Prolonged expression of lipopolysaccharide (LPS)-induced inflammatory genes in whole blood requires continual exposure to LPS

Abstract

Blood-borne lipopolysaccharide (LPS) is thought to be a major inducer of sepsis; however, it remains controversial whether an ongoing exposure to LPS is required to maintain the underlying systemic inflammatory response. To address this question, we have studied the expression of tumor necrosis factor alpha (TNF-alpha), interleukin 1-beta (IL-1 beta), and the procoagulant protein tissue factor induced by LPS ex vivo in whole human blood. The addition of a 1-ng/ml bolus of LPS to blood rapidly induced mRNA expression of all three genes. The mRNA levels peaked after 1 to 2 h, depending on the gene, and then declined to baseline after approximately 5 h. The decline in mRNA expression was not caused by a loss of responsiveness of the blood cells to LPS but rather correlated with the neutralization of LPS inflammatory activity by plasma components. Furthermore, administering a 1-ng/ml dose of LPS in six hourly aliquots of 167 pg/ml greatly prolonged the expression of mRNAs and induced a much greater release of TNF-alpha and IL-1 beta protein than did a single bolus. Dosing by repeated additions was more effective than a single bolus in inducing secretion of TNF-alpha and IL-1 beta at LPS levels of < or = 10 ng/ml, which corresponded to the LPS neutralization capacity of plasma. Finally, both mRNA expression and protein secretion induced by repeated administration of LPS were rapidly reversed by the addition of the LPS-neutralizing protein, bactericidal/permeability-increasing protein, even after several hours of stimulation. These results indicate that continuous or repeated exposure to LPS is required to maintain the expression of inflammatory genes and that the activated state is rapidly reversed with LPS neutralization.