Exosomes as a short-range mechanism to spread alloantigen between dendritic cells during T cell allorecognition

Recent advances in mesenchymal stem cell immunomodulation: the role of microvesicles

Mesenchymal stem cells are the most widely used cell phenotype for therapeutic applications, the main reasons being their well-established abilities to promote regeneration of injured tissues and to modulate immune responses. Efficacy was reported in the treatment of several animal models of inflammatory and autoimmune diseases and, in clinical settings, for the management of disorders such as GVHD, systemic lupus erythematosus, multiple sclerosis, and inflammatory bowel disease. The effects of mesenchymal stem cells are believed to be largely mediated by paracrine signals, and several secreted molecules have been identified as contributors to the net biological effect. Recently, it has been recognized that bioactive molecules can be shuttled from cell to cell packed in microvesicles, tiny portions of cytoplasm surrounded by a membrane. Coding and noncoding RNAs are also carried in such microvesicles, transferring relevant biological activity to target cells. Several reports indicate that the regenerative effect of mesenchymal stem cells can be reproduced by microvesicles isolated from their culture medium. More recent evidence suggests that the immunomodulatory effects of mesenchymal stem cells are also at least partially mediated by secreted microvesicles. These findings allow better understanding of the mechanisms involved in cell-to-cell interaction and may have interesting implications for the development of novel therapeutic tools in place of the parent cells.

Origin and biology of the allogeneic response

The recognition by the immune system of nonself determinants on cells, tissues, or organs transplanted between genetically disparate members of the same species can lead to a potent allogeneic response that is responsible for rejection. We review here fundamental concepts that underlie the origins and biology of allorecognition in the mammalian immune system. We examine why and how T cells are alloreactive and discuss emerging evidence of allorecognition by innate immune cells. The nature of T cells (naïve vs. memory) and the alloantigen presentation pathways (direct, indirect, and semidirect) that initiate the allogeneic response are outlined.

The origin, function, and diagnostic potential of RNA within extracellular vesicles present in human biological fluids

We have previously demonstrated that tumor cells release membranous structures into their extracellular environment, which are termed exosomes, microvesicles or extracellular vesicles depending on specific characteristics, including size, composition and biogenesis pathway. These cell-derived vesicles can exhibit an array of proteins, lipids and nucleic acids derived from the originating tumor. This review focuses of the transcriptome (RNA) of these extracellular vesicles. Based on current data, these vesicular components play essential roles as conveyers of intercellular communication and mediators of many of the pathological conditions associated with cancer development, progression and therapeutic failures. These extracellular vesicles express components responsible for angiogenesis promotion, stromal remodeling, signal pathway activation through growth factor/receptor transfer, chemoresistance, and genetic exchange. These tumor-derived extracellular vesicles not only to represent a central mediator of the tumor microenvironment, but their presence in the peripheral circulation may serve as a surrogate for tumor biopsies, enabling real-time diagnosis and disease monitoring.

Human placental trophoblasts confer viral resistance to recipient cells

Placental trophoblasts form the interface between the fetal and maternal environments and serve to limit the maternal-fetal spread of viruses. Here we show that cultured primary human placental trophoblasts are highly resistant to infection by a number of viruses and, importantly, confer this resistance to nonplacental recipient cells by exosome-mediated delivery of specific microRNAs (miRNAs). We show that miRNA members of the chromosome 19 miRNA cluster, which are almost exclusively expressed in the human placenta, are packaged within trophoblast-derived exosomes and attenuate viral replication in recipient cells by the induction of autophagy. Together, our findings identify an unprecedented paracrine and/or systemic function of placental trophoblasts that uses exosome-mediated transfer of a unique set of placental-specific effector miRNAs to directly communicate with placental or maternal target cells and regulate their immunity to viral infections.

Transfer of extracellular vesicles during immune cell-cell interactions

The transfer of molecules between cells during cognate immune cell interactions has been reported, and recently a novel mechanism of transfer of proteins and genetic material such as small RNA between T cells and antigen-presenting cells (APCs) has been described, involving exchange of extracellular vesicles (EVs) during the formation of the immunological synapse (IS). EVs, a term that encompasses exosomes and microvesicles, has been implicated in cell-cell communication during immune responses associated with tumors, pathogens, allergies, and autoimmune diseases. This review focuses on EV transfer as a mechanism for the exchange of molecules during immune cell-cell interactions.

Exosomes and the kidney: blaming the messenger

Exosomes are membrane-bound vesicles of endosomal origin, present in a wide range of biological fluids, including blood and urine. They range between 30 and 100 nm in diameter, and consist of a limiting lipid bilayer, transmembrane proteins and a hydrophilic core containing proteins, mRNAs and microRNAs (miRNA). Exosomes can act as extracellular vehicles by which cells communicate, through the delivery of their functional cargo to recipient cells, with many important biological, physiological and pathological implications. The exosome release pathway contributes towards protein secretion, antigen presentation, pathogen transfer and cancer progression. Exosomes and exosome-mediated signalling have been implicated in disease processes such as atherosclerosis, calcification and kidney diseases. Circulating levels of exosomes and extracellular vesicles can be influenced by the progression of renal disease. Advances in methods for purification and analysis of exosomes are leading to potential diagnostic and therapeutic avenues for kidney diseases. This review will focus on biophysical properties and biogenesis of exosomes, their pathophysiological roles and their potential as biomarkers and therapeutics in kidney diseases.

