Dendritic cell longevity and T cell persistence is controlled by CD154-CD40 interactions

Persistent depots of influenza antigen fail to induce a cytotoxic CD8 T cell response

Encounter with Ag during chronic infections results in the generation of phenotypically and functionally heterogeneous subsets of Ag-specific CD8 T cells. Influenza, an acute infection, results in the generation of similar CD8 T cell heterogeneity, which may be attributed to long-lived depots of flu Ags that stimulate T cell proliferation well after virus clearance. We hypothesized that the heterogeneity of flu-specific CD8 T cells and maintenance of T cell memory required the recruitment of new CD8 T cells to persistent depots of flu Ag, as was the case for flu-specific CD4 T cell responses. However, robust expansion and generation of highly differentiated cytolytic effectors and memory T cells only occurred when naive CD8 T cells were primed during the first week of flu infection. Priming of new naive CD8 T cells after the first week of infection resulted in low numbers of poorly functional effectors, with little to no cytolytic activity, and a negligible contribution to the memory pool. Therefore, although the presentation of flu Ag during the late stages of infection may provide a mechanism for maintaining an activated population of CD8 T cells in the lung, few latecomer CD8 T cells are recruited into the functional memory T cell pool.

CD40-CD40 ligand interaction between dendritic cells and CD8+ T cells is needed to stimulate maximal T cell responses in the absence of CD4+ T cell help

Stimulation of CD40 on APCs through CD40L expressed on helper CD4+ T cells activates and "licenses" the APCs to prime CD8+ T cell responses. Although other stimuli, such as TLR agonists, can also activate APCs, it is unclear to what extent they can replace the signals provided by CD40-CD40L interactions. In this study, we used an adoptive transfer system to re-examine the role of CD40 in the priming of naive CD8+ T cells. We find an approximately 50% reduction in expansion and cytokine production in TCR-transgenic T cells in the absence of CD40 on all APCs, and on dendritic cells in particular. Moreover, CD40-deficient and CD40L-deficient mice fail to develop endogenous CTL responses after immunization. Surprisingly, the role for CD40 and CD40L are observed even in the absence of CD4+ T cells; in this situation, the CD8+ T cell itself provides CD40L. Furthermore, we show that although TLR stimulation improves T cell responses, it cannot fully substitute for CD40. Altogether, these results reveal a direct and unique role for CD40L on CD8+ T cells interacting with CD40 on APCs that affects the magnitude and quality of CD8+ T cell responses.

Autoimmune Uveitis Elicited with Antigen-Pulsed Dendritic Cells Has a Distinct Clinical Signature and Is Driven by Unique Effector Mechanisms: Initial Encounter with Autoantigen Defines Disease Phenotype

Human autoimmune uveitis is a heterogeneous group of potentially blinding ocular diseases in which most patients who exhibit immunity recognize the same retinal Ag. It is represented by the model of experimental autoimmune uveitis (EAU) induced in mice by immunization with retinal Ag in CFA. Murine EAU is characterized by a Th1/Th17 response pattern, which may not represent all types of human uveitis. We report in this study a new model of EAU induced by injection of matured dendritic cells loaded with a uveitogenic retinal peptide. Dendritic cell-induced EAU demonstrated unique characteristics compared with traditional EAU in terms of clinical manifestations, the nature of the inflammatory infiltrating cells, the cytokine response profile, and a strict requirement for IFN-gamma, whereas IL-17 appeared to play a minor role. Disease was self-limiting, but could be reinduced with the same Ag in CFA, albeit with reduced severity, suggesting post-recovery resistance. Our study demonstrates in a disease setting that the context in which the same autoantigen is initially presented to the immune system precipitates distinct forms of pathology via a distinct pathogenic pathway on the same genetic background. These findings may shed new light on the complex biology and the heterogeneous nature of human uveitis, and provide an alternative model for uveitic diseases of immune origin.

Visfatin, an adipocytokine with proinflammatory and immunomodulating properties

Adipocytokines are mainly adipocyte-derived cytokines regulating metabolism and as such are key regulators of insulin resistance. Some adipocytokines such as adiponectin and leptin affect immune and inflammatory functions. Visfatin (pre-B cell colony-enhancing factor) has recently been identified as a new adipocytokine affecting insulin resistance by binding to the insulin receptor. In this study, we show that recombinant visfatin activates human leukocytes and induces cytokine production. In CD14(+) monocytes, visfatin induces the production of IL-1beta, TNF-alpha, and especially IL-6. Moreover, it increases the surface expression of costimulatory molecules CD54, CD40, and CD80. Visfatin-stimulated monocytes show augmented FITC-dextran uptake and an enhanced capacity to induce alloproliferative responses in human lymphocytes. Visfatin-induced effects involve p38 as well as MEK1 pathways as determined by inhibition with MAPK inhibitors and we observed activation of NF-kappaB. In vivo, visfatin induces circulating IL-6 in BALB/c mice. In patients with inflammatory bowel disease, plasma levels of visfatin are elevated and its mRNA expression is significantly increased in colonic tissue of Crohn's and ulcerative colitis patients compared with healthy controls. Macrophages, dendritic cells, and colonic epithelial cells might be additional sources of visfatin as determined by confocal microscopy. Visfatin can be considered a new proinflammatory adipocytokine.

Improvements in mucopolysaccharidosis I mice after adult retroviral vector-mediated gene therapy with immunomodulation

Mucopolysaccharidosis I (MPS I) is caused by deficient alpha-L-iduronidase (IDUA) activity and results in the accumulation of glycosaminoglycans and multisystemic disease. Gene therapy could program cells to secrete mannose 6-phosphate-modified IDUA, and enzyme in blood could be taken up by other cells. Neonatal retroviral vector (RV)-mediated gene therapy has been shown to reduce the manifestations of murine MPS I; however, intravenous injection of RV into adults was ineffective owing to a cytotoxic T lymphocyte (CTL) response against transduced cells. In this study, prolonged inhibition of CD28 signaling with CTLA4-Ig, or transient administration of CTLA4-Ig with an anti-CD40 ligand antibody or with an anti-CD4 antibody, resulted in stable expression in most mice that received RV as adults. Mice with stable expression had 81 +/- 41U/ml IDUA activity in serum. This resulted in reductions in bone disease, improvements in hearing and vision, and reductions in biochemical and pathological evidence of lysosomal storage in most organs. Improvements in brain were likely due to diffusion of enzyme from blood. However, aortic disease was refractory to treatment. This demonstrates that most manifestations of MPS I can be prevented using adult gene therapy if an immune response is blocked.

