Characterization of the human transferrin and lactoferrin receptors in Haemophilus influenzae

The expression of human transferrin and lactoferrin binding activity in Haemophilus influenzae, detected by a binding assay using human transferrin or lactoferrin conjugated to peroxidase, was regulated by the level of available iron in the medium. Transferrin binding activity was present in all H. influenzae isolates tested but not detected in other Haemophilus species or in species of Pasteurella or Actinobacillus. Lactoferrin binding activity was only detected in 1/15 H. influenzae isolates tested. The transferrin and lactoferrin receptors were shown to be specific for the respective human proteins by means of a competition binding assay. Competition binding assays also showed that iron-loaded transferrin was more effective at blocking the transferrin receptor than apotransferrin, but no differences in receptor blocking were observed between iron-loaded lactoferrin and apolactoferrin.