Polymyxin B stimulates production of complement components and cytokines in human monocytes.

Polymyxin B (PmB), an agent often used to neutralize the effects of bacterial lipopolysaccharide (LPS), was shown to exert a dose-dependent stimulatory effect on the biosynthesis of C3, factor B, interleukin-6 (IL-6), and granulocyte-macrophage colony-stimulating factor (GM-CSF) in human monocytes. A low dose of PmB (1 to 5 micrograms/ml) efficiently suppressed the LPS-induced (1 or 100 ng/ml) production of IL-6, GM-CSF, and factor B, but not the C3 production induced by 100 ng of LPS per ml. A reduced level of GM-CSF may have contributed to the persisting high C3 concentrations and the apparent lack of LPS inhibition in the latter situation, since GM-CSF is an inhibitor of monocyte C3 biosynthesis.