Ethanol promotes T cell apoptosis through the mitochondrial pathway

Clinical reports suggest that acute ethanol intoxication is often associated with lymphopenia. Previously, ethanol was reported to invoke thymocyte apoptosis. We studied the effect of ethanol on T cell apoptosis. In addition, we evaluated the molecular mechanism of ethanol-induced T cell apoptosis. Human T cells harvested from healthy subjects after an alcohol drinking binge showed enhanced T cell apoptosis (before, 0.4 +/- 0.2% versus after, 19.6 +/- 2.5% apoptotic lymphocytes/field; P < 0.001). In in vitro studies, ethanol in a concentration of 50 mm and higher enhanced the apoptosis of Jurkat cells. DNA isolated from ethanol-treated Jurkat cells displayed integer multiples of 180 base pairs. Ethanol decreased Jurkat cell expression of Bcl-2, whereas ethanol increased Jurkat cell expression of Bax. Jurkat cells treated with ethanol also showed translocation of cytochrome C into cytosol. Moreover, a caspase-9 inhibitor partially inhibited ethanol-induced Jurkat cell apoptosis. In in vivo studies, after binge drinking, T cell expression of Bcl-2 also decreased. In addition, binge drinking induced the cleavage of caspase-3, suggesting activation of caspase-3 in T cells. These results suggest that ethanol promotes T cell apoptosis through the activation of intrinsic or mitochondrial pathway.     

线粒体凋亡途径在槲皮素诱导U937 细胞凋亡中的作用研究

目的:观察槲皮素对人急性髓系白血病U937 细胞增殖、凋亡、线粒体膜电位(Δφm),、人B 细胞淋巴瘤因子- 2、人Bcl-2 相关X 蛋白、细胞色素c 表达及半胱氨酸蛋白酶鄄3、半胱氨酸蛋白酶鄄9 活性的影响,探讨线粒体凋亡途径在槲皮素 诱导U937 细胞凋亡中的作用。方法:体外培养U937 细胞,分别以0、10、20、40、80、160μmol/ L 浓度的槲皮素处理24、48 和 72 h,采用细胞计数试剂盒(Cell counting kit-8,CCK-8) 检测槲皮素对U937 细胞增殖的抑制作用;实验随机分为对照组 (Control)、槲皮素10、20 组和40 μmol/ L 组。采用流式细胞仪AnnexinV-FITC/ PI 双染法检测U937 细胞凋亡情况;采用荧光染 料3,3‘-二己基含氧碳菁碘代物[3,3‘-dihexyloxacarbocyanine iodide,DiOC6(3)]染色,流式细胞仪检测U937 细胞的变化; 采用Western blot 检测U937 细胞Bcl-2、Bax、Cytc 表达;采用比色法检测U937 细胞Caspase-3、Caspase-9 活性。结果:槲皮素可 抑制U937 细胞的增殖,抑制率显著升高,且呈时间-剂量依赖性。槲皮素亦可显著抑制U937 细胞凋亡率,与对照组比较(P< 0.01)。槲皮素显著降低U937 细胞驻鬃m,与对照组比较(P<0.01)。槲皮素显著下调U937 细胞Bcl-2 表达,上调Bax 表达,下 调Bcl鄄2/ Bax 比值及上调Cyt c 表达,与对照组比较(P<0.01)。槲皮素显著升高U937 细胞Caspase-3、Caspase-9 活性,与对照 组比较(P<0.01)。结论:槲皮素可显著抑制U937 细胞增殖,线粒体凋亡途径激活是槲皮素诱导U937 细胞凋亡的途径之一。     

TRAIL诱导Hela细胞凋亡的线粒体信号通路

 目的 探讨 TRAIL 诱导人宫颈癌 Hela 细胞凋亡的线粒体通路。 方法 琼脂糖凝胶电泳判断细胞凋亡;激光共聚焦、Western blot、荧光免疫和 caspase-3 活性检测测定细胞线粒体膜电位 (∆Ψm) 、Bcl-2 蛋白、细胞色素 c (Cyt c) 和凋亡诱导因子 (AIF) 蛋白在细胞中的定位以及 caspase-3 活性。结果 TRAIL 能诱导 Hela 细胞凋亡,有凋亡细胞特有的 DNA 梯状条带。同时,TRAIL具有时间依赖性致 ∆Ψm 和 Bcl-2 蛋白含量明显下降,线粒体 Cyt c 蛋白释放,AIF 蛋白向细胞质、细胞核转移,caspase-3 活性增强。结论 TRAIL 诱导 Hela 细胞凋亡途径之一是通过线粒体信号通路进行的。     

Polyunsaturated fatty acids trigger apoptosis of colon cancer cells through a mitochondrial pathway

Introduction

Colorectal cancer is common in developed countries. Polyunsaturated fatty acids (PUFAs) have been reported to possess tumoricidal action, but the exact mechanism of their action is not clear.

