Activation of the classical pathway of complement by binding of bovine lactoferrin to unencapsulated Streptococcus agalactiae.

The ability of lactoferrin (Lf) bound to Streptococcus agalactiae to interfere with the deposition of complement components on the bacterial surface was investigated by enzyme-linked immunosorbent assay (ELISA). By using a strain of S. agalactiae which activates the alternative pathway of complement in the absence of antibodies, it was found that pretreatment of bacteria with Lf shortened the lag phase preceding the deposition of C3 on bacteria. The kinetics of C3 deposition was comparable to that obtained by adding antibodies against S. agalactiae to agammaglobulinaemic precolostral calf serum (PCS) heated at 56 degrees for 3 min to inactivate the alternative pathway. Accelerated C3 deposition did not occur in the absence of Ca2+ ions. Deposition of C4 on bacteria occurred only when either antibodies or Lf were added to PCS. These results demonstrate that the interaction of lactoferrin with bacteria activated the classical pathway of complement in the absence of antibodies. The binding of purified C1q to bacteria was promoted in a dose-dependent manner by Lf, suggesting that recruitment of classical pathway of complement resulted from the interaction of C1q with Lf adsorbed to the bacterial surface. Phagocytosis of bacteria opsonized with heated PCS (at 56 degrees for 3 min) and Lf was comparable to that occurring in the presence of heated PCS and antibodies. In conclusion, Lf was able to substitute for antibodies in order to activate the classical pathway of complement and to opsonize unencapsulated S. agalactiae efficiently.