"Tolerosomes" are produced by intestinal epithelial cells

Extracellular plasma RNA from colon cancer patients is confined in a vesicle-like structure and is mRNA-enriched

Little is yet known about the origin and protective mechanism of free nucleic acids in plasma. We investigated the possibility of these free nucleic acids being particle associated. Plasma samples from colon cancer patients and cell culture media were subjected to various antibody incubations, ultracentrifugation, and RNA extraction protocols for total RNA, epithelial RNA, and mRNA. Flow cytometry using a Ber-EP4 antibody and confocal laser microscopy after staining with propidium iodide were also performed. mRNA levels of the LISCH7 and SDHA genes were determined in cells and in culture media. Ber-EP4 antibody and polystyrene beads coated with oligo dT sequences were employed. We observed that, after incubation, total RNA and mRNA were always detected after membrane digestion, and that epithelial RNA was detected before this procedure. In ultracentrifugation, mRNA was caught in the supernatant only if a former lysis mediated or in the pellet if there was no previous digestion. Flow cytometry determinations showed that antibody-coated microbeads keep acellular structures bearing epithelial antigens apart. Confocal laser microscopy made 1- to 2-microm-diameter particles perceptible in the vicinity of magnetic polystyrene beads. Relevant differences were observed between mRNA of cells and culture media, as there was a considerable difference in LISCH7 mRNA levels between HT29 and IMR90 cell co-cultures and their culture media. Our results support the view that extracellular RNA found in plasma from cancer patients circulates in association with or is protected in a multiparticle complex, and that an active release mechanism by tumor cells may be a possible origin.

Serum-derived exosomes from antigen-fed mice prevent allergic sensitization in a model of allergic asthma

Oral tolerance is an active process that starts with sampling of luminal antigens by the intestinal epithelial cells (IEC), followed by processing and assembly with major histocompatibility complex class II and subsequently a release of tolerogenic exosomes (tolerosomes) from the IEC. We have previously shown that tolerosomes can be isolated from serum shortly after an antigen feed, and will potently transfer antigen-specific tolerance to naive recipients. Here we study the capacity of the tolerosomes to protect against allergic sensitization in a mouse model of allergic asthma. Serum or isolated serum exosomes from tolerized BALB/c donor mice were transferred to syngeneic recipients followed by sensitization and intranasal exposure to ovalbumin (OVA). Blood, bronchoalveolar lavage (BAL) and lymph nodes were sampled 24 hr after the final exposure. The number of eosinophils was counted in BAL fluid and the levels of immunoglobulin E (IgE) and OVA-specific IgE were measured in serum. Mediastinal and coeliac lymph nodes were analysed by flow cytometry. The animals receiving serum from OVA-fed mice displayed significantly lower numbers of airway eosinophils and lower serum levels of total IgE as well as of OVA-specific IgE compared with controls. Moreover, the tolerant animals showed a significantly higher frequency of activated T cells with a regulatory phenotype in both mediastinal and coeliac lymph nodes. The results show that serum or isolated serum exosomes obtained from OVA-fed mice and administered intraperitoneally to naive recipient mice abrogated allergic sensitization in the recipients.

Exosome function: from tumor immunology to pathogen biology

Exosomes are the newest family member of ‘bioactive vesicles’ that function to promote intercellular communication. Exosomes are derived from the fusion of multivesicular bodies with the plasma membrane and extracellular release of the intraluminal vesicles. Recent studies have focused on the biogenesis and composition of exosomes as well as regulation of exosome release. Exosomes have been shown to be released by cells of hematopoietic and non‐hematopoietic origin, yet their function remains enigmatic. Much of the prior work has focused on exosomes as a source of tumor antigens and in presentation of tumor antigens to T cells. However, new studies have shown that exosomes might also promote cell‐to‐cell spread of infectious agents. Moreover, exosomes isolated from cells infected with various intracellular pathogens, including Mycobacterium tuberculosis and Toxoplasma gondii, have been shown to contain microbial components and can promote antigen presentation and macrophage activation, suggesting that exosomes may function in immune surveillance. In this review, we summarize our understanding of exosome biogenesis but focus primarily on new insights into exosome function. We also discuss their possible use as disease biomarkers and vaccine candidates.

Proteomic analysis of secreted exosomes

This chapter focuses on the contribution of proteomic analysis to the understanding of the process of exosome secretion and the mechanism and function of exosomes. It also describes the potential of exosome proteomic analysis to aid in the development of exosomes for therapeutic use.

Proteomic analysis of microvesicles derived from human colorectal cancer cells

Microvesicles (MV) are membrane vesicles secreted from the plasma and endosomal membrane compartment by various cell types such as hematopoietic, epithelial, and tumor cells. Actively growing tumor cells shed MV, and the rate of shedding increases in malignant tumors. Although recent progress in this area has revealed that tumor-derived MV play multiple roles in tumor growth and metastasis via immune escape, tumor invasion, and angiogenesis, the mechanism of vesicle formation and the biological roles of tumor-derived MV are not understood. Here, we report the first global proteomic analysis of highly purified MV from human colorectal cancer cells. Using 1D SDS gel electrophoresis and nano-LC-MS/MS analyses, we identified a total of 547 microvesicular proteins from three independent experiments with high confidence; 416 proteins were identified at least in two trials, including 181 as yet unreported proteins. We identified 49 proteins involved in the biogenesis of MV, including annexins, ADP-ribosylation factors, and Rab proteins. We also identified 28 proteins that may function in tumorigenesis via promotion of migration, invasion, and growth of tumor cells, immune modulation, metastasis, and angiogenesis. Taken together with previously reported results, our observations suggest that tumor-derived MV may act as communicasomes, that is, extracellular organelles that play diverse roles in intercellular communication. This information will help elucidate the biogenesis and functions of tumor-derived MV, and aid in the development of effective vaccines for various cancers, including colorectal cancer.

Different types of in vitro generated human monocyte-derived dendritic cells release exosomes with distinct phenotypes

Human in vitro generated dendritic cells and the exosomes they release are potential tools for the modulation of immune responses. Here, we characterized differently generated monocyte-derived dendritic cells (MDDCs) and their exosomes. Culturing of peripheral CD14+ cells from the same individuals with either interleukin (IL)-4 and granulocyte-macrophage colony-stimulating factor (GM-CSF) (conventional MDDCs) or alternatively with IL-4 and IL-3 generated immature MDDCs in 7 days. Fluorescence-activated cell sorting (FACS) analysis showed that the IL-4/IL-3-generated MDDCs had significantly lower percentages of CD1a+, CD40+ and CD80+ cells and a higher percentage of CD86+ cells as compared with conventional MDDCs. In addition, IL-4/IL-3-generated MDDCs had significantly higher densities of major histocompatibility complex (MHC) class I [human leucocyte antigen (HLA)-ABC], MHC class II (HLA-DR), CD11c and the tetraspanin CD81 as compared with conventional MDDCs. In a comparison of their ability to stimulate CD8+ T cells, we found that the IL-4/IL-3 MDDCs were slightly more efficient than the conventional MDDCs at inducing interferon (IFN)-gamma release in response to viral peptides. Exosome morphology was confirmed by electron microscopy and exosome phenotypes were analysed by flow cytometry and western blot. In comparison to exosomes from conventional MDDCs, exosomes from IL-4/IL-3-generated MDDCs showed significantly stronger signals for HLA-ABC, HLA-DR, CD11c, CD63 and CD81. Thus, phenotypically the exosomes largely reflected their MDDCs of origin. When exosomes were loaded with viral peptides, both types of exosomes induced IFN-gamma release from CD8+ T cells. Our findings might have significance for the development of DC- and exosome-based therapies.