Material and methods

In the present study, we studied the effect of various n-6 and n-3 fatty acids on the survival of the colon cancer cells LoVo and RKO and evaluated the possible involvement of a mitochondrial pathway in their ability to induce apoptosis.

Results

It was observed that n-3 α-linolenic acid, eicosapentaenoic acid and docosahexaenoic acid (ALA, EPA and DHA respectively) and n-6 linoleic acid, gamma-linolenic acid and arachidonic acid (LA, GLA and AA respectively) induced apoptosis of the colon cancer cells LoVo and RKO at concentrations above 120 μM (p < 0.01 compared to control). The semi-differentiated colon cancer cell line RKO was more sensitive to the cytotoxic action of PUFAs compared to the undifferentiated colon cancer cell line LoVo. PUFA-treated cells showed an increased number of lipid droplets in their cytoplasm. PUFA-induced apoptosis of LoVo and RKO cells is mediated through a mitochondria-mediated pathway as evidenced by loss of mitochondrial membrane potential, generation of ROS, accumulation of intracellular Ca²⁺, activation of caspase-9 and caspase-3, decreased ATP level and increase in the Bax/Bcl2 expression ratio.

Conclusions

PUFAs induced apoptosis of colon cancer cells through a mitochondrial dependent pathway.     

MHC class II-mediated apoptosis in dendritic cells: a role for membrane-associated and mitochondrial signaling pathways

Cytotoxic elimination of dendritic cells (DC) in lymphoid tissue represents an important pathway of immune regulation. However, the mechanism of DC removal is still controversial since mature DC are insensitive to death receptor-mediated killing and other surface or soluble molecules mediating DC death in vivo have yet to be characterized. Class II ligation is the only known signal that induces rapid cell death in mature DC, thus our studies have now focused on the requirements for this cell death using the advantages of tools available for both the mouse and human systems. Anti-class II mAb could be grouped into (i) mAb that both bound to class II and caused class II-mediated cell death as well as (ii) those that bound to class II, but did not cause apoptosis. mAb binding stable class II dimers as well as those mAb recognizing either the alpha or beta chains of class II were found in both groups. Whereas class II-mediated death was enhanced by DC-DC homotypic interactions, DC clustering itself was insufficient to induce apoptosis. Although DC death could be inhibited by uncoupling actin filament bundling, the inhibition of various proteases, including the caspases, and protein transport mediators failed to inhibit class II-mediated cell death. Neither Bid, poly-ADP-ribose polymerase, caspases-3, -7 and -8 nor FLICE-inhibitory protein were found to be cleaved during class II apoptosis. Lastly, although class II mAb induced a rapid mitochondrial membrane depolarization in DC, cell death was not inhibited by Bcl-2 over-expression in DC. The independence of this form of apoptosis from protein or RNA synthesis, coupled to the rapidity of the mitochondrial depolarization and the lack of protection by Bcl-2, suggests that mature DC express pre-formed pro-apoptotic molecules that are involved in class II-mediated death.     

Cyt-C 依赖的线粒体途径在T42诱导肝癌HepG2细胞凋亡中的作用

目的 检测重组肿瘤抑素42肽(T42)诱导肝癌HepG2细胞凋亡及与线粒体凋亡途径的关系,探讨T42诱导肿瘤细胞凋亡的可能机制.方法 吖啶橙/溴化乙锭(AO/EB)荧光染色观察细胞凋亡的形态学变化;流式细胞仪检测凋亡率;JC-1荧光染色检测线粒体膜电位的变化;Western印迹检测细胞色素C(Cyt-C)的分布.结果 18 μmol/LT42作用下,HepG2细胞出现明显凋亡形态学变化,凋亡率为22.4%,与对照组比较差异有统计学意义(t=7.75,P＜0.05);T42降低了HepG2细胞线粒体膜电势,明显减少了线粒体Cyt-C.结论 T42通过降低线粒体膜电势,促进Cyt-C由线粒体膜释放到胞浆中,激活 caspase-3途径诱导人肝癌细胞系HepG2细胞凋亡.

