Opsonophagocytic Antibody Titers for S. aureus

Serologic responses to capsular polysaccharides CP5 and CP8 were measured by an opsonophagocytic activity (OPA) assay, as previously described [14 (link)]. Briefly, serial dilutions of heat-inactivated immune sera were mixed with either a CP5-expressing or CP8-expressing clinical isolate of S. aureus and allowed to opsonize the bacteria. The reaction mixtures were then mixed with baby rabbit complement (Pel-Freez, Rogers, AR, USA) and neutrophil-like HL-60 cells (ATCC^® CCL-240™, Manassas, VA, USA). An OPA antibody titer was defined as the reciprocal of the highest serum dilution resulting in 50% reduction of the number of bacterial colonies after incubation for 60 min at 37 °C when compared to the background control from which serum was omitted. Samples without detectable activity at the lowest serum dilution of 100 were assigned OPA titer values of 50.