Characterization of binding of human lactoferrin to pneumococcal surface protein A

Human lactoferrin is an iron-binding glycoprotein that is particularly prom inent in exocrine secretions and leukocytes and is also found in serum, esp ecially during inflammation, It is able to sequester iron from microbes and has immunomodulatory functions, including inhibition of both complement ac tivation and cytokine production. This study used mutants lacking pneumococ cal sur face protein A (PspA) and PspC to demonstrate that the binding of h uman lactoferrin to the surface of Streptococcus pneumoniae was entirely de pendent on PspA, Lactoferrin bound both family 1 and family 2 PspAs. Bindin g of lactoferrin to PspA was shown by surface colocalization with PspA and was verified by the lack of binding to PspA-negative mutants, Lactoferrin w as expressed on the body of the cells but was largely absent from tile pole s. PspC showed exactly the same distribution on the pneumococcal surface as PspA but did not bind lactoferrin, PspA's binding site for lactoferrin was mapped using recombinant fragments of PspA of families 1 and 2, Binding of human lactoferrin was detected primarily in the C-terminal half of the a-h elical domain of PspA (amino acids 167 to 288 of PspA/Rx1), with no binding to the N-terminal 115 amino acids in either strain. The interaction was hi ghly specific. As observed preciously, bovine lactoferrin bound poorly to P spA. Human transferrin did not bind PspA at all. The binding of lactoferrin to S, pneumoniae might provide a way for the bacteria to interfere with ho st immune functions or to aid in the acquisition of iron at the site of inf ection.