Regulation of T cell cytokine production by dendritic cells

Previous work has established that the dendritic cells (DC) of mouse spleen regulate the IL-2 production, and hence the extent of proliferation, of the CD8 T cells they activate. It is now reported here that interaction of primary CD8 T cells with splenic CD8alpha- DC induced much higher production of IL-3, IFN-gamma and granulocyte-macrophage colony-stimulating factor (GM-CSF), as well as IL-2, than did interaction with CD8alpha+ splenic DC. Furthermore, the CD8alpha- DC also induced higher levels of IL-2, IL-3 and IL-10 production in primary CD4 T cells, compared with that induced by CD8alpha+ DC. These quantitative differences did not involve qualitative shifts in the type of cytokine produced. Interleukin-4 production remained low in all the primary T cell cultures and restimulation experiments in secondary cultures did not reveal any bias in the cytokine production profile. When exogenous IL-2 was added to the primary cultures to ensure equal proliferation in response to CD8alpha- or CD8alpha+ DC, the higher level of production of IL-3, IFN-gamma and GM-CSF induced by CD8alpha- DC was maintained. Thus, this general control of T cell cytokine production by splenic DC involves factors additional to those that govern activation of T cells into cell cycle.     

Regulation of follicular dendritic cell networks by activated T cells: the role of CD137 signaling

B cells, but not T cells, are considered to be important for the formation of follicular dendritic cell (FDC) clusters. Stimulation with agonist mAbs against CD137 (4-1BB), a TNFR family member primarily expressed on activated T cells, was effective in promoting T cell responses, but paradoxically suppressed T-dependent humoral immunity and autoantibody production in autoimmune disease models. Our present study shows that agonistic anti-CD137 treatment activates T cells, resulting in diminished FDC networks in B cell follicles, which are important components in T-dependent humoral immune responses both before and after the initiation of an immune response. Pretreatment with anti-CD137 before the secondary immunization inhibited memory Ab responses. Interestingly, CD137 costimulation-induced diminishment of FDC is T cell dependent. In addition, both CD4(+) and CD8(+) T cells are recruited into FDC area and are able to regulate FDCs by CD137 costimulation through a direct or indirect mechanism. These studies have revealed a previously unappreciated role of T cells in the regulation of FDC networks.     

Human lymphocyte activation gene-3 molecules expressed by activated T cells deliver costimulation signal for dendritic cell activation

Data have been reported on the in vivo adjuvant role of soluble lymphocyte activation gene-3 (LAG-3) recombinant protein in mouse models and on its ability to support the in vitro generation of human, tumor-specific CTLs. In this study, we show that soluble human rLAG-3 protein (hLAG-3Ig) used in vitro as a single maturation agent induces phenotypic maturation of monocyte-derived dendritic cells and promoted the production of chemokines and TNF-alpha inflammatory cytokine. When given in association with optimal or suboptimal doses of CD40/CD40L, hLAG-3Ig functions as a strong costimulatory factor and induces full functional activation of monocyte-derived dendritic cells that includes the production of high level of IL-12p70. Moreover, evidence is here provided that this costimulatory function licensing dendritic cells to produce IL-12p70 is also a functional property of LAG-3 molecules when expressed in a physiological context by CD4(+) activated T cells. Altogether, these data show for the first time a role of LAG-3 in mediating dendritic cell activation when expressed on the T cell surface or released after specific Ag stimulation in the interspaces of immunological synapses.     

Cutting edge: CCR4 mediates antigen-primed T cell binding to activated dendritic cells.

The binding of a T cell to an Ag-laden dendritic cell (DC) is a critical step of the acquired immune response. Herein, we address whether a DC-produced chemokine can induce the arrest of T cells on DC under dynamic flow conditions. Ag-primed T cells and a T cell line were observed to rapidly ( approximately 0.5 s) bind to immobilized DC at low shear stress (0.1-0.2 dynes/cm(2)) in a pertussis toxin-sensitive fashion. Quantitatively, Ag-primed T cells displayed 2- to 3-fold enhanced binding to DC compared with unprimed T cells (p < 0.01). In contrast to naive T cells, primed T cell arrest was largely inhibited by pertussis toxin, neutralization of the CC chemokine, macrophage-derived chemokine (CCL22), or by desensitization of the CCL22 receptor, CCR4. Our results demonstrate that DC-derived CCL22 induces rapid binding of activated T cells under dynamic conditions and that Ag-primed and naive T cells fundamentally differ with respect to chemokine-dependent binding to DC.     

