Inhibition of the macrophage-induced oxidation of low density lipoprotein by interferon-gamma.

The regulation of the macrophage-induced oxidation of low density lipoprotein (LDL) by cytokines was investigated. As an initial source of cytokines, medium from an activated type 2 helper T-cell clone was tested. This cell-free supernatant inhibited the subsequent oxidation of LDL by mouse peritoneal macrophages. The inhibition was concentration- and time-dependent as measured by changes in thiobarbituric acid (TBA) reactive substances. In addition, there were decreases in conjugated diene formation as well as the generation of LDL particles with an increased net negative charge that were recognized by the scavenger receptor. The inhibition was not due to a decrease in cell viability or to nonspecific antioxidant activity, as assessed by measuring phagocytic activity and metal ion-induced oxidation of LDL, respectively. Using antibodies that inactivate specific cytokines, the role of select individual cytokines in this inhibition was investigated. Addition of antibodies against interleukin-3 (IL-3), granulocyte/macrophage-colony stimulating factor (GM-CSF), or tumor necrosis factor alpha (TNF alpha) to the media had little or no effect on the ability of the cytokines to affect oxidation by macrophages, whereas anti-interferon-gamma (IFN-gamma) antibodies completely reversed the inhibition induced by the T-cell supernatant. A role for this cytokine was confirmed using recombinant IFN-gamma. A concentration-dependent inhibition was produced with a maximum inhibition to 24% of control cells, whereas smooth muscle cell-dependent LDL oxidation was not affected. To examine the cellular basis for the inhibition, the effect of IFN-gamma on oxidant activities (O2- production, lipoxygenase activity, and thiol production) were measured. IFN-gamma at concentrations that maximally inhibit LDL oxidation stimulated the phorbol myristate acetate (PMA)-induced production of O2- 1.4-times greater than control cells after one hour. Similarly, thiol production was increased 29% by IFN-gamma pretreatment. In contrast, macrophage lipoxygenase was inhibited approximately 21%. Based on these in vitro findings, the potential regulation of macrophage LDL oxidation by IFN-gamma in vivo was also investigated. Macrophages from Toxoplasma gondii-infected mice have been shown previously to be activated in situ by an IFN-gamma-dependent mechanism. These were tested for their ability to oxidize LDL. Macrophages from these mice oxidized LDL to a much lesser extent than cells from age-matched control mice, demonstrating that the ability of macrophages to oxidize lipoprotein may also be susceptible to regulation possibly also by IFN-gamma in vivo. Together these studies demonstrate that the cell-mediated oxidation of LDL can be regulated by cytokines, specifically IFN-gamma. This mode of regulation may play a role in regulating this process in the developing atherosclerotic lesion.