Bacterial Surface Protein L Binds and Inactivates Neutrophil Proteins S100A8/A9

Finegoldia magna is an anaerobic bacterial species that is part of the normal human flora on all nonsterile body surfaces, but it is also a significant opportunistic pathogen causing a wide range of infections. Some isolates of F. magna that are more frequently associated with clinical infection express protein L, a surface protein containing multiple homologous domains (B1-B5) that bind Igs through interactions with Ig L chains. The present study shows that the N-terminal A domain of protein L binds S100A8/A9, antibacterial proteins present in large amounts in the cytoplasm of neutrophils, but also extracellularly in tissues during inflammation. As a result, protein L-expressing F. magna are protected against killing by S100A8/A9. Igs and S100A8/A9 were found to interact independently with protein L, demonstrating that this bacterial surface protein is capable of manipulating both adaptive and innate immune defense mechanisms. The Journal of Immunology, 2009, 183: 4583-4592.