Endocytosis by antigen presenting cells: dendritic cells are as endocytically active as other antigen presenting cells

Enhanced Donor Antigen Presentation by B Cells Predicts Acute Cellular Rejection and Late Outcomes After Transplantation

Background

Enhanced B-cell presentation of donor alloantigen relative to presentation of HLA-mismatched reference alloantigen is associated with acute cellular rejection (ACR), when expressed as a ratio called the antigen presenting index (API) in an exploratory cohort of liver and intestine transplant (LT and IT) recipients.

Methods

To test clinical performance, we measured the API using the previously described 6-h assay in 84 LT and 54 IT recipients with median age 3.3 y (0.05-23.96). Recipients experiencing ACR within 60 d after testing were termed rejectors.

Results

We first confirmed that B-cell uptake and presentation of alloantigen induced and thus reflected the alloresponse of T-helper cells, which were incubated without and with cytochalasin and primaquine to inhibit antigen uptake and presentation, respectively. Transplant recipients included 76 males and 62 females. Rejectors were tested at median 3.6 d before diagnosis. The API was higher among rejectors compared with nonrejectors (2.2 ± 0.2 versus 0.6 ± 0.04, P value = 1.7E-09). In logistic regression and receiver-operating-characteristic analysis, API ≥1.1 achieved sensitivity, specificity, and positive and negative predictive values for predicting ACR in 99 training set samples. Corresponding metrics ranged from 80% to 88% in 32 independent posttransplant samples, and 73% to 100% in 20 independent pretransplant samples. In time-to-event analysis, API ≥1.1 predicted higher incidence of late donor-specific anti-HLA antibodies after API measurements in LT recipients (P = 0.011) and graft loss in IT recipients (P = 0.008), compared with recipients with API <1.1, respectively.

Conclusions

Enhanced donor antigen presentation by circulating B cells predicts rejection after liver or intestine transplantation as well as higher incidence of DSA and graft loss late after transplantation.

Development and Evaluation of a Physiologically Based Pharmacokinetic Model for Predicting the Effects of Anti-FcRn Therapy on the Disposition of Endogenous IgG in Humans

This work scaled up a previously developed physiologically based pharmacokinetic model to predict the effects of anti-FcRn agents on the disposition of endogenous IgG in human subjects. Simulations were performed with the scaled model to predict the effects of single- and multiple-dose administration of anti-FcRn monoclonal antibodies (1-256 mg/kg) and high-dose intravenous immune globulin (0.4-2 g/kg). The model was evaluated for prediction accuracy through comparison to the effects of rozanolixizumab, an anti-FcRn monoclonal antibodies under current clinical evaluation, on the disposition of endogenous IgG in healthy human subjects. The model provided reasonably accurate predictions of the effects of rozanolixizumab. Prediction errors for the maximum reduction in endogenous IgG concentrations were -8.50% (90% model prediction interval: -14.0% to 1.44%), 3.33% (90% model prediction interval: -13.9% to 21.2%), and 6.85% (90% model prediction interval: -35.2% to 10.5%) for rozanolixizumab doses of 1, 4, and 7 mg/kg, respectively. Model simulations predict that anti-FcRn therapies will exhibit greater dose potency in healthy volunteers than in patients with elevated IgG production rates (e.g., as typically found in autoimmune disease). The model appears to have potential for use in assessing and predicting novel dosing strategies for anti-FcRn therapies.

Application of Physiologically Based Pharmacokinetic Modeling to Predict the Effects of FcRn Inhibitors in Mice, Rats, and Monkeys

There is a growing interest in developing inhibitors of the neonatal Fc-receptor, FcRn, for use in the treatment for humoral autoimmune conditions. We have developed a new physiologically based pharmacokinetic model that is capable of characterizing the pharmacokinetics and pharmacodynamics of anti-FcRn monoclonal antibodies (mAb) in mice, rats, and monkeys. The model includes incorporation of FcRn recycling of immune gamma globulin (IgG) in hematopoietic cells in addition to FcRn recycling of IgG in vascular endothelial cells and considers FcRn turnover and intracellular cycling. The model captured antibody disposition in wild-type and FcRn-knockout mice and rats, and also predicted the effects of intravenous immune globulin and anti-FcRn mAb on IgG disposition. Simulations predicted the change in IgG clearance in response to intravenous immune globulin with good accuracy in rats (mean prediction error of 7.15% ± 7.67%). In monkeys, prediction windows for simulated IgG concentration versus time data, as generated through Monte Carlo simulation, were able to capture the effects of anti-FcRn mAb on endogenous IgG. The model may have utility in guiding preclinical evaluations of anti-FcRn therapies in development, potentially assisting in the identification of optimal dosing strategies for this emerging class of immunosuppressive drugs.

A Quantitative Systems Pharmacology Platform to Investigate the Impact of Alirocumab and Cholesterol-Lowering Therapies on Lipid Profiles and Plaque Characteristics

Reduction in low-density lipoprotein cholesterol (LDL-C) is associated with decreased risk for cardiovascular disease. Alirocumab, an antibody to proprotein convertase subtilisin/kexin type 9 (PCSK9), significantly reduces LDL-C. Here, we report development of a quantitative systems pharmacology (QSP) model integrating peripheral and liver cholesterol metabolism, as well as PCSK9 function, to examine the mechanisms of action of alirocumab and other lipid-lowering therapies, including statins. The model predicts changes in LDL-C and other lipids that are consistent with effects observed in clinical trials of single or combined treatments of alirocumab and other treatments. An exploratory model to examine the effects of lipid levels on plaque dynamics was also developed. The QSP platform, on further development and qualification, may support dose optimization and clinical trial design for PCSK9 inhibitors and lipid-modulating drugs. It may also improve our understanding of factors affecting therapeutic responses in different phenotypes of dyslipidemia and cardiovascular disease.

Enhanced B Cell Alloantigen Presentation and Its Epigenetic Dysregulation in Liver Transplant Rejection

T cell suppression prevents acute cellular rejection but causes life-threatening infections and malignancies. Previously, liver transplant (LTx) rejection in children was associated with the single-nucleotide polymorphism (SNP) rs9296068 upstream of the HLA-DOA gene. HLA-DOA inhibits B cell presentation of antigen, a potentially novel antirejection drug target. Using archived samples from 122 white pediatric LTx patients (including 77 described previously), we confirmed the association between rs9296068 and LTx rejection (p = 0.001, odds ratio [OR] 2.55). Next-generation sequencing revealed that the putative transcription factor (CCCTC binding factor [CTCF]) binding SNP locus rs2395304, in linkage disequilibrium with rs9296068 (D' 0.578, r(2) = 0.4), is also associated with LTx rejection (p = 0.008, OR 2.34). Furthermore, LTx rejection is associated with enhanced B cell presentation of donor antigen relative to HLA-nonidentical antigen in a novel cell-based assay and with a downregulated HLA-DOA gene in a subset of these children. In lymphoblastoid B (Raji) cells, rs2395304 coimmunoprecipitates with CTCF, and CTCF knockdown with morpholino antisense oligonucleotides enhances alloantigen presentation and downregulates the HLA-DOA gene, reproducing observations made with HLA-DOA knockdown and clinical rejection. Alloantigen presentation is suppressed by inhibitors of methylation and histone deacetylation, reproducing observations made during resolution of rejection. Enhanced donor antigen presentation by B cells and its epigenetic dysregulation via the HLA-DOA gene represent novel opportunities for surveillance and treatment of transplant rejection.

