Unequal death in T helper cell (Th)1 and Th2 effectors: Th1, but not Th2, effectors undergo rapid Fas/FasL-mediated apoptosis

Glial progenitor-based repair of demyelinating neurological diseases

Demyelinating diseases of the brain and spinal cord affect more than one-quarter million of Americans, with numbers reaching more than two million across the world. These patients experience not only the vascular, traumatic, and inflammatory demyelinations of adulthood but the congenital and childhood dysmyelinating syndromes of the pediatric leukodystrophies. Several disease-modifying strategies have been developed that slow disease progression, especially in the inflammatory demyelinations and in multiple sclerosis in particular. Yet, currently available disease modifiers typically influence the immune system and are neither intended to nor competent to reverse the structural neurologic damage attending acquired demyelination. Fortunately, however, the disorders of myelin lend themselves well to attempts at structural repair, because central oligodendrocytes are the primary, and often sole, victims of the underlying disease process. Given the relative availability and homogeneity of human oligodendrocyte progenitor cells, the disorders of myelin formation and maintenance may be especially compelling targets for cell-based neurologic therapy.

Autophagy is induced in CD4+ T cells and important for the growth factor-withdrawal cell death

Autophagy is a tightly regulated catabolic mechanism that degrades proteins and organelles. Autophagy mediates programmed cell death under certain conditions. To determine the role of autophagy in T cells, we examined, in mouse CD4+ T cells, conditions under which autophagy is induced and alterations of the cell fate when autophagy is blocked. We have found that resting naive CD4+ T cells do not contain detectable autophagosomes. Autophagy can be observed in activated CD4+ T cells upon TCR stimulation, cytokine culturing, and prolonged serum starvation. Induction of autophagy in T cells requires JNK and the class III PI3K. Autophagy is inhibited by caspases and mammalian target of rapamycin in T cells. Interestingly, more Th2 cells than Th1 cells undergo autophagy. Th2 cells become more resistant to growth factor-withdrawal cell death when autophagy is blocked using either chemical inhibitors 3-methyladenine, or by RNA interference knockdown of beclin 1 and Atg7. Therefore, autophagy is an important mechanism that controls homeostasis of CD4+ T cells.

Cross-talk between apoptosis and cytokines in the regulation of parasitic infection

Parasitic diseases have worldwide medical and economical impact. Host T lymphocytes and the cytokines they produce determine the outcome of parasitic infections. Programmed cell death by apoptosis is induced in the course of parasitic infections, and affects cytokine production by removing activated effector T and B cells. In addition, engulfment of apoptotic cells promotes the secretion of cytokines that regulate intracellular replication of protozoan parasites. In this review, we discuss how the cross-talk between apoptosis and cytokines regulates parasitic infection.

The Fas death pathway controls coordinated expansions of type 1 CD8 and type 2 CD4 T cells in Trypanosoma cruzi infection

We investigated the role of the Fas ligand (FasL)/Fas death pathway on apoptosis and cytokine production by T cells in Trypanosoma cruzi infection. Anti-FasL, but not anti-TNF-alpha or anti-TRAIL, blocked activation-induced cell death of CD8 T cells and increased secretion of IL-10 and IL-4 by CD4 T cells from T. cruzi-infected mice. CD4 and CD8 T cells up-regulated Fas/FasL expression during T. cruzi infection. However, Fas expression increased earlier in CD8 T cells, and a higher proportion of CD8 T cells was activated and expressed IFN-gamma compared with CD4 T cells. Injection of anti-FasL in infected mice reduced parasitemia and CD8 T cell apoptosis and increased the ratio of CD8:CD4 T cells recovered from spleen and peritoneum. FasL blockade increased the number of activated T cells, enhanced NO production, and reduced parasite loads in peritoneal macrophages. Injection of anti-FasL increased IFN-gamma secretion by splenocytes responding to T. cruzi antigens but also exacerbated production of type 2 cytokines IL-10 and IL-4 at a late stage of acute infection. These results indicate that the FasL/Fas death pathway regulates apoptosis and coordinated cytokine responses by type 1 CD8 and type 2 CD4 T cells in T. cruzi infection.

CD4+ T-cell memory: generation and multi-faceted roles for CD4+ T cells in protective immunity to influenza

We have outlined the carefully orchestrated process of CD4+ T-cell differentiation from naïve to effector and from effector to memory cells with a focus on how these processes can be studied in vivo in responses to pathogen infection. We emphasize that the regulatory factors that determine the quality and quantity of the effector and memory cells generated include (i) the antigen dose during the initial T-cell interaction with antigen-presenting cells; (ii) the dose and duration of repeated interactions; and (iii) the milieu of inflammatory and growth cytokines that responding CD4+ T cells encounter. We suggest that heterogeneity in these regulatory factors leads to the generation of a spectrum of effectors with different functional attributes. Furthermore, we suggest that it is the presence of effectors at different stages along a pathway of progressive linear differentiation that leads to a related spectrum of memory cells. Our studies particularly highlight the multifaceted roles of CD4+ effector and memory T cells in protective responses to influenza infection and support the concept that efficient priming of CD4+ T cells that react to shared influenza proteins could contribute greatly to vaccine strategies for influenza.

CD4+ memory T cells: functional differentiation and homeostasis

CD4+ T cells are central regulators of both humoral and cellular immune responses. There are many subsets of CD4+ T cells, the most prominent being T-helper 1 (Th1), Th2, Th-17, and regulatory T cells, specialized in regulating different aspects of immunity. Without participation by these CD4+ T-cell subsets, B cells cannot undergo isotype switching to generate high-affinity antibodies, the microbicidal activity of macrophages is reduced, the efficiency of CD8+ T-cell responses and CD8+ T-cell memory are compromised, and downregulation of effector responses is impaired. It therefore stands to reason that memory CD4+ T cells are likely to fulfill an important facilitator role in the maintenance and control of protective immune responses. This review discusses some issues of importance for the generation of memory CD4+ T cells and focuses in particular on their heterogeneity and plasticity, with respect to both phenotypic characteristics and function. Finally, we discuss a number of factors that affect long-term maintenance of memory CD4+ T cells.

Comparison of gene expression by co-cultured WC1+ gammadelta and CD4+ alphabeta T cells exhibiting a recall response to bacterial antigen

Immunization of cattle with a Leptospira borgpetersenii serovar hardjo-bovis vaccine results in the development of a recall response by WC1(+) gammadelta T cells and CD4(+) alphabeta T cells characterized by proliferation and interferon-gamma production. It was hypothesized that these two T cell subpopulations had largely redundant effector functions, principally differing in their requirements for activation. To test this, gene expression in cells proliferating to antigen were compared utilizing RT-PCR and bovine microarrays. Both T cell populations had similar transcript profiles for effector molecules, including IFN-gamma, FasL and granzyme B. In contrast, transcripts for costimulatory receptors and ligands were notably different following activation, as WC1(+) T cells expressed no or lower levels of transcripts for CD28 and CD40L, while CD4(+) T cells expressed substantial levels of both. However, both cell types had high levels of CTLA-4 transcript suggesting the cells may be regulated similarly following activation but differ in their need for and ability to provide costimulation. Microarray analyses to extend the number of genes examined revealed that while both subpopulations upregulated anti-apoptotic genes as well as those involved in cell activation and protein biosynthesis, overall there were limited differences between the two antigen-activated cell populations. Those genes that did differ were involved in cell signaling, protein production and intracellular protein trafficking. These results strengthen the hypothesis that these particular activated WC1(+) and CD4(+) T cells have overlapping effector functions and therefore may differ principally with regard to how they are recruited into immune responses.

