P2X Receptor-Dependent Erythrocyte Damage by α-Hemolysin from Escherichia coli Triggers Phagocytosis by THP-1 Cells

The pore-forming exotoxin alpha-hemolysin from E. coli causes a significant volume reduction of human erythrocytes that precedes the ultimate swelling and lysis. This shrinkage results from activation of Ca2+-sensitive K+ (K(Ca)3.1) and Cl- channels (TMEM16A) and reduced functions of either of these channels potentiate the HlyA-induced hemolysis. This means that Ca2+-dependent activation of K(Ca)3.1 and TMEM16A protects the cells against early hemolysis. Simultaneous to the HlyA-induced shrinkage, the erythrocytes show increased exposure of phosphatidylserine (PS) in the outer plasma membrane leaflet, which is known to be a keen trigger for phagocytosis. We hypothesize that exposure to HlyA elicits removal of the damaged erythrocytes by phagocytic cells. Cultured THP-1 cells as a model for erythrocytal phagocytosis was verified by a variety of methods, including live cell imaging. We consistently found the HlyA to very potently trigger phagocytosis of erythrocytes by THP-1 cells. The HlyA-induced phagocytosis was prevented by inhibition of K(Ca)3.1, which is known to reduce PS-exposure in human erythrocytes subjected to both ionomycin and HlyA. Moreover, we show that P2X receptor inhibition, which prevents the cell damages caused by HlyA, also reduced that HlyA-induced PS-exposure and phagocytosis. Based on these results, we propose that erythrocytes, damaged by HlyA-insertion, are effectively cleared from the blood stream. This mechanism will potentially reduce the risk of intravascular hemolysis.