Isolation and characterization of sea star factor.

Sea star factor (SSF) harvested from the echinoderm Asterias forbesi celomocytes is a potent inhibitor of the primary immune response to T-dependent antigens and also suppresses concanavalin-A-induced mitogenesis. We have isolated and purified a basic protein from this source which has a molecular weight of approximately 38, 000 daltons and is responsible for these biological activities. After complete reduction, SSF was shown to be composed of a single pair of heavy and light chains. Preincubation of spleen cells with concanavalin A does not inhibit binding of SSF; however, specific binding can be inhibited by preincubation with unlabeled SSF. These data suggest that the binding site for SSF is not the same as that for concanavalin A.