Standardization of sample collection, isolation and analysis methods in extracellular vesicle research

The emergence of publications on extracellular RNA (exRNA) and extracellular vesicles (EV) has highlighted the potential of these molecules and vehicles as biomarkers of disease and therapeutic targets. These findings have created a paradigm shift, most prominently in the field of oncology, prompting expanded interest in the field and dedication of funds for EV research. At the same time, understanding of EV subtypes, biogenesis, cargo and mechanisms of shuttling remains incomplete. The techniques that can be harnessed to address the many gaps in our current knowledge were the subject of a special workshop of the International Society for Extracellular Vesicles (ISEV) in New York City in October 2012. As part of the “ISEV Research Seminar: Analysis and Function of RNA in Extracellular Vesicles (evRNA)”, 6 round-table discussions were held to provide an evidence-based framework for isolation and analysis of EV, purification and analysis of associated RNA molecules, and molecular engineering of EV for therapeutic intervention. This article arises from the discussion of EV isolation and analysis at that meeting. The conclusions of the round table are supplemented with a review of published materials and our experience. Controversies and outstanding questions are identified that may inform future research and funding priorities. While we emphasize the need for standardization of specimen handling, appropriate normative controls, and isolation and analysis techniques to facilitate comparison of results, we also recognize that continual development and evaluation of techniques will be necessary as new knowledge is amassed. On many points, consensus has not yet been achieved and must be built through the reporting of well-controlled experiments.

BM mesenchymal stromal cell-derived exosomes facilitate multiple myeloma progression

BM mesenchymal stromal cells (BM-MSCs) support multiple myeloma (MM) cell growth, but little is known about the putative mechanisms by which the BM microenvironment plays an oncogenic role in this disease. Cell-cell communication is mediated by exosomes. In this study, we showed that MM BM-MSCs release exosomes that are transferred to MM cells, thereby resulting in modulation of tumor growth in vivo. Exosomal microRNA (miR) content differed between MM and normal BM-MSCs, with a lower content of the tumor suppressor miR-15a. In addition, MM BM-MSC-derived exosomes had higher levels of oncogenic proteins, cytokines, and adhesion molecules compared with exosomes from the cells of origin. Importantly, whereas MM BM-MSC-derived exosomes promoted MM tumor growth, normal BM-MSC exosomes inhibited the growth of MM cells. In summary, these in vitro and in vivo studies demonstrated that exosome transfer from BM-MSCs to clonal plasma cells represents a previously undescribed and unique mechanism that highlights the contribution of BM-MSCs to MM disease progression.

Endothelium and Regulatory Inflammatory Mechanisms During Organ Rejection

Endothelial integrity is mandatory for physiologic organ function; however, endothelium dysfunction can be caused by systemic inflammation, occurring during sepsis or organ rejection after transplantation. This article will address our current understanding of endothelial involvement in organ transplantation and rejection. Overall, more detailed studies focusing on the endothelial modulation after organ transplantation would be necessary to investigate the role of endothelium activation during organ rejection.

Copresentation of intact and processed MHC alloantigen by recipient dendritic cells enables delivery of linked help to alloreactive CD8 T cells by indirect-pathway CD4 T cells

In transplantation, direct-pathway CD8 T cells that recognize alloantigen on donor cells require CD4 help for activation and cytolytic function. The ability of indirect-pathway CD4 T cells to provide this help remains unexplained, because a fundamental requirement for epitope linkage is seemingly broken. The simultaneous presentation, by host dendritic cells (DCs), of both intact MHC class I alloantigen and processed alloantigen would deliver linked help, but has not been demonstrated definitively. In this study, we report that following in vitro coculture with BALB/c DCs, small numbers (~1.5%) of C57BL/6 (B6) DCs presented acquired H-2(d) alloantigen both as processed allopeptide and as unprocessed Ag. This represented class I alloantigen provides a conformational epitope for direct-pathway allorecognition, because B6 DCs isolated from cocultures and transferred to naive B6 mice provoked cytotoxic CD8 T cell alloimmunity. Crucially, this response was dependent upon simultaneous presentation of class II-restricted allopeptide, because despite acquiring similar amounts of H-2(d) alloantigen upon coculture, MHC class II-deficient B6 DCs failed to elicit cytotoxic alloimmunity. The relevance of this pathway to solid-organ transplantation was then confirmed by the demonstration that CD8 T cell cytotoxicity was provoked in secondary recipients by transfer of DCs purified from wild-type, but not from MHC class II-deficient, C57BL/6 recipients of BALB/c heart transplants. These experiments demonstrate that representation of conformationally intact MHC alloantigen by recipient APC can induce cytotoxic alloimmunity, but simultaneous copresentation of processed allopeptide is essential, presumably because this facilitates linked recognition by indirect-pathway CD4 Th cells.

Signaling pathways in exosomes biogenesis, secretion and fate

Exosomes are small extracellular vesicles (30–100 nm) derived from the endosomal system, which have raised considerable interest in the last decade. Several studies have shown that they mediate cell-to-cell communication in a variety of biological processes. Thus, in addition to cell-to-cell direct interaction or secretion of active molecules, they are now considered another class of signal mediators. Exosomes can be secreted by several cell types and retrieved in many body fluids, such as blood, urine, saliva and cerebrospinal fluid. In addition to proteins and lipids, they also contain nucleic acids, namely mRNA and miRNA. These features have prompted extensive research to exploit them as a source of biomarkers for several pathologies, such as cancer and neurodegenerative disorders. In this context, exosomes also appear attractive as gene delivery vehicles. Furthermore, exosome immunomodulatory and regenerative properties are also encouraging their application for further therapeutic purposes. Nevertheless, several issues remain to be addressed: exosome biogenesis and secretion mechanisms have not been clearly understood, and physiological functions, as well as pathological roles, are far from being satisfactorily elucidated.

