T cell antigenicity and immunogenicity of allogeneic exosomes

Intercellular transfer of MHC molecules in T cell alloimmunity and allotransplantation

After transplantation of allogeneic tissues and organs, recognition by recipient T cells of donor MHC molecules initiates the pro-inflammatory adaptive immune response leading to allograft rejection. T cell allorecognition has long been known to be mediated via two distinct pathways: the direct pathway in which T cells recognize intact allogeneic MHC molecules displayed on donor cells and the indirect pathway whereby T cells recognize donor MHC peptides processed and presented by recipient antigen-presenting cells (APCs). It is believed that direct allorecognition is the driving force behind early acute allograft rejection while indirect allorecognition is involved in chronic allograft rejection, a progressive condition characterized by graft vasculopathy and tissue fibrosis. Recently, we and others have reported that after transplantation of allogeneic skin and organs, donor MHC molecules are transferred from donor cells to the host's APCs via trogocytosis or extracellular vesicles. Recipient APCs having captured donor MHC molecules can either present them to T cells in the intact form on their surface (semi-direct pathway) or the form of peptides bound to self-MHC molecules (indirect pathway). The present article provides an overview of recent studies evaluating the role of intercellular exchange of MHC molecules in T cell alloimmunity and its contribution to allograft rejection and tolerance.

Rescuing Photoreceptors in RPE Dysfunction-Driven Retinal Degeneration: The Role of Small Extracellular Vesicles Secreted from Retinal Pigment Epithelium

Dysfunction of the retinal pigment epithelium (RPE) is a common shared pathology in major degenerative retinal diseases despite variations in the primary etiologies of each disease. Due to their demanding and indispensable functional roles throughout the lifetime, RPE cells are vulnerable to genetic predisposition, external stress, and aging processes. Building upon recent advancements in stem cell technology for differentiating healthy RPE cells and recognizing the significant roles of small extracellular vesicles (sEV) in cellular paracrine and autocrine actions, we investigated the hypothesis that the RPE-secreted sEV alone can restore essential RPE functions and rescue photoreceptors in RPE dysfunction-driven retinal degeneration. Our findings support the rationale for developing intravitreal treatment of sEV. We demonstrate that intravitreally delivered sEV effectively penetrate the full thickness of the retina. Xenogenic intraocular administration of human-derived EVs did not induce acute immune reactions in rodents. sEV derived from human embryonic stem cell (hESC)-derived fully differentiated RPE cells, but not sEV-depleted conditioned cell culture media (CCM minus sEV), rescued photoreceptors and their function in a Royal College of Surgeons (RCS) rat model. This model is characterized by photoreceptor death and retinal degeneration resulting from a mutation in the MerTK gene in RPE cells. From the bulk RNA sequencing study, we identified 447 differently expressed genes in the retina after hESC-RPE-sEV treatment compared with the untreated control. Furthermore, 394 out of 447 genes (88%) showed a reversal in expression toward the healthy state in Long-Evans (LE) rats after treatment compared to the diseased state. Particularly, detrimental alterations in gene expression in RCS rats, including essential RPE functions such as phototransduction, vitamin A metabolism, and lipid metabolism were partially reversed. Defective photoreceptor outer segment engulfment due to intrinsic MerTK mutation was partially ameliorated. These findings suggest that RPE-secreted sEV may play a functional role similar to that of RPE cells. Our study justifies further exploration to fully unlock future therapeutic interventions with sEV in a broad array of degenerative retinal diseases.

Unveiling the Complex World of Extracellular Vesicles: Novel Characterization Techniques and Manufacturing Considerations

Extracellular vesicles (EVs) function as potent mediators of intercellular communication for many in vivo processes, contributing to both health and disease related conditions. Given their biological origins and diverse functionality from correspondingly unique "cargo" compositions, both endogenous and modified EVs are garnering attention as promising therapeutic modalities and vehicles for targeted therapeutic delivery applications. Their diversity in composition, however, has revealed a significant need for more comprehensive analytical-based characterization methods, and manufacturing processes that are consistent and scalable. In this review, we explore the dynamic landscape of EV research and development efforts, ranging from novel isolation approaches, to their analytical assessment through novel characterization techniques, and to their production by industrial-scale manufacturing process considerations. Expanding the horizon of these topics to EVs for in-human applications, we underscore the need for stringent development and adherence to Good Manufacturing Practice (GMP) guidelines. Wherein, the intricate interplay of raw materials, production in bioreactors, and isolation practices, along with analytical assessments compliant with the Minimal Information for Studies of Extracellular Vesicles (MISEV) guidelines, in conjunction with reference standard materials, collectively pave the way for standardized and consistent GMP production processes.

