One-Way Synergistic Effect of Low Superantigen Concentrations on Lipopolysaccharide-Induced Cytokine Production

Lipopolysaccharides (LPS) of gram-negative bacteria and superantigens of gram-positive bacteria are among the main causes of sepsis and septic shock. Symptoms are initiated primarily by the release of endogenous mediators, especially cytokines. In the last few years, increasing evidence for the clinical relevance of mixed sepsis caused by coinfections with both types of bacteria has been found. Therefore, we developed an in vitro mixed sepsis model investigating the effect of different superantigen doses, in combination with different LPS concentrations, on cytokine production in human PBMCs using ELISA and RT-PCR. Low, in vivo relevant concentrations of the superantigen toxic shock syndrome toxin-1 (TSST-1) synergistically enhance LPS-induced production of interferon-γ (IFN-γ) interleukin-1β (IL-1β), IL-6, and IL-10, but low LPS has no comparable effect. Signal transduction studies with different inhibitors suggest that this one-way synergism is caused by an interaction between the cAMP and the PIP₂ signaling pathway. Furthermore, our findings support the idea that this interaction is one important crossover point of signal transduction pathways by LPS and superantigens, which seems to be predominantly regulated by IFN-γ and PGE-2. The identification of additional crossover points in the genesis of a mixed sepsis and their selective influence could lead to identical treatment of both gram-negative and gram-positive sepsis.