Apoptotic Cell Clearance and its Failure

Apoptotic cell death is a critical and evolutionarily conserved process for the elimination of unnecessary cells. After cells undergo apoptosis, apoptotic cell corpses are rapidly recognized and phagocytosed by professional phagocytes such as macrophages and dendritic cells. The rapid removal of apoptotic cells by phagocytes prevents the release of potentially toxic or immunogenic materials from dying cells. In mammals, the recognition and engulfment of apoptotic cells by phagocytes is an intricate process, and a number of molecules on phagocytes are involved in this phenomenon. We previously showed that the milk fat globule-EGF-factor 8 (MFG-E8), a bridging molecule between apoptotic cells and phagocytes, is expressed in tingible body macrophages located in the germinal center (GC) of spleen and lymph nodes. MFG-E8-deficient mice showed failure of apoptotic cell clearance in GC, and spontaneously developed autoantibody production. We also found that the intravenous injection of an MFG-E8 mutant protein, D89E protein, induced the production of autoantibodies in mice by inhibiting apoptotic cell clearance. These results indicate that the complete removal of apoptotic cells by phagocytes plays a critical role in the maintenance of self-tolerance. Our laboratory is investigating the molecular mechanisms for the recognition and phagocytosis of dying cells by phagocytes, and the pathological relevance of impaired phagocytosis to inflammatory disorders including autoimmune diseases.

 

Critical Roles of Sinus Macrophages in Anti-tumor Immunity

   Phagocytes, such as macrophages and DCs, swiftly phagocytose apoptotic cell corpses by recognizing molecules exposed only on the surface of the corpses. Failure of apoptotic cell clearance results in autoimmune disorder, indicating that apoptotic cell clearance by phagocytes essentially contributes to the maintenance of self-tolerance in physiological conditions. Consistent with these findings, the intravenous injection of apoptotic cells induced cell associated antigen-specific immunosuppression or tolerance. For the tolerance induction, not only rapid corpse clearance by splenic macrophages in the marginal zone (MZ), but also selective phagocytosis and subsequent antigen presentation by the splenic DC subpopulation located in the MZ are required.

   On the contrary, subcutaneously injected apoptotic cells are often immunogenic and researchers have taken advantage of immunogenic tumor corpses for tumor vaccination. These findings suggest that apoptotic cells in periphery are cleared and processed in a different way from blood-borne apoptotic cells in spleen. The antigen presentation in peripheral LNs is thought to be coordinately performed by migratory DCs from peripheral tissues and LN-resident APCs. However, little is known about the role of different APCs in the clearance of dead cells and the presentation of dead cell-associated antigens in peripheral tissue or LNs.

   We recently documented how tumor cell-associated antigens derived from dead tumor cells were crosspresented in the draining LNs. We concluded that macrophages that reside in LN sinus take up dead tumor cells and directly crossprime CTLs. Mice lacking sinus macrophages at the time of dead tumor cell vaccination failed to induce anti-tumor immunity. These findings are expected to improve our understanding of how to generate and activate tumor- specific T cell immunity in response to tumor cell death.

 

CD169-positive macrophages as first-line defenders

   The LN sinus is a highway that links afferent and efferent lymph and is believed to be a filtering zone for lymph-borne molecules. Large particles, including cellular antigens in the lymphatic fluid make initial contact with CD169+ macrophages that serve as sentinels located along the length of the LN sinus. In the case of spleen, CD169+ marginal metallophilic macrophages as well as marginal zone macrophages are located between the lymphoid compartment of the spleen and the scavenging red pulp compartment, and are well equipped to constantly screen the blood for foreign particles and organisms, as well as aberrant molecular debris and dying cells. We can detect the CD169+ macrophage subpopulation in the boundary border between outer environment and tissue in several organs. We speculate that the macrophage subpopulation plays a critical role as sentinel macrophages in each organ, and are studying physiological and pathological roles of these macrophages.