Distinct mRNA microarray profiles of tolerogenic dendritic cells

AIRE is not essential for the induction of human tolerogenic dendritic cells

Loss-of-function mutations of the Autoimmune Regulator (AIRE) gene results in organ-specific autoimmunity and disease Autoimmune Polyendocrinopathy type 1 (APS1)/Autoimmune Polyendocrinopathy Candidiasis Ectodermal Dystrophy (APECED). The AIRE protein is crucial in the induction of central tolerance, promoting ectopic expression of tissue-specific antigens in medullary thymic epithelial cells and enabling removal of self-reactive T-cells. AIRE expression has recently been detected in myeloid dendritic cells (DC), suggesting AIRE may have a significant role in peripheral tolerance. DC stimulation of T-cells is critical in determining the initiation or lack of an immune response, depending on the pattern of costimulation and cytokine production by DCs, defining immunogenic/inflammatory (inflDC) and tolerogenic (tolDC) DC. In AIRE-deficient patients and healthy controls, we validated the role of AIRE in the generation and function of monocyte-derived inflDC and tolDCs by determining mRNA and protein expression of AIRE and comparing activation markers (HLA-DR/DP/DQ,CD83,CD86,CD274(PDL-1),TLR-2), cytokine production (IL-12p70,IL-10,IL-6,TNF-α,IFN-γ) and T-cell stimulatory capacity (mixed lymphocyte reaction) of AIRE+ and AIRE- DCs. We show for the first time that: (1) tolDCs from healthy individuals express AIRE; (2) AIRE expression is not significantly higher in tolDC compared to inflDC; (3) tolDC can be generated from APECED patient monocytes and (4) tolDCs lacking AIRE retain the same phenotype and reduced T-cell stimulatory function. Our findings suggest that AIRE does not have a role in the induction and function of monocyte-derived tolerogenic DC in humans, but these findings do not exclude a role for AIRE in peripheral tolerance mediated by other cell types.

MERTK as negative regulator of human T cell activation

The aim of this study was to test the hypothesis whether MERTK, which is up-regulated in human DCs treated with immunosuppressive agents, is directly involved in modulating T cell activation. MERTK is a member of the TAM family and contributes to regulating innate immune response to ACs by inhibiting DC activation in animal models. However, whether MERTK interacts directly with T cells has not been addressed. Here, we show that MERTK is highly expressed on dex-induced human tol-DCs and participates in their tolerogenic effect. Neutralization of MERTK in allogenic MLR, as well as autologous DC-T cell cultures, leads to increased T cell proliferation and IFN-γ production. Additionally, we identify a previously unrecognized noncell-autonomous regulatory function of MERTK expressed on DCs. Mer-Fc protein, used to mimic MERTK on DCs, suppresses naïve and antigen-specific memory T cell activation. This mechanism is mediated by the neutralization of the MERTK ligand PROS1. We find that MERTK and PROS1 are expressed in human T cells upon TCR activation and drive an autocrine proproliferative mechanism. Collectively, these results suggest that MERTK on DCs controls T cell activation and expansion through the competition for PROS1 interaction with MERTK in the T cells. In conclusion, this report identified MERTK as a potent suppressor of T cell response.

Efferocytosis promotes suppressive effects on dendritic cells through prostaglandin E2 production in the context of autoimmunity

Introduction

Efferocytosis is a crucial process by which apoptotic cells are cleared by phagocytes, maintaining immune tolerance to self in the absence of inflammation. Peripheral tolerance, lost in autoimmune processes, may be restored by the administration of autologous dendritic cells loaded with islet apoptotic cells in experimental type 1 diabetes.

Objective

To evaluate tolerogenic properties in dendritic cells induced by the clearance of apoptotic islet cells, thus explaining the re-establishment of tolerance in a context of autoimmunity.

Methods

Bone marrow derived dendritic cells from non-obese diabetic mice, a model of autoimmune diabetes, were generated and pulsed with islet apoptotic cells. The ability of these cells to induce autologous T cell proliferation and to suppress mature dendritic cell function was assessed, together with cytokine production. Microarray experiments were performed using dendritic cells to identify differentially expressed genes after efferocytosis.

Results

Molecular and functional changes in dendritic cells after the capture of apoptotic cells were observed. 1) Impaired ability of dendritic cells to stimulate autologous T cell proliferation after the capture of apoptotic cells even after proinflammatory stimuli, with a cytokine profile typical for immature dendritic cells. 2) Suppressive ability of mature dendritic cell function. 3) Microarray-based gene expression profiling of dendritic cells showed differential expression of genes involved in antigen processing and presentation after efferocytosis. 4) Prostaglandin E2 increased production was responsible for immunosuppressive mechanism of dendritic cells after the capture of apoptotic cells.

