Phylogenetic aspects of the cyclostomes with special attention to the primitive complement sysytem.

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Hagfish is the most, primitive extant vertebrate and thought to defend itself against pathogens only by innate immune system. With a view to understanding the evolution of the innate immune system, we have isolated complement C3 from the serum of hagfish. In this study, our attempt to identify a hagfish C3 convertase was carried out by RT-PCR using degenerative primers designed on the basis of the conserved amino acid stretches among the several kinds of serine proteases. Analyses of a full-length cDNA clone isolated from a hagfish liver cDNA library using one of the cloned RT-PCR products as a probe indicated that this cDNA encoded hagfish mannose-binding lectin-associated serine protease WASP)1. This evidence strongly suggests that the hagfish defends itself against pathogens at least by the complement system composed of lectin pathway