IL3-dependent cells die by apoptosis on removal of their growth factor

Restimulation-induced cell death: new medical and research perspectives

In the periphery, homeostasis of the immune system depends on the equilibrium of expanding and contracting T lymphocytes during immune response. An important mechanism of lymphocyte contraction is clonal depletion of activated T cells by cytokine withdrawal induced death (CWID) and TCR restimulation induced cell death (RICD). Deficiencies in signaling components for CWID and RICD leads to autoimmunune lymphoproliferative disorders in mouse and human. The most important feature of CWID and RICD is clonal specificity, which lends great appeal as a strategy for targeted tolerance induction and treatment of autoimmune diseases, allergic disorders, and graft rejection by depleting undesired disease-causing T cells while keeping the overall host immunity intact.

Apoptosis and cancer: the genesis of a research field

In multicellular organisms, the total number of cells is a balance between the cell-generating effects of mitosis and cell death that is induced through apoptosis. A disruption of this delicate balance can lead to the development of cancer. This Timeline article focuses on how the field of apoptosis biology has developed in the context of its contribution to our understanding of cell death, or lack of it, in the development of malignant disease. It traces the course of research from key discoveries in fundamental biology to potential therapeutic applications.

JAK1 N-terminus binds to conserved Box 1 and Box 2 motifs of cytokine receptor common beta subunit but signal activation requires JAK1 C-terminus

The human interleukin-3 receptor (hIL-3R) consists of a unique alpha subunit (hIL-3Ralpha) and a common beta subunit (betac). Binding of IL-3 to IL-3R activates Janus kinases JAK1 and JAK2. Our previously study showed that JAK2 and JAK1 were constitutively associated with the hIL-3Ralpha and betac subunits, respectively. In this study, we further demonstrate that JAK2 binds to the intracellular domain of hIL-3Ralpha and JAK1 binds to the Box 1 and Box 2 motifs of betac using GST-hIL-3R fusion proteins in pull-down assays. JAK1 mutational analysis revealed that its JH7-3 domains bound directly to the Box 1 and Box 2 motifs of betac. We further examined the role of JAK1 JH7-3 domains in JAK1 and JAK2-mediated signaling using the CDJAKs fusion proteins, which consisted of a CD16 extracellular domain, a CD7 transmembrane domain, and either JAK1 (CDJAK1), JAK2 (CDJAK2), or JAK1-JH7-3 domains (CDJAK1-JH7-3) as intracellular domains. Anti-CD16 antibody crosslinking of wild type fusion proteins CDJAK1 with CDJAK2 could mimic IL-3 signaling, however, the crosslinking of fusion proteins CDJAK1-JH7-3 with CDJAK2 failed to activate downstream proteins. These results suggest that the JAK1-JH7-3 domains are required for betac interaction and abolish wild type JAK1 and JAK2-mediated signaling.

Simultaneous activation of JAK1 and JAK2 confers IL-3 independent growth on Ba/F3 pro-B cells

JAK1 and JAK2 are tyrosine kinases involved in the regulation of cell proliferation, differentiation, and survival. These proteins may play a key role in mediating the effects of the cytokine IL-3 on hematopoietic cells. IL-3 induces tyrosine phosphorylation of both JAK1 and JAK2. However, it is not clear whether the activation of JAK1, JAK2, or both is sufficient to confer factor-independent growth in IL-3 dependent cells. To address this issue, fusion proteins CD16/CD7/JAK (CDJAK), comprised of a CD16 extracellular domain, a CD7 transmembrane domain, and a JAK cytoplasmic region (either a wild-type JAK or a dominant negative mutant of JAK) were constructed. We established several Ba/F3 derivatives that stably overexpress the conditionally active forms of either CDJAK1, CDJAK2, or both these fusion proteins. In this study, the autophosphorylation of CDJAK1 or CDJAK2 was induced by crosslinking with anti-CD16 antibody. We demonstrated that, like their wild-type counterparts, CDJAK1 and CDJAK2 were preassociated with the IL-3 receptor beta and alpha subunits, respectively. Furthermore, the simultaneous activation of both CDJAK1 and CDJAK2 fusion proteins, but not either one alone, led to the tyrosine phosphorylation of the IL-3 receptor beta subunit, the activation of downstream signaling molecules, including STAT5, Akt, and MAPK, and the conferring of factor-independent growth to IL-3-dependent Ba/F3 cells. Coexpression of dominant negative mutants CDJAK1KE or CDJAK2KE with wild type CDJAK2 or CDJAK1, respectively, inhibited these activation activities. These results suggest that JAK1 and JAK2 must work cooperatively and not independently and that their actions are dependent on having normal kinase activity to trigger downstream signals leading to IL-3 independent proliferation and survival of Ba/F3 cells.