Effect of influenza A infection on maturation and function of neonatal monocyte-derived dendritic cells

Dendritic cells (DC) are essential for the first-line innate defense against influenza infection. The greater susceptibility to severe influenza infection in young infants and neonates may be attributed in part to their defective DC function. We sought to investigate the effect of influenza A virus (IAV) infection on the maturation, apoptosis, and function of monocyte-derived dendritic cells (MoDCs) from umbilical cord blood (UCB) and compared this with responses from adult peripheral blood (APB). Our findings were as follows. First, MoDCs derived from UCB showed deficient CD40, CD80, CD86, and HLA-DR upregulation following IAV infection compared to APB MoDCs. Second, IAV induced a multiplicity of infection (MOI)-dependent increase of apoptosis in UCB MoDCs, similar to that observed with APB. Third, the ability of UCB MoDCs to uptake dextran is decreased following IAV infection. Fourth, deficient TNF-α, but not IL-6, IFN-α response was induced by IAV infection of UCB MoDCs. Fifth, the ability of UCB MoDCs to promote allogeneic CD3 T-cell proliferation is inhibited by IAV infection. Taken together, we demonstrated a differential response of UCB and APB MoDCs following IAV infection, which may contribute in part to the increased susceptibility to severe influenza infection observed in young infants and neonates.

Targeted delivery of antigen processing inhibitors to antigen presenting cells via mannose receptors

Improved chemical inhibitors are required to dissect the role of specific antigen processing enzymes and to complement genetic models. In this study we explore the in vitro and in vivo properties of a novel class of targeted inhibitor of aspartic proteinases, in which pepstatin is coupled to mannosylated albumin (MPC6), creating an inhibitor with improved solubility and the potential for selective cell tropism. Using these compounds, we have demonstrated that MPC6 is taken up via mannose receptor facilitated endocytosis, leading to a slow but continuous accumulation of inhibitor within large endocytic vesicles within dendritic cells and a parallel inhibition of intracellular aspartic proteinase activity. Inhibition of intracellular proteinase activity is associated with reduction in antigen processing activity, but this is epitope-specific, preferentially inhibiting processing of T cell epitopes buried within compact proteinase-resistant protein domains. Unexpectedly, we have also demonstrated, using quenched fluorescent substrates, that little or no cleavage of the disulfide linker takes place within dendritic cells. This does not appear to affect the activity of MPC6 as an inhibitor of cathepsins D and E in vitro and in vivo. Finally, we have shown that MPC6 selectively targets dendritic cells and macrophages in spleen in vivo. Preliminary results suggest that access to nonlymphoid tissues is very limited in the steady state but is strongly enhanced at local sites of inflammation. The strategy adopted for MPC6 synthesis may therefore represent a more general way to deliver chemical inhibitors to cells of the innate immune system, especially at sites of inflammation.

The effects of dietary lipids on dendritic cells in perinodal adipose tissue during chronic mild inflammation

The effects of dietary lipids on the abundance of dendritic cells in adipose tissue in anatomically defined relationships to chronically inflamed lymph nodes were investigated in mature male rats fed plain chow or chow plus 20 % sunflower-seed or fish oil. The popliteal lymph nodes were stimulated by local subcutaneous injection of 20 μg lipopolysaccharide to both hindlegs three times/week for 2 weeks. The masses of the major adipose depots and the numbers of dendritic cells emerging from perinodal adipose tissue and samples 5 and 10 mm from the popliteal lymph nodes were measured, and those from omental and mesenteric adipose tissue around and remote from lymphoid tissue, and mesenteric and popliteal lymph nodes. Dendritic cells were most numerous in the perinodal adipose tissue, with the corresponding ‘remote’ samples containing 25–50 % fewer such cells under all conditions studied. Dietary sunflower-seed oil increased the numbers of dendritic cells by about 17 % in all adipose samples and fish oil reduced the numbers in perinodal tissue by about 5 %. The fish-oil diet diminished responses of the intra-abdominal adipose depots to local stimulation of the popliteal node. Correlations in dendritic cell numbers were stronger between perinodal samples from different depots than between remote and perinodal samples from the same depot and after the sunflower-seed-oil diet compared with fish oil. These data show that dietary lipids modulate the number of dendritic cells in lymphoid tissue-containing adipose depots and support the hypothesis that perinodal adipose tissue interacts locally with lymphoid cells.

Mannose–pepstatin conjugates as targeted inhibitors of antigen processing

The molecular details of antigen processing, including the identity of the enzymes involved, their intracellular location and their substrate specificity, are still incompletely understood. Selective inhibition of proteolytic antigen processing enzymes such as cathepsins D and E, using small molecular inhibitors such as pepstatin, has proven to be a valuable tool in investigating these pathways. However, pepstatin is poorly soluble in water and has limited access to the antigen processing compartment in antigen presenting cells. We have synthesised mannose-pepstatin conjugates, and neomannosylated BSA-pepstatin conjugates, as tools for the in vivo study of the antigen processing pathway. Conjugation to mannose and to neomannosylated BSA substantially improved the solubility of the conjugates relative to pepstatin. The mannose-pepstatin conjugates showed no reduction in inhibition of cathepsin E, whereas the neomannosylated BSA-pepstatin conjugates showed some loss of inhibition, probably due to steric factors. However, a neomannosylated BSA-pepstatin conjugate incorporating a cleavable disulfide linkage between the pepstatin and the BSA showed the best uptake to dendritic cells and the best inhibition of antigen processing.

Dendritic cell-based approaches to cancer immunotherapy

Immunologic approaches to the treatment of malignancies are currently enjoying a resurgence of enthusiasm due to the discovery of tumour-associated antigens and the requirements for stimulating a tumour antigen-specific immune response. The goal of the newer strategies is to stimulate immunity against specific tumour-associated antigens, rather than to broadly, but non-specifically, stimulate the immune system. Since dendritic cells, professional antigen-presenting cells, play a central role in stimulating immune responses in vivo, there is considerable interest in immunising patients with autologous dendritic cells loaded with tumour antigens of interest. Methods for generating large numbers of dendritic cells under clinically-applicable conditions have been developed and it has been shown that they may be loaded with antigen in many different forms including proteins or peptides, RNA or DNA and cellular extracts. Ongoing research is seeking to optimise the purity, antigen loading and maturation of the dendritic cells. Phase I clinical trials have been initiated in order to study the safety and feasibility of immunisations with dendritic cells in humans with various malignancies. Phase II studies will be performed to establish which tumours and clinical scenarios will be most relevant for dendritic cell immunotherapy. Although the commercial applicability of dendritic cell-based immunotherapy has been recognised by the biotechnology industry, commercial availability of dendritic cell vaccines await phase III studies.

Site-Specific Differences in Fatty Acid Composition of Dendritic Cells and Associated Adipose Tissue in Popliteal Depot, Mesentery, and Omentum and Their Modulation by Chronic Inflammation and Dietary Lipids

BACKGROUND

This study explores the role of lymphatics-associated adipocytes in determining the lipid composition of dendritic cells.

METHODS AND RESULTS

Adult male rats were fed plain chow, or chow supplemented with 20% sunflower or fish oil. Chronic local inflammation was induced by subcutaneous injection of 20 microg lipopolysaccharide three times a week for 2 weeks near the popliteal lymph nodes. Chemokine-stimulated dendritic cells were collected over 4 hours from popliteal and mesenteric lymph nodes, and perinodal and other samples of mesenteric, popliteal, and omental adipose tissue. Fatty acids extracted from triacylglycerols and/or phospholipids were separated and quantified by gas chromatography from each sample of dendritic cells and intracellular lipids, membranes, stroma and isolated adipocytes from the adipose tissue. Dendritic cells from lymph nodes and adipose tissue samples differed in fatty acid composition, and were modulated by diet. The site-specific differences of dendritic cells correlated with those of the contiguous adipocytes. Chronic mild stimulation altered the lipid composition of dendritic cells near the inflamed site and elsewhere; changes were minimal after the fish-oil diet. The composition of adipocyte triacylglycerol and phospholipid fatty acids also changed near the stimulation site in ways that counteracted alterations induced by the experimental diets.