CCR4 participation in Th type 1 (mycobacterial) and Th type 2 (schistosomal) anamnestic pulmonary granulomatous responses

CCR4 is purported to be a Th type 2 (Th2) cell-biased receptor but its functional role is unclear. Recent studies suggest that chemokine receptor expression and function are more complex in vivo and raise doubts regarding restricted CCR4 expression by Th2 cells. To address these issues, we analyzed the role of CCR4 in highly polarized models of Th type 1 (Th1) and Th2 cell-mediated pulmonary granulomas, respectively, elicited by i.v. challenge of primed mice with either mycobacterial purified protein derivative or schistosomal egg Ag-coated beads. CCR4 agonists were expressed during both responses, correlating with a shift of CCR4+ CD4+ T cells from blood to lungs. CCL22 dominated in draining nodes during the Th1 response. Analysis of CD4+ effector T cells revealed CCR4 expression and CCR4-mediated chemotaxis by both IFN-gamma and IL-4 producers. Studies of CCR4 knockout (CCR4(-/-)) mice showed partial impairment of the local type-2 cytokine response and surprisingly strong impairment of the Th1 response with abrogated IFN-gamma production during secondary but not primary challenge. Adoptive transfer indicated CCR4(-/-)CD4+ Th1 cell function was defective but this could not be reconstituted with wild-type (CCR4(+/+)) CD4+ T cells indicating involvement of another CCR4+ population. Coculture of CCR4(+/+)CD4+ T cells and CCR4(-/-) dendritic cells revealed intact IL-2 but impaired IFN-gamma production, pointing to a role for CCR4+ dendritic cells in effector cell expression. Therefore, CCR4 is not Th2-restricted and was required for sustenance and expression of the Th1 effector/memory response to mycobacterial Ags.

T cell tolerance induced by histone deacetylase inhibitor is mediated by P21cip1

MEB [n-butyrate 2-(4-morpholinyl) ethyl butyrate hydrochloride], a histone deacetylase inhibitor and G1 blocker, has been shown to induce unresponsiveness in antigen-activated Th1 cells. MEB was tested for here for its ability to inactivate naive alloantigen-specific T cells from DBA/2 and C57BL/10 mice. Since T cells from these two strains of mice have been shown to differ in their cell cycle regulation, it we hoped that this comparison would provide information concerning the role of cycle regulatory proteins in mediating MEB-induced T cell unresponsiveness. MEB inhibited proliferation in a one-way mixed lymphocyte reaction (MLR) in which spleen cells from DBA/2 mice (H-2d) or C57BL/10 mice (H-2b) were stimulated with spleen cells from C57BL/10 or DBA/2 mice, respectively. C57BL/10 responder T cells isolated from the MEB-treated primary MLR remained unresponsive to alloantigen following restimulation in a secondary MLR that did not contain MEB. T cells from DBA/2 mice were less sensitive to MEB-induced unresponsiveness and required a longer exposure or pretreatment with IL-2 to become tolerant. In all cases responsiveness to MEB-induced tolerance in the alloantigen-stimulated T cells corresponded with the levels of the cyclin-dependent kinase inhibitor p21cip1. Additional experiments showed that T cells from p21cip1-deficient mice, unlike T cells from p21cip1 wild-type littermates, were resistant to MEB-induced tolerance. These results underscore the role of p21cip1 in mediating T cell tolerance induced by the histone deacetylase inhibitor MEB.

T-cell subsets in the pathogenesis of human asthma

Genetic predisposition and environmental instructions tune thresholds for activation of T cells, other inflammatory cells, and resident tissue cells in asthmatic inflammation. Selective migration of peripheral-blood T cells to the lungs, their survival and reactivation in the submucosa, and their effector functions represent sequential immunologic events. Activation-induced T-cell death and peripheral T-cell tolerance are critical events in disease pathogenesis. As a mechanism for peripheral Th2 response in atopic diseases, particularly, the high interferon (IFN)-gamma-producing Th1 compartment of activated effector T cells shows increased activation-induced cell death, skewing the immune response toward surviving Th2 cells in allergic asthma. After migration to asthmatic lung, these cells switch on effector cytokines and induce bronchial epithelial apoptosis with mainly IFN-gamma, tumor necrosis factor (TNF)-alpha, and Fas-ligand. In addition, skewing of allergen-specific effector T cells to T-regulatory cells appears to be an essential event in the control of harmful immune response induced by allergens as a possible means for remedy.

Elevated levels of soluble Fas (APO-1, CD95), soluble Fas ligand, and matrix metalloproteinase-3 in sera from patients with active untreated adult onset Still’s disease

To investigate serum levels of soluble Fas (sFas), soluble Fas ligand (sFas-L), and matrix metalloproteinase-3 (MMP-3) in patients with active untreated adult onset Still's disease (AOSD). Serum levels of sFas, sFas-L, and MMP-3 were determined by enzyme-linked immunosorbent assays in 20 patients with active untreated AOSD, 20 patients with active rheumatoid arthritis (RA), and 20 healthy controls. Linear regression was used to evaluate the correlation between clinical activity scores and serum levels of sFas, sFas-L, and MMP-3 in AOSD patients. Significantly higher levels of sFas, sFas-L, and MMP-3 in sera were found in active untreated AOSD patients compared to healthy controls. Serum levels of sFas, sFas-L, and MMP-3 correlated well with clinical activity scores of AOSD patients (r=0.467, 0.694, and 0.798, respectively). There was a significant correlation between CRP values and serum levels of sFas-L as well as MMP-3 (r=0.583 and r=0.582, respectively, both p<0.01), and a positive correlation between serum sFas-L levels and serum MMP-3 levels (r=0.726, p<0.001) in AOSD patients. Significantly higher levels of serum sFas-L were found in AOSD patients than in RA patients. Serum levels of sFas, sFas-L, and MMP-3 fluctuated and were found to be parallel to disease activity of AOSD. sFas, sFas-L, and MMP-3, which were significantly elevated in sera of active untreated AOSD patients and paralleled disease activity, may be involved in the pathogenesis of this disease. Further studies are needed to determine the pathophysiologic role of sFas, sFas-L, and MMP-3 in AOSD.

Seasonal changes of proapoptotic soluble Fas ligand level in allergic rhinitis combined with asthma