Luminal antigens access late endosomes of intestinal epithelial cells enriched in MHC I and MHC II molecules: in vivo study in Crohn's ileitis

In contrast to healthy conditions, intestinal epithelial cells (IECs) stimulate proinflammatory CD4+ and CD8+ T cells during Crohn's disease (CD). The underlying regulatory mechanisms remain unknown. Here we investigated the epithelial expression of major histocompatibility complex (MHC) I and MHC II and its interference with endocytic pathways, in vivo. During ileoscopy, ovalbumin (OVA) was sprayed onto ileal mucosa of CD patients (ileitis and remission) and controls. The epithelial traffic of OVA and MHC I/II pathways were studied in biopsies using fluorescence and electron microscopy. We found MHC I and MHC II to accumulate within multivesicular late endosomes (MVLE) of IECs. Faint labeling for these molecules was seen in early endosomes and lysosomes. MVLE were entered by OVA 10 min after exposure. Exosomes carrying MHC I, MHC II, and OVA were detected in intercellular spaces of the epithelium. OVA trafficking and labeling patterns for MHC I and MHC II in IECs showed no differences between CD patients and controls. Independent of inflammatory stimuli, MHC I and MHC II pathways intersect MVLE in IECs, which were efficiently targeted by luminal antigens. Similar to MHC II-enriched compartments in professional antigen presenting cells, these MVLE might be critically involved in MHC I- and MHC II-related antigen processing in IECs and the source of epithelial-released exosomes. The access of luminal antigens to MHC I in MVLE might indicate that the presentation of exogenous antigens by IECs must not be restricted to MHC II but might also occur as "cross-presentation" via MHC I to CD8+ T cells.

Exosomes with immune modulatory features are present in human breast milk

Breast milk is a complex liquid with immune-competent cells and soluble proteins that provide immunity to the infant and affect the maturation of the infant's immune system. Exosomes are nanovesicles (30-100 nm) with an endosome-derived limiting membrane secreted by a diverse range of cell types. Because exosomes carry immunorelevant structures, they are suggested to participate in directing the immune response. We hypothesized that human breast milk contain exosomes, which may be important for the development of the infant's immune system. We isolated vesicles from the human colostrum and mature breast milk by ultracentrifugations and/or immuno-isolation on paramagnetic beads. We found that the vesicles displayed a typical exosome-like size and morphology as analyzed by electron microscopy. Furthermore, they floated at a density between 1.10 and 1.18 g/ml in a sucrose gradient, corresponding to the known density of exosomes. In addition, MHC classes I and II, CD63, CD81, and CD86 were detected on the vesicles by flow cytometry. Western blot and mass spectrometry further confirmed the presence of several exosome-associated molecules. Functional analysis revealed that the vesicle preparation inhibited anti-CD3-induced IL-2 and IFN-gamma production from allogeneic and autologous PBMC. In addition, an increased number of Foxp3(+)CD4(+)CD25(+) T regulatory cells were observed in PBMC incubated with milk vesicle preparations. We conclude that human breast milk contains exosomes with the capacity to influence immune responses.

B cell-derived exosomes can present allergen peptides and activate allergen-specific T cells to proliferate and produce TH2-like cytokines

BACKGROUND

Exosomes are vesicles of 30 to 100 nm produced by inward budding of endosomal compartments and are released by a range of different cell types. Exosomes from antigen-presenting cells carry immunorelevant molecules like MHC class I and II and costimulatory molecules and thus are suggested to have a role in immune modulation.

OBJECTIVE

To investigate the role of antigen-presenting cell derived exosomes in allergen presentation and T-cell stimulation.

METHODS

Exosomes were isolated from supernatants of B-cell lines derived from patients with birch pollen allergy. The exosomes were characterized with regard to the expression of surface molecules by flow cytometry. Moreover, exosomes were loaded with T-cell-activating peptides from the major birch allergen Bet v 1, and binding was tested with ELISA. Loaded exosomes were used for stimulation of Bet v 1-specific T-cell lines. Cell proliferation and cytokine production were assessed.

RESULTS

The exosomes had a phenotype typical of B cell-derived exosomes with expression of MHC, costimulatory molecules like CD86, tetraspanin proteins such as CD81, and CD19. Furthermore, B cell-derived exosomes bound Bet v 1-derived peptides and subsequently induced a dose-dependent T-cell proliferation. In addition to proliferation, T cells synthesized the cytokines IL-5 and IL-13 in response to peptide-loaded exosomes.

CONCLUSION

These results demonstrate for the first time that exosomes isolated from B cells can present allergen-derived peptides and thereby induce T-cell proliferation and T(H)2-like cytokine production.

CLINICAL IMPLICATIONS

Our data suggest that exosomes from B lymphocytes are an immunostimulatory factor in allergic immune responses.

Human tumor-derived exosomes selectively impair lymphocyte responses to interleukin-2

Exosomes are nanometer-sized vesicles, secreted by normal and neoplastic cells. The outcome following interaction between the cellular immune system and cancer-derived exosomes is not well understood. Interleukin-2 (IL-2) is a key factor supporting expansion and differentiation of CTL and natural killer (NK) cells but can also support regulatory T cells and their suppressive functions. Our study examined whether tumor-derived exosomes could modify lymphocyte IL-2 responses. Proliferation of healthy donor peripheral blood lymphocytes in response to IL-2 was inhibited by tumor exosomes. In unfractionated lymphocytes, this effect was seen in all cell subsets. Separating CD4(+) T cells, CD8(+) T cells, and NK cells revealed that CD8(+) T-cell proliferation was not inhibited in the absence of CD4(+) T cells and that NK cell proliferation was only slightly impaired. Other exosome effects included selective impairment of IL-2-mediated CD25 up-regulation, affecting all but the CD3(+)CD8(-) T-cell subset. IL-2-induced Foxp3 expression by CD4(+)CD25(+) cells was not inhibited by tumor exosomes, and the suppressive function of CD4(+)CD25(+) T cells was enhanced by exosomes. In contrast, exosomes directly inhibited NK cell killing function in a T-cell-independent manner. Analysis of tumor exosomes revealed membrane-associated transforming growth factor beta(1) (TGFbeta(1)), which contributed to the antiproliferative effects, shown by using neutralizing TGFbeta(1)-specific antibody. The data show an exosome-mediated mechanism of skewing IL-2 responsiveness in favor of regulatory T cells and away from cytotoxic cells. This coordinated "double hit" to cellular immunity strongly implicates the role of exosomes in tumor immune evasion.

Effective Treatment of Inflammatory Disease Models with Exosomes Derived from Dendritic Cells Genetically Modified to Express IL-4

In this study, we demonstrate that genetically modified bone marrow-derived dendritic cells (DC) and exosomes derived from the DC, expressing either secreted IL-4 or membrane-bound IL-4, can reduce the severity and the incidence of established collagen-induced arthritis and inhibit inflammation of delayed-type hypersensitivity (DTH) in mice. The ability of the DC and DC-derived exosomes to suppress the DTH response was MHC class II and, in part, Fas ligand/Fas dependent. The DC-derived exosomes were internalized by CD11c(+) DC in the dermis at the site of injection and in the draining lymph node as well as by CD11c(+) DC and F4/80(+) macrophages in the spleen. Moreover, adoptive transfer of CD11c(+) or CD3(+) splenic cells from mice treated with exosomes showed significant reduction of footpad swelling in the DTH model. These results demonstrate that administration of DC/IL-4 or exosomes derived from DC/IL-4 are able to modulate the activity of APC and T cells in vivo through a MHC class II and partly Fas ligand/Fas-dependent mechanism, resulting in effective treatment of established collagen-induced arthritis and suppression of the DTH inflammatory response. Thus, APC-derived exosomes could be used therapeutically for the treatment of autoimmune disease and inflammatory disorders.

MHC Class II+ Exosomes in Plasma Suppress Inflammation in an Antigen-Specific and Fas Ligand/Fas-Dependent Manner

Exosomes are 50- to 100-nm vesicles that are formed within the late endocytic compartment and released from a variety of cell types. Previously, we demonstrated that exosomes derived from dendritic cells transduced with adenoviral vectors expressing IL-10, IL-4, or Fas ligand (FasL) produce anti-inflammatory exosomes able to reduce inflammation in a murine paw delayed-type hypersensitivity model, suppress the onset on murine collagen-induced arthritis, and reduce the severity of established collagen-induce arthritis. In this study, we examined the ability of endogenous, blood-borne exosomes to regulate the immune response. Exosomes isolated from plasma of mice immunized to keyhole limpet hemocyanin, but not from naive or OVA-immunized mice, were able to suppress the keyhole limpet hemocyanin-specific delayed-type hypersensitivity inflammatory response. The anti-inflammatory effect was mediated by MHC class II(+) plasma exosomes that were also FasL(+) and CD11b(+), but CD11c(-). Moreover, the anti-inflammatory effect of the MHC class II(+) plasma-derived exosomes was, in part, dependent upon the presence of FasL in the exosomes and Fas in the recipient mouse. These results suggest that exosomes in the plasma, produced by MHC class II(+) and CD11b(+) cells, have the ability to suppress the immune response in an Ag-specific manner in part through a Fas/FasL-dependent manner.