CONCLUSIONS

Fatty acids in dendritic cells differed with anatomical site, and were determined by the adjacent adipocytes, which actively regulated their own lipid composition. These findings demonstrated functional bases for the anatomical associations between adipose and lymphoid tissues and may be a mechanism by which dietary lipids modulate the immune system.

Modulation of dendritic cell endocytosis and antigen processing pathways by Escherichia coli heat-labile enterotoxin and mutant derivatives

Escherichia coli heat-labile enterotoxin (LT) is known to be a potent adjuvant of both the mucosal and systemic immune systems but the mechanism of action leading to adjuvant activity remains incompletely understood. This study investigates the action of LT and LT mutants with impaired enzymatic activity, on the function of dendritic cells. Wild-type LT and LTR72, which retains some ADP ribosyltransferase activity, induced a selective increase in cell surface expression of B7.1, and a selective decrease of CD40 expression on mouse bone marrow derived dendritic cells. LTK63 and LT-B had no obvious effect on the expression of these antigens on similar dendritic cells. LT-treated dendritic cells also showed a profoundly impaired ability to present protein antigen (ovalbumin) to cognate T cells, although this effect was not observed with non-toxic LT mutants. LT and LTR72-treated cells showed a slower rate of receptor-mediated endocytosis as measured by flow cytometric analysis of uptake of fluorescently labelled dextran. Furthermore, confocal microscopy showed changes in the intracellular distribution of endocytosed molecules, and of the class II containing acidic antigen processing compartments. This response of dendritic cells to toxin is likely to play an important role in determining the adjuvant activity of these molecules.

Primary dendritic cells phagocytose Cryptococcus neoformans via mannose receptors and Fcgamma receptor II for presentation to T lymphocytes

Different "professional" antigen-presenting cells (APC) have unique characteristics that favor or restrict presentation of microbial antigens to T cells, depending on the organism. Cryptococcus neoformans is a pathogenic yeast that presents unique challenges to APC, including its large size, its rigid cell wall, and its ability to stimulate T cells as a mitogen. T-cell proliferation in response to the C. neoformans mitogen (CnM) requires phagocytosis and processing of the organisms by accessory cells prior to presentation of CnM to T cells. Because of the requirement for uptake of the organism and more limited costimulatory requirements of mitogens, macrophages might be the most likely cellular source for the accessory cell. However, the present study demonstrates that a transiently adherent cell that was CD3(-), CD14(-), CD19(-), CD56(-), HLA-DR(+), and CD83(+) with a dendritic morphology, rather than monocyte-derived or tissue (alveolar) macrophages, was the most efficient APC for presentation of CnM. A large number of these cells bound and internalized the organism, and only a small number of dendritic cells were required for presentation of the mitogen to T cells. Further, the mannose receptor and Fcgamma receptor II were required for presentation of C. neoformans, as blocking either of these receptors abrogated both uptake of C. neoformans and lymphocyte proliferation in response to CnM. These studies demonstrate the surprising fact that dendritic cells are the most efficient accessory cells for CnM.

Efficiency of protein transduction is cell type-dependent and is enhanced by dextran sulfate

Protein transduction domains (PTDs), both naturally occurring and synthetic, have been increasingly utilized to deliver biologically active agents to a variety of cell types in vitro and in vivo. We report that in addition to previously characterized arginine-rich PTDs, including TAT, lysine homopolymers were able to mediate transduction of a wide variety of cell types, as measured by flow cytometric and enzymatic assays. The efficiency of PTD-mediated transduction was influenced by the cell type tested, although polylysine homopolymers demonstrate levels of internalization that consistently exceeded those of TAT and arginine homopolymers. Transduction of arginine/lysine-rich PTDs occurred at 4 degrees C and following depletion of cellular ATP pools, albeit generally at reduced levels. Although transduction was reduced in Chinese hamster ovary mutant lines deficient in either heparan sulfate or glycosaminoglycan synthesis, uptake was restored to wild-type levels by incubating target cells with dextran sulfate. The enhancement of transduction by dextran sulfate suggests that electrostatic interactions play an important first step in the process by which PTDs and their cargo traverse the plasma membrane.

Bipartite regulation of different components of the MHC class I antigen-processing machinery during dendritic cell maturation

Dendritic cells (DC) are professional antigen-presenting cells (APC) which proceed from immature to a mature stage during their final differentiation. Immature DC are highly effective in terms of antigen uptake and processing, whereas mature DC become potent immunostimulatory cells. Until now, the expression profiles of the major components of the MHC class I antigen-processing machinery (APM) during DC development have not been well characterized. In this study, the mRNA and protein expression levels of the IFN-gamma inducible proteasome subunits, of the proteasome activators PA28, and of key components required for peptide transport and MHC class I-peptide complex assembly have been evaluated in immature and mature stages of human monocyte-derived DC using semiquantitative RT-PCR and Western blot analyses. The IFN-gamma-responsive immunoproteasome subunits LMP2, LMP7 and MECL1 are up-regulated in immature DC, whereas the other components of the MHC class I presentation machinery, such as PA28, TAP, tapasin, and HLA heavy and light chains, were found to be more abundant in mature DC. These findings support the hypothesis that immature DC produced by the differentiation of monocytes in response to IL-4 and granulocyte macrophage colony stimulating factor first increase their capacity to capture antigens and process them into peptides, thereby switching from housekeeping to immunoproteasomes, while mature DC rather up-regulate the components required for peptide translocation and MHC class I-peptide complex formation, and thus specialize in antigen presentation. Our results establish that MHC class I, like MHC class II surface expression, is markedly regulated during DC development and maturation.

Antibodies against listerial protein 60 act as an opsonin for phagocytosis of Listeria monocytogenes by human dendritic cells

Human-monocyte-derived dendritic cells (MoDC) are very efficient in the uptake of Listeria monocytogenes, a gram-positive bacterium which is an important pathogen in humans and animals causing systemic infections with symptoms such as septicemia and meningitis. In this work, we analyzed the influence of blood plasma on the internalization of L. monocytogenes into human MoDC and compared the uptake of L. monocytogenes with that of Salmonella enterica serovar Typhimurium and Yersinia enterocolitica. While human plasma did not significantly influence the uptake of serovar Typhimurium and Y. enterocolitica by human MoDC, the efficiency of the uptake of L. monocytogenes by these phagocytes was strongly enhanced by human plasma. In plasma-free medium the internalization of L. monocytogenes was very low, whereas the addition of pooled human immunoglobulins resulted in the internalization of these bacteria to a degree comparable to the highly efficient uptake observed with human plasma. All human plasma tested contained antibodies against the 60-kDa extracellular protein of L. monocytogenes (p60), and anti-p60 antibodies were also found in the commercially available pooled immunoglobulins. Strikingly, in contrast to L. monocytogenes wild type, an iap deletion mutant (totally deficient in p60) showed only a minor difference in the uptake by human MoDC in the presence or the absence of human plasma. These results support the assumption that antibodies against the listerial p60 protein may play an important role in Fc-receptor-mediated uptake of L. monocytogenes by human MoDC via opsonization of the bacteria. This process may have a major impact in preventing systemic infection in L. monocytogenes in immunocompetent humans.

