Characterization of Alpha-ToxinhlaGene Variants, Alpha-Toxin Expression Levels, and Levels of Antibody to Alpha-Toxin in Hemodialysis and Postsurgical Patients with Staphylococcus aureus Bacteremia

Abstract

Alpha-toxin is a majorStaphylococcus aureusvirulence factor. This study evaluated potential relationships betweenin vitroalpha-toxin expression ofS. aureusbloodstream isolates, anti-alpha-toxin antibody in serum of patients withS. aureusbacteremia (SAB), and clinical outcomes in 100 hemodialysis and 100 postsurgical SAB patients. Isolates underwentspatyping andhlasequencing. Serum anti-alpha-toxin IgG and neutralizing antibody levels were measured by using an enzyme-linked immunosorbent assay and a red blood cell (RBC)-based hemolysis neutralization assay. Neutralization of alpha-toxin by an anti-alpha-toxin monoclonal antibody (MAb MEDI4893) was tested in an RBC-based lysis assay. Most isolates encodedhla(197/200; 98.5%) and expressed alpha-toxin (173/200; 86.5%).In vitroalpha-toxin levels were inversely associated with survival (cure, 2.19 μg/ml, versus failure, 1.09 μg/ml;P< 0.01). Both neutralizing (hemodialysis, 1.26 IU/ml, versus postsurgical, 0.95;P< 0.05) and IgG (hemodialysis, 1.94 IU/ml, versus postsurgical, 1.27;P< 0.05) antibody levels were higher in the hemodialysis population. Antibody levels were also significantly higher in patients infected with alpha-toxin-expressingS. aureusisolates (P< 0.05). Levels of both neutralizing antibodies and IgG were similar among patients who were cured and those not cured (failures). Sequence analysis ofhlarevealed 12 distincthlagenotypes, and all genotypic variants were susceptible to a neutralizing monoclonal antibody in clinical development (MEDI4893). These data demonstrate that alpha-toxin is highly conserved in clinicalS. aureusisolates. Higherin vitroalpha-toxin levels were associated with a positive clinical outcome. Although patients infected with alpha-toxin-producingS. aureusexhibited higher anti-alpha-toxin antibody levels, these levels were not associated with a better clinical outcome in this study.