Exosomes as intercellular signaling organelles involved in health and disease: basic science and clinical applications

Cell to cell communication is essential for the coordination and proper organization of different cell types in multicellular systems. Cells exchange information through a multitude of mechanisms such as secreted growth factors and chemokines, small molecules (peptides, ions, bioactive lipids and nucleotides), cell-cell contact and the secretion of extracellular matrix components. Over the last few years, however, a considerable amount of experimental evidence has demonstrated the occurrence of a sophisticated method of cell communication based on the release of specialized membranous nano-sized vesicles termed exosomes. Exosome biogenesis involves the endosomal compartment, the multivesicular bodies (MVB), which contain internal vesicles packed with an extraordinary set of molecules including enzymes, cytokines, nucleic acids and different bioactive compounds. In response to stimuli, MVB fuse with the plasma membrane and vesicles are released in the extracellular space where they can interact with neighboring cells and directly induce a signaling pathway or affect the cellular phenotype through the transfer of new receptors or even genetic material. This review will focus on exosomes as intercellular signaling organelles involved in a number of physiological as well as pathological processes and their potential use in clinical diagnostics and therapeutics.

Anti-donor immune responses elicited by allogeneic mesenchymal stem cells: what have we learned so far?

Mesenchymal stem (stromal) cells (MSCs) have potent anti-inflammatory/immunosuppressive properties which underlie much of their therapeutic potential. This fact has led to the widely accepted belief that MSCs from genetically unrelated individuals (allogeneic (allo)-MSCs) can be used therapeutically with equal efficacy to autologous MSCs and without triggering the donor-specific immune responses that are typically associated with allo-transplants. In this article, we critically review available experimental data to determine whether good in vivo evidence exists in support of the 'immune privileged' status of allo-MSCs. We also examine published studies regarding the immunogenicity of allo-MSCs following activation ('licensing') by inflammatory stimuli or following differentiation. Among the identified studies which have addressed in vivo immunogenicity of allo-MSCs, there was substantial variability as regards experimental species, disease model, route of MSC administration, cell dose and stringency of the immunological assays employed. Nonetheless, the majority of these studies has documented specific cellular (T-cell) and humoral (B-cell/antibody) immune responses against donor antigens following administration of non-manipulated, interferon-γ-activated and differentiated allo-MSCs. The consequences of such anti-donor immune responses were also variable and ranged from reduced in vivo survival of allo-MSCs with accelerated rejection of subsequent allogeneic transplants to apparent promotion of donor-specific tolerance. On the basis of these findings and on existing knowledge of allo-antigen recognition from the field of transplant immunology, we propose that the concept of the immune privileged nature of allo-MSCs should be reconsidered and that the range and clinical implications of anti-donor immune responses elicited by allo-MSCs be more precisely studied in human and animal recipients.

Tolerance induction by exosomes from immature dendritic cells and rapamycin in a mouse cardiac allograft model

Background

Dendritic cells (DCs) release bioactive exosomes that play an important role in immune regulation. Because they express low levels of class I major histocompatibility complex (MHC) and co-stimulatory molecules, exosomes derived from donor immature DCs (imDex) prolong allograft survival by inhibiting T-cell activation. However, this effect is limited and does not induce immunological tolerance when imDex are administered alone. Thus, we tested the effect of combined treatment with donor imDex and low-dose rapamycin on inducing tolerance in a mouse cardiac transplantation model.

Methods

ImDex were obtained from the culture supernatant of immature DCs derived from donor mouse (C57BL/6) bone marrow and were injected with suboptimal doses of rapamycin into recipient mouse (BALB/c) before and after transplantation. The capacity of this treatment to induce immune tolerance was analyzed in vitro and in vivo using the mouse cardiac transplantation model.

Results

Donor imDex expressed moderate levels of MHC class II and low levels of MHC class I and co-stimulatory molecules, but neither imDex nor subtherapeutic rapamycin dose alone induced cardiac allograft tolerance. Combined treatment with imDex and rapamycin, however, led to donor specific cardiac allograft tolerance. This effect was accompanied by decreased anti-donor antigen cellular response and an increased percentage of spleen CD4⁺CD25⁺ T cells in recipients. Furthermore, this donor specific tolerance could be further transferred to naïve allograft recipients through injection of splenocytes, but not serum, from tolerant recipients.

Conclusion

Combined with immunosuppressive treatment, donor imDex can prolong cardiac allograft survival and induce donor specific allograft tolerance.

Nanoparticle analysis of circulating cell-derived vesicles in ovarian cancer patients

Cell-derived vesicles are recognized as essential components of intercellular communication, and many disease processes are associated with their aberrant composition and release. Circulating tumor-derived vesicles have major potential as biomarkers; however, the diagnostic use of exosomes is limited by the technology available for their objective characterization and measurement. In this study, we compare nanoparticle tracking analysis (NTA) with submicron particle analysis (SPA), dynamic light scattering (DLS), and electron microscopy (EM) to objectively define size distribution, number, and phenotype of circulating cell-derived vesicles from ovarian cancer patients. Using the NanoSight LM10 instrument, cell-derived vesicles were visualized by laser light scattering and analyzing Brownian motion of these vesicles captured by video. The NTA software calculates the size and total concentration of the vesicles in solution. Using vesicles isolated from ovarian cancer patients, we demonstrate that NTA can measure the size distributions of cell-derived vesicles comparable to other analysis instrumentation. Size determinations by NTA, SPA, and DLS were more objective and complete than that obtained with the commonly used EM approach. NTA can also define the total vesicle concentration. Furthermore, the use of fluorescent-labeled antibodies against specific markers with NTA allows the determination of the "phenotype" of the cell-derived vesicles.