Osteoprotegerin expression in dendritic cells increases with maturation and is NF-κB-dependent

Dendritic cells (DC) comprise a unique leukocyte population which controls primary immune responses. Recent studies indicate that DC express osteoprotegerin (OPG), a secreted tumor necrosis factor receptor homolog, which regulates DC survival, monocyte chemotaxis, and B cell development and function by ligating TNF family member receptor activator of NF-kappaB ligand (RANKL). The precise regulators of OPG expression in DC have not been investigated. In this study, we assessed OPG mRNA steady state levels by Northern blot analysis and OPG protein secretion by an immunoassay in monocyte-derived DC of different maturation, and the effect of different cytokines and hormones on OPG expression. OPG was upregulated with maturation of DC, whereas pretreatment of DC with 1alpha,25(OH)(2) vitamin D(3), tamoxifen, or dexamethasone, agents that inhibit differentiation of DC, decreased OPG expression. In vivo, OPG was found to be colocalized with mature CD83(+) DC in human tonsils by immunofluorescence confocal microscopy analysis. Furthermore, OPG was upregulated by TNF superfamily members TNF-alpha, anti-CD40, and RANKL, and by ligands of the Toll-like/IL-1 receptor family including IL-1beta, double-stranded RNA (poly I:C), or lipopolysaccharide (LPS), all of which induce maturation of DC. Gene silencing by small interfering RNA (siRNA) directed against transcription factor NF-kappaB abrogated the expression of OPG as demonstrated by real-time PCR. In summary, we describe that the expression of OPG by DC increases with maturation and is NF-kappaB-dependent, possibly regulating immune responses in lymphoid tissues.

An essential role for Akt1 in dendritic cell function and tumor immunotherapy

Current dendritic cell (DC) vaccine preparations involving ex vivo differentiation and maturation produce short-lived, transiently active DCs that may curtail T-cell responses in vivo. We demonstrate that Akt1, downregulation of which decreases DC lifespan, is critical for proinflammatory signal-mediated DC survival and maturation. Lipopolysaccharide or CD40 signaling stabilizes Akt1, promoting both activation and Bcl-2-dependent survival of DCs. Expression of a potent allele encoding a lipid raft-targeted Akt1, M(F)-DeltaAkt, is sufficient for maturation and survival of murine bone marrow-derived DCs in vivo. M(F)-DeltaAkt-transduced DCs enhanced T-cell proliferation, activation and long-term memory responses, enabling eradication of large pre-established lymphomas and aggressive B16 melanomas. Human myeloid DCs expressing constitutively active M(F)-DeltahAkt also survived significantly longer and promoted antigen-specific T-cell responses. Thus, Akt1 is a critical regulator of DC lifespan, and its manipulation in DCs can improve the clinical efficacy of DC-based tumor vaccines.

Altered CD4+T Cell Phenotype and Function Determine the Susceptibility to Mucosal Candidiasis in Transgenic Mice Expressing HIV-1

The impairments of protective mucosal immunity which cause susceptibility to oropharyngeal candidiasis (OPC) in HIV infection remain undefined. This study used a model of OPC in CD4C/HIV MutA transgenic (Tg) mice expressing Rev, Env, and Nef of HIV-1 to investigate the role of transgene expressing dendritic cells (DCs) and CD4+ T cells in maintenance of chronic oral carriage of Candida albicans. DCs were depleted in the Tg mice and had an immature phenotype, with low expression of MHC class II and IL-12. CD4+ T cells were quantitatively reduced in the oral mucosa, cervical lymph nodes (CLNs) and peripheral blood of the Tg mice, and displayed a polarization toward a nonprotective Th2 response. Proliferation of CLN CD4+ T cells from infected Tg mice in response to C. albicans Ag in vitro was abrogated and the cells failed to acquire an effector phenotype. Coculture of C. albicans-pulsed DCs with CD4+ T cells in vitro showed that Tg expression in either or both of these cell populations sharply reduced the proliferation of CD4+ T cells and their production of IL-2. Finally, transfer of naive non-Tg CD4+ T cells into these Tg mice restored proliferation to C. albicans Ag and sharply reduced oral burdens of C. albicans. Overall, these results indicate that defective CD4+ T cells primarily determine the susceptibility to chronic carriage of C. albicans in these Tg mice.

CD4 memory T cells survive and proliferate but fail to differentiate in the absence of CD40

Secondary T cell responses are enhanced because of an expansion in numbers of antigen-specific (memory) cells. Using major histocompatibility complex class II tetramers we have tracked peptide-specific endogenous (non–T cell receptor transgenic) CD4 memory T cells in normal and in costimulation-deficient mice. CD4 memory T cells were detectable after immunization for more than 200 days, although decay was apparent. Memory cells generated in CD40 knockout mice by immunization with peptide-pulsed wild-type dendritic cells survived in the absence of CD40 and proliferated when boosted with peptide (plus adjuvant) in a CD40-independent fashion. However, differentiation of the memory cells into cytokine-producing effector cells did not occur in the absence of CD40. The data indicate that memory cells can be generated without passing through the effector cell stage.

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.

Circumventing tolerance at the T cell or the antigen-presenting cell surface: Antibodies that ligate CD40 and OX40 have different effects

An adjuvant can be defined as an agent that non-specifically promotes the immune response to an accompanying antigen. Ligation of CD40 on the surface of the antigen-presenting cell leads to upregulation of OX40 ligand which, in turn, ligates OX40 on the activated T cell resulting in prolonged T cell proliferation/survival, boosting the immune response. Thus agonistic anti-CD40 and anti-OX40 might be viewed as "adjuvant antibodies" and have been shown in diverse experimental systems to either boost immune responses or prevent the establishment of immunological tolerance. Here we describe that both these antibodies are able to prevent the induction of tolerance induced using soluble peptide antigen. However, unlike lipopolysaccharide, they are not sufficient to convert tolerance to immunity (i.e. they are not true adjuvants in this system). Using mice that are prone to either Th1 or Th2 immunity under identical immunization conditions, we show that the effects of anti-OX40 are quantitative -- boosting whichever response is dominant. In contrast, anti-CD40 boosts Th1 immunity and converts a Th2 response to Th1. We conclude that, although these two antibodies seem to impact on the same molecular pathway of costimulation to prevent tolerance, their effects are qualitatively distinct and their use cannot be viewed as interchangeable.

CD38 orchestrates migration, survival, and Th1 immune response of human mature dendritic cells

CD38, an ectoenzyme and a signaling receptor, is a novel marker of human mature monocyte-derived dendritic cells (MDDCs). The working hypothesis is that CD38 is not only a marker but also contributes to functions specifically gained by MDDCs with maturation. This was tested by assessing the role(s) of CD38 after signaling with agonistic anti-CD38 monoclonal antibodies or by blocking the interactions taking place between CD38 and CD31, its counterreceptor. The results indicate the following: (1) CD38 engagement in MDDCs ensures efficient chemotaxis and transendothelial migration driven by CC chemokine ligand 21 (CCL21); (2) CD38 is laterally associated with the CCL21-specific CC chemokine receptor 7 and with CD83 and CD11b; (3) CD38 localizes in membrane lipid domains; (4) CD38 signaling contributes to support longevity of lipopolysaccharide (LPS)-matured MDDCs after growth factor withdrawal; and (5) IFN-gamma is produced by cocultured T lymphocytes, thus affecting T-helper 1 (Th1) polarization. These data suggest that the localization of CD38 in lipid rafts and its multiple interactions with signaling receptors rule innate and adaptive immune responses by tuning DC migration, survival, and Th1-polarization ability. These findings may lay out the basis to assess the functional role(s) of human CD38 in infections, autoimmune diseases, and neoplastic disorders.

