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Influence of Actinobacillus pleuropneumoniae serotype 2 and its cytolysins on porcine neutrophil chemiluminescence

The effects of Actinobacillus pleuropneumoniae serotype 2 and its metabolites on the oxidative activity of porcine neutrophils were studied by using a chemiluminescence technique. Viable A. pleuropneumoniae stimulated the production of oxygen radicals by neutrophils. After having reached a peak value, the oxidative activity decreased until a total inhibition of the oxidative activity of the neutrophils was achieved. All effects were neutralized with homologous convalescent-phase pig sera which had been adsorbed by heat-inactivated A. pleuropneumoniae. Inactivated bacteria and bacteria in the presence of chloramphenicol each had no influence on the oxidative activity of neutrophils. In contrast, a heat-labile factor in A. pleuropneumoniae culture supernatants stimulated and inhibited the oxidative activity of the neutrophils in a dose-dependent manner. Undiluted and low dilutions of culture supernatants were toxic for the phagocytes, while high dilutions stimulated the oxygen radical production of the neutrophils. These effects were neutralized with homologous convalescent-phase pig sera. In order to investigate whether the heat-labile factors in the culture supernatant could be cytolysins, we repeated the experiments with cytolysin II and cytolysin III produced by recombinant Escherichia coli. It was demonstrated that stimulation and inhibition could be reproduced by both cytolysins. In conclusion, the observations made in this study showed that A. pleuropneumoniae secretes heat-labile metabolites that stimulate neutrophil-oxidative metabolism at relatively low concentrations and kill the neutrophils at higher concentrations. Cytolysins may be responsible, at least in part, for these effects.