T cell activation pathways: B7, LFA-3, and ICAM-1 shape unique T cell profiles

Evolution of biologic therapies for the treatment of psoriasis

Over the past three decades, laboratory and clinical research findings have shown that T cells are the primary mediators of psoriasis pathogenesis and that psoriasis can be treated by eliminating these T cells or interfering with their activation or activity. Based on these observations, many new biologic therapies to treat psoriasis are now in development. These agents, developed primarily through recombinant DNA techniques, are designed to target T cells and the immunologic cascade associated with their activation. Four basic strategic approaches that focus on the steps involved in the immunopathology of psoriasis are: 1) elimination of the pathogenic T cells; 2) inhibition of T-cell activation, proliferation, and migration; 3) immune deviation to down-regulate the type 1 (TH1) response predominant in psoriasis; and 4) blockade of cytokine production. The goal of these new therapies is to improve the treatment of psoriasis, particularly moderate to severe disease, with agents that are well tolerated and safe for long-term use.

The immunology of psoriasis and biologic immunotherapy

In this review, we will discuss the immunological basis for psoriasis with special emphasis on the role of effector T cells. With the understanding of this immunologic process, we will present a model for the development of targeted immune response modifiers, termed biologic immunotherapies, and their potential role for the benefit of patients with psoriasis.

The role of T cells in psoriasis

Evidence for a key role of T cells in the pathogenesis of psoriasis has come from both experimental and clinical data. Initially, generalized immunosuppressants, intended for use in transplant settings, were found to improve clinical signs and symptoms of psoriasis. Their efficacy attracted attention to the activated T cells that are a major component of the inflammatory infiltrate of psoriatic lesions. Further research determined that T cells from patients with psoriasis could transmit disease in animal models. These findings laid the groundwork for characterizing the pathogenesis of psoriasis as immune mediated with skin-directed T cells playing a central role. Once these pathogenic T cells have entered the skin, they become activated and release cytokines and chemokines to attract other immune cells to perpetuate the inflammatory cascade. As the role of the T cell in psoriasis has evolved and understanding of immunopathology has increased, a multitude of biologic targets have been revealed. Newer strategies for the treatment of psoriasis have therefore focused on modifying T cells in this disease through direct elimination of activated T cells, inhibition of T-cell activation, or inhibition of cytokine secretion or activity. The mechanisms by which these new biologic agents act on psoriasis will affect their profile of efficacy and safety. Important selection criteria for optimal antipsoriatic therapies include long-term safety and tolerability, ability to produce long-lasting remissions, and convenient dosing regimens.

A triad of costimulatory molecules synergize to amplify T-cell activation in both vector-based and vector-infected dendritic cell vaccines

The activation of a T cell has been shown to require two signals via molecules present on professional antigen presenting cells: signal 1, via a peptide/MHC complex, and signal 2, via a costimulatory molecule. Here, the role of three costimulatory molecules in the activation of T cells was examined. Poxvirus (vaccinia and avipox) vectors were employed because of their ability to efficiently express multiple genes. Murine cells provided with signal 1 and infected with either recombinant vaccinia or avipox vectors containing a TRIad of COstimulatory Molecules (B7-1/ICAM-1/LFA-3, designated TRICOM) induced the activation of T cells to a far greater extent than cells infected with vectors expressing any one or two costimulatory molecules. Despite this T-cell "hyperstimulation" using TRICOM vectors, no evidence of apoptosis above that seen using the B7-1 vector was observed. Results employing the TRICOM vectors were most dramatic under conditions of either low levels of first signal or low stimulator cell to T-cell ratios. Experiments employing a four-gene construct also showed that TRICOM recombinants could enhance antigen-specific T-cell responses in vivo. These studies thus demonstrate the ability of vectors to introduce three costimulatory molecules into cells, thereby activating both CD4+ and CD8+ T-cell populations to levels greater than those achieved with the use of only one or two costimulatory molecules. This new threshold of T-cell activation has broad implications in vaccine design and development. Dendritic cells infected with TRICOM vectors were found to greatly enhance naïve T-cell activation, and peptide-specific T-cell stimulation. In vivo, peptide-pulsed DCs infected with TRICOM vectors induced cytotoxic T lymphocyte activity markedly and significantly greater than peptide-pulsed DCs.

Vector-based delivery of tumor-associated antigens and T-cell co-stimulatory molecules in the induction of immune responses and anti-tumor immunity

It has now been demonstrated in both experimental models and recent clinical trials that certain "self" antigens, which are functionally non-immunogenic in the host, can become immunogenic if presented to the immune system in a certain way. Here, we describe recombinant vaccines and vaccine strategies that have been developed to induce and potentiate T-cell responses of the host to such self-antigens. These strategies include: (a) the use of recombinant poxvirus vectors in which the tumor-associated antigen (TAA) is inserted as a transgene. Recombinant vaccinia vaccines and recombinant avipox (replication-defective) vaccines have been employed to break tolerance to a self-antigen; (b) the use of diversified prime and boost strategies using different vaccines; and (c) the insertion of multiple T-cell co-stimulatory molecules into recombinant poxvirus vectors, along with the TAA gene, to enhance T-cell immune responses to the TAA and induce anti-tumor immunity.

Antitumor immunity after vaccination with B lymphoma cells overexpressing a triad of costimulatory molecules

BACKGROUND

The costimulatory molecules B7-1, intercellular adhesion molecule-1 (ICAM-1), and leukocyte function-associated antigen-3 (LFA-3) play pivotal roles in the activation of T cells. We investigated whether in vivo vaccination with lymphoma cells infected with a recombinant, nonreplicating fowlpox (FP) virus encoding this triad of costimulatory molecules (TRICOM) could stimulate lymphoma-specific immunity.

METHODS

TRICOM-infected A20 B lymphoma cells were analyzed for expression of B7-1, ICAM-1, and LFA-3. Mice (10 per group) were vaccinated with irradiated A20 cells infected with either the TRICOM vector or the wild-type FP virus (WT-FP), challenged with live A20 tumor cells, and followed for survival. Mice with established A20 tumors were also treated with irradiated TRICOM-infected A20 cells. Survival curves were compared with the log-rank statistic. The mechanism of the antitumor effect was studied by in vivo depletion of CD4(+) and CD8(+) T cells and in vitro cytotoxicity assays. All statistical tests were two-sided.

RESULTS

A20 tumor cells infected with TRICOM expressed high levels of B7-1, ICAM-1, and LFA-3. Mice vaccinated with irradiated TRICOM-infected A20 cells had prolonged survival relative to mice vaccinated with WT-FP-infected cells (80% versus 20% survival at 110 days; P<.001). In mice with established tumors, tumor growth was slower in those treated with TRICOM-infected tumor cells than in those treated with WT-FP-infected cells, and this treatment provided a survival advantage (P<.001). Depletion of CD4(+) or CD8(+) T cells reduced the antitumor immunity provided by the tumor cell-TRICOM vaccine, and lymphocytes from vaccinated mice displayed in vitro cytotoxic activity toward A20 cells.