Conclusions

The tolerogenic behaviour of dendritic cells after islet cells efferocytosis points to a mechanism of silencing potential autoreactive T cells in the microenvironment of autoimmunity. Our results suggest that dendritic cells may be programmed to induce specific immune tolerance using apoptotic cells; this is a viable strategy for a variety of autoimmune diseases.

An gene expression pattern

DNA microarray technology has become a powerful tool in the arsenal of the molecular biologist. Capitalizing on high-precision robotics and the wealth of DNA sequences annotated from the genomes of a large number of organisms, the manufacture of microarrays is now possible for the average academic laboratory with the funds and motivation. Microarray production requires attention to both biological and physical resources, including DNA libraries, robotics, and qualified personnel. Although the fabrication of microarrays is a very labor-intensive process, production of quality microarrays individually tailored on a project-by-project basis will help researchers shed light on future scientific questions.

CD8+ suppressor and cytotoxic T cells recognize the same human leukocyte antigen-A2 restricted cytomegalovirus peptide

We explored the possibility that antigen-specific human CD8(+) T cells, which display cytotoxic or suppressor function, can recognize the same peptide epitope. Using the human leukocyte antigen-A0201 restricted immunodominant cytomegalovirus epitope pp65-NLVPMVATV for pulsing either mature/immunogenic or ILT3(high)ILT4(high) tolerogenic dendritic cells (DC), we generated cytotoxic and suppressor CD8(+) T-cell lines, respectively. Our data indicate that modulating the functional state of DC is crucial to the development of tolerogenic or immunogeneic peptide-specific vaccines.

CD8+ T suppressor cells and the ILT3 master switch

Similar to helper and cytotoxic T cells, CD8(+) T suppressor cells (Ts) acquire antigen specificity via direct interaction with antigen-presenting cells (APC). They induce the upregulation of the inhibitory receptor immunoglobulin-like transcript (ILT)3 on professional and nonprofessional APC, rendering these cells tolerogenic and able to induce the differentiation of further waves of regulatory and suppressor T cells. This review sums up evidence that ILT3 is the centerpiece of CD8(+) Ts-driven suppression and acts as a master switch in the regulation of CD8(+) and CD4(+) T-cell responses to antigens in transplantation, autoimmunity, allergy, and cancer.

Central role of ILT3 in the T suppressor cell cascade

CD8+ T suppressor cells differentiate both in vivo and in vitro upon chronic exposure of responding T cells to allogeneic APC. These Ts are allospecific and exhibit their function interacting directly with priming APC which they render tolerogenic. Tolerogenicity of professional and non-professional human APC, such as dendritic cells and endothelial cells, respectively is due to the upregulation of the inhibitory receptors ILT3 and ILT4. ILT3 signals both intracellularly, inhibiting NF-kappaB activation, and transcription of costimulatory molecules, and extracellularly, inducing anergy and regulatory function in T cells with cognate specificity. Both membrane and soluble ILT3 are proteins with potent immunosuppressive activity which are of importance for treatment of rejection, autoimmunity and cancer.

Dendritic cells and myeloid leukaemias: plasticity and commitment in cell differentiation

Dendritic cells (DCs) are key antigen-presenting cells (APCs), which link innate and adaptive immunity, ultimately activating antigen-specific T cells. This review examines the relationship between the acute and chronic myeloid leukaemias and cells with DC properties. DCs are non-dividing terminally differentiated cells, and ex vivo leukaemic cells or cell lines show little similarity to DCs. However, many leukaemias differentiate further in response to defined stimuli, and retain a degree of lineage plasticity. Therefore, several studies have explored the response of leukaemic cells to the in vitro regimens used to differentiate ex vivo primary DCs. Recent data suggest that the most 'dendritic-like' cells can be derived from more undifferentiated myeloid leukaemias, such as the myelomonocytic Mutz-3 cell line. These findings have important implications for understanding the developmental origins of DCs, for harnessing the APC properties of this class of tumour to stimulate the therapeutic anti-tumour immunity, and for developing useful models for the study of human DC physiology and pathology. There is a strong rationale for further exploration of this class of tumour and its relationship to the normal DC.

