Role of interleukin 12 and costimulators in T cell anergy in vivo.

The induction of T cell anergy in vivo is thought to result from antigen recognition in the absence of co-stimulation and inflammation, and is associated with a block in T cell proliferation and Th1 differentiation. Here we have examined the role of interleukin (IL)-12, a potent inducer of Th1 responses, in regulating this process. T cell tolerance was induced by the administration of protein antigen without adjuvant in normal mice, and in recipients of adoptively transferred T cells from T cell receptor transgenic mice. The administration of IL-12 at the time of tolerance induction stimulates Th1 differentiation, but does not promote antigen-specific T cell proliferation. Conversely, inhibiting CTLA-4 engagement during anergy induction reverses the block in T cell proliferation, but does not promote full Th1 differentiation. T cells exposed to tolerogenic antigen in the presence of both IL-12 and anti-CTLA-4 antibody are not anergized, and behave identically to T cells which have encountered immunogenic antigen. These results suggest that two processes contribute to the induction of anergy in vivo; CTLA-4 engagement, which leads to a block in the ability of T cells to proliferate to antigen, and the absence of a prototypic inflammatory cytokine, IL-12, which prevents the differentiation of T cells into Th1 effector cells. The combination of IL-12 and anti-CTLA-4 antibody is sufficient to convert a normally tolerogenic stimulus to an immunogenic one.

Homeostasis and self-tolerance in the immune system: turning lymphocytes off.

The immune system responds in a regulated fashion to microbes and eliminates them, but it does not respond to self-antigens. Several regulatory mechanisms function to terminate responses to foreign antigens, returning the immune system to a basal state after the antigen has been cleared, and to maintain unresponsiveness, or tolerance, to self-antigens. Here, recent advances in understanding of the molecular bases and physiologic roles of the mechanisms of immune homeostasis are examined.

Role of Fas-mediated cell death in the regulation of immune responses.

Interaction of the cell-surface molecule Fas (CD95 APO-1) with its specific ligand triggers apoptotic death of T and B lymphocytes. This pathway is important for eliminating self-reactive lymphocytes and thus preventing autoimmunity. Fas is also involved in controlling injurious lymphocyte reactions in immunologically 'privileged' tissues, and may provide a strategy for reducing graft rejection.

Retroviruses as tools to study the immune system.

Retrovirus-based vectors provide an efficient means to introduce and express genes in cells of the immune system and have become a popular tool to study immune function. They are easy to manipulate and provide stable, long-term gene expression because they integrate into the genome. Current retroviral vectors do have limitations that affect their usefulness in certain applications. However, recent advances suggest a number of ways in which these vectors might be improved to extend their utility in immunological research.

Retroviral expression in embryonic stem cells and hematopoietic stem cells.

Achieving long-term retroviral expression in primary cells has been problematic. De novo DNA methylation of infecting proviruses has been proposed as a major cause of this transcriptional repression. Here we report the development of a mouse stem cell virus (MSCV) long terminal repeat-based retroviral vector that is expressed in both embryonic stem (ES) cells and hematopoietic stem (HS) cells. Infected HS cells and their differentiated descendants maintained long-term and stable retroviral expression after serial adoptive transfers. In addition, retrovirally infected ES cells showed detectable expression level of the green fluorescent protein (GFP). Moreover, GFP expression of integrated proviruses was maintained after in vitro differentiation of infected ES cells. Long-term passage of infected ES cells resulted in methylation-mediated silencing, while short-term expression was methylation independent. Tissues of transgenic animals, which we derived from ES cells carrying the MSCV-based provirus, did not express GFP. However, treatment with the demethylating agent 5-azadeoxycytidine reactivated the silent provirus, demonstrating that DNA methylation is involved in the maintenance of retroviral repression. Our results indicate that retroviral expression in ES cells is repressed by methylation-dependent as well as methylation-independent mechanisms.

Mechanisms of peripheral T cell tolerance.

