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Sir,—We would like to endorse much of the editorial on apoptosis in cardiovascular disease,1 but feel it appropriate to add some further points. The original description of apoptosis was a morphological description, and morphology of cells remains the gold standard for detection of apoptosis. The morphological changes observed in apoptosis are a continuous process from retraction of cells as the first indicator to phagocytosis of the apoptotic bodies. In contrast, the TUNEL technique detects DNA strand breaks, which may occur in non-apoptotic states and should be correlated with morphological evidence of apoptosis. It is not yet certain how much of the apoptotic process is labelled by TUNEL, or even whether apoptotic bodies ingested by phagocytes remain TUNEL positive. Therefore, although it is possible to determine the percentage of cells that are undergoing apoptosis in a defined tissue, it is difficult to establish a rate of apoptosis based on TUNEL.
We disagree that the presence of interleukin converting enzyme (ICE) in a cell confirms that apoptosis is occurring. At present, there is no unique immunoreactive or biochemical marker of apoptosis. Demonstration that the apoptosis machinery is present in cells is not evidence that the cell is undergoing apoptosis. In fact, the machinery (the CASPase proteases) is present in (almost) all cells as inactive zymogens. Thus, whether a cell is ICE or CPP32 positive by immunocytochemistry does not indicate that it is undergoing apoptosis or, if the cell is undergoing apoptosis, that these proteases are responsible. For example, ICE cleavage is a necessary event in the activation of this enzyme in the processing of interleukin (IL) 1β. Clearly not all cells that can make IL-1β undergo apoptosis when they synthesise this product. There are substrates that are cleaved in apoptosis (such as poly ADP ribose polymerase (PARP)), but the presence of PARP itself does not indicate apoptosis, although the presence of a cleaved form of PARP may help substantiate the observation that death occurs by apoptosis.