SAA increases neutrophil respiratory burst in response to E coli but not S pneumoniae. Activation of NADPH oxidase was determined by FACS analysis of rhodamine fluorescence (RHO⁺) in response to E coli or S pneumoniae in the absence or presence of SAA. (A) Typical FACS profiles of neutrophil respiratory burst when neutrophils are incubated with medium alone (control), SAA alone, E coli alone, or E coli + SAA (1 μg/mL). (B) Time course of reactive oxygen production expressed as percentage of RHO⁺ cells following incubation of neutrophils from a typical donor with E coli alone (▴) or E coli + SAA 1 μg/mL (▾), 10 μg/mL (▪), or 50 μg/mL (○). (C) Relative ratio of percentage of RHO⁺ cells in region denoted by bar calculated from values obtained with bacteria + SAA over values obtained with bacteria alone or values from SAA alone over values obtained with medium alone. SAA alone () had no effect on activation of NADPH oxidase at any of the concentrations. SAA enhanced percentage of RHO⁺ neutrophils in response to E coli (▪) but not S pneumoniae (□). The dotted line represents unaltered respiratory burst in the absence of SAA. Data represent mean ± SEM of 10 donors for E coli and 8 for S pneumoniae. Statistical significance of samples with bacteria + SAA versus bacteria alone, or bacteria with 1 μg/mL SAA versus bacteria with 10 μg/mL SAA, was calculated from raw percentage of RHO⁺ cells by Wilcoxon matched pair test (^*P < .05).