Structure and differential target sensitivity of the stimulable cytotoxic complex from hemolymph of the Mediterranean mussel Mytilus galloprovincialis

A cytotoxic protein complex of 320 kDA was isolated from dialyzed plasma of the edible mussel, Mytilus galloprovincialis. Constituted by the assembly of several different proteins, the complex exhibits selective killing against eukaryotic cells, including erythrocytes, mouse tumor cells and protozoan parasites. High variability, which was not correlated with protein concentration, suggested that the immune response of naive mussels was in various stages of activation. Stimulation assays by different treatments in vivo resulted in significant increases in the activity of the plasma suggesting that cytotoxic complexes are involved in immune defense. Lytic activity appears to involve binding of cytotoxic complexes onto target cell membranes and the formation of transmembrane pores. This research provides more evidence that the innate immune system of invertebrates involves large cytotoxic proteins with a broad range of recognitive specificities in addition to small antibacterial, antifungal peptides.