Reticulocyte-secreted exosomes bind natural IgM antibodies: involvement of a ROS-activatable endosomal phospholipase iPLA2

Reticulocytes release small membrane vesicles termed exosomes during their maturation into erythrocytes. It has been suggested that reticulocytes remodel the plasma membrane of the immature red cell during erythropoiesis by specifically eliminating various proteins. We report here that exosome release is associated with a physiologic cascade induced by the expression of a 15-lipoxygenase at the reticulocyte stage. We found that the phospholipase iPLA2 specifically associated with the endosomal and exosomal membranes could be activated by reactive oxygen species (ROSs) produced during mitochondria degeneration induced by 15-lipoxygenase. Since iPLA2 has recently been demonstrated to participate in the clearance of apoptotic cells, we investigated its role in vesicle removal. We found that exosomes isolated directly from the blood of an anemic rat or released during in vitro maturation of rat reticulocytes bind IgM antibodies on their surface, in contrast to immature and mature red cells. These natural IgM antibodies recognize lysophosphatidylcholine and are able to specifically bind to apoptotic cells. Finally, evidence of C3 deposition on the exosome surface leads us to hypothesize that this cascade may favor the clearance of exosomes by cells once released into the bloodstream, via a mechanism similar to that involved in the elimination of apoptotic cells.

Tumor-released microvesicles as vehicles of immunosuppression

Tumor-released microvesicles, or exosomes, which are abundant in the body fluids of patients with cancer, are likely to be involved in tumor progression. We recently showed that microvesicles released by human melanoma and colorectal carcinoma cells can promote the differentiation of monocytes to myeloid-derived suppressor cells which support tumoral growth and immune escape. These findings underscore an important role for these extracellular organelles in remodeling tumor-stromal interactions to promote malignancy.

Exosome-mediated transfer of mRNAs and microRNAs is a novel mechanism of genetic exchange between cells

Exosomes are vesicles of endocytic origin released by many cells. These vesicles can mediate communication between cells, facilitating processes such as antigen presentation. Here, we show that exosomes from a mouse and a human mast cell line (MC/9 and HMC-1, respectively), as well as primary bone marrow-derived mouse mast cells, contain RNA. Microarray assessments revealed the presence of mRNA from approximately 1300 genes, many of which are not present in the cytoplasm of the donor cell. In vitro translation proved that the exosome mRNAs were functional. Quality control RNA analysis of total RNA derived from exosomes also revealed presence of small RNAs, including microRNAs. The RNA from mast cell exosomes is transferable to other mouse and human mast cells. After transfer of mouse exosomal RNA to human mast cells, new mouse proteins were found in the recipient cells, indicating that transferred exosomal mRNA can be translated after entering another cell. In summary, we show that exosomes contain both mRNA and microRNA, which can be delivered to another cell, and can be functional in this new location. We propose that this RNA is called "exosomal shuttle RNA" (esRNA).

Digestion, Absorption and Tissue Distribution of Ovalbumin and Palmitoyl-ovalbumin: Impact on Immune Responses Triggered by Orally Administered Antigens

Previous work in this laboratory has demonstrated that ovalbumin coupled to palmitoyl residues (palmitoyl-Ova) does not induce oral tolerance. The present study sought to determine whether this coupling affects digestion, absorption and transfer of antigen. Ova and palmitoyl-Ova were shown to be digested differently in vitro by proteolytic enzymes and presented different tissue distribution kinetics after being labelled with (99m)technetium and orally administered to animals. Palmitoyl-Ova remained longer in the stomach, while native Ova was quickly transferred to the gut and other organs. After 3 h, higher levels of palmitoyl-Ova were found in the blood, Peyer's patches, mesenteric lymph nodes, liver and, especially, the spleen, which appears to be essential for immunization with palmitoyl-Ova. In fact, splenectomized mice treated orally with palmitoyl-Ova became tolerant, while tolerance to Ova was not affected. Thus, palmitoyl coupling was demonstrated to affect antigen digestion, absorption and transport. This is the first time that the spleen has been shown to be required for oral immunization with palmitoyl-Ova.

Modulation of the immune response using dendritic cell-derived exosomes

Initial studies in our laboratory were focused on the use of dendritic cells (DC) genetically modified to express Th2-derived cytokines (i.e., interleukin [IL]-4 and IL-10) or apoptotic proteins (i.e., Fas Ligand [FasL]) to reduce inflammation in a mouse model of experimentally induced arthritis. Exosomes are nano-sized vesicles (40-100 nm diameter) released by different cell types, including DC, that contain many of the proteins thought to be involved in regulating the immune response. We have demonstrated that exosomes derived from immature DC treated with immunomodulatory cytokines (i.e., IL-10, IL-4) are able to inhibit inflammation in a murine footpad model of delayed-type hypersensitivity (DTH) and reduce the severity of established collagen-induced arthritis (CIA). In fact, the exosomes were as therapeutic as the parental DC. Because purified DC-derived exosomes are very stable vesicles, they may be a better approach for future treatment of arthritis and other autoimmune disorders than the more unstable DC. In this chapter we detail a protocol for preparing the exosomes produced by murine bone marrow-derived DC. We also review methods to assess the purity and concentration of purified exosomes, by using electron microscopy, Western blot analysis, and flow cytometry. Finally, we describe methods to assess the function of exosomes in vitro, using the mixed leukocytes reaction, and in vivo by means of DTH and an experimental model of CIA.

Circulating, soluble forms of major histocompatability complex antigens are not exosome-associated

In vitro studies have shown that soluble MHC (sMHC) released by cell lines is bound to nano-vesicles termed exosomes. It is thought that exosomes may represent the major reservoir of sMHC class I and II molecules in biological fluids. However, most studies have been confined to in vitro assays performed with cell lines. We show here that sMHC in the serum or plasma differs from exosome-bound sMHC in five ways: In contrast to exosome-associated sMHC, circulating sMHC is of low density, has a low apparent molecular mass (40-300 kDa) and is not detergent-labile. Moreover, the majority of MHC class II isoforms and MHC class I in blood are not physically linked and circulating HLA-DR is accessible to an antibody specific for the HLA-DR alpha-chain intracellular epitope, which is masked by its association with cellular or exosomal membranes. Finally, utilizing transcriptional activator of murine MHC class II (C2ta) promoter-mutant mice, we showed that the release of sMHC class II into the circulation is dependent on the C2ta pI promoter, but not pIII or pIV. This suggests that myeloid dendritic cells and/or macrophages, which preferentially use promoter pI of the C2ta gene, produce most of the sMHC class II found in the circulation.

Efficient induction of antitumor T cell immunity by exosomes derived from heat-shocked lymphoma cells

Exosomes secreted by tumor cells could serve as a promising immunotherapeutic tumor vaccine. Heat shock proteins (HSP) induced in tumor cells by heat shock are molecular chaperones with potent adjuvant activity in the induction of antigen-specific T cell responses. To improve exosome-based tumor vaccines, we have investigated the efficacy of exosomes derived from heat-shocked mouse B lymphoma cells (HS-Exo) in the induction of antitumor immune responses. We found that HS-Exo, compared with control exosomes derived from the same cells (Exo), contain more HSP60 and HSP90 and increased amounts of molecules involved in immunogenicity including MHC class I, MHC class II, CD40, CD86, RANTES and IL-1beta. Furthermore, HS-Exo induce both phenotypic and functional maturation of dendritic cells more efficiently. HS-Exo immunization activates T cell responses more potently. Importantly, HS-Exo induce dramatically increased antitumor immune responses compared to control exosomes from the same cells in prophylaxis and therapeutic in vivo lymphoma models. We further demonstrate that CD8(+) T cells are the predominant T cell subset responsible for the antitumor effect of HS-Exo and that CD4(+) T cells are necessary in the induction phase of tumor rejection in a prophylaxis model. These findings provide a novel strategy to improve the efficacy of exosome-based tumor vaccines.