Endogenous dendritic cells are required for amplification of T cell responses induced by dendritic cell vaccines in vivo

Dendritic cells (DCs) loaded in vitro with Ag are used as cellular vaccines to induce Ag-specific immunity. These cells are thought to be responsible for direct stimulation of Ag-specific T cells, which may subsequently mediate immunity. In this study, in transgenic mouse models with targeted MHC class II expression specifically on DCs, we show that the DC vaccine is responsible only for partial CD4(+) T cell activation, but to obtain optimal expansion of T cells in vivo, participation of endogenous (resident) DCs, but not endogenous B cells, is crucial. Transfer of Ag to endogenous DCs seems not to be mediated by simple peptide diffusion, but rather by DC-DC interaction in lymph nodes as demonstrated by histological analysis. In contrast, injection of apoptotic or necrotic DC vaccines does not induce T cell responses, but rather represents an immunological null event, which argues that viability of DC vaccines can be crucial for initial triggering of T cells. We propose that viable DCs from the DC vaccine must migrate to the draining lymph nodes and initiate a T cell response, which thereafter requires endogenous DCs that present transferred Ag in order induce optimal T cell expansion. These results are of specific importance with regard to the applicability of DC vaccinations in tumor patients, where the function of endogenous DCs is suppressed by either tumors or chemotherapy.     

Ligand-activated T cell growth factor-induced proliferation: absorption of T cell growth factor by activated T cells.

The fate in culture of the T cell growth factor (TCGF), which is required for continued growth of human cultured T cells (CTC) in vitro, was studied. TCGF activity was stable for 7 days at 37 degrees C. However, it was no longer detectable after incubation with actively growing CTC at 37 degrees C for 3 days. This loss of TCGF activity also occurred quite rapidly and was detectable within 1 hr of incubation of 0.3 ml supernatant with 2 to 5 x 10(7) CTC at 23 degrees C. 2 x 10(8) mononuclear peripheral blood leukocytes were not effective in removing TCGF activity, and incubation with similar numbers of cells from B and T cell lines had no effect. Three-day-old concanavalin A and phytohemagglutinin blasts were very reactive with TCGF, so that 10(7) or 2 x 10(7) cells consistently removed TCGF activity. These experiments suggested specific absorption of TCGF by activated T cells, and led us to develop a model of ligand-activated TCGF-induced proliferation of T cells: Ligands induce production of TCGF by T-producer cells and deliver a first signal to the T-responder cells. This causes a receptor for TCGF to appear on T-responder cells. Only then does TCGF deliver the obligatory second signal that is needed to drive the T-responder cells into proliferation.     

Mechanical interactions between dendritic cells and T cells correlate with T cell responsiveness

Ag recognition is achieved through the communication across intercellular contacts between T cells and APCs such as dendritic cells (DC). Despite remarkable progress in delineating detailed molecular components at the intercellular contacts, little is known about the functional roles of physical cross-junctional adhesion between T and DC in shaping T cell responses. In addition, the mechanisms underlying sensitivity and specificity of Ag discrimination by T cells at intercellular contacts remain to be elucidated. In this study, we use single-cell force spectroscopy to probe the mechanical interactions between DC and T cells in response to stimulation with a panel of altered peptide ligands. The results show that intercellular interactions of DC-T cell conjugates exhibited different ranges of interaction forces in peptide-dependent manners that match the ability of the peptides to activate T cells. Elevated calcium mobilization and IL-2 secretion by T cells were only promoted in response to antigenic peptides that induce strong interaction forces, suggesting that mechanically stable DC-T cell contacts are crucial for driving T cell activation. Strong interactions were not solely dependent on cell-surface molecules such as TCRs and the adhesion molecule LFA-1, but were also controlled by cytoskeletal dynamics and the integrity of membrane lipid rafts. These data provide novel mechanical insights into the effect of Ag affinity on intercellular contacts that align with T cell responsiveness.     

Regulation of T helper cell differentiation by respiratory tract dendritic cells.

Studies on dendritic cells (DC) of the respiratory and gastric mucosae have identified an extensive network of cells that represent the predominant antigen-presenting cell type at these sites. Under steady-state conditions, respiratory tract DC (RTDC) are specialized for antigen uptake and spontaneously migrate to local lymph nodes, although in vivo transfer studies have shown that the T-cell priming activity of these cells is restricted to low-level, Th2-skewed responses. Following exposure to inflammatory stimuli, the migration of RTDC to lymph nodes is accelerated and is associated with a rapid and dramatic increase in the ability of these cells to induce both Th1- and Th2-dependent immunity. Under normal circumstances, however, responsiveness of epithelial RTDC to maturation stimuli is regulated by locally produced micro-environmental factors, including pro-inflammatory cytokines, reactive oxygen species and prostanoids. These studies have led to a greater understanding of airway DC function and their role in T helper cell differentiation and provide the basis for future studies to determine the role of the cells in the aetiology and pathogenesis of respiratory immunoinflammatory disorders.     

