Tethering and tickling: a new role for the phosphatidylserine receptor

Contribution of phosphatidylserine receptor to apoptotic cell uptake Apoptosis or programmed cell death occurs during embryogenesis, normal cell turnover, and as a consequence of immune mediated, infectious, or inflammatory effects. Clearance of intact apoptotic cells (ACs)* by phagocytes protects surrounding tissues from intracellular factors and reduces the likelihood of tissue damage caused by inappropriate autoimmune responses. ACs exhibit numerous changes including the surface exposure of phosphatidylserine (PS) and alteration of membrane carbohydrates. Multiple ligands and receptors have been implicated in the recognition and uptake of ACs (Fig. 1), but attempts to assign function to discrete receptors has proven difficult. In this issue, Hoffmann et al. (2001) assess the relative role of various receptors through AC surrogates that ligate individual receptors. These surrogates consist of biotinylated human erythrocytes coated with avidin (Eba), which are bound to a biotinylated protein ligand or antireceptor antibody, creating Ebab-X. Binding to and internalization of Ebab-X by macrophages and other cells is evaluated in the absence of serum (to prevent interference of serum proteins) and is distinguishable by microscopy. Surprisingly, individual or multiple engagement of the AC receptors CD36, the v 3 and v 5 integrins, CD14, and CD68 caused tethering of erythrocytes but little internalization. In contrast, engagement of the PS receptor (PSR) alone through PS-coated erythrocytes induced neither tethering nor uptake. However, ligation of both PSR and other receptors (including receptors not normally involved in phagocytosis) converted the adhesion mediated by the latter to ingestion. Direct ligation of PSR produced TGF , supporting earlier data that PSR modulation is critical for inducing immunosuppressive cytokines. Based upon these studies, in this issue Hoffmann et al. (2001) propose a new paradigm for AC uptake by phagocytes. Ligation of PSR on phagocytes delivers a “tickle” signal, which stimulates the internalization of ACs, including bystander cells, that are “tethered” through other recognition receptors. Simultaneously, the immune response is modulated through secretion of immunosuppressive cytokines (Fig. 2). The observation that ACs which do not express PS are poorly phagocytosed (Fadok et al., 2001) suggests that PS–PSR interactions play a crucial regulatory role in dead cell clearance. The “tether and tickle” mechanism is attractive for many reasons. The tethering step makes it feasible for signaling to proceed through low avidity PS–PSR interactions. Specificity and regulation is provided to the multiple recognition mechanisms for AC uptake uncovered in mammalians. Because cells can transiently express PS during their activation, the two step model protects against accidental uptake (Henson et al., 2001b). Finally, clearance of apoptotic cells would suppress any potential autoimmmue responses directed toward self antigens through inhibition of monocyte and T cell activation and maturation of professional antigen-presenting cells like dendritic cells (DCs). New paradigms generate a host of new questions. For instance, what is the role of receptors which potentially bind PS directly (CD14, scavenger receptors) or through bridging molecules such as thrombospondin, lactadherin, iC3b, and 2glycoprotein I? Do they simply provide tethering signals or do they deliver both signals? Since PSR expression may be a function of the activation status, geographic location and nature of the phagocyte (absent on fresh monocytes but upregulated on activated macrophages [Fadok et al., 2000] and present on immature DCs [unpublished data]), it remains to be established when this PSR-dependent mechanism is dominant. Is it only involved in the removal of ACs during normal cell homeostasis, or does it also play a role in clearing cells generated during immune or inflammatory responses when responses must be subsequently downregulated? PSR-independent pathways clearly exist (Fig. 1). Defense collagens such as the collectins (surfactant binding protein Address correspondence to Nina Bhardwaj, The Laboratory of Molecular Neuro-Oncology, The Rockefeller University, Room 41, Box #176, 1230 York Ave., New York, NY 10021. Tel.: (212) 327-8332. Fax: (212) 327-7232. E-mail [email protected]