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REVIEW ARTICLE
Year : 2016  |  Volume : 29  |  Issue : 2  |  Page : 187-191

Exosomes: clinical implications and therapeutic potentials


Department of Internal Medicine, Faculty of Medicine, Menoufia University, Menoufia, Egypt

Date of Submission15-Jan-2015
Date of Acceptance12-Mar-2015
Date of Web Publication18-Oct-2016

Correspondence Address:
Mohammed Al-Houseiny Ata
Desouk, Kafr Alsheikh, 33611
Egypt
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/1110-2098.192402

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  Abstract 

The aim of this review was to assess the role of exosome microvesicles in medicine. Exosomes are spherical nanosized extracellular membrane vesicles that act as regulators of cell-to-cell communication and as immune modulators with immunosuppressive or immune-stimulating properties by delivering proteins or nucleic acid to recipient cells. Data were obtained from medical text books, medical journals, and all materials available on the internet from 2006 to 2014. We selected systematic reviews that addressed exosomes as well as studies that addressed the clinical implications and therapeutic potential of exosomes. A special search was conducted in Medline with the keywords in the title of papers; suitable studies were extracted, after assessing the quality and validity of the papers. Each study was reviewed independently. The obtained data were translated into the language understood by the researcher and have been arranged in topics throughout the article. Exosomes are believed to possess a powerful capacity to regulate cell survival/death, inflammation, and tumor metastasis, depending on their molecular content. The clinical application of exosomes as a drug delivery system for the treatment of autoimmune/inflammatory diseases and cancer is promising. Exosomes provide a novel minimally invasive approach to diagnosis, prognosis, and therapy.

Keywords: cell-to-cell communication, exosomes, microvesicles, tumor-derived exosomes


How to cite this article:
Galal AZ, Shoieb SA, Abdel Hafez MA, Ata MA. Exosomes: clinical implications and therapeutic potentials. Menoufia Med J 2016;29:187-91

How to cite this URL:
Galal AZ, Shoieb SA, Abdel Hafez MA, Ata MA. Exosomes: clinical implications and therapeutic potentials. Menoufia Med J [serial online] 2016 [cited 2019 Nov 13];29:187-91. Available from: http://www.mmj.eg.net/text.asp?2016/29/2/187/192402


  Introduction Top


Exosomes are small nanovesicles of endocytic origin, which are released into the extracellular environment by many different cell types, including mast cells, mesenchymal stem cells, platelets, dendritic cells (DCs), epithelial cells, tumor cells, and endothelial cells and have been shown to be secreted by a variety of cell types including B-lymphocyte and T-lymphocyte cells. They are distinguished in their genesis by being budded into endosomes to form multivesicular bodies (MVBs) in the cytoplasm. These MVBs are released into extracellular fluids by fusion of these MVBs with the cell surface, resulting in secretion in bursts. Exosomes are found in different body fluids such as blood plasma, urine, saliva, breast milk, bronchial lavage fluid, cerebral spinal fluid, amniotic fluid, and malignant ascites [1].

Exosomes contain RNA that can be shuttled between cells. Thus, one cell can produce exosomes that influence another cell, which has opened a new research field into cell-to-cell communication. Originally exosomes were believed to be involved in the removal of unwanted protein from cells. Current opinion suggests they play a variety of roles, including signaling in immune cells, and having virus-like properties that allow gene regulation in the recipient cell [2].


  Exosomes characterization and Molecular Structure Top


Exosomes are defined as small, spherical nanosized rigid lipid bilayer membrane vesicles of endocytic origin, ∼30–100 nm in diameter and with densities ranging between 1.10 and 1.21 g/ml, composed of cholesterol, sphingomyelin, and ceramide-rich lipid rafts, including proteins and RNA. A large diversity of proteins can be found in exosomes, of which many are common for all exosomes, regardless of their cellular origin. These conserved proteins, enriched in exosomes, include proteins involved in MVB biogenesis (e.g. Alix, clathrin, and tumor susceptibility gene 101, Tsg101), membrane transport and fusion proteins (e.g. annexins and Rabs), tetraspanins (e.g. CD9, CD63, CD81), cytoskeleton proteins (e.g. ezrin and actins), and heat shock proteins Hsp (e.g. Hsp60, Hsp70, and Hsp90). Exosomes contain not only proteins but also a large diversity of RNA, both microRNA (miRNA) and mRNA, with the exosomal mRNA content differing substantially from the mRNA profile of the donor cells. The miRNA content of exosomes has been shown to be similar to the originating cell [3].


