Purinergic receptor agonists modulate phagocytosis and clearance of apoptotic cells in macrophages

Phagocytosis plays an important role in controlling inflammation and antigen cross-presentation through the uptake of apoptotic bodies from dying cells. As dying cells are known to release nucleotides and other "danger signals", we investigated whether extracellular nucleotides may affect phagocytosis through binding to P2 purinergic receptors on phagocytic cells. We here show that the purinergic receptor agonists, ATP, ADP, α,β-methylene ATP (α,β-meATP), 3'-O-(4-benzoyl)benzoyl ATP, UTP and UDP, increased phagocytosis of latex beads, and some of them increased endocytosis and/or macropinocytosis of dextran by macrophages. The enhanced phagocytosis could be inhibited by pre-treatment with the P2X and P2Y antagonists, pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid and suramin, and the P2Y₁-selective antagonist, MRS2179. The nucleotides induced upregulation in macrophages of the β2 integrin CD11b/CD18 (Mac-1) and the vitronectin receptor (α_vβ3, CD51/CD61), both of which are involved in recognition and internalization of apoptotic cells. In addition, ATP and α,β-meATP increased adhesion of apoptotic cells to macrophages, both in vitro and in vivo, and α,β-meATP had a small effect on adhesion of necrotic cells. The nucleotides had no effect on adhesion of viable cells. We propose that engagement of the P2 receptors (P2X₁, or P2X₃) by extracellular nucleotides released from dying cells increases the ability of macrophages to bind apoptotic bodies, thus enhancing their ability to internalize and present antigens from the dying cells.