Dendritic cell therapies in transplantation revisited: deletion of recipient DCs deters the effect of therapeutic DCs

A critical goal in transplantation is the achievement of donor-specific tolerance, minimizing the use of immunosuppressants. Dendritic cells (DCs) are antigen (Ag) presenting cells (APCs) with capability to promote immunity or tolerance. The immune-regulatory properties of DCs have been exploited for generation of tolerogenic/immunosuppressive (IS) DCs that, when transfer systemically, prolong allograft survival in murine models. Surprisingly, the in vivo mechanisms of therapies based on (donor- or recipient-derived) ISDCs in transplantation remain unknown, given that previous studies investigated their effects in vitro, or ex vivo after transplantation. Since once injected, ISDCs are short-lived and transfer Ag to recipient APCs, we assessed the role of recipient DCs by depleting them at the time of ISDC-therapy in a mouse model of cardiac transplantation. The results indicate that, contrary to the accepted paradigm, systemically administered ISDCs reduce the alloresponse and prolong allograft survival, not by themselves, but through conventional DCs (cDCs) of the recipient. These findings raise doubts on the advantages of the currently used ISDC-therapies, since the immune-regulatory properties of the injected ISDC do not seem to be functionally relevant in vivo, and the quiescent/pro-tolerogenic status of cDCs may be compromised in patients with end-stage diseases that require transplantation.

The molecular legacy of apoptosis in transplantation

Transplanted organs have to cope with diverse immunologic and metabolic stressors that augment the percentage of stressed and dying cells. Cell death, whether apoptotic or necrotic, is crucial in various transplantation-associated conditions. Necrosis, a proinflammatory type of cell death classically considered as accidental, is increasingly recognized as a highly controlled death program. Apoptosis, the classical programmed cell death mode program, is tightly orchestrated and culminates in the activation of caspases. Apoptosis was classically regarded as a silent form of cell death, but mounting evidence indicates that apoptotic cells "don't go silently" and leave a heritage to the local microenvironment. This apoptotic legacy, embedded within the effector phase of apoptosis, is aimed, at least in part, at controlling leukocyte trafficking and fostering tissue remodeling at sites of apoptotic cell deletion and can promote maladaptive remodeling pathways of importance for obliterative vascular remodeling. Moreover, apoptotic cells can transfer bioactive molecules by the release of apoptotic membrane vesicles that, in turn, shapes the phenotype and functions of immune cells. In this review, we summarize recent data highlighting the importance of apoptosis-associated intercellular communication networks in the regulation of allograft remodeling and immune responses in transplantation.

The ailing gut: a therapeutic role for bone marrow cells?

It is obvious that the BM does more than simply supply the GIT with cells of the innate and adaptive immune system. A growing number of studies suggest that BMCs can differentiate into ISEMFs (Lee et al., PLOS ONE 2011;6:e26082) and in the setting of inflammation can be contributors to all lineages of the neovasculature. The role of BMCs in epithelial turnover is more problematic; their contribution after transient mucosal injury seems negligible, but a number of studies in both rodents and man suggest that small numbers of BMCs can be incorporated into the epithelial compartment with more chronic injury (e.g., GvHD in man and chemically induced colitis in rodents); commonly cell fusion seems to be responsible for this. Significantly, this engraftment does not seem to occur in the stem-cell compartment, with the notable single report of the chronically infected murine gastric mucosa, where the BM origin of the stem cells can be the only rational explanation for the complete colonization of the mucosa by BMDCs. In the clinical setting, a role for MSCs in ameliorating colitis seems promising, though the mechanisms by which this is achieved remain somewhat unclear, though both immunomodulatory and regenerative effects of BMCs are likely to be important.

The expression profile of C19MC microRNAs in primary human trophoblast cells and exosomes

The largest gene cluster of human microRNAs (miRNAs), the chromosome 19 miRNA cluster (C19MC), is exclusively expressed in the placenta and in undifferentiated cells. The precise expression pattern and function of C19MC members are unknown. We sought to profile the relative expression of C19MC miRNAs in primary human trophoblast (PHT) cells and exosomes. Using high-throughput profiling, confirmed by PCR, we found that C19MC miRNAs are among the most abundant miRNAs in term human trophoblasts. Hypoxic stress selectively reduced miR-520c-3p expression at certain time-points with no effect on other C19MC miRNAs. Similarly, differentiation in vitro had a negligible effect on C19MC miRNAs. We found that C19MC miRNAs are the predominant miRNA species expressed in exosomes released from PHT, resembling the profile of trophoblastic cellular miRNA. Predictably, we detected the similar levels of circulating C19MC miRNAs in the serum of healthy pregnant women at term and in women with pregnancies complicated by fetal growth restriction. Our data define the relative expression levels of C19MC miRNAs in trophoblasts and exosomes, and suggest that C19MC miRNAs function in placental-maternal signaling.

Mechanism of transfer of functional microRNAs between mouse dendritic cells via exosomes

Dendritic cells (DCs) are the most potent APCs. Whereas immature DCs down-regulate T-cell responses to induce/maintain immunologic tolerance, mature DCs promote immunity. To amplify their functions, DCs communicate with neighboring DCs through soluble mediators, cell-to-cell contact, and vesicle exchange. Transfer of nanovesicles (< 100 nm) derived from the endocytic pathway (termed exosomes) represents a novel mechanism of DC-to-DC communication. The facts that exosomes contain exosome-shuttle miRNAs and DC functions can be regulated by exogenous miRNAs, suggest that DC-to-DC interactions could be mediated through exosome-shuttle miRNAs, a hypothesis that remains to be tested. Importantly, the mechanism of transfer of exosome-shuttle miRNAs from the exosome lumen to the cytosol of target cells is unknown. Here, we demonstrate that DCs release exosomes with different miRNAs depending on the maturation of the DCs. By visualizing spontaneous transfer of exosomes between DCs, we demonstrate that exosomes fused with the target DCs, the latter followed by release of the exosome content into the DC cytosol. Importantly, exosome-shuttle miRNAs are functional, because they repress target mRNAs of acceptor DCs. Our findings unveil a mechanism of transfer of exosome-shuttle miRNAs between DCs and its role as a means of communication and posttranscriptional regulation between DCs.

Bone marrow cells in murine colitis: multi-signal analysis confirms pericryptal myofibroblast engraftment without epithelial involvement

Background

The contribution of bone marrow-derived cells to epithelial tissues in the inflamed gut remains controversial. Recent reports have suggested that cell fusion between bone marrow-derived cells and the intestinal epithelium takes place in inflammatory conditions.

