Effects of polymyxin-B on TNF-α production in equine whole blood stimulated with three different bacterial toxins.

Abstract

Polymyxin-B is used to treat equine systemic inflammation. Bacterial toxins other than lipopolysaccharide (LPS) contribute to systemic inflammation but the effects of polymyxin-B on these are poorly defined. Whole blood aliquots from six healthy horses diluted 1:1 with RPMI were incubated for 21hr with 1μg/ml of LPS, lipoteichoic acid (LTA) or peptidoglycan (PGN) in the presence of increasing concentrations of polymyxin-B (10-3000μg/ml). A murine L929 fibroblast bioassay was used to measure TNF-α activity. Polymyxin-B significantly inhibited the effects of all three bacterial toxins. Analysis of variance showed the IC50 value for polymyxin-B for TNF-α inhibition caused by LTA (11.19±2.89μg/ml polymyxin-B) was significantly lower (p=.009) than the values for LPS (46.48±9.93μg/ml) and PGN (54.44±8.97μg/ml). There was no significant difference in IC50 values between LPS and PGN (p>.05). Maximum inhibition of TNF-α was 77.4%, 73.0% and 82.7% for LPS, PGN and LTA, respectively and was not significantly different between toxins. At the two highest concentrations of polymyxin-B, TNF-α began to increase. These data suggest that polymyxin-B may inhibit the effects of bacterial toxins other than LPS and might be a more potent inhibitor of LTA than LPS or PGN.