The function of apoptosis is to eliminate unnecessary or dangerous cells. The balance between production and death is important in the control of cell numbers within physiological ranges. Cells involved in allergic reactions may have altered apoptosis. The aim of this study was to examine the seasonal changes of programmed cell death in children with pollen allergy. We measured serum levels of soluble Fas (sFas) and soluble Fas ligand (sFasL), and examined whether there was any correlation between soluble apoptosis markers and development of asthma and or rhinitis in children with pollen allergy. We examined two groups of patients with ragweed pollen allergy. The first group consisted of 17 children with 'rhinitis only'. The second group consisted of 16 children with 'asthma + rhinitis'. For seasonal analysis we pooled the two groups and termed this the 'ragweed sensitive' group (n = 33, 5-18 yr, 25 boys, eight girls). Measurements (sFas and sFasL) were taken during the ragweed pollen allergy season, while control measurements were performed during the symptom-free period. There was no difference in sFas levels measured during and after [1941 +/- 68, 1963 +/- 83 pg/ml (mean+/-s.e.m, respectively)] the pollen season in the 'ragweed sensitive' group. The sFasL level showed seasonal change, which was significantly higher (p = 0.0086) in the symptomatic period compared to the symptom-free state (99 +/- 13 and 53 +/- 16 pg/ml, respectively). There was a difference between the 'rhinitis only' and the 'asthma + rhinitis' groups in the measured parameters of apoptosis. Children having allergic rhinitis combined with asthma had a significantly (p = 0.03) higher sFas level in the symptom-free state than the 'rhinitis only' group did (2115 +/- 156 and 1820 +/- 52 pg/ml, respectively). During the allergic symptom state the sFasL level of the 'asthma + rhinitis' group was significantly higher (p = 0.025) than that of the 'rhinitis only' group (125 +/- 20 and 75 +/- 14 pg/ml, respectively). In conclusion, the increased level of sFasL during the pollen season may signal its role in the pathogenesis of allergic airway diseases. There was no seasonal change in sFas levels in the examined ragweed allergic group, however in the symptomatic period we observed a diminished level of antiapoptotic factor (sFas) and an elevated level of proapoptotic factor (sFasL) if there was a combined disease with pollen allergic asthma. We suggest that there is a deviation in the apoptotic reaction in children that may increase the seasonal allergic inflammation.

Effect of interleukin-15 on effector and regulatory function of anti-CD3/anti-CD28-stimulated CD4(+) T cells

Autologous transfer of anti-CD3/anti-CD28 (CD3/CD28)-activated CD4(+) T cells may benefit patients receiving autologous stem cell transplant with severe CD4 lymphopenia. Interleukin (IL)-15, an IL-2-like cytokine that promotes T cell survival may enhance immune reconstitution in conjunction with adoptive immunotherapy. We investigated the effect of IL-15 on effector and regulatory function of CD3/CD28-activated CD4(+) T cells. IL-15 upregulated CD45RO and CD25 whereas it down regulated CD62L expression of CD3/CD28-stimulated CD4(+) T cells. Both type 1 (IFN-gamma, tumor necrosis factor (TNF)-alpha) and type 2 (IL-5 and IL-10) production by CD3/CD28-activated CD4(+) T cells was further enhanced by IL-15. Co-culture experiments revealed that CD3/CD28-activated CD4(+) T cells down regulated proliferation of autologous peripheral blood lymphocytes (PBLs) and CD8(+) PBL subsets upon TCR ligation, a contact-dependent effect that was further enhanced by pretreatment with IL-15. Flow cytometric analysis of cell mixture with carboxyfluorescein diacetate succinimidyl ester and Annexin-V-PE staining revealed that CD3/CD28+IL-15-activated CD4(+) T cells showed increased apoptosis over CD4(+) T cells stimulated with CD3/CD28 alone. Taken together, pretreatment of CD3/CD28-activated CD4(+) T cells with IL-15 may increase regulatory function but may aggravate activation-induced apoptosis of CD3/CD28 CD4(+) T cells.

Granzyme B is critical for T cell receptor-induced cell death of type 2 helper T cells

Although CD95L is required for T cell receptor (TCR)-induced cell death (TCR-ICD) in T helper 1 cells, the molecular mechanisms mediating TCR-ICD in Th2 cells are unknown. We found that death receptors were not involved in TCR-ICD of Th2 cells because blocking their cognate ligands had no effect on apoptosis of activated Th2 cells. Furthermore, we showed that caspases were not actively involved in TCR-ICD of Th2 cells. However, inhibition of granzyme B (GrB) activity abolished TCR-ICD in Th2 cells but not Th1 cells. Likewise, Th2 cells derived from GrB-deficient mice were resistant to TCR-ICD, and GrB deficiency or inhibition of GrB activity consequently enhanced the production of Th2 cytokines. GrB-deficient mice exhibited increased susceptibility to allergen-induced asthma. Thus, GrB plays a critical role in the TCR-ICD of Th2 cells.

Persistence and function of central and effector memory CD4+ T cells following infection with a gastrointestinal helminth

Immunity in the gastrointestinal tract is important for resistance to many pathogens, but the memory T cells that mediate such immunity are poorly characterized. In this study, we show that following sterile cure of a primary infection with the gastrointestinal parasite Trichuris muris, memory CD4+ T cells persist in the draining mesenteric lymph node and protect mice against reinfection. The memory CD4+ T cells that developed were a heterogeneous population, consisting of both CD62L(high) central memory T cells (T(CM)) and CD62L(low) effector memory T cells (T(EM)) that were competent to produce the Th type 2 effector cytokine, IL-4. Unlike memory T cells that develop following exposure to several other pathogens, both CD4+ T(CM) and T(EM) populations persisted in the absence of chronic infection, and, critically, both populations were able to transfer protective immunity to naive recipients. CD62L(high)CD4+ T(CM) were not apparent early after infection, but emerged following clearance of primary infection, suggesting that they may be derived from CD4+ T(EM). Consistent with this theory, transfer of CD62L(low)CD4+ T(EM) into naive recipients resulted in the development of a population of protective CD62L(high)CD4+ T(CM). Taken together, these studies show that distinct subsets of memory CD4+ T cells develop after infection with Trichuris, persist in the GALT, and mediate protective immunity to rechallenge.

A network-based analysis of allergen-challenged CD4+ T cells from patients with allergic rhinitis

We performed a network-based analysis of DNA microarray data from allergen-challenged CD4(+) T cells from patients with seasonal allergic rhinitis. Differentially expressed genes were organized into a functionally annotated network using the Ingenuity Knowledge Database, which is based on manual review of more than 200,000 publications. The main function of this network is the regulation of lymphocyte apoptosis, a role associated with several genes of the tuber necrosis factor superfamily. The expression of TNFRSF4, one of the genes in this family, was found to be 48 times higher in allergen-challenged cells than in diluent-challenged cells. TNFRSF4 is known to inhibit apoptosis and to enhance Th2 proliferation. Examination of a different material of allergen-stimulated peripheral blood mononuclear cells showed a higher number of interleukin-4(+) type 2 CD4(+) T (Th2) cells in patients than in controls (P<0.01), as well as a higher number of non-apoptotic Th2 cells in patients (P<0.01). The number of Th2 cells expressing TNFRSF4, TNFSF7 and TNFRSF1B was also significantly higher in patients. Treatment with anti-TNFSF4 resulted in a significantly decreased number of Th2 cells (P<0.05). A logical inference from all this is that the proliferation of allergen-challenged Th2 cells is associated with a decreased apoptosis of Th2 cells and an increase in TNFRSF4 signalling.

CD1d expression on hemopoietic cells promotes CD4+ Th1 response in coxsackievirus B3 induced myocarditis

Coxsackievirus B3 induced murine myocarditis depends upon CD1d expression and upon a population of CD1d-restricted Vgamma4+ T cells. Infection upregulates CD1d expression in CD4+ T cells. Bone marrow chimeras were made between BALB/c and BALB/c CD1d-/- mice and showed that CD1d expression in either hemopoietic and non-hemopoietic cells induces myocarditis, although CD1d expression on hemopoietic cells was more effective in increasing Vgamma4+ cell numbers and activation, and CD4+ IFNgamma+ cell response than CD1d expression on non-hemopoietic cells. Co-culture of enriched CD4+ cells from infected CD1d-/- and BALB/c mice with Vgamma4+ T cells demonstrated that the Vgamma4+ cells bias the CD4+ cell response to the Th1 phenotype through CD1d. Anti-CD1d antibody effectively blocked promotion of IFNgamma expression by the CD4+ cell population. These results show that Vgamma4+ cells modulate developing adaptive immunity through recognition of CD1d on CD4+ T cells, and that this interaction, more than Vgamma4+ cell interaction with infected cardiocytes, determines pathogenicity.