Differential endocytotic characteristics of a novel human B/DC cell line HBM-Noda: effective macropinocytic and phagocytic function rather than scavenging function

In order to characterize a novel human B cell-lineage dendritic cell line (B/DC line) as an antigen-presenting cell (APC), we compared three types of endocytosis (micropinocytosis via a clathrin-coated pit, macropinocytosis via membrane ruffling, and phagocytosis) among myeloid-related, macrophage (Mphi) cell lines and a B/DC line. In the present examination, we used a unique human dendritic cell (DC) line, HBM-Noda (Noda). Flow cytometric and immunocytochemical analyses revealed that Noda not only expresses some DC markers, but also it expresses some B-cell associated markers. Noda shows strong capacities to stimulate allogenic T cells, to produce immunoglobulin G (IgG), and to perform immunoglobulin gene rearrangement. These data strongly suggest that Noda is a B-cell lineage DC line. The endocytic differences among these cell lines were as follows. (1) The level of micropinocytosis of Noda was significantly less than that of conventional human Mphi cell lines, and the formation of a clathrin-coated pit was not observed in Noda. (2) The level of macropinocytosis of Noda was also smaller than that of conventional Mphi cells indicating that the active membrane ruffling of Noda induces rapid recycling. (3) Phagocytosis of opsonized sheep red blood cells (SRBC) was performed more efficiently in Noda than in other Mphi cell lines. Collectively, these data suggest that in human bone marrow cells, we can identify a unique DC subtype, B/DC line, which develops through a lymphoid DC-differentiation pathway, and DC in this lineage plays an important role in the host immune response because of its effective uptake of a variety of size of antigens by using the skillful membrane ruffling and surface receptors

Targeting antigen in mature dendritic cells for simultaneous stimulation of CD4+ and CD8+ T cells

Due to their potent immunostimulatory capacity, dendritic cells (DC) have become the centerpiece of many vaccine regimens. Immature DC (DCimm) capture, process, and present Ags to CD4(+) lymphocytes, which reciprocally activate DCimm through CD40, and the resulting mature DC (DCmat) loose phagocytic capacity, but acquire the ability to efficiently stimulate CD8(+) lymphocytes. Recombinant vaccinia viruses (rVV) provide a rapid, easy, and efficient method to introduce Ags into DC, but we observed that rVV infection of DCimm results in blockade of DC maturation in response to all activation signals, including CD40L, monocyte-conditioned medium, LPS, TNF-alpha, and poly(I:C), and failure to induce a CD8(+) response. By contrast, DCmat can be infected with rVV and induce a CD8(+) response, but, having lost phagocytic activity, fail to process the Ag via the exogenous class II pathway. To overcome these limitations, we used the CMV protein pp65 as a model Ag and designed a gene containing the lysosomal-associated membrane protein 1 targeting sequence (Sig-pp65-LAMP1) to target pp65 to the class II compartment. DCmat infected with rVV-Sig-pp65-LAMP1 induced proliferation of pp65-specific CD4(+) clones and efficiently induced a pp65-specific CD4(+) response, suggesting that after DC maturation the intracellular processing machinery for class II remains intact for at least 16 h. Moreover, infection of DCmat with rVV-Sig-pp65-LAMP1 resulted in at least equivalent presentation to CD8(+) cells as infection with rVV-pp65. These results demonstrate that despite rVV interference with DCimm maturation, a single targeting vector can deliver Ags to DCmat for the effective simultaneous stimulation of both CD4(+) and CD8(+) cells.

Listeria monocytogenes-infected human dendritic cells: uptake and host cell response

Dendritic cells (DCs) are potent antigen-presenting cells and play a crucial role in initiation and modulation of specific immune responses. Various pathogens are able to persist inside DCs. However, internalization of the gram-positive bacterium Listeria monocytogenes into human DCs has not yet been shown. In the present study, we demonstrate that human monocyte-derived immature DCs can efficiently phagocytose L. monocytogenes. This uptake is independent of listerial adhesion factors internalin A and internalin B but requires cytoskeletal motion and factors present in human plasma. A major portion of internalized bacteria is found in membrane-bound phagosomes and is rarely free in the cytosol, as shown by transmission electron microscopy and by using an L. monocytogenes strain expressing green fluorescent protein when in the host cell cytosol. The infection caused maturation of the immature DCs into mature DCs displaying high levels of CD83, CD25, major histocompatibility complex class II, and the CD86 costimulator molecule. This effect appeared to be largely mediated by listerial lipoteichoic acid. Although L. monocytogenes infection is known to induce death in other cell types, infection of human DCs was found to induce necrotic but not apoptotic death in fewer than 20% of DCs. Therefore, the ability of DCs to act as effective antigen-presenting cells for listerial immunity is probably enhanced by their resistance to cell death, as well as their ability to rapidly differentiate into mature, immunostimulatory DCs upon encountering bacteria.

Identification of progenitor cells in long-term spleen stromal cultures that produce immature dendritic cells

Dendritic cells (DC) are produced continuously by a unique, long-term culture (LTC) system in which hemopoiesis is supported by a splenic stromal cell layer in the absence of added growth factors. Flow cytometric analysis reveals the production of two distinct cell subsets. The more predominant large-cell subset resembles highly endocytic DC that are large, granular, and possess membrane extensions. They also express high levels of the DC markers CD11c, CD11b, DEC-205, and CD80 on their cell surface. They do not resemble mature DC because they express low levels of MHC type II and CD86 molecules, as well as c-kit and Fc receptor (FcR). These are known characteristics of immature DC. Small cells are smaller and less granular than large cells, with negative to low expression of CD11c, DEC-205, and CD86. A majority of small cells express varying levels of CD11b and CD80. Subpopulations of small cells express low levels of c-kit, FcR, and MHC type II, and only a 20% subpopulation is weakly endocytic. Upon transfer to an irradiated stromal layer, cells within the small subset proliferate and differentiate to resemble the large cells in size, complexity, membrane extensions, and CD11c and CD86 expression. The two cell subsets produced in LTC are developmentally linked, with the heterogeneous small-cell subset containing progenitors of the larger homogeneous, immature DC subset. LTC represent a valuable model system for studying DC development from hemopoietic progenitors.

IFN-γ Down-Regulates MHC Expression and Antigen Processing in a Human B Cell Line

IFN-γ is a crucial mediator in the induction of cell-mediated Th1-type responses but is predominantly a negative regulator of B cell differentiation and proliferation. This cytokine is therefore a key factor in determining Th1 vs Th2 differentiation. This study investigates the action of IFN-γ in modulation of HLA-DR expression and Ag presentation by EBV-transformed human B cell lines. In contrast to its action on the monocyte/macrophage, IFN-γ down-regulates surface MHC expression on these B cells, and this regulation is posttranscriptional. In parallel with MHC down-regulation, there is a reduced capability to process and present exogenous protein and peptide Ag to T cell hybridomas. IFN-γ does not change the rates of fluid phase endocytosis or exocytosis in this model system but correlates with an up-regulation of the lysosomal enzymes cathepsins B and D.