Paired immunoglobin-like receptors A and B are new targets for inducing dendritic cells tolerance in mice

The expression of paired immunoglobin-like receptors A (PIR-A) and B (PIR-B) and their relationship with tolerogenic dendritic cells (T-DC) in mice were investigated. The mouse DCs line, DC2.4 cells were cultured with the recombinant murine interleukin-10 (IL-10) and recombinant human transforming growth factor beta1 (TGF-beta1) respectively to develop the T-DC and stimulated with lipopolysaccharide (LPS) for 48 h to induce the mature dendritic cells (LPS-DC). Special small interfering RNAs (siRNA) molecule for PIR-B was chemically synthesized and transfected into DC2.4 cells (Si-DC) by lip2000. The expression of PIRs on DC2.4 cells were detected by semi-quantitative reverse transcription polymerase chain reaction (RT-PCR), flow cytometry (FCM) and Western blot. Realtime reverse transcriptase-polymerase chain reaction (Realtime-PCR) was applied for measurement of PIR-A and CD80, CD86, MHC-II mRNA expression. The allogeneic stimulating capacity of DCs was measured by mixed lymphocyte reaction (MLR) using (3)H-thymidine incorporation test. The concentration of IFN-gamma in supernatants of MLR from distinct groups was analyzed by ELISA. The results showed that PIR positive rate was (28.65+/-8.12)% examined by FCM on DC2.4 cells. PIR positive rate was increased dramatically to (54.21+/-6.34)%, (58.78+/-4.70)%, (48.24+/-6.75)% respectively for IL-10, TGF-beta1 and LPS induction (P<0.01), but there was no significantly different among the three groups (P>0.01). The semi-quantitative RT-PCR and Western blot revealed that IL-10 and TGF-beta1 induced the higher PIR-B level and lower PIR-A level. On the contrary, the LPS down-regulated the PIR-B expression and up-regulated the PIR-A expression. Realtime PCR examination demonstrated that PIR-A and co-stimulating molecules such as CD80, CD86 and MHC-II were increased significantly after stimulation with LPS. Compared with the DC2.4 cells and the LPS-DC, the T-DCs inhibited alloactivated T cell proliferation and down-regulated the IFN-gamma secretion in MLR supernatant. Si-DC promoted the T cell proliferation (P<0.01) and enhanced the IFN-gamma secretion (P<0.01). It was concluded that up-regulating the PIR-B and down-regulating the PIR-A expression were the general feature of phenotype and constructed the new targets for dendritic cells to acquire immune tolerance in mice. Overexpression of PIR-B can inhibit the up-regulation of the PIR-A, CD80, CD86 and MHC-II expression, which might be the molecular mechanism for the T-DC.

Tumor immunology

Advances in tumor immunology are supporting the clinical implementation of several immunological approaches to cancer in the clinical setting. However, the alternate success of current immunotherapeutic regimens underscores the fact that the molecular mechanisms underlying immune-mediated tumor rejection are still poorly understood. Given the complexity of the immune system network and the multidimensionality of tumor/host interactions, the comprehension of tumor immunology might greatly benefit from high-throughput microarray analysis, which can portrait the molecular kinetics of immune response on a genome-wide scale, thus accelerating the discovery pace and ultimately catalyzing the development of new hypotheses in cell biology. Although in its infancy, the implementation of microarray technology in tumor immunology studies has already provided investigators with novel data and intriguing new hypotheses on the molecular cascade leading to an effective immune response against cancer. Although the general principles of microarray-based gene profiling have rapidly spread in the scientific community, the need for mastering this technique to produce meaningful data and correctly interpret the enormous output of information generated by this technology is critical and represents a tremendous challenge for investigators, as outlined in the first section of this book. In the present Chapter, we report on some of the most significant results obtained with the application of DNA microarray in this oncology field.

CD28-mediated regulation of multiple myeloma cell proliferation and survival

Although interactions with bone marrow stromal cells are essential for multiple myeloma (MM) cell survival, the specific molecular and cellular elements involved are largely unknown, due in large part to the complexity of the bone marrow microenvironment itself. The T-cell costimulatory receptor CD28 is also expressed on normal and malignant plasma cells, and CD28 expression in MM correlates significantly with poor prognosis and disease progression. In contrast to T cells, activation and function of CD28 in myeloma cells is largely undefined. We have found that direct activation of myeloma cell CD28 by anti-CD28 mAb alone induces activation of PI3K and NFkappaB, suppresses MM cell proliferation, and protects against serum starvation and dexamethasone (dex)-induced cell death. Coculture with dendritic cells (DCs) expressing the CD28 ligands CD80 and CD86 also elicits CD28-mediated effects on MM survival and proliferation, and DCs appear to preferentially localize within myeloma infiltrates in primary patient samples. Our findings suggest a previously undescribed myeloma/DC cell-cell interaction involving CD28 that may play an important role in myeloma cell survival within the bone marrow stroma. These data also point to CD28 as a potential therapeutic target in the treatment of MM.