CONCLUSIONS

Increasing expression of costimulatory molecules on B lymphoma cells by infection with a recombinant FP virus encoding B7-1, ICAM-1, and LFA-3 stimulates antitumor immune responses in vivo and may provide a novel strategy for treating patients with B-cell malignancies.

A recombinant vector expressing transgenes for four T-cell costimulatory molecules (OX40L, B7-1, ICAM-1, LFA-3) induces sustained CD4+ and CD8+ T-cell activation, protection from apoptosis, and enhanced cytokine production

The role of OX40L on the activation of T cells was investigated using poxvirus vectors expressing OX40L alone or in combination with three other T-cell costimulatory molecules: B7-1, ICAM-1, and LFA-3. Poxvirus vector-infected cells were used to stimulate nai;ve or activated CD4(+) and CD8(+) T cells. These studies demonstrate that (a) OX40L plays a role in sustaining the long-term proliferation of CD8(+) T cells in addition to the known effect on CD4(+) T cells following activation, (b) OX40L enhances the production of Th1 cytokines (IL-2, IFN-gamma, and TNF-alpha) from both CD4(+) and CD8(+) while no change in IL-4 expression was observed, and (c) the anti-apoptotic effect of OX40L on T cells is likely the result of sustained expression of anti-apoptotic genes while genes involved in apoptosis are inhibited. In addition, these are the first studies to demonstrate that the combined use of a vector driving the expression of OX40L with three other costimulatory molecules (B7-1, ICAM-1, and LFA-3) both enhances initial activation and then further potentiates sustained activation of nai;ve and effector T cells.

Pharmacokinetics, biologic activity, and tolerability of alefacept by intravenous and intramuscular administration

Alefacept, human LFA-3/IgG1 fusion protein, is currently under clinical development for the treatment of chronic plaque psoriasis and other T cell mediated disorders. This recombinant protein binds CD2 on T cells and Fc gamma RIII on accessory cells (e.g., natural killer cells, macrophages), inhibiting T cell activation/proliferation and inducing selective T cell apoptosis. These effects are associated with selective reductions in memory-effector (CD4+ CD45RO+ and CD8+ CD45RO+) T cells. Two open-label studies were conducted in healthy male volunteers to evaluate the pharmacokinetics, biologic activity, and tolerability of a single dose of alefacept when administered as a 0.15 mg/kg 30-sec i.v. bolus (n = 12), 0.04 mg/kg intramuscular (i.m.) injection (n = 8), or 0.04 mg/kg 30-min intravenous (i.v.) infusion (n = 8). i.v. infusion produced a higher Cmax (0.96 +/- 0.26 mcg/ml vs. 0.36 +/- 0.19 mcg/ml) and a shorter Tmax (2.8 +/- 1.9 hr vs. 86 +/- 60 hr) when compared to i.m. injection. Based on AUC0-last and AUC0-infenity values, the relative bioavailability of i.m. to i.v. infusion was approximately 60%. After absorption from the i.m. injection was complete, the rate of alefacept elimination from the serum appeared consistent with the i.v. infusion half-life (approximately 12 days). Biologic activity was demonstrated by transient reductions in absolute number of CD2+ lymphocytes, with notable specificity for memory T-cell subsets. Alefacept was well tolerated; the most common adverse effects were headache, pharyngitis, rash, and myalgia. IM administration was not associated with significant local reactions. Results of these studies support i.v. bolus or i.m. administration of alefacept. An i.m. dose of approximately 150 to 200% of the i.v. dose is an appropriate and convenient alternative to i.v. administration.

Critical roles of c-Rel in autoimmune inflammation and helper T cell differentiation

Different members of the Rel/NF-kappaB family may play different roles in immunity and inflammation. We report here that c-Rel-deficient mice are resistant to autoimmune encephalomyelitis and are defective in Th1, but not Th2 responses. The Th1 deficiency appears to be caused by selective blockade of IL-12 production by c-Rel-deficient antigen-presenting cells, as well as by a complete abrogation of IFN-gamma expression in c-Rel-deficient T cells. Interestingly, c-Rel deficiency does not affect T-bet expression, suggesting that c-Rel may act downstream of T-bet during Th1 cell differentiation. Thus, unlike NF-kappaB1, which selectively regulates Th2 cell differentiation, c-Rel is essential for Th1 cell differentiation and Th1 cell-mediated autoimmune inflammation.

A role for inducible costimulator protein in the CD28- independent mechanism of resistance to Toxoplasma gondii

Long-term resistance to Toxoplasma gondii is dependent on the development of parasite-specific T cells that produce IFN-gamma. CD28 is a costimulatory molecule important for optimal activation of T cells, but CD28(-/-) mice are resistant to T. gondii, demonstrating that CD28-independent mechanisms regulate T cell responses during toxoplasmosis. The identification of the B7-related protein 1/inducible costimulator protein (ICOS) pathway and its ability to regulate the production of IFN-gamma suggested that this pathway may be involved in the CD28-independent activation of T cells required for resistance to T. gondii. In support of this hypothesis, infection of wild-type or CD28(-/-) mice with T. gondii resulted in the increased expression of ICOS by activated CD4(+) and CD8(+) T cells. In addition, both costimulatory pathways contributed to the in vitro production of IFN-gamma by parasite-specific T cells and when both pathways were blocked, there was an additive effect that resulted in almost complete inhibition of IFN-gamma production. Although in vivo blockade of the ICOS costimulatory pathway did not result in the early mortality of wild-type mice infected with T. gondii, it did lead to increased susceptibility of CD28(-/-) mice to T. gondi associated with reduced serum levels of IFN-gamma, increased parasite burden, and increased mortality compared with the control group. Together, these results identify a critical role for ICOS in the protective Th1-type response required for resistance to T. gondii and suggest that ICOS and CD28 are parallel costimulatory pathways, either of which is sufficient to mediate resistance to this intracellular pathogen.

Current and future applications of immunological attenuation via pegylation of cells and tissue

Prevention of immunological rejection of transplanted tissues is of crucial importance in transplantation medicine. Current procedures primarily use pharmacological agents such as cyclosporin, which, while effective, must be typically administered for the life of the individual. Furthermore, the drug-induced global immunosuppression of the patient predisposes the individual to infection and enhances their risk of developing certain forms of cancer. Hence, additional methods are needed to both enhance tissue engraftment and diminish the adverse effects of current immunosuppressive therapy. Studies from blood transfusion (i.e. a specialised form of cellular transplantation) suggest that covalent modification of cells and tissues with methoxypoly(ethylene glycol) [mPEG] can significantly diminish rejection episodes and may further enhance the induction of tolerance to donor tissues. The mechanisms underlying mPEG-mediated immunocamouflage are the loss of antigen recognition, impaired cell-cell interaction, and an inability of endogenous antibodies (e.g. immunoglobulin G) to effectively recognise and bind foreign epitopes. As a consequence of the global camouflage imparted by mPEG, the weak co-stimulation of alloreactive T cells may subsequently induce apoptosis, thus leading to tolerance. Initial studies on the transplantation of pegylated isogeneic rat pancreatic islets demonstrates that mPEG-derivatisation does not impair in vivo cellular signalling and function. Thus, in contrast to the pharmacological inhibition of the recipient's immune response, the mPEG-mediated immunocamouflage directly addresses the inherent antigenicity and immunogenicity of the donor tissue itself while leaving the recipient a fully competent immune system.

