Oxidized low-density lipoprotein promotes mature dendritic cell transition from differentiating monocyte.

Proinflammatory oxidized phospholipids are generated during oxidative modification of low-density lipoproteins (LDL). The production of these proinflammatory oxidized phospholipids is controlled by secreted enzymes that circulate as proteins complexed with LDL and high-density lipoprotein. During the acute phase response to tissue injury, profound changes occur in lipoprotein enzymatic composition that alter their anti-inflammatory function. Monocytes may encounter oxidized phospholipids in vivo during their differentiation to macrophages or dendritic cells (DC). In this study we show that the presence of oxidized LDL (oxLDL) at the first day of monocyte differentiation to DC in vitro yielded phenotypically atypical cells with some functional characteristics of mature DC. Addition of oxLDL during the late stage of monocyte differentiation gave rise directly to phenotypically mature DC with reduced uptake capacity, secreting IL-12 but not IL-10, and supporting both syngeneic and allogeneic T cell stimulation. In contrast to known mediators of DC activation, oxLDL did not trigger maturation of immature DC. An intriguing possibility is that a burst of oxidized phospholipids is an endogenous activation signal for the immune system, which is tightly controlled by lipoproteins during the acute phase response.

Purification of intracellular compartments involved in antigen processing: a new method based on magnetic sorting.

In the present study, we describe a method to specifically isolate intracellular compartments containing endocytosed antigen. We have demonstrated that isolated compartments represent a small proportion of the intracellular material, highly enriched in antigen. Antigen-containing vesicles are specifically sorted from other intracellular compartments, such as endoplasmic reticulum or Golgi apparatus, and from the plasma membrane. They remain functional in vitro since they can be acidified, and the antigen inside has been found to be partially proteolysed. In macrophages, kinetic analysis has revealed that the antigen is first found in compartments of endosomal density, carrying Rab 5 and Rab 7, then in late compartments of lysosomal density, which are rich in proteases. The global protein content of the compartments was mapped by two-dimensional electrophoresis. In B lymphocytes, this method has allowed the isolation of endocytic compartments emerging from receptor-mediated endocytosis of the antigen. After 2 h of chase, the antigen reached vesicles containing large amounts of MHC-class II molecules, invariant chain and human leucocyte antigen-DM, where peptide loading can occur.