Abstract：

Objective The aim of the present article is to detect the apoptosis of hepatocarcinoma cells HepG2 induced by recombinant tumor endostatin 42 peptide (T42) ,with an emphasis on the signaling pathways involved. Methods Observed the morphological changes associated with the apoptosis of HepG2 cells by using AO/EB. Apoptosis rate were dentified by using flow cytometry. Mitochondrial membrane potential was evaluated by using JC-1 fluorescent staining. The distribution of cytochrome C(Cytc ) was estimated by using western blot. Results Compared with the control group, there was significant difference in apoptosis rate of cells HepG2 under 18μmol /L of T42. (22.4% vs 3.70% ,t =7.75, P＜0.05). Mitochondrial membrane potential was decreased by T42, and cytochrome c was reduced significantly compared with the control group. Conclusions The result demonstrated that the T42 enhanced the apoptosis of HepG2 cells and its potential mechanism was related to the decreased of mitochondrial membrane potential, an increase in Cytochrome C released into the cytosol, and reduced activation of Caspase-3 channels.     

Baicalein induces apoptosis through ROS-mediated mitochondrial dysfunction pathway in HL-60 cells

Baicalein is one component of the dried root of Scutellaria Baicalensis Georgi. (Huang Qin) which is widely used in the traditional Chinese herbal medicine. In this study, we report that baicalein was able to induce apoptosis in human promyelocytic leukemia cells (HL-60), as characterized by poly-(ADP-ribose) polymerase (PARP) cleavage and DNA fragmentation. The efficacious induction of apoptosis was observed at 100 microM for 6 h. Mechanistic analysis demonstrated that baicalein induced the cleavage of Bid protein, cytochrome c release from mitochondria into cytosol, and activation of caspase-3, -8 and -9. Moreover, baicalein caused elevation of intracellular hydrogen peroxide level. Catalase could effectively block baicalein-induced DNA fragmentation. These data indicate that baicalein may trigger an apoptotic death program through reactive oxygen species (ROS)-mediated mitochondrial dysfunction pathway. The findings enhance our understanding of anticancer function of baicalein in herbal medicine.     

Hydrogen peroxide induces apoptosis through the mitochondrial pathway in rat Schwann cells

Oxidative stress is one of the several mechanisms that induces apoptosis in cells. It has been shown that hydrogen peroxide (H₂O₂) induces apoptosis in several kinds of cells; however, the role of H₂O₂ in the apoptosis of Schwann cells (SCs) is currently unclear. The objective of this study was to determine whether H₂O₂ is capable of inducing apoptosis in SCs and whether or not such an effect is associated with the activation of mitochondrial pathway. We demonstrated that H₂O₂ induces apoptosis in SCs, and is associated with increased release of cytochrome c from mitochondria and the activation of caspase-3 and -9 by up-regulation of Bax and down-regulation of Bcl-2. These results suggest a potential role for H₂O₂ in SC injury by triggering apoptosis via the mitochondrial pathway under oxidative stress.     

The molecular basis of MHC-restricted antigen recognition by T cells

In order to determine the contribution of the clonotypic T cell receptor (Ti) alpha beta heterodimer to the antigen/MHC specificity of mature T cells, we have transfected cloned Ti alpha and/or beta genes into either human or mouse T cells, and analyzed the transfectants for Ti-T3 expression and responses to antigen and Ia molecules. Our analysis establishes that a single receptor structure (the Ti alpha beta heterodimer) is necessary and sufficient to define the dual specificity of T cell antigen recognition and suggests that in at least certain instances Ti beta chains play a predominant role in MHC restriction specificity, raising the possibility of a "one receptor, two sites" model of T cell recognition.     

