A terminal degradation product (C3d) of mammalian complement component C3 plays an important role in modulation of the adaptive immune response through the interaction with complement receptor type 2 (CR2) on B cells. The present study is aimed at determining whether this is a functional bridge between the innate and adaptive immune systems in bony fish. The fragmentation of the complement component C3 in carp (Cyprinus carpio) serum, activated with zymosan, was analysed to ascertain if carp C3 also generates a mammalian C3d-like fragment under physiological conditions. A 35 kDa peptide reactive to the anti-carp C3 α-chain was detected on the zymosan particles and in the activated serum. Its N-terminal amino acid sequence identified it as carp C3d derived from the C3-H1 isoform. Another C3 isoform, C3-S, of carp was found to yield a C3d fragment at lower efficiency than C3-H1. Recombinant C3d fragments derived from C3-H1 and C3-S were produced in Escherichia coli as fusion proteins with glutathione-S-transferase (GST), and used for ligands to examine the presence of a possible CR2-like C3d receptor on carp lymphocytes. An enzyme-linked immunoadsorbent assay system, using the recombinant C3d proteins and anti-GST on a microplate to which was attached carp peripheral lymphocytes, detected a significant binding of carp C3d to the lymphocyte. The degree of binding of C3-H1-derived C3d was higher than that of C3-S-derived C3d. In addition, the binding of both ligands was inhibited by anti-C3 α-chain, but not by EDTA or EGTA, indicating that the putative C3d receptor does not require divalent cation. These properties agree well with those reported for mammalian CR2.
All Science Journal Classification (ASJC) codes
- Environmental Chemistry
- Aquatic Science