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First published online May 13, 2004
Journal of Experimental Biology 207, 2147-2155 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.01001

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The sea urchin complement homologue, SpC3, functions as an opsonin

Lori A. Clow^1,*,, David A. Raftos^3,*, Paul S. Gross⁴ and L. Courtney Smith^1,2,

¹ Graduate Program in Genetics, Institute of Biomedical Sciences, George Washington University, Washington DC, USA
² Department of Biological Sciences, George Washington University, Washington DC, USA
³ Department of Biological Sciences, Macquarie University, Sydney, Australia
⁴ Department of Biochemistry, Medical University of South Carolina, Charleston SC, USA

Author for correspondence (e-mail: csmith{at}gwu.edu)

Accepted 29 March 2004

The purple sea urchin Strongylocentrotus purpuratus expresses a homologue of complement component C3 (SpC3), which acts as a humoral opsonin. Significantly increased phagocytic activity was evident when yeast target cells were opsonized after incubation with coelomic fluid containing SpC3. SpC3 could be detected on the surface of yeast, and phagocytic activity could be inhibited by an anti-SpC3 antibody. This indicates that SpC3 promotes phagocytosis by physically tagging target cells for ingestion. Confocal microscopy showed that opsonized yeast were phagocytosed by a single coelomocyte type (polygonal phagocytes), presumably because these cells express SpC3 receptors. Overall, these data indicate that SpC3 is a major humoral opsonin in S. purpuratus coelomic fluid.

Key words: phagocyte, opsonin, sea urchin, Strongylocentrotus purpuratus, echinoderm, complement, innate immunity, evolution

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