Identification of gene expression changes induced by chemical allergens in dendritic cells: opportunities for skin sensitization testing

Cellular changes within resident skin dendritic cells (DCs) after allergen uptake and processing are critical events in the acquisition of skin sensitization. Here we describe the development of a set of selection criteria to derive a list of potential target genes from previous microarray analyses of human peripheral blood-derived (peripheral blood mononuclear cells (PBMCs)-DCs) treated with dinitrobenzene sulfonic acid for predicting skin-sensitizing chemicals. Based on those criteria, a probing evaluation of the target genes has been conducted using an extended chemical data set, comprising five skin irritants and 11 contact allergens. PBMCs-DCs were treated for 24 hours with various concentrations of chemicals and in each instance the expression of up to 60 genes was examined by real-time PCR analysis. Consistent allergen-induced changes in the expression of many genes were observed and further prioritization of the targets was conducted by analysis of the same genes in DCs treated with non-sensitizing chemicals to determine their specificity for skin sensitization. Real-time PCR analyses of multiple chemical allergens, irritants, and non-sensitizers have identified 10 genes that demonstrate reproducibly high levels of selectivity, specificity, and dynamic range consistent with providing the basis for robust and sensitive alternative approaches for the identification of skin-sensitizing chemicals.

Overlap between molecular markers expressed by naturally occurring CD4+CD25+ regulatory T cells and antigen specific CD4+CD25+ and CD8+CD28- T suppressor cells

Alloantigen specific CD8+CD28- T suppressor (TS) cells differ from naturally occurring CD4+CD25+ T-regulatory (natural TR) cells not only by their phenotype but also by their mechanism of action. Natural TR have been extensively studied, leading to the identification of characteristic "molecular markers" such as Forkhead box P3 (FOXP3), glucocorticoid-induced tumor necrosis factor receptor-related protein (GITR) and cytotoxic T lymphocyte-associated antigen 4 (CTLA-4). We have investigated the expression of these genes in alloantigen specific TS and CD4+CD25+ T regulatory (TR) cells and found that they are expressed at levels similar to those observed in natural TR. Furthermore, similar to natural CD4+CD25+ TR, antigen-specific CD8+CD28-CD62L+ TS cells have more suppressive capacity than CD8+CD28-CD62L- TS cells. In spite of these similarities, natural TR are not antigen-specific and inhibit other T cells by T cell-to-T cell interaction, whereas TS are antigen-specific and exert their inhibitory function by interacting with antigen-presenting cells and render them tolerogenic to other T cells. The molecular characterization of TS cells may contribute to a better understanding of mechanisms involved in inhibition of immune responses in autoimmunity, transplantation, and chronic viral infection.

Expression analysis of immune-related genes in CD34+ progenitor-derived dendritic cells after exposure to the chemical contact allergen DNCB

We studied the changes in gene expression after exposure of human dendritic cells (DCs) to the model allergen dinitrochlorobenzene (DNCB). DCs were derived from CD34(+) progenitor cells of three different donors and exposed to 10 microM DNCB or solvent for several time intervals (3, 6 and 12h). cDNA microarrays were used to assess the transcriptional activity of 11,000 human genes. Compared to control gene expression, changes larger than +/-two-fold were observed for 241 genes after exposure to DNCB. Of these genes, 137 were up-regulated and 104 down-regulated. Twenty of these genes encode proteins that are related to the immune response (cytokines, chemokines, their receptors, cytokine/chemokines-related genes, transcription and signal transduction genes) and are discussed in more detail. Our data indicate that exposure to DNCB does not induce a typical maturation pattern in DCs.

