Characterization of the mechanism of protection mediated by CS-D7, a monoclonal antibody to Staphylococcus aureus iron regulated surface determinant B (IsdB)

Abstract

We previously reported the development of a human monoclonal antibody (CS-D7, IgG1) with specificity and affinity for the iron regulated surface determinant B (IsdB) of Staphylococcus aureus. CS-D7 mediates opsonophagocytic killing in vitro and protection in a murine sepsis model. In light of recent data indicating that IsdB specific T cells (CD4+, Th17), not Ab, mediate protection after vaccination with IsdB, it is important to investigate the mechanism of protection mediated by CS-D7. The mAb was examined to determine if it blocked heme binding to IsdB in vitro. The mAb was not found to have heme blocking activity, nor did it prevent bacterial growth under in vivo conditions, in an implanted growth chamber. To assess the role of the mAb Fc a point mutation was introduced at aa 297 (CS-D7·N297A). This point mutation removes Fc effector functions. In vitro analysis of the mutein confirmed that it lacked measurable binding to FcγR, and that it did not fix complement. The mutein had dramatically reduced in vitro opsonic OP activity compared to CS-D7. Nonetheless, the mutein conferred protection equivalent to the wild type mAb in the murine sepsis model. Both wild type and mutein mAbs were efficacious in FcγR deletion mice (including both FcγRII−/− mice and FcγRIII−/− mice), indicating that these receptors were not essential for mAb mediated protection in vivo. Protection mediated by CS-D7 was lost in Balb/c mice depleted of C3 with cobra venom factor (CFV), was lost in mice depleted of superoxide dismutase (SOD) in P47phox deletion mice, and as previously reported, was absent in SCID mice (Joshi et al., 2012). Enhanced clearance of S. aureus in the liver of CS-D7 treated mice and enhanced production of IFN-γ, but not of IL17, may play a role in the mechanism of protection mediated by the mAb. CS-D7 apparently mediates survival in challenged mice through a mechanism involving complement, phagocytes, and lymphocytes, but which does not depend on interaction with FcγR, or on blocking heme uptake.

Naturally occurring IgG antibody levels to theStaphylococcus aureusprotein IsdB in humans

“…8 The IsdB antigen is highly conserved and expressed on all isolates examined to date. 9 Due to upregulation in iron-limited environments, 9,10 IsdB is theoretically available for immune recognition during infection in the human host, where iron is highly sequestered. 8,11,12 Expression of IsdB was highly upregulated by essentially 100% of strain Becker grown under in vivo conditions in rats.…”

“…8,11,12 Expression of IsdB was highly upregulated by essentially 100% of strain Becker grown under in vivo conditions in rats. 10 Multiple reports have demonstrated IsdB to be a potential vaccine candidate for the prevention of S. aureus infection 9,[13][14][15][16] in rodent challenge models. Importantly, enhanced protection from lethal sepsis in rodent models was mediated by both IsdB-specific CD4+ T cells, 16 and IsdB-specific mAb.…”

“…Importantly, enhanced protection from lethal sepsis in rodent models was mediated by both IsdB-specific CD4+ T cells, 16 and IsdB-specific mAb. 10,13,[17][18][19] The protection afforded by vaccination with IsdB in rodents was lost if the animals were challenged with an IsdB fluid (9), surgical tissue (22), synovial fluid (17), sputum (9), or other sites (5) during routine clinical care. Of the 52 S. aureus isolates, 51 were spa typed; a single isolate failed to grow sufficiently in culture for spa determination.…”

“…Although in vivo expression of IsdB has been demonstrated in rodent models, 10,12 expression in human hosts during infection has not been investigated. To address this question, an investigation was conducted to determine if antibody titers to IsdB increase during the course of S. aureus infection in non-vaccinated patients.…”

The Staphylococci and staphylococcal pathogenesis

“…Cue et al (2012) the vaccine failures that have plagued human clinical trials, and offer optimism that success can be achieved with an appropriate multivalent vaccine that stimulates cellular immunity and opsonophagocytosis, while inhibiting bacterial viability and/or toxicity. Pancari et al (2012) outline the vaccine strategy and progress at Merck Research labs, defining the mechanistic basis of protection offered by a vaccine that targets the IsdB protein, which is expressed in vivo under iron-limited growth conditions.…”