Human SAP Is a Novel Peptidoglycan Recognition Protein That Induces Complement-Independent Phagocytosis of Staphylococcus aureus

The human pathogen Staphylococcus aureus is responsible for many community-acquired and hospital-associated infections and is associated with high mortality. Concern over the emergence of multidrug-resistant strains has renewed interest in the elucidation of host mechanisms that defend against S. aureus infection. We recently demonstrated that human serum mannose-binding lectin binds to S. aureus wall teichoic acid (WTA), a cell wall glycopolymer-alpha discovery that prompted further screening to identify additional serum proteins that recognize S. aureus cell wall components. In this report, we incubated human serum with 10 different S. aureus mutants and determined that serum amyloid P component (SAP) bound specifically to a WTA-deficient S. aureus Delta tagO mutant, but not to tagO-complemented, WTA-expressing cells. Biochemical characterization revealed that SAP recognizes bacterial peptidoglycan as a ligand and that WTA inhibits this interaction. Although SAP binding to peptidoglycan was not observed to induce complement activation, SAP-bound Delta tagO cells were phagocytosed by human polymorphonuclear leukocytes in an Fc gamma R-dependent manner. These results indicate that SAP functions as a host defense factor, similar to other peptidoglycan recognition proteins and nucleotide-binding oligomerization domain-like receptors.