Regulation of RelB expression during the initiation of dendritic cell differentiation

The transcription factor RelB is required for proper development and function of dendritic cells (DCs), and its expression is upregulated early during differentiation from a variety of progenitors. We explored this mechanism of upregulation in the KG1 cell line model of a DC progenitor and in the differentiation-resistant KG1a subline. RelB expression is relatively higher in untreated KG1a cells but is upregulated only during differentiation of KG1 by an early enhancement of transcriptional elongation, followed by an increase in transcription initiation. Restoration of protein kinase CbetaII (PKCbetaII) expression in KG1a cells allows them to differentiate into DCs. We show that PKCbetaII also downregulated constitutive expression of NF-kappaB in KG1a-transfected cells and restores the upregulation of RelB during differentiation by increased transcriptional initiation and elongation. The two mechanisms are independent and sensitive to PKC signaling levels. Conversely, RelB upregulation was inhibited in primary human monocytes where PKCbetaII expression was knocked down by small interfering RNA targeting. Altogether, the data show that RelB expression during DC differentiation is controlled by PKCbetaII-mediated regulation of transcriptional initiation and elongation.

Molecular characterization of allospecific T suppressor and tolerogenic dendritic cells: review

T suppressor and regulatory cells have been shown to play an important role in the maintenance of central and peripheral tolerance thereby preventing allograft rejection, autoimmunity and allergy. We have previously described a distinct population of antigen-specific CD8(+)CD28(-) T suppressor cells (T(S)). These CD8(+)CD28(-) T(S) cells can be generated in vitro after multiple rounds of stimulation of human peripheral blood mononuclear cells (PBMCs) with either allogenic- or xenogeneic-donor APCs. CD8(+)CD28(-) T(S) cells are FOXP3+, MHC class I-restricted and tolerize both professional antigen presenting cells, such as dendritic cells (DC) and nonprofessional APC such as endothelial cells (EC) by up-regulating the cell surface expression of inhibitory receptors immunoglobulin-like transcript (ILT)-3 and ILT4 and down-regulating the expression of costimulatory molecules such as CD58 and CD86. Tolerized ILT3(high), ILT4(high) APC anergize CD4(+) T(H) cells and can induce the generation of antigen-specific CD4(+)CD25(+) T regulatory cells (T(R)) cells and CD8(+)CD28(-) T(S) cells. In this review, we present our recent studies on the molecular characterization of these antigen specific T suppressor cells and tolerogenic APC.

License to heal: bidirectional interaction of antigen-specific regulatory T cells and tolerogenic APC

Naturally occurring CD4(+)CD25(+) regulatory T (T(R)) cells, a component of the innate immune response, which play a key role in the maintenance of self-tolerance, have become the focus of numerous studies over the last decade. These cells inhibit the immune response in an Ag-nonspecific manner, interacting with other T cells. Much less is known about adaptive T(R) cells, which develop in response to chronic antigenic stimulation, and act directly on professional and nonprofessional APC, rendering them tolerogenic and able to elicit the differentiation of CD8(+) and CD4(+) T cells with suppressive activity. In this review, we will discuss data pertaining to the bidirectional interaction between Ag-specific T(R) with APC and their clinical relevance.

Protein kinase C betaII plays an essential role in dendritic cell differentiation and autoregulates its own expression

Dendritic cells (DC) arise from a diverse group of hematopoietic progenitors and have marked phenotypic and functional heterogeneity. The signal transduction pathways that regulate the ability of progenitors to undergo DC differentiation, as well as the specific characteristics of the resulting DC, are only beginning to be characterized. We have found previously that activation of protein kinase C (PKC) by cytokines or phorbol esters drives normal human CD34(+) hematopoietic progenitors and myeloid leukemic blasts (KG1, K562 cell lines, and primary patient blasts) to differentiate into DC. We now report that PKC activation is also required for cytokine-driven DC differentiation from monocytes. Of the cPKC isoforms, only PKC-betaII was consistently activated by DC differentiation-inducing stimuli in normal and leukemic progenitors. Transfection of PKC-betaII into the differentiation-resistant KG1a subline restored the ability to undergo DC differentiation in a signal strength-dependent fashion as follows: 1) by development of characteristic morphology; 2) the up-regulation of DC surface markers; 3) the induction of expression of the NFkappaB family member Rel B; and 4) the potent ability to stimulate allo-T cells. Most unexpectedly, the restoration of PKC-betaII signaling in KG1a was not directly due to overexpression of the transfected classical PKC (alpha, betaII, or gamma) but rather through induction of endogenous PKC-beta gene expression by the transfected classical PKC. The mechanism of this positive autoregulation involves up-regulation of PKC-beta promoter activity by constitutive PKC signaling. These findings indicate that the regulation of PKC-betaII expression and signaling play critical roles in mediating progenitor to DC differentiation.