Failure of HIV-exposed CD4+ T cells to activate dendritic cells is reversed by restoration of CD40/CD154 interactions

Because interactions between activated CD4+ T cells and antigen-presenting cells (APCs) are crucial for optimal APC function, defective CD4+ T-cell activation may contribute to APC dysregulation in HIV infection. Here, we show that CD4+ T cells exposed during stimulation to noninfectious HIV having functional envelope glycoproteins failed to provide activation signals to autologous dendritic cells (DCs). Consequently, important DC functions, including production of immunoregulatory cytokines (interleukin-12 p40 and interleukin-10) and up-regulation of costimulatory molecules (CD86, CD40, CD83), as well as the capacity to stimulate naive allogeneic T cells, were all adversely affected. The blunted up-regulation of CD154 in CD4+ T cells that were activated in the presence of noninfectious viruses is likely to be the major underlying mechanism for these defects. Addition of recombinant trimeric CD154 could restore production of cytokines by DCs cocultured with HIV-exposed T cells. Moreover, the functional defects mediated by coculture with HIV-exposed T cells were similar to those following antibody blockade of CD40-CD154 interactions. HIV-mediated blunted CD154 expression may thus play an important role in the suppression of cell-mediated immunity seen in HIV infection.

Aberrancies in the differentiation and maturation of dendritic cells from bone-marrow precursors are linked to various genes on chromosome 4 and other chromosomes of the BB-DP rat

BB-Diabetes Prone (BB-DP) rats, a model for endocrine autoimmune diseases, are severely lymphopenic, especially lacking ART2+ regulatory T cells. BB-Diabetes Resistant (DR) rats are not lymphopenic and do not develop autoimmunity. BB-DP and BB-DR rats only differ at the lymphopenia (lyp) gene (iddm2) on chromosome 4. Since BB-DP rats also show aberrancies in the differentiation of dendritic cells (DC) from bone-marrow precursors, we tested the hypothesis that F344 rats congenic for a BB-DP chromosome 4 region (42.5-93.6Mb; including the lyp gene, but also iddm4) display an in vitro DC differentiation different from normal F344 rats. Here we show that the 42.5-93.6Mb BB-DP chromosome 4 region is linked to an increased DC precursor apoptosis, a low MHC class II expression, a reduced IL-10 production and a reduced T cell stimulatory capacity of DC. From our previous report on DC differentiation defects in BB rats (only differing in iddm2) and the present report, we deduce that the abnormal apoptosis and low MHC class II expression is linked to iddm2. The reduced T cell stimulatory capacity is linked to other genes on chromosome 4 (candidate gene: iddm4). The reduced IL-10 production has a complex linkage pattern.

Employing the immunological synapse in AML: Development of leukemic dendritic cells for active specific immunization

Cytotoxic T cells directed against leukemic blasts have been observed in patients with acute myeloid leukemia (AML). However, generation of efficient T-cell responses is hampered due to several factors that enable AML blasts to protect themselves from the patients immune system. Improved immune responses can be established by the differentiation of AML blasts into AML-derived dendritic cells (DC) thereby conserving their intrinsic leukemia specific antigens and obtaining full capacity to present these antigens to naive T cells. This review discusses increased immunogenicity of AML blasts by differentiation into AML-DC and describes ways to augment the AML-DC vaccination approach.

Antigen persistence is required throughout the expansion phase of a CD4(+) T cell response

For CD8(+) T cells, a relatively short antigen pulse seems sufficient for antigen-presenting cells to drive clonal expansion and differentiation. It is unknown whether the requirement for antigen is similarly ephemeral for CD4(+) T cells. To study the dependence of a CD4(+) T cell response on antigen persistence in a quantitatively and temporally controlled manner in vivo, we engineered a mouse line expressing a major histocompatibility complex class II-restricted epitope in dendritic cells under the control of a tetracycline-inducible promoter. Experiments tracking the proliferation of CD4(+) T cells exposed to their cognate antigen in various amounts for different time periods revealed that the division of such cells was contingent on the presence of antigen throughout their expansion phase, even in the presence of an inflammatory stimulus. This previously unrecognized feature of a CD4(+) T cell response contrasts with the proliferative behavior of CD8(+) T cells that has been documented, and it implies that the two T cell subsets might require different strategies for efficient vaccination.

Balance between NF-κB and JNK/AP-1 activity controls dendritic cell life and death

The life cycle of dendritic cells (DCs) must be precisely regulated for proper functioning of adaptive immunity. However, signaling pathways actively mediating DC death remain enigmatic. Here we describe a novel mechanism of hierarchical transcriptional control of DC life and death. Ligation of tumor necrosis factor receptor superfamily (TNFR-SF) members on DCs and cognate contact with T cells resulted in quantitatively balanced nuclear factor-κB (NF-κB) and c-Jun N-terminal kinase (JNK)–mediated activator protein-1 (AP-1) induction and strongly enhanced DC longevity. Specific blockade of NF-κB in DCs induced strongly augmented JNK/AP-1 activity because of elevated levels of reactive oxygen species. In this scenario, DC activation by TNFR-SF members or T cells induced DC apoptosis. Specific inhibition of JNK/AP-1 rescued DCs from this activation-induced cell death program and restored TNFR-SF member- and T-cell–mediated survival. We conclude that JNK/AP-1 activity is under negative feedback control of NF-κB and can execute apoptosis in DCs. Thus, feedback-controlled signaling amplitudes of 2 transcriptional pathways decide the fate of a DC.

Roles of lymphoid cells in the differentiation of Langerhans dendritic cells in mice

Langerhans dendritic cells are antigen presenting cells (APC) that reside within the epidermis and are capable of stimulating naive T cells. Reciprocally, lymphocytes may play a role in Langerhans cells (LC) differentiation. Our results show that the differentiation of skin LC is unaffected in the absence of lymphocytes and/or signaling through the common cytokine receptor gamma chain (gammac) required for IL-2, IL-4, IL-7, IL-9, IL-15 and IL-21 signaling. Migration of LC and other dendritic cells (DC) from the skin to the draining lymph nodes (LNs) after FITC skin sensitization, is unaffected in the absence of lymphocytes or CD40. FITC+ LC/DC sorted from the LNs of lymphoid deficient or control mice stimulated naive T cells with similar efficiency. However, while the absence of lymphocytes did not appear to affect the phenotype or number of emigrating LN DC/LC, their persistence in the LN appears to depend on alphabeta T cells. Thus, DC are strikingly reduced in numbers in the peripheral LNs of T-cell deficient mice. Finally, CD8alpha expression on skin emigrants was low and dependent on the presence of CD8+ lymphocytes, while spleen CD8+ DC were present in the absence of lymphocytes. We conclude that the presence of T cells is not required for the differentiation and migration of resident skin DC but is critical for the maintenance of DC and LC migrating into the LNs.

CD40/CD40L interaction regulates CD4+CD25+ T reg homeostasis through dendritic cell-produced IL-2

CD4+CD25+ regulatory T cells (T reg) development and homeostasis require IL-2 and costimulation through same TNF-receptor family members. CD40KO mice have reduced number of T reg in peripheral blood, thymus and spleen. Herein we show that naive T reg express low basal level of CD40L that is upregulated upon TCR-triggered mediated activation. Treatment of wt mice with Ab blocking CD40/CD40L interaction results in a fast decrease in T reg number that rapidly recovers upon Ab withdrawal. CFSE-labeled T reg from wt mice injected into CD40KO, but not wild-type (wt) mice, showed reduced survival and proliferation in homeostatic setting. In vitro, dendritic cells from CD40KO mice but not wt mice produce diminished amount of IL-2 upon T reg encounter and are impaired in expanding T reg, a defect corrected by the addition of rIL-2. Accordingly, four daily IL-2 administrations to CD40KO mice normalize T reg number by promoting both their survival and homeostatic proliferation. Such IL-2 effect is transient since T reg number returns to the low constitutive level described in CD40KO mice within 5 days upon IL-2 withdrawal thus suggesting that IL-2 is persistently needed to assure T reg homeostasis.