Methods

In attempts to confirm this, we have undertaken gender mis-matched bone marrow (BM) transplants from male Swiss Webster (SWR) mice to B and T cell-deficient female Rag2 KO mice which, 4 weeks later, were given 5% dextran sodium sulphate in drinking water to induce acute colitis. A further BM-treated group of animals with a graft versus host-like condition was also studied. We developed a new method to combine up to three brightfield or fluorescent lectin- or immuno-histochemical signals with fluorescent in situ hybridisation for the Y and X chromosomes to enable us unequivocally to identify BM-derived male cells which presented as different cell types in the gastrointestinal tract.

Principal Findings

In rolled preparations of whole intestines we scanned around 1.5 million crypts at many tissue levels. In no instance did we see a Y chromosome-positive cell in the epithelial compartment, which was not also CD45-positive. We saw no evidence of cell fusion, based on combined X and Y chromosome analysis. Levels of CD45-positive stromal and lymphoid cells and pericryptal myfibroblasts (positive for α-smooth muscle actin) increased with time up to a plateau, which resembled the level seen in untreated control grafted animals. We saw very few Y chromosome-positive endothelial cells in intestinal stromal vessels.

Conclusions

We conclude that whole BM transplantation does not result in intestinal epithelial engraftment in this model. Our new methods can usefully assist in multi-signal analyses of cell phenotypes following BM transplant and in models of chimaerism and regenerative medicine.

Exosomes/microvesicles: mediators of cancer-associated immunosuppressive microenvironments

Cancer cells, both in vivo and in vitro, have been demonstrated to release membranous structures, defined as microvesicles or exosomes, consisting of an array of macromolecules derived from the originating cells, including proteins, lipids, and nucleic acids. While only recently have the roles of these vesicular components in intercellular communication become elucidated, significant evidence has demonstrated that tumor exosomes can exert a broad array of detrimental effects on the immune system-ranging from apoptosis of activated cytotoxic T cells to impairment of monocyte differentiation into dendritic cells, to induction of myeloid-suppressive cells and T regulatory cells. Immunosuppressive exosomes of tumor origin can be found within neoplastic lesions and in biologic fluids from cancer patients, implying a potential role of these pathways in in vivo tumor progression and systemic paraneoplastic syndromes. Through the expression of molecules involved in angiogenesis promotion, stromal remodeling, signaling pathway activation through growth factor/receptor transfer, chemoresistance, and genetic intercellular exchange, tumor exosomes could represent a central mediator of the tumor microenvironment. By understanding the nature of these tumor-derived exosomes/microvesicles and their roles in mediating cancer progression and modulating the host immune response will significantly impact therapeutic approaches targeting exosomes.

Toward managing chronic rejection after lung transplant: the fate and effects of inhaled cyclosporine in a complex environment

The fate and effects of inhaled cyclosporine A (CsA) are considered after deposition on the lung surface. Special emphasis is given to a post-lung transplant environment and to the potential effects of the drug on the various cell types it is expected to encounter. The known stability, metabolism, pharmacokinetics and pharmacodynamics of the drug have been reviewed and discussed in the context of the lung microenvironment. Arguments support the contention that the immuno-inhibitory and anti-inflammatory effects of CsA are not restricted to T-cells. It is likely that pharmacologically effective concentrations of CsA can be sustained in the lungs but due to the complexity of uptake and action, the elucidation of effective posology must ultimately rely on clinical evidence.

Exosomes: immune properties and potential clinical implementations

To communicate, cells are known to release in their environment proteins which bind to receptors on surrounding cells. But cells also secrete more complex structures, called membrane vesicles, composed of a lipid bilayer with inserted transmembrane proteins, enclosing an internal content of hydrophilic components. Exosomes represent a specific subclass of such secreted membrane vesicles, which, despite having been described more than 20 years ago by two groups studying reticulocyte maturation, have only recently received attention from the scientific community. This renewed interest originated first from the description of exosome secretion by antigen-presenting cells, suggesting a potential role in immune responses, and very recently by the identification of the presence of RNA (both messenger and microRNA) in exosomes, suggesting a potential transfer of genetic information between cells. In this review, we will describe the conclusions of 20 years of studies on the immune properties of exosomes and the most recent advances on their roles and potential uses as markers or as therapeutic tools during pathologies, especially in cancer.

Exosomes/microvesicles as a mechanism of cell-to-cell communication

Microvesicles (MVs) are circular fragments of membrane released from the endosomal compartment as exosomes or shed from the surface membranes of most cell types. An increasing body of evidence indicates that they play a pivotal role in cell-to-cell communication. Indeed, they may directly stimulate target cells by receptor-mediated interactions or may transfer from the cell of origin to various bioactive molecules including membrane receptors, proteins, mRNAs, microRNAs, and organelles. In this review we discuss the pleiotropic biologic effects of MVs that are relevant for communication among cells in physiological and pathological conditions. In particular, we discuss their potential involvement in inflammation, renal disease, and tumor progression, and the evidence supporting a bidirectional exchange of genetic information between stem and injured cells. The transfer of gene products from injured cells may explain stem cell functional and phenotypic changes without the need of transdifferentiation into tissue cells. On the other hand, transfer of gene products from stem cells may reprogram injured cells to repair damaged tissues.