Schwann Cell-Derived Exosomal Vesicles: A Promising Therapy for the Injured Spinal Cord

Exosomes are nanoscale-sized membrane vesicles released by cells into their extracellular milieu. Within these nanovesicles reside a multitude of bioactive molecules, which orchestrate essential biological processes, including cell differentiation, proliferation, and survival, in the recipient cells. These bioactive properties of exosomes render them a promising choice for therapeutic use in the realm of tissue regeneration and repair. Exosomes possess notable positive attributes, including a high bioavailability, inherent safety, and stability, as well as the capacity to be functionalized so that drugs or biological agents can be encapsulated within them or to have their surface modified with ligands and receptors to imbue them with selective cell or tissue targeting. Remarkably, their small size and capacity for receptor-mediated transcytosis enable exosomes to cross the blood-brain barrier (BBB) and access the central nervous system (CNS). Unlike cell-based therapies, exosomes present fewer ethical constraints in their collection and direct use as a therapeutic approach in the human body. These advantageous qualities underscore the vast potential of exosomes as a treatment option for neurological injuries and diseases, setting them apart from other cell-based biological agents. Considering the therapeutic potential of exosomes, the current review seeks to specifically examine an area of investigation that encompasses the development of Schwann cell (SC)-derived exosomal vesicles (SCEVs) as an approach to spinal cord injury (SCI) protection and repair. SCs, the myelinating glia of the peripheral nervous system, have a long history of demonstrated benefit in repair of the injured spinal cord and peripheral nerves when transplanted, including their recent advancement to clinical investigations for feasibility and safety in humans. This review delves into the potential of utilizing SCEVs as a therapy for SCI, explores promising engineering strategies to customize SCEVs for specific actions, and examines how SCEVs may offer unique clinical advantages over SC transplantation for repair of the injured spinal cord.

Exosomal Cargo: Pro-angiogeneic, anti-inflammatory, and regenerative effects in ischemic and non-ischemic heart diseases - A comprehensive review

Heart diseases are the primary cause of mortality and morbidity worldwide which inflict a heavy social and economic burden. Among heart diseases, most deaths are due to myocardial infarction (MI) or heart attack, which occurs when a decrement in blood flow to the heart causes injury to cardiac tissue. Despite several available diagnostic, therapeutic, and prognostic approaches, heart disease remains a significant concern. Exosomes are a kind of small extracellular vesicles released by different types of cells that play a part in intercellular communication by transferring bioactive molecules important in regenerative medicine. Many studies have reported the diagnostic, therapeutic, and prognostic role of exosomes in various heart diseases. Herein, we reviewed the roles of exosomes as new emerging agents in various types of heart diseases, including ischemic heart disease, cardiomyopathy, arrhythmia, and valvular disease, focusing on pathogenesis, therapeutic, diagnostic, and prognostic roles in different areas. We have also mentioned different routes of exosome delivery to target tissues, the effects of preconditioning and modification on exosome's capability, exosome production in compliance with good manufacturing practice (GMP), and their ongoing clinical applications in various medical contexts to shed light on possible clinical translation.

The Transplant Bellwether: Endothelial Cells in Antibody-Mediated Rejection

Ab-mediated rejection of organ transplants remains a stubborn, frequent problem affecting patient quality of life, graft function, and grant survival, and for which few efficacious therapies currently exist. Although the field has gained considerable knowledge over the last two decades on how anti-HLA Abs cause acute tissue injury and promote inflammation, there has been a gap in linking these effects with the chronic inflammation, vascular remodeling, and persistent alloimmunity that leads to deterioration of graft function over the long term. This review will discuss new data emerging over the last 5 y that provide clues into how ongoing Ab-endothelial cell interactions may shape vascular fate and propagate alloimmunity in organ transplants.