Repeated stimulation of CD4 effector T cells can limit their protective function

Chronic infections often result in CD8 T-cell deletion or functional nonresponsiveness. However, to date no definitive studies have attempted to determine the impact of repeated T cell receptor stimulation on CD4 effector T cell generation. We have determined that when antigen presentation is limited to 2 d, optimum in vitro CD4 effector generation is achieved. Alternatively, repeated stimulation results in decreased CD4 effector expansion, decreased cytokine production, and altered migration. Similarly, functionally impaired effectors develop in vivo when antigen-pulsed antigen-presenting cells are replenished every 24 h during a primary immune response. CD4 effectors that are generated with repeated stimulation provide no protection during influenza infection, and have an impaired ability to provide cognate help to B cells. These results suggest that duration of antigen presentation dictates CD4 effector function, and repeated T cell receptor stimulation in vitro and in vivo that exceeds an optimal threshold results in effectors with impaired function.

bcl-xL is critical for dendritic cell survival in vivo

Dendritic cells (DC) are important regulators of immune function, transporting Ags from the periphery to draining lymph nodes (dLN) where they prime Ag-specific T lymphocytes. The magnitude of the immune response generated depends upon the longevity of the Ag-bearing DC in lymphoid tissues. We hypothesized that the control of DC survival is regulated by the antiapoptotic factor bcl-x(L). Gene gun immunization of dual-expression DNA vaccines into a bcl-x(fl/fl) mouse resulted in the delivery of Ag, as well as selective deletion of the bcl-x gene in directly transfected, skin-residing DC. bcl-x-deficient DC failed to mount effective immune responses, and this corresponded to their rapid disappearance from the dLN due to apoptosis. We confirmed these results using RNA interference to specifically silence the antiapoptotic bcl-x(L) isoform in targeted skin-residing DC of C57BL/6 mice. In addition, delivery of bcl-x(L) in trans complemented the bcl-x deficiency in DC of bcl-x(fl/fl) mice, resulting in the maintenance of normal levels of Ag-bearing DC in the dLN. Taken together, our work demonstrates that the bcl-x(L) isoform is critical for survival of skin-derived, Ag-bearing DC in vivo.

Visualizing the first 50 hr of the primary immune response to a soluble antigen

Recently, static and dynamic imaging methods have produced the first glimpses of the interactions between antigen-specific T cells and peptide-MHC-bearing antigen-presenting cells in the lymph nodes. Using data from these experiments, we produced a numerically, spatially, and temporally scaled simulation of the first 50 hr of the primary T cell-dependent immune response. The simulation highlights how lymph node structure facilitates antigen presentation to rare, naive, antigen-specific CD4+ T cells.

An Evaluation of Enforced Rapid Proteasomal Degradation as a Means of Enhancing Vaccine-Induced CTL Responses

The HIV-1 Gag protein is an attractive target for CTL-based vaccine strategies because it shows less sequence variability than other HIV-1 proteins. In an attempt to increase the immunogenicity of HIV-1 Gag, we created Gag variants that were targeted to the proteasomal pathway for rapid degradation. This enhanced rate of degradation was associated with increased presentation of MHC class I-associated antigenic peptides on the cell surface. Despite this, immunizing mice with either plasmid DNA or recombinant vaccinia vectors expressing unstable Gag failed to produce significant increases in bulk CTL responses or Ag-specific production of IFN-gamma by CD8(+) T cells compared with mice immunized with stable forms of Gag. Production of IFN-gamma by CD4(+) T cells was also impaired, and we speculate that the abrogation of CD4(+) T cell help was responsible for the impaired CTL response. These results suggest that vaccine strategies designed to increase the density of peptide-MHC class I complexes on the surfaces of APC may not necessarily enhance immunogenicity with respect to CTL responses.

CD40/CD154 interactions at the interface of tolerance and immunity

Development of the acquired immune response is dependent on the signaling of CD40 by its ligand, CD154. These molecules govern both the magnitude and quality of humoral- and cell-mediated immunity. A litany of studies have conclusively documented that blockade of this ligand-receptor pair can prevent, and also intervene in, the progression of antibody- and cell-mediated autoimmune diseases, and can instill long-lived allogeneic and xenogeneic graft tolerance. Many effector mechanisms of inflammation are abolished as a result of CD154 blockade, but we are now beginning to understand that CD154 blockade may, in some instances, engender long-lived, antigen-specific tolerance. In the context of transplantation tolerance, we present a hypothesis that alpha CD154 blockade is most effective at inducing long-lived allospecific tolerance if anergy and regulation can be elicited prior to the onslaught of inflammation that is induced by grafting (preemptive tolerance). This facet of alpha CD154-induced tolerance appears to co-opt the normal processes of peripheral tolerance induced by immature DCs and can be exploited to induce long-lived antigen-specific tolerance. The underlying science and the prospects for inducing long-lived antigen-specific tolerance in a model of allograft tolerance through CD154 blockade are presented and discussed.

The CD40/CD40L costimulatory pathway in inflammatory bowel disease

The CD154-CD40 ligand pair interaction plays a central role in both induction of the immune response and in immune effector functions. Indeed, many animal disease models and human autoimmune diseases have demonstrated a central role for CD154 expression. The expression of CD154 is very tightly regulated by the immune system through a number of non-redundant overlapping mechanisms that ensure its limited initial induction, along with its temporal maintenance and rapid elimination from the cell surface, and its functional neutralization by the release of soluble CD40. In this review, we discuss the current state of understanding of CD154 regulation during the activation of the immune system and describe numerous strategic mechanisms by which modulation of CD154-CD40 interactions may be applied to treat autoimmune disease.

Expression of apoptosis-regulatory proteins in lesions of pulmonary Langerhans cell histiocytosis

AIMS

Pulmonary Langerhans cell histiocytosis (PLCH) is characterized by the presence of lesions containing numerous activated Langerhans cells (LCs). An uncontrolled immune response sustained by activated LCs seems to be involved in the pathogenesis of the disease. The aim of this study was to establish whether disruption of LC apoptosis related to the expression of the Bcl-2 family proteins is implicated in the maintenance of PLCH lesions.

METHODS

Six patients with PLCH were evaluated by morphological and immunohistochemical techniques to explore the incidence of apoptosis in pathological LCs and to characterize the expression of Bcl-2-related proteins by these cells.

RESULTS

Very few LCs present in PLCH lesions exhibited nuclear apoptotic changes or expressed cleaved caspase-3, whereas they all strongly expressed the anti-apoptotic molecule Bcl-x(L). Interestingly, pulmonary LCs present in intervening lung tissue not involved by the pathological process and known to be immature dendritic cells did not express Bcl-2 family proteins.

