative pathway of complement activation, lectins and associated serine proteases exert important roles in the recognition of non-self and activation of the effectors. In the colonial tunicate Botryllus schlosseri, we identified, characterized and studied the expression of three orthologues of genes involved in the lectin pathway of complement activation of vertebrates, i.e., genes for a mannose-binding lectin (MBL), a ficolin and a mannose-associated serine protease 1 (MASP1). All the genes are transcribed by hemocytes, and specifically by morula cells, the same immunocytes responsible for the transcription of C3 and Bf orthologues. The transcription levels of MASP1 and ficolin orthologues are not affected by zymosan challenge, indicating a constitutive expression of complement system associated serine proteases, whereas the MBL orthologue is up-regulated after 15 min of zymosan exposure. Collectively, our data suggest the presence of a complete lectin activation pathway in Botryllus.