Differential dependence of the ingestion of necrotic cells and TNF-α/IL-1β production by murine macrophages on lipid rafts

Monocytes and macrophages may encounter both pro-inflammatory and anti-inflammatory signals during their lifetime, in the form of micro-organisms or their products or as cytokines. In addition, macrophages are also exposed to apoptotic and necrotic cells. Apoptosis or 'programmed cell death' is thought to be the physiological end of developing or maturing cells, whereas necrosis is regarded as 'accidental death' or injury-associated cell death. Apoptotic cells are cleared from tissues by phagocytic cells without eliciting an inflammatory response, while necrotic cells elicit inflammation. Several cell membrane molecules from apoptotic and necrotic, as well as from phagocytic cells, have been shown to participate in the process of endocytosis of dying and potentially harmful cells. Apart from an array of cell surface receptors, it is also known that lipid rafts are key components of cell-cell communication and signalling. By using the interaction of BALB/c mice thymus-derived apoptotic or necrotic cells with murine macrophages of the J774 cell line as a model system, we provide evidence that endocytosis of apoptotic but not of necrotic cells is inhibited by methyl-beta-cyclodextrin, a cholesterol sequestering agent, able to disrupt lipid rafts. However, necrotic but not apoptotic cells co-localize with lipid rafts within macrophages. Interestingly, necrotic cell-induced secretion of TNF-alpha and IL-1 beta was also inhibited by methyl-beta-cyclodextrin, thus suggesting a role for lipid rafts in the signalling of this particular inflammatory response. Taken together, our results argue in favour of differential macrophage recognition of apoptotic and necrotic cells at the level of lipid rafts, and endocytosis versus signalling for TNF-alpha and IL-1 beta synthesis.