Endogenous dendritic cells mediate the effects of intravenously injected therapeutic immunosuppressive dendritic cells in transplantation

The prevailing idea regarding the mechanism(s) by which therapeutic immunosuppressive dendritic cells (DCs) restrain alloimmunity is based on the concept that they interact directly with antidonor T cells, inducing anergy, deletion, and/or regulation. However, this idea has not been tested in vivo. Using prototypic in vitro-generated maturation-resistant (MR) DCs, we demonstrate that once MR-DCs carrying donor antigen (Ag) are administered intravenously, they decrease the direct and indirect pathway T-cell responses and prolong heart allograft survival but fail to directly regulate T cells in vivo. Rather, injected MR-DCs are short-lived and reprocessed by recipient DCs for presentation to indirect pathway CD4(+) T cells, resulting in abortive activation and deletion without detrimental effect on the number of indirect CD4(+) FoxP3(+) T cells, thus increasing the regulatory to effector T cell relative percentage. The effect on the antidonor response was independent of the method used to generate therapeutic DCs or their viability; and in accordance with the idea that recipient Ag-presenting cells mediate the effects of therapeutic DCs in transplantation, prolongation of allograft survival was achieved using donor apoptotic MR-DCs or those lacking surface major histocompatibility complex molecules. We therefore conclude that therapeutic DCs function as Ag-transporting cells rather than Ag-presenting cells to prolong allograft survival.

Exosome secretion ameliorates lysosomal storage of cholesterol in Niemann-Pick type C disease

Niemann-Pick type C1 disease is an autosomal-recessive lysosomal storage disorder. Loss of function of the npc1 gene leads to abnormal accumulation of free cholesterol and sphingolipids within the late endosomal and lysosomal compartments resulting in progressive neurodegeneration and dysmyelination. Here, we show that oligodendroglial cells secrete cholesterol by exosomes when challenged with cholesterol or U18666A, which induces late endosomal cholesterol accumulation. Up-regulation of exosomal cholesterol release was also observed after siRNA-mediated knockdown of NPC1 and in fibroblasts derived from NPC1 patients and could be reversed by expression of wild-type NPC1. We provide evidence that exosomal cholesterol secretion depends on the presence of flotillin. Our findings indicate that exosomal release of cholesterol may serve as a cellular mechanism to partially bypass the traffic block that results in the toxic lysosomal cholesterol accumulation in Niemann-Pick type C1 disease. Furthermore, we suggest that secretion of cholesterol by exosomes contributes to maintain cellular cholesterol homeostasis.

Intercellular exchange of surface molecules and its physiological relevance

For many decades, cellular immunologists have relied on the expression of various cell surface molecules to divide cells into different types and subtypes to study their function. However, in recent years, a large and fast-expanding body of work has described the transfer of surface molecules, including MHC class I and II molecules, between cells, both in vitro and in vivo. The function of this process is still largely unknown, but it is likely to have a significant role in the control of the immune system. It is also likely that this process takes place in a regulated rather than stochastic manner, thus providing another way for the immune system to orchestrate its function. In this review we will summarize the key findings so far, examining the mechanisms of transfer, the consequences of this transfer as shown by in vitro experiments, and possible consequences for the wider immune response.

Advances in the design and delivery of peptide subunit vaccines with a focus on toll-like receptor agonists

Considerable success has been made with many peptide antigen formulations, and peptide-based vaccines are emerging as the next generation of prophylactic and remedial immunotherapy. However, finding an optimal platform balancing all of the requirements for an effective, specific and safe immune response remains a major challenge for many infectious and chronic diseases. This review outlines how peptide immunogenicity is influenced by the way in which peptides are presented to the immune system, underscoring the need for multifunctional delivery systems that couple antigen and adjuvant into a single construct. Particular attention is given to the ability of Toll-like receptor agonists to act as adjuvants. A survey of recent approaches to developing peptide antigen delivery systems is given, many of which incorporate Toll-like receptor agonists into the design.

Functional delivery of viral miRNAs via exosomes

Noncoding regulatory microRNAs (miRNAs) of cellular and viral origin control gene expression by repressing the translation of mRNAs into protein. Interestingly, miRNAs are secreted actively through small vesicles called "exosomes" that protect them from degradation by RNases, suggesting that these miRNAs may function outside the cell in which they were produced. Here we demonstrate that miRNAs secreted by EBV-infected cells are transferred to and act in uninfected recipient cells. Using a quantitative RT-PCR approach, we demonstrate that mature EBV-encoded miRNAs are secreted by EBV-infected B cells through exosomes. These EBV-miRNAs are functional because internalization of exosomes by MoDC results in a dose-dependent, miRNA-mediated repression of confirmed EBV target genes, including CXCL11/ITAC, an immunoregulatory gene down-regulated in primary EBV-associated lymphomas. We demonstrate that throughout coculture of EBV-infected B cells EBV-miRNAs accumulate in noninfected neighboring MoDC and show that this accumulation is mediated by transfer of exosomes. Thus, the exogenous EBV-miRNAs transferred through exosomes are delivered to subcellular sites of gene repression in recipient cells. Finally, we show in peripheral blood mononuclear cells from patients with increased EBV load that, although EBV DNA is restricted to the circulating B-cell population, EBV BART miRNAs are present in both B-cell and non-B-cell fractions, suggestive of miRNA transfer. Taken together our findings are consistent with miRNA-mediated gene silencing as a potential mechanism of intercellular communication between cells of the immune system that may be exploited by the persistent human gamma-herpesvirus EBV.

Antihuman leukocyte antigen antibody-induced autoimmunity: role in chronic rejection

PURPOSE OF REVIEW

We provide evidence for the role of de-novo development of immune responses to self-antigens in the posttransplant period and its possible induction by alloimmunity in the pathogenesis of chronic rejection following lung, heart and kidney transplantation. The present review details recent findings for the two distinct yet interdependent immune processes in the immunopathogenesis of chronic rejection.