Extracellular ATP inhibits apoptosis and maintains cell viability by inducing autocrine production of interleukin-4 in a myeloid progenitor cell line

Interleukin-3 (IL-3)-dependent myeloid progenitor cell FDC.P2 is induced to undergo apoptotic cell death upon IL-3 depletion. Extracellular adenosine triphosphate (ATP) was found to prevent apoptosis and maintain cell viability of FDC.P2 cells upon IL-3 withdrawal. The antiapoptotic effect of ATP required extracellular Ca2+. Furthermore, FK506, a specific inhibitor of calcium/calmodulin-dependent protein phosphatase calcineurin, inhibited the antiapoptotic effect of ATP. As one of cytokines whose expression is dependent on the activation of calcineurin, interleukin-4 (IL-4) played a critical role in ATP-mediated cell survival of FDC.P2 cells because neutralizing antibody against IL-4 effectively abrogated the antiapoptotic activity of ATP. Moreover, ATP treatment induced a significant amount of secreted IL-4 that was sufficient to maintain cell viability. Taken together, our present results demonstrate that extracellular ATP triggers autocrine production of IL-4 through calcium-dependent activation of calcineurin and secreted IL-4 substitutes IL-3 in protecting FDC.P2 cells from apoptosis even in the absence of IL-3.

Cyclin B2-null mice develop normally and are fertile whereas cyclin B1-null mice die in utero

Two B-type cyclins, B1 and B2, have been identified in mammals. Proliferating cells express both cyclins, which bind to and activate p34(cdc2). To test whether the two B-type cyclins have distinct roles, we generated lines of transgenic mice, one lacking cyclin B1 and the other lacking cyclin B2. Cyclin B1 proved to be an essential gene; no homozygous B1-null pups were born. In contrast, nullizygous B2 mice developed normally and did not display any obvious abnormalities. Both male and female cyclin B2-null mice were fertile, which was unexpected in view of the high levels and distinct patterns of expression of cyclin B2 during spermatogenesis. We show that the expression of cyclin B1 overlaps the expression of cyclin B2 in the mature testis, but not vice versa. Cyclin B1 can be found both on intracellular membranes and free in the cytoplasm, in contrast to cyclin B2, which is membrane-associated. These observations suggest that cyclin B1 may compensate for the loss of cyclin B2 in the mutant mice, and implies that cyclin B1 is capable of targeting the p34(cdc2) kinase to the essential substrates of cyclin B2.

Human T-Cell Leukemia Virus Type II Directly Acts on CD34+ Hematopoietic Precursors by Increasing Their Survival Potential. Envelope-Associated HLA Class II Molecules Reverse This Effect

The role of human T-cell leukemia virus type II (HTLV-II) in human lymphoproliferative and hematopoietic abnormalities in which the retrovirus can be isolated is still elusive. Here we show that the C344 T-cell–derived lymphotropic HTLV-II type IIa Mo strain acts directly on CD34+ hematopoietic precursors by rescuing them from apoptosis induced by interleukin-3 (IL-3) deprivation. This effect is viral strain-specific, as it is not observed with the B-lymphotropic HTLV-II type IIb Gu strain, it does not require infection of the hematopoietic precursors, and, interestingly, it is strongly dependent on the infected cellular host from which the virus was derived. Indeed, growth adaptation of the Mo strain to the permissive B-cell line, BJAB, renders the virus no longer capable of mediating the antiapoptotic effect. However, pretreatment of the BJAB-adapted Mo strain with antibodies specific for HLA class II, but not class I, histocompatibility antigens restores the antiapoptotic potential of the virus. These results constitute the first evidence that HTLV-II retrovirus can directly influence the homeostasis of human progenitors, without infecting them, and that this crucial activity is strongly inhibited by the presence of host-derived envelope-associated HLA class II antigens.