Activation of the integrated stress response during T helper cell differentiation

Adaptive immune responses require clonal expansion and differentiation of naive T cells into cytokine-secreting effector cells. After priming via signals through the T cell receptor, naive T helper cells express cytokine mRNA but do not secrete cytokine protein without additional T cell receptor stimulation. Here we show that primed T cells demonstrated phosphorylation of eukaryotic initiation factor 2-alpha (eIF2alpha), a 'collapsed' polysome profile, increased expression of stress-response genes and accumulation of cytoplasmic granules associated with RNA-binding proteins, all features of the integrated stress response. Restimulation of the cells resulted in rapid eIF2alpha dephosphorylation, ribosomal mRNA loading and cytokine secretion. Interference with the function of granule-associated proteins or accumulation of phosphorylated eIF2alpha enhanced release of interleukin 4 during T helper type 2 priming. Therefore, T lymphocytes require components of the integrated stress response to uncouple differentiation from the execution of effector functions.

Chronic Inflammation, Immune Response, and Infection in Abdominal Aortic Aneurysms

Abdominal aortic aneurysms (AAA) are associated with atherosclerosis, transmural degenerative processes, neovascularization, decrease in content of vascular smooth muscle cells, and a chronic infiltration, mainly located in the outer aortic wall. The chronic infiltration consists mainly of macrophages, lymphocytes, and plasma cells. The dominant cells are Th2 restricted CD3+ lymphocytes expressing interleukine 4, 5, 8, and 10, and tumor necrosis factor-alpha for regulation of the local immune response. They also produce interferon-gamma and CD40 ligand to stimulate surrounding cells to produce matrix metalloproteases and cysteine proteases for aortic matrix remodeling. The lymphocyte activation may be mediated by microorganisms as well as autoantigens generated from vascular structural proteins, perhaps through molecular mimicry. As in autoimmune diseases, the risk of AAA is increased by certain genotypes concerning human leucocyte antigen class II. These types are also associated with increased aneurysmal inflammation indicating a genetic susceptibility to aortic inflammation. Chlamydia pneumoniae is often detected in AAA but the validity of the methods can be questioned, and two small antibiotic trials have been disappointing. However, serum antibodies against C. pneumoniae have been associated with AAA growth and cross-react with AAA wall proteins. Thus, immune responses mediated by microorganisms and autoantigens may play a pivotal role in AAA pathogenesis.

Fas-positive T cells regulate the resolution of airway inflammation in a murine model of asthma

Persistent airway inflammation, mucus production, and airway hyperreactivity are the major contributors to the frequency and severity of asthma. Why lung inflammation persists in asthmatics remains unclear. It has been proposed that Fas-mediated apoptosis of inflammatory cells is a fundamental mechanism involved in the resolution of eosinophilic airway inflammation. Because infiltrating eosinophils are highly sensitive to Fas-mediated apoptosis, it has been presumed that direct ligation of Fas on eosinophils is involved. Here, we utilize adoptive transfers of T cells to demonstrate that the delayed resolution of eosinophilia in Fas-deficient mice is a downstream effect of Fas deficiency on T cells, not eosinophils. Interestingly, the mice that received Fas-deficient T cells, but not the controls, developed a persistent phase of inflammation that failed to resolve even 6 wk after the last challenge. This persistent phase correlated with decreased interferon (IFN)γ production by Fas-deficient T cells and could be reproduced with adoptive transfer of IFNγ-deficient T cells. These data demonstrate that Fas deficiency on T cells is sufficient for the development of long-term allergic airway disease in mice and implies that deregulation of death receptors such as Fas on human T cells could be an important factor in the development and/or chronic nature of asthma.

SUMOylation Interferes with CCAAT/Enhancer-Binding Protein β-Mediated c-mycRepression, but Not IL-4 Activation in T Cells

The transcription factor C/EBPbeta transactivates the IL-4 gene in murine T lymphocytes and facilitates Th2 cell responses. In this study, we demonstrate that C/EBPbeta also acts as a repressor of T cell proliferation. By binding to the c-myc promoter(s), C/EBPbeta represses c-Myc expression and, therefore, arrests T cells in the G1 phase of the cell cycle. For C/EBPbeta-mediated repression, the integrity of its N-terminal transactivation domain is essential whereas the central regulatory domain is dispensable. This central regulatory domain is sumoylated in vivo which leads to an alteration of the activity of C/EBPbeta. Whereas sumoylation does not affect the C/EBPbeta-mediated activation of the IL-4 gene, it relieves its repressive effect on c-Myc expression and T cell proliferation. Similar to several other transcription factors, sumoylation redistributes nuclear C/EBPbeta and targets it to pericentric heterochromatin. These results suggest an important role of sumoylation in adjusting the finely tuned balance between proliferation and differentiation in peripheral T cells which is controlled by C/EBPbeta.

In silico simulations suggest that Th‐cell development is regulated by both selective and instructive mechanisms

Th-cell differentiation is highly influenced by the local cytokine environment. Although cytokines such as IL-12 and IL-4 are known to polarize the Th-cell response towards Th1 or Th2, respectively, it is not known whether these cytokines instruct the developmental fate of uncommitted Th cells or select cells that have already been committed through a stochastic process. We present an individual based model that accommodates both stochastic and deterministic processes to simulate the dynamic behaviour of selective versus instructive Th-cell development. The predictions made by each model show distinct behaviours, which are compared with experimental observations. The simulations show that the instructive model generates an exclusive Th1 or Th2 response in the absence of an external cytokine source, whereas the selective model favours coexistence of the phenotypes. A hybrid model, including both instructive and selective development, shows behaviour similar to either the selective or the instructive model dependent on the strength of activation. The hybrid model shows the closest qualitative agreement with a number of well-established experimental observations. The predictions by each model suggest that neither pure selective nor instructive Th development is likely to be functional as exclusive mechanisms in Th1/Th2 development.

cFLIP regulation of lymphocyte activation and development

Cellular caspase-8 (FLICE)-like inhibitory protein (cFLIP) was originally identified as an inhibitor of death-receptor signalling through competition with caspase-8 for recruitment to FAS-associated via death domain (FADD). More recently, it has been determined that both cFLIP and caspase-8 are required for the survival and proliferation of T cells following T-cell-receptor stimulation. This paradoxical finding launched new investigations of how these molecules might connect with signalling pathways that link to cell survival and growth following antigen-receptor activation. As discussed in this Review, insight gained from these studies indicates that cFLIP and caspase-8 form a heterodimer that ultimately links T-cell-receptor signalling to activation of nuclear factor-kappaB through a complex that includes B-cell lymphoma 10 (BCL-10), mucosa-associated-lymphoid-tissue lymphoma-translocation gene 1 (MALT1) and receptor-interacting protein 1 (RIP1).

Unexpected prolonged presentation of influenza antigens promotes CD4 T cell memory generation

The kinetics of presentation of influenza virus–derived antigens (Ags), resulting in CD4 T cell effector and memory generation, remains undefined. Naive influenza-specific CD4 T cells were transferred into mice at various times after influenza infection to determine the duration and impact of virus-derived Ag presentation. Ag-specific T cell responses were generated even when the donor T cells were transferred 3–4 wk after viral clearance. Transfer of naive CD4 T cells during early phases of infection resulted in a robust expansion of highly differentiated effectors, which then contracted to a small number of memory T cells. Importantly, T cell transfer during later phases of infection resulted in a modest expansion of effectors with intermediate phenotypes, which were capable of persisting as memory with high efficiency. Thus, distinct stages of pathogen-derived Ag presentation may provide a mechanism by which T cell heterogeneity is generated and diverse memory subsets are maintained.