CONCLUSIONS

These findings suggest that activated LCs present within PLCH lesions are poorly susceptible to apoptosis and, thus, are able to sustain the pathological process by causing continuous local stimulation of T cells. Functional studies are needed, however, to demonstrate that they are actually resistant to programmed cell death.

A Bcl-2-dependent molecular timer regulates the lifespan and immunogenicity of dendritic cells

The lifespan of antigen-bearing dendritic cells (DCs) is determined by signals from pathogens and T cells. These signals regulate DC survival by modulating expression of Bcl-2 family proteins. Toll-like receptors and T cell costimulatory molecules both trigger a DC survival pathway that is dependent on Bcl-x(L). However, Toll-like receptors uniquely increase expression of Bim and trigger cell death by a pathway that is blocked by Bcl-2. This pathway serves as a molecular 'timer' that sets the lifespan of DCs and regulates the magnitude of T cell responses in vivo. Thus, signals derived from the innate and acquired immune systems control DC lifespan and immunogenicity by distinct molecular mechanisms.

T-cell costimulatory capacity of oral and skin epithelial cells in vitro: presence of suppressive activity in supernatants from skin epithelial cell cultures

Oral Langerhans cells (LC) have better T-cell costimulatory capacity than skin LC. In this study factors affecting this capacity have been assessed in a mixed epithelial cell lymphocyte reaction (MELR) assay. Flow cytometry analysis of freshly recovered cells revealed major histocompatibility complex (MHC) class II molecule expression on 7.5% of the oral epithelial cells and 9.7% of the skin epithelial cells. Monoclonal anti class II antibodies significantly reduced the T-cell proliferation in the MELR. Pretreatment of skin epithelial cells with interleukin-1beta, tumour necrosis factor-alpha or interferon (IFN)-gamma did not affect the MELR proliferation, but incubation with IFNgamma significantly suppressed the T-cell response. Transfer of supernatants from cultures of skin epithelial cells and allogeneic T cells to cultures of oral epithelial cells and T cells resulted in a reduced T-cell proliferation while supernatants from oral epithelial cells and T cells did not reduce proliferation. The higher proliferation in cultures of T cells and oral epithelial cells than in cultures containing skin epithelial cells may be due to the presence of a suppressive factor in the skin epithelial cell suspensions.

Dendritic cells: emerging pharmacological targets of immunosuppressive drugs

Immunosuppressive drugs have revolutionized organ transplantation and improved the therapeutic management of autoimmune diseases. The development of immunosuppressive drugs and understanding of their action traditionally has been focused on lymphocytes, but recent evidence indicates that these agents interfere with immune responses at the earliest stage, targeting key functions of dendritic cells (DCs). Here, we review our present understanding of how classical and new immunosuppressive agents interfere with DC development and function. This knowledge might provide a rational basis for the selection of immunosuppressive drugs in different clinical settings and for the generation of tolerogenic DCs in the laboratory.

Prostate Tumor Microenvironment Alters Immune Cells and Prevents Long-Term Survival in an Orthotopic Mouse Model Following flt3-Ligand/CD40-Ligand Immunotherapy

A novel orthotopic metastatic model of mouse prostate cancer was developed using MHC-negative TRAMP-C1P3 (transgenic adenocarcinoma of mouse prostate) cells derived by serial passage of the parental TRAMP-C1 line in mouse prostate glands. TRAMP-C1P3 cells grew efficiently in mouse prostate glands and reproducibly metastasized to draining lymph nodes. Using this model, we show that Fms-like tyrosine kinase-3 ligand (flt3-L) dramatically inhibited growth of preexisting orthotopic TRAMP-C1P3 tumors and the development of metastatic disease. Mice remained in remission for several months following termination of flt3-L treatment but eventually relapsed and died of progressive disease. flt3-ligand treatment induced a pronounced mixed inflammatory cell infiltrate that consisted of CD8alpha-CD4- dendritic cells (CD11c+), macrophages, granulocytes (Gr-1+) and to a lesser extent T cells (CD4+ and CD8+). Dendritic cells isolated from TRAMP-C1P3 tumors were phenotypically immature (CD11c+ B7.2-I-A-CD40-), and this phenotype was also predominant in peripheral organs of mice treated with flt3-L alone or in combination with the DC maturation factor, CD40-L. Diminished expression of TCR-beta, CD3-epsilon, and CD3-zeta was also observed on intratumoral T cells, although these signaling proteins were reexpressed following in vitro culture with IL-2. The TCR/CD3 complex remained intact on peripheral T cells except in mice treated with flt3-L where CD3-zeta loss was observed. In contrast to alphabeta-T cells, tumor-infiltrating gammadelta-T cells maintained expression of their antigen receptors but not CD3epsilon. Thus, TRAMP-C1P3 tumors quickly establish a microenvironment that profoundly diminishes expression of molecules critical for normal dendritic cell and T cell function, thus limiting the efficacy of flt3-L and CD40-L immunotherapy. Overall, these data suggest that long-term cures of established MHC-negative tumors may not be achieved until therapeutic interventions are engineered to overcome this immunosuppressive microenvironment.

CD40, but Not CD154, Expression on B Cells Is Necessary for Optimal Primary B Cell Responses

CD40 is an important costimulatory molecule for B cells as well as dendritic cells, monocytes, and other APCs. The ligand for CD40, CD154, is expressed on activated T cells, NK cells, mast cells, basophils, and even activated B cells. Although both CD40(-/-) and CD154(-/-) mice have impaired ability to isotype switch, form germinal centers, make memory B cells, and produce Ab, it is not entirely clear whether these defects are intrinsic to B cells, to other APCs, or to T cells. Using bone marrow chimeric mice, we investigated whether CD40 or CD154 must be expressed on B cells for optimal B cell responses in vivo. We demonstrate that CD40 expression on B cells is required for the generation of germinal centers, isotype switching, and sustained Ab production, even when other APCs express CD40. In contrast, the expression of CD154 on B cells is not required for the generation of germinal centers, isotype switching, or sustained Ab production. In fact, B cell responses are completely normal when CD154 expression is limited exclusively to Ag-specific T cells. These results suggest that the interaction of CD154 expressed by activated CD4 T cells with CD40 expressed by B cells is the primary pathway necessary to achieve B cell activation and differentiation and that CD154 expression on B cells does not noticeably facilitate B cell activation and differentiation.

Role of CD40 ligand dysregulation in HIV-associated dysfunction of antigen-presenting cells

Cellular interactions between antigen-presenting cells and activated CD4+ T cells are central to the regulation of adaptive immunity. Among the many receptor-ligand pairs involved, the critical importance of CD40-CD40 Ligand (CD40L) interactions has been demonstrated in many experimental systems. Dysregulation of antigen-presenting cell function is a hallmark of HIV-associated defects in cell-mediated immunity. Much evidence suggests a mechanistic role for defective CD40-CD40L interactions in such a defect. Consistent with this hypothesis, the capacity to upregulate CD40L on purified CD4+ T cells becomes progressively impaired in HIV infection, in parallel with the progression of clinical immunosuppression. The mechanisms underlying CD40L dysregulation in HIV infection remain unknown. Because CD40L expression is tightly regulated (transcriptionally, post-transcriptionally and post-translationally), HIV may interfere at several levels. However, a transcriptional defect in CD40L expression, mediated by the engagement of CD4 by HIV gp120, appears to play a primary role. Clear elucidation of mechanism may well lead to the development of novel immunotherapeutic approaches to HIV infection.