RECENT FINDINGS

The contribution of both humoral and cell-mediated alloimmune responses against mismatched donor histocompatibility antigens (HLA) in the pathogenesis of chronic rejection is well established. Recent studies have focused on development of immune responses to self-antigens during the posttransplant period and its correlation with chronic rejection. These self-antigens include myosin and vimentin in cardiac, K-alpha-1-tubulin and collagen-V in lung and angiotensin II type 1 receptor, collagen-IV and VI in kidney transplants. During the posttransplant period, the development of immune responses to self-antigens is facilitated by induction of a distinct subset of autoreactive T-helper cells referred to as Th17 cells.

SUMMARY

Following organ transplantation, tissue injury and remodeling inflicted by antibodies (Abs) to HLA antigens is conducive to develop autoimmunity. Abs to HLA and self-antigens are detectable in the serum of transplant recipients who develop chronic rejection. Anti-HLA Abs are often present transiently but precede the development of Abs to self-antigens.

NK cell patrolling and elimination of donor-derived dendritic cells favor indirect alloreactivity

Direct presentation of foreign MHC molecules expressed by donor-derived dendritic cells (DCs) has generally been considered the dominant pathway of allorecognition in acute transplant rejection. However, recent studies implicate preferential activation of the indirect pathway by host DCs. The respective importance of each pathway and the mechanisms that determine their relative contributions remain to be clearly established. In this study, using two-photon microscopy, we visualized host NK cell interactions with syngeneic and allogeneic DCs within intact lymph nodes of mice. Upon contact with allogeneic DCs, NK cells formed prolonged interactions that led directly to target cell lysis. This rapid elimination limited the ability of allogeneic DCs to stimulate primary and recall T cell responses. To discriminate whether donor or host DCs are principally involved in presenting Ag derived from allografts, we used CD11c-diphtheria toxoid receptor mice to conditionally ablate CD11c(+) DCs and to show that direct presentation by donor DCs is alone insufficient to elicit acute allograft rejection. We thus propose that rapid elimination of allogeneic DCs limits direct Ag presentation and thereby favors the indirect pathway of alloreactivity.

Immunogenicity of allo-vesicle carrying ERBB2 tumor antigen for dendritic cell-based anti-tumor immunotherapy

Dendritic cells (DCs) are able to orchestrate innate and acquired immunity and can activate and sustain a long-lasting anti-tumor immune response in vivo when used as anti-tumor cell therapy. The selection of the antigen and the choice of its formulation are key points in designing anti-cancer DC-based vaccines. Cell released vesicles/exosomes have been shown to transfer antigens, HLAI/peptide complexes and co-stimulatory molecules to recipient cells. In this study we describe the generation of an allogenic microvesicle cell factory in which the expression of a specific tumor antigen was combined to the expression of co-stimulatory and allogeneic molecules. The DG75 lymphoblastoid cell line was selected as microvesicle producer and transfected with ErbB2, as tumor antigen prototype. The shed microvesicles transferred antigenic components to recipient DCs, increasing their immunogenicity. DC pulsing resulted in cross-presentation of ErbB2 both in HLAI and HLAII compartments, and ErbB2-specific CD8+ T cells from cancer patients were activated by DCs pulsed with vesicle-bound ErbB2. The microvesicle cell factory proposed may represent a source of cell free immunogen to be used for DC-based cancer therapy.

Role of secondary lymphoid tissues in primary and memory T-cell responses to a transplanted organ

Secondary lymphoid tissues are the hub of adaptive immune responses wherein rare cognate lymphocytes encounter dendritic cells bearing antigen from peripheral tissues and differentiate into effector and memory cells that eliminate antigen. It is accepted that immune responses against microbial and tumor antigens are initiated within secondary lymphoid tissues. There is less agreement on whether the same principle applies to immune responses to a transplanted organ because an allograft expresses foreign major histocompatibility complex and contains donor antigen presenting cells that could activate T cells directly in situ leading to rejection. Recent studies confirm that although naïve T cells can be primed within the allograft, their differentiation to effect rejection is dependent on secondary lymphoid tissues. Antigen-experienced memory T cells, unlike Naïve T cells, function largely independent of secondary lymphoid tissues to cause allograft rejection. In an alloimmune response, secondary lymphoid tissues support not only immune activation but also immune regulation essential for allograft survival. Here, we will review recent findings and discuss the role of secondary lymphoid tissues in primary and memory alloimmune responses.

Membrane vesicles as conveyors of immune responses

In multicellular organisms, communication between cells mainly involves the secretion of proteins that then bind to receptors on neighbouring cells. But another mode of intercellular communication - the release of membrane vesicles - has recently become the subject of increasing interest. Membrane vesicles are complex structures composed of a lipid bilayer that contains transmembrane proteins and encloses soluble hydrophilic components derived from the cytosol of the donor cell. These vesicles have been shown to affect the physiology of neighbouring recipient cells in various ways, from inducing intracellular signalling following binding to receptors to conferring new properties after the acquisition of new receptors, enzymes or even genetic material from the vesicles. This Review focuses on the role of membrane vesicles, in particular exosomes, in the communication between immune cells, and between tumour and immune cells.

P2X7 receptor-stimulated secretion of MHC class II-containing exosomes requires the ASC/NLRP3 inflammasome but is independent of caspase-1