  Isolation and visualization of Exosomes Top


Several methods have been described for purifying exosomes, but each presents unique challenges for clinical translation. However, there are still no isolation methods that can guarantee total purity of the yielded vesicles. Cell culture supernatants and biological fluids contain a mixture of extracellular vesicles, and thus isolation of these vesicles cannot distinguish their origin but can separate them according to properties such as size, density, morphology, and composition. It is therefore important to use high-quality isolation and identification to distinguish exosomes from other extracellular vesicles [4].

The most commonly used procedure for exosome purification is based on differential centrifugations, with or without a filtration step. However, when isolating exosomes based on their density or by immunoaffinity capture, only a subpopulation may be purified. Additional isolation methods may include using a nanomembrane ultrafiltration concentrator or a microfiltration approach [5].


  Exosomes and Maintenance of Normal Physiological Processes Top


Exosomes are emerging as important mediators of cell–cell communication to maintain the physiological function. Exosomes derived from a range of different immune cells can harbor immunologically relevant molecules such as major histocompatibility complex (MHC) class II, cluster of differentiation 86 (CD86), lymphocyte function-associated antigen 1, and intercellular adhesion molecule 1, which impact on a variety of immunological functions, including T-cell activation, tolerance induction, and DC maturation. Besides immune cells, exosomes from other cell sources such as breast tissues have also been found to modulate immune responses, including immune stimulation and tolerance induction [3].

Exosomes derived from neurones have been shown to transmit information in the form of proteins to facilitate neural circuit function. For example, cortical neurone-derived exosomes transfer newly synthesized proteins to the presynaptic terminals of connected neurones and contribute to the local synaptic plasticity. Further, increasing evidence of esRNA (endogenous short RNAs) transfer within the nervous system suggests that mRNAs and miRNAs are also transferred to regulate neuronal function [6].


  Exosomes and Pathological Disease Spread Top


Many studies have noted that the production of exosomes rises sharply in diseased versus nondiseased states. One mechanism of disease spreading is through the vesicle-mediated receptor transfer – for example, transfer of chemokine receptors to aid the spread of the HIV. Another mechanism could involve the transfer of miRNAs and oncogenic proteins through microvesicles to facilitate local or metastatic tumor spread [7].

Moreover, neighboring nontumor cells such as tumor-associated macrophages can secrete microvesicles with high levels of miR-223 (miRNA-223 identified in exosomes derived from B cells and DCs). miR-223 can bind to target sites, causing an increased accumulation of β-catenin in the nuclei of breast cancer cells to encourage local invasion. Further, microvesicles have been postulated to reshape the local tumor environment into a more favorable niche for tumor growth, invasion, and spread of metastasis [8].

Besides cancer, exosomes are also implicated in the local spread of neurodegenerative diseases, as increasing evidence indicates that exosomes released from neurons can be transferred to other brain cells locally and at a distance. Thus, exosomes containing toxic proteins can transport these and lead to pathogenic amyloid deposition in other parts of the brain [9].


  Exosomes for Genetic Information Transfer and Gene Therapy Top


Exosomes have been reported as containing not only proteins but also nucleic acids such as DNA, RNA, mRNA, and long noncoding RNAs such as small regulatory miRNAs as well as proteins that can be functionally delivered between different cell types and across species [0].

This natural ability of exosomes to transfer genetic information might instead facilitate the spread of disease through the delivery of genetic material and/or pathogenic proteins. It has been noted that greater numbers of exosomes can be isolated from diseased patients, some of which contain elevated levels of specific miRNAs, which may be involved in the cause and spread of diseases such as cancer [1].


  Functional Relevance of Exosomes in Cancer Top


Exosomes can have a bimodal role in cancer. They can manipulate the local and systemic environment to aid in cancer growth and dissemination. Exosomes may also program the immune system to elicit an antitumor response with a network of interactions as follows.