DNA array-based gene profiling in tumor immunology

Recent advances in tumor immunology have fostered the clinical implementation of different immunotherapy modalities. However, the alternate success of such regimens underscores the fact that the molecular mechanisms underlying tumor immune rejection are still poorly understood. Given the complexity of the immune system network and the multidimensionality of tumor-host interactions, the comprehension of tumor immunology might greatly benefit from high-throughput DNA array analysis, which can portray the molecular kinetics of immune response on a genome-wide scale, thus accelerating the accumulation of knowledge and ultimately catalyzing the development of new hypotheses in cell biology. Although in its infancy, the implementation of DNA array technology in tumor immunology studies has already provided investigators with novel data and intriguing hypotheses on the cascade of molecular events leading to an effective immune response against cancer. Although the principles of DNA array-based gene profiling techniques have become common knowledge, the need for mastering this technique to produce meaningful data and correctly interpret this enormous output of information is critical and represents a tremendous challenge for investigators. In the present work, we summarize the main technical features and critical issues characterizing this powerful laboratory tool and review its applications in the fascinating field of cancer immunogenomics.

Antigen presentation by MART-1 adenovirus-transduced interleukin-10-polarized human monocyte-derived dendritic cells

Dendritic cells (DC) play critical roles in generating an immune response and in inducing tolerance. Diverse microenvironmental factors can 'polarize' DC toward an immunogenic or non-immunogenic phenotype. Among the various microenvironmental factors, interleukin-10 (IL-10) exhibits a potent immunosuppressive effect on antigen-presenting cells (APC). Here, we show that monocyte-derived DC generated in the presence of IL-10 exhibit a profound down-regulation of many genes that are associated with immune activation and show that the IL-10-grown DC are poor stimulators of CD8(+) T cells in a strictly autologous and major histocompatibility complex (MHC) class I-restricted melanoma antigen recognized by T cells (MART-1) epitope presentation system. However, these IL-10-grown DC can efficiently activate the epitope-specific CD8(+) T cells when they are made to present the epitope following transduction with an adenoviral vector expressing the MART-1 antigen. In addition, we show that the MART-1 protein colocalizes with the MHC class I protein, equally well, in the iDC and in the DC cultured in presence of IL-10 when both DC types are infected with the viral vector. We also show that the vector transduced DC present the MART-1(27-35) epitope for a sustained period compared to the peptide pulsed DC. These data suggest that although DCs generated in the presence of IL-10 tend to be non-immunogenic, they are capable of processing and presenting an antigen when the antigen is synthesized within the DC.

Alloantigen specific CD8+CD28- FOXP3+ T suppressor cells induce ILT3+ ILT4+ tolerogenic endothelial cells, inhibiting alloreactivity

Endothelial cells have been shown to activate T cell responses to alloantigens, triggering transplant rejection. However, they may also play a role in tolerance induction. Using RT-PCR we show here that alloantigen specific CD8(+)CD28(-) T suppressor cells generated in vitro are FOXP3 positive and interact with human endothelial cells. This interaction results in the induction of inhibitory receptors and down-regulation of costimulatory and adhesion molecules, thus rendering endothelial cells tolerogenic. In turn, tolerized endothelial cells elicit the differentiation of CD8(+)CD28(-) FOXP3(+) T suppressor cells. Taken together our data demonstrate a functional and phenotypic overlap between tolerogenic dendritic cells and endothelial cells. Furthermore, alloantigen specific CD8(+)CD28(-) FOXP3(+) T cells, which trigger the upregulation of inhibitory receptors in endothelial cells, are present in the circulation of heart allograft recipients in quiescence as demonstrated by flow cytometry, RT-PCR and luciferase transcription assays. Their detection facilitates the identification of patients who may benefit from partial or complete cessation of immunosuppressive therapy, a goal of obvious importance given the morbidity and mortality associated with chronic immunosuppression. Modulation of endothelial cells in favor of promoting tolerance may be important for long-term survival of organ allografts.

New phenotypic, functional and electrophysiological characteristics of KG-1 cells

