Clearance of Apoptotic Cells by Macrophages Induces Regulatory Phenotype and Involves Stimulation of CD36 and Platelet-Activating Factor Receptor

Abstract

Phagocytosis of apoptotic cells (efferocytosis) induces macrophage differentiation towards a regulatory phenotype (IL-10high/IL-12p40low). CD36 is involved in the recognition of apoptotic cells (AC), and we have shown that the platelet-activating factor receptor (PAFR) is also involved. Here, we investigated the contribution of PAFR and CD36 to efferocytosis and to the establishment of a regulatory macrophage phenotype. Mice bone marrow-derived macrophages were cocultured with apoptotic thymocytes, and the phagocytic index was determined. Blockage of PAFR with antagonists or CD36 with specific antibodies inhibited the phagocytosis of AC (~70–80%). Using immunoprecipitation and confocal microscopy, we showed that efferocytosis increased the CD36 and PAFR colocalisation in the macrophage plasma membrane; PAFR and CD36 coimmunoprecipitated with flotillin-1, a constitutive lipid raft protein, and disruption of these membrane microdomains by methyl-β-cyclodextrin reduced AC phagocytosis. Efferocytosis induced a pattern of cytokine production, IL-10high/IL-12p40low, that is, characteristic of a regulatory phenotype. LPS potentiated the efferocytosis-induced production of IL-10, and this was prevented by blocking PAFR or CD36. It can be concluded that phagocytosis of apoptotic cells engages CD36 and PAFR, possibly in lipid rafts, and this is required for optimal efferocytosis and the establishment of the macrophage regulatory phenotype.

Inflammatory Cytokine-Mediated Regulation of Thrombospondin-1 and CD36 in Conjunctival Cells

“…9 In addition, our results indicate unaltered TSP-1 expression in EDE conjunctiva with significantly reduced CD36 expression, in contrast to the increased CD36 expression in TSP-1-deficient conjunctiva. Several in vitro studies have reported expression of both proinflammatory 37,38 and antiinflammatory [39][40][41] cytokines in macrophages resulting from the ligation of CD36 by ligands, such as apoptotic cells, oxidized low-density lipoprotein, and TSP-1, but not on epithelial cells. However, in vivo studies appear to predominantly support the anti-inflammatory role of CD36.…”

Oncogenic effects of PAFR ligands produced in tumours upon chemotherapy and radiotherapy

“…In a separate study, it was shown that clearance of dying cells by tumour macrophages shifts them towards the pro tumorigenic M2 phenotype by mechanisms involving the receptor for the lipid mediator plateletactivating factor (PAF), a member of the 1alkyl,2acylglycerophosphocholine subclass of lipids 4 . Lipid mediators are generated in the tumour microenvironment and can modu late tumour growth.…”

PAFR in adipose tissue macrophages is associated with anti-inflammatory phenotype and metabolic homoeostasis

“…Our previous studies pointed to a central role for PAFR in this process. We found that CD36 co-localizes with PAFR in macrophage plasma membrane lipid rafts for optimal oxLDL uptake and apoptotic cells phagocytosis (14,15). The association of these receptors shifted macrophages towards the anti-inflammatory phenotype.…”

“…The adipose tissue presented higher mRNA levels of some M1 markers, increased IL-12 and reduced IL-10 levels. The balance of IL-12 and IL-10 had been previously employed to establish the macrophages phenotype: the classically activated or M1 macrophages and the alternatively activated or M2 (14,16,30). The deficiency of PAFR also resulted in weight gain, reduced glucose tolerance, increased expression of Ldlr in the liver and hepatocyte disarrangement, hepatic insulin resistance, and increased glucose levels.…”

“…The PAF receptor (PAFR) is a G-protein coupled receptor expressed in plasma and nuclear membranes of many cell types, including macrophages, that binds not only to PAF but also to a wide range of non-enzymatically generated oxidized phospholipids (13). We have previously shown that apoptotic cells and oxidized low density lipoproteins express moieties that bind to PAFR and are cleared by macrophages through the scavenger receptor CD36 in association with PAFR (14,15). We also found that this process induces functional changes in macrophages, compatible with the regulatory phenotype or M2 proposed by Mosser and Edwards (16), with the secretion of anti-inflammatory mediators/cytokines.…”