The formation of extracellular traps (ETs) by neutrophils and mast cells is an important mechanism in the innate immune response. These structures consist of a chromatin-DNA backbone with attached antimicrobial peptides and enzymes that trap and kill microbes. After stimulation of neutrophils and mast cells with phorbol esters, chemoattractant peptides, or chemokines, the generation of reactive oxygen species (ROS), such as hydrogen peroxide, by NAPDH [nicotinamide adenine dinucleotide phosphate (reduced form)] oxidase initiates a signaling cascade that leads to the disintegration of the nuclear and cellular membranes and the formation of ETs. This form of cell death is neither apoptotic nor necrotic, but whether it occurs because of the oxidation of phosphatases and kinases, as in other ROS-mediated signaling cascades, remains to be elucidated. These findings implicate "ETosis" as a novel cell death pathway in leukocytes.

ABSTRACT: BACKGROUND: Atherosclerosis is still the leading cause of death in the western world. Besides known risk factors studies demonstrating Chlamydophila pneumoniae (C. pneumoniae) to be implicated in the progression of the disease, little is known about C. pneumoniae infection dynamics. We investigated whether C. pneumoniae induce cell death of human aortic endothelial cells, a cell type involved in the initiation of atherosclerosis, and whether chlamydial spots derive from inclusions. RESULTS: Lactate dehydrogenase release revealed host cell death to be dependent on the amounts of Chlamydia used for infection. The morphology of lysed human aortic endothelial cells showed DNA strand breaks simultaneously with cell membrane damage exclusively in cells carrying Chlamydia as spots. Further ultrastructural analysis revealed additional organelle dilation, leading to the definition as aponecrotic cell death of endothelial cells. Exclusive staining of the metabolic active pathogens by chlamydial heat shock protein 60 labelling and ceramide incorporation demonstrated that the bacteria responsible for the induction of aponecrosis had resided in former inclusions. Furthermore, a strong pro-inflammatory molecule, high mobility group box protein 1, was shown to be released from aponecrotic host cells. CONCLUSIONS: From the data it can be concluded that aponecrosis inducing C. pneumoniae stem from inclusions, since metabolically active bacterial spots are strongly associated with aponecrosis late in the infectious cycle in vascular endothelial cells and metabolic activity was exclusively located inside of inclusions in intact cells. Vice versa initial spot-like infection with metabolically inert bacteria does not have an effect on cell death induction. Hence, C. pneumoniae infection can contribute to atherosclerosis by initial endothelial damage.

Patterns of change in cell volume and plasma membrane phospholipid distribution during cell death are regarded as diagnostic means of distinguishing apoptosis from necrosis, the former being associated with cell shrinkage and early phosphatidylserine (PS) exposure, whereas necrosis is associated with cell swelling and consequent lysis. We demonstrate that cell volume regulation during lymphocyte death stimulated via the purinergic receptor P2X(7) is distinct from both. Within seconds of stimulation, murine lymphocytes undergo rapid shrinkage concomitant with, but also required for, PS exposure. However, within 2 min shrinkage is reversed and swelling ensues ending in cell rupture. P2X(7)-induced shrinkage and PS translocation depend upon K(+) efflux via K(Ca)3.1, but use a pathway of Cl(-) efflux distinct from that previously implicated in apoptosis. Thus, P2X(7) stimulation activates a novel pathway of cell death that does not conform to those conventionally associated with apoptosis and necrosis. The mixed apoptotic/necrotic phenotype of P2X(7)-stimulated cells is consistent with a potential role for this death pathway in lupus disease.

Human solid tumors are invariably less well-oxygenated than the normal tissues from which they arose. This so-called tumor hypoxia leads to resistance to radiotherapy and anticancer chemotherapy as well as predisposing for increased tumor metastases. In this chapter, we examine the resistance of tumors to radiotherapy produced by hypoxia and, in particular, address the question of whether this resistance is the result of the physicochemical free radical mechanism that produces resistance to radiation killing of cells in vitro. We conclude that a major part of the resistance, though perhaps not all, is the result of the physicochemical free radical mechanism of the oxygen effect in sensitizing cells to ionizing radiation. However, in modeling studies used to evaluate the effect of fractionated irradiation on tumor response, it is essential to consider the fact that the tumor cells are at a wide range of oxygen concentrations, not just at the extremes of oxygenated and hypoxic. Prolonged hypoxia of the tumor tissue also leads to necrosis, and necrotic regions are also characteristic of solid tumors. These two characteristics-hypoxia and necrosis-represent clear differences between tumors and normal tissues and are potentially exploitable in cancer treatment. We discuss strategies for exploiting these differences. One such strategy is to use drugs that are toxic only under hypoxic conditions. The second strategy is to take advantage of the selective induction under hypoxia of the hypoxia-inducible factor (HIF)-1. Gene therapy strategies based on this strategy are in development. Finally, tumor hypoxia can be exploited using live obligate anaerobes that have been genetically engineered to express enzymes that can activate nontoxic prodrugs into toxic chemotherapeutic agents. PMID: 17998060 [PubMed - in process]