Sequence of events terminating in death of leucocytes

Role of apoptosis in biology and pathology: resistance to apoptosis in colon carcinogenesis

The overview of apoptosis presented here emphasizes cell deletion in the immune system, with particular reference to T- and B-lymphocyte development, and the in vivo and in vitro senescence of human neutrophils. Some biochemical criteria that are used to identify apoptotic cells are described. Pitfalls in using agarose gel electrophoresis as the sole method for the identification of apoptotic cells are discussed. There are multiple modes of cell death that can be identified at the morphologic level. Thus the central role of microscopic methods, and in particular, electron microscopy, as an important tool in the study of cell death mechanisms, is presented. Apoptosis has a protective role against disease and could, a priori, have an important role in either the initiation or progression of cancer. Two paradoxes concerning the relationship of tumor aggressiveness at the clinical level to mitotic activity have been explained by an evaluation of apoptotic index. In the first case, basal cell carcinomas grow slowly but show a high rate of mitosis. Here, the apoptotic rate is quite high, but just below the mitotic rate, thereby accounting for the slow rate of growth. A second instance is follicular lymphoma, which has a low rate of mitosis that is less than that described for reactive germinal centers. However, apoptosis is markedly reduced in follicular lymphomas compared with that seen in reactive germinal centers, thus providing an explanation for the progressive growth of the follicle. We present a brief description of recent work from our laboratory that indicates that apoptosis may play an important role in colon carcinogenesis. We have shown that sodium deoxycholate, the particular bile salt present in highest concentration in the colon, induces apoptosis in the goblet cells of the human colonic mucosa in an in vitro assay. The intriguing finding is that cells of the normal-appearing mucosa of colon cancer patients are resistant to bile salt-induced apoptosis. This suggests a novel hypothesis about the etiologic role of bile salts in colon cancer. The chronic presence of bile salts that accompany a high-fat diet could select for apoptosis-resistant epithelial cells in the colon over time. Thus, a resistance-to-apoptosis bioassay may prove useful as an intermediate biomarker for determining which individuals are at high risk for colon cancer.

Programmed cell death of the normal human neutrophil: an in vitro model of senescence

The present study provides experimental data which indicate that the neutrophil is ideal for studying programmed cell death or apoptosis in vitro. Neutrophils can be obtained from human peripheral blood in large numbers with minimal experimental manipulation and are easily separated from other leukocytes, providing nearly pure cell suspensions. The neutrophil life span in vitro is sufficiently short to allow observations to be made within eight hours after experimental manipulation. Neutrophils can also be easily maintained in serum-free, chemically defined media which can be systematically altered, thereby defining specific variables that influence the apoptotic process. Since the neutrophils do not need an exogenous trigger to undergo programmed cell death, it is also an excellent model to study senescence. It was determined from this study that neutrophils undergo apoptosis most efficiently at 37 degrees C, a temperature requirement for physiologic cell death. Neutrophils undergo apoptosis at a slightly faster rate and maintain membrane integrity better when incubated in a tissue culture medium (e.g., RPMI 1640) compared with a balanced salt solution (e.g., HBBB). Cycloheximide, an inhibitor of protein synthesis, was shown to accelerate apoptosis in a dose-dependent manner. The presence of Zn++ significantly decreased the rate of apoptosis, whereas the presence of Ca++ and Mg++ had no apparent effect. These studies indicate that the process of senescence, culminating in cell death, is subject to modulation by a variety of agents and experimental conditions. In addition, the ultrastructural features of neutrophils undergoing programmed cell death in vitro were compared in detail to those occurring in vivo and were found to be comparable.

Ultrastructure of effector--target cell interaction in secondary cell-mediated lympholysis

The ultrastructure of the secondary cell-mediated lympholysis (CML) reaction and the effects on interacting lymphocytes of colchicine, cytochalasin B, and effector cell-specific antisera were examined using transmission and scanning electron microscopy. Surface labelling of cytotoxic secondary effector cells with cationized ferritin allowed them to be distinguished from unlabelled target lymphocytes. Effector--target interactions were characterized by intercellular junctions involving extensive areas of membrane apposition and interdigitation and extension of pseudopod-like processes by the effector cell. The abolition of such interactions when effector populations were pretreated with anti-Ly2 sera plus complement demonstrated target cell destruction in secondary CML to be dependent on the activity of restimulated cytotoxic T lymphocytes. Cytochalasin B and colchicine dramatically decreased the numbers of specific effector--target cell interactions observed. Although the data presented do not allow the possible activity of soluble lytic factors associated with the effector cell surface to be ruled out, they suggest that target cell lysis in the secondary CML system examined results from immune-specific binding of alloantigen-sensitized effectors to targets and osmotic effects which follow localized disruption of the target cell membrane.