Passive immunization with anti‐glucosaminidase monoclonal antibodies protects mice from implant‐associated osteomyelitis by mediating opsonophagocytosis of Staphylococcus aureus megaclusters

Towards the development of a methicillin‐resistant Staphylococcus aureus (MRSA) vaccine we evaluated a neutralizing anti‐glucosaminidase (Gmd) monoclonal antibody (1C11) in a murine model of implant‐associated osteomyelitis, and compared its effects on LAC USA300 MRSA versus a placebo and a Gmd‐deficient isogenic strain (ΔGmd). 1C11 significantly reduced infection severity, as determined by bioluminescent imaging of bacteria, micro‐CT assessment of osteolysis, and histomorphometry of abscess numbers (p < 0.05). Histology also revealed infiltrating macrophages, and the complete lack of staphylococcal abscess communities (SAC), in marrow abscesses of 1C11 treated mice. In vitro, 1C11 had no direct effects on proliferation, but electron microscopy demonstrated that 1C11 treatment phenocopies ΔGmd defects in binary fission. Moreover, addition of 1C11 to MRSA cultures induced the formation of large bacterial aggregates (megaclusters) that sedimented out of solution, which was not observed in ΔGmd cultures or 1C11 treated cultures of a protein A‐deficient strain (ΔSpa), suggesting that the combined effects of Gmd inhibition and antibody‐mediated agglutination are required. Finally, we demonstrated that macrophage opsonophagocytosis of MRSA and megaclusters is significantly increased by 1C11 (p < 0.01). Collectively, these results suggest that the primary mechanism of anti‐Gmd humoral immunity against MRSA osteomyelitis is macrophage invasion of Staphylococcal abscess communities (SAC) and opsonophagocytosis of megaclusters. © 2014 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 32:1389–1396, 2014.