Exosome-mediated transfer of oncogenic proteins between cancer cells

Through exosome-mediated transfer, tumor cells exchange proteins with oncogenic activity. The functional purpose for this is still largely undetermined [2].

Exosome-mediated modulation of the tumor microenviroment and angiogenesis

Myofibroblasts are a key source of matrix-remodeling proteins within the tumor microenvironment and participate in tumor angiogenesis. In this regard, tumor exosome-induced recruitment of fibroblasts could support tumor angiogenesis. Furthermore, cancer cells transfer membrane-bound epidermal growth factor receptor (known as EGFRvIII) to endothelial cells through exosomes. This transfer activates the vascular endothelial growth factor (VEGF)/VEGFR-2 pathway in endothelial cells, which supports tumor angiogenesis [3].

Modulation of immune system by cancer-derived exosomes

Cancer cells recruit immune cells to enhance tumor invasion, tumor angiogenesis, and dissemination. Exosome-mediated communication between tumor cells and the immune system is involved in recruiting protumorigenicimmune cells. In a murine breast cancer model, 4T1 cancer cells release exosomes in a Rab27a-dependent manner. Blockade of exosome secretion by inhibiting Rab27a is associated with a decreased mobilization of neutrophils. Such impairment results in decreased primary tumor growth and lung metastasis [4].

Tumor-derived exosomes as a source of antigens associated with tumor rejection

Exosomes can transport antigens from tumor cells to antigen-presenting DCs. Through MHC class-I molecules, DCs' prime cytotoxic T lymphocytes evoke an antitumor response and suppress tumor growth in vivo [5].

Exosomes as cancer biomarkers

Exosomes are often reflective of disease state and can be easily detected in bodily fluids, even after extended cryostorage, making these small nanovesicles an ideal candidate for a noninvasive biomarker of tumor progression. Exosomes for cancer biomarkers (particularly miRNA) have demonstrated enough promise to be implemented clinically. Further investigation of the dynamic expressional profile of exosomal contents with improved collection methods is needed before the clinical implementation of exosomes as a biomarker for either diagnosis or prognosis [6].

Exosomes in cancer therapy

Exosomes represent a vehicle that is well tolerated, bioavailable, targetable to specific tissues, resistant to metabolic processes, and membrane permeable. The removal of exosomes from the circulatory system is an attractive therapeutic option in mitigating the metastatic effect of exosomes. Prevention of exosome production can inhibit tumorigenesis, and a range of methods have been suggested for the inhibition of exosome production, including the targeting of microtubule assembly and stability, endosomal sorting pathways, and the use of proton pump inhibitors [7].

The immunostimulatory potential of exosomes led to phase I clinical trials that have demonstrated that antitumor immune responses can be induced using DC-derived exosomes (dexosomes). This has been further extended using peptide-loaded dexosomes as a cancer vaccine and is now in phase II clinical trials [8].


  The Role of Exosomes in Infectious Diseases Top


Exosomes and viral infection

In viral infections, exosomes and other microvesicles contribute to two main processes: the communication between cells and the modulation of immune responses in the host. Although infected cells secrete specific exosomes that include functional proteins, mRNA/miRNA, and infectious agent particles, in each viral infection, the effects of specific exosomes on the host immune system differ according to the type of virus and its life cycle stage. To overcome the host antiviral immunity, two main strategies are required, which are controversial:

  1. Incomplete elimination of viral infection through the production of exosomes enriched with proteins and glycoproteins, and
  2. Suppression of antigen presenting on the surface of antigen presenting cells. Exosomes derived from cells infected by virus give rise to systematic distribution of viral agents that encounter the host immune cells.


These released exosomes can also participate in priming antiviral immunity through anergy or antigen tolerance [9].

HIV and epstein-barr virus use another mechanism based on an exosome named 'a Trojan exosome'. In this mechanism, HIV-1 internalizes within the DC, recruits its intracellular vesicle trafficking pathways, packages antigens and particles of virus in exosome-like vesicles, and subsequently releases them into the extracellular space. Investigation of each type of exosome may be useful for designing an efficient vaccine to treat infections, especially for dangerous viruses such as HIV and HCV [0].