Prevention of CD40-triggered dendritic cell maturation and induction of T-cell hyporeactivity by targeting of Janus kinase 3

Pharmacological targeting of Janus kinase 3 (JAK3) has been employed successfully to control allograft rejection and graft-vs.-host disease (GVHD). Recent evidence suggests that in addition to its involvement in common-gamma chain (cgamma) signaling of cytokine receptors, JAK3 is also engaged in the CD40 signaling pathway of peripheral blood monocytes. In this study, we assessed the consequences of JAK3 inhibition during CD40-induced maturation of myeloid dendritic cells (DCs), and tested the impact thereof on the induction of T-cell alloreactivity. Dendritic cells triggering through CD40 induced JAK3 activity, the expression of costimulatory molecules, production of IL-12, and potent allogeneic stimulatory capacity. In contrast, JAK3 inhibition with the rationally designed JAK3 inhibitor WHI-P-154 prevented these effects arresting the DCs at an immature level. Interestingly, DCs exposed to the JAK3-inhibitor during CD40-ligation induced a state of hyporeactivity in alloreactive T cells that was reversible upon exogenous IL-2 supplementation to secondary cultures. These results suggest that immunosuppressive therapies targeting the tyrosine kinase JAK3 may also affect the function of myeloid cells. This property of JAK3 inhibitors therefore represents a further level of interference, which together with the well-established suppression of cgamma signaling could be responsible for their clinical efficacy.

TRAF6 is a critical factor for dendritic cell maturation and development

IL-1 receptor (IL-1R)/Toll-like receptor (TLR) family and TNF receptor (TNFR) superfamily members are critical for regulating multiple aspects of dendritic cell (DC) biology. Several signaling pathways associated with each family utilize the adapter molecule, TRAF6, but its role in DCs is unclear. By examining TRAF6-deficient mice and bone marrow (BM) chimeras reconstituted with TRAF6-deficient fetal liver cells, we show that proper DC maturation requires TRAF6. In response to either microbial components or CD40L, TRAF6-deficient DCs fail to upregulate surface expression of MHCII and B7.2, or produce inflammatory cytokines. Moreover, LPS-treated TRAF6-deficient DCs do not exhibit an enhanced capacity to stimulate naive T cells. Interestingly, a major population of splenic DCs, the CD4(+)CD8alpha(-) subset, is nearly absent in both TRAF6-deficient mice and BM chimeras. Together these results indicate that TRAF6 regulates the critical processes required for maturation, activation, and development of DCs, the primary cellular bridge between innate and adaptive immunity.

Mechanisms of donor-specific transfusion tolerance: preemptive induction of clonal T-cell exhaustion via indirect presentation

Induction of transplantation tolerance to alloantigens without general immunosuppression remains an enduring challenge. Injecting a donor-specific transfusion (DST) of spleen cells together with blocking alphaCD154 antibody prior to graft transplantation is an effective way to induce long-lived graft acceptance. Using a novel T-cell receptor (TCR) transgenic (Tg) model of CD4+ T-cell-mediated rejection, this study sheds new insights into the cellular basis for enhanced graft survival induced by DST and alphaCD154. The study shows that DST and alphaCD154 induce an early, robust, abortive expansion of the Tg T cells that results in profound anergy. This is contrasted with the more delayed, regional, productive response elicited by an allogeneic graft. Studies show that the induction of tolerance to the allograft induced by DST is mediated by indirect presentation by host antigen-presenting cells. Based on these observations, we conclude that DST and alphaCD154 preemptively tolerize the alloreactive T-cell compartment to prohibit subsequent responses to the immunogenic allograft.

Impact of the tumor microenvironment on host infiltrating cells and the efficacy of flt3-ligand combination immunotherapy evaluated in a treatment model of mouse prostate cancer

We have previously reported that Fms-like tyrosine kinase-3 ligand (flt3-L) induced tumor stabilization and regression of palpable ectopic prostate tumors (TRAMP-C1). Although some mice remained "tumor free" for several months following termination of therapy, tumors invariably reappeared and grew progressively in all animals. The lack of a curative response suggests that TRAMP-C1 tumors may inhibit the development of a flt3-L-induced anti-tumor immune response. Consistent with this view, we demonstrate herein that TRAMP-C1 tumors isolated from flt3-L treated animals contained a marked dendritic cell (DC) infiltrate that was temporally correlated with tumor regression. However, tumor-associated DCs, especially in a flt3-L setting, progressively lost MHC class II antigen expression during tumor growth. Treatment with the DC maturation factor trimeric CD40 ligand (CD40-L) either alone or in combination with fl3-L neither prevented loss of DC class II antigens nor disease relapse. Because loss of class II antigens would prevent CD4+ helper T (Th) cell development, we treated tumor-bearing mice with agonistic anti-4-1BB antibody (Ab), which can promote cytotoxic T lymphocyte (CTL) development independent of Th cell function. However, anti-4-1BB Ab alone did not alter TRAMP-C1 growth kinetics, and, when used in combination, was no more effective than flt3-L alone. The inability of the 4-1BB co-stimulatory signal to promote tumor regression may have been related to two additional features of TRAMP-C1 tumors. First, tumor-associated T cells, but not splenic T cells from tumor-bearing animals, were profoundly deficient in expression of CD3-epsilon (CD3epsilon) and T cell receptor-beta chain (TCRbeta). Second, CTLs required 24 h to efficiently kill TRAMP-C1 target cells even after up-regulation of MHC class I antigens by interferon-gamma. This rate of tumor cell destruction by CTLs may not be sufficient to prevent tumor progression. Taken together, these data reveal several important immunosuppressive characteristics of the prostate tumor microenvironment (TME) that immunotherapeutic interventions must first overcome to achieve longterm cures. These data also highlight the importance of utilizing treatment versus vaccination models in the evaluation of immunotherapeutic modalities.

Hematopoietic chimerism and central tolerance created by peripheral-tolerance induction without myeloablative conditioning

Allogeneic hematopoietic chimerism leading to central tolerance has significant therapeutic potential. Realization of that potential has been impeded by the need for myeloablative conditioning of the host and development of graft-versus-host disease (GVHD). To surmount these impediments, we have adapted a costimulation blockade-based protocol developed for solid organ transplantation for use in stem cell transplantation. The protocol combines donor-specific transfusion (DST) with anti-CD154 mAb. When applied to stem cell transplantation, administration of DST, anti-CD154 mAb, and allogeneic bone marrow leads to hematopoietic chimerism and central tolerance with no myeloablation and no GVHD. Tolerance in this system results from deletion of both peripheral host alloreactive CD8+ T cells and nascent intrathymic alloreactive CD8+ T cells. In the absence of large numbers of host alloreactive CD8+ T cells, the transfusion that precedes transplantation need not be of donor origin, suggesting that both allospecific and non-allospecific mechanisms regulate engraftment. Agents that interfere with peripheral transplantation tolerance impair establishment of chimerism. We conclude that robust allogeneic hematopoietic chimerism and central tolerance can be established in the absence of host myeloablative conditioning using a peripheral transplantation tolerance protocol.