Immune regulation through mitochondrion-dependent dendritic cell death induced by T regulatory cells

Dendritic cells (DCs) harbor an active mitochondrion-dependent cell death pathway regulated by Bcl-2 family members and undergo rapid turnover in vivo. However, the functions for mitochondrion-dependent cell death of DCs in immune regulation remain to be elucidated. In this article, we show that DC-specific knockout of proapoptotic Bcl-2 family members, Bax and Bak, induced spontaneous T cell activation and autoimmunity in mice. In addition to a defect in spontaneous cell death, Bax(-/-)Bak(-/-) DCs were resistant to killing by CD4(+)Foxp3(+) T regulatory cells (Tregs) compared with wild-type DCs. Tregs inhibited the activation of T effector cells by wild-type, but not Bax(-/-)Bak(-/-), DCs. Bax(-/-)Bak(-/-) DCs showed increased propensity for inducing autoantibodies. Moreover, the autoimmune potential of Bax(-/-)Bak(-/-) DCs was resistant to suppression by Tregs. Our data suggested that Bax and Bak mediate intrinsic spontaneous cell death in DCs, as well as regulate DC killing triggered by Tregs. Bax- and Bak-dependent cell death mechanisms help to maintain DC homeostasis and contribute to the regulation of T cell activation and the suppression of autoimmunity.     

Simultaneous induction of CD4 T cell tolerance and CD8 T cell immunity by semimature dendritic cells

Previous studies suggested that depending on their maturation state, dendritic cells (DC) could either induce T cell tolerance (immature and semimature DC) or T cell activation (mature DC). Pretreatment of C57BL/6 mice with encephalitogenic myelin oligodendrocyte glycoprotein (MOG)(35-55) peptide-loaded semimature DC protected from MOG-induced autoimmune encephalomyelitis. This protection was mediated by IL-10-producing CD4 T cells specific for the self Ag. Here we show that semimature DC loaded with the MHC class II-restricted nonself peptide Ag (OVA) induce an identical regulatory T cell cytokine pattern. However, semimature DC loaded simultaneously with MHC class II- and MHC class I-restricted peptides, could efficiently initiate CD8 T cell responses leading to autoimmune diabetes in a TCR-transgenic adoptive transfer model. Double-peptide-loaded semimature DC also induced simultaneously in the same animal partially activated CD8 T cells with cytolytic function as well as protection from MOG-induced autoimmune encephalomyelitis. Our study suggests that the decision between tolerance and immunity not only depends on the DC, but also on the type and activation requirements of the responding T cell.     

Regulatory T cells target chemokine secretion by dendritic cells independently of their capacity to regulate T cell proliferation

The clinical manipulation of regulatory T cells (Tregs) represents a promising strategy for the regulation of unwanted immune responses. It is now becoming clear that Tregs exert multiple effects on different cell targets under particular conditions; however, the interplay between these different factors remains unclear. Using mouse Tregs of known Ag specificity, we report in this study two different levels of Treg-mediated suppression: one that targets T cell proliferation and one that targets dendritic cell-mediated proinflammatory chemokine (CCL3 and CCL4) production. These two effects can be dissociated, and whereas modulation of T cell proliferation depends on the strength of the antigenic stimulus, modulation of chemokine production by dendritic cells does not. We also provide evidence that the bystander effect of Tregs on immune responses observed in vivo may be in great part explained by a decrease in the recruitment of target T cells, and therefore in the magnitude of the response, rather than by a direct effect on their priming or proliferation. Overall, our results shed some light on the different aspects that need to be considered when attempting to modulate Tregs for clinical purposes.     

T cell proliferation induced by autologous non-T cells is a response to apoptotic cells processed by dendritic cells