Exosomes derived from infected cells by bacteria, fungus, and parasites

The exosomes originate from the endosomal compartment where the phagosome-enriched foreign proteins enter the endosomic network and become involved in exosomal contents. These phagosomes have the intracellular pathogen-associated molecular patterns. The influence of this type of exosome on immune response – for example, to Mycobacterium tuberculosis infection – is the induction of IFN-γ secretion and macrophage activation. Many infected macrophages are resistant to it; therefore, they are unable to kill M. tuberculosis. Two paradoxical functions are claimed for this exosome:

  1. Induction of proinflammatory secretions such as TNF-α macrophages and triggering of naive T-cell activation, and
  2. An increased possibility of M. tuberculosis survival by adjusting the macrophage functions [1].



  Exosomes in Therapeutic Medicine Top


Phase I clinical trials in human cancer evaluated the effectiveness of patient specific exosomes released by DCs and loaded with tumor antigen derived peptides (dexosomes) for melanoma and non-small-cell lung cancer and showed that dexosome immunotherapy was feasible, safe, and led to the induction of both innate and adaptive immune responses, disease stabilization, and long term survival for several patients [2]. Also, ascites derived exosomes derived from colorectal cancer patients were shown to be safe, nontoxic, and tolerable when used as a cancer vaccine, and in combination with GM CSF can efficiently induce potent carcinoembryonic antigen-specific antitumor immunity in advanced colorectal cancer patients [3].

Exosomes are favorable as vaccine candidates in infections such as toxoplasmosis, diphtheria, tuberculosis, and atypical severe acute respiratory syndrome [4]. It has been shown that murine bone marrow-derived DCs pulsed in vitro with intact diphtheria toxin released exosomes, which upon injection into mice induce immunoglobulin G (IgG)2b and IgG2a responses specific for diphtheria toxin [5]. Studies demonstrated that exosomes containing spike S protein SARS associated coronavirus (SARS CoV) induced neutralizing antibody titers, which enhanced by priming with the SARS S exosomal vaccine and then boosting with the currently used adenoviral vector vaccine [6].

The role of exosomes in healing of wounds and reconstruction of hypoxic injury while hampering neovascularization delays tumor development. Endothelial derived exosomes carrying proteins such as delta like and matrix metalloproteinases help in angiogenesis [7].


  Discussion Top


Exosomes play a variety of roles, including signaling in immune cells and having virus-like properties that allow gene regulation in the recipient cell [2]. Exosomes derived from a range of different immune cells can harbor MHC, which impacts on a variety of immunological functions, including T-cell activation, tolerance induction, and DC maturation, which provide critical regulatory signals for appropriate cell maturation [3]. The natural ability of exosomes to transfer genetic information might facilitate the spread of disease through the delivery of genetic material and/or pathogenic proteins [2].

The use of exosomes as a vehicle for the administration of antitumor compounds, either as cell-derived material or as therapeutic drug, makes exosomes ideal candidates for delivery of drugs, proteins, miRNA/siRNA, and other molecules. The removal of exosomes from the circulatory system is an attractive therapeutic option in mitigating the metastatic effect of exosomes [8]. The immunostimulatory potential of exosomes induce antitumor immune responses. These promising results led to phase I clinical trials that have demonstrated that antitumor immune responses can be induced using dexosomes as a cancer vaccine, and is now in phase II clinical trials [9].


  Conclusion Top


Exosome secretion in the normal physiological state and during cancer development and progression, as well as the specific content of exosomes, will increase our understanding of their role in intercellular communication and tumorigenesis. Investigation into exosome biology has been a relatively new area of research, and much work remains to be done to ensure the safe and effective use of exosomes for therapeutic applications as exosomes appear to be noncytotoxic and well tolerated. The role of exosomes as a next-generation drug delivery system appears to be advantageous over existing drug delivery systems because of their small size, lack of toxicity, and target specificity, although loading of exosomes without compromising their biological properties remains a challenge.

Conflicts of interest

There are no conflicts of interest.[27]

 
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  In this article
Abstract
Introduction
Exosomes charact...
Isolation and vi...
Exosomes and Mai...
Exosomes and Pat...
Exosomes for Gen...
Functional Relev...
The Role of Exos...
Exosomes in Ther...
Discussion
Conclusion
References

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