Oral tolerance induction by mucosal administration of cholera toxin B-coupled antigen involves T-cell proliferation in vivo and is not affected by depletion of CD25+ T cells

Oral administration of antigens coupled to the B subunit of the cholera toxin (CTB) can dramatically reduce the amount of antigen needed for tolerance induction and has been used in several animal models to suppress conditions where the immune system overreacts to foreign and self-antigens. In this study, the cellular events following oral administration of CTB-coupled antigen was investigated. As a model system, limited numbers of CSFE-labelled cells from influenza haemagglutinin peptide (HApep) T-cell transgenic mice were transferred to wild type mice and the mice were then given CTB-coupled HApep orally. The inductive events of CTB-induced tolerance was characterized by extensive proliferation of HApep-specific T cells in the mesenteric lymph nodes (MLNs) and in the spleen. The proliferating cells up-regulated the gut homing molecule alpha4beta7 and down-regulated the high endothelial venule binding molecule L-selectin. Addition of the whole cholera toxin (CT) to CTB-HApep showed a similar pattern as CTB-HApep feeding, with antigen-specific proliferation in the MLN and spleen and expression of alpha4beta7 on the proliferating cells. However, addition of CT to CTB-HApep, produced a stronger and faster proliferative response and abrogated CTB-HA mediated oral tolerance. Feeding of CTB-HApep expanded CD25+ cells in the MLNs. CTB-induced oral tolerance could, however, not be explained by CD25+ dependent regulatory activity, as oral administration of CTB-HApep to mice depleted of CD25+ cells still gave rise to systemic tolerance. Thus, several mechanisms might co-orchestrate the systemic tolerance seen in response to feeding with CTB-coupled antigen.

Direct exosome stimulation of peripheral humanT cells detected by ELISPOT

Exosomes from APC are nano-vesicles that can induce antigen-specific T cell responses and are presently explored as therapeutic tools in different clinical settings. Investigations of the capacity of exosomes to stimulate T cells in vitro have mostly been performed on T cell hybridomas, clones or lines. Whether exosomes can stimulate T cells directly or need the presence of dendritic cells (DC) is debated. We could detect exosome-induced antigen-specific CD8(+) T cell responses in peripheral blood from humans. Exosomes from monocyte-derived DC (MDDC) were loaded with a mix of 23 immunogenic peptides from EBV, CMV and influenza virus, and added to autologous peripheral CD8(+) T cells. IFN-gamma-producing cells were detected by enzyme-linked immunospot assay (ELISPOT). MDDC-exosomes induced IFN-gamma production in CD8(+) T cells without addition of DC. The response was exosome dose dependent, and dependent on exosomal MHC class I. Furthermore, we detected an enhanced T cell stimulatory capacity by exosomes from lipopolysaccharide-matured MDDC compared to exosomes from immature MDDC. Exosomes could also induce TNF-alpha production. These results show, for the first time, that exosomes can directly stimulate human peripheral CD8(+) T cells in an antigen-specific manner and that ELISPOT is a suitable method for detecting exosome-induced peripheral T cell responses. This system may provide a useful tool when developing exosomes as therapeutic agents.

Importance of gastrointestinal ingestion and macromolecular antigens in the vein for oral tolerance induction

Oral administration of a certain dose of antigen can generally induce immunological tolerance against the same antigen. In this study, we showed the temporal appearance of ovalbumin (OVA) antigens in both portal and peripheral blood of mice after the oral administration of OVA. Furthermore, we detected 45,000 MW OVA in mouse serum 30 min after the oral administration of OVA. Based on this observation, we examined whether the injection of intact OVA into the portal or peripheral vein induces immunological tolerance against OVA. We found that the intravenous injection of intact OVA did not induce immunological tolerance but rather enhanced OVA-specific antibody production in some subclasses, suggesting that OVA antigens via the gastrointestinal tract but not intact OVA may contribute to establish immunological tolerance against OVA. Therefore, we examined the effects of digesting intact OVA in the gastrointestinal tract on the induction of oral tolerance. When mice were orally administered or injected into various gastrointestinal organs, such as the stomach, duodenum, ileum, or colon and boosted with intact OVA, OVA-specific antibody production and delayed-type hypersensitivity (DTH) response were significantly enhanced in mice injected into the ileum or colon, compared with orally administered mice. These results suggest that although macromolecular OVA antigens are detected after oral administration of OVA in tolerant-mouse serum, injection of intact OVA cannot contribute to tolerance induction. Therefore, some modification of macromolecular OVA in the gastrointestinal tract and ingestion may be essential for oral tolerance induction.

Escape strategies and reasons for failure in the interaction between tumour cells and the immune system: how can we tilt the balance towards immune-mediated cancer control?

The last decade has witnessed an exponential increase in the attempts to demonstrate that adaptive immunity can effectively detect cancer cells and impair their growth in vivo in cancer patients. However, clinical trials of immunotherapy with a broad array of immunisation strategies have depicted a rather disappointing scenario, suggesting that successful control of tumour growth by immunotherapeutic treatments may not be an easy task to achieve. The attention of tumour immunologists has thus been switched to the potential reasons of failure, and extensive efforts are being made in defining the cellular and molecular pathways interfering with the capacity of the immune system to develop powerful immunological reactions against tumour cells. Although many of these pathways have been well characterised in murine models, little and controversial information about their role in determining neoplastic progression in cancer patients is available. This discrepancy at the moment represents one of the major limitations in understanding the obstacles to the in vivo development of protective T cell-mediated immune responses against tumours, and how pharmacological or biological interventions aimed at bypassing tumour escape mechanisms would indeed result in a clinical benefit. The study of the reasons for the failure of the immune system to control tumour growth, which have to be ascribed to highly interconnected phenomena occurring at both tumour and immune levels, could in the near future provide adequate tools to fight cancer by finely tuning the host environment through biological therapies.

Pregnancy-associated exosomes and their modulation of T cell signaling

Exosome release by viable cells is a feature of activated cell types, including tumors, fetal cells, and cells of the immune system. Exosomes critically regulate immune activation, by mediating activation-induced cell death. Fetal cells may mimic these events to selectively delete reactive lymphocytes. In this study the presence and composition of placenta-derived exosomes are demonstrated in the maternal circulation along with their consequences on T cell activation markers. For all pregnant patients, exosomes were isolated from sera obtained between 28 and 30 wk gestation. For pregnant women, subsequently delivering at term, circulating levels of placental exosomes were 1.8 times greater than those delivering preterm (p < 0.0001). Exosomes isolated from pregnancies subsequently delivering at term expressed significantly higher levels of biologically active components, including Fas ligand (FasL) and HLA-DR, than those from pregnancies delivering preterm. Standardizing for protein concentrations, exosomes from term-delivering pregnancies exhibited greater suppression of CD3-zeta and JAK3 than those delivering preterm. The suppression of CD3-zeta and JAK3 correlated with exosome expression levels of FasL (r2= 0.92 and r2= 0.938, respectively). Fractionation of exosomes from term-delivering pregnancies by continuously eluting electrophoresis indicated that intact 42 kD FasL and an unidentified 24-kDa protein were associated with CD3-zeta suppression. Our results demonstrated that exosomes from pregnancies ultimately delivering at term are present at significantly greater concentrations than those from pregnancies delivering preterm; however, exosomes from term-delivering pregnancies also exhibit significantly greater suppression of CD3-zeta and JAK3.

