1. Age-Related Macular Degeneration

Age-related macular degeneration (AMD) is a prevalent eye disorder that affects individuals over 50 years old, causing vision loss and blindness in the elderly. AMD can be classified into three stages: early, intermediate, and late. Early-stage AMD presents with medium-sized drusen deposits without pigment changes or vision loss. As AMD progresses to the intermediate stage, large drusen deposits and/or pigment changes may cause mild vision loss. Late-stage AMD can be dry or wet, with wet AMD causing rapid vision decline due to fluid and blood leakage from abnormal blood vessel growth beneath the macula, known as choroidal neovascularization (CNV). Several targets exist for AMD treatment, such as reducing inflammation, inhibiting angiogenesis, and treating CNV in wet AMD. Treatment options depend on the type and severity of AMD. While dry AMD can be managed through monitoring and nutritional supplements, wet AMD typically requires frequent intravitreal injections of anti-VEGF drugs. However, not all patients respond favorably to this treatment, and vision-threatening complications such as endophthalmitis and retinal detachment may occur. MSC-derived exosomes offer potential advantages in addressing these issues. They may significantly reduce the frequency of intravitreal injections due to better biocompatibility and longer duration of action, and they have the potential to be delivered topically instead of intravitreally. Therefore, optimizing therapies that target both inflammation and neovascularization with the use of MSC-derived exosomes could provide a more effective and less burdensome treatment solution ^[1][2].

Hajrasouliha and colleagues were the first to demonstrate the potential therapeutic effects of exosomes in age-related macular degeneration by suppressing retinal vessel leakage and inhibiting choroidal neovascularization in 2013 ^[3]. In a subsequent study in 2018, He and colleagues investigated the effects of human umbilical cord MSC-derived exosomes on age-related macular degeneration (AMD) and the development of choroidal neovascularization (CNV) using a blue light injured human retinal pigment epithelial (RPE) cell model and a laser-induced CNV rat model ^[4]. The authors found that the MSC-derived exosomes downregulated VEGF-A expression in RPE cells and improved the histological structures of CNV for better visual function in vivo. The study suggested that MSC-derived exosomes have potential as a treatment for AMD and CNV.

Li and colleagues (2021) investigated the use of human umbilical cord-derived mesenchymal stem cells (hUCMSCs) in vivo and in vitro to attenuate subretinal fibrosis. Laser-induced choroidal neovascularization (CNV) and subretinal fibrosis models were established in mice, and upon intravitreal injection of hUCMSC-exosomes, alleviation in subretinal fibrosis was observed in vivo. Additionally, hUCMSC-exosomes suppressed the migration of RPE cells and promoted the mesenchymal–epithelial transition via miR-27-3p. In addition, intravitreal injection of hUCMSC-exosome effectively ameliorated laser-induced CNV and subretinal fibrosis via the suppression of epithelial–mesenchymal transition (EMT) process. These studies elucidated the potential use of MSC-exosomes as an alternative treatment for wet AMD and CNV, potentially reducing the need for frequent anti-VEGF injections prescribed by ophthalmologists.

2. Retinitis Pigmentosa

Retinitis pigmentosa (RP) is a hereditary disorder that causes progressive vision loss due to the degeneration of photoreceptor cells, primarily rods, resulting from genetic mutations. Unfortunately, conventional treatments for RP are limited, mainly providing symptom management, eye complication prevention, and slow disease progression. Voretigene neparvovec is a targeted gene therapy that halts and cures RP caused by the RPE65 gene mutation, present in only 0.3% to 1% of RP patients. Researchers have focused on preventing retinal degeneration by targeting the posterior eye through neuroprotective agents or mesenchymal stem cell transplantation (MSCT). These treatments aim to prevent photoreceptor death, promote its survival, and potentially regenerate functional cells and tissues ^[5].

