Bacterial superantigens enhance the in vitro proinflammatory response and in vivo lethality of the TLR2 agonist bacterial lipoprotein

Bacterial superantigens are Gram-positive exotoxins that induce proinflammatory cytokine release in vitro, cause lethal shock in vivo, and can be detected in the bloodstream of critically ill patients. They also have a powerful priming effect on the TLR4 agonist LPS. The aim of this study was to investigate the relationship between superantigens and the TLR2 agonist bacterial lipoprotein (BLP). Priming of human monocytes or PBMCs with superantigens significantly enhanced proinflammatory cytokine TNF-α and IL-6 release in response to BLP stimulation. The priming effect of superantigens could be blocked by inhibiting p38 MAPK during the priming phase as opposed to NF-κB or ERK inhibition. This was consistent with higher expression of the phosphorylated p38 after superantigen priming and BLP or LPS stimulation. C57BL/6 mice with superantigen priming (10 μg/mouse) when challenged with BLP (600 μg/mouse) exhibited substantially higher mortality (100%) compared with mice without superantigen priming (zero). Mice given superantigen alone did not demonstrate any signs of illness. Mice challenged with both superantigen and BLP had significantly higher levels of serum TNF-α and IL-6 compared with those of mice challenged with either agent alone. Depletion of the monocyte/macrophage subpopulation significantly reduced the mortality rate from 100 to 20% in superantigen-primed, BLP-challenged C57BL/6 mice, with a 5- to 10-fold decrease in serum TNF-α and IL-6. Our results demonstrate that bacterial superantigens enhance the in vitro proinflammatory cytokine release and in vivo lethality of BLP. This novel finding may help to explain the massive proinflammatory cytokine release seen in superantigen-mediated septic shock.