Control of signaling-mediated clearance of apoptotic cells by the tumor suppressor p53

INTRODUCTION Programmed cell death occurs throughout life in all tissues of the body, and more than a billion cells die every day as part of normal processes. Thus, rapid and efficient clearance of cell corpses is a vital prerequisite for homeostatic maintenance of tissue health. Failure to clear dying cells can lead to the accumulation of autoantigens in tissues that foster diseases, such as chronic inflammation, autoimmunity, and developmental abnormalities. In the normal immune system, phagocytic engulfment of apoptotic cells is accompanied by induction of a certain degree of immune tolerance in order to prevent self-antigen recognition. Over the past few decades, enormous efforts have been made toward understanding various mechanisms of tumor suppressor p53–mediated apoptosis. However, the involvement of p53 in postapoptosis has yet to be addressed. RATIONALE One of the most intriguing, yet enigmatic, questions in studying homeostatic control of efficient dead cell clearance and proper immune tolerance is how these two essential activities are interrelated: The complexity of these processes is demonstrated by the many receptors and signaling pathways involved in the engulfment of apoptotic cells and stringent discrimination of self antigens from nonself antigens. Thus, there must be key connection(s) linking the balance between immune homeostasis and inflammation. In addition to the antitumor functions of p53, p53 has been implicated in immune responses and inflammatory diseases, with various roles in the immune system becoming apparent. We identified a postapoptotic target gene of p53, Death Domain1α ( DD1α ), that is responsive to genotoxic stresses and expressed in immune cells. DD1α appears to function as an immunoregulator of T cell tolerance. We hypothesized that p53 controls signaling-mediated phagocytosis of apoptotic cells through its target, DD1α. We determined that DD1α functions as an engulfment ligand or receptor that is involved in homophilic intermolecular interaction at intercellular junctions of apoptotic cells and macrophages. We also addressed whether DD1 α deficiency caused any defects in dead cell clearance in vivo. RESULTS DD1α has similarity with several members of the immunoglobulin superfamily with the extracellular immunoglobulin V (IgV) domain, such as TIM family proteins and an immune checkpoint regulator, PD-L1. We found that the p53 induction and maintenance of DD1α expression in apoptotic cells and its subsequent functional intercellular homophilic interaction between apoptotic cells and macrophages are required for engulfment of apoptotic cells. DD1 α-deficient mice showed less reduction in organ size and cell number after ionizing radiation (IR), owing to defective dead cell clearance. DD1 α-null mice are viable and indistinguishable in appearance from wild-type littermates at an early age. However, at a later age, DD1 α deficiency resulted in the development of autoimmune phenotypes and prominent formation of immune infiltrates in the skin, lung, and kidney, which indicated an immune dysregulation and breakdown of self-tolerance in DD1 α-null mice. We demonstrated that DD1α also plays an important role as an intercellular homophilic receptor on T cells, which suggests that DD1α is a key-connecting molecule linking postapoptotic processes to immune surveillance. We found that DD1α deficiency in T cells impaired DD1α-mediated inhibitory activity of T cell proliferation. These data indicate that potential homophilic DD1α interactions are important for the DD1α-mediated T cell inhibitory role. Therefore, the results indicate a role for p53 in regulating expression of immune checkpoint regulators, including PD-1, PD-L1, and DD1α. CONCLUSION We found that the tumor suppressor p53 controls signaling-mediated phagocytosis of apoptotic cells through its target DD1 α, which suggests that p53 promotes both the proapoptotic pathway and postapoptotic events. DD1α functions as an engulfment ligand that engages in homophilic intermolecular interaction at intercellular junctions of apoptotic cells and macrophages. DD1 α-deficient mice showed in vivo defects in clearing dying cells that led to damage to multiple organs indicative of immune dysfunction. p53-induced expression of DD1α is a vital phase for the phagocytic engulfment process of dead cells and then facilitates the stepwise priming of immune surveillance. As a downstream target of the tumor suppressor p53, DD1α activation may extend the repertoire of p53 activities to “guardian of the immune integrity.”