Resting respiratory tract dendritic cells preferentially stimulate T helper cell type 2 (Th2) responses and require obligatory cytokine signals for induction of Th1 immunity

Consistent with their role in host defense, mature dendritic cells (DCs) from central lymphoid organs preferentially prime for T helper cell type 1 (Th1)-polarized immunity. However, the "default" T helper response at mucosal surfaces demonstrates Th2 polarity, which is reflected in the cytokine profiles of activated T cells from mucosal lymph nodes. This study on rat respiratory tract DCs (RTDCs) provides an explanation for this paradox. We demonstrate that freshly isolated RTDCs are functionally immature as defined in vitro, being surface major histocompatibility complex (MHC) II lo, endocytosishi, and mixed lymphocyte reactionlo, and these cells produce mRNA encoding interleukin (IL)-10. After ovalbumin (OVA)-pulsing and adoptive transfer, freshly isolated RTDCs preferentially stimulated Th2-dependent OVA-specific immunoglobulin (Ig)G1 responses, and antigen-stimulated splenocytes from recipient animals produced IL-4 in vitro. However, preculture with granulocyte/macrophage colony stimulating factor increased their in vivo IgG priming capacity by 2-3 logs, inducing production of both Th1- and Th2-dependent IgG subclasses and high levels of IFN-gamma by antigen-stimulated splenocytes. Associated phenotypic changes included upregulation of surface MHC II and B7 expression and IL-12 p35 mRNA, and downregulation of endocytosis, MHC II processing- associated genes, and IL-10 mRNA expression. Full expression of IL-12 p40 required additional signals, such as tumor necrosis factor alpha or CD40 ligand. These results suggest that the observed Th2 polarity of the resting mucosal immune system may be an inherent property of the resident DC population, and furthermore that mobilization of Th1 immunity relies absolutely on the provision of appropriate microenvironmental costimuli.

The Role of Tumor Necrosis Factor α in Modulating the Quantity of Peripheral Blood-Derived, Cytokine-Driven Human Dendritic Cells and Its Role in Enhancing the Quality of Dendritic Cell Function in Presenting Soluble Antigens to CD4+ T Cells In Vitro

Because dendritic cells (DC) are critically involved in both initiating primary and boosting secondary host immune responses, attention has focused on the use of DC in vaccine strategies to enhance reactivity to tumor-associated antigens. We have reported previously the induction of major histocompatibility complex class II-specific T-cell responses after stimulation with tumor antigen-pulsed DC in vitro. The identification of in vitro conditions that would generate large numbers of DC with more potent antigen-presenting cell (APC) capacity would be an important step in the further development of clinical cancer vaccine approaches in humans. We have focused attention on identifying certain exogenous cytokines added to DC cultures that would lead to augmented human DC number and function. DC progenitors from peripheral blood mononuclear cells (PBMC) were enriched by adherence to plastic, and the adherent cells were then cultured in serum-free XVIVO-15 medium (SFM) for 7 days with added granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-4 (IL-4). At day 7, cultures contained cells that displayed the typical phenotypic and morphologic characteristics of DC. Importantly, we have found that the further addition of tumor necrosis factor α (TNFα) at day 7 resulted in a twofold higher yield of DC compared with non–TNFα-containing DC cultures at day 14. Moreover, 14-day cultured DC generated in the presence of TNFα (when added at day 7) demonstrated marked enhancement in their capacity to stimulate a primary allogeneic mixed leukocyte reaction (8-fold increase in stimulation index [SI]) as well as to present soluble tetanus toxoid and candida albicans (10- to 100-fold increases in SI) to purified CD4+ T cells. These defined conditions allowed for significantly fewer DC and lower concentrations of soluble antigen to be used for the pulsing of DC to efficiently trigger specific T-cell proliferative responses in vitro. When compared with non–TNFα-supplemented cultures, these DC also displayed an increased surface expression of CD83 as well as the costimulatory molecules, CD80 and CD86. Removal of TNFα from the DC cultures after 2 or 4 days reduced its enhancing effect on DC yield, phenotype, and function. Thus, the continuous presence of TNFα over a 7-day period was necessary to achieve the maximum enhancing effect observed. Collectively, our findings point out the importance of exogenous TNFα added to cultures of cytokine-driven human DC under serum-free conditions, which resulted in an enhanced number and function of these APC. On the basis of these results, we plan to initiate clinical vaccine trials in patients that use tumor-pulsed DC generated under these defined conditions.

The Role of Tumor Necrosis Factor α in Modulating the Quantity of Peripheral Blood-Derived, Cytokine-Driven Human Dendritic Cells and Its Role in Enhancing the Quality of Dendritic Cell Function in Presenting Soluble Antigens to CD4+ T Cells In Vitro

Because dendritic cells (DC) are critically involved in both initiating primary and boosting secondary host immune responses, attention has focused on the use of DC in vaccine strategies to enhance reactivity to tumor-associated antigens. We have reported previously the induction of major histocompatibility complex class II-specific T-cell responses after stimulation with tumor antigen-pulsed DC in vitro. The identification of in vitro conditions that would generate large numbers of DC with more potent antigen-presenting cell (APC) capacity would be an important step in the further development of clinical cancer vaccine approaches in humans. We have focused attention on identifying certain exogenous cytokines added to DC cultures that would lead to augmented human DC number and function. DC progenitors from peripheral blood mononuclear cells (PBMC) were enriched by adherence to plastic, and the adherent cells were then cultured in serum-free XVIVO-15 medium (SFM) for 7 days with added granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-4 (IL-4). At day 7, cultures contained cells that displayed the typical phenotypic and morphologic characteristics of DC. Importantly, we have found that the further addition of tumor necrosis factor α (TNFα) at day 7 resulted in a twofold higher yield of DC compared with non–TNFα-containing DC cultures at day 14. Moreover, 14-day cultured DC generated in the presence of TNFα (when added at day 7) demonstrated marked enhancement in their capacity to stimulate a primary allogeneic mixed leukocyte reaction (8-fold increase in stimulation index [SI]) as well as to present soluble tetanus toxoid and candida albicans (10- to 100-fold increases in SI) to purified CD4+ T cells. These defined conditions allowed for significantly fewer DC and lower concentrations of soluble antigen to be used for the pulsing of DC to efficiently trigger specific T-cell proliferative responses in vitro. When compared with non–TNFα-supplemented cultures, these DC also displayed an increased surface expression of CD83 as well as the costimulatory molecules, CD80 and CD86. Removal of TNFα from the DC cultures after 2 or 4 days reduced its enhancing effect on DC yield, phenotype, and function. Thus, the continuous presence of TNFα over a 7-day period was necessary to achieve the maximum enhancing effect observed. Collectively, our findings point out the importance of exogenous TNFα added to cultures of cytokine-driven human DC under serum-free conditions, which resulted in an enhanced number and function of these APC. On the basis of these results, we plan to initiate clinical vaccine trials in patients that use tumor-pulsed DC generated under these defined conditions.

Depression of T-cell epitope generation by stabilizing hen lysozyme

Conformational stability of proteins is an important factor that determines their resistance/susceptibility to proteolytic digestion. Intracellular proteolysis is the key step in antigen presentation events for protein antigens; hence, it is likely that increasing protein stability reduces the antigenicity of proteins. We prepared three hen egg white lysozyme derivatives possessing different stabilities by chemical modification to clarify the relationship between conformational stability and the antigenicity of the protein. One of the derivatives was conformationally unstabilized by removing one intramolecular disulfide bond, whereas the two others were stabilized by the addition of an intramolecular cross-link. The antigenicity of these derivatives was evaluated using hen egg white lysozyme-specific T-cell hybridoma cells and a B-lymphoma cell line, A20, as antigen-presenting cells. With an increase in conformational stability, the T-cell response decreased. However, the reduction was not derived from the inefficiency of internalization to A20 cells nor the alteration of antigenicity by chemical modifications. Moreover, from analyses of their susceptibility to proteolysis and the kinetics of presentation of the T-cell epitope, it was confirmed that increasing protein stability led to the depression of T-cell epitope generation by increasing resistance to proteolysis. These results have an important implication in devising a new strategy for manipulating T-cell response by the stability of protein antigen.