Self-reactive T cells are present in the mature immune repertoire as demonstrated by T cell proliferation induced by autologous non-T cells in the autologous mixed lymphocyte reaction. This reaction generates regulatory T cells in vitro and may reflect immune regulatory pathways in vivo, but the antigenic peptides recognized remain uncharacterized. We revisited this issue in light of the importance of apoptosis in immune regulation. We found that apoptosis among peripheral blood non-T stimulator cells is associated with augmented induction of autologous T cell proliferation. Our data show that caspase activity in the non-T stimulator population is essential for induction of autologous T cell proliferation, suggesting that cellular components in the non-T cell fraction are enzymatically modified, most likely by effector caspases, and have a direct or indirect effect on autoreactive T cell activation. Furthermore, exposure of macrophage-derived dendritic cells to apoptotic non-T cells augments autologous T cell proliferation, and blockade of alpha(v)beta(5) integrin, but not alpha(v)beta(3), inhibits the capacity of irradiated non-T cells or dendritic cells to stimulate autologous T cell proliferation. These experiments, using an entirely autologous system, suggest the interpretation that autoreactive T cells may recognize self-Ags modified through the actions of caspases and presented to T cells by dendritic cells. Induction of an in vivo autologous mixed lymphocyte reaction by caspase-modified self-Ags present in apoptotic cells may represent a mechanism to maintain peripheral immune tolerance.     

Newly activated T cells promote maturation of bystander dendritic cells but not IL-12 production

The activation of dendritic cells (DC) leads to increased costimulatory activity (termed DC maturation) and, in some instances, production of immunomodulatory cytokines such as IL-12. Both innate and T cell-derived signals can promote DC activation but it is unclear to what extent the two classes of stimuli are interchangeable or regulate distinct aspects of DC function. In this study, we show that signals from newly activated CD4(+) T cells cannot initiate IL-12 synthesis although they can amplify secretion of bioactive IL-12 p70 by DC exposed to an appropriate innate stimulus. This occurs exclusively in cis and does not influence IL-12 synthesis by bystander DC that do not present Ag. In marked contrast, signals from newly activated CD4(+) T cells can induce an increase in DC costimulatory activity in the absence of any innate priming. This occurs both in cis and in trans, affecting all DC in the microenvironment, including those that do not bear specific Ag. Consistent with the latter, we show that newly activated CD4(+) T cells in vivo can deliver "help" in trans, effectively lowering the number of MHC/peptide complexes required for proliferation of third-party naive CD4(+) T cells recognizing Ag on bystander DC. These results demonstrate that DC maturation and cytokine production are regulated distinctly by innate stimuli vs signals from CD4(+) T cells and reveal a process of trans activation of DC without secretion of polarizing cytokines that takes place during T cell priming and may be involved in amplifying immune responses.     

Human dendritic cells are activated by chimeric human papillomavirus type-16 virus-like particles and induce epitope-specific human T cell responses in vitro

Human papillomavirus (HPV)-derived chimeric virus-like particles (VLPs) are the leading candidate vaccine for the treatment or prevention of cervical cancer in humans. Dendritic cells (DCs) are the most potent inducers of immune responses and here we show for the first time evidence for binding of chimeric HPV-16 VLPs to human peripheral blood-derived DCS: Incubation of immature human DCs with VLPs for 48 h induced a significant up-regulation of the CD80 and CD83 molecules as well as secretion of IL-12. Confocal microscopy analysis revealed that cell surface-bound chimeric VLPs were taken up by DCS: Moreover, DCs loaded with chimeric HPV-16 L1L2-E7 VLPs induced an HLA-*0201-restricted human T cell response in vitro specific for E7-derived peptides. These results clearly demonstrate that immature human DCs are fully activated by chimeric HPV-16 VLPs and subsequently are capable of inducing endogenously processed epitope-specific human T cell responses in vitro. Overall, these findings could explain the high immunogenicity and efficiency of VLPs as vaccines.     

Glutamate released by dendritic cells as a novel modulator of T cell activation

Adaptive immune responses begin after productive immunosynaptic contacts formation established in secondary lymphoid organs by dendritic cells (DC) presenting the Ag to T lymphocytes. Despite its resemblance to the neurosynapse, the participation of soluble small nonpeptidic mediators in the intercellular cross-talk taking place during T cell-DC interactions remains poorly studied. In this study, we show that human DC undergoing maturation and in contact with T cells release significant amounts of glutamate, which is the main excitatory neurotransmitter in mammalians. The release of glutamate is nonvesicular and mediated by the DC-expressed Xc- cystine/glutamate antiporter. DC-derived glutamate stimulating the constitutively expressed metabotropic glutamate receptor 5 impairs T cell activation. However, after productive Ag presentation, metabotropic glutamate receptor 1 is expressed in T cells to mediate enhanced T cell proliferation and secretion of Th1 and proinflammatory cytokines. These data suggest that, during T cell-DC interaction, glutamate is a novel and highly effective regulator in the initiation of T cell-mediated immune responses.     