In preclinical studies, mesenchymal stem cell transplantation (MSCT) and its derived exosomes (MSC-exosomes) have shown potential as treatment options for photoreceptor apoptosis in retinitis pigmentosa (RP) by preventing photoreceptor death, promoting its survival, and potentially regenerating functional cells and tissues. Deng et al. (2020) used mouse bone marrow MSCT in an N-methyl-N-nitrosourea (MNU) driven photoreceptor injury mouse model and found that MSC-exosomes were critical for preventing photoreceptor apoptosis and alleviating retinal morphological and functional degeneration ^[6]. Liu et al. (2019) treated Rd10 mutated mice with human bone marrow MSC-exosomes and observed improvements in electroretinogram (ERG) and optokinetic tracking response (OKT), as well as the inhibition of pro-inflammatory cytokine expression, indicating relief of neuroinflammation ^[7]. Similarly, Zhang et al. (2022) investigated the use of MSC-exosomes in RP and observed increased photoreceptor survival, preservation of their structure, and improved visual function. Additionally, MSC-exosomes were found to inhibit inflammation through overexpression of the miR-146a-Nr4a3 axis ^[8]. Although these preclinical studies were conducted using animal models, they elucidated the potential long-term benefits of using MSC-exosomes in the treatment of RP. However, future clinical studies are necessary to determine whether the same effects can be achieved in humans.

3. Diabetic Retinopathy

Diabetic retinopathy, a chronic eye condition, involves progressive damage to the blood vessels of the retina, with two types of the disease existing: non-proliferative diabetic retinopathy (NPDR) and proliferative diabetic retinopathy (PDR). NPDR is characterized by vascular permeability, blockage, and the formation of various abnormalities. PDR occurs in the advanced stages of diabetic retinopathy and is caused by ongoing damage to the retinal blood vessels, which results in significant retinal ischemia. The release of pro-angiogenic factors, including vascular endothelial growth factor (VEGF), from ischemic retinal tissue stimulates the growth of new, abnormal blood vessels, leading to vision-threatening complications such as neovascularization. Inhibiting the activity of VEGF through laser photocoagulation or intravitreal anti-VEGF injections is the primary management strategy for PDR, as these methods work by binding to VEGF and preventing it from interacting with its receptor ^[9].

Preclinical studies have investigated the therapeutic potential of MSC-exosomes to treat diabetic retinopathy (DR) and its complications. In a streptozotocin-induced diabetes mellitus (DM) rabbit model, Safwat et al. (2018) injected MSC-exosomes derived from the adipose tissue of rabbits via different routes (intravenous (IV), subconjunctival (SC), and intraocular (IO)) ^[7]. Retinal regeneration was observed at 12 weeks post-administration, with the IV route resulting in irregular ganglionic layer and increased retinal thickness, SC administration leading to well-defined layers, and IO administration resulting in morphologically and functionally analogous layers to those of a normal retina. The role of micRNA-222 in DR was also explored, with the findings showing that its underexpression in hyperglycemic conditions resulted in increased retinal damage and hemorrhage. However, this damage was ameliorated through MSC-exosomal transfer of micRNA-222 ^[10]. Additionally, Zhang et al. (2019) investigated the role of miR-126 transfer via human umbilical cord mesenchymal stem cell (MSC)-derived exosomes (hUCMSC-Exos) in regulating hyperglycemia-induced retinal inflammation ^[11]. The administration of hUCMSC-exosomes effectively reversed inflammation in diabetic rats, with hUCMSC-exosomes overexpressing miR-126 more successfully suppressing inflammation compared to control hUCMSC-exosomes. This highlights the key role of miR-126 in attenuating DR.

Studies have identified individual microRNAs involved in the amelioration of DR by MSC-exosomes. Li et al. (2021) showed the potential of bone marrow mesenchymal stem cell (BMSC)-induced exosomal microRNA-486-3p (miR-486-3p) in treating DR in mice ^[12]. Histological analysis of Muller cells injected with streptozotocin (STZ) confirmed DR pathology, with upregulated Toll-like receptor 4 (TLR4) and nuclear factor-kappa B (NF-κB). Exposure to BMSC-exosomes in vitro promoted Muller cell proliferation and inhibited inflammation, oxidative stress, and apoptosis. Upregulating miR-486-3p or downregulating TLR4 further inhibited oxidative stress, inflammation, and apoptosis, indicating that miR-486-3p targets TLR4. Li et al. (2021) then demonstrated the role of microRNA-17-3p in targeting the signal transducer and activator of transcription 1 (STAT1) in a DR mouse model ^[1]. An overexpression of miR-17-3p in hUCMSC-exosomes decreased STAT1 expression, while a depletion of miR-17-3p in exosomes exerted an inverse effect. Overall, injection of MSC-exosomes overexpressing miR-17-3p reduced blood glucose and HbA1c, increased body weight and Hb content, decreased inflammatory factors and VEGF, alleviated oxidative injury, and inhibited retinal cell apoptosis in DR mice through the inhibition of STAT1.

