Aging Cell (2004), pp145–149 Doi: 10.1111/j.1474-9728.2004.00100.x © Blackwell Publishing Ltd/Anatomical Society of Great Britain and Ireland 2004 145 Blackwell Science, Ltd REVIEW Signalling shutdown strategies in aging immune cells Khurram Ayub and Maurice B. Hallett Neutrophil Signalling Group, University Department of Surgery, University of Wales College of Medicine, Heath Park, Cardiff CF14 4XN, UK Summary It is important for the resolution of inflammation that the number and activity of immune cells are reduced. Clearance of immune cells may be achieved by apoptosis and phagocytosis of cell fragments by macrophages. However, signalling shutdown by immune cells committed to apop- tosis occurs early in the progression of these cells towards fragmentation and, it could be argued, is a key feature of apoptosis. There is surprisingly little known about the mechanisms that underlie this signalling shutdown, in particular the shutdown of Ca 2+ influx. The conse- quences and the potential mechanisms by which Ca 2+ influx shutdown is achieved are discussed. In addition, the potential consequences for cell signalling of cyto- chrome c release from mitochondria and of phosphatidyl- serine externalization are discussed. The aim of the review is therefore to highlight the evidence for various signalling shutdown strategies in immune cells that may limit their activity during progression towards apoptosis. Key words: aging; apoptosis; Ca 2+ signalling; fas; neutro- phils; phagocytosis. Signalling shutdown strategies The activity of immune cells such as neutrophils and lym- phocytes that have accumulated locally at sites of infection must be suppressed once the microbial threat to the host has been overcome. This is important because the activity of these cells themselves represents a potential threat. For example, neu- trophils have proteases and hydrolases within granules that, if released, would cause damage to the extracellular matrix and neighbouring cells. The most widely recognized mechanism for limiting the activity of immune cells is, of course, apoptosis. Neutrophils, for example, undergo spontaneous apoptosis after about 20 h of isolation from the blood. The rate of apoptosis of both lymphocytes and neutrophils is accelerated by ligation of fas (CD95), suggesting that progression to apoptosis is also actively signalled during resolution of inflammation. It is usually thought that cell fragmentation and phagocytosis by macrophages is the ‘goal’ of the sequence of events triggered either by fas ligation or triggered spontaneously and that this represents the way in which the activity of these cells is switched off. However, we have suggested that the activity of these cells is down-regulated and switched-off during progression towards apoptosis (Ayub & Hallett, 2004a) and that fragmentation of the resultant ineffective cells represents the mechanism by which these defunct cells are cleared. Indeed, it could be argued that there would be danger to the macrophage of a neutrophil producing ‘eat me’ signals while it was still capable of responding. Although there is clear evidence of loss of function of immune cells as they progress towards apoptosis, until recently there has been little study of the mechanisms that may underlie this. In this article, we review some of the recent evidence for specific signalling shutdown strategies in immune cells, particularly neutrophils, as part of progression towards apoptosis. Although there are obvious differences between lymphocytes and neutrophils, many of the features discussed are common to both immune cell types. Here we highlight the consequences and mechanisms for three signalling shutdown strategies: (i) Ca 2+ influx shutdown, (ii) release of cytochrome c and (iii) phosphatidylserine (PS) externalization. Cell aging and cell death In this review, we discuss the events that occur as the cell pro- gresses towards apoptosis. We have chosen to call this process ‘cell aging’. Although apoptosis is often described as ‘cell suicide’, it would be more appropriate to describe it as ‘natural cell death’ because it involves mechanisms within the cell that have evolved for the purpose. This would distinguish it from necrosis, which can be caused by external toxic agents, and would thus be unnatural cell death (or ‘cell murder’). If apoptosis is natural cell death then, by analogy, the process leading to it may be equated with cell aging (Fig. 1). It is important that this concept of aging of an individual cell as it progresses towards apoptosis is not confused with other concepts of cell aging. For example, the telomere length of cells in a population may decrease at each cell doubling and the cell population may be described as senescing or aging. This leads to the idea that newly created individual cells at high passage numbers are ‘older’ than the ‘young’ cells from which they arose. In this review, we use the term cell aging to describe the progress towards natural death (apoptosis) of individual immune cells. Correspondence Maurice B. Hallett, Neutrophil Signalling Group, University Department of Surgery, University of Wales College of Medicine, Heath Park, Cardiff CF14 4XN, UK. Tel.: +44 29 20742748; fax: +44 29 20761623; e-mail: hallettmb@cf.ac.uk Accepted for publication 21 May 2004