Structural and Kinetic Basis for Selective Inhibition of Canine Myeloperoxidase by Staphylococcal Peroxidase Inhibitor (SPIN) Orthologs.

Staphylococcus aureus and related species are highly adapted to their hosts and have evolved numerous strategies to evade the innate immune system. The Staphylococcal Peroxidase Inhibitor (SPIN) is a small, secreted protein that inhibits myeloperoxidase (MPO). SPIN blocks the MPO active site and prevents production of cytotoxic hypohalous acids within the neutrophil phagosome, thereby enabling evasion of oxidative killing. S. aureus is primarily a human pathogen, and its SPIN protein cannot inhibit MPO from most non-human species. This raises questions as to whether staphylococci adapted to alternative mammalian hosts might produce SPIN proteins with similarly strict preferences for MPO from those species. We tested this hypothesis by isolating canine MPO (cMPO) from abscess samples obtained for veterinary diagnostic purposes and exploring its inhibition profile by a panel of SPIN orthologs. Across two independent trials, SPIN proteins from known canine pathogens bound to and inhibited canine MPO (cMPO) much more potently than human MPO (hMPO). We also found that this increased affinity and potency arises from corresponding increases in the association rate constant (ka) rate constant for cMPO as compared to hMPO. A 2.10 Å resolution co-crystal structure of cMPO inhibited by SPIN from S. delphini provided insights into the physical basis for our functional observations. Along our previous studies on the SPIN/MPO interaction, this work lays a foundation for broadly defining the structure/function principles that underlie host-specific evolution of immune evasion proteins.