Antigen nonspecific suppression of T cell responses by activated stimulation-refractory CD4+ T cells

Several classes of anergic T cells are capable of suppressing naive T cell proliferation and thereby limiting immune responses. Activated T cells, although not anergic, are transiently refractory to restimulation with Ag. We examine in this study whether activated refractory murine T cells can also suppress naive T cell responses. We find that they can, and that they exhibit many of the suppressive properties of anergic T cells. The activated cells strongly diminish Ag-mediated T cell proliferation, an activity that correlates with their refractory period. Suppression is independent of APC numbers and requires cell contact or proximity. Naive T cells stimulated in the presence of activated refractory cells up-regulate CD25 and CD69, but fail to produce IL-2. The addition of IL-2 to culture medium, however, does not prevent the suppression, which is therefore not solely due to the absence of this growth factor. Persistence of the suppressor cells is also not essential. T cells stimulated in their presence and then isolated from them and cultured do not divide. The suppressive cells, however, do not confer a refractory or anergic state on the target T lymphocytes, which can fully respond to antigenic stimulation if removed from the suppressors. Our results therefore provide evidence that activated T cells act as transient suppressor cells, severely constraining bystander T cell stimulation and thereby restricting their response. These results have potentially broad implications for the development and regulation of immune responses.     

Generation of activated and antigen-specific T cells with cytotoxic activity after co-culture with dendritic cells

Co-culturing of immunological effector cells with antigen-pulsed DC leads to an increase of cytotoxic activity against antigen-expressing tumour cells. Using this approach, we could detect up to 2.8% antigen-specific CTLs after co-culture with antigen-pulsed DC. However, the required high effector cell numbers remain a major obstacle in immunotherapy. In this study, we show an approach for generating activated and antigen-specific effector cells that enables us to decrease effector to target cell ratios. We used an interferon-gamma secretion assay to enrich activated effector cells after co-culture with antigen-pulsed dendritic cells (DC). Purified immunological effector cells lysed 58.3% of antigen-expressing tumour cells at an effector to target ratio of 1:1. Furthermore, using MHC-IgG complexes, we enriched effector cells expressing antigen-specific T-cell receptor after co-culture with DC. Performing ELISpot, flow cytometry and TCR analysis, we could show a significant increase of activated and specific TCR-expressing effector cells after co-culture with DC.     

Regulation of T cell cytokine production by dendritic cells generated in vitro from hematopoietic progenitor cells

When dendritic cells (DC) present antigens to T cells, reciprocal cellular interactions occur that lead to cytokine production. This cytokine response is regulated by specific properties of DC, notably their maturation/activation status and perhaps their origin. The latter possibility prompted us to determine if DC produced along distinct developmental pathways induced distinct T cell responses. Hematopoietic progenitor cells with the potential to differentiate into multiple lineages of cells were induced to differentiate into DC along two pathways. One leads to the formation of lymphoid-related DC but not of monocyte-derived DC and is induced by culture of CD34(+) cells with flt-3 ligand (F), c-kit ligand (K), GM-CSF (Gm), IL-1beta ("1"), and IL-7 ("7") (FKGm17). Another pathway with distinct molecular requirements supports in part monocyte-derived DC and is induced by the cytokines F, K, Gm, TNF-alpha (T), and IL-4 ("4") (FKGmT4). DC produced along these two pathways were isolated by flow cytometry and compared. They differed only slightly in phenotype and morphology and both induced Th1-type cytokine production in MLR (mixed lymphocyte reactions). However, on a cell-per-cell basis, FKGm17-DC produced more IL-18 or IL-12 and induced more IFN-gamma by T cells in MLR. Such superior properties were not intrinsically determined by the origin of the DC but were induced by FKGm17 cytokines. We conclude that lymphoid-related DC have the potential to induce Th1 T cell responses but that environmental signals strongly influence T-cell-stimulating properties of DC.     

Suppressor T cells activated by lymphoblastoid cell lines inhibit pokeweed mitogen-lnduced immunoglobulin synthesis

EBV-transformed B-cell lines of normal or malignant origin suppressed pokeweed mitogen-induced immunoglobulin synthesis of normal B cells, measured by the protein A plaque assay method. Autologous and allogeneic LCL were equally effective. Allogeneic irradiated B cells were ineffective or slightly enhancing. EBV-negative hematopoietic cell lines derived from myeloid leukemia (K562 and HL-60) were ineffective or slightly enhancing. The suppressive effect of the Burkitt lymphoma line Raji was partly due to released soluble suppressor factors, but to a larger extent to the generation of radiosensitive suppressor T cells in the responder population. Autologous and allogeneic LCL were equally effective in generating suppressor T cells. It is postulated that the suppressor circuit reflects the existence of regulatory mechanisms that govern the proliferation of B lymphocytes.