Detection of complement receptors on head kidney phagocytes of the common carp Cyprinus carpio

Opsonization is a crucial function of the complement system in immune clearance. To identify receptors responsible for the complement-mediated opsonization in teleost, head kidney leukocytes from β-1,3-glucan-injected and non-injected common carp (Cyprinus carpio) were subjected to a rosette-forming assay with fixed sheep erythrocytes (E^f). Leukocyte fractions rich in granulocytes and macrophages from the glucan-injected carp had an ability to form the rosette with complement-coated E^f ut the corresponding fractions from non-injected fish did not, suggesting that β-1,3-glucan elicits expression of complement receptors on those cells. While sensitization of E^f with antibodies slightly elevated the rosette formation, complement-coated E^f showed a significant increase in rosette formation and this formation was inhibited with anti-carp C3. Rosette formation was also inhibited by EDTA but not by EGTA. These results suggest that complement-dependent opsonization in carp is mediated by CS-receptors analogous to mammalian complement receptor type 3 (CD11b/CD18 or integrin α_M/β₂), which recognizes the iC3b fragment in a divalent cation-dependent manner.