Gu and colleagues (2022) investigated the use of bone marrow mesenchymal stem cell (BMSC) exosomal miR-146a to reduce inflammation in DR mice ^[2]. The researchers found that exposing microglial cells of DR mice to BMSC exosomal miR-146a reduced levels of proliferating cell antigen and B-cell lymphoma-2, as well as inflammatory cytokines TNF-α, IL-1β, and IL-6. The study also showed an inverse association between miR-146a and TLR4, as overexpression of TLR4 reversed the effects of miR-146a on the proliferation, apoptosis, and inflammation of microglia. Another study conducted by Ebrahim and colleagues (2022) investigated the Wnt/b-catenin signaling pathway in DR using rat bone marrow-derived mesenchymal stem cell exosomes (BMMSCs) ^[13]. The researchers treated rats with STZ-induced DR with intravitreal administration of BMMSC-exosomes to block the wnt/b-catenin pathway. They found that this treatment resulted in a significant decrease in retinal mRNA markers indicative of oxidative stress, inflammation, and angiogenesis and vascular leakage in DR compared to diabetic controls. This effect was achieved by targeting the miR-129–5p and miR-34a exosomal microRNA.

Cao and colleagues (2021) investigated the involvement of long non-coding RNA (lncRNAs) small nucleolar RNA host gene (SNHG7) in DR pathogenesis ^[14]. They used human retinal microvascular endothelial cells (HRMECs) treated with high glucose (HG) to establish a DR cell model. Their findings suggest that MSC-exosomal lncRNA SNHG7 downregulates miR-34a-5p and inhibits hyperglycemia-induced endothelial–mesenchymal transition (EndMT) and tube formation of HRMECs. Overexpression of miR-34a-5p reversed these benefits, while knockdown of miR-34a-5p repressed HG-induced EndMT and tube formation. Overall, these results highlight the potential of MSC-exosomal lncRNA SNHG7 in suppressing EndMT and tube formation in HRMECs via miR-34a-5p/XBP1 downregulation.

4. Retinal Ischemia

Retinal ischemia is a condition caused by insufficient blood flow to the retina, leading to a lack of oxygen and nutrients and potential vision loss. Both systemic and ocular conditions can contribute to its development. Systemic causes include diabetes mellitus, hypertension, and blood dyscrasias, while ocular causes include retinal vascular occlusions and carotid artery stenosis. Inflammatory and degenerative eye diseases can also induce retinal ischemia by compromising ocular circulation. Treatment options depend on the underlying cause, but anti-VEGF medications are commonly prescribed to prevent abnormal blood vessel growth in the retina.

Several studies have investigated the potential of MSC-exosomes in treating retinal ischemic injuries. Mosseiev et al. (2017) demonstrated the protective effect of human mesenchymal stem cells (hMSCs) administered intravitreally in the murine model. The study induced oxygen-induced retinopathy in two groups of mice, followed by administration of saline and MSC-exosomes, respectively. Results indicated that hMSC-exosomes significantly reduced retinal thinning and neovascularization, suggesting the therapeutic effect of paracrine factors and miRNAs. In a laser-induced injury and ischemia model, Yu et al. (2016) also found that intravitreal injection of adipose-derived and human umbilical cord-derived MSC-exosomes reduced damage, inhibited apoptosis, and suppressed inflammatory responses in mice. Moreover, the downregulation of monocyte chemotactic protein (MCP)-1 in the retina suggests that MSC-exosomes may ameliorate laser-induced retinal injury. These studies demonstrate promising results, and the use of MSC-exosomes could potentially be extended to other ischemic retinal diseases, such as retinopathy of prematurity, ocular ischemic syndrome, and retinal vein and artery occlusion.