The mannose receptor functions as a high capacity and broad specificity antigen receptor in human dendritic cells

Dendritic cells, in contrast to B lymphocytes, must be able to efficiently internalize a diverse array of antigens for processing and loading onto major histocompatibility complex (MHC) class II molecules. Here we characterize the mannose receptor pathway in dendritic cells and show that mannose receptor-mediated uptake of antigens results in a approximately 100-fold more efficient presentation to T cells, as compared to antigens internalized via fluid phase. Immunocytochemistry as well as subcellular fractionation revealed the localization of the mannose receptor and MHC class II molecules in distinct subcellular compartments. The mannose receptor thus functions in rapid internalization and concentration of a variety of glycosylated antigens that become available for processing and presentation. This may contribute to the unique capacity of dendritic cells to generate primary T cell responses against infectious agents.

Synthesis, stability, and subcellular distribution of major histocompatibility complex class II molecules in Langerhans cells infected with Leishmania major

Protozoan parasites of the genus Leishmania exist as obligatory intracellular amastigotes and invade macrophages and Langerhans cells, the dendritic cells of the skin. Langerhans cells are much more efficient in presenting Leishmania major antigen to T cells than macrophages are and have the unique ability to retain parasite antigen in immunogenic form for prolonged periods. To analyze the mechanisms that are responsible for this potency, we defined the synthesis, turnover, conformation, and localization of major histocompatibility complex (MHC) class II molecules in Langerhans cells. Hence, Langerhans cells were pulse-labeled; immunoprecipitation of MHC class II molecules and gel electrophoresis followed. In addition, the subcellular distribution of MHC class II molecules in L. major-infected Langerhans cells was analyzed by confocal microscopy. The results show that (i) newly synthesized MHC class II molecules are required for L. major antigen presentation by Langerhans cells, (ii) MHC class II-peptide complexes in Langerhans cells are long-lived, (iii) phagocytosis of L. major modulates MHC class II biosynthesis by reducing its downregulation during Langerhans cell differentiation, and (iv) newly synthesized MHC class II molecules are associated with the parasitophorous vacuole of infected Langerhans cells. These findings support the conclusion that the traits of MHC class II expression correspond to the highly specialized functions of Langerhans cells in the immunoregulation of cutaneous leishmaniasis.

Immunocompetent cells in rat periodontal ligament and their recruitment incident to experimental orthodontic tooth movement

The aims of this study were to evaluate the number and distribution of immunocompetent cells in normal rat periodontal ligament (PDL) and to quantify their recruitment incident to experimental tooth movement. 27 young animals had the 1st right maxillary molar moved mesially by an orthodontic appliance for 1, 3, 7 and 14 days, respectively. 4 animals served as untreated controls. An immunohistochemical procedure was carried out on alternate serial cryostat sections, and monoclonal antibodies against CD11b (macrophages, dendritic cells), CD43 (lymphocytes, polymorphs), CD4 (helper T-lymphocytes), and class II MHC molecules were used. Mean counts of the immunolabeled cells in the control group showed the highest number of CD11b+ and class II molecule expressing cells, while CD4+ and CD43+ cells were scarcely found. Significant increase in the number of CD11b+, CD43+ cells and class II molecules was found in the PDL of the experimentally moved 1st molars compared with the contralateral side and the control group, while CD4+ cells showed no significant increase. CD11b+ and cells expressing class II molecules were found around hyalinized tissue, between dentin and cellular cementum and close to Malassez' epithelial cells. In conclusion, normal rat PDL has high number of macrophage and dendritic-like cells, but few lymphocytes and granulocytes. Furthermore, experimental tooth movement leads to significant recruitment of cells belonging to the mononuclear phagocytic system, but has no significant effect on the number of lymphocytes and granulocytes in the rat PDL.

CD14+ blood monocytes can differentiate into functionally mature CD83+ dendritic cells

Dendritic cells are potent antigen-presenting cells that initiate primary immune responses. Although dendritic cells derive from bone marrow stem cells, the intermediate stages in their development remain unknown. In this study, plastic-adherent blood monocytes (CD14+, CD1a-) cultured for 7 days with granulocyte-monocyte colony-stimulating factor, interleukin 4, and tumor necrosis factor alpha were shown to differentiate into CD1a+ CD83+ dendritic cells. These cells displayed all phenotypic and morphologic characteristics of mature dendritic cells and were the most potent stimulatory cells in allogeneic mixed leukocyte reactions. The identification of specific culture conditions that generate large numbers of dendritic cells from purified monocytes uncovers an important step in dendritic cell maturation that will allow the further characterization of their role in autoimmune diseases, graft rejection, and human immunodeficiency virus infection.

Different cytokines regulate antigen uptake and presentation of a precursor dendritic cell line

Langerhans cells (LC) and dendritic cells (DC) need to be activated in order to perform their antigen-presenting function. In this study, we explored the influence of cytokines on the uptake and presentation of protein antigens by the retrovirally immortalized myeloid cell line FSDC. This cell line was generated from mouse fetal skin and was previously shown to have the characteristics of early DC precursors. Both FSDC and bone marrow-derived DC (BM-DC) were more effective in the pinocytosis of FITC-conjugated ovalbumin (FITC-OVA) and dextran (FITC-DX) than B cells or macrophages. Pretreatment of FSDC with granulocyte/macrophage colony-stimulating factor (GM-CSF) +/- interleukin (IL)-4 enhanced the pinocytic uptake of FITC-OVA and FITC-DX, but did not induce antigen-presenting capacity. In contrast, untreated FSDC or FSDC pre-incubated with GM-CSF +/- IL-4 suppressed T cell responses. Treatment of FSDC with IFN-gamma reduced pinocytosis but increased the expression of MHC and co-stimulatory/adhesion molecules and promoted efficient presentation of OVA protein or peptide to the specific DO11.10 T cell hybridoma or to naive CD4+ T cells from DO11.10 TCR-transgenic mice. The results suggest that antigen uptake and antigen presentation in DC are regulated by different cytokine signals provided by the surrounding tissue.

Dendritic cells: origin and differentiation

Dendritic cells (DC) are bone marrow-derived cells that are specialized to take up, process and present antigen, and have the capacity to stimulate resting T cells in the primary immune response. DC are a unique population that is likely to derive from a myeloid precursor cell. DC differentiation from bone marrow precursors in enhanced by the cytokines GM-CSF and tumor necrosis factor-alpha. In contrast, it has been proposed that thymic DC and T cells arise from a common stem cell, and that these DC play a specific role in the negative selection of thymic T cells. A number of post-bone marrow differentiation stages can be defined phenotypically and functionally. Undifferentiated DC have very active endocytic pathways, including receptor-mediated endocytosis involving a mannose/beta glucan receptor, and macropinocytosis of soluble antigen. In contrast, later stages of maturation are associated with a decreased ability to take up and process antigen, and increasing expression of major histocompatibility complex, adhesion and costimulatory molecules. Finally, activation of DC for full antigen-presenting cell function can be identified by the expression of CD28 ligands. The inflammatory site in rheumatoid arthritis is a human model of DC differentiation in response to a chronic antigenic stimulus. The features of this DC model are discussed.

Expression of a mannose/fucose membrane lectin on human dendritic cells

Dendritic cells (CD) are the most efficient antigen presenting cells for T lymphocytes. CD1a+ CD14- CD with high antigen-presenting capacities can now be obtained easily from adherent peripheral blood monocytes by culture in the presence of granulocyte/macrophage colony-stimulating factor and interleukin-4 (Sallusto et al., J. Exp. Med. 1994. 179: 1109). Human macrophages express a membrane lectin, or sugar-specific receptor, which specifically mediates the binding and endocytosis of mannose- and fucose-terminated glycoproteins and is involved in the phagocytosis of pathogens. A similar lectin activity was sought on cultured human DC using flow cytometry and confocal microscopy to detect binding and internalization of fluoresceinated neoglycoproteins [bovine serum albumin (BSA) substituted with sugar residues]. Several neoglycoproteins, especially alpha-L-fucosyl-, alpha-D-mannosyl-, N,N'-di-acetyl-beta-chitobiosyl- and beta-D-glucosyl-BSA, were endocytosed by cultured human CD1a+ DC as well as by CD1a- CD14- cells which were also obtained in the culture. Fuc-BSA and Man-BSA had the same number of binding sites (1.7 x 10(6)/cell) on CD1a+ DC, and bound with an affinity constant close to 10(7) 1/mol. Inhibition experiments indicated that these two neoglycoproteins bound to the same membrane lectin. CD1a+ and CD1a- cells were both labeled by an antiserum specific for the human macrophage mannose receptor. The membrane lectin specific for mannose and fucose that is evidenced in these experiments on cultured DC may be similar to the macrophage membrane lectin or may share functional and structural properties with it.

