Inhibition of C3 deposition on solid-phase bound immune complexes by lactoferrin

In the study described here, the effect of human tears and purified lactoferrin was investigated on the deposition of complement components on solid-phase bound immune complexes. After incubating immune complexes with fresh normal human serum, the deposition of complement components (C3, C4 and C5) was measured with an ELISA technique. Rabbit antibodies were used as a constituent of the immune complexes, and so both alternative and classical complement pathway activation could be studied. The addition of human tears or purified lactoferrin to this system resulted in the inhibition of classical pathway deposition of C3 and C5, while C4 deposition was not affected. High concentrations of human tears also inhibited alternative pathway C3 deposition on immune complexes, whereas lactoferrin did not detectably affect this pathway. The inhibition of complement activation by tears was not due to a masking of the immune complexes or the already deposited C3. Experiments with purified lactoferrin furthermore showed that lactoferrin did not bind to the complexes, either before or during complement activation. These findings suggest that the complement inhibitory effect is probably taking place in the fluid phase. Saturation of lactoferrin with iron or copper ions resulted in a markedly diminished effect on the capacity of lactoferrin to inhibit complement activation. C4 deposition on immune complexes was not affected by lactoferrin, which suggested that the inhibition of the classical pathway was due to an effect on the classical C3 convertase. The fact that lactoferrin inhibits the classical, but not the alternative C3 convertase, suggests that the effect is probably not mediated through a competition for certain trace metal ions, but may be caused by protein-protein interactions. The findings reported here indicate that lactoferrin may play an important anti-inflammatory role by modulating activation of the complement system. This observation adds a new property to the already described functions of the iron-binding protein lactoferrin.