Impact of human bladder cancer cell architecture on autologous T-lymphocyte activation

To investigate the influence of tumor cell architecture on T-cell activation, we used an autologous human model based on 2 bladder tumor cell lines as targets for cytotoxic tumor-infiltrating lymphocytes (TILs). These tumor cell lines were grown in vitro as either standard 2-dimensional (2D) monolayers or 3-dimensional (3D) spheroids. T-cell activation was determined by measuring the production of three major cytokines (tumor necrosis factor, granulocyte/macrophage colony-stimulating factor and interferon-gamma), known to be secreted by most activated TILs. Changes in the architecture of target cells from 2D to 3D induced a dramatic decrease in their capacity for stimulating TILs. Interestingly, neither TIL infiltration nor MHC class I, B7.1 costimulatory or lymphocyte function-associated factor-3 adhesion molecule downregulation played a major role in this decrease. These findings demonstrate that tumor architecture has a major impact on T-cell activation and might be implicated in the escape of tumor cells from the immune system.

The use of HLA-A*0201-transfected K562 as standard antigen-presenting cells for CD8(+) T lymphocytes in IFN-gamma ELISPOT assays

ELISPOT assays are increasingly used for a direct detection and quantification of single antigen-specific T cells in freshly isolated peripheral blood mononuclear cells (PBMC). They are particularly attractive for the monitoring of specific T lymphocyte responses in clinical trials assessing antigen-specific immunizations in patients with cancer or chronic viral infections. However, one major limitation for the broad clinical implementation of ELISPOT assays is the lack of an inexhaustible source of suitable HLA-matched antigen-presenting cells (APC). Currently available allogeneic or xenogeneic APC (such as the human lymphoid hybrid T2 or HLA-transfected insect cells) can either lead to strong background spot production by APC-reactive T lymphocytes or have a low antigen presentation capability. Both phenomena can prevent the detection of low frequency T cell responses in PBMC. In search of alternative APC for ELISPOT assays, the human chronic myelogenous leukemia cell line K562 that per se does not express HLA class I and class II molecules on the cell surface was transfected with the HLA-A*0201 gene. Clonal HLA-A*0201-expressing K562 (K562/A*0201) cells were able to process and present endogenously expressed and exogenously loaded melanoma peptide antigens to HLA-A*0201-restricted cytolytic T lymphocyte clones in cytotoxicity and IFN-gamma ELISPOT assays. K562/A*0201 cells were then used as APC in IFN-gamma spot assays to detect ex vivo CD8(+) T lymphocytes responsive to known HLA-A*0201-binding peptide epitopes derived from cytomegalovirus, Epstein-Barr virus, influenza virus and melanoma in PBMC from HLA-A*0201-positive donors. In the majority of cases, peptide-pulsed K562/A*0201 cells were similarly efficient in the ability to visualize single antigen-specific CD8(+) T lymphocytes when compared to T2 cells. However, in contrast to T2, background reactivity of CD8(+) T cells responsive to unpulsed K562/A*0201 was regularly found to be negligible, thereby enhancing the sensitivity of the ELISPOT assay, particularly in donors with strong anti-T2 reactivity. K562 cells transfected with HLA-A*0201 or other HLA genes can serve as standard APC for monitoring T lymphocyte responses against tumor and viral peptide antigens.

Enhanced activation of rhesus T cells by vectors encoding a triad of costimulatory molecules (B7-1, ICAM-1, LFA-3)

Since the rhesus is often used as a "gatekeeper" model for the evaluation of malaria and simian immunodeficiency virus (SIV)/HIV vaccines, the identification of strategies to enhance the activation of rhesus T cells would potentially aid in the generation of more potent vaccines directed against these infectious agents. Several molecules normally found on the surface of professional human APCs are capable of providing the second signals critical for T cell activation: B7-1 (CD80), ICAM-1 (CD54), and LFA-3 (CD58). With the exception of B7, T cell costimulatory molecules in the rhesus have not been identified. We have recently designed and characterized both recombinant vaccinia and recombinant avipox vectors containing the transgenes for a triad of human T cell costimulatory molecules (B7-1, ICAM-1, LFA-3; designated TRICOM). Here, we demonstrate the enhanced activation of rhesus T cells stimulated with rhesus APCs infected with TRICOM vectors in the presence of signal 1. Infection with TRICOM vectors led to significant improvement of APC capabilities in terms of reduction of the amount of signal 1 needed to activate naive T cells, and reduction in the amount of APCs required to activate T cells using a constant amount of signal 1. Antibody blocking studies demonstrated that each of the three costimulatory molecule transgenes contributed to the enhanced proliferation of T cells. TRICOM-enhanced T cell activation was shown to correspond to increases in type 1 cytokines and a reduced level of apoptosis. TRICOM-infected autologous B cells from rhesus immunized with either an SIV vaccine or a malaria vaccine stimulated significantly greater levels of IFN-gamma in response to specific peptide than stimulation with uninfected autologous B cells or B cells infected with wild-type vector. The ability to augment immune responses using poxvirus-based vaccines containing multiple costimulatory molecule transgenes can now be addressed in the rhesus macaque model.

Enhancing the potency of peptide-pulsed antigen presenting cells by vector-driven hyperexpression of a triad of costimulatory molecules

Recombinant orthopox vectors (both replication-defective fowlpox [rF], and replication competent vaccinia [rV] have been developed that simultaneously express three T-cell costimulatory molecule transgenes. The constituents of this triad of costimulatory molecules (designated TRICOM) are B7-1, ICAM-1, and LFA-3. We have previously shown that infection of murine dendritic cells (DCs) with TRICOM vectors increases their level of expression of the triad of costimulatory molecules and enhances the efficacy of DCs to activate T cells. While DCs are arguably the most potent antigen presenting cell (APC), limitations clearly exist in their use due to the level of effort and cost for their generation. The studies reported here demonstrate that a generic APC population, murine splenocytes, can be made markedly more efficient as APCs by infection with either rF-TRICOM or rV-TRICOM vectors. Infection of splenocytes with either TRICOM vector led to significant improvement of APC capabilities in terms of: (a) enhancement of mixed lymphocyte reactions; (b) a reduction in the amount of signal 1 to activate naive T cells; and (c) a reduction in the amount of APCs required to activate T cells using a constant amount of signal 1. TRICOM-enhanced T-cell activation was shown to correspond to increases in type-1 cytokines and a reduced level of apoptosis, compared with T cells activated with uninfected or control vector-infected splenocytes. In vitro and in vivo experiments compared DCs with TRICOM-infected splenocytes. Infection of splenocytes with TRICOM vectors markedly enhanced their ability to activate T cells to levels approaching that of DCs. These studies thus demonstrate for the first time that an abundant and accessible population of APCs obtainable without lengthy culture or the use of costly exogenous cytokines (in contrast to that of DCs) can be made more potent as APCs with the use of vectors that express a triad of costimulatory molecules.