Human T-Cell Leukemia Virus Type II Directly Acts on CD34+ Hematopoietic Precursors by Increasing Their Survival Potential. Envelope-Associated HLA Class II Molecules Reverse This Effect

The role of human T-cell leukemia virus type II (HTLV-II) in human lymphoproliferative and hematopoietic abnormalities in which the retrovirus can be isolated is still elusive. Here we show that the C344 T-cell–derived lymphotropic HTLV-II type IIa Mo strain acts directly on CD34+ hematopoietic precursors by rescuing them from apoptosis induced by interleukin-3 (IL-3) deprivation. This effect is viral strain-specific, as it is not observed with the B-lymphotropic HTLV-II type IIb Gu strain, it does not require infection of the hematopoietic precursors, and, interestingly, it is strongly dependent on the infected cellular host from which the virus was derived. Indeed, growth adaptation of the Mo strain to the permissive B-cell line, BJAB, renders the virus no longer capable of mediating the antiapoptotic effect. However, pretreatment of the BJAB-adapted Mo strain with antibodies specific for HLA class II, but not class I, histocompatibility antigens restores the antiapoptotic potential of the virus. These results constitute the first evidence that HTLV-II retrovirus can directly influence the homeostasis of human progenitors, without infecting them, and that this crucial activity is strongly inhibited by the presence of host-derived envelope-associated HLA class II antigens.

Fetuses from preeclamptic mothers show reduced hepatic erythropoiesis

The fetal liver is the main hematopoietic organ during intrauterine life. Morphometrical studies were performed on liver sections to detect changes occurring with intrauterine growth retardation and preeclampsia. Compared with the controls (n = 10), fetuses from preeclamptic mothers showed a severe reduction of erythroid cells by 60% on average (n = 18). Closer examination revealed that the erythroid cells at early stages of differentiation were more affected (80% reduction) than at later stages (55%). Seven out of 18 fetuses from preeclamptic mothers did not show growth retardation but exhibited severely reduced hepatic erythropoiesis. We suggest that the prime factor for impaired red blood cell production is preeclampsia itself rather than intrauterine growth retardation. Regulation of erythropoiesis in utero might depend on the interaction of many hematopoietic growth factors, and preeclampsia might alter the balance. To test this notion, we quantitated erythropoietin in fetal blood and various cytokines in the amniotic fluid. An elevation of erythropoietin and interleukin (IL)-3 levels was seen in babies born under the conditions of preeclampsia, whereas the concentrations of granulocyte/macrophage-colony-stimulating factor (CSF), granulocyte-CSF, and IL-1 beta were reduced, and the levels of IL-6 and IL-8 remained constant. With preeclampsia, a discrepancy between elevation of erythrocyte numbers in peripheral blood and depression of hematopoiesis at the main production site, the fetal liver, is seen. Concomitantly, there is elevation of some but reduction of other hematopoietic cytokines. We envision that during the course of preeclampsia quantitation of hematopoietic growth factors might allow to predict the deterioration of in utero life conditions.

Apoptosis and cell culture technology

The importance of apoptosis in cell culture has now been widely recognised. Recent work has shown that this process is widely manifested during the in vitro cultivation of commercially important mammalian cell lines. In this review I summarise what is now known of the characteristics, significance and regulatory mechanisms of apoptosis. As the process of cell proliferation and cell death are now considered intimately related, particular attention is paid to highlight the progress and opportunities in the field of cell culture engineering. The strategies that have been undertaken to prevent the induction of apoptosis in cell culture and those which have been suggested as possibilities to improve culture productivity through the apoptosis route are discussed with given examples.

Thrombopoietin Upregulates the Promoter Conformation of p53 in a Proliferation-Independent Manner Coincident With a Decreased Expression of Bax: Potential Mechanisms for Survival Enhancing Effects

Thrombopoietin (Tpo) has proliferative and maturational effects on immature and more committed cells, respectively. We previously reported a role for Tpo as a survival factor in the factor-dependent human cell line M07e by demonstrating that Tpo suppresses apoptosis in the absence of induced proliferation. Wild-type p53 is a tumor suppressor gene that can play a vital role in mediating growth factor withdrawal-induced apoptosis in factor-dependent hematopoietic cells. Wild-type p53 can switch from a suppressor conformation, with an antiproliferative, pro-apoptotic phenotype, to a promoter conformation that has a diminished ability to mediate cell cycle arrest and apoptosis. In an effort to elucidate the mechanisms through which Tpo suppresses apoptosis, we investigated the effects of Tpo treatment on p53-mediated apoptosis in M07e cells. Tpo upregulated the expression of the promoter conformation of p53 in M07e cells coincident with a downregulation of Bax and Mdm2 protein levels. Protein levels of Bcl-2 and Bcl-xL did not significantly vary as a function of growth-factor stimulation. Conversely, the levels of suppressor conformation p53 were maximal when M07e was in a growth arrested state and decreased during factor stimulation. Furthermore, Tpo treatment induced an extranuclear buildup and greatly weakened the DNA binding capacity of p53. p53-specific antisense oligonucleotide treatment recapitulated the effects of Tpo treatment on the levels of Bax, Mdm-2, and Bcl-2. These results suggest that Tpo is suppressing growth factor withdrawal induced-apoptosis, at least in part, by downregulating the expression of pro-apoptotic Bax protein levels, through modulating the conformation of p53, which results in a functional inactivation of its pro-apoptotic abilities.

