Myeloperoxidase secreted by phagocytes in the artery wall may be a catalyst for lipoprotein oxidation. High density lipoprotein (HDL) oxidized by peroxidase-generated tyrosyl radical has a markedly enhanced ability to deplete cultured cells of cholesterol. We have investigated the structural modifications in tyrosylated HDL responsible for this effect. Spherical reconstituted HDL (rHDL) containing the whole apolipoprotein (apo) fraction of tyrosylated HDL reproduced the ability of intact tyrosylated HDL to enhance cholesterol efflux from cholesterol-loaded human fibroblasts when reconstituted with the whole lipid fraction of either HDL or tyrosylated HDL. Free apoAI or apoAII showed no increased capacity to induce cholesterol efflux from cholesterolloaded fibroblasts following oxidation by tyrosyl radical, either in their lipid-free forms or in rHDL. The product of oxidation of a mixture of apoAI and apoAII (1:1 molar ratio) by tyrosyl radical, however, reproduced the enhanced ability of tyrosylated HDL to induce cholesterol efflux when reconstituted with the whole lipid fraction of HDL. HDL containing only apoAI or apoAII showed no enhanced ability to promote cholesterol efflux following oxidation by tyrosyl radical, whereas HDL containing both apoAI and apoAII did. rHDL containing apoAI-apoAII monomer and apoAI-(apoAII) 2 heterodimers showed a markedly increased ability to prevent the accumulation of LDL-derived cholesterol mass by sterol-depleted fibroblasts compared with other apolipoprotein species of tyrosylated HDL. These results indicate a novel product of HDL oxidation, apoAIapoAII heterodimers, with a markedly enhanced capacity to deplete cells of the regulatory pool of free cholesterol and total cholesterol mass. The recent observation of tyrosyl radical-oxidized LDL in vivo suggests that a similar modification of HDL would significantly enhance its ability to deplete peripheral cells of cholesterol in the first step of reverse cholesterol transport.Lipoprotein oxidation is felt to be pivotal for the formation of macrophage foam cells in the developing atherosclerotic lesion (1). Low density lipoprotein (LDL) 1 particles containing modifications consistent with in vivo oxidation have been isolated from human atheromatous lesions (2-5). Numerous potential mechanisms for the oxidation of lipoproteins in vivo have been proposed (6). One such pathway is the release of myeloperoxidase and oxygen radicals by activated phagocytes at sites of inflammation, such as the intima of the atherosclerotic artery (7). Myeloperoxidase employs hydrogen peroxide to catalyze the oxidation of substrates, including chloride ion and L-tyrosine, in the extracellular fluid (8). The likelihood of oxidation of lipoproteins by myeloperoxidase-generated tyrosyl radical in vivo is suggested by the presence of the active enzyme (9), micromolar concentrations of L-tyrosine (10), and tyrosyl radical-modified LDL (5) in the artery wall.High density lipoprotein (HDL) is felt to protect against atherosclerosis in part by stimulating the ...

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Natural Killer (NK) cells have been implicated in the response to poxviruses, but the interaction between NK and infected cells is not well characterized. We show that downregulation of class I major histocompatibility complex (MHC-I) molecules in human cells by vaccinia virus (VV) sensitizes the cells to lysis by NK cells. We provide evidence suggesting that NK cells are infected as a consequence of co-culture with infected target cells. We also show that infection of NK cells leads to a marked depression of cytotoxicity. Moreover, the effect on NK cytotoxicity occurs within hours of infection and is prevented by UV inactivation of the virus but is only partially prevented by blocking late gene expression. VV infection also renders the NK cells more sensitive to inhibitory signals. Together our observations suggest that VV infection of NK cells can modulate their signaling in a manner that prevents them from acting on infected target cells.

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