Mathew and colleagues (2019) investigated the neuroprotective effects of mesenchymal stem cell (MSC)-exosomes in retinal ischemia. They used an in vitro oxygen-glucose deprivation (OGD) model of retinal ischemia with R28 cell line derived from rats and found that MSC-exosomes reduced cell death, attenuated loss of cell proliferation, and decreased neuroinflammation and apoptosis in the rat model when injected into the vitreous humor 24 h post-ischemia development. MSC-exosomes were present in the vitreous humor for four weeks after intravitreal administration ^[15]. Yu and colleagues (2022) explored the neuroprotective effects of human gingival MSCs (hGMSC) derived exosomes in retinal ischemia-reperfusion injury ^[16]. They injected hGMSC-exosomes into the vitreous of mice and found that the injection of exosomes transfected with siRNA-maternally expressed gene 3 (siRNA-MEG3) that were stimulated by TNF-α (TG-exos) significantly reduced inflammation and cell loss compared to unstimulated exosomes (G-exos) in mice with retinal ischemia. Furthermore, miR-21-5p acted as a crucial factor in TG-exos for neuroprotection and anti-inflammation. These findings suggest that MSC-exosomes have potential as a therapeutic option for retinal ischemic injuries.

Finally, Ma and colleagues (2020) investigated the potential therapeutic effects of using rat bone marrow-derived MSC-exosomes in a rat retinal detachment model ^[17]. The subretinal administration of MSC-exosomes at the time of retinal separation reduced the expression of proinflammatory cytokines at day seven and suppressed photoreceptor cell apoptosis, resulting in the maintenance of normal retinal structure. This study highlights the potential therapeutic effects of MSC-exosomes on retinal ischemia secondary to retinal detachment.

5. Uveitis

Uveitis is an ocular inflammation that affects the uvea, which encompasses the iris, ciliary body, and choroid. The type of uveitis depends on the part of the eye involved, with anterior, intermediate, posterior, and panuveitic forms being the most common types. The causes of uveitis can be diverse, with non-infectious autoimmune or idiopathic sources being the most prevalent, while infectious causes are typically less frequent but more severe. Non-infectious anterior uveitis is usually treated with glucocorticoid steroids, either topically or orally, and sometimes in combination with non-steroidal therapies. However, posterior uveitis presents a greater challenge, as topical eye drops cannot reach the critical posterior segments of the eye, including the macula, optic nerve, and retinal vessels, where rapid vision loss and blindness can occur. In these instances, more invasive administration methods, such as periocular or intravitreal injection, are often required. The use of MSC-derived exosomes as a drug-delivery system is promising, as they may effectively penetrate barriers and protect their cargo from degradation, leading to increased bioavailability.

In 2014, Oh and colleagues demonstrated the potential benefits of using mesenchymal stem cells (MSCs) to prevent experimental autoimmune uveitis (EAU) in mice. The administration of intraperitoneal hMSCs resulted in a reduction in proinflammatory cytokines in the eye and a marked suppression of Th1 and Th17 cells in draining lymph nodes, protecting the retina and attenuating EAU. More recently, Zhang et al. (2018) investigated the efficacy of human umbilical cord-derived MSC-exosomes in treating large and refractory macular holes (MHs) in seven patients who underwent vitrectomy and internal limiting membrane peeling. Six of the MHs closed post-treatment, indicating the potential benefits of MSC and MSC-exosome therapy in promoting functional and anatomic recovery from MH. Compared to MSCT therapy, MSC-exosome therapy was found to be safer and easier to administer due to its lower risk of developing proliferations and immune responses, and the small size of MSCs allows for various administration routes. However, a lack of a control group and limited patient numbers prevented conclusions about the superiority of MSC-exosome therapy over MSCT treatment for MH closure.

Recent studies have focused on exploring MSC-exosomes as potential therapeutic agents for EAU, since their therapeutic effects are mediated via the transport and transfer of exosomes containing various miRNAs. Shigemoto-Kuroda et al. (2017) investigated the use of MSC-exosomes for T1D and EAU by harvesting exosomes from human bone marrow MSCs and administering them intravenously into established mouse models. The study found that MSC-exosomes prevented the onset of T1D and EAU by inhibiting the activation of antigen-presenting cells and suppressing the activation of Th1 and Th17 cells. Similar findings were reported by Bai et al. (2017) when exploring the potential therapeutic effects of human umbilical cord-derived MSC-exosomes on EAU. In vivo administration of MSC-exosomes demonstrated a milder development of EAU compared to control rats, and in vitro assays demonstrated a marked reduction in the intensity of EAU following MSC-exosomes administration. MSC-exosomes effectively inhibited the migration of CD4+T cells, neutrophils, NK cells, and macrophage cells, reduced the percentage of CD4+IFN-γ+ and CD4+IL-17+ cells in the retina, and reduced unchecked inflammation. Although MSC-exosomes treatment reduced the concentration of Tregs, the reduction was not significant enough to eliminate the impending beneficial role of Tregs in the suppression of EAU. Overall, preclinical studies suggest that MSC-exosomes could serve as a potential treatment for T1D and EAU, reducing the need for conventional treatments such as topical steroids and systemic immunosuppressants.