Dendritic cell immaturity during infancy restricts the capacity to express vaccine-specific T-cell memory

The capacity of the immune system in infants to develop stable T-cell memory in response to vaccination is attenuated, and the mechanism(s) underlying this developmental deficiency in humans is poorly understood. The present study focuses on the capacity for expression of in vitro recall responses to tetanus and diphtheria antigens in lymphocytes from 12-month-old infants vaccinated during the first 6 months of life. We demonstrate that supplementation of infant lymphocytes with "matured" dendritic cells (DC) cultured from autologous CD14+ precursors unmasks previously covert cellular immunity in the form of Th2-skewed cytokine production. Supplementation of adult lymphocytes with comparable prematured autologous DC also boosted vaccine-specific T-cell memory expression, but in contrast to the case for the infants, these cytokine responses were heavily Th1 skewed. Compared to adults, infants had significantly fewer circulating myeloid DC (P < 0.0001) and plasmacytoid DC (P < 0.0001) as a proportion of peripheral blood mononuclear cells. These findings suggest that deficiencies in the numbers of antigen-presenting cells and their functional competence at 12 months of age limit the capacity to express effector memory responses and are potentially a key factor in reduced vaccine responsiveness in infants.

Granzyme B, a new player in activation-induced cell death, is down-regulated by vasoactive intestinal peptide in Th2 but not Th1 effectors

Following antigenic stimulation and differentiation, Th1 and Th2 effector cells contribute differently to cellular and humoral immunity. Vasoactive intestinal peptide (VIP) induces Th2 responses by promoting Th2 differentiation and survival. In this study, we investigate the mechanisms for the protective effect of VIP against activation-induced cell death (AICD) of Th2 effectors. Surprisingly, microarray and protein data indicate that VIP prevents the up-regulation of granzyme B (GrB) in Th2 but not Th1 effectors. This is the first report of GrB expression in Th cells and of its involvement in activation-induced apoptosis. The enhanced responsiveness of Th2 cells to VIP is probably due to the higher expression of VIP receptors. The effect of VIP on Th2 survival and GrB expression is mediated through the VIP receptors 1 and 2 and cAMP signaling through exchange protein activated by cAMP and, to a lesser degree, protein kinase A. In addition to effects on GrB, VIP also down-regulates Fas ligand (FasL) and perforin (Pfr) expression. The extrinsic Fas/FasL pathway and the intrinsic GrB-dependent pathway act independently in inducing AICD. The mechanisms by which GrB induces cell death in Th1/Th2 effectors include both fratricide and suicide. Fratricide killing, prevalent in wild-type cells, is calcium and Pfr dependent, whereas the cell death of Pfr-deficient Th cells involves Fas and GrB but is calcium independent. This study identifies GrB as a new significant player in Th1/Th2 AICD and characterizes two mechanisms for the protective effect of VIP on Th2 survival, i.e., the down-regulation of GrB and FasL expression.

The immunoregulatory effects of gangliosides involve immune deviation favoring type-2 T cell responses

Gangliosides, sialic acid-containing glycosphingolipids present in most cell membranes, are thought to participate in the maintenance of immune privilege and tumor-induced immunosuppression. However, the mechanisms responsible for their immunomodulatory activity remain poorly understood. The purpose of this study was to investigate whether gangliosides are able to modulate the balance of type-1/type-2 T cell responses and to characterize the cellular mechanisms involved. The effects of different gangliosides on anti-CD3-stimulated murine splenocytes and purified T cells were studied. The presence of gangliosides during T cell activation reduced the expression of interferon-gamma (IFN-gamma) and enhanced that of interleukin (IL)-4, suggesting a shift toward a type-2 response. Intracellular cytokine staining demonstrated that gangliosides inhibited IFN-gamma production in CD4+, CD8+, and natural killer (NK)1.1+ cell populations and enhanced IL-4 in CD4+ T cells. The ganglioside-mediated enhancement in IL-4 production was independent of changes in endogenous IFN-gamma, did not occur with cells from CD1d-deficient mice, and was partially inhibited by anti-CD1d antibodies. The inhibitory effects on IFN-gamma were independent of endogenous IL-4 or the presence of NKT cells and were unaffected by anti-CD1d antibodies. These results suggest that gangliosides may modify the immunological environment by promoting immune deviation in favor of type-2 T cell responses.

T cell-mediated activation and regulation of anti-chromatin B cells

We have taken an immunoglobulin transgenic approach to study how self-reactive B cells are held in check in healthy mice and what parameters contribute to their activation in autoimmunity. Using this strategy, we have documented that a population of anti-chromatin B cells migrate to the periphery. In a healthy background, these cells have a reduced lifespan, appear developmentally arrested, and localize primarily to the T/B cell interface in the spleen. Importantly, they are capable of differentiating into antibody-forming cells when provided with T cell help. T(H)1 and T(H)2 cells induce IgG2a and IgG1 autoantibodies, respectively. In the context of the autoimmune-prone lpr/lpr or gld/gld mutations, these autoreactive B cells populate the B cell follicle, and this is dependent upon CD4 T cells. However, after 10 weeks of age serum autoantibodies are produced. We hypothesize that control of autoantibody production in young autoimmune-prone mice is regulated by the counterbalancing influence of regulatory T cells. We show that while autoantibody production is blocked in the context of regulatory T cells, early events characterizing a productive T cell-B cell interaction are not disturbed, with the notable exceptions of T(H) ICOS levels and IFN-gamma and IL-10 production.

Systemic dissemination and persistence of Th2 and type 2 cells in response to infection with a strictly enteric nematode parasite

Oral infection with the nematode parasite Heligmosomoides polygyrus H. polygyrus is entirely restricted to the small intestine. Although the evoked Th2 response has been extensively studied in secondary lymphoid organs, little is known about the systemic dissemination of Th2 cells or type 2 associated eosinophils and basophils. In this study we use bicistronic 4get IL-4 reporter mice to directly visualize the type 2 response to H. polygyrus infection. We observed that CD4(+)/GFP(+) Th2 cells spread systemically and found that these cells accumulated in nonlymphoid "hot spots" in the liver, the lung airways, and the peritoneal cavity. Interestingly, the total number of Th2 cells in the peritoneal cavity was comparable to those found in the draining mesenteric lymph node or the spleen. Peritoneal Th2 cells were distinguished by an exceptionally low apoptotic potential and high expression of the intestinal homing receptor alpha(4)beta(7) integrin. CD4(+)/GFP(+) Th2 cells from these peripheral sites were fully functional as indicated by rapid IL-4 production upon polyclonal or Ag-specific restimulation. Th2 cells persisted in the intestinal tissue and the peritoneal cavity of drug-cured mice for weeks. The presence of peripheral memory Th2 cells in the intestine might be crucial for immunity to recall infections. These findings have important implications for the design of vaccination strategies because it may be necessary to establish and maintain memory CD4(+) T cells at the potential future site of infection.