We recently reported that P2X7 receptor (P2X7R)-induced activation of caspase-1 inflammasomes is accompanied by release of MHC class II (MHC-II) protein into extracellular compartments during brief stimulation of murine macrophages with ATP. Here we demonstrate that MHC-II containing membranes released from macrophages or dendritic cells (DCs) in response to P2X7R stimulation comprise two pools of vesicles with distinct biogenesis: one pool comprises 100- to 600-nm microvesicles derived from direct budding of the plasma membrane, while the second pool is composed of 50- to 80-nm exosomes released from multivesicular bodies. ATP-stimulated release of MHC-II in these membrane fractions is observed within 15 min and results in the export of approximately 15% of the total MHC-II pool within 90 min. ATP did not stimulate MHC-II release in macrophages from P2X7R knockout mice. The inflammasome regulatory proteins, ASC (apoptosis-associated speck-like protein containing a caspase-recruitment domain) and NLRP3 (NLR family, pyrin domain containing 3), which are essential for caspase-1 activation, were also required for the P2X7R-regulated release of the exosome but not the microvesicle MHC-II pool. Treatment of bone marrow-derived macrophages with YVAD-cmk, a peptide inhibitor of caspase-1, also abrogated P2X7R-dependent MHC-II secretion. Surprisingly, however, MHC-II release in response to ATP was intact in caspase-1(-/-) macrophages. The inhibitory actions of YVAD-cmk were mimicked by the pan-caspase inhibitor zVAD-fmk and the serine protease inhibitor TPCK, but not the caspase-3 inhibitor DEVD-cho. These data suggest that the ASC/NLRP3 inflammasome complexes assembled in response to P2X7R activation involve protease effector(s) in addition to caspase-1, and that these proteases may play important roles in regulating the membrane trafficking pathways that control biogenesis and release of MHC-II-containing exosomes.

Preferential priming of alloreactive T cells with indirect reactivity

The relative contributions of the direct and indirect pathways in alloimmune responses have not been fully elucidated. We report a novel murine TCR transgenic system that can simultaneously track the CD4-direct (CD4-d), CD4-indirect (CD4-i) and CD8-direct (CD8-d) pathways after transplantation. Using this system, we have observed a profoundly greater proliferation of CD4-i T cells relative to CD4-d and CD8-d T cells after transplantation. Furthermore, a much larger proportion of CD4-i T cells attain an effector phenotype. We also analyzed endogenous, wild-type T cells using enzyme-linked immunospot analysis. In naïve mice, T cells with indirect reactivity were undetectable, but T cells with direct reactivity were abundant. However, 10 days after skin or heterotopic heart transplantation, CD4-i T cells comprised approximately 10% of the CD4+ response. Consistent with increased priming of the CD4-i pathway, we observed that the CD4-i T cells were further enriched in the effector cells migrating to the allograft and in memory-like T cells persisting after rejection. Thus, priming of the CD4-i pathway is favored after transplantation, allowing a rare population to rapidly become a major component of the CD4+ T-cell response in acute allograft rejection. The generalizability of this observation to other models remains to be determined.

No significant CTL cross-priming by dendritic cell-derived exosomes during murine lymphocytic choriomeningitis virus infection

Exosomes are small membrane vesicles of endocytic origin that are secreted by most cells in culture, but are also present in serum. They contain a wide array of protein ligands on their surface, which has led to the hypothesis that they might mediate intercellular communication. Indeed, data support that exosomes can transfer Ags to dendritic cells (DC), and, interestingly, that these DC can subsequently induce T cell priming or tolerance. We have investigated whether this concept can be expanded to antiviral immunity. We isolated exosomes from supernatant of cultured bone marrow-derived DC (BMDC) that were infected with lymphocytic choriomeningitis virus (LCMV) or loaded with an immunodominant LCMV peptide, and characterized them by flow cytometry upon binding to beads. We then incubated the exosome preparations with BMDC and looked at their potential to activate LCMV gp33-specific naive and memory CD8 T cells. We found that exosomes do not significantly contribute to CD8 T cell cross-priming in vitro. Additionally, exosomes derived from in vitro-infected BMDC did not exhibit significant in vivo priming activity, as evidenced by the lack of protection following exosome vaccination. Thus, DC-derived exosomes do not appear to contribute significantly to CTL priming during acute LCMV infection.

Rapid deletional peripheral CD8 T cell tolerance induced by allogeneic bone marrow: role of donor class II MHC and B cells

Mixed chimerism and donor-specific tolerance are achieved in mice receiving 3 Gy of total body irradiation and anti-CD154 mAb followed by allogeneic bone marrow (BM) transplantation. In this model, recipient CD4 cells are critically important for CD8 tolerance. To evaluate the role of CD4 cells recognizing donor MHC class II directly, we used class II-deficient donor marrow and were not able to achieve chimerism unless recipient CD8 cells were depleted, indicating that directly alloreactive CD4 cells were necessary for CD8 tolerance. To identify the MHC class II(+) donor cells promoting this tolerance, we used donor BM lacking certain cell populations or used positively selected cell populations. Neither donor CD11c(+) dendritic cells, B cells, T cells, nor donor-derived IL-10 were critical for chimerism induction. Purified donor B cells induced early chimerism and donor-specific cell-mediated lympholysis tolerance in both strain combinations tested. In contrast, positively selected CD11b(+) monocytes/myeloid cells did not induce early chimerism in either strain combination. Donor cell preparations containing B cells were able to induce early deletion of donor-reactive TCR-transgenic 2C CD8 T cells, whereas those devoid of B cells had reduced activity. Thus, induction of stable mixed chimerism depends on the expression of MHC class II on the donor marrow, but no requisite donor cell lineage was identified. Donor BM-derived B cells induced early chimerism, donor-specific cell-mediated lympholysis tolerance, and deletion of donor-reactive CD8 T cells, whereas CD11b(+) cells did not. Thus, BM-derived B cells are potent tolerogenic APCs for alloreactive CD8 cells.

The importance of the indirect pathway of allorecognition in clinical transplantation

The immune system mounts a response to non-self transplanted tissue through a number of mechanisms. The indirect pathway of allorecognition, in which cells of the adaptive immune system recognize MHC alloantigen-derived peptide on self-MHC molecules, has emerged as a potent inducer of allograft rejection. In particular, recent evidence convincingly connects the indirect pathway with chronic rejection, including antibody-mediated and CD8(+) T cell-mediated rejection. However, the indirect pathway can also promote the generation of regulatory T cells, which have emerged as crucial suppressors of the alloresponse, and hold much promise in the quest for clinical tolerance. An improved understanding of the indirect pathway is likely to bring important benefits to transplant recipients.