Costimulation of memory T-cells by ICOS: a potential therapeutic target for autoimmunity?

Approaches that target costimulatory receptors are independent of T-cell receptor specificity and may be useful for T-cell-mediated diseases in which the antigens involved are not well defined. However, the proper costimulatory pathways need to be targeted. For example, therapies for human T-cell-mediated diseases need to be effective against previously activated memory cells. In this review, we use autoimmune demyelination as a paradigm for established immune-mediated pathogenesis. Studies with the human disease multiple sclerosis and the rodent model experimental autoimmune encephalomyelitis have suggested that the effectiveness of CD28 blockade, as a therapeutic strategy for established autoimmune demyelination, may be limited. ICOS, a receptor that appears to be involved in the costimulation of previously activated T-cells, may be an attractive alternative.

Schiff base-mediated co-stimulation primes the T-cell-receptor-dependent calcium signalling pathway in CD4 T cells

In addition to macromolecular interactions that provide co-stimulation during antigen-presenting cell (APC) and CD4+ T-cell conjugation, covalent chemical events between specialized ligands have been implicated in T-cell co-stimulation. These take the form of transient Schiff base formation between carbonyls and amines expressed on APC and T-cell surfaces. Small Schiff base-forming molecules, such as tucaresol, can substitute for the physiological donor of carbonyl groups and provide co-stimulation to T cells, thereby functioning as orally active immunopotentiatory drugs. The Schiff base co-stimulatory pathway in T cells has been partially characterized in terms of changes in Na+ and K+ transport, and activation of the mitogen activated protein kinase (MAPK) ERK2. In the present study, the effects of Schiff base co-stimulation by tucaresol on the T-cell receptor (TCR)-dependent pathway leading to Ca2+ release were investigated. Schiff base co-stimulation by tucaresol was found to prime for enhanced TCR-dependent phospholipase C-gamma phosphorylation, inositol 1,4,5-triphosphate production, and Ca2+ mobilization that correlated with functional enhancement of interleukin-2 production in primary T cells. The effects on Ca2+ occurred comparably in Jurkat and primary CD4+ T cells responding to anti-CD3 monoclonal antibody. Enhancement of the Ca2+ response required a 10-min priming period and was prevented by prior covalent ligation of cell-surface free amino groups by sulpho-N-hydroxy succinimido-biotin; clofilium-mediated inhibition of tucaresol-induced changes in intracellular K+; and selective inhibition of the MAPK pathway. The data are consistent with a priming mechanism in which late co-stimulation-triggered events exert a positive influence on early TCR-triggered events. In additional studies of murine T cells expressing trans-gene TCRs, tucaresol was likewise shown to prime for enhanced Ca2+ mobilization in response to physiological TCR-engagement by MHC-peptide complexes.

Vector-driven hyperexpression of a triad of costimulatory molecules confers enhanced T-cell stimulatory capacity to DC precursors

Activation of T cells requires at least two signals: signal 1, via the T-cell receptor, and signal 2, in which a costimulatory molecule on the antigen presenting cell (APC) interacts with a ligand on the T cell. Dendritic cells (DCs) are the most potent APCs in part due to their expression of costimulatory molecules. DCs, however, constitute only a minor percentage of APCs in the body, and the in vitro preparation of DCs is both costly and time consuming. The studies reported here demonstrate that one can utilize other APCs, such as bone marrow progenitor cells (BMPCs) and make them markedly more effective as APCs; this was accomplished by their infection with recombinant poxviruses (either the replication-defective avipox or vaccinia), which contain transgenes for a triad of costimulatory molecules (B7-1, ICAM-1 and LFA-3, designated TRICOM). APCs infected with TRICOM vectors are shown to significantly enhance the activation of both naive and effector CD4(+) and CD8(+) T-cell populations. The use of TRICOM vectors in vaccine strategies is discussed.

Melatonin provides signal 3 to unprimed CD4(+) T cells but failed to stimulate LPS primed B cells

Growing evidence has supported the conclusion that melatonin, a pineal hormone, modulates the immune function. In our previous study, we evaluated in vivo the potential role of melatonin in the regulation of the antigen specific T and B cells. In the present study, we observe that melatonin down-regulated the expression of the co-stimulatory molecule B7-1 but not B7-2 on macrophages. Further, melatonin encouraged the proliferation of anti-CD3 antibody activated CD4(+) T cells only in the presence of antigen-presenting cells and promoted the production of Th2-like cytokines. Furthermore, it failed to influence the activity of B cells in a T-independent manner. Melatonin suppressed the release of TNF-alpha by LPS or IFN-gamma activated macrophages but failed to inhibit nitric oxide (NO) release. Thus the study shows that melatonin can engineer the growth of unprimed CD4(+) T cells if both the signals are provided by antigen-presenting cells. However, it could not regulate the function of B cells.

Immunological principles of adverse drug reactions: the initiation and propagation of immune responses elicited by drug treatment

Adverse drug reactions account for between 2 to 5% of all hospital admissions and can prevent the administration of an otherwise effective therapeutic agent. Hypersensitivity or immune-mediated reactions, although less common, tend to be proportionately more serious. There is convincing evidence to implicate the immune system in the pathogenesis of hypersensitivity reactions. Our understanding of the way in which the immune system recognises drugs is based on the hapten hypothesis; the onset of hypersensitivity involves drug bioactivation, covalent binding to proteins, followed by uptake, antigen processing and T cell proliferation. Central to this hypothesis is the critical role of drug metabolism, with the balance between metabolic bioactivation and detoxification being one important component of individual susceptibility. The purpose of this review is to classify drug hypersensitivity reactions in terms of their clinical presentation, and also to consider recent advances in our understanding of the chemical, biochemical and, in particular, cellular immunological mechanisms of hypersensitivity. The following topics are reviewed: (i) drug disposition and cellular metabolism; (ii) mechanisms of antigen processing and presentation; (iii) the role of cytokines and co-stimulatory molecules in the induction and maintenance of a polarised immune response; and (iv) the application of the hapten hypothesis, danger hypothesis and serial triggering model to drug hypersensitivity. A greater understanding of the mechanism(s) of hypersensitivity may identify novel therapeutic strategies and help to combat one of the more severe forms of adverse reactions to drugs.

Enhanced activation of T cells by dendritic cells engineered to hyperexpress a triad of costimulatory molecules

BACKGROUND

Activation and proliferation of T cells are essential for a successful cellular immune response to an antigen. Antigen-presenting cells (APCs) activate T cells through a two-signal mechanism. The first signal is antigen specific and causes T cells to enter the cell cycle. The second signal involves a costimulatory molecule that interacts with a ligand on the T-cell surface and leads to T-cell cytokine production and their proliferation. Dendritic cells express several costimulatory molecules and are believed to be the most potent APCs. Two recombinant poxvirus vectors (replication-defective avipox [fowlpox; rF] and a replication-competent vaccinia [rV]) have been engineered to express a triad of costimulatory molecules (B7-1, intercellular adhesion molecule-1, and leukocyte function-associated antigen-3; designated TRICOM). This study was designed to determine if dendritic cells infected with these vectors would have an enhanced capacity to stimulate T-cell responses.