Chronic morphine treatment promotes specific Th2 cytokine production by murine T cells in vitro via a Fas/Fas ligand-dependent mechanism

Improper homeostasis of Th1 and Th2 cell differentiation can promote pathological immune responses such as autoimmunity and asthma. A number of factors govern the development of these cells including TCR ligation, costimulation, death effector expression, and activation-induced cell death (AICD). Although chronic morphine administration has been shown to selectively promote Th2 development in unpurified T cell populations, the direct effects of chronic morphine on Th cell skewing and cytokine production by CD4(+) T cells have not been elucidated. We previously showed that morphine enhances Fas death receptor expression in a T cell hybridoma and human PBL. In addition, we have demonstrated a role for Fas, Fas ligand (FasL), and TRAIL in promoting Th2 development via killing of Th1 cells. Therefore, we analyzed whether the ability of morphine to affect Th2 cytokine production was mediated by regulation of Fas, FasL, and TRAIL expression and AICD directly in purified Th cells. We found that morphine significantly promoted IL-4 and IL-13 production but did not alter IL-5 or IFN-gamma. Furthermore, morphine enhanced the mRNA expression of Fas, FasL and TRAIL and promoted Fas-mediated AICD of CD4(+) T cells. Additionally, blockade of Fas/FasL interaction by anti-FasL inhibited the morphine-induced production of IL-4 and IL-13 and AICD of CD4(+) T cells. These results suggest that morphine preferentially enhances Th2 cell differentiation via killing of Th1 cells in a Fas/FasL-dependent manner.

CD4-dependent signaling is required for a late checkpoint during Th2 development associated with resistance to activation-induced cell death

Previous studies have found that class II-restricted T cells from CD4-deficient mice reconstituted with a tail-less CD4 transgene have a specific defect in the development of Th2 effector cells; however, the reason for this defect was not clear. Following stimulation with a high potency peptide and exogenous IL-4, CD4-dependent signaling is required for optimal generation of a Th2 effector population. However, initial IL-4 and GATA-3 transcription is appropriately induced, suggesting that the initial stages of Th2 development are intact and independent of CD4 after priming with a strong agonist peptide. In addition to the defect in Th2 development, CD4 mutant T cells are also relatively resistant to activation-induced cell death (AICD). Furthermore, inhibition of AICD in wild-type T cells causes a defect in Th2 development similar to that seen in the CD4 mutant T cells. These data support the hypothesis that CD4-dependent signaling pathways regulate a distinct checkpoint in the expansion and commitment phase of Th2 development, which is related to dysregulation of AICD.

Feedback regulation of autoimmunity via TCR-centered regulation

The complexity of a self-regulatory system demands a balance between effectors and regulators; that is, it is necessary for both cell types to exist. Regulation of self-reactive T cells can occur at several complementary but different levels: (1) at the level of priming itself: for example, inhibition of expansion of antigen-reactive T cells by regulatory CD4+ CD25+ T cells; (2) after the priming of self-reactive T cells, regulatory T-cell populations with reactivity to distinct self-determinants derived from the T-cell receptor (TCR) can be engaged via a negative feedback mechanism. Thus, these mechanisms ensure induction of effective and appropriately limited responses against foreign antigens while preventing autoreactivity from inflicting self-damage.

The influence of effector T cells and Fas ligand on lupus-associated B cells

Circulating autoantibodies against dsDNA and chromatin are a characteristic of systemic lupus erythematosus in humans and many mouse models of this disease. B cells expressing these autoantibodies are normally regulated in nonautoimmune-prone mice but are induced to secrete Abs following T cell help. Likewise, anti-chromatin autoantibody production is T cell-dependent in Fas/Fas ligand (FasL)-deficient (lpr/lpr or gld/gld) mice. In this study, we demonstrate that Th2 cells promote anti-chromatin B cell survival and autoantibody production in vivo. FasL influences the ability of Th2 cells to help B cells, as Th2-gld/gld cells support higher titers of anti-chromatin Abs than their FasL-sufficient counterparts and promote anti-chromatin B cell participation in germinal centers. Th1 cells induce anti-chromatin B cell germinal centers regardless of FasL status; however, their ability to stimulate anti-chromatin Ab production positively correlates with their level of IFN-gamma production. This distinction is lost if FasL-deficient T cells are used: Th1-gld/gld cells promote significant titers of anti-chromatin Abs regardless of IFN-gamma production levels. Thus, FasL from effector T cells plays an important role in determining the fate of anti-chromatin B cells.

Targeting memory Th2 cells for the treatment of allergic asthma

Th2 memory cells play an important role in the pathogenesis of allergic asthma. Evidence from patients and experimental models indicates that memory Th2 cells reside in the lungs during disease remission and, upon allergen exposure, become activated effectors involved in disease exacerbation. The inhibition of memory Th2 cells or their effector functions in allergic asthma influence disease progression, suggesting their importance as therapeutic targets. They are allergen specific and can potentially be suppressed or eliminated using this specificity. They have distinct activation, differentiation, cell surface phenotype, migration capacity, and effector functions that can be targeted singularly or in combination. Furthermore, memory Th2 cells residing in the lungs can be treated locally. Capitalizing on these unique attributes is important for drug development for allergic asthma. The aim of this review is to present an overview of therapeutic strategies targeting Th2 memory cells in allergic asthma, emphasizing Th2 generation, differentiation, activation, migration, effector function, and survival.

Acellular Bordetella pertussis vaccine enhances mucosal interleukin-10 production, induces apoptosis of activated Th1 cells and attenuates colitis in Galphai2-deficient mice

Mice deficient for the inhibitory G protein subunit alpha2 (Galphai2(-/-)) spontaneously develop a progressive inflammatory bowel disease resembling ulcerative colitis, and have a T helper 1 (Th1)-dominated immune response prior to onset of colitis, which is further augmented after the onset of disease. The present study was performed to investigate whether the Galphai2(-/-) mice were able to down-regulate the Th1-dominated inflammatory mucosal immune response and/or induce an anti-inflammatory Th2/T regulatory response and thereby diminish the severity of colitis following treatment with acellular Bordetella pertussis vaccine. The acellular vaccine against B. pertussis, the causative agent of whooping cough, has been demonstrated to induce a Th2-mediated response in both man and mice. We therefore treated Galphai2(-/-) mice intraperitoneally with a three-component acellular B. pertussis vaccine. The treated Galphai2(-/-) mice showed significantly increased interleukin (IL)-10 production in intestinal tissue, associated with significantly reduced colitis and decreased mortality, compared to untreated Galphai2(-/-) mice. The attenuation of colitis in Galphai2(-/-) mice was due, at least partly, to the B. pertussis surface antigen filamentous haemagglutinin (FHA), which almost completely inhibited proliferation of CD4(+) T cells and stimulated apoptosis of activated CD4(+) T helper 1 cells. In conclusion, the three-component acellular B. pertussis vaccine containing filamentous haemagglutinin increases the production of IL-10 in the intestinal mucosa, induces apoptosis of activated Th1 cells and attenuates colitis in Galphai2(-/-) mice.

Neurosphere-derived multipotent precursors promote neuroprotection by an immunomodulatory mechanism

In degenerative disorders of the central nervous system (CNS), transplantation of neural multipotent (stem) precursor cells (NPCs) is aimed at replacing damaged neural cells. Here we show that in CNS inflammation, NPCs are able to promote neuroprotection by maintaining undifferentiated features and exerting unexpected immune-like functions. In a mouse model of chronic CNS inflammation, systemically injected adult syngeneic NPCs use constitutively activated integrins and functional chemokine receptors to selectively enter the inflamed CNS. These undifferentiated cells survive repeated episodes of CNS inflammation by accumulating within perivascular areas where reactive astrocytes, inflamed endothelial cells and encephalitogenic T cells produce neurogenic and gliogenic regulators. In perivascular CNS areas, surviving adult NPCs induce apoptosis of blood-borne CNS-infiltrating encephalitogenic T cells, thus protecting against chronic neural tissue loss as well as disease-related disability. These results indicate that undifferentiated adult NPCs have relevant therapeutic potential in chronic inflammatory CNS disorders because they display immune-like functions that promote long-lasting neuroprotection.

