(A-F) OpaD+ Gc were exposed to primary human neutrophils and ROS production was measured as in Fig 1C. Before addition to neutrophils, OpaD+ Gc was incubated with serum from the indicated species (final concentration, 25%) (A); the indicated molecular weight fraction of human serum (B); human serum that was intact (solid orange; UVA), treated with trypsin (dotted red), or treated sequentially with trypsin inhibitor then trypsin (purple) (C); the ≥ 100 kD human serum fraction from (B) (orange), or serum depleted of immunoglobulins G, M, and A (“Ig-depl serum”, dotted blue) (D); human serum fractions generated by anion exchange chromatography via elution with the indicated molarity of NaCl (E); or the indicated suppressive serum fraction from B and E that was pre-incubated with Gc (“Gc-depl”) or not (“no-depl”) (F). Incubation of OpaD+ Gc with PBS+ was used as a positive control for neutrophil ROS production in all conditions (grey).