Juglone-induced apoptosis in human gastric cancer SGC-7901 cells via the mitochondrial pathway

This study was designed to investigate the effect of juglone on the apoptosis of human gastric cancer SGC-7901 cells. The cytotoxic activity of juglone on SGC-7901 cells was tested by the sulforhodamine B (SRB) assay. The morphological changes in the cells were observed by transmission electron microscopy (TEM). The apoptotic rate, the level of reactive oxygen species (ROS), mitochondrial transmembrane potential and the expression of cytochrome c protein were detected by flow cytometry (FCM). The expression of Bcl-2 and Bax proteins were examined by Western blot. Caspase 3 activity was determined with a microplate reader. Our results were as follows: the GI₅₀ values for SGC-7901 cells were 36.51±1.05 μmol/L (24 h) and 25.37±1.19 μmol/L (48 h). After 24 h of exposure to juglone (5, 10, 15 and 20 μmol/L), the cells presented the typical morphological changes of apoptosis, and the rate of apoptosis was found to increase in a dose-dependent manner. After cells were treated with juglone at the same dose for 24 h, the level of ROS was significantly higher, the expression of Bcl-2 was significantly down-regulated and the expression of Bax was significantly up-regulated compared to the control. The mitochondrial transmembrane potential was significantly lower, and the expression of the cytochrome c protein was significantly higher relative to the control. Caspase 3 was activated in a concentration-dependent manner. In conclusion, juglone can induce apoptosis in SGC-7901 cells through a mitochondrial pathway that seems to be mediated by the generation of ROS and a reduction in the Bcl-2/Bax ratio.     

靛红衍生物IF203通过线粒体途径诱导人肝癌HepG2细胞凋亡

目的 探讨靛红衍生物IF203诱导人肝癌HepG2细胞凋亡及线粒体凋亡途径.方法 体外培养HepG2细胞,应用倒置相差显微镜观察细胞形态变化;采用酸性磷酸酶法(APA)、流式细胞术(FCM)和免疫印迹法(Western blotting),检测IF203对HepG2细胞生长、细胞凋亡率、线粒体膜电位、Caspase-9...     

The basis of autoimmunity in MRL-lpr/lpr mice: a role for self Ia-reactive T cells

Murine models of systemic lupus eRythematosus (SLE) have significantly contributed to our understanding of human autoimmunity. One such strain, the MRL-lpr/lpr, spontaneously develops an autoimmune disease manifested clinically by arthritis, vasculitis, immune-complex glomerulonephritis and autoantibody production^1–3. In this article Yvonne Rosenberg and her colleagues suggest a theoretical basis for the development of autoimmunity in MRL-lpr/lpr mice.     

Intervertebral disc degeneration: the role of the mitochondrial pathway in annulus fibrosus cell apoptosis induced by overload

Degeneration of the intervertebral disk (IVD) is a major pathological process implicated in low back pain and is a prerequisite to disk herniation. Although mechanical stress is an important modulator of the degeneration, the underlying molecular mechanism remains unclear. The association of human IVD degeneration, assessed by magnetic resonance imaging, with annulus fibrosus cell apoptosis and anti-cytochrome c staining revealed that the activation of the mitochondria-dependent apoptosome was a major event in the degeneration process. Mouse models of IVD degeneration were used to investigate the role of the mechanical stress in this process. The application of mechanical overload (1.3 MPa) for 24 hours induced annulus fibrosus cell apoptosis and led to severe degeneration of the mouse disks. Immunostaining revealed cytochrome c release but not Fas-L generation. The role of the caspase-9-dependent mitochondrial pathway in annulus fibrosus cell apoptosis induced by overload was investigated further with the use of cultured rabbit IVD cells in a stretch device. Mechanical overload (15% area change) induced apoptosis with increased caspase-9 activity and decreased mitochondrial membrane potential. Furthermore, Z-LEHD-FMK, a caspase-9 inhibitor, but not Z-IETD-FMK, a caspase-8 inhibitor, attenuated the overload-induced apoptosis. Our results from human samples, mouse models, and annulus fibrosus culture experiments demonstrate that the mechanical overload-induced IVD degeneration is mediated through the mitochondrial apoptotic pathway in IVD cells.     

