Masking of phosphatidylserine inhibits apoptotic cell engulfment and induces autoantibody production in mice

Apoptosis: a basic biological phenomenon with wide-ranging implications in human disease

Apoptosis is a highly regulated process of cell deletion and plays a fundamental role in the maintenance of tissue homeostasis in the adult organism. Numerous studies in recent years have revealed that apoptosis is a constitutive suicide programme expressed in most, if not all cells, and can be triggered by a variety of extrinsic and intrinsic signals. Many human diseases can be attributed directly or indirectly to a derangement of apoptosis, resulting in either cell accumulation, in which cell eradication or cell turnover is impaired, or cell loss, in which the apoptotic programme is inadvertently triggered. In addition, defective macrophage engulfment and degradation of cell corpses may also contribute to a dysregulation of tissue homeostasis. An increased understanding of the signalling pathways that govern the execution of apoptosis and the subsequent clearance of dying cells may thus yield novel targets for therapeutic intervention in a wide range of human maladies.

Apoptosis and autoimmunity

Autoimmune diseases reflect the confluence of genetic, environmental and stochastic events. Recent studies have implicated apoptotic cell death pathways in initiating and propagating autoimmune diseases, as well as in rendering individuals susceptible to such diseases. Similar to autoimmunity, apoptosis is a multistep process, affecting immune and target cells, integrating numerous intrinsic and extrinsic signals, and requiring the actions of multiple gene products. Particularly relevant to the complexity of autoimmunity are the recent observations that apoptotic death might provide a primary source of tolerogen to shape the immune repertoire, or be the target of the immune response in autoimmunity, and that apoptosis is both required for lymphocyte selection and immunoregulation, and is a prominent outcome of immune and inflammatory effector pathways.

Phosphatidylserine-dependent engulfment by macrophages of nuclei from erythroid precursor cells

Definitive erythropoiesis usually occurs in the bone marrow or fetal liver, where erythroblasts are associated with a central macrophage in anatomical units called 'blood islands'. Late in erythropoiesis, nuclei are expelled from the erythroid precursor cells and engulfed by the macrophages in the blood island. Here we show that the nuclei are engulfed by macrophages only after they are disconnected from reticulocytes, and that phosphatidylserine, which is often used as an 'eat me' signal for apoptotic cells, is also used for the engulfment of nuclei expelled from erythroblasts. We investigated the mechanism behind the enucleation and engulfment processes by isolating late-stage erythroblasts from the spleens of phlebotomized mice. When these erythroblasts were cultured, the nuclei protruded spontaneously from the erythroblasts. A weak physical force could disconnect the nuclei from the reticulocytes. The released nuclei contained an undetectable level of ATP, and quickly exposed phosphatidylserine on their surface. Fetal liver macrophages efficiently engulfed the nuclei; masking the phosphatidylserine on the nuclei with the dominant-negative form of milk-fat-globule EGF8 (MFG-E8) prevented this engulfment.

Neutrophils Accelerate Macrophage-Mediated Digestion of Apoptotic Cells In Vivo as Well as In Vitro

It is generally believed that the clearance of apoptotic cells does not lead to inflammation. In contrast, we previously found that injection of apoptotic cells into the peritoneal cavity induced the expression of an inflammatory chemokine, MIP-2, and infiltration of neutrophils, and that anti-MIP-2 Abs suppressed the infiltration significantly. Because our previous study showed that whole-body x-irradiation caused neutrophil infiltration into the thymus along with T cell apoptosis, we examined the role of neutrophils in apoptotic cell clearance. Neutrophil infiltration reached a peak 12 h after irradiation with 1 Gy of x-rays. Immunohistological analysis revealed that apoptotic cells disappeared dramatically from 10.5 to 12 h after x-irradiation. As neutrophils moved from an inner area of the cortex to the periphery, apoptotic cells disappeared concomitantly. Either anti-MIP-2 or anti-CXCR2 Abs suppressed neutrophil infiltration significantly, and the suppression of neutrophil infiltration by anti-MIP-2 Abs delayed the disappearance of apoptotic cells. Moreover, macrophage-mediated digestion of apoptotic thymocytes was accelerated in vitro on coculturing with neutrophils, even if neutrophils were separated from macrophages. These results suggest that neutrophils are recruited to the thymus mainly by MIP-2 after whole-body x-irradiation and that such neutrophils may not induce inflammation but rather accelerate complete digestion of apoptotic cells by macrophages.