Tregs integrate native and CAR-mediated costimulatory signals for control of allograft rejection

Tregs expressing chimeric antigen receptors (CAR-Tregs) are a promising tool to promote transplant tolerance. The relationship between CAR structure and Treg function was studied in xenogeneic, immunodeficient mice, revealing advantages of CD28-encoding CARs. However, these models could underrepresent interactions between CAR-Tregs, antigen-presenting cells (APCs), and donor-specific Abs. We generated Tregs expressing HLA-A2-specific CARs with different costimulatory domains and compared their function in vitro and in vivo using an immunocompetent model of transplantation. In vitro, the CD28-encoding CAR had superior antigen-specific suppression, proliferation, and cytokine production. In contrast, in vivo, Tregs expressing CARs encoding CD28, ICOS, programmed cell death 1, and GITR, but not 4-1BB or OX40, all extended skin allograft survival. To reconcile in vitro and in vivo data, we analyzed effects of a CAR encoding CD3ζ but no costimulatory domain. These data revealed that exogenous costimulation from APCs can compensate for the lack of a CAR-encoded CD28 domain. Thus, Tregs expressing a CAR with or without CD28 are functionally equivalent in vivo, mediating similar extension of skin allograft survival and controlling the generation of anti-HLA-A2 alloantibodies. This study reveals a dimension of CAR-Treg biology and has important implications for the design of CARs for clinical use in Tregs.

Human PSC-derived cardiac cells and their products: therapies for cardiac repair

Despite the dramatic improvements in the management of patients with chronic heart failure which have occurred over the last decades, some of them still exhaust conventional drug-based therapies without being eligible for more aggressive options like heart transplantation or implantation of a left ventricular assist device. Cell therapy has thus emerged as a possible means of filling this niche. Multiple cell types have now been tested both in the laboratory but also in the clinics and it is fair to acknowledge that none of the clinical trials have yet conclusively proven the efficacy of cell-based approaches. These clinical studies, however, have entailed the use of cells from various sources but of non-cardiac lineage origins. Although this might not be the main reason for their failures, the discovery of pluripotent stem cells capable of generating cardiomyocytes now raises the hope that such cardiac-committed cells could be therapeutically more effective. In this review, we will first describe where we currently are with regard to the clinical trials using PSC-differentiated cells and discuss the main issues which remain to be addressed. In parallel, because the capacity of cells to stably engraft in the recipient heart has increasingly been questioned, it has been hypothesized that a major mechanism of action could be the cell-triggered release of biomolecules that foster host-associated reparative pathways. Thus, in the second part of this review, we will discuss the rationale, clinically relevant advantages and pitfalls associated with the use of these PSC "products".

Immune engineered extracellular vesicles to modulate T cell activation in the context of type 1 diabetes

Extracellular vesicles (EVs) can affect immune responses through antigen presentation and costimulation or coinhibition. We generated designer EVs to modulate T cells in the context of type 1 diabetes, a T cell-mediated autoimmune disease, by engineering a lymphoblast cell line, K562, to express HLA-A*02 (HLA-A2) alongside costimulatory CD80 and/or coinhibitory programmed death ligand 1 (PD-L1). EVs presenting HLA-A2 and CD80 activated CD8+ T cells in a dose, antigen, and HLA-specific manner. Adding PD-L1 to these EVs produced an immunoregulatory response, reducing CD8+ T cell activation and cytotoxicity in vitro. EVs alone could not stimulate T cells without antigen-presenting cells. EVs lacking CD80 were ineffective at modulating CD8+ T cell activation, suggesting that both peptide-HLA complex and costimulation are required for EV-mediated immune modulation. These results provide mechanistic insight into the rational design of EVs as a cell-free approach to immunotherapy that can be tailored to promote inflammatory or tolerogenic immune responses.

Physiological and pathological consequences of exosomes at the blood-brain-barrier interface

Blood-brain barrier (BBB) interface with multicellular structure controls strictly the entry of varied circulating macromolecules from the blood-facing surface into the brain parenchyma. Under several pathological conditions within the central nervous system, the integrity of the BBB interface is disrupted due to the abnormal crosstalk between the cellular constituents and the recruitment of inflammatory cells. Exosomes (Exos) are nano-sized extracellular vesicles with diverse therapeutic outcomes. These particles transfer a plethora of signaling molecules with the potential to modulate target cell behavior in a paracrine manner. Here, in the current review article, the therapeutic properties of Exos and their potential in the alleviation of compromised BBB structure were discussed. Video Abstract.