Mobilization of annexin V during the uptake of DNP-albumin by human dendritic cells

Dendritic cells play a crucial role in antigen presentation in various tissues. The endocytic capacity of these cells has been regarded as minimal, but recent work on dendritic cells from mouse spleen has disclosed that the fluid-phase traffic through late endosomes is as active in dendritic cells as in other antigen-presenting cell types. We show that cultured human dendritic cells express the annexins I, III, IV, V and VI, as detected by immunofluorescence staining. The annexins are cytosolic Ca(2+)-dependent proteins with the ability to promote vesicle aggregation and membrane fusion through their capacity to bind to membrane phospholipids. Annexin I and VI appeared to outline the cytoskeleton and the plasma membrane in cultured human dendritic cells. Studies using confocal laser scanning microscopy showed that during the endocytosis of fluorescent dinitrophenyl-conjugated albumin by dendritic cells, there was a redistribution of annexin V which was found to colocalize with vesicles containing dinitrophenyl-FITC-conjugated albumin.

A novel antigen-processing-defective phenotype in major histocompatibility complex class II-positive CIITA transfectants is corrected by interferon-gamma

Presentation of exogenous protein antigens to T lymphocytes is based on the intersection of two complex pathways: (a) synthesis, assembly, and transport of major histocompatibility complex (MHC) class II-invariant chain complexes from the endoplasmic reticulum to a specialized endosomal compartment, and (b) endocytosis, denaturation, and proteolysis of antigens followed by loading of antigenic peptides onto newly synthesized MHC class II molecules. It is believed that expression of MHC class II heterodimers, invariant chain and human leukocyte antigen-DM is both necessary and sufficient to reconstitute a functional MHC class II loading compartment in antigen-presenting cells. Expression of each of these essential molecules is under the control of the MHC class II transactivator CIITA. Unexpectedly, however, whereas interferon gamma stimulation does confer effective antigen-processing function to nonprofessional antigen presenting cells, such as melanoma cells, expression of the CIITA transactivator alone is not sufficient. Activation of antigen-specific T cells thus requires additional CIITA-independent factor(s), and such factor(s) can be induced by interferon gamma.

Processing and presentation of type II collagen, a fibrillar autoantigen, by H-2q antigen-presenting cells

Native type II collagen (CII) is a high molecular-weight fibrillar molecule which induces a chronic polyarthritis in mice expressing the H-2q haplotype. The present study was initiated to analyze the processing and the presentation of this nonglobular protein by H-2q antigen-presenting cells (APC). Efficiency of presentation was assessed by the ability of antigen-pulsed APC to activate collagen-specific CD4+ T cell hybridomas. Fixation of APC or the presence of chloroquine completely blocked the reactivity of the T cell hybrids to native, denatured and cyanogen bromide (CB) degraded CII, thus indicating the requirement of intracellular processing for adequate presentation of CII peptides to T cells. In the presence of various processing inhibitors (brefeldin A, leupeptin and N-tosyl-L-phenylalanine chloromethylketone) stimulation of T hybrids by CII-pulsed APC was reduced, pointing to the need of newly synthesized class II molecules, the use of several intracellular compartments and the implications of different proteases in the generation of CII peptides. Peritoneal macrophages and, to a lesser extent, total spleen cells, presented native and denatured CII with higher efficiency than purified splenic dendritic cells, naive or even immune B cells from CII-primed mice. In contrast, these dendritic and B cells were fully competent to present intact ovalbumin to a specific T cell hybrid. The stimulation by dendritic cells and immune B cells was greater when CB peptides of CII were added instead of the native molecule. Similarly, the cleavage of CII was an absolute requirement for its presentation by epidermal cells and B cell lymphomas to the T cell hybridomas. Taken together, these findings emphasize the crucial role of intracellular processing for recognition of soluble CII, similar to most antigens. However, in contrast to ovalbumin, the size and fibrillar nature of the native CII molecule influences its capture by the APC, thus limiting the type of APC able to present this antigen.

Antigen presentation by naive macrophages, dendritic cells and B cells to primed T lymphocytes and their cytokine production following exposure to immunostimulating complexes

Influenza virus envelope proteins incorporated into immunostimulating complexes (iscoms) are taken up and processed by various kinds of antigen-presenting cells (APC), encompassing peritoneal cells (PEC), unfractionated splenocytes, splenic dendritic cells (DC) or B cells. The iscom-pulsed naive APC stimulated primed T cells to proliferate and produce cytokine in vitro. In contrast, only DC and B cells pulsed with the same antigen (Ag) in the micelle form functioned as accessory cells stimulating the primed T cells to proliferate and produce cytokine. In general, iscoms were better inducers of cell proliferation than micelles. Iscoms stimulated more secretion of IL-2 and interferon-gamma (IFN-gamma) than the micelles, but both antigenic forms stimulated secretion of IL-4. DC and B cells pulsed with iscoms stimulated most efficiently the secretion of IL-2 and IFN-gamma. DC were superior to the other APC in stimulating primed T cells to secrete IFN-gamma. On the other hand, micelles stimulated more efficiently than iscoms splenic T cells from micelle-primed as well as iscom-primed mice to secrete IL-10. These data indicate that influenza virus envelope proteins incorporated in iscoms stimulate a broad T cell response, possibly emphasizing a Th1 type of response. The same Ag in a micelle form induce a more prominent Th2 type of T cell response. The results indicate that the administration of an Ag in an adjuvant formulation can superimpose a different cytokine profile on the immune response than that induced by the protein Ag alone.

Contact sensitizers modulate mechanisms of receptor-mediated endocytosis but not fluid-phase endocytosis in murine epidermal Langerhans cells

In order to define the influence of contact allergens on the fluid-phase endocytosis (FPE) of soluble molecules of murine epidermal Langerhans cells (LC), we studied the internalization of FITC-labeled bovine serum albumin (FITC-BSA), TRITC-labeled dextrane (TRITC-DEX) as well as horseradish peroxidase by LC. A 3-parameter flow-cytometric technique was performed for quantification of internalized FITC-BSA in LC using quantum red-labeled reagents for detection of Ia-antigen expression by LC and propidium iodide for exclusion of dead cells from analysis. A temperature-dependent rapid accumulation of FITC-BSA was noticed in time-course studies reaching a plateau between 1 and 2 h of in vitro culture at 37 degrees C. The quantity of FPE under stimulation with phorbol 12-myristate 13-acetate (PMA), concanavalin A (Con A), staphylococcal enterotoxin B (SEB) and contact sensitizers (DNFB, Kathon CG, K2Cr2O7) as well as the irritant SLS was determined. Treatment of LC with PMA and Con A resulted in a significant increase of total FITC-BSA uptake. The contact sensitizers as well as SEB and SLS failed to mediate augmented fluid-phase endocytosis. By use of the pH-insensitive soluble marker, TRITC-DEX and a microscope photometer for evaluation these findings could be confirmed. This excluded any artificial influence of differences in pH values in endocytotic compartments which might have influenced the fluorescence intensity of the pH-sensitive fluorochrome FITC. For qualitative analysis of FPE, the intracellular distribution of internalized horseradish peroxidase in LC was studied. An aggregated pattern became apparent in untreated LC and did not change under stimulation with any of the substances used.(ABSTRACT TRUNCATED AT 250 WORDS)

