Search

Dying cells, such as apoptotic and necrotic cells, are cleared rapidly from the site of cell death to prevent the exposure of intracellular antigenic and immunostimulatory molecules that may cause tissue injury or facilitate the development of autoimmune diseases. For the immune system to recognize and remove dying cells efficiently, professional phagocytes use a variety of mechanisms that distinguish healthy cells from dying cells. HRG, a relatively abundant heparin/HS-binding protein in human plasma, has been shown recently to tether IgG specifically to necrotic cells and aid the phagocytic uptake of necrotic cells via a FcgammaRI-dependent pathway. In this study, we provide direct evidence that HRG can function cooperatively with cell surface HS on the monocytic cell line THP-1 to promote necrotic cell removal. In addition, we found that the presence of heparin can markedly inhibit HRG-enhanced necrotic cell clearance by THP-1 cells, possibly by blocking the ability of HRG to interact with necrotic cells as well as THP-1 cells. Thus, these data suggest that HRG can aid the phagocytosis of necrotic cells via a HS-dependent pathway, and this process can be regulated by the presence of certain HRG ligands, such as heparin.

Under normal physiologic conditions, necrotic cells resulting from tissue injury are rapidly removed from the circulation and tissues by phagocytes, thus preventing the exposure of intracellular antigenic and immunostimulatory molecules that can aid the development of autoimmune disease. Histidine-rich glycoprotein (HRG), a relatively abundant plasma glycoprotein, has a multidomain structure that can interact with many ligands including components of the fibrinolytic and immune systems. Recently, it has been reported that HRG can bind strongly to cytoplasmic ligand(s) exposed in necrotic cells to enhance clearance by phagocytes. Here we describe the molecular mechanisms underpinning this process. A complex consisting of both HRG and immunoglobulin G (IgG) was found as necessary to aid necrotic cell uptake by monocytes, predominantly via an FcgammaRI-dependent mechanism. The findings in this study also show that HRG can potentially interact with anionic phospholipids exposed in necrotic cells. Furthermore, the enhanced phagocytosis of necrotic cells induced by HRG-IgG complexes triggers phagocytes to release proinflammatory cytokines such as interleukin-8 and tumor necrosis factor. Thus, HRG has the unique property of complexing with IgG and facilitating a proinflammatory innate immune response to promote the clearance of necrotic cells.

Phagocytosis serves as one of the key processes involved in development, maintenance of tissue homeostasis, as well as in eliminating pathogens from an organism. Under normal physiological conditions, dying cells (e.g., apoptotic and necrotic cells) and pathogens (e.g., bacteria and fungi) are rapidly detected and removed by professional phagocytes such as macrophages and dendritic cells (DCs). In most cases, specific receptors and opsonins are used by phagocytes to recognize and bind their target cells, which can trigger the intracellular signalling events required for phagocytosis. Depending on the type of target cell, phagocytes may also release both immunomodulatory molecules and growth factors to orchestrate a subsequent immune response and wound healing process. In recent years, evidence is growing that opsonins and receptors involved in the removal of pathogens can also aid the disposal of dying cells at all stages of cell death, in particular plasma membrane-damaged cells such as late apoptotic and necrotic cells. This review provides an overview of the molecular mechanisms and the immunological outcomes of late apoptotic/necrotic cell removal and highlights the striking similarities between late apoptotic/necrotic cell and pathogen clearance.

Cells that become necrotic or apoptotic through tissue damage or during normal cellular turnover are usually rapidly cleared from the circulation and tissues by phagocytic cells. A number of soluble proteins have been identified that facilitate the phagocytosis of apoptotic cells, but few proteins have been defined that selectively opsonize necrotic cells. Previous studies have shown that histidine-rich glycoprotein (HRG), an abundant (approximately 100 microg/ml) 75-kDa plasma glycoprotein, binds to cell surface heparan sulfate on viable cells and cross-links other ligands, such as plasminogen, to the cell surface. In this study we have demonstrated that HRG also binds very strongly, in a heparan sulfate-independent manner, to cytoplasmic ligand(s) exposed in necrotic cells. This interaction is mediated by the amino-terminal domain of HRG and results in enhanced phagocytosis of the necrotic cells by a monocytic cell line. In contrast, it was found that HRG binds poorly to and does not opsonize early stage apoptotic cells. Thus, HRG has the unique property of selectively recognizing necrotic cells and may play an important physiological role in vivo by facilitating the uptake and clearance of necrotic, but not apoptotic, cells by phagocytes.