Genetic disassociation of autoimmunity and resistance to costimulation blockade-induced transplantation tolerance in nonobese diabetic mice

Curing type 1 diabetes by islet transplantation requires overcoming both allorejection and recurrent autoimmunity. This has been achieved with systemic immunosuppression, but tolerance induction would be preferable. Most islet allotransplant tolerance induction protocols have been tested in nonobese diabetic (NOD) mice, and most have failed. Failure has been attributed to the underlying autoimmunity, assuming that autoimmunity and resistance to transplantation tolerance have a common basis. Out of concern that NOD biology could be misleading in this regard, we tested the hypothesis that autoimmunity and resistance to transplantation tolerance in NOD mice are distinct phenotypes. Unexpectedly, we observed that (NOD x C57BL/6)F(1) mice, which have no diabetes, nonetheless resist prolongation of skin allografts by costimulation blockade. Further analyses revealed that the F(1) mice shared the dendritic cell maturation defects and abnormal CD4(+) T cell responses of the NOD but had lost its defects in macrophage maturation and NK cell activity. We conclude that resistance to allograft tolerance induction in the NOD mouse is not a direct consequence of overt autoimmunity and that autoimmunity and resistance to costimulation blockade-induced transplantation tolerance phenotypes in NOD mice can be dissociated genetically. The outcomes of tolerance induction protocols tested in NOD mice may not accurately predict outcomes in human subjects.

NF-κB activation and zinc finger protein A20 expression in mature dendritic cells derived from liver allografts undergoing acute rejection

AIM: To investigate the role of NF-κB activation and zinc finger protein A20 expression in the regulation of maturation of dendritic cells (DCs) derived from liver allografts undergoing acute rejection.

METHODS: Sixty donor male SD rats and sixty recipient male LEW rats weighing 220-300 g were randomly divided into whole liver transplantation group and partial liver transplantation group. Allogeneic (SD rat to LEW rat) whole and 50% partial liver transplantation were performed. DCs from liver grafts 0 hour and 4 d after transplantation were isolated and propagated in the presence of GM-CSF in vitro. Morphological characteristics and phenotypical features of DCs propagated for 10 d were analyzed by electron microscopy and flow cytometry, respectively. NF-κB binding activity, IL-12 p70 protein and zinc finger protein A20 expression in these DCs were measured by EMSA and Western blotting, respectively. Histological grading of rejection was determined.

RESULTS: Allogeneic whole liver grafts showed no signs of rejection on day 4 after the transplantation. In contrast, allogeneic partial liver grafts demonstrated moderate to severe rejection on day 4 after the transplantation. After propagation for 10 d in the presence of GM-CSF in vitro, DCs from allogeneic whole liver grafts exhibited features of immature DC with absence of CD40 surface expression, these DCs were found to exhibit detectable but very low level of NF-κB activity, IL-12 p70 protein and zinc finger protein A20 expression. Whereas, DCs from allogeneic partial liver graft 4 d after transplantation displayed features of mature DC, with high level of CD40 surface expression, and as a consequence, higher expression of IL-12p70 protein, higher activities of NF-κB and higher expression of zinc finger protein A20 compared with those of DCs from whole liver grafts (P < 0.001).

CONCLUSION: These results suggest that A20 expression is up-regulated in response to NF-κB activation in mature DCs derived from allogeneic liver grafts undergoing acute rejection. Given the NF-κB inhibition function of this gene, it is suggested that their expression survives to limit NF-κB activation and maturation of DCs, and consequently inhibits the acute rejection and induces acceptance of liver graft.

Feedback regulation of pathogen-specific T cell priming

MHC class I-restricted antigen presentation is an essential step in the priming of CD8 T lymphocytes during immune responses to infection. While microbial growth and clearance have been accurately measured in mammalian hosts, the duration of functional antigen presentation during infection remains undefined in vivo. Herein we characterize the activation of naive and memory T cells at different times during bacterial infection. Surprisingly, the host's ability to prime T cells is of much shorter duration than bacterial infection, inversely correlating with the development of pathogen-specific cytolytic T lymphocytes. Our studies demonstrate a feedback mechanism that limits the duration of effective in vivo antigen presentation, thereby modulating T cell responses by temporally restricting recruitment of naive T cells into the immune response.

Cognate interactions between memory T cells and tumor antigen-presenting dendritic cells from bone marrow of breast cancer patients: bidirectional cell stimulation, survival and antitumor activity in vivo

Dendritic cells (DC) and T cells were generated from Ficoll separated bone marrow (BM) mononuclear cells of primary operated breast cancer patients according to new cell culture protocols. BM-DC were capable of functioning as professional antigen-presenting cells (APCs) and of inducing autologous antigen-specific memory T-cell responses to either tetanus toxoid recall antigen or to breast cancer antigens. Treatment with lipopolysaccharide (LPS) resulted in phenotypic and functional maturation of BM-DC. When BM-DC, pulsed with breast cancer-associated tumor antigens, were cocultured with autologous patient-derived BM-T cells to allow for cognate breast cancer antigen recognition and stimulation, apoptosis of T cells-which occurred in noncognate coculture systems-was inhibited. Furthermore, in cocultures allowing for antigen-specific cognate interactions, the expression on BM-DC of CD83, MHC class II, CD40 and CD86 molecules was upregulated and the cytokines IL-12 and IFN-alpha were produced in significantly elevated amounts. Adoptive transfer of breast cancer-reactive memory T cells together with APCs into human breast cancer-bearing NOD/SCID mice caused a regression of the tumor and prolonged survival of the animals. This was not the case when such animals had been treated by transfer of reactivated BM T cells without BM-DCs. Our findings suggest that cognate interactions between cancer patient-derived memory BM-T cells and tumor antigen-presenting BM-DCs are important for reciprocal cell stimulation, survival and therapeutic activity.

2,3,7,8-Tetrachlorodibenzo-p-dioxin suppresses tumor necrosis factor-alpha and anti-CD40-induced activation of NF-kappaB/Rel in dendritic cells: p50 homodimer activation is not affected

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) suppresses many immune responses, both innate and adaptive. Suppression is mediated by the aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor. The AhR mediates TCDD toxicity presumably through the alteration of transcriptional events, either by promoting gene expression or potentially by physically interacting with other transcription factors. Another transcription factor, NF-kappaB/Rel, is involved in several signaling pathways in immune cells and is crucial for generating effective immune responses. Dendritic cells (DCs), considered to be the "pacemakers" of the immune system, were recently recognized as targets of TCDD and are also dependent on NF-kappaB/Rel for activation and survival. In these studies, we investigated whether TCDD would alter the activation of NF-kappaB/Rel in DCs. The dendritic cell line DC2.4 was exposed to TCDD before treatment with tumor necrosis factor alpha (TNF-alpha) or anti-CD40, and NF-kappaB/Rel activation was measured by electrophoretic mobility shift assay and immunoblotting. TCDD suppressed the binding of NF-kappaB/Rel to its cognate response element in TNF-alpha- and anti-CD40-treated cells and blocked translocation to the nucleus. The AhR was shown to associate with RelA, after coimmunoprecipitation, and seemed to block its binding to DNA. It is noteworthy that p50 homodimers freely bound to DNA. These results suggest that TCDD may alter the balance between NF-kappaB/Rel heterodimers and transcriptional inhibitory p50 homodimers in DCs, leading to defects in the DCs and suppression of the immune response.