Exosomes Derived from Genetically Modified DC Expressing FasL Are Anti-inflammatory and Immunosuppressive

We previously have demonstrated the ability of primary murine bone marrow-derived DC (BM-DC), genetically modified by adenoviral infection to express FasL, to inhibit progression of established collagen-induced arthritis (CIA) following systemic delivery. Here we demonstrate that exosomes derived from genetically modified BM-DC expressing FasL are able to inhibit inflammation in a murine footpad model of delayed-type hypersensitivity (DTH). Local administration of exosomes derived from DC expressing FasL (Exo/FasL) as well as the parental DC/FasL resulted in a significant reduction in swelling in both the treated and the untreated distal paw. However, both the DC/FasL and the Exo/FasL were unable to suppress the DTH response in lpr (Fas-deficient) mice. Gene transfer of FasL to BM-DC from gld (FasL-deficient) mice resulted in restoration of the ability of DC as well as DC-derived exosomes to suppress DTH. The ability of DC-derived exosomes and DC to suppress DTH responses was antigen specific and MHC class II dependent, but class I independent. The injected exosomes were found to be internalized into CD11c(+) cells at the site of injection and in the draining popliteal lymph node. Systemic injection of exosome/FasL into mice with established CIA resulted in significant disease amelioration. These results demonstrate that both systemic and local administration of exosomes derived from FasL-expressing DC are able to suppress antigen-specific immune responses through an MHC class II-dependent pathway, resulting in effective and sustained treatment of established collagen-induced arthritis and suppression of the DTH inflammatory response. These results suggest that DC/FasL-derived exosomes could be used clinically for the treatment of inflammatory and autoimmune diseases.

Tumor exosomes expressing Fas ligand mediate CD8+ T-cell apoptosis

Tumor-derived immune suppression is considered to be a major mechanism of tumor evasion from the immune system destruction, however, little is known regarding the induction of T-cell functional suppression by tumor-derived exosomes. Herein, we investigate tumor-derived exosomes involved in normal immunological communications as means of inhibiting an antitumor T-cell response. Exosomes derived from LNCaP, a human prostate cancer cell line, were visualized by FACS and identified based on size (80-200 nm) in comparison to marker beads. Exosomes from tumor cell line inhibited T-cell proliferation. Dose-dependent apoptosis of T cells was induced by co-culture with tumor exosomes. Addition of anti-FasL antibody blocked the apoptosis induction by tumor exosomes. This study suggests that induction of T-cell apoptosis by tumor-derived exosomes appears to be a novel mechanism of tumor immune evasion.

Human small intestinal epithelial cells constitutively express the key elements for antigen processing and the production of exosomes

In humans, the small intestinal epithelial cells (IEC) have a high constitutive expression of MHC class II (MHC II), and contains lysosomes. The IEC also contains MHC II rich multivesicular compartments and has been shown to produce exosomes. This suggests a role for the IEC in antigen processing and presentation either directly or indirectly by the production of exosomes. However, the presence and localisation in the IEC of other key molecules involved in this process has not been studied previously. In the present work, we have investigated small intestinal biopsies from healthy adults and the HT29 IEC cell line with monoclonal antibodies against molecules involved in the antigen processing/presenting systems and molecules typically found on exosomes derived from professional APCs and IECs. Immunohistology was performed to study the expression and localisation of MHC II (HLA-DR), HLA-DM, MHC I (HLA-ABC), CD1d, Invariant chain, Lamp-1, CD68, CD63, B7.1, B7.2, ICAM-1, Cathepsin D/S/L and the IEC specific marker A33 in the IECs. We found that the IECs from the biopsies constitutively express MHC II, HLA-DM, MHC I, Invariant chain, Lamp-1, CD 68, CD63 and A33, and these markers were also found in the IFN-g treated HT-29 cells. All these molecules were found apically in the IECs of the biopsies, localised mainly in vesicular structures. Interestingly, in the baso-latereral area of the IEC, only MHC II, MHC I, Lamp 1, CD68, CD63 and A33 were found and also here with vesicular staining pattern which matches the molecules previously found on exosomes derived professional APCs and human IEC lines. CD1d, B7, ICAM-1, CD9 and cathepsin D and L were absent in the IEC compartment, but cathepsin S showed a relatively weak staining in the apical part of the IEC. The staining pattern and the morphological localisation of these markers suggest a prominent antigen processing/loading and trafficking compartment, and a possible baso-lateral release of exosomes in the normal human IEC.

Tolerosome-induced oral tolerance is MHC dependent

Oral administration of a protein antigen generates a serum factor that induces tolerance when transferred into naïve recipients. This serum factor has been described in rats as consisting of exosome-like structures or tolerosomes, which express major histocompatibility complex class II molecules (MHCII) and mediate antigen-specific tolerance. In this study, we investigated the functions of serum-derived tolerosomes both in vivo and in vitro. Tolerosomes were purified from the 100,000 g pellet fraction of serum from ovalbumin (OVA)-fed mice. When transferred into naïve recipient mice, the tolerosomes mediated OVA-specific tolerance. We also found that tolerosomes from OVA-fed mice induced the activation of OVA-specific T cells both in vivo and in vitro. The inoculation of severe combined immunodeficiency (SCID) mice with an interferon-gamma-producing cell line normalized the expression of MHCII in the intestinal epithelial cells and restored their ability to generate tolerosomes. Syngeneic but not allogeneic transfer of tolerosomes from OVA-fed donors induced tolerance in the recipients. Our results show that tolerosomes can be isolated from mouse serum, that tolerosome-induced oral tolerance requires MHCII expression in intestinal epithelial cells, and that tolerosomes are functional only in syngeneic recipients.

Antigen targeting to MHC class II-enriched late endosomes in colonic epithelial cells: trafficking of luminal antigens studied in vivo in Crohn's colitis patients

In Crohn's disease (CD), colonic epithelial cells (CECs) are suggested to stimulate pro-inflammatory CD4+ T cells. However, the endocytic pathways of luminal antigens involved in underlying MHC class II presentation by CECs remain unknown. Our aim was to elucidate antigen trafficking and associated MHC class II expression in CECs of CD patients in vivo. In CD patients (Crohn's colitis and remission) and healthy controls undergoing colonoscopy, ovalbumin (OVA) was sprayed onto inflamed or healthy mucosa. The subcellular localization of OVA and MHC class II was visualized in biopsies taken from OVA-incubated mucosa using fluorescence and cryoelectron microscopy. Targeting of OVA into late endosomes of CECs was found in healthy (controls and CD in remission) and inflamed mucosa (Crohn's colitis). MHC class II expression in CECs was not detected in healthy mucosa but strongly up-regulated during CD inflammation. Induced MHC class II in CECs was predominantly seen at basolateral membranes and in late endosomes, which were efficiently accessed by internalized OVA. Our data provide in vivo evidence that the endocytic pathway of luminal antigens in CECs of Crohn's colitis patients intersects MHC class II-enriched late endosomes and support the postulated role of CECs in MHC class II-associated antigen presentation during CD.

Induction of antigen-specific regulatory T cells in the liver-draining celiac lymph node following oral antigen administration

Regulatory T cells are induced by oral administration of an antigen, but the physiological requirements and localization of the inductive sites are largely unknown. Using an adoptive transfer system of cells transgenic for ovalbumin T-cell receptor (OVA TCR tg), we found that antigen-specific CD4+ T cells were activated in the liver-draining celiac lymph node (CLN) shortly after ovalbumin feeding, and that a significantly higher proportion of the T cells in the CLN developed into the putative regulatory phenotype [co-expressing CD25 with the glucocortico-induced tumour necrosis factor (TNF) receptor family related gene (GITR), cytotoxic T-lymphocyte antigen (CTLA)-4 and CD103] than in Peyer's patches, the mesenteric and peripheral lymph nodes and the spleen. In addition, a particularly high level of expression of CD103 on the OVA-specific T cells in the CLN may favour homing to the epithelium of the intestine. While equally suppressive, OVA tg T cells isolated from the CLN of OVA-fed DO11.10 mice were less dependent on transforming growth factor (TGF)-beta for suppression than cells isolated from the peripheral and mesenteric lymph nodes, which indicates the involvement of an additional suppressive mechanism. The expression of FoxP3 was not up-regulated in any of the lymph node compartments studied. Our phenotypic and functional findings suggest that the induction of regulatory T cells in the CLN may be relevant in the control of the immune response to dietary antigens.