Dendritic cells use macropinocytosis and the mannose receptor to concentrate macromolecules in the major histocompatibility complex class II compartment: downregulation by cytokines and bacterial products

We have previously demonstrated that human peripheral blood low density mononuclear cells cultured in granulocyte/macrophage colony-stimulating factor (GM-CSF) and interleukin (IL)-4 develop into dendritic cells (DCs) that are extremely efficient in presenting soluble antigens to T cells. To identify the mechanisms responsible for efficient antigen capture, we studied the endocytic capacity of DCs using fluorescein isothiocyanate-dextran, horseradish peroxidase, and lucifer yellow. We found that DCs use two distinct mechanisms for antigen capture. The first is a high level of fluid phase uptake via macropinocytosis. In contrast to what has been found with other cell types, macropinocytosis in DCs is constitutive and allows continuous internalization of large volumes of fluid. The second mechanism of capture is mediated via the mannose receptor (MR), which is expressed at high levels on DCs. At low ligand concentrations, the MR can deliver a large number of ligands to the cell in successive rounds. Thus, while macropinocytosis endows DCs with a high capacity, nonsaturable mechanism for capture of any soluble antigen, the MR gives an extra capacity for antigen capture with some degree of selectivity for non-self molecules. In addition to their high endocytic capacity, DCs from GM-CSF + IL-4-dependent cultures are characterized by the presence of a large intracellular compartment that contains high levels of class II molecules, cathepsin D, and lysosomal-associated membrane protein-1, and is rapidly accessible to endocytic markers. We investigated whether the capacity of DCs to capture and process antigen could be modulated by exogenous stimuli. We found that DCs respond to tumor necrosis factor alpha, CD40 ligand, IL-1, and lipopolysaccharide with a coordinate series of changes that include downregulation of macropinocytosis and Fc receptors, disappearance of the class II compartment, and upregulation of adhesion and costimulatory molecules. These changes occur within 1-2 d and are irreversible, since neither pinocytosis nor the class II compartment are recovered when the maturation-inducing stimulus is removed. The specificity of the MR and the capacity to respond to inflammatory stimuli maximize the capacity of DCs to present infectious non-self antigens to T cells.

Antigen capture and major histocompatibility class II compartments of freshly isolated and cultured human blood dendritic cells

Dendritic cells (DC) represent potent antigen-presenting cells for the induction of T cell-dependent immune responses. Previous work on antigen uptake and presentation by human DC is based largely on studies of blood DC that have been cultured for various periods of time before analysis. These cultured cells may therefore have undergone a maturation process from precursors that have different capacities for antigen capture and presentation. We have now used immunoelectron microscopy and antigen presentation assays to compare freshly isolated DC (f-DC) and cultured DC (c-DC). f-DC display a round appearance, whereas c-DC display characteristic long processes. c-DC express much more cell surface major histocompatibility complex (MHC) class II than f-DC. The uptake of colloidal gold-labeled bovine serum albumin (BSA), however, is greater in f-DC, as is the presentation of 65-kD heat shock protein to T cell clones. The most striking discovery is that the majority of MHC class II molecules in both f-DC and c-DC occur in intracellular vacuoles with a complex shape (multivesicular and multilaminar). These MHC class II enriched compartments (MIIC) represent the site to which BSA is transported within 30 min. Although MIIC appear as more dense structures with less MHC class II molecules in f-DC than c-DC, the marker characteristics are very similar. The MIIC in both types of DC are acidic, contain invariant chain, and express the recently described HLA-DM molecule that can contribute to antigen presentation. CD19+ peripheral blood B cells have fewer MIIC and surface MHC class II expression than DCs, while monocytes had low levels of MIIC and surface MHC class II. These results demonstrate in dendritic cells the elaborate development of MIIC expressing several of the components that are required for efficient antigen presentation.

Uptake of microparticle-adsorbed protein antigen by bone marrow-derived dendritic cells results in up-regulation of interleukin-1 alpha and interleukin-12 p40/p35 and triggers prolonged, efficient antigen presentation

Dendritic cells synthesize and express major histocompatibility complex (MHC) class II peptide-binding elements constitutively and, therefore, belong to the category of professional antigen-presenting cells. Unlike other cells that show constitutive class II expression, such as B cells and certain T cell clones, dendritic cells possess the unique capacity to activate naive T cells. Using dendritic cells generated in vitro by culture of mouse bone marrow in the presence of low doses of recombinant mouse granulocyte/macrophage colony-stimulating factor, we found that discrete maturation stages of these cells can be distinguished which were correlated with defined functional capabilities. The striking observation was the presence of a progenitor dendritic cell expressing low levels of class II which, unlike its differentiated counterpart in vitro, possessed pronounced phagocytic activity. Adding protein antigen to dendritic cells in a particle-adsorbed form, as compared to a soluble form, we demonstrate that phagocytosis of the particle-adsorbed protein by progenitor dendritic cells involves an activation event. This is evidenced by the de novo synthesis of transcripts of interleukin-1 alpha and interleukin-12 p40/p35 as well as transcripts of MHC class II. Most importantly, an augmented and prolonged antigen-presentation capacity was observed when the antigen was given in particle-adsorbed instead of soluble form. These findings indicate that progenitor dendritic cells are functionally more flexible and potent than fully differentiated dendritic cells and that they play a crucial role in antigen presentation. It is suggested that these findings will open up new possibilities to devise strategies for vaccine development.

Modulation of antigen processing and presentation by covalently linked complement C3b fragment

Ligands such as complement fragments (C3, C4), IgG or alpha 2-macroglobulin, which bind antigen (Ag) before their uptake by antigen-presenting cells (APC), are likely to modulate the different steps of Ag processing and presentation. These ligands contribute to internalization and endosomal targeting of Ag; they also influence its processing and, consequently, the binding of resulting peptides to major histocompatibility complex (MHC) class II molecules before presentation to T cells. Complement protein C3 contains, like other members of the alpha 2-macroglobulin family, an intrachain thiolester bond. Conformational alteration or limited proteolysis of C3 into C3b leads to breaking of the thiolester with transient capacity of the revealed carbonyl group to esterify hydroxyl groups of Ag. Ester-linked complexes including tetanus toxin (TT) and C3b were prepared to analyse the influence of bound C3b on TT processing and presentation by APC. Covalent binding of C3b to TT resulted in increased and prolonged stimulation of specific T-cell proliferation. This effect was observed with non-specific B cells, as well as with a TT-specific B-cell clone, as APC. On the other hand, SDS-PAGE analysis of proteolysates of TT or C3b-TT, obtained with endosome/lysosome-enriched subcellular fractions prepared from human Epstein-Barr virus (EBV)-transformed B cells, indicated a delay of TT proteolysis when TT was associated to C3b. Treatment of APC with protease inhibitors, before and during exposure of the cells to Ag, resulted in differences in the inhibition of TT and C3b-TT proteolysis. Using purified cathepsins B and D, we demonstrated that covalent binding of C3b to TT totally abolished TT proteolysis by cathepsin D, while proteolysis by cathepsin B was preserved. This finding and the absence of cathepsin B in endosomes may explain a delay in TT processing when it is associated to C3b. Confirming these data, presentation by formaldehyde-fixed cells of C3b-TT proteolysates showed higher stimulation of specific T-cell clones than formaldehyde-fixed TT proteolysates.