Exosomes in transplantation: Role in allograft rejection, diagnostic biomarker, and therapeutic potential

Exosomes are 50-200 nm-sized extracellular vesicles that are secreted by cells to transfer signals and communicate with other cells. Recent research has revealed that allograft-specific exosomes containing proteins, lipids, and genetic materials are released into circulation post-transplantation which are powerful indicators of graft failure in solid-organ and tissue transplantations. The macromolecular content of exosomes released by the allograft and the immune cells serve as potential biomarkers for assessing the function and the acceptance/rejection status of the transplanted grafts. Identifying these biomarkers could aid in the development of therapeutic strategies to improve graft longevity. Exosomes can be used to deliver therapeutic agonists/antagonists to grafts and prevent rejection. Inducing long-term graft tolerance has been demonstrated in many studies using exosomes from immunomodulatory cells such as immature DCs, T regulatory cells, and MSCs. The use of graft-specific exosomes for targeted drug therapy has the potential to reduce the unwanted side effects of immunosuppressive drugs. Overall, in this review, we have explored the critical role of exosomes in the recognition and cross-presentation of donor organ-specific antigens during allograft rejection. Additionally, we have discussed the potential of exosomes as a biomarker for monitoring graft function and damage, as well as their potential therapeutic applications in mitigating allograft rejection.

Pathways of Antigen Recognition by T Cells in Allograft Rejection

The adaptive immune response leading to the rejection of allogeneic transplants is initiated and orchestrated by recipient T cells recognizing donor antigens. T-cell allorecognition is mediated via 3 distinct mechanisms: the direct pathway in which T cells recognize allogeneic major histocompatibility complex (MHC) molecules on donor cells, the indirect pathway through which T cells interact with donor peptides bound with self-MHC molecules on recipient antigen-presenting cells, and the recently described semidirect pathway whereby T cells recognize donor MHC proteins on recipient antigen-presenting cells. In this article, we present a description of each of these allorecognition pathways and discuss their role in acute and chronic rejection of allogeneic transplants.

Treg Therapy for the Induction of Immune Tolerance in Transplantation-Not Lost in Translation?

The clinical success of solid organ transplantation is still limited by the insufficiency of immunosuppressive regimens to control chronic rejection and late graft loss. Moreover, serious side effects caused by chronic immunosuppressive treatment increase morbidity and mortality in transplant patients. Regulatory T cells (Tregs) have proven to be efficient in the induction of allograft tolerance and prolongation of graft survival in numerous preclinical models, and treatment has now moved to the clinics. The results of the first Treg-based clinical trials seem promising, proving the feasibility and safety of Treg therapy in clinical organ transplantation. However, many questions regarding Treg phenotype, optimum dosage, antigen-specificity, adjunct immunosuppressants and efficacy remain open. This review summarizes the results of the first Treg-based clinical trials for tolerance induction in solid organ transplantation and recapitulates what we have learnt so far and which questions need to be resolved before Treg therapy can become part of daily clinical practice. In addition, we discuss new strategies being developed for induction of donor-specific tolerance in solid organ transplantation with the clinical aims of prolonged graft survival and minimization of immunosuppression.

Extracellular vesicles and their cells of origin: Open issues in autoimmune diseases

The conventional therapeutic approaches to treat autoimmune diseases through suppressing the immune system, such as steroidal and non-steroidal anti-inflammatory drugs, are not adequately practical. Moreover, these regimens are associated with considerable complications. Designing tolerogenic therapeutic strategies based on stem cells, immune cells, and their extracellular vesicles (EVs) seems to open a promising path to managing autoimmune diseases' vast burden. Mesenchymal stem/stromal cells (MSCs), dendritic cells, and regulatory T cells (Tregs) are the main cell types applied to restore a tolerogenic immune status; MSCs play a more beneficial role due to their amenable properties and extensive cross-talks with different immune cells. With existing concerns about the employment of cells, new cell-free therapeutic paradigms, such as EV-based therapies, are gaining attention in this field. Additionally, EVs' unique properties have made them to be known as smart immunomodulators and are considered as a potential substitute for cell therapy. This review provides an overview of the advantages and disadvantages of cell-based and EV-based methods for treating autoimmune diseases. The study also presents an outlook on the future of EVs to be implemented in clinics for autoimmune patients.