Myeloid dendritic cells (DC) are representatives of a rare and phenotypically diverse population of professional antigen presenting cells possessing high functional heterogeneity and flexibility. Here we studied the phenotypic, functional and electrophysiological characteristics of KG-1 cells, an erythroleukemia model cell line, which shares morphological and physiological similarities with immature and mature myeloid DC. We compared the expression of internalizing receptors and other cell surface molecules, antigen uptake and migration of unstimulated and activated KG-1 cells with the characteristics of immature and mature DC. Unstimulated KG-1 cells were less potent in capturing extracellular materials than immature DC. In contrast to monocyte-derived DC KG-1 cells stimulated by PMA and ionomycin ceased to migrate along the MIP-3beta chemokine gradient despite their high expression of CCR7 chemokine receptor and MDR, a transporter implicated in DC migration. Moreover, we determined the ion channel repertoire of KG-1 cells before and after treatment with PMA and ionomycin by using the patch-clamp technique. We found that both unstimulated and activated KG-1 cells expressed time- and voltage-independent, ChTx sensitive intracellular Ca(2+)-gated potassium conductance suggesting the presence of K(Ca) channels in their membranes. Based on our results we propose that KG-1 cells resemble myeloid DC but also possess unique phenotypic, functional and electrophysiological characteristics.

Immunophenotype and functional properties of feline dendritic cells derived from blood and bone marrow

Dendritic cells (DCs) are a heterogeneous population of cells of fundamental importance in initiating innate as well as specific immune responses. The identity and function of DCs in the cat are unknown, although they are likely pivotal in the response to infection. In this study, feline DCs were derived by 3-10-day culture of adherent blood mononuclear cells (PBMCs) and bone marrow mononuclear cells (BMMCs) in the presence of IL 4 and GM-CSF. BMMC consistently yielded a greater number of DCs than PBMC, and there were fewer macrophages than DC from both compartments. DCs expressed a distinct constellation of surface molecules, which included CD1a, CD1b, and CD1c, CD11b, CD14, and 2-3-fold higher levels of MHC class I and II molecules than co-cultured macrophages or fresh blood monocytes. DCs displayed typical cytoplasmic processes, limited non-specific esterase activity, and acquired antigen by phagocytosis, pinocytosis, and binding to specific receptors. Cytokine-exposed cells induced proliferation of allogeneic lymphocytes. Thus, the cells derived by these culture conditions had markers and functions analogous to immature myeloid DCs. Availability of feline DCs will enable investigation of their role in infectious disease and their potential therapeutic application.

Generation and function of antigen-specific suppressor and regulatory T cells

The identification and characterization of regulatory and suppressor T cells that control immune responsiveness to self and non-self antigens has become the focus of innumerable studies. There are two broad categories of naturally occurring and induced CD4(+)CD25(+) regulatory T cells. Naturally occurring T(R) are antigen non-specific and interact directly with other T cells inhibiting their activation. Induced T(R) are either CD4(+)CD25(+) or CD8(+), produce immunosuppressive cytokines such as IL-10, act directly on other T cells or APC and are antigen specific in some but not in all systems. Finally, a distinct subset of T suppressor cells, characterized by their CD8(+)CD28(-) phenotype have been shown to be antigen-specific, recognizing HLA class I/peptide complexes. T(S) act directly on APC inducing the up-regulation of inhibitory receptors ILT3 and ILT4, which render the APC tolerogenic. Tolerized APC, which expresses high ILT3 and ILT4, trigger the generation of antigen-specific CD4(+) T(R) propagating antigen-specific suppression. Up-regulation of ILT3 and ILT4 appears to be a general characteristic of tolerogenic DC since it is also induced by use of vit D3, IL-10 and/or IFN-alpha. The clinical relevance of these inhibitory receptors is in the maintenance of transplantation tolerance as well as in progression of AIDS has been demonstrated.

High expression of ILT3 and ILT4 is a general feature of tolerogenic dendritic cells

The direct interaction between antigen specific CD8(+) CD28(-) T suppressor cells (T(S)) with dendritic cells (DC) results in the tolerization of DC by inducing the upregulation of immunologlobulin like transcript 3 (ILT3) and ILT4. We show here that such tolerogenic DC anergize alloreactive CD4(+) CD45RO(+) CD25(+) T cells converting them into regulatory T cells (T(R)), which in turn, continue the cascade of suppression by tolerizing other DC. Interleukin 10 (IL-10) and interferon-alpha (IFN-alpha) also induce ILT3 and ILT4 upregulation in DC, rendering them tolerogenic. This implies a common mechanism of DC-mediated suppression. This finding and the observation that in organ allograft recipients quiescence is associated with the presence in the circulation of donor-specific T(S) and T(R) emphasize the importance of the cross talk between tolerogenic DC and T cells in suppression of the immune response.

