Common Cell‐Surface Antigens Functioning in Self‐Recognition Reactions by Both Somatic Cells and Gametes in the Solitary Ascidian Halocynthia roretzi

The “contact reaction” is an extremely rapid allogeneic cytotoxic reaction (ACR) mediated by hemocytes in the solitary ascidian Halocynthia roretzi. It has been proposed that regulation of the alloreactivity of hemocytes may be involved in preference for fertilization or self‐sterility in this species. To identify the receptors and target ligands involved both in self‐recognition by somatic cells and self‐discrimination by gametes, we produced monoclonal antibodies (mAbs) that inhibit the ACR mediated by hemocytes and tested their effects on fertilization. Six different mAbs that inhibit the ACR were prepared and categorized into three groups. Although all three mAbs seemed to have the same ability to inhibit the ACR, almost constant and statistically significant inhibition (CRB1.1) and infrequent but significant inhibition (CRB2.1, and CRB3.1) of the ACR were observed in the same pairs of animals. Pretreatment of the unfertilized eggs with CRB1.1, CRB2.1, and CRB3.1, resulted in the constant and statistically significant inhibition, infrequent but significant inhibition, and no inhibition, respectively, of fertilization. Antigens recognized by CRB1.1 (CRB1.1 antigens) were detected on the cell surface of all types of hemocytes and on the vitelline coat and follicle cells of unfertilized eggs. CRB2.1 and CRB3.1 antigens were detected on the surface of certain types of hemocytes and follicle cells, but not on the vitelline coat. CRB mAbs were directed against different epitopes in the N‐linked glycan on glycoproteins. These common carbohydrate antigens on somatic cells and gametes may function in some recognition processes in ACR and fertilization in H. roretzi.