Analysis of T-cell alloantigen response via a direct pathway in kidney transplant recipients with donor-specific antibodies

Donor-specific antibodies (DSAs) are the main cause of graft loss over time. The direct pathway of alloantigen recognition is important in the pathogenesis of acute rejection. Recent studies have suggested that the direct pathway also contributes to the pathogenesis of chronic injury. Nevertheless, there are no reports on T-cell alloantigen response via the direct pathway in kidney recipients with DSAs. We analyzed the T-cell alloantigen response via the direct pathway in kidney recipients with DSAs (DSA+) or without DSAs (DSA-). A mixed lymphocyte reaction assay was implemented to assess the direct pathway response. DSA+ patients showed significantly higher CD8⁺ and CD4⁺ T cell responses to donor cells than DSA- patients. Furthermore, proliferating CD4⁺ T cells showed a marked increase in Th1 and Th17 responses in DSA+ patients than in DSA- patients. In a comparison between anti-donor and third-party responses, the anti-donor CD8⁺ and CD4⁺ T cell response was significantly lower than the anti-third-party response. In contrast, the donor-specific hyporesponsiveness was absent in DSA+ patients. Our study demonstrated that DSA+ recipients have a greater potential for developing immune responses against the donor tissues via the direct alloantigen recognition pathway. These data contribute to an understanding of DSAs pathogenicity during kidney transplantation.

Recent Advances in Extracellular Vesicle-Based Therapies Using Induced Pluripotent Stem Cell-Derived Mesenchymal Stromal Cells

Extracellular vesicles (EVs) are being widely investigated as acellular therapeutics in regenerative medicine applications. EVs isolated from mesenchymal stromal cells (MSCs) are by far the most frequently used in preclinical models for diverse therapeutic applications, including inflammatory, degenerative, or acute diseases. Although they represent promising tools as cell-free therapeutic agents, one limitation to their use is related to the batch-to-batch unreliability that may arise from the heterogeneity between MSC donors. Isolating EVs from MSCs derived from induced pluripotent stem cells (iMSCs) might allow unlimited access to cells with a more stable phenotype and function. In the present review, we first present the latest findings regarding the functional aspects of EVs isolated from iMSCs and their interest in regenerative medicine for the treatment of various diseases. We will then discuss future directions for their translation to clinics with good manufacturing practice implementation.

The Therapeutic Potential of Exosomes in Soft Tissue Repair and Regeneration

Soft tissue defects are common following trauma and tumor extirpation. These injuries can result in poor functional recovery and lead to a diminished quality of life. The healing of skin and muscle is a complex process that, at present, leads to incomplete recovery and scarring. Regenerative medicine may offer the opportunity to improve the healing process and functional outcomes. Barriers to regenerative strategies have included cost, regulatory hurdles, and the need for cell-based therapies. In recent years, exosomes, or extracellular vesicles, have gained tremendous attention in the field of soft tissue repair and regeneration. These nanosized extracellular particles (30-140 nm) can break the cellular boundaries, as well as facilitate intracellular signal delivery in various regenerative physiologic and pathologic processes. Existing studies have established the potential of exosomes in regenerating tendons, skeletal muscles, and peripheral nerves through different mechanisms, including promoting myogenesis, increasing tenocyte differentiation and enhancing neurite outgrowth, and the proliferation of Schwann cells. These exosomes can be stored for immediate use in the operating room, and can be produced cost efficiently. In this article, we critically review the current advances of exosomes in soft tissue (tendons, skeletal muscles, and peripheral nerves) healing. Additionally, new directions for clinical applications in the future will be discussed.