Accumulation of MFG-E8/lactadherin on exosomes from immature dendritic cells

Exosomes are vesicles of endocytic origin secreted spontaneously by dendritic cells (DCs). We have shown previously that exosomes can transfer antigen or MHC-peptide complexes between DCs, thus potentially amplifying the immune response. We had also identified milk fat globule EGF/factor VIII (MFG-E8), also called lactadherin, as one of the major exosomal proteins. MFG-E8 has two domains: an Arg-Gly-Asp sequence that binds integrins alphavbeta3 and alphavbeta5 (expressed by human DCs and macrophages) and a phosphatidyl-serine (PS) binding sequence through which it associates to PS-containing membranes (among which exosomes). MFG-E8 is thus a good candidate molecule to address exosomes to DCs. Here, we show that MFG-E8 is expressed by immature bone-marrow-derived DCs (BMDCs) and secreted in association with exosomes in vitro. We have generated mice expressing an inactive form of MFG-E8, fused to beta-galactosidase. Analyzing these mice, we demonstrate that MFG-E8 is expressed in vivo in splenic DCs. In a mouse DC-dependent, antigen-specific, CD4 T cell-stimulation assay, exosomes produced by MFG-E8-deficient BMDCs were barely less efficient than exosomes bearing MFG-E8. We conclude that MFG-E8 is efficiently targeted to exosomes but is not essential to address exosomes to mouse BMDCs. Involvement of MFG-E8/lactadherin in exosome targeting to other DC subpopulations, or to human DCs, is still possible.

Phagocytosis of apoptotic cells is regulated by a UNC-73/TRIO-MIG-2/RhoG signaling module and armadillo repeats of CED-12/ELMO

BACKGROUND

Phagocytosis of cells undergoing apoptosis is essential during development, cellular turnover, and wound healing. Failure to promptly clear apoptotic cells has been linked to autoimmune disorders. C. elegans CED-12 and mammalian ELMO are evolutionarily conserved scaffolding proteins that play a critical role in engulfment from worm to human. ELMO functions together with Dock180 (a guanine nucleotide exchange factor for Rac) to mediate Rac-dependent cytoskeletal reorganization during engulfment and cell migration. However, the components upstream of ELMO and Dock180 during engulfment remain elusive.

RESULTS

Here, we define a conserved signaling module involving the small GTPase RhoG and its exchange factor TRIO, which functions upstream of ELMO/Dock180/Rac during engulfment. Complementary studies in C. elegans show that MIG-2 (which we identify as the homolog of mammalian RhoG) and UNC-73 (the TRIO homolog) also regulate corpse clearance in vivo, upstream of CED-12. At the molecular level, we identify a novel set of evolutionarily conserved Armadillo (ARM) repeats within CED-12/ELMO that mediate an interaction with activated MIG-2/RhoG; this, in turn, promotes Dock180-mediated Rac activation and cytoskeletal reorganization.

CONCLUSIONS

The combination of in vitro and in vivo studies presented here identify two evolutionarily conserved players in engulfment, TRIO/UNC73 and RhoG/MIG-2, and the TRIO --> RhoG signaling module is linked by ELMO/CED-12 to Dock180-dependent Rac activation during engulfment. This work also identifies ARM repeats within CED-12/ELMO and their role in linking RhoG and Rac, two GTPases that function in tandem during engulfment.