Towards a molecular basis of oligometastatic disease: potential role of micro-RNAs

Oligometastasis is a cancer disease state characterized by a limited number of metastatic tumors involving single or few organs and with biological properties that make them potentially amenable to locoregional antitumor therapy. Current clinical data show that they are potentially curable with surgical resection or/and radiotherapy. Yet, mechanisms of progression from primary tumor to oligometastasis, rather than to polymetastases, is lacking in detail. In the current review we focus on the role of micro-RNAs in the regulation of metastases development and the role they may play in the differentiation of oligometastatic from polymetastatic progression. We also discuss the analyses of metastatic samples from oligo-and polymetastatic patients, which suggest that oligometastasis is a distinct biologic entity regulated in part by micro-RNAs. In addition, a review of the known functions of oligometastatic-specific micro-RNAs suggest that they regulate multiple steps in the metastatic cascade, including epithelial–mesenchymal transition, tumor invasion, intravasation, distant vascular extravasation and proliferation in a distant organ. Understanding the role of micro-RNAs and their target genes in oligometastatic disease may allow for the development of targeted therapies to effectively conrol the spread of metastases.     

Advanced glycation end products induce T cell apoptosis: Involvement of oxidative stress, caspase and the mitochondrial pathway

Accumulation of advanced glycation end products (AGEs) is a hallmark in aged people. T cells play important roles in maintaining homeostasis of immune function. This study investigated the effects of AGEs-bovine serum albumin (AGEs) in human T cells. Incubation of Jurkat and several immortalized T cell lines with AGEs resulted in cell death dose-dependently. AGEs-induced cell death was partially but significantly blocked by neutralizing antibodies recognizing receptor of AGEs. In addition to detecting DNA nick, simultaneous stainings of annexin V with 7-amino-actinomycin D further confirmed the apoptotic nature of cell death. AGEs also caused apoptosis in purified T cells. Although AGEs-induced apoptosis could be blocked by the pan-caspase inhibitor, Ala-Asp-fluomethyl ketone (Z-VAD-fmk), there was no activation of caspase-3, -5, -8 and -9. AGEs caused mitochondrial outer membrane permeabilization and this process was prevented by an antioxidant or Z-VAD-fmk. Furthermore, AGEs treatment led to translocation of apoptosis inducing factor (AIF) from the mitochondria into the nucleus. Altogether, this report demonstrated that AGEs induced T cell apoptosis in an oxidative stress-associated and caspase-dependent manner with involvement of the mitochondrial pathway. It is likely that AGEs-induced T cell apoptosis may play a role in T cell homeostasis in ageing.     

Small intestinal submucosa induces loss of mitochondrial integrity and caspase-dependent apoptosis in human T cells

Porcine small intestinal submucosa (SIS) is a cell-free biomaterial used in humans for wound healing and as scaffold material for constructive remodeling of damaged or missing tissue. We have previously shown that SIS contains a factor that suppresses human helper T cell subset differentiation and expansion by inducing programmed cell death. Our aims here were to identify in detail the processes involved in SIS-induced T cell apoptosis and to perform the first characterization of the apoptosis-inducing factor present in SIS. In in vitro experiments, we utilized human T cell lines, Jurkat and CEM, to identify the processes involved in SIS-induced T cell apoptosis. Two types of sterile SIS material were used: hydrated sheets and rehydrated clinical-grade sheets. We found that SIS-mediated apoptosis as detected by induction of membrane annexin V staining involved the loss of mitochondrial membrane potential and was dependent on caspase activation. We eliminated transforming growth factor beta (TGF-beta), Fas ligand (FasL), and galectin family members as factors in SIS-mediated T cell apoptosis. We further established that processes required to prepare SIS for clinical use, freeze-drying, and gas sterilization destroyed the apoptosis-inducing factor. SIS contains a factor that induces loss of mitochondrial integrity and caspase-dependent apoptosis in human T cells. This factor is destroyed by freeze-drying and gas sterilization and is not TGF-beta, FasL, or a galectin family member. Normal T cell homeostasis in gut-associated tissues may be regulated in part by this unknown factor.     

Glycodelin A triggers mitochondrial stress and apoptosis in T cells by a mechanism distinct and independent of TCR signaling

