Immune Reactions in the Vertebrates Closest Relatives the Urochordates

Konstantin Khalturin(*ü), Ulrich Kürn, and Thomas C.G. Bosch

1 Introduction 100

2 Urochordates are at the Root of Vertebrate Evolution 100

3 Natural History and Ecology of Urochordates 101

4 Immunity in Urochordates 102

4.1 Antimicrobial Peptides from Urochordates 103

4.2 Allorecognition in Urochordates 104

4.3 Complement in Urochordates 106

4.4 Despite the Absence of MHC, Urochordate Blood Contains NK-Like Cells 107

5 Conclusion 108

References 108

Abstract Currently existing urochordates (sea squirts) and vertebrates diverged in evolution around 570 million years (myr) ago. Phylogenetic analyses based on molecular data provide compelling evidence that these animals are the closest living relatives of vertebrates. Urochordares, therefore, are of critical importance for understanding the origin of vertebrate immune system. For a number of species a large body of molecular data is now available. An extensive EST project and the draft genome sequences of Ciona intestinalis and C. savignyi allow rapid "in silico" searches for immunorelevant molecules. Recent data convincingly demonstrate that urochordates possess nearly full repertoire of vertebrate innate immune system, but totally lack most of the components of an adaptive immunity such as MHC, TCRs and antibodies. In this review we show that knowledge of immunity in lower vertebrate and invertebrate species is now rapidly increasing. Elucidating the details of the origin of the immune systems from a comparative point of view in vertebrate's closest relatives, may finally lead to a better understanding of our own immune system.

Zoological Institute, Christian-Albrechts-University, Olshausenstrasse 40, 24098 Kiel, Germany, [email protected]

H. Heine (ed.), Innate Immunity of Plants, Animals, and Humans. Nucleic Acids and Molecular Biology 21. © Springer-Verlag Berlin Heidelberg 2008

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