Enigmatic role of exosomes in breast cancer progression and therapy

Breast cancer (BC) is reported to be the leading cause of mortality in females worldwide. At the beginning of the year 2021, about 7.8 million women were diagnosed with BC in past 5 years. High prevalence and poor neoadjuvant chemotherapeutic efficacy has motivated the scientists around the globe to investigate alternative management strategies. In recent years, there has been an exponential rise in the scientific studies reporting the role of tumor derived exosomes (TDEs) in the BC pathophysiology and management. TDEs play an important role in the intercellular communication and transportation of biomolecules. This manuscript reviews the role of exosomes in the BC pathophysiology, diagnosis, and therapy. Role of TDEs in the mechanistic pathways of BC metastasis, immunosuppression, migration, dormancy and chemo-resistance is extensively reviewed. We have also highlighted the epigenetic modulations orchestrated by exosomal miRNAs and long noncoding RNAs (lnc RNAs) in the BC environment. Liquid biopsies analyzing blood circulating exosomes for early and accurate detection of the BC have been discussed. Characterization of exosomes, strategies to use them in BC chemotherapy, BC immunotherapy and potential challenges that will present themselves in translating exosomes based technologies to market are discussed.

Functional Recovery Caused by Human Adipose Tissue Mesenchymal Stem Cell-Derived Extracellular Vesicles Administered 24 h after Stroke in Rats

Ischemic stroke is a major cause of death and disability, intensely demanding innovative and accessible therapeutic strategies. Approaches presenting a prolonged period for therapeutic intervention and new treatment administration routes are promising tools for stroke treatment. Here, we evaluated the potential neuroprotective properties of nasally administered human adipose tissue mesenchymal stem cell (hAT-MSC)-derived extracellular vesicles (EVs) obtained from healthy individuals who underwent liposuction. After a single intranasal EV (200 µg/kg) administered 24 h after a focal permanent ischemic stroke in rats, a higher number of EVs, improvement of the blood-brain barrier, and re-stabilization of vascularization were observed in the recoverable peri-infarct zone, as well as a significant decrease in infarct volume. In addition, EV treatment recovered long-term motor (front paws symmetry) and behavioral impairment (short- and long-term memory and anxiety-like behavior) induced by ischemic stroke. In line with these findings, our work highlights hAT-MSC-derived EVs as a promising therapeutic strategy for stroke.

C1q as a potential tolerogenic therapeutic in transplantation

In 1963, Lepow and colleagues resolved C1, the first component of the classical pathway, into three components, which they named C1q, C1r, and C1s. All three of these components were demonstrated to be involved in causing hemolysis in vitro. For over 30 years after that seminal discovery, the primary function attributed to C1q was as part of the C1 complex that initiated the classical pathway of the complement cascade. Then, a series of papers reported that isolated C1q could bind to apoptotic cells and facilitate their clearance by macrophages. Since then, rheumatologists have recognized that C1q is an important pattern recognition receptor (PRR) that diverts autoantigen containing extracellular vesicles from immune recognition. This critical function of C1q as a regulator of immune recognition has been largely overlooked in transplantation. Now that extracellular vesicles released from transplants have been identified as a major agent of immune recognition, it is logical to consider the potential impact of C1q on modulating the delivery of allogeneic extracellular vesicles to antigen presenting cells. This concept has clinical implications in the possible use of C1q or a derivative as a biological therapeutic to down-modulate immune responses to transplants.

Applications, challenges and prospects of mesenchymal stem cell exosomes in regenerative medicine

Recent advances in the identification and application of different stem cell types have offered alternative therapeutic approaches for clinicians. The lack of successful engraftment, migration into the injured site, loss of functionality and viability, ethical issues, shortage of donated allogeneic stem cells and the possibility of transmission of infectious are the main challenges associated with direct cell transplantation. The discovery and research on exosomes have led to the rise of hopes for the alleviation of different pathologies in regenerative medicine. Exo are nano-sized extracellular vesicles (40-150 nm) and released by each type. These nanoparticles participate in cell-to-cell communication in a paracrine manner. It is thought that the application of Exo can circumvent several drawbacks related to whole-cell therapies. Because of their appropriate size and stability, Exo are touted as therapeutic bullets transferring signaling factors into the acceptor cells in a paracrine manner. Despite these advantages, technologies associated with Exo isolation and purification are challenging because of heterogeneity in exosomal size and cargo. The lack of standard GMP-grade protocols is the main hurdle that limits the extensive application of Exo in the clinical setting. Here, the authors aimed to inspire a logical and realistic vision about problems associated with Exo application in regenerative medicine.