METHODS

Murine dendritic cells (of both intermediate maturity and full maturity) were infected with rF-TRICOM or rV-TRICOM and were used in vitro to stimulate naive T cells with the use of a pharmacologic agent as signal 1, to stimulate T cells in allospecific mixed lymphocyte cultures, and to stimulate CD8(+) T cells specific for a peptide from the ovalbumin (OVA) protein. In addition, dendritic cells infected with TRICOM vectors were pulsed with OVA peptide and used to vaccinate mice to examine T-cell responses in vivo. All statistical tests were two-sided.

RESULTS

Dendritic cells infected with either rF-TRICOM or rV-TRICOM were found to greatly enhance naive T-cell activation (P<.001), allogeneic responses of T cells (P<.001), and peptide-specific T-cell stimulation in vitro (P<.001). Peptide-pulsed dendritic cells infected with rF-TRICOM or rV-TRICOM induced cytotoxic T-lymphocyte activity in vivo to a markedly greater extent than peptide-pulsed dendritic cells (P =.001 in both).

CONCLUSIONS

The ability of dendritic cells to activate both naive and effector T cells in vitro and in vivo can be enhanced with the use of poxvirus vectors that potentiate the hyperexpression of a triad of costimulatory molecules. Use of either rF-TRICOM or rV-TRICOM vectors significantly improved the efficacy of dendritic cells in priming specific immune responses. These studies have implications in vaccine strategies for both cancer and infectious diseases.

Impaired antigen presentation by human monocytes during endotoxin tolerance

Endotoxin tolerance (ET) has been described as a temporary alteration in the lipopolysaccharide (LPS) response of monocytic cells after an initial LPS exposure with respect to the production of soluble immunomodulators. Apart from the LPS response, monocytic cells play an important role in initiation of the specific immune response as antigen-presenting cells. This study investigated the capacity of human blood monocytes to induce T-cell stimulation in ET. First, the expression of monocyte surface molecules, important for T-cell interaction, was analyzed by flow cytometry. In vitro priming of peripheral blood mononuclear cells with LPS clearly down-regulates major histocompatibility complex class II molecules and the costimulatory molecule CD86. Both changes were dependent on the endogenous interleukin (IL)-10 and less so on the transforming growth factor-β. In contrast, other accessory molecules on monocytes were only marginally down-regulated (CD58), were not significantly changed during ET (CD40), or even remained up-regulated after initial LPS priming (CD54, CD80). Second, an impact of these phenotypic alterations on the accessory function of monocytes was observed. This was manifested as diminished T-cell proliferation and interferon (IFN)-γ release in response to the presence of different recall antigens. Neutralizing IL-10 during LPS priming prevented the diminished T-cell IFN-γ production but had little effect on T-cell proliferation. These data confirm that ET is an appropriate model of the monocyte functional state in immunoparalysis, which is frequently observed in patients after septic shock, trauma, or major surgery.

Impaired antigen presentation by human monocytes during endotoxin tolerance

Endotoxin tolerance (ET) has been described as a temporary alteration in the lipopolysaccharide (LPS) response of monocytic cells after an initial LPS exposure with respect to the production of soluble immunomodulators. Apart from the LPS response, monocytic cells play an important role in initiation of the specific immune response as antigen-presenting cells. This study investigated the capacity of human blood monocytes to induce T-cell stimulation in ET. First, the expression of monocyte surface molecules, important for T-cell interaction, was analyzed by flow cytometry. In vitro priming of peripheral blood mononuclear cells with LPS clearly down-regulates major histocompatibility complex class II molecules and the costimulatory molecule CD86. Both changes were dependent on the endogenous interleukin (IL)-10 and less so on the transforming growth factor-β. In contrast, other accessory molecules on monocytes were only marginally down-regulated (CD58), were not significantly changed during ET (CD40), or even remained up-regulated after initial LPS priming (CD54, CD80). Second, an impact of these phenotypic alterations on the accessory function of monocytes was observed. This was manifested as diminished T-cell proliferation and interferon (IFN)-γ release in response to the presence of different recall antigens. Neutralizing IL-10 during LPS priming prevented the diminished T-cell IFN-γ production but had little effect on T-cell proliferation. These data confirm that ET is an appropriate model of the monocyte functional state in immunoparalysis, which is frequently observed in patients after septic shock, trauma, or major surgery.

Treatment of established relapsing experimental autoimmune encephalomyelitis with the proteasome inhibitor PS-519

PLP139-151-induced relapsing experimental autoimmune encephalomyelitis (R-EAE) in SJL mice is a Th1-mediated autoimmune demyelinating disease model for multiple sclerosis (MS) in which the primary disease relapse is mediated by T cells specific for the endogenous PLP178-191 epitope. This complex inflammatory process requires the co-ordinated expression of a wide variety of immune-related genes active at a variety of stages of the autoimmune process which are regulated, in part, by the transcription factor nuclear factor (NF)-kappaB which is activated via the ubiquitin-proteasome pathway. We asked if in vivo administration of a selective inhibitor of the ubiquitin-proteasome pathway, PS-519, which downregulates activation of NF-kappaB, could downregulate ongoing R-EAE. Administration of PS-519 during the remission phase, following acute clinical disease was effective in significantly reducing the incidence of clinical relapses, CNS histopathology, and T cell responses to both the initiating and relapse-associated PLP epitopes. The inhibition of clinical disease was dependent upon continuous administration of PS-519 in that recovery of T cell function and onset of disease relapses developed within 10-14 days of drug withdrawal. The data suggest that targeting the ubiquitin proteasome pathway, in particular NF-kappaB, may offer a novel and efficacious approach for the treatment of progressive autoimmune diseases, including MS.

Immune-inflammatory mechanisms in IFNgamma-mediated anti-tumor activity

IFNgamma is a functionally pleiotropic cytokine which shows considerable potency in promoting anti-tumor functions in vivo. Despite limited efficacy when delivered systemically either to experimental animals or patients, IFNgamma appears to play an important and perhaps critical role in directing the development of immune-mediated tumor destruction when expressed within the tumor bed. This has been demonstrated both by use of tumor cells transduced to express IFNgamma and by the use of IL-12 which is able, at least is murine models, to promote an IFNgamma-dependent, T cell mediated anti-tumor response. Recent studies indicate that the therapeutic efficacy of IFNgamma in tumor models depends critically upon the ability of the tumor cells themselves to respond to IFNgamma. Though IFNgamma is able to induce anti-viral activity and has direct anti-proliferative effects on some tumor cell lines, immunomodulatory function also appears to be an important component of its anti-tumor action. This is mediated through the action of several different classes of IFNgamma-inducible gene expression which control antigen processing and presentation, leukocyte trafficking, and indirect tumor cytotoxicity.