Peripheral tolerance to self proteins is induced because these antigens are presented to T lymphocytes under conditions that do not allow effective immune responses to develop, or because the responses of the specific T cells are tightly regulated. The two principal mechanisms of peripheral tolerance are activation-induced cell death (AICD) and anergy. In CD4+ T lymphocytes, AICD is induced by repeated stimulation, with high levels of interleukin (IL)-2 production. Under these conditions, the T cells co-express Fas (CD95) and Fas ligand (FasL), and engagement of Fas triggers apoptotic death of the T cells. Mice with defects in Fas, FasL, IL-2R alpha or beta chain exhibit defects in AICD and develop autoimmune disease. The induction of T cell anergy is dependent on the recognition of B7 co-stimulators by the inhibitory T cell counter-receptor, CTLA-4. Failure of anergy is the likely basis for the fatal autoimmune disease of CTLA-4 knockout mice. The single-gene defects that result in autoimmunity are all defects in lymphocyte regulation, indicating that tolerance is often maintained by the control of lymphocyte responses to self antigen. The existence of distinct pathways of T cell tolerance suggest that different types of antigens induce tolerance by distinct mechanisms.

[Health education. Educational approach to change]. / Education pour la santé. Une pédagogie de changement.

There is currently a widespread interest in health education and a need to better understand the bases of the educational approach to health problems. This paper examines therefore some concepts and methods underlying health education work. It is important that health education objectives are closely related to the effects of human behaviour (what people do for their health) on health outcomes. One can, for example, identify health behaviours which are consequential to health promotion, disease prevention and health care utilization. They provide a focus for educational interventions. Secondly, the strategy of health education is based on the assumption that health practices (or their underlying factors) are subject to change through educational and information methods. This health education is essentially a process of change through education and may involve a more general process of educating the public for health, or a more specific attempt at modifying particular health practices. Applying health education to health problems requires an understanding of the communication process and its components. A number of characteristics which may facilitate or hinder communication are discussed. Finally, health education is more than a services of unstructured influences on people's behaviour. It is a planned strategy involving several stages of change. Also, to be fully effective, health education interventions should be integrated in health programmes and services together with other public health mesures.

Reflections on the medical model and the objectives of health education.

A recent article in the IJHE deals with the linkage of health education to a medical model philosophy and discusses the origins and consequences of medical dominance on health education practice (Vuori, 1980). This paper questions some of the observations and conclusions on which the medical dominance premise is based and argues that historical developments and the current state of affairs of health education do not fully support a medical dominance thesis. For example, there is a historical omission concerning the growth and influence of social and behavioural sciences on health education theory and practice. Also, current health education systems and services in European countries do not unequivocally support the medical influence premise, nor do training programmes and research. The paper further re-examines some statements about the possible consequences of medical dominance on current health education practice, especially as regards the usefulness of single-theme campaigns, the use of health services and the dissemination of medical knowledge. Opinions about the use of individual versus social means of action are discussed, as well as the need for health education to invest more energy in creating favourable attitudes towards the use of legislative and administrative measures. Finally, the discussion centres on the proposition that health education should be more concerned with the internalization of health values. It is argued that it would be useful to re-examine the conceptual base of health education in developmental terms and proposed that we reflect on the organization of health education activities and service around five sectors of socialization. These are interdependent, yet play a specific role in health education throughout the various stages of life.

Public education in cancer prevention.

Life-style is now recognized as a main determinant of cancer risk. Public education is an important component of cancer control programmes and has been shown to be effective in leading to life-style changes. Four basic types of education programmes are reviewed: for increasing the public's awareness of cancer, for changing specific risk behaviour (such as stopping smoking), for learning self-examination skills (such as breast self-examination), and for promoting early cancer detection in the community.To change human behaviour it is best to approach the risk habit through the same forces that develop and sustain the habit. Simply giving information of an association between specific habits and cancer, even if repeated several times, will lead to increased public awareness and encourage some to make a minimal effort to change their behaviour, but in general the new habit does not persist and continuing and intensifying this approach are ineffective. An alternative strategy utilizes socially active forces to support the prevention practice and remove possible barriers to action. For example, an antismoking programme should create a favourable social image of the non-smoker. Although a culturally and socially relevant mass media campaign can influence knowledge and beliefs and induce people to participate in a screening activity, this needs to be supplemented over a period of time by personal contact methods, such as group discussions, telephone conversations and home visits, in order to promote a regular screening habit. Contrary to popular opinion, mass communication methods can be expensive on a per person cost-effectiveness basis because of low participation rates and weakness in sustaining healthy behaviour.

Genetic models of abnormal apoptosis in lymphocytes.

Apoptosis is a critical mechanism for regulating cell numbers during development, normal responses to hormones and other stimuli, and immune and inflammatory reactions. Recent advances in defining the biochemical mechanisms of cell death, and the development of animal models with isolated defects in cell death pathways, have led to an increasing appreciation of the pathophysiologic importance of lymphocyte apoptosis. In this article, we review our current understanding of the pathways and roles of apoptosis in lymphocytes, with an emphasis on transgenic and knockout models. We also summarize the relevance of these animal models to human diseases.

The roles of costimulation and Fas in T cell apoptosis and peripheral tolerance.

Using cells from TCR transgenic mice that do or do not express Fas, we show that there are two mechanistically distinct forms of apoptosis in CD4+ T cells. Naive T cells undergo apoptosis if cultured in the absence of antigen or costimulation. This form of programmed cell death (PCD) is not dependent on Fas, and is prevented by CD28-mediated signals, which lead to the secretion of growth factors and the expression of survival genes, such as bcl-xL. Recently activated T cells undergo apoptotic death upon repeated stimulation. This activation-induced cell death (AICD) is mediated by Fas, but is independent of costimulation and is not prevented by IL-2 or bcl-xL. Finally, we show that peripheral tolerance may be induced in vivo independent of Fas-mediated cell death.

The Fas/Fas ligand pathway and Bcl-2 regulate T cell responses to model self and foreign antigens.

We have examined the role of Fas and Bcl-2 in T cell survival and responses to antigen in vivo using T cells that express a transgenic antigen receptor specific for hen egg lysozyme (HEL) and that either lack functional Fas or Fas ligand (FasL) or overexpress Bcl-2 as a transgene. HEL-specific, Bcl-2-transgenic T cells showed prolonged responses to immunization with cognate peptide but were eliminated rapidly when exposed to HEL expressed systemically as a self antigen. In contrast, Fas- and FasL-defective T cells did not display exaggerated responses to immunization with HEL peptide, but did show increased expansion and survival in response to systemic self antigen and were able to activate anti-HEL (self) antibody-forming cells. Thus, Bcl-2 and Fas play different roles in the regulation of T cell responses to antigen in vivo and in self tolerance.

Functional roles of Fas and Bcl-2-regulated apoptosis of T lymphocytes.

Apoptotic cell death is an important mechanism for maintaining homeostasis in the immune system and for regulating the fates of lymphocytes following encounters with self and foreign Ags. To study the physiologic roles of the proapoptotic Fas pathway and the antiapoptotic protein, Bcl-2, in T cell maturation and homeostasis, a TCR transgene has been bred into mice lacking functional Fas and mice that express Bcl-2 constitutively. In vitro, Fas-deficient T cells are resistant to activation-induced cell death, whereas Bcl-2-overexpressing T cells are resistant to death induced by withdrawal of growth factors. In vivo, Bcl-2-overexpressing mice accumulate T cells in the thymus and peripheral lymphoid tissues in the absence of Ag, but these cells are deleted normally after Ag administration. In contrast, Fas-deficient mature T cells are present in normal numbers in the absence of Ag, but are resistant to Ag-induced deletion. Both Fas-deficient and Bcl-2 overexpressing thymocytes are deleted when exposed to transgene-encoded circulating self Ag, indicating that the pathways of apoptosis controlled by these proteins are not critical for negative selection of developing thymocytes. Moreover, deficiency of Fas, but not Bcl-2 overexpression, results in the accumulation of autoreactive T cells in peripheral lymphoid tissues. These results demonstrate that Fas and Bcl-2 regulate different pathways of apoptosis that may serve distinct functions in lymphocyte homeostasis and in the maintenance of T cell tolerance.

Differentiation and tolerance of CD4+ T lymphocytes.

The development of effector and memory populations of T lymphocytes is determined by antigen-induced growth and differentiation of naive T cells, and it is regulated by antigen-induced functional tolerance and cell death. CD4+ helper T lymphocytes that vary in their profiles of cytokine production and in effector functions also show distinct responses to antigens and co-stimulatory signals, and they differ in their sensitivity to tolerance induction. Thus, stimuli that trigger T cell growth and differentiation, as well as mechanisms that inhibit T cell expansion, determine both the magnitude and the nature of T cell-dependent immune responses to protein antigens.

Biochemical mechanisms of IL-2-regulated Fas-mediated T cell apoptosis.

Activation-induced cell death (AICD) of lymphocytes is an important mechanism of self-tolerance. In CD4+ T cells, AICD is mediated by the Fas pathway and is enhanced by IL-2. To define the mechanisms of this pro-apoptotic action of IL-2, we analyzed CD4+ T cells from wild-type and IL-2-/- mice expressing a transgenic T cell receptor. T cells become sensitive to AICD after activation by antigen and IL-2. IL-2 increases transcription and surface expression of Fas ligand (FasL) and suppresses transcription and expression of FLIP, the inhibitor of apoptosis. The ability of IL-2 to enhance expression of a pro-apoptotic molecule, FasL, and to suppress an inhibitor of Fas signaling, FLIP, likely accounts for the role of this cytokine in potentiating T cell apoptosis.

Autoimmunity as a consequence of retrovirus-mediated expression of C-FLIP in lymphocytes.

The induction of apoptosis by death receptors serves to regulate immune responses by eliminating unwanted and harmful cells. Mature lymphocytes express FLICE inhibitory proteins (FLIPs) that block death receptor-induced cell death. Here, we show that both B and T cells downregulate c-FLIP upon activation in vitro. Retrovirus-mediated expression of c-FLIP blocks Fas-induced apoptosis of activated lymphocytes but does not affect cell death resulting from cytokine withdrawal. In vivo, c-FLIP expression results in defective superantigen-mediated elimination of T cells, the accumulation of activated B cells, the production of autoantibodies, and the development of autoimmune disease. No effect was seen on negative selection of thyomocytes. These results suggest that activation-dependent downregulation of c-FLIP renders mature lymphocytes sensitive to death receptor-mediated apoptosis and is required to maintain self-tolerance.

Uncoupling IL-2 signals that regulate T cell proliferation, survival, and Fas-mediated activation-induced cell death.

IL-2 is an important growth and survival factor for T lymphocytes but also sensitizes these cells to Fas-mediated activation-induced cell death (AICD). The molecular basis of these different effects of IL-2 was studied by introducing wild-type and mutant forms of the IL-2 receptor beta (IL-2Rbeta) chain that lacked specific signaling capacities into receptor-deficient T cells by retroviral gene transfer. Activation of Stat5 by IL-2 was found to be involved in T cell proliferation and promoted Fas ligand (FasL) expression and AICD. T cell survival was dependent on a receptor region that activated Akt and the expression of Bcl-2. Thus, distinct IL-2Rbeta chain signaling modules regulate T cell fate by stimulating growth and survival or by promoting apoptosis.

Serum lipids and apoproteins in students whose parents suffered prematurely from a myocardial infarction.

Lipids and apoproteins as well as other coronary risk factors were measured in offspring of patients who suffered from a myocardial infarction before the age of 50 years; the results are compared with the results of a control group matched for age and sex. Significant differences were observed in the apoprotein A1 level, in the protein/fat ratios of high- and low-density lipoproteins, and in smoking habits. In a multivariate analysis, the offspring group was found to be different from the control group in nonhigh-density lipoprotein cholesterol/apoprotein B ratio, high-density lipoprotein cholesterol/apoprotein A1 ratio, smoking habits, apoprotein A1, and apoprotein A2. By means of these variables a total of 85% of all subjects could be correctly classified. We conclude that as early as age 21 years the offspring of patients with premature coronary heart disease differ from matched control subjects in lipoprotein measurements and in smoking habits.