Gut barrier dysfunction in food allergy

Intestinal permeability tests used in the diagnosis of allergic diseases in response to oral food challenge have led to the conclusion that constitutive defects of the intestinal barrier are not the primary cause of allergic diseases. However, perturbation of environmental factors (infection, stress), by increasing intestinal permeability and enhancing danger signals, may favour food allergy in susceptible individuals. The mechanisms of enhanced permeability to specific and bystander antigens have been delineated as well as the molecular events involved in the sequential phases of allergic reactions. Intestinal absorption of food antigens and immune responses are mutually dependent, and luminal (environmental) and serosal (intrinsic) factors synergize to maintain a self-perpetuating cycle in which antigens penetrate the mucosa and induce allergic inflammation.

Cross-presentation of oral antigens by liver sinusoidal endothelial cells leads to CD8 T cell tolerance

After ingestion, oral antigens distribute systemically and provoke T cell stimulation outside the gastrointestinal tract. Within the liver, scavenger liver sinusoidal endothelial cells (LSEC) eliminate blood-borne antigens and induce T cell tolerance. Here we investigated whether LSEC contribute to oral tolerance. Oral antigens were efficiently cross-presented on H-2K(b) by LSEC to naive CD8 T cells. Cross-presentation efficiency in LSEC but not dendritic cells was increased by antigen-exposure to heat or low pH. Mechanistically, cross-presentation in LSEC requires endosomal maturation, involves hsc73 and proteasomal degradation. H-2K(b)-restricted cross-presentation of oral antigens by LSEC in vivo induced CD8 T cell priming and led to development of CD8 T cell tolerance in two independent experimental systems. Adoptive transfer of LSEC from mice fed with antigen (ovalbumin) into RAG2-/- knockout mice, previously reconstituted with naive ovalbumin-specific CD8 T cells, prevented development of specific cytotoxicity and expression of IFN-gamma in CD8 T cells. Using a new transgenic mouse line expressing H-2K(b) only on endothelial cells, we have demonstrated that oral antigen administration leads to tolerance in H-2K(b)-restricted CD8 T cells. Collectively, our data demonstrate a participation of the liver, in particular scavenger LSEC, in development of CD8 T cell tolerance towards oral antigens.

The potential of exosomes in immunotherapy

In 1996, B cell-derived exosome immunogenicity was first reported. In 1998, dendritic cell-derived exosomes were also shown to have immunogenic properties in mice, leading to tumour eradication. These observations showed for the first time that exosomes were bioactive vesicles with immuno-regulatory roles and potent antitumour effects. Exosomes are secreted vesicles formed in late endocytic compartments (multivesicular bodies). Vesicles exocytosed from multivesicular bodies into the extracellular medium are referred to as 'exosomes' and should not be confused with the more recently described 'ribonuclease complex' also named exosome. Exosomes are vesicles of 50-100 nm in diameter, formed by inward budding of the endosomal membrane in a process that sequesters particular proteins and lipids. This review will describe the biogenesis, the protein and lipid compositions, and the biological functions of exosomes; this review will also present their immunostimulatory functions in mice and humans, and will discuss their possible use as cancer vaccines.

Phenotypic and functional characterization of intestinal epithelial exosomes

Intestinal epithelial cells (IEC) are located at a strategic position between the external environment and the most extended lymphoid tissue in the body. Besides their central role in the absorption of nutrients, IEC also provide antigenic information to the immune system and are involved in the balance tolerance/allergy to food antigens. Like professional antigen presenting cells, IEC have been shown to secrete 30- to 90-nm diameter vesicles named exosomes, in a polarized way, either from their apical or basolateral side. These vesicles carry molecules involved in adhesion and antigen presentation, comprising major histocompatibility complex (MHC) class I and class II molecules, tetraspan proteins, CD26/dipeptidyl-peptidase IV, and A33 antigen, a molecule essentially restricted to the intestinal epithelium. Invariant chain, transferrin receptor, and Na-K-ATPase are not expressed on epithelial exosomes. In vivo, in mice, epithelial exosomes carrying MHC/ovalbumin peptide complexes induce specific immune responses when injected intraperitoneally. A33 antigen, an Ig-like molecule highly specific for intestinal epithelial cells and enriched in epithelial exosomes, is found at the surface of cells entering mesenteric lymph nodes suggesting exosome migration from the epithelial layer to the gut associated lymphoid system. Taken together, intestinal epithelial exosomes released at the basolateral surface of enterocytes could be antigen-carrying structures constituting a link between luminal antigens and the local immune system and acting as sensors of the antigenic information present in the intestinal lumen.

The influence of environment on development of the mucosal immune system

The mucosal immune system expresses active responses against pathogens and also tolerance against harmless food and commensal bacterial antigens. The mechanisms that determine which of these outcomes occur after recognition of antigens by T-cells are not clear. One possibility is that it is determined by the initial interaction between a dendritic and a naïve T-cell in organised lymphoid tissue. However, such organised structures are, evolutionarily, quite recent and the original immune system must have made appropriate responses in more diffuse immunological architecture; a second possibility is that the critical interaction is between primed T-cells and their environment, in the lamina propria of the intestine. The mucosal immune system of neonates is poorly developed and inefficient at expressing appropriate immune responses. Development is influenced by a range of environmental factors including maternally derived antigen or antibody and commensal flora and pathogens. The intestine is a complex immunological structure in which the immune system and the macro- and microenvironment interact.

Exosomal-like vesicles are present in human blood plasma

Exosomes are small membrane vesicles (50-90 nm in diameter) secreted by most hematopoietic cells. We provide here the first evidence for the presence of exosomes in vivo, in the blood. Plasma samples of all healthy donors tested (n = 15) contain vesicles that are similar in shape, size and density to the previously described exosomes. They were clearly identified by electron microscopy after isolation by differential ultracentrifugation or immunoisolation with CD63-coated latex beads. We performed their biochemical characterization by western blot analysis and by flow cytometry after vesicle adsorption onto latex beads using a panel of mAbs. We observed that these plasma-derived vesicles contain tetraspanin molecules such as CD63, CD9, CD81 as well as class I and class II MHC molecules and Lamp-2 (i.e. proteins that are known to be enriched in exosomes). In addition, these vesicles float on sucrose gradient at a density similar to exosomes. Our results demonstrate that blood is a physiological fluid for exosome circulation in the body, suggesting their role in cell-cell or organ-organ communications as carriers for molecules that need to reach distant cell targets.

A Full Flora, but not Monocolonization by Escherichia coli or Lactobacilli, Supports Tolerogenic Processing of a Fed Antigen

Fed protein undergoes processing and coupling to major histocompatibility complex (MHC) II molecules during passage through the intestinal epithelium, generating a tolerogenic form of the antigen in serum. Transfer of this factor to naïve animals induces tolerance in the recipient. In this study, we investigate what impact colonization with Gram-positive (Lactobacillus plantarum) or Gram-negative (Escherichia coli) bacteria has on tolerogenic processing in the gut. Germ-free (GF), monocolonized or conventional mice were fed ovalbumin (OVA), and their serum was collected and transferred to naïve conventional recipients that were tested for delayed-type hypersensitivity against OVA after parenteral immunization. A transferable tolerogenic factor was produced by conventional mice, but not by mice that were germ free or monocolonized with either E. coli or L. plantarum. Conventional, but neither GF nor monocolonized mice showed upregulation of MHCII expression in the epithelium of small intestine. The results suggest that a complex intestinal microflora is needed to support oral tolerance development.

Human colorectal cancer cells induce T-cell death through release of proapoptotic microvesicles: role in immune escape

BACKGROUND & AIMS

Normal and neoplastic cells release microvesicles, whose effects on the immune system still need to be elucidated. Because human colorectal cancer cells are hypothesized to escape immune recognition by expressing proapoptotic molecules, we investigated whether microvesicles bearing Fas ligand and tumor necrosis factor-related apoptosis-inducing ligand and inducing apoptosis of activated T cells are secreted by colorectal cancer cells both in vitro and in affected patients.