Human antigen-presenting cell/tumour cell hybrids stimulate strong allogeneic responses and present tumour-associated antigens to cytotoxic T cells in vitro

Most tumours do not stimulate effective antitumour immune responses in vivo. In order to enhance the immunogenicity of human tumour cells, we fused a variety of tumour cell lines with an Epstein-Barr virus transformed B-lymphoblastoid cell line (EBV B-LCL) in vitro, to produce stable hybrid cells. Hybrid cell lines showed a marked increase in their ability to stimulate primary allogeneic T-cell responses in vitro, as compared with the parent tumour cells. The hybrid cells induced proliferation of naive (CD45RA+) as well as memory (CD45RO+) T lymphocytes, and both CD4+ and CD8+ subpopulations of T cells were directly stimulated. The stimulatory hybrids expressed human leucocyte antigen (HLA) class I and II, and a wide range of surface accessory molecules, including the T-cell co-stimulatory ligand molecules CD40, CD80 (B7.1) and CD86 (B7.2), the expression of which was required for optimal stimulation of T-cell responses. Fusion of the EBVB-LCL with a melanoma cell line (518.A2) yielded hybrid cells that expressed the melanoma-associated antigens MAGE-1 and MAGE-3, and presented these antigens to antigen-specific, HLA class I-restricted cytotoxic T-lymphocyte clones with greater efficiency than the parent melanoma cell line. These findings suggest that the generation of human antigen-presenting cell/tumour cell hybrids offers promise as an approach to cancer immunotherapy.

Ligation of CD28 In Vivo Induces CD40 Ligand Expression and Promotes B Cell Survival

Functional activation of T cells requires ligation of Ag receptors with specific peptides presented by MHC molecules on APCs concurrent with appropriate contacts of cell surface accessory molecules. Among these accessory molecules, interactions between CD28/CTLA-4 with B7 family members (CD80 and CD86) and CD40 with CD40 ligand (CD40L) play a decisive role in regulating the progression of balanced immune responses. However, most information regarding the role of accessory molecules in immune responses has been derived in the context of signals from the TCRs. Little understanding has been achieved regarding the consequence of ligation of costimulation molecules in absence of signals from the TCR. By employing an in vivo murine system, we show, herein, that ligation of CD28 alone with anti-CD28 Abs leads to a dramatic enlargement of the peripheral lymphoid organs characterized primarily by the expansion of B cells. B cells from anti-CD28-treated mice are resistant to spontaneous and anti-IgM-induced apoptosis. These cells are also unsusceptible to FasL-mediated apoptosis. Interestingly, this in vivo effect of CD28 on B cells is largely mediated by inducing the expression of CD40L, since coadministration of a blocking Ab against CD40L inhibited CD28-mediated B cell survival and expansion. Therefore, CD28-mediated expression of CD40L may play an important role in the regulation of lymphocyte homeostasis.

Stealth Cells: Prevention of Major Histocompatibility Complex Class II-Mediated T-Cell Activation by Cell Surface Modification

Transfusion or transplantation of T lymphocytes into an allogeneic recipient can evoke potent immune responses including, in immunocompromised patients, graft-versus-host disease (GVHD). As our previous studies demonstrated attenuated immunorecognition of red blood cells covalently modified with methoxy(polyethylene glycol) (mPEG), we hypothesized that T-cell activation by foreign antigens might similarly be prevented by mPEG modification. Mixed lymphocyte reactions (MLR) using peripheral blood mononuclear cells (PBMC) from HLA class II disparate donors demonstrate that mPEG modification of PBMC effectively inhibits T-cell proliferation (measured by 3H-thymidine incorporation) in a dose-dependent manner. Even slight derivatization (0.4 mmol/L mPEG per 4 × 106 cells) resulted in a ≥75% decrease, while higher concentrations caused ≥96% decrease in proliferation. Loss of PBMC proliferation was not due to either mPEG-induced cytotoxicity, as viability was normal, or cellular anergy, as phytohemagglutinin (PHA)-stimulated mPEG-PBMC demonstrated normal proliferative responses. Addition of exogenous interleukin (IL)-2 also had no proliferative effect, suggesting that the mPEG-modified T cells were not antigen primed. Flow cytometric analysis demonstrates that mPEG-modification dramatically decreases antibody recognition of multiple molecules involved in essential cell:cell interactions, including both T-cell molecules (CD2, CD3, CD4, CD8, CD28, CD11a, CD62L) and antigen-presenting cell (APC) molecules (CD80, CD58, CD62L) likely preventing the initial adhesion and costimulatory events necessary for immune recognition and response.

Stealth Cells: Prevention of Major Histocompatibility Complex Class II-Mediated T-Cell Activation by Cell Surface Modification

Transfusion or transplantation of T lymphocytes into an allogeneic recipient can evoke potent immune responses including, in immunocompromised patients, graft-versus-host disease (GVHD). As our previous studies demonstrated attenuated immunorecognition of red blood cells covalently modified with methoxy(polyethylene glycol) (mPEG), we hypothesized that T-cell activation by foreign antigens might similarly be prevented by mPEG modification. Mixed lymphocyte reactions (MLR) using peripheral blood mononuclear cells (PBMC) from HLA class II disparate donors demonstrate that mPEG modification of PBMC effectively inhibits T-cell proliferation (measured by 3H-thymidine incorporation) in a dose-dependent manner. Even slight derivatization (0.4 mmol/L mPEG per 4 × 106 cells) resulted in a ≥75% decrease, while higher concentrations caused ≥96% decrease in proliferation. Loss of PBMC proliferation was not due to either mPEG-induced cytotoxicity, as viability was normal, or cellular anergy, as phytohemagglutinin (PHA)-stimulated mPEG-PBMC demonstrated normal proliferative responses. Addition of exogenous interleukin (IL)-2 also had no proliferative effect, suggesting that the mPEG-modified T cells were not antigen primed. Flow cytometric analysis demonstrates that mPEG-modification dramatically decreases antibody recognition of multiple molecules involved in essential cell:cell interactions, including both T-cell molecules (CD2, CD3, CD4, CD8, CD28, CD11a, CD62L) and antigen-presenting cell (APC) molecules (CD80, CD58, CD62L) likely preventing the initial adhesion and costimulatory events necessary for immune recognition and response.

Experimental Autoimmune Encephalomyelitis in NF-κB- Deficient Mice: Roles of NF-κB in the Activation and Differentiation of Autoreactive T Cells

Experimental autoimmune encephalomyelitis (EAE) is an inflammatory disease of the CNS, which has long been used as an animal model for human multiple sclerosis. Development of autoimmune disease requires coordinated expression of a number of genes that are involved in the activation and effector functions of inflammatory cells. These include genes that encode costimulatory molecules, cytokines, chemokines, and adhesion molecules. Activation of these genes is regulated at the transcriptional level by several families of transcription factors. One of these is the NF-κB family, which is present in a variety of cell types and becomes highly activated at sites of inflammation. To test the roles of NF-κB in the development of autoimmune diseases, we studied EAE in mice deficient in one of the NF-κB isoforms, i.e., NF-κB1 (p50). We found that NF-κB1-deficient mice were significantly resistant to EAE induced by myelin oligodendrocyte glycoprotein. The resistance was primarily evidenced by a decrease in disease incidence, clinical score, and the degree of CNS inflammation. Furthermore, we established that the resistance to EAE in NF-κB1-deficient mice was associated with a deficiency of myelin oligodendrocyte glycoprotein-specific T cells to differentiate into either Th1- or Th2-type effector cells in vivo. These results strongly suggest that NF-κB1 plays crucial roles in the activation and differentiation of autoreactive T cells in vivo and that blocking NF-κB function can be an effective means to prevent autoimmune encephalomyelitis.

