MECHANISM OF ACTIVATION OF MACROPHAGES BY BACTERIAL LIPOPOLYSACCHARIDES

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The long-term goal of this research is an understanding of molecular mechanisms of lipopolysaccharide (LPS)-initiated signalling as a model system by which the tumoricidal potential of the activated macrophage in treating cancer may be assessed. LPS has been recognized for several decades as among the most potent stimuli for the activation of macrophages for tumor cell killing. Increasing experimental evidence has implicated several macrophage membrane structures as potentially important receptors for activation and resultant signal transduction. These include the phosphatidyl-inositol linked CD14 glycoprotein, the p73 LPS binding protein and the CD11/18 adhesins. The precise contribution of each of these molecules to the actual activation event has not yet been established and Specific Aim #1 of the proposed research is to define the relative role of these various membrane LPS binding proteins and mechanisms of macrophage activation. It is our hypothesis that activation will depend upon the macromolecular structure of the LPS, and the intrinsic accessibility of lipid A for binding and the prior influence of environmental factors; we also hypothesize that there is one central pathway for LPS activation events. LPS-dependent macrophage activation results in the production of multiple proinflammatory mediators, including tumor necrosis factor (TNF) and nitric oxide (NO). Both of these have been shown to contribute to killing of tumor cells by LPS activated macrophages. Recent studies from this laboratory have shown that macrophages, pretreated in vitro with very low concentrations of LPS (which are not sufficient to induce either TNF or NO secretion) can nevertheless induce profound alterations in these cells such that subsequent activation with LPS or other stimuli results in markedly altered responses in terms of TNF and NO secretion. These alterations in TNF and NO responses are biophasic and reciprocal, and represent components of earlier-described macrophage desensitization and priming. We have termed this process macrophage "reprogramming" and a second Specific Aim of the proposed research will be to investigate the biochemical and cellular basis for this phenomenon. Particular attention will focus upon the potential role of a pertussis-toxin sensitive G- protein, which has been shown by others to contribute to LPS-dependent signalling of macrophages and by us to parallel many aspects of LPS- dependent macrophage reprogramming. We also hypothesize that reprogramming of macrophages occurs in vivo and that such events are of importance in the ability of the host to kill tumors. Studies in the third Specific Aim, therefore, will seek to establish macrophage reprogramming events as the primary explanation for induction of LPS tolerance or tumor-mediated LPS hypersensitivity reactions.