Tailoring of immunosuppression in renal and liver allograft recipients displaying donor specific T-suppressor cells

Although transplantation tolerance cannot be yet reliably achieved in humans, there is evidence that active immunosuppression contributes to the maintenance of quiescence. However, the mechanism that underlies quiescence and the precise identity of regulatory cells are not completely understood. We have demonstrated that allograft recipients who remain rejection-free display allospecific T-suppressor cells (Ts). Ts express the CD8(+) CD28(-) phenotype, recognize major histocompatibility complex (MHC) class I antigens, and suppress the up-regulation of costimulatory molecules induced by CD40 ligation of donor antigen presenting cells. The presence of Ts is inversely correlated with T cell alloreactivity to donor MHC peptides, alloantibody production, and rejection. Monitoring of Ts has been successfully used in our studies for tailoring immunosuppression in kidney and liver allograft recipients.

Role of dendritic cells in graft-versus-host disease

A major barrier to successful allogeneic hematopoietic stem cell transplantation is graft-versus-host disease (GVHD). Until recently, the role of antigen presentation in the development of this disorder was unknown. The experimental finding that recipient antigen-presenting cells (APCs) were required for the development of CD8(+) T cell-dependent GVHD has led to a fundamental reappraisal of our ideas concerning the pathogenesis of this disease. Following transplantation, the origin (donor or recipient), number, lineage, and function of APCs within the recipient are altered significantly. Studies that test the influence of each of these factors upon graft-versus-host responses, including graft-versus-tumor responses, are beginning to emerge and suggest that APCs, such as dendritic cells, constitute a potential target for therapeutic manipulation.

Dendritic cells crossprime allo-specific self-restricted CD4(+) T cells after coculture with dead allogeneic cells

Indirect alloreactivity, i.e., the recognition of allopeptides on self-MHC molecules, contributes both to acute and chronic rejection of transplants. The antigen presenting cell priming these allo-specific self-restricted T cells is unknown. We demontrate that dendritic cells, which have been matured in the presence of necrotic allogeneic cells, can crossprime allo-specific self-restricted CD4(+) T cells in vitro. We demonstrate dendrtitic cell mediated crosspriming of HLA-DR13 specific, HLA-DR7 restricted and HLA-DR1 specific, HLA-DR11 restricted CD4(+) T cells. The allo-specific self-restricted CD4(+) T cells primed in our culture system secrete predominantly Th1 and not Th2 cytokines. The use of dendritic cells to monitor the indirect pathway of alloreactivity should help to design and understand interventions against acute and chronic transplant rejection.

Tolerization of dendritic cells by T(S) cells: the crucial role of inhibitory receptors ILT3 and ILT4

Immunoglobulin-like transcript 3 (ILT3) and ILT4 belong to a family of inhibitory receptors expressed by human monocytes and dendritic cells. We show here that CD8+CD28(-) alloantigen-specific T suppressor (TS) cells induce the up-regulation of ILT3 and ILT4 on monocytes and dendritic cells, rendering these antigen-presenting cells (APCs) tolerogenic. Tolerogenic APCs show reduced expression of costimulatory molecules and induce antigen-specific unresponsiveness in CD4+ T helper cells. Studies of human heart transplant recipients showed that rejection-free patients have circulating TS cells, which induce the up-regulation of ILT3 and ILT4 in donor APCs. These findings demonstrate an important mechanism of immune regulation.

Avoiding horror autotoxicus: the importance of dendritic cells in peripheral T cell tolerance

The immune system generally avoids horror autotoxicus or autoimmunity, an attack against the body's own constituents. This avoidance requires that self-reactive T cells be actively silenced or tolerized. We propose that dendritic cells (DCs) play a critical role in establishing tolerance, especially in the periphery, after functioning T cells have been produced in the thymus. In the steady state, meaning in the absence of acute infection and inflammation, DCs are in an immature state and not fully differentiated to carry out their known roles as inducers of immunity. Nevertheless, immature DCs continuously circulate through tissues and into lymphoid organs, capturing self antigens as well as innocuous environmental proteins. Recent experiments have provided direct evidence that antigen-loaded immature DCs silence T cells either by deleting them or by expanding regulatory T cells. This capacity of DCs to induce peripheral tolerance can work in two opposing ways in the context of infection. In acute infection, a beneficial effect should occur. The immune system would overcome the risk of developing autoimmunity and chronic inflammation if, before infection, tolerance were induced to innocuous environmental proteins as well as self antigens captured from dying infected cells. For chronic or persistent pathogens, a second but dire potential could take place. Continuous presentation of a pathogen by immature DCs, HIV-1 for example, may lead to tolerance and active evasion of protective immunity. The function of DCs in defining immunologic self provides a new focus for the study of autoimmunity and chronic immune-based diseases.