Interactions of HIV-1 with antigen-presenting cells

There is currently much interest in the numerical and functional loss of antigen-presenting cells (APC) in HIV-1 disease and the contribution that this may make to HIV-1 pathology. The HIV-1 virus can interfere with the normal function of APC in a number of ways involving inappropriate signalling. These include changes in cytokine balance, cell-surface molecule expression and intracellular signalling pathways. This review examines how HIV-1 is able to disregulate APC function and discusses possible outcomes for the function of the immune system.

The diversity of T-cell co-stimulation in the induction of antitumor immunity

T-cell activation has now been shown to require at least two signals. The first signal is antigen-specific, is delivered through the T-cell receptor (TCR) via the peptide/major histocompatibility complex (MHC), and causes the T cell to enter the cell cycle. The second, or co-stimulatory, signal is required for cytokine production and proliferation, and is mediated through ligand interaction on the surface of the T cell. This chapter deals with: 1) comparative studies on the use of a dual-gene construct of a recombinant vaccinia (rV) vector containing a tumor-associated antigen (TAA) gene and a co-stimulatory molecule gene vs the use of admixtures of rV-TAA and rV containing the co-stimulatory molecule to induce anti-tumor immunity; 2) the use of an admixture of vaccinia viruses containing a TAA gene and the B7-1 co-stimulatory molecule gene to induce a therapeutic response in a lung metastasis tumor model; 3) the antitumor efficacy of whole-tumor-cell vaccines in which the B7-1 co-stimulatory molecule is expressed in a tumor-cell vaccine via a vaccinia vs a retroviral vector; 4) the use of recombinant poxviruses containing the genes for the co-stimulatory molecules ICAM-1 or LFA-3 to induce antitumor immunity; and 5) the use of poxvirus vectors containing a triad of co-stimulatory molecules (B7-1, ICAM-1 and LFA-3) that synergize to enhance both CD4+ and CD8+ T-cell responses to a new threshold.

Functional glycan-free adhesion domain of human cell surface receptor CD58: design, production and NMR studies

A general strategy is presented here for producing glycan-free forms of glycoproteins without loss of function by employing apolar-to-polar mutations of surface residues in functionally irrelevant epitopes. The success of this structure-based approach was demonstrated through the expression in Escherichia coli of a soluble 11 kDa adhesion domain extracted from the heavily glycosylated 55 kDa human CD58 ectodomain. The solution structure was subsequently determined and binding to its counter-receptor CD2 studied by NMR. This mutant adhesion domain is functional as determined by several experimental methods, and the size of its binding site has been probed by chemical shift perturbations in NMR titration experiments. The new structural information supports a 'hand-shake' model of CD2-CD58 interaction involving the GFCC'C" faces of both CD2 and CD58 adhesion domains. The region responsible for binding specificity is most likely localized on the C, C' and C" strands and the C-C' and C'-C" loops on CD58.

Polarized expression and function of the costimulatory molecule CD58 on human intestinal epithelial cells

BACKGROUND & AIMS

Intestinal epithelial cells (IECs) can process foreign protein antigens and display antigenic peptides to CD4(+) T lymphocytes via HLA class II molecules. The purpose of this study was to determine the nature of the second, or costimulatory, signal provided by IECs.

METHODS

We investigated surface expression of the costimulatory molecules CD58 (LFA-3), CD80 (B7-1), and CD86 (B7-2) by using flow cytometry, confocal microscopy, and vectorial biotinylation. Antibodies specific for CD58, CD80, and CD86 were used in blocking experiments to assess the role of these molecules in providing a costimulatory signal to CD4(+) T cells by IECs.

RESULTS

CD58, but not CD80 or CD86, was observed to be expressed constitutively on both native IECs and in the IEC lines T84 and HT-29. The surface expression of CD58 was highly polarized and restricted to the basolateral surface of the cell. Antibodies against CD58, but not CD80 or CD86, inhibited the stimulation of CD4(+) T-cell proliferation mediated by IECs.

CONCLUSIONS

CD58 is expressed by polarized IECs in a topologically restricted manner at the region of T-cell contact and can function as a costimulatory molecule in HLA class II-mediated antigen presentation.

Ikaros sets thresholds for T cell activation and regulates chromosome propagation

T cell activation involves the sustained accumulation of T cell receptor (TCR) and IL-2 receptor (IL-2R) mediated signaling events that promote cell cycle entry and progression. The Ikaros family of nuclear factors regulate this process by providing thresholds overcome by receptor signaling. T cells with reduced levels of Ikaros activity require fewer TCR engagement events for activation, exhibit a greater proliferative response to IL-2, and are less sensitive to inhibitors of TCR and IL-2R signaling. Upon T cell activation, Ikaros proteins localize in a higher-order chromatin structure where they colocalize with components of the DNA replication machinery. Proliferating T cells with reduced Ikaros activity display chromosome abnormalities. We propose that participation of Ikaros in higher-order chromatin structures controls cell cycle transitions and restricts DNA replication.

Signaling through the tetraspanin CD82 triggers its association with the cytoskeleton leading to sustained morphological changes and T cell activation

In this report, we provide new evidence of a crosstalk between T cell activation and adhesion processes through a functional cytokeleton. We show that CD82 signaling induces long-lasting adhesion, spreading and development of membrane extensions, involving actin polymerization. Addition of various co-stimuli (phorbol 12-myristate 13-acetate or monoclonal antibodies to CD3 or CD2) increases the CD82-induced morphological alterations and, reciprocally, CD82 engagement synergizes with these stimuli to induce T cell activation as indicated by both primary tyrosine phosphorylation and IL-2 production. Different kinases are involved in both processes. CD82 co-signaling involves src kinases including p56 Ick. On the other hand, the CD82-induced alterations of cell morphology are negatively regulated by cAMP-dependent kinases independently of activation of src kinases. Simultaneously with cytoskeletal rearrangements, we observed an inducible association of CD82 with the cytoskeletal matrix. In addition, the potentiating and stabilizing effects induced by CD82 cross-linking on tyrosine phosphorylation were abolished by cytoskeleton-disrupting agents. These results suggest that the actin polymerization triggered by CD82, through its ability to associate with the cytoskeletal matrix, is the primary step involved in the CD82 induced co-stimulatory activity. Our data provide further evidence for a direct role of the actin cytoskeleton as a major component for sustained signal transduction in T cells and suggest that tetraspanins could be "membrane organizers" connecting both surface and intracellular molecules.

