Identification of pneumococcal surface protein a as a lactoferrin-binding protein of Streptococcus pneumoniae

Lactoferrin (Lf), an iron-sequestering glycoprotein, predominates in mucosa l secretions, where the level of free extracellular iron (10(-18) M) is not sufficient for bacterial growth. This represents a mechanism of resistance to bacterial infections by prevention of colonization of the host by patho gens. In this study we were able to show that Streptococcus pneumoniae spec ifically recognizes and binds the iron carrier protein human Lf (hLf), Pret reatment of pneumococci with proteases reduced hLf binding significantly, i ndicating that the hLf receptor is proteinaceous. Binding assays performed with 63 clinical isolates belonging to different serotypes showed that 88% of the tested isolates interacted with hLf. Scatchard analysis showed the e xistence of two hLf-binding proteins with dissociation constants of 5.7 x 1 0(-8) and 2.74 x 10(-7) M. The receptors were purified by affinity chromato graphy, and internal sequence analysis revealed that one of the S. pneumoni ae proteins was homologous to pneumococcal surface protein A (PspA). The fu nction of PspA as an hLf-binding protein,vas confirmed by the ability of pu rified PspA to bind hLf and to competitively inhibit hLf binding to pneumoc occi. S. pneumoniae may use the hLf-PspA interaction to overcome the iron l imitation at mucosal surfaces, and this might represent a potential virulen ce mechanism.