Xie et al. (2018) conducted a study to investigate the effects of MSC-exosomes on EAU in a rat model. The experimental group received periocular injection of MSC-exosomes while the control group received phosphate buffer ^[18]. The study observed that CD68 cell expression was significantly lower in the experimental group, even 15 days after administration of MSC-exosomes. The retinal pathological score was also significantly lower in the experimental group, along with a decrease in the number of Th1, Th17, and Tregs. The study also showed that the experimental group had better retinal function than the control group 15 days post MSC-exosomes administration. Similarly, Yongtao et al. (2021) confirmed the therapeutic effects of MSC-exosomes on EAU through intravenous injection of hUCMSCs in mice. In this study, the treatment group showed significantly lower inflammation and pathological scores compared to the control group. There was also a decrease in the number of Th1 and Th17 cells, as well as reduced proliferation of other T cell subtypes in the experimental group. Based on these findings, Yongtao et al. (2022) conducted a subsequent study to investigate the potential therapeutic role of IL-10-overexpressing MSC-exosomes on EAU ^[19]. This study revealed that IL-10-overexpressing MSC-exosomes effectively suppressed the proliferation and differentiation of Th1 and Th17 cells with increased Treg cells in the spleen and DLN. Overall, these preclinical studies suggest that MSC-exosomes have the potential to be a novel therapy for treating EAU.

Li and colleagues (2022) investigated the use of Rapamycin loaded MSC-exosomes (Rapa-MSC exosomes) as a conjugate therapy for EAU and ocular complications caused by frequent intravitreal injections, given the successful penetration of MSC-exosomes into the eye. In mice with EAU, Rapa-MSC exosomes were found to significantly reduce ocular inflammatory cell infiltration, protect the retinal structure and improve drug delivery of Rapamycin into the eye within 24 h of subconjunctival injection. This study highlights the potential of MSC-exosomes in improving drug delivery and efficacy in the eye ^[19].

6. Idiopathic Macular Hole

An idiopathic macular hole, a tear in the center of the retina caused by pathological vitreoretinal traction, can result in central vision loss or distortion. Treatment options depend on the severity of the condition, with stage 2 or above requiring a surgical intervention called pars plana vitrectomy (PPV). However, PPV is invasive and carries several risks, including tearing or detachment of the retina, increased eye pressure, reinfection, and permanent vision loss. Additionally, the postoperative period demands strict compliance with a prone position for over a week, which can be uncomfortable for patients ^[20].

Intravitreal ocriplasmin provides a less invasive treatment option for macular holes, but it still carries certain risks associated with the injection and the drug itself. Risks may include lens subluxation or phacodonesis, dyschromatopsia, transient changes in the electroretinogram, retinal tear or detachment, and decreased visual acuity ^[21].

Zhang et al. (2018) investigated the efficacy of human umbilical cord-derived MSC-exosomes to treat large and refractory macular holes (MHs). In a study of seven patients, all underwent vitrectomy and internal limiting membrane peeling, with two receiving MSCT therapy and five receiving MSC-exosome treatment. Six of the MHs closed post-treatment, while one remained flat-open. MSC-exosome therapy was found to be safer and easier to administer than MSCT therapy due to its lower risk of developing proliferations and immune responses, as well as the small size of MSC allowing for various administration routes. However, a lack of a control group and a limited number of patients did not allow for conclusions about the superiority of MSC-exosome therapy over MSCT treatment for MH closure. Nonetheless, the findings indicate the potential benefits of MSC and MSC-exosome therapy in promoting functional and anatomic recovery from MH.