Death receptor-mediated suicide: a novel target of autoimmune disease treatment

In the thymus, based on the reactivity of their T-cell receptor with self-MHC and antigenic peptides, developing immature T-cells undergo positive and negative selection. Cells recognising self-peptides and MHC with high affinity are considered autoreactive, and thus potentially harmful, and are eliminated by induction of apoptotic cell death. Thymic negative selection is, however, only incomplete and autoreactive T-cells escape into the periphery. It is not the presence of autoreactive mature T- and B-lymphocytes as the underlying cause of tissue destruction and development of autoimmune diseases, but their uncontrolled and excessive clonal expansion upon activation by self-antigen. Thus, potent regulatory mechanisms must keep these autoreactive cells under control to avoid their inappropriate activation. Recent evidence indicates that death receptors of the tumour necrosis factor receptor family play a central role in mediating antigen receptor-induced suicide of autoreactive T-lymphocytes. Defects in these apoptosis-inducing regulatory mechanisms may result in the development of autoimmune diseases. Therefore, enhancing the cell's own suicide program, offers a most attractive therapeutic target for the treatment of autoimmune diseases.

Th2-type immune response observed in healthy individuals to sonicate antigen prepared from the most prevalentMycobacterium tuberculosisstrain with single copy of IS6110

Different Mycobacterium tuberculosis strains operate different immune evasion strategies for their survival in the host. This mainly depends on the virulence of the strain and the host immune responses. The most virulent strains are actively involved in the transmission, widely spread in the community and induce differential immune responses. We evaluated the immune response of a sonicate antigen prepared from one predominant strain (S7) from M. tuberculosis harbouring a single copy of IS6110. Significant lymphoproliferative response against purified protein derivative from tubercle bacillus (PPD) and H37Rv antigens was observed in PPD positive normal individuals and tuberculosis patients. Interferon-gamma (IFN-gamma) levels against these antigens were significantly increased in normal individuals but not in tuberculosis patients. The antigen S7 showed marginal T-cell proliferation but did not induce IFN-gamma secretion in both groups. Conversely, it induced significantly high levels of cytokine interleukin 4 (IL-4) in normal individuals. The macrophage cytokines, IL-12 and tumour necrosis factor alpha (TNF-alpha), did not show S7 antigen specific stimulation. The intracellular cytokine further confirmed an increase in IL-4(+)/CD4+ T-cells and a decrease in IFN-gamma(+)/CD4+ T-cells upon stimulation. The antibody response showed an increase in IgG and IgA levels against this antigen in normal individuals. These observations suggest that antigen S7 modulates the immune response towards T helper cell type 2 by suppressing T helper cell type 1 protective immune response in PPD positive normal individuals. We speculate that some components of this sonicate antigen are associated with immunosuppressive response.

Effects of dietary n-3 polyunsaturated fatty acids on T-cell membrane composition and function

Dietary n-3 PUFA have been shown to attenuate T-cell-mediated inflammation, in part, by suppressing T-cell activation and proliferation. n-3 PUFA have also been shown to promote apoptosis, another important mechanism for the prevention of chronic inflammation by maintaining T-cell homeostasis through the contraction of populations of activated T cells. Recent studies have specifically examined Fas death receptor-mediated activation-induced cell death (AICD), since it is the form of apoptosis associated with peripheral T-cell deletion involved in immunological tolerance and T-cell homeostasis. Data from our laboratory indicate that n-3 PUFA promote AICD in T helper 1 polarized cells, which are the mediators of chronic inflammation. Since Fas and components of the death-inducing signaling complex are recruited to plasma membrane microdomains (rafts), the effect of dietary n-3 PUFA on raft composition and resident protein localization has been the focus of recent investigations. Indeed, there is now compelling evidence that dietary n-3 PUFA are capable of modifying the composition of T-cell membrane microdomains (rafts). Because the lipids found in membrane microdomains actively participate in signal transduction pathways, these results support the hypothesis that dietary n-3 PUFA influence signaling complexes and modulate T-cell cytokinetics in vivo by altering T-cell raft composition.

T cells in G1 provide a memory-like response to secondary stimulation

The commitment of naive T cells to proliferate is a function of the strength and duration of stimuli mediated by the TCR and coreceptors. Ranges of 2-20 h of stimulation have been reported as necessary in vitro. Whether T cells actually experience uninterrupted stimulation for such long periods under physiological conditions is controversial. Here we ask whether commitment to proliferate requires continuous stimulation, or can T cells integrate intermittent periods of stimulation. T cells were stimulated for two short-term (subthreshold) periods (5-7 h) either sequentially or separated by an interval of rest. Naive lymph node T cells were able to integrate interrupted stimulation, even when the duration of rest was as long as 2 days. Furthermore, when short-term-stimulated T cells were separated by density, three populations were observed: low density blasts, intermediate density G(1) cells, and high density G(0) cells. Low density cells progressed to division without further stimulation, whereas G(0) and G(1) cells remained undivided. However, after a period of rest, a second subthreshold stimulation caused the G(1) but not the G(0) fraction to quickly proceed through the cell cycle. We conclude that noncycling T cells in the G(1) phase of the cell cycle remain in a state of readiness for prolonged periods of time, and may represent a population of memory-like effectors capable of responding rapidly to antigenic challenge.

Effector CD4+ T cells generate intermediate caspase activity and cleavage of caspase-8 substrates

Caspase-8 activation promotes cell apoptosis but is also essential for T cell activation. The extent of caspase activation and substrate cleavage in these divergent processes remains unclear. We show that murine effector CD4(+) T cells generated levels of caspase activity intermediate between unstimulated T cells and apoptotic populations. Both caspase-8 and caspase-3 were partially activated in effector T cells, which was reflected in cleavage of the caspase-8 substrates, c-FLIP(L), receptor interacting protein 1, and to a lesser extent Bid, but not the caspase-3 substrate inhibitor of caspase-activated DNase. Th2 effector CD4(+) T cells manifested more caspase activity than did Th1 effectors, and caspase blockade greatly decreased initiation of cell cycling. The current findings define the level of caspase activity and substrates during initiation of T cell cycling.

Fas ligand in the uterus of the non-pregnant mouse induces apoptosis of CD4+ T cells

The role of FasL in the reproductive tissues of the non-pregnant mouse may be the induction of apoptosis of activated T cells for the prevention of unwanted inflammatory responses secondary to infection. This study was undertaken to identify cell and tissue types that typically express FasL in the uterus and oviduct of the mouse and to establish whether FasL on the surface of these cells was able to induce T cell apoptosis. FasL in the mouse uterus and oviduct was demonstrated using three independent methods: RT-PCR, Western blot and immunocytochemistry. The protein was present in the epithelial and mesenchymal cells of the uterus and showed a granular pattern in the apical epithelial portion. Although this suggests the presence of vesicles, surface expression was also detected by flow cytometry of isolated uterine cells. Exogenous administration of estradiol and progesterone had no significant effect on the expression and localization of FasL. The ability of uterine cells to induce FasL-dependent apoptosis of activated CD4+T cells was examined by incubation of phytohemagglutinin-treated T cells with cultured uterine cells. TUNEL and flow cytometric analyses showed that CD4+T cells experienced apoptosis after 5h of co-incubation. Neutralizing antibodies inhibited apoptosis demonstrating that a biologically active FasL is present in the reproductive tissues of the mouse. Results indicate that FasL is a biologically active molecule present in epithelial and mesenchymal cells of the uterus and the oviduct of the non-pregnant mouse that might restrain local immune response by induction of apoptosis of CD4+T lymphocytes.