Experimental autoimmune encephalomyelitis in intercellular adhesion molecule-1-deficient mice

Intercellular adhesion molecule (ICAM)-1, or CD54, is a member of the immunoglobulin superfamily that binds to lymphocyte function-associated antigen-1 and macrophage-1 antigen. ICAM-1:LFA-1/Mac-1 interaction may be involved in both activation and extravasation of leukocytes. To determine the roles of ICAM-1 in the development of autoimmune disease, we studied experimental autoimmune encephalomyelitis (EAE) in mice deficient in ICAM-1. We found that T cell proliferation and TH1-type cytokine production in response to myelin antigen were significantly reduced in ICAM-1-deficient mice, whereas TH2-type cytokine IL-10 was increased. Unexpectedly, EAE induced by either myelin oligodendrocyte glycoprotein or myelin basic protein was significantly enhanced in mice deficient in ICAM-1. The enhancement was evidenced primarily by the increase in disease severity, mortality, and the degree of central nervous system inflammation. The cellular composition of the inflammatory infiltrates in the central nervous system is similar in control and ICAM-1-deficient mice. These results suggest that (1) ICAM-1 is involved in the activation of autoreactive TH-1, but not TH2 cells, and (2) ICAM-1 plays an important role in down-regulating autoimmune inflammation in the central nervous system.

Antibodies to TR2 (Herpesvirus Entry Mediator), a New Member of the TNF Receptor Superfamily, Block T Cell Proliferation, Expression of Activation Markers, and Production of Cytokines

TR2 (TNFR-related 2) is a recently identified member of the TNFR family with homology to TNFRII. We have demonstrated previously that TR2 mRNA is expressed in resting and activated human T cells and that TR2-Ig partially inhibits an allogeneic mixed leukocyte proliferation response. We now characterize TR2 further by the use of specific mAbs. Flow-cytometry analysis using TR2 mAbs confirmed that resting PBL express high levels of cell surface TR2, and that TR2 is widely expressed on all freshly isolated lymphocyte subpopulations. However, stimulation of purified T cells with either PHA or PHA plus PMA resulted in decreased surface expression within 48 h of activation before returning to resting levels at 72 h. TR2 mAbs inhibited CD4+ T cell proliferation in response to stimulation by immobilized CD3 or CD3 plus CD28 mAbs. Assay of culture supernatants by ELISA showed inhibition of TNF-α, IFN-γ, IL-2, and IL-4 production, which, for IL-2 and TNF-α was also confirmed by intracellular cytokine staining. Furthermore, expression of activation markers on CD4+ T cells, including CD25, CD30, CD69, CD71, and OX40 (CD134), was inhibited. TR2 mAbs inhibited proliferation in a three-way MLR, and a response to soluble recall Ag, tetanus toxoid. In conclusion, these results suggest that TR2 is involved in the activation cascade of T cell responses and TR2 mAbs prevent optimal T cell proliferation, cytokine production, and expression of activation markers.

Human T Lymphocyte Proliferative Response to Resting Porcine Endothelial Cells Results from an HLA-Restricted, IL-10-Sensitive, Indirect Presentation Pathway But Also Depends on Endothelial-Specific Costimulatory Factors

To investigate the mechanisms of cellular rejection in pig-to-human xenotransplantation, the proliferation of different human purified lymphocyte subpopulations in response to swine leukocyte Ag class II-negative porcine aortic endothelial cells (PAEC) was measured in the presence or absence of human autologous adherent cells (huAPC). CD8+ lymphocytes proliferated moderately in the absence of huAPC, and the immune response was slightly increased when huAPC were added. CD56+ lymphocytes failed to proliferate in response to PAEC whether huAPC were present or not. CD4+ lymphocytes alone did not proliferate in response to PAEC, but a strong proliferative response was observed in the presence of metabolically active huAPC. This response was totally abolished by mAbs directed against HLA class II molecules or by pretreatment of huAPC by human IL-10. Even in the presence of huAPC, CD4+ lymphocytes failed to respond to fixed PAEC or to PAEC-lysates, suggesting that PAEC must be viable to support lymphocyte proliferation. Finally, none of the nonendothelial porcine adherent cells tested was able to induce human lymphocyte proliferation, despite the fact that they also provided a large set of xenogeneic peptides. Our results show that the indirect presentation pathway of xenoantigens by huAPC to CD4+ lymphocytes is crucial in the response to porcine endothelial cells, and that IL-10 could be of therapeutic interest to prevent human lymphocyte activation by this pathway. Furthermore, we demonstrated that stimulatory signals specifically provided by endothelial cells are also necessary for this huAPC-restricted proliferative response.

Redox signals and NF-kappaB activation in T cells

Accumulating data from a number of laboratories have recently indicated that the response of transcription factor NF-kappaB to alterations in the redox homeostasis of cells may play an important role in modulating immune function. The activation of NF-kappaB has been recognized to regulate a number of genes necessary for normal T cell responses including IL-2, IL-6, IL-8, and several T cell surface receptors. Diminished NF-kappaB activity has been shown to occur in T cells with aging, suggesting that impaired activation of NF-kappaB might occur during cellular senescence. In addition, aberrancies in NF-kappaB activity have been implicated in the immunopathogenesis of diseases involving immune or inflammatory processes such as atherosclerosis and HIV-1 infection. The role of H2O2 and other reactive oxygen species (ROS) as an integratory secondary messenger for divergent T cell signals has been complicated by the fact that various T cell lines and peripheral blood T cells differ markedly in the levels of NF-kappaB activation induced by oxidant stress. Additionally, proposed pathways of NF-kappaB activation have been based on indirect evidence provided by experiments which used antioxidants to inhibit active NF-kappaB formation. Further, complete activation of T cells requires at least two signals, one that stimulates an increase in intracellular calcium and one that stimulates enzymatic processes including kinases. Similarly, substantial evidence indicates that full activation of NF-kappaB requires dual signals. The ability of H2O2 or other ROS to induce T cell signals and functional responses by these two mechanisms is reviewed and the specific response of NF-kappaB to redox changes in T cells is examined. Data are also presented to suggest that the redox regulation in NF-kappaB activation may be relevant to immune-related diseases and to aging.

Trypanosoma cruzi-infected macrophages are defective in major histocompatibility complex class II antigen presentation

Trypanosoma cruzi, the intracellular protozoan parasite that causes Chagas' disease, interferes with the host immune response to establish a persistent infection. In this report, we demonstrate that macrophages infected with T. cruzi are unable to effectively present antigens to CD4 T cells. The interference is due to defective antigen-presenting cell (APC) function, as antigen-independent stimulation of the T cell in the presence of infected macrophages is not affected. The defect is distal to antigen processing and is not due to decreased major histocompatibility complex (MHC) class II expression, decreased viability, defective peptide loading in the infected macrophages, nor absence of CD28 co-stimulation. There was a role for gp39: CD40 co-stimulation during antigen presentation to the T cells we studied, but the expression of CD40 on T. cruzi-infected macrophages was not decreased. Antigen-specific adhesion between macrophages and T cells was reduced by infection. Equivalent levels of the adhesion molecules lymphocyte function-associated antigen-1, intercellular adhesion molecule-1, vascular cell adhesion molecule-1 or very late antigen-4 are found on infected and uninfected APC, suggesting that reduced expression of these adhesion molecules was not responsible for the defect in antigen-specific adhesion. The defective T cell:macrophage adhesion may be due to the reduced expression of other adhesion molecules or other changes in the cell induced by infection. Interfering with MHC class II antigen presentation in infected macrophages may help T. cruzi to blunt the immune response by the host.