Thrombopoietin Upregulates the Promoter Conformation of p53 in a Proliferation-Independent Manner Coincident With a Decreased Expression of Bax: Potential Mechanisms for Survival Enhancing Effects

Thrombopoietin (Tpo) has proliferative and maturational effects on immature and more committed cells, respectively. We previously reported a role for Tpo as a survival factor in the factor-dependent human cell line M07e by demonstrating that Tpo suppresses apoptosis in the absence of induced proliferation. Wild-type p53 is a tumor suppressor gene that can play a vital role in mediating growth factor withdrawal-induced apoptosis in factor-dependent hematopoietic cells. Wild-type p53 can switch from a suppressor conformation, with an antiproliferative, pro-apoptotic phenotype, to a promoter conformation that has a diminished ability to mediate cell cycle arrest and apoptosis. In an effort to elucidate the mechanisms through which Tpo suppresses apoptosis, we investigated the effects of Tpo treatment on p53-mediated apoptosis in M07e cells. Tpo upregulated the expression of the promoter conformation of p53 in M07e cells coincident with a downregulation of Bax and Mdm2 protein levels. Protein levels of Bcl-2 and Bcl-xL did not significantly vary as a function of growth-factor stimulation. Conversely, the levels of suppressor conformation p53 were maximal when M07e was in a growth arrested state and decreased during factor stimulation. Furthermore, Tpo treatment induced an extranuclear buildup and greatly weakened the DNA binding capacity of p53. p53-specific antisense oligonucleotide treatment recapitulated the effects of Tpo treatment on the levels of Bax, Mdm-2, and Bcl-2. These results suggest that Tpo is suppressing growth factor withdrawal induced-apoptosis, at least in part, by downregulating the expression of pro-apoptotic Bax protein levels, through modulating the conformation of p53, which results in a functional inactivation of its pro-apoptotic abilities.

Purines and their roles in apoptosis

Purines are ubiquitous endogenous metabolites, and their roles as signalling molecules, especially in the case of adenosine and ATP, are well documented. The release of purines is increased when cells are highly activated, stressed or damaged, and this is known to have profound effects on various organ systems. Recently, purines like adenosine and ATP have been shown to be cytotoxic. Current evidence suggests that adenosine induces cell death by apoptosis, whereas ATP appears to cause both necrosis and apoptosis. Apoptosis is an important physiological process during normal tissue turnover and in the maturation of the immune system, embryogenesis, metamorphosis, endocrine-dependent tissue atrophy, etc. Recently, many of the key components of the apoptotic cell death cascade have become unravelled. In particular, proteases belonging to the interleukin-1 beta-converting (ICE) enzyme family, also known as caspases, have been shown to act as an intracellular convergence point that orchestrates the morphological and biochemical features of apoptosis. However, little is known about the signalling or the biochemical mechanisms of purine-mediated cell death. Adenosine appears to act through P1 purinoceptors, although the subtype involved remains controversial, whereas ATP may involve both P2X1 and P2X7 purinoceptors. More recent evidence suggests that the intracellular levels of purines, in addition to the cell surface receptor-mediated responses, may also play a critical role by modulating other apoptotic cell death signals. Here, we review our current understanding about purines in mediating cell death and raise a number of questions as to the possible mechanisms involved.

Apoptosis induced in human colorectal carcinoma by anti-Fas antibody

BACKGROUND

Apoptosis or programmed cell death has been shown to play an important role in the progression from polyps to carcinomas. Fas/APO-1 is a cell surface protein that can induce apoptosis in a variety of cell types upon specific binding. In this study seven human colorectal carcinoma (HCRC) cell lines of varying differentiation were analyzed for cell surface Fas expression. Fas-mediated apoptosis, and correlation of apoptosis with bcl-2 expression.