Early proliferation of CCR5(+) CD38(+++) antigen-specific CD4(+) Th1 effector cells during primary HIV-1 infection

We investigated whether HIV-1 antigen-specific CD4(+) T cells expressed the viral coreceptor CCR5 during primary HIV-1 infection (PHI). In the peripheral blood of subjects with very early PHI (< 22 days after onset of symptoms), there was a 10- to 20-fold increase in the proportion of highly activated (CD38(+++)) and proliferating (Ki-67(+)) CD4(+) T cells that expressed CCR5(+), and were mostly T-cell intracellular antigen-1 (TIA-1)(+) perforin(+) granzyme B(+). Inthe same patient samples, CD4(+) T cells producing interferon (IFN)-gamma in response to HIV group-specific antigen (Gag) peptides were readily detected (median, 0.58%) by intracellular cytokine assay-these cells were again predominantly CD38(+++), Ki-67(+), and TIA-(++), as well as Bcl-2(low). On average, 20% of the Gag-specific CD4(+) T cells also expressed interleukin-2 (IL-2) and were CD127 (IL-7R)(+). Taken together, these results suggest that Gag-specific T-helper 1 (Th1) effector cells express CCR5 during the primary response and may include precursors of long-term self-renewing memory cells. However, in PHI subjects with later presentation, antigen-specific CD4(+) T cells could not be readily detected (median, 0.08%), coinciding with a 5-fold lower level of the CCR5(+)CD38(+++) CD4(+) T cells. These results suggest that the antiviral response to HIV-1 infection includes highly activated CCR5(+)CD4(+) cytotoxic effector cells, which are susceptible to both apoptosis and cytopathic infection with HIV-1, and rapidly decline.

A role for caspases in controlling IL-4 expression in T cells

Although caspase activation is critical for T cell proliferation following activation, the role of caspases in T cell differentiation is unclear. In this study, we have examined the effect of inhibition of caspases on the process of Th1/Th2 differentiation. Naive CD4+ T cells activated under neutral differentiation conditions in the presence of the pan caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp (Z-VAD) fluoromethylketone showed increased Th2 cell differentiation concomitant with an up-regulation of GATA-3. Z-VAD induced optimal Th2 differentiation when T cells were stimulated under strong primary activation conditions. Treatment of naive CD4+ T cells with Z-VAD under strong activation conditions led to a 6-fold increase in IL-4 mRNA compared with control-treated T cells. The Z-VAD-induced increase in IL-4 transcription occurred within 24 h of activation and was independent of Stat6. IFN-gamma mRNA expression was not affected by Z-VAD at the 24-h time point. Z-VAD did not augment IL-4 expression from a committed Th2 cell, suggesting that caspases regulate IL-4 expression specifically during primary T cell activation. Z-VAD did not augment IL-12-driven Th1 differentiation. Activation of T cells in the presence of Z-VAD led to a specific increase in the expression of the transcription factor c-fos. Lastly, retrovirus-mediated expression of the antiapoptotic protein Bcl-2 resulted in an enhancement of Th2 cytokine expression, suggesting that inhibition of caspase activation by Bcl-2 can also modulate IL-4 expression. These findings reveal a novel regulatory mechanism of cytokine expression by caspases, and may explain how signaling pathways that inhibit apoptosis tend to promote Th2 differentiation.

Increased specific T cell cytokine responses in patients with active pulmonary tuberculosis from Central Africa

An understanding of T cell responses that are crucial for control of Mycobacterium tuberculosis (MTB) has major implications for the development of immune-based interventions. We studied the frequency of purified protein derivative (PPD)-specific CD3) cells expressing interleukin-2 (IL)-2, gamma interferon (IFN)-gamma, tumor necrosis factor (TNF)-alpha and IL-10 in HIV-negative pulmonary tuberculosis patients (TB, n=30) as well as in healthy individuals (controls, n=21) from Central Africa. Increased frequencies of PPD-stimulated CD3+ cells expressing IL-2, IFN-gamma, and TNF-alpha in TB were seen when compared with frequencies of controls. The presence of type 1 cytokine biased responses in TB patients was supported by a shift in the distribution pattern of cytokine expression from exclusively IL-2 or TNF-alpha expression seen in controls towards an increased frequency of IFN-gamma/IL-2 or IFN-gamma/TNF-alpha co-expression in TB. Higher levels of PPD-induced IFN-gamma in the supernatants from TB patients than from controls were found, which correlated with its intracellular expression. PPD was a weak inducer of IL-10 in T cells and insufficient in promoting cytokine production in TCRgammadelta+CD3+ cells. Non-specific stimulation with PMA and ionomycin revealed increased frequencies of CD4+ cells expressing IFN-gamma in controls, while expression of IL-2, IL-4, IL-10, IL-13, and TNF-alpha was not different. Non-specific cytokine responses of TCRgammadelta+CD3+ cells were similar in all groups. Pulmonary TB in Central Africa is associated with enhanced expression and secretion of specifically induced cytokines that are frequently implicated in host defense against MTB.

Apoptosis in tissue inflammation and allergic disease

Genetic predisposition and environmental instructions tune thresholds for the activation, effector functions and lifespan of T cells, other inflammatory cells and resident tissue cells. Defects in apoptosis and peripheral tolerance in T cells define different allergic phenotypes. In individuals with atopic allergic disease, activated allergen-specific T cells expressing high levels of IFN-gamma predominantly undergo apoptosis in the circulation, skewing the immune response to surviving T helper type 2 (Th2) cells. In affected tissues, these cells switch on effector cytokines and induce the activation and apoptosis of epithelial cells. In individuals with non-atopic monoallergic disease, by contrast, a disturbed balance towards allergen-specific Th2 cells instead of T regulatory type 1 (Tr1) cells characterizes the T cell response.

Induction of tolerance using Fas ligand: a double-edged immunomodulator

Apoptosis mediated by Fas ligand (FasL) interaction with Fas receptor plays a pivotal regulatory role in immune homeostasis, immune privilege, and self-tolerance. FasL, therefore, has been extensively exploited as an immunomodulatory agent to induce tolerance to both autoimmune and foreign antigens with conflicting results. Difficulties associated with the use of FasL as a tolerogenic factor may arise from (1) its complex posttranslational regulation, (2) the opposing functions of different forms of FasL, (3) different modes of expression, systemic versus localized and transient versus continuous, (4) the level and duration of expression, (5) the sensitivity of target tissues to Fas/FasL-mediated apoptosis and the efficiency of antigen presentation in these tissues, and (6) the types and levels of cytokines, chemokines, and metalloproteinases in the extracellular milieu of the target tissues. Thus, the effective use of FasL as an immunomodulator to achieve durable antigen-specific immune tolerance requires careful consideration of all of these parameters and the design of treatment regimens that maximize tolerogenic efficacy, while minimizing the non-tolerogenic and toxic functions of this molecule. This review summarizes the current status of FasL as a tolerogenic agent, problems associated with its use as an immunomodulator, and new strategies to improve its therapeutic potential.