The Staphylococcus aureus superantigen SElX is a bifunctional toxin that inhibits neutrophil function
Fig 5
SElX inhibits IgG-mediated neutrophil phagocytosis and reduces killing by human neutrophils.
(i) Phagocytosis of fluorescent-labelled S. aureus opsonized with 150 μg/ml of purified human IgG or 1% (v/v) complement-inactivated human pooled serum (ΔHPS), in the presence of SElX at various concentrations. Gates indicate the populations of neutrophils that have phagocytosed bacteria or not (+/-). (ii) Inhibition by recombinant SElX protein was compared to the IgG-mediated phagocytosis inhibitor FLIPr and the neutrophil-binding deficient mutant SElX EKQD-A. Phagocytosis was calculated as the percentage of cells with fluorescent bacteria and expressed relative to buffer-treated cells with 75 μg/ml human IgG. Results shown are the means of three different human donors (error bars SE of mean). Results between SElX wild-type and SElX EKQD-A protein were tested by two-way ANOVA and found to be significantly different, *indicates at which concentration a significant difference in phagocytosis was observed. (iii) S. aureus USA300 wild-type, selx deletion and site-directed mutants were incubated with isolated human neutrophils for 60 min. Following incubation neutrophils were lysed with Triton-X 100 and surviving bacteria plated and enumerated. Killing was calculated as the difference between the no neutrophil control and the surviving CFU. Data shown are the mean % surviving bacteria incubated with cells from 5 donors ± SD. CFU data were tested by students t-test (unpaired, two-tailed) with Welches correction, (* indicates a p value < 0.05, ** indicates a p value < 0.01).