METHODS

Fas ligand and tumor necrosis factor-related apoptosis-inducing ligand expression were analyzed in colorectal cancer cells and purified microvesicles by flow cytometry, Western blotting, and immunoelectron microscopy. Microvesicle tumor origin was assessed through simultaneous detection of lysosomal (CD63) and adenocarcinoma (carcinoembryonic antigen) markers. Proapoptotic activity of microvesicles was evaluated by annexin V/propidium iodide staining and caspase activation in T cells, including CD8+ T lymphocytes from colorectal cancer patients.

RESULTS

Colorectal cancer cells showed a granular pattern of tumor necrosis factor-related apoptosis-inducing ligand and Fas ligand expression, suggesting a secretory behavior. These proapoptotic molecules were detected on isolated microvesicles, together with class I HLA, CD63, and carcinoembryonic antigen. Microvesicles induced Fas ligand-mediated and tumor necrosis factor-related apoptosis-inducing ligand-mediated apoptosis of activated CD8+ T cells generated from colorectal cancer patients. Microvesicles with comparable phenotypes and functions were found in plasma from patients with advanced disease, whereas vesicular structures expressing Fas ligand and tumor necrosis factor-related apoptosis-inducing ligand were also detected in colorectal cancer specimens.

CONCLUSIONS

These data show that colorectal cancer induces T-cell apoptosis through the release of Fas ligand-bearing and tumor necrosis factor-related apoptosis-inducing ligand-bearing microvesicles both in vitro and in vivo. This mechanism of immune escape has potential implications as a prognostic factor and could be targeted for the development of new antitumor therapies in colorectal cancer patients.

Exosomes Derived from IL-10-Treated Dendritic Cells Can Suppress Inflammation and Collagen-Induced Arthritis

We have demonstrated previously that local, adenoviral-mediated gene transfer of viral IL-10 to a single joint of rabbits and mice with experimental arthritis can suppress disease in both the treated and untreated contralateral joints. This contralateral effect is mediated in part by APCs able to traffic from the treated joint to lymph nodes as well as to untreated joints. Moreover, injection of dendritic cells (DC) genetically modified to express IL-4 or Fas ligand was able to reverse established murine arthritis. To examine the ability of exosomes derived from immunosuppressive DCs to reduce inflammation and autoimmunity, murine models of delayed-type hypersensitivity and collagen-induced arthritis were used. In this study, we demonstrate that periarticular administration of exosomes purified from either bone marrow-derived DCs transduced ex vivo with an adenovirus expressing viral IL-10 or bone marrow-derived DCs treated with recombinant murine IL-10 were able to suppress delayed-type hypersensitivity responses within injected and untreated contralateral joints. In addition, the systemic injection of IL-10-treated DC-derived exosomes was able suppress the onset of murine collagen-induced arthritis as well as reduce severity of established arthritis. Taken together, these data suggest that immature DCs are able to secrete exosomes that are involved in the suppression of inflammatory and autoimmune responses. Thus DC-derived exosomes may represent a novel, cell-free therapy for the treatment of autoimmune diseases.

Interferon-gamma mediates antigen trafficking to MHC class II-positive late endosomes of enterocytes

MHC class II-positive late endosomes of enterocytes are thought to be involved in antigen presentation to CD4(+) T cells. In contrast to enterocytes of BALB/c mice, severe combined immunodeficiency (SCID) enterocytes lack MHC class II expression and fail to transport internalized ovalbumin (OVA) into late endosomes. IFN-gamma is known to induce MHC class II in enterocytes and antigen targeting to late endosomes in macrophages. In this study, we investigated the influence of IFN-gamma and MHC class II on the processes of antigen traffic in enterocytes. Subcellular targeting of OVA and MHC class II expression within enterocytes were examined in SCID, IFN-gamma-treated SCID, BALB/c and C57BL/6 MHC class II knockout (KO) mice after a single feed with OVA. Sorting of OVA into late endosomes was found in enterocytes from BALB/c, C57BL/6 KO and IFN-gamma-stimulated SCID mice, but not from untreated SCID mice. MHC class II expression was restricted to enterocytes of IFN-gamma-treated SCID and BALB/c mice, present at basolateral membranes and within endosomal compartments. These enterocytes further revealed colocalization of class II antigens and OVA in endosomes. We suggest that antigen trafficking into late endosomes of enterocytes is mediated by IFN-gamma and occurs in the absence of MHC class II.

Differentiation of the follicle-associated epithelium in ileal Peyer?s patch and production of 50-nm particles are maintained in B-cell-depleted fetal sheep

To evaluate the dependence of the differentiation of the follicle-associated epithelium (FAE) on the presence of follicular B-cells, the FAE of ileal Peyer's patch follicles was examined in B-cell-depleted fetal lambs. The FAE of these rudimentary follicles, which are devoid of lymphocytes, showed normal differentiation, including carbonic anhydrase reactivity and ultrastructural characteristics of transcytosis, extensive interdigitation of the lateral plasma membrane and the shedding of membrane-bounded particles, approximately 50 nm in size, resembling exosomes. These 50-nm membrane-bounded particles were abundant in the extracellular space of the epithelium and the dome but no particles were found in the rudimentary follicles. This study confirms that the rudimentary follicles consist of clusters of follicular dendritic cells. Our findings suggest that the differentiation of FAE of ileal Peyer's patch and the production of the 50-nm particles constitute features that appear to be independent of B-cells.

Proteomic analysis of melanoma-derived exosomes by two-dimensional polyacrylamide gel electrophoresis and mass spectrometry

Exosomes are 40-100 nm vesicles released by numerous cell types and are thought to have a variety of roles depending on their origin. Exosomes derived from antigen presenting cells have been shown to be capable of initiating immune responses in vivo and eradicating established tumours in murine models. Tumour-derived exosomes can be utilised as a source of tumour antigen for cross-priming to T-cells and are thus of interest for use in anti-tumour immunotherapy. Further exploration into the protein composition of exosomes may increase our understanding of their potential roles in vivo and this study has examined the proteome of exosomes purified from cell supernatants of the melanoma cell lines MeWo and SK-MEL-28. The vesicular nature and size (30-100 nm) of the purified exosomes was confirmed by electron microscopy and sucrose density gradient centrifugation. Western blotting demonstrated the absence of calnexin and cytochrome c, verifying the purity of the exosome preparations, as well as enrichment of MHC class I and the tumour-associated antigens Mart-1 and Mel-CAM. The two-dimensional polyacrylamide gel electrophoresis (2-D PAGE) protein profiles of exosomes from the two cell lines were highly comparable and strikingly different from the profiles of the total cell lysates. Mass spectrometric sequencing identified proteins present in 49 protein spots in the exosome lysates. Several of these have been identified previously in exosomes but some are novel, including p120 catenin, radixin, and immunoglobulin superfamily member 8 (PGRL). Proteins present in whole-cell lysates that were significantly reduced or excluded from exosomes were also identified and included several mitochondrial and lysosomal proteins, again confirming the proposed endosomal origin of exosomes. This study presents a starting point for future more in-depth protein studies of tumour-derived exosomes which will aid the understanding of their biogenesis and targeting for use in anti-tumour immunotherapy protocols.

Exosome Secretion: The Art of Reutilizing Nonrecycled Proteins?

Multivesicular bodies contain membrane vesicles which either undergo lysosomal digestion or are released in the extracellular environment as exosomes. Evidence is accumulating that supports a physiological role for exosomes in, for example, antigen presentation or removal of transferrin receptor during reticulocyte development. Here, inspired by observations on exosomal release from reticulocytes, we discuss the potential involvement of the so-called ESCRT mechanism in the entrapment of both lysosomal and exosomal cargo within the intralumenal vesicles of multivesicular bodies. We propose that this mechanism operates at different sites in the endocytic itinerary in different cells, thereby providing a tool for directional sorting. We also explore the possibility that the efficiency of sorting of molecules into exosomes increases when the recycling kinetics of molecules decreases, exosomal sorting being favored by intermolecular interactions occurring within lipid domains, or with protein webs, that slow lateral mobility. These considerations are mirrored in the context of current knowledge on the mechanism of protein sorting for degradation in lysosomes, and the hijacking of such mechanisms by some retroviruses for particle budding.

