7and and and and < 0.05; **< 0.01. Discussion Our findings indicate that macrophage PPAR and RXR are required for the efficient disposal of apoptotic bodies, which prevents the release of intracellular components and the consequent loss of SB 334867 immunological self-tolerance, and might explain the development of an SLE-like nephritis in mice lacking macrophage PPAR or RXR. vascular disease, and increases the risk of cardiovascular mortality (2). The disease mechanism of SLE was shown recently to involve impaired clearance of apoptotic cells, which has led to a conceptual change in our understanding of the disease (3). The nuclear DNA fragments and nuclear proteins of dying cells are self-Ags, and they are important triggers of autoreactive Ig production (4). The safe removal of apoptotic cell debris is a SB 334867 dedicated function of phagocytic cells, particularly tissue-resident macrophages (5, 6). Signals from apoptotic cells direct the activation of macrophages toward a so-called regulatory or deactivated phenotype, thereby dampening inflammation (7C9). Macrophages reprogrammed by apoptotic cells control and deactivate immunity and inflammation by triggering the release of anti-inflammatory cytokines, such as TGF- and IL-10, and inhibiting the production of proinflammatory cytokines. In pathologies in which apoptotic cell clearance is disrupted, as in SLE, the absence of this suppressive effect on inflammatory cytokine production by macrophages could aggravate chronic inflammation (10). To correctly engulf apoptotic cells, macrophages express a large set of cell surface receptors, including scavenger receptors (e.g., CD36, CD14), integrins (e.g., ITGA5, ITGB3), Ig supergene family receptors (e.g., FcRs), receptors for complement factors (e.g., LRP1/Calr, the low-density lipoprotein receptor-related protein 1/calreticulin complex), complement receptor 3 (CR3, also known as ITGM), complement receptor 4 (CR4, also known as ITGX), and Tyro3/Axl/Mertk (TAM) family tyrosine kinase cell surface receptor. Apoptotic cell removal is enhanced by bridging molecules (opsonins), which interact with macrophage receptors and apoptotic cell molecules. Opsonins include components of the complement system (C1q, C3b, C4b, and iC3b), immunoglobulins (IgG and IgM), MFG-E8, THBS1, and GAS6. Altered expression of some cell-surface receptors and opsonins has been proposed to influence the development of SLE (11C13). Ligands of peroxisome proliferator activated receptor (PPAR) modulate macrophage function and regulate the expression of a range of inflammatory genes, making them potential drugs for the treatment of inflammation and immune responses (14). Activators of PPAR include lipid metabolites and synthetic thiazolidinediones such as rosiglitazone (RSG) (15). PPAR belongs to the nuclear receptor superfamily of ligand-dependent transcription factors and affects various cellular processes at the level of gene expression regulation (16, 17). PPAR controls macrophage polarization and inhibits expression of inflammatory genes (14); therefore, it is a key regulator of macrophage-mediated immune responses (15, 18). PPAR forms a permissive heterodimer with retinoid X receptors (RXRs). These nuclear receptors, encoded by three distinct genes (tests. Quantitative PCR data were analyzed by unpaired Student test for comparison of WT and KO samples and by paired Student test for comparison of untreated and treated WT cells. The normal distribution of data were checked by the Kolmogorov-Smirnov test, with = 0.05. Results Mice lacking macrophage expression of PPAR or RXR develop glomerulonephritis Conditional KO mice lacking macrophage expression of PPAR (18) or RXR (24) were generated by crossing mice bearing the lox-P-targeted PPAR (PPARf/f) or RXR (RXRf/f) alleles with mice bearing the lysozyme-M Cre (LysMCre) recombinase transgene. We refer to LysM Cre-negative PPARf/f and RXRf/f mice as WT, and their PPARf/fLysMCre+ or RXRf/fLysMCre+ littermates as KO animals (PPAR-KO and RXR-KO). Female mice lacking macrophage expression of PPAR or RXR developed severe nephropathy at 4C6 mo old. An early sign of nephritis is enlargement of the renal glomeruli (12), so we analyzed the kidney angioarchitecture by OPT. OPT imaging of renal arteriolar casts revealed enlarged glomeruli in focal regions within the kidneys of SB 334867 PPAR-KO and RXR-KO animals (Fig. 1< 0.05; **< 0.01. Silver impregnation of kidney sections from 4C6-mo-old mice. Arrowheads indicate basement membranes. Scale bar, 20 m. < 0.05; **< 0.01. Macrophage PPAR and RXR are required for normal apoptotic cell clearance Impaired clearance of apoptotic cells within the kidney glomeruli is thought to contribute to autoimmune GN (12, 38). A significant increase in the HDM2 number of apoptotic cells in the glomeruli of PPAR-KO and RXR-KO mice was detected by TUNEL (Fig. 3< 0.05; **< 0.01. In WT mice, peritoneal macrophages ingested most ATs (white arrows), whereas noningested ATs were abundant (white arrowheads) in PPAR-KO.