Recognition and phagocytosis of dying cells

Abstract

Apoptosis and necrosis may share initiation, signaling and execution mechanisms. Therefore, it is difficult to find unambiguous markers for the mode of cell death. The most clearly distinguishing features of apoptosis were the activation of caspases, and the selective uptake by phagocytes. The present data now show that phagocytosis is principally possible also in caspase-independent cell death. Jurkat cells induced to undergo necrosis by combining toxins with ATP-depleting agents were phagocytosed in a PS-dependent fashion barely after plasma membrane lysis. Lysed Jurkat cells showed a caspase- and Ca2+-independent PS exposure on their surface. In addition, such cells were preferred by macrophages and were removed more efficiently than apoptotic cells. Jurkat cell death triggered with staurosporine in the presence of the pan-caspase inhibitor zVAD-fmk was characterized by delayed necrosis and the absence of PS-translocation. Nevertheless, such cells where removed by macrophages before plasma membrane lysis occurred by mechanisms that might involve CD14. Caspase-independent PS exposure could be provoked by Ca2+ influx in Jurkat cells using an ionophore or in neurons by glutamate receptor stimulation, respectively. Ca2+-stressed cells were efficiently phagocytosed before membrane lysis. Protein kinase C inhibitors prevented both Ca2+-mediated PS exposure and phagocytosis. MCF7 and LoVo36 cells induced to undergo a caspase-independent apoptosis-like cell death by HSP70-depletion were efficiently removed by macrophages. Overexpression of caspase-3 in MCF7 cells did not influence this phagocytosis. The TNFa-response of macrophages to pro-inflammatory stimuli could be abolished by Jurkat and LoVo36 cells dying by classical apoptosis and ATP-depleted necrotic Jurkat cells, but not by heat-killed Jurkat cells or HSP70-depleted LoVo36-cells, suggesting that some necrotic cells may provoke pro-inflammatory responses.