Abstract
Commercially available pokeweed mitogen (PWM) has been reported to activate macrophages, leading to production of proinflammatory cytokines and nitric oxide (NO). However, we found that polymyxin B (PMB), a specific inhibitor of endotoxin activity, inhibited the PWM-induced expression of proinflammatory cytokines and NO and the activation of Toll-like receptor 4 (TLR4). A kinetic-turbidimetric Limulus amebocyte lysate assay demonstrated that commercial PWM contained substantial endotoxin, over 10(4) endotoxin units/mg of the PWM. A PWM repurified by PMB-coupled beads no longer induced the expression of proinflammatory cytokines, TLR4 activation, or dendritic cell maturation. However, the repurified PWM remained able to induce proliferation of human lymphocytes, which is a representative characteristic of PWM. These results suggest that commercial PWM might be contaminated with a large amount of endotoxin, resulting in the attribution of misleading immunological properties to PWM.