METHODS AND RESULTS

Using flow cytometry, all seven lines expressed varying amounts of cell surface Fas antigen. Exposure to anti-Fas antibody induced cell death in all the cell lines, albeit to varying degrees. The rate of apoptosis was quantitated using flow cytometry with propidium iodide staining of nuclear DNA. The poorly differentiated cell lines had a significantly decreased (p < 0.05) anti-Fas sensitivity as compared with the well-differentiated lines. Measurement of bcl-2 expression by flow cytometry showed an inverse correlation with anti-Fas sensitivity.

CONCLUSIONS

This study confirms that HCRC cell lines express Fas antigen and, more importantly, provides the first evidence that exposure to anti-Fas antibody can induce apoptosis. Fas-mediated apoptosis in HCRC cell lines may be regulated by bcl-2 and may correlate with the degree of differentiation.

Neuronal cell death in the mammalian nervous system: the calmortin hypothesis

1. This review is concerned with the calcium-dependent mechanisms involved in neuronal cell death. To this end, it provides definitions of the major types of cell death and then describes what is known of their occurrence during development and degeneration of the mammalian nervous system. 2. An analysis is presented of the different sources and compartments of calcium in neurons and of how these are related to the known calcium-dependent enzymes whose excess activation will lead to cell death. 3. The review uses the relatively large amount of pertinent information now available for other cell types, especially thymocytes, to reveal our limited knowledge of how calcium controls neuronal cell death. 4. In the final section, consideration is given to the identification of those factors that may mitigate against the calcium-dependent pathways leading to neuronal degeneration.

Evolutionary conservation of function among mammalian, avian, and viral homologs of the Bcl-2 oncoprotein

The bcl-2 gene was originally cloned because of its involvement in B-cell lymphomas and encodes a 25-kD integral membrane protein that has been shown to inhibit programmed cell death (also termed apoptosis) in a wide variety of circumstances. The Epstein-Barr Virus (EBV) also has been implicated in B-cell malignancies and interestingly contains an open reading frame (BHRF-1) predicting a 19-kD protein with 22% homology to Bcl-2. To compare the functions of p26-Bcl-2 and p19-BHRF-1, we stably introduced expression plasmids encoding these proteins into a murine interleukin-3 (IL-3)-dependent hemopoietic cell line, 32D. Removal of IL-3 from cultures of control-transfected 32D cells resulted in internucleosomal DNA cleavage (a hallmark of programmed cell death) and loss of cell survival. In contrast, 32D cells containing high levels of p26-Bcl-2 or p19-BHRF-2 proteins exhibited prolonged survival and markedly delayed DNA degradation under the same conditions of IL-3 deprivation. As a first attempt to determine the functional importance of amino acid sequences that are conserved between the Bcl-2 and BHRF-1 proteins, we used site-specific mutagenesis to replace two conserved cysteine residues with alanines (positions 158 and 219) in the human Bcl-2 protein. Comparisons of the wild-type and cysteine-minus human Bcl-2 proteins in S49 lymphoma cells revealed equivalent ability to block glucocorticoid-induced cell death and DNA fragmentation, indicating that these two conserved cysteines are not critical for Bcl-2 oncoprotein function. Investigations in 32D cells of an avian homolog of Bcl-2 cloned from the chicken also revealed conservation of function with the human Bcl-2 protein, despite the presence of a 48-amino-acid region of divergent sequence. Taken together, these data demonstrate that despite marked differences in their predicted amino-acid sequences, the human, chicken, and EBV versions of Bcl-2 have retained the structural characteristics necessary to interface with pathways involved in the regulation of programmed cell death in murine cells. The findings thus contribute to the mapping of functional domains in Bcl-2 proteins, and raise the possibility that the EBV-encoded p19-BHRF-1 protein may be able to substitute for p26-Bcl-2 in the development of some types of cancer.