Endocytosis, intracellular sorting, and processing of exosomes by dendritic cells

Exosomes are nanovesicles released by leukocytes and epithelial cells. Although their function remains enigmatic, exosomes are a source of antigen and transfer functional major histocompatibility complex (MHC)-I/peptide complexes to dendritic cells (DCs) for CD8(+) T-cell activation. Here we demonstrate that exosomes also are internalized and processed by immature DCs for presentation to CD4(+) T cells. Endocytosed exosomes are sorted into the endocytic compartment of DCs for processing, followed by loading of exosome-derived peptides in MHC-II molecules for presentation to CD4(+) T cells. Targeting of exosomes to DCs is mediated via milk fat globule (MFG)-E8/lactadherin, CD11a, CD54, phosphatidylserine, and the tetraspanins CD9 and CD81 on the exosome and alpha(v)/beta(3) integrin, and CD11a and CD54 on the DCs. Circulating exosomes are internalized by DCs and specialized phagocytes of the spleen and by hepatic Kupffer cells. Internalization of blood-borne allogeneic exosomes by splenic DCs does not affect DC maturation and is followed by loading of the exosome-derived allopeptide IEalpha(52-68) in IA(b) by host CD8alpha(+) DCs for presentation to CD4(+) T cells. These data imply that exosomes present in circulation or extracellular fluids constitute an alternative source of self- or allopeptides for DCs during maintenance of peripheral tolerance or initiation of the indirect pathway of allorecognition in transplantation.

Control of Immune Responses by Trafficking Cell Surface Proteins, Vesicles and Lipid Rafts to and from the Immunological Synapse

Supramolecular clusters at the immunological synapse provide a mechanism for structuring complex communication networks between cells of the immune system. Regulating intra- and intercellular trafficking of proteins and lipids to and from the immunological synapse provides an additional level of complexity in determining the functional outcome of immune cell interactions. An emergent principle is that molecules requiring tightly regulated cell surface expression, e.g. negative regulators of cell activation or molecules promoting cytotoxicity, are trafficked to the immunological synapse from intracellular secretory as required lysosomes. Many molecules required for the early stages of the intercellular communication are already present at the cell surface, sometimes in lipid rafts, and are rapidly translocated laterally to the intercellular contact. Our understanding of these events critically depends on utilizing appropriate technologies for probing supramolecular recognition in live cells. Thus, we also present here a critical discussion of the technologies used to study lipid rafts and, more broadly, a map of the spatial and temporal dimensions covered by current live cell physical techniques, highlighting where advances are needed to exceed current spatial and temporal boundaries.

Mechanisms of natural tolerance in the intestine: implications for inflammatory bowel disease

Tolerance, the regulated inability to respond to a specific immunologic stimulant, is a physiological event important to normal immune function. Just as loss of tolerance to self-proteins results in autoimmune diseases, we assert that loss of tolerance to commensal flora in the intestinal lumen leads to inflammatory bowel disease (IBD). Mechanisms through which the mucosal immune system establishes and remains hyporesponsive toward the presence of food proteins and commensal flora, which we define as natural tolerance, are discussed. In addition to the contributions by commensal flora, the innate host defense and the adaptive immune systems promote natural tolerance to sustain normal mucosal homeostasis. Understanding the molecular and cellular events that mediate natural tolerance will lead to more advanced insights into IBD pathogenesis and improved therapeutic options.

Allergen-responsive CD4+CD25+ regulatory T cells in children who have outgrown cow's milk allergy

Cow's milk allergy in children is often of short duration, which makes this disorder an interesting clinical model for studies of tolerance to dietary antigens. Here, we studied T cell responses in 21 initially allergic children who, after a milk-free period of >2 mo, had cow's milk reintroduced to their diet. Children who outgrew their allergy (tolerant children) had higher frequencies of circulating CD4⁺CD25⁺ T cells and decreased in vitro proliferative responses to bovine β-lactoglobulin in peripheral blood mononuclear cells (PBMCs) compared with children who maintained clinically active allergy. No significant difference in proliferative activity stimulated by the polyclonal mitogen phytohemagglutinin was observed between the two groups. Depletion of CD25⁺ cells from PBMCs of tolerant children led to a fivefold increase in in vitro proliferation against β-lactoglobulin. This suggests that tolerance is associated with the appearance of circulating CD4⁺CD25⁺ regulatory T (Treg) cells that are capable of suppressing the effector T cells generated 1 wk after reintroduction of cow's milk. The suppressive function of the CD4⁺CD25⁺ Treg cells was shown to be partly cell contact dependent. Collectively, our study provides human data to suggest that mucosal induction of tolerance against dietary antigens is associated with the development of CD4⁺CD25⁺ Treg cells.

Targeting dendritic cells for priming cellular immune responses

The cardinal role of dendritic cells (DC) in priming adaptive immunity and in orchestrating immune responses against all classes of pathogens and also against tumors is well established. Their unique potential both to maintain self-tolerance and to initiate protective immune responses against foreign and/or dangerous structures is based on the functional diversity and flexibility of these cells. Tissue DC lining antigenic portals such as mucosal surfaces and the skin are specialized to take up a wide array of compounds including proteins, lipids, carbohydrates, glycoproteins, glycolipids and oligonucleotides, particles carrying such structures and apoptotic or necrotic cells. This process is facilitated by specialized receptors with high endocytic capacity, which provides potential targets for delivering designed molecules. The best route for targeting B- and/or T cell epitopes, however, is still the subject of intense investigation. Immature DC, which reside in various tissues, can be activated by pathogens, stress and inflammation or modified metabolic products, which induce mobilization of cells to draining lymph nodes where they act as highly potent professional antigen presenting cells. This is brought about by the ability to present their accumulated intracellular content for both CD4+ helper (Th) and CD8+ cytotoxic/cytolytic T lymphocytes (Tc/CTL). Engulfed proteins are processed intracellularly and their peptide fragments are transported to the cell surface in the context of major histocompatibility complex encoded class I and II molecules for presentation to Th cells and CTLs, respectively. The T cell priming capacity of DC, however, depends not only on antigen presentation but also on other features of DC. Human monocyte-derived DC provide an excellent tool to study the internalizing, antigen-presenting and T cell-activating functions of DC at their immature and activated differentiation states. These biological activities of DC, however, are highly dependent on their migratory potential from the peripheral non-lymphoid tissues to the lymph nodes, on the expression of adhesion molecules, which support the interaction of DC with T lymphocytes, and the cytokines secreted by DC, which polarize immune responses to Th1-mediated cellular or Th2-mediated antibody responses. These results altogether demonstrate that monocyte-derived DC are useful candidates for in vitro or in vivo targeting of antigens to induce efficient adaptive immune responses against pathogens and also against tumors.

Adhesion and signaling by B cell-derived exosomes: the role of integrins

Exosomes are nanometer-sized vesicles secreted by various cells, with potentially diverse roles in physiology. Although emphasis has been placed on their involvement in immune modulation, their potential for more wide-ranging biological effects has not been appreciated. A common exosome feature is the expression of adhesion molecules, which include the integrin family. We have for the first time addressed the possible function of B cell-derived exosome-integrins by examining adhesive interactions of exosomes (immobilized onto beads) with extracellular matrix (ECM) components and cytokine-treated fibroblasts. Integrin (beta1 and beta2) expression was demonstrated by Western blotting and flow cytometry. Binding studies (with blocking antibodies) demonstrated their function in adhesion to collagen-I, fibronectin, and tumor necrosis factor (TNF)-alpha-activated fibroblasts. Exosome adhesion to TNF-alpha-activated fibroblasts also triggered integrin-dependent changes in cytosolic calcium, measured by single cell imaging. Thus, B cell-derived exosomes express functional integrins, which are capable of mediating anchorage to ECM and cell-surface adhesion molecules, and may be a novel mode of delivering adhesion signals at distances beyond that of direct cell-cell contact during inflammation.