Impaired Th2 development and increased mortality during Schistosoma mansoni infection in the absence of CD40/CD154 interaction

The role of CD40/CD154 interaction during infection has primarily focused on pathogens that drive inflammatory Th1 responses. In this study, we show that CD40/CD154 interaction is a fundamental requirement for Th2 response development to the parasitic helminth Schistosoma mansoni. Compared with infected wild-type mice, greatly reduced levels of Th2-associated cytokines were measured both in vitro and in vivo, and no IgE or IgG1 was detected in infected CD154(-/-) mice. In the absence of an overt Th2 response, no exaggerated Th1 response was mounted by CD154(-/-) mice. Infected CD154(-/-) mice suffered severe morbidity and mortality, even though parasitemias in wild-type and CD154(-/-) mice did not differ significantly. These data indicate that CD40/CD154 interaction is required to allow development of a Th2-dominated immune response to S. mansoni and support the view that failure to develop such a response can have fatal consequences.

The growth of cutaneous T-cell lymphoma is stimulated by immature dendritic cells

In the initial stage of cutaneous T-cell lymphoma (CTCL), proliferating CTCL cells are concentrated in the epidermis in close association with an immature dendritic cell (DC), the Langerhans cell. Because long-term in vitro culture of CTCL cells has proven difficult, the in vivo association with the major antigen-presenting cell (APC) of the epidermis has been postulated to play a role in directly stimulating the clonal T-cell proliferation. We report that CTCL cells can be reproducibly grown in culture for 3 months when cocultured with immature DCs. CTCL cells retain the phenotype and genotype of the initial malignant clone, whereas the APCs are a mixture of immature and mature DCs. CTCL cell and DC survival was dependent on direct membrane contact. Growth was inhibited by antibodies that bound to the T-cell receptor (TCR) or interfered with the interaction of CD40 with its ligand on the CTCL cell. Addition of antibody to CD3 or the clonotypic TCR caused rapid CTCL cell apoptosis followed by engulfment by avidly phagocytic immature DCs and subsequent DC maturation. The opportunity to study CTCL cells and immature DCs for prolonged periods will facilitate studies of tumor cell biology and will allow investigation of the intriguing hypothesis that CTCL cell growth is driven through TCR recognition of class II–presented self-peptides. In addition, the culture of CTCL cells will permit evaluation of therapies in vitro before clinical intervention, thereby improving safety and efficacy.

The growth of cutaneous T-cell lymphoma is stimulated by immature dendritic cells

In the initial stage of cutaneous T-cell lymphoma (CTCL), proliferating CTCL cells are concentrated in the epidermis in close association with an immature dendritic cell (DC), the Langerhans cell. Because long-term in vitro culture of CTCL cells has proven difficult, the in vivo association with the major antigen-presenting cell (APC) of the epidermis has been postulated to play a role in directly stimulating the clonal T-cell proliferation. We report that CTCL cells can be reproducibly grown in culture for 3 months when cocultured with immature DCs. CTCL cells retain the phenotype and genotype of the initial malignant clone, whereas the APCs are a mixture of immature and mature DCs. CTCL cell and DC survival was dependent on direct membrane contact. Growth was inhibited by antibodies that bound to the T-cell receptor (TCR) or interfered with the interaction of CD40 with its ligand on the CTCL cell. Addition of antibody to CD3 or the clonotypic TCR caused rapid CTCL cell apoptosis followed by engulfment by avidly phagocytic immature DCs and subsequent DC maturation. The opportunity to study CTCL cells and immature DCs for prolonged periods will facilitate studies of tumor cell biology and will allow investigation of the intriguing hypothesis that CTCL cell growth is driven through TCR recognition of class II–presented self-peptides. In addition, the culture of CTCL cells will permit evaluation of therapies in vitro before clinical intervention, thereby improving safety and efficacy.

Recent advances in understanding the mechanisms of TCDD immunotoxicity

TCDD is a highly immunosuppressive chemical that induces potent suppression of immune responses in laboratory animals. However, apart from the requisite role of the AhR and the identification of bone-marrow-derived cells as critical AhR-expressing targets, the specific cells and the underlying biochemical mechanisms by which TCDD disrupts immunological functions remain unclear. Recent data suggest that a new paradigm for the mechanism of immunotoxic action of TCDD may be more accurate, moving from one focused on the suppression of immune functions to one focused on the inappropriate activation of cells, leading to anergy or death, and the consequent premature termination of the immune response. Enhanced activation of B cells, DC and CD4+ T cells by TCDD has been described as well as the earlier disappearance of the latter two populations from peripheral lymphoid organs. Although much remains to be learned about how inappropriate cellular activation via the AhR induces immune suppression, deducing this mechanism of action and the signaling pathways involved, should lead to new insight into basic mechanisms of immune regulation.

Dendritic cell development and survival require distinct NF-kappaB subunits

Despite the established role of dendritic cells (DCs) in regulating T lymphocyte activation, intracellular mechanisms responsible for controlling DC function are largely undefined. Here, we have studied DCs from mice deficient in the p50, RelA, and cRel subunits of the immunomodulatory NF-kappaB transcription factor. Although DC development and function was normal in mice lacking individual NF-kappaB subunits, development of doubly deficient p50(-/-)RelA(-/-) DCs was significantly impaired. In contrast, DCs from p50(-/-)cRel(-/-) mice developed normally, but CD40L- and TRANCE-induced survival and IL-12 production was abolished. Surprisingly, no significant impairment in MHC and costimulatory molecule expression was seen, despite significantly reduced kappaB site binding activity. These results therefore indicate essential, subunit-specific functions for NF-kappaB proteins in regulating DC development, survival, and cytokine production.

Induction of MHC class I presentation of exogenous antigen by dendritic cells is controlled by CD4+ T cells engaging class II molecules in cholesterol-rich domains

We investigated interactions between CD4+ T cells and dendritic cells (DC) necessary for presentation of exogenous Ag by DC to CD8+ T cells. CD4+ T cells responding to their cognate Ag presented by MHC class II molecules of DC were necessary for induction of CD8+ T cell responses to MHC class I-associated Ag, but their ability to do so depended on the manner in which class II-peptide complexes were formed. DC derived from short-term mouse bone marrow culture efficiently took up Ag encapsulated in IgG FcR-targeted liposomes and stimulated CD4+ T cell responses to Ag-derived peptides associated with class II molecules. This CD4+ T cell-DC interaction resulted in expression by the DC of complexes of class I molecules and peptides from the Ag delivered in liposomes and permitted expression of the activation marker CD69 and cytotoxic responses by naive CD8+ T cells. However, while free peptides in solution loaded onto DC class II molecules could stimulate IL-2 production by CD4+ T cells as efficiently as peptides derived from endocytosed Ag, they could not stimulate induction of cytotoxic responses by CD8+ T cells to Ag delivered in liposomes into the same DC. Signals requiring class II molecules loaded with endocytosed Ag, but not free peptide, were inhibited by methyl-beta-cyclodextrin, which depletes cell membrane cholesterol. CD4+ T cell signals thus require class II molecules in cholesterol-rich domains of DC for induction of CD8+ T cell responses to exogenous Ag by inducing DC to process this Ag for class I presentation.