Preliminary characterization of complement in a colonial tunicate: C3, Bf and inhibition of C3 opsonic activity by compstatin

• Complement C3 and Bf of Botryllus schlosseri are characterized.
• Compstatin, the C3 specific inhibitor, inhibits Botryllus complement-mediated phagocytosis.
• Only circulating cytotoxic morula cells express BsC3 and BsBf mRNA.

The complement system is a fundamental effector mechanism of the innate immunity in both vertebrates and invertebrates. The comprehension of its roots in the evolution is a useful step to understand how the main complement-related proteins had changed in order to adapt to new environmental conditions and life-cycles or, in the case of vertebrates, to interact with the adaptive immunity.Data on organisms evolutionary close to vertebrates, such as tunicates, are of primary importance for a better understanding of the changes in immune responses associated with the invertebrate-vertebrate transition. Here we report on the characterization of C3 and Bf transcripts from the colonial ascidian Botryllus schlosseri (BsC3 and BsBf, respectively), a reliable model organism for immunobiological research, and present a comparative analysis of amino acid sequences of C3s and Bfs suggesting that, in deuterostomes, the structure of these proteins remained largely unchanged. We also present new data on the cells responsible of the expression of BsC3 and BsBf showing that cytotoxic immunocytes are the sole cells where the relative transcripts can be found. Finally, using the C3 specific inhibitor compstatin, we demonstrate the opsonic role of BsC3 in accordance with the idea that promotion of phagocytosis is one of the main function of C3 in metazoans.