Monophosphoryl lipid A stimulated up-regulation of reactive oxygen intermediates in human monocytes in vitro

The production of reactive oxygen and nitrogen intermediates is a common re sponse to infectious challenge in vivo. These agents have been implicated i n the modulation of cytokine responses and are produced in large amounts in response to endotoxins produced by a number of infectious agents. The anti gen-presenting cell activation caused by these lipopolysacchardies (LPS) ha s been exploited in the use of these agents as adjuvants. In recent years, less-toxic derivatives have been sought. One such agent, monophosphoryl lip id A (MPL), has been used increasingly in vivo as an adjuvant and as a modu lator of the inflammatory process. It is known that this agent modulates th e inflammatory response and cytokine production. In addition, we have shown its effect on the production of reactive nitrogen intermediates. In this p aper, we show that MPL stimulates the release of high levels of superoxide (O-2(-)) and hydrogen peroxide (H2O2), the latter being greater than that s een with LPS and appearing to be related to die inability of MPL to stimula te catalase activity. When cells were pretreated with LPS or MPL and subseq uently challenged with LPS, the production of O-2(-) and H2O2 was inhibited significantly by LPS and MPL. The concentration of MPL required to induce significant hyporesponsiveness to subsequent LPS challenge was 10 times low er than that of LPS. Hyporesponsiveness was greatest when induced by 10 mug /ml MPL, the same concentration that induced the maximum release of H2O2 in primary stimulation. In addition, we have shown that following MPL pretrea tment, LPS stimulation does not cause the loss of cytoplasmic I kappaB alph a, which occurs when human monocytes are cultured with LPS. From our result s, we propose a model for the reduced toxicity of MPL.