Apoptosis. Its significance in cancer and cancer therapy

Apoptosis is a distinct mode of cell death that is responsible for deletion of cells in normal tissues; it also occurs in specific pathologic contexts. Morphologically, it involves rapid condensation and budding of the cell, with the formation of membrane-enclosed apoptotic bodies containing well-preserved organelles, which are phagocytosed and digested by nearby resident cells. There is no associated inflammation. A characteristic biochemical feature of the process is double-strand cleavage of nuclear DNA at the linker regions between nucleosomes leading to the production of oligonucleosomal fragments. In many, although not all of the circumstances in which apoptosis occurs, it is suppressed by inhibitors of messenger RNA and protein synthesis. Apoptosis occurs spontaneously in malignant tumors, often markedly retarding their growth, and it is increased in tumors responding to irradiation, cytotoxic chemotherapy, heating and hormone ablation. However, much of the current interest in the process stems from the discovery that it can be regulated by certain proto-oncogenes and the p53 tumor suppressor gene. Thus, c-myc expression has been shown to be involved in the initiation of apoptosis in some situations, and bcl-2 has emerged as a new type of proto-oncogene that inhibits apoptosis, rather than stimulating mitosis. In p53-negative tumor-derived cell lines transfected with wild-type p53, induction of the gene has, in rare cases, been found to cause extensive apoptosis, instead of growth arrest. Finally, the demonstration that antibodies against a cell-surface protein designated APO-1 or Fas can enhance apoptosis in some human lymphoid cell lines may have therapeutic implications.

Control of lens epithelial cell survival

We have studied the survival requirements of developing lens epithelial cells to test the hypothesis that most cells are programmed to kill themselves unless they are continuously signaled by other cells not to do so. The lens cells survived for weeks in both explant cultures and high-density dissociated cell cultures in the absence of other cells or added serum or protein, suggesting that they do not require signals from other cell types to survive. When cultured at low density, however, they died by apoptosis, suggesting that they depend on other lens epithelial cells for their survival. Lens epithelial cells cultured at high density in agarose gels also survived for weeks, even though they were not in direct contact with one another, suggesting that they can promote one another's survival in the absence of cell-cell contact. Conditioned medium from high density cultures promoted the survival of cells cultured at low density, suggesting that lens epithelial cells support one another's survival by secreting survival factors. We show for the first time that normal cell death occurs within the anterior epithelium in the mature lens, but this death is strictly confined to the region of the anterior suture.

A free-radical hypothesis for the instability and evolution of genotype and phenotype in vitro

It has been known for several decades that cultured murine cells undergo a defined series of changes, i.e., an in vitro evolution, which includes crisis, spontaneous transformation ('immortalization'), aneuploidy, and spontaneous neoplastic transformation. These changes have been shown to be caused by the in vitro environment rather than an inherent instability of the murine phenotype or genotype. Serum amine oxidases were recently identified as a predominant cause of crisis. These enzymes generate hydrogen peroxide from polyamine substrates that enter the extracellular milieu. This finding implicates free-radical toxicity as the underlying cause of in vitro evolution. We propose an oxyradical hypothesis to explain each of the stages of in vitro evolution and discuss its significance for cytotechnology and long-term cultivation of mammalian cell types.

Apoptosis

Cell death can be accidental or programmed in a multicellular organism. Evidence supports the proposition that there is a 'suicide program' inherent in vertebrate cells which can be activated when the cell's death is desirable for the good of the rest of the community. The morphology of such death is usually that of apoptosis, rather than of necrosis. Here, John Cohen describes the changes of apoptosis, and discusses progress on the identification of regulatory mechanisms and genes.

Apoptosis in the development of the immune system: growth factors, clonal selection and bcl-2

The mammalian immune system is essential for surviving challenge infections with a great range of potential pathogens. The protective effect produced is dependent on many different types of cells which require flexible and independent production and regulation. In particular, many important responses are carried out by lymphocytes, which recognise foreign antigen through exquisitely specific receptors: i.e. surface immunoglobulin (sIg) on B lymphocytes and the T cell receptor (TCR) on T lymphocytes. Each lymphocyte displays receptors with a single specificity, allowing cells with particular specificities to be regulated independently. Since millions of different Igs and TCRs are expressed, the precise selection and regulation of each T and B cell population to produce a useful self-tolerant repertoire is a very complex process. Control of cell populations can, in theory, be exercised at a number of levels, including modulation of active cell death by apoptosis. Recent research has demonstrated that regulation of apoptosis is indeed a crucial element in the control of the immune system in general, and in the development of the TCR and Ig repertoires in particular. The molecular analysis of apoptosis now takes a high priority and the proto-oncogene bcl-2 appears to be responsible for specific suppression of apoptosis in several important situations. It is also clear that malfunctions affecting apoptosis, and in particular bcl-2, can result in significant progression towards malignancy.