Glycodelin A is one of the progesterone inducible endometrial factors that protect the fetal semiallograft from maternal immune rejection. Our previous studies demonstrate that glycodelin A induces apoptosis in activated T lymphocytes. Here, we report that glycodelin A initiates the intrinsic apoptotic program in T cells. Glycodelin A treatment triggers a stress response leading to mitochondrial membrane permeabilization and activation of initiator caspase 9. The kinetics of mitochondrial depolarization precede onset of DNA fragmentation in both Jurkat cells and peripheral blood T cells treated with glycodelin A. Overexpression of the antiapoptotic protein Bcl-2 is sufficient to protect from glycodelin A-induced cell death. It has been reported earlier that glycodelin A desensitizes T cell receptor (TCR) signaling, probably by its association with the tyrosine phosphatase CD45. Here, we provide evidence that the apoptogenic activity of glycodelin A is not a consequence of this phenomenon. Glycodelin A-induced apoptosis does not depend on components of the TCR signal cascade, including CD45. We observe that glycodelin A is inhibitory to T cells even upon phorbol ester and ionophore stimulation which bypasses the TCR-proximal signaling events, and that glycodelin A treatment does not interfere with T cell activation as evidenced from induction of the activation marker CD69. Thus, glycodelin A initiates mitochondrial stress-mediated apoptosis in T cells by a pathway that is distinct and independent from the TCR signaling pathway.     

肺炎链球菌溶血素经死亡受体/Fas途径和线粒体途径诱导RAW264.7细胞凋亡

目的:探讨肺炎链球菌溶血素(Pneumolysin,Ply)对小鼠RAW264.7细胞的增殖抑制和诱导凋亡的作用及机制。方法:Ply蛋白加入RAW264.7细胞培养上清与细胞共孵育。倒置显微镜观察Ply对RAW264.7细胞形态的影响。MTT法检测Ply对RAW264.7细胞的增殖抑制。Annexin V法检测细胞凋亡率。分光光度法检测Caspase-3、8、9活性。免疫细胞化学法检测到Bax、Fas、Bcl-2蛋白的表达。结果:Ply对小鼠RAW264.7细胞有明显的增殖抑制作用,呈剂量和时间依赖性;1μg/ml Ply处理RAW264.7细胞24小时后,可见典型的凋亡形态学改变;1μg/ml Ply处理RAW264.7细胞1小时和3小时后,细胞凋亡率分别为32.90%和51.56%(P<0.05);1μg/ml Ply蛋白处理RAW264.7细胞24小时,Caspase-3、8、9活性均比对照组升高(P<0.05);免疫细胞化学法检测到Bax、Fas表达较对照组增强,Bcl-2表达减弱(P<0.01)。结论:Ply可诱导小鼠RAW264.7细胞凋亡,诱导凋亡的机制可能是通过死亡受体/Fas途径和线粒体途径双重机制的介导实现。     

Involvement of the mitochondrial pathway in bruceine D-induced apoptosis in Capan-2 human pancreatic adenocarcinoma cells

The fruit of Brucea javanica L. is a common herb used in Chinese medicine for the treatment of a variety of cancers. Our research group has previously identified bruceine D (BD), a quassinoid found abundantly in B. javanica, to have potent cytotoxic effect on a number of pancreatic cancer cell lines, including Panc-1, SW1990 and Capan-1 cells. In the present study, we showed that BD was also able to inhibit the growth of the Capan-2 human pancreatic adenocarcinoma cell line, but it exerted only modest cytotoxicity on the WRL68 human hepatocyte cell line and a human pancreatic progenitor cell line. The antiproliferative effects of BD were comparable to those exhibited by camptothecin and gemcitabine in our culture system. We found a dose-dependent decrease of the mitochondrial membrane potential in BD-treated Capan-2 cells as measured by the JC-1 assay. BD exposure was able to attenuate the expression of Bcl-2 protein in Capan-2 cells as detected by western blot analysis. In addition, the expression of both caspase 9 and caspase 3 in BD-treated Capan-2 cells was significantly accentuated. Moreover, BD was capable of inducing the fragmentation of genomic DNA in Capan-2 cells as evidenced by Hoechst staining. Cell cycle analysis demonstrated that BD could increase the percentage of Capan-2 cells in the subG1 phase in a dose-related manner. An increase in the apoptosis of Capan-2 cells was also observed by Annexin V and PI staining. These results unequivocally indicate that BD induces cytotoxicity in Capan-2 cells via the induction of cellular apoptosis involving the mitochondrial pathway.     

The molecular basis of T cell differentiation

Our laboratory has studied the molecular basis of T helper cell differentiation. We have used reporter transgenic mice, selective hybridization techniques, and studies of cell signaling to show that a complex pattern of gene expression is reprogrammed as the decision is made to become either a Th1 or Th2 cell. Many of these components have been identified, and their mechanisms of action elucidated. Understanding these mechanisms is likely to lead in the long-